The standard solution must be put in a burette with a glass stopcock, as

it attacks india-rubber. The assay should be contained in a pint flask,

and be cooled before titrating. The standard solution must be run in

until a pinkish tinge permeates the whole solution; this must be taken

as the finishing point. When certain interfering bodies are present this

colour quickly fades, but the fading must be ignored. With pure

solutions the colour is fairly permanent, and a single drop of the

potassium permanganate solution is sufficient to determine the finishing

point.

2. _With Bichromate of Potash._--Prepare a standard solution by

dissolving 4.39 grams of the powdered and dried salt in water, and

diluting to 1 litre. This solution is permanent, its strength is

determined by dissolving 0.2 gram of iron wire in 10 c.c. of dilute

sulphuric acid, diluting to about a quarter of a litre, and titrating.

Also prepare a test solution by dissolving 0.1 gram of ferricyanide of

potassium in 100 c.c. of water. This solution does not keep well and

must be freshly prepared.

An ordinary burette is used. The assay is best contained in a glazed

earthenware dish, and may be titrated hot or cold. To determine the

finishing point, place a series of drops of the ferricyanide solution on

a dry white glazed plate. The drops should be of about the same size and

be placed in lines at fairly equal distances. The bichromate is run in,

in a steady stream, the assay solution being continuously stirred until

the reaction is sensibly slackened. Then bring a drop of the assay with

the stirrer in contact with one of the test drops on the plate. The

standard can be safely run in 1 c.c. at a time, so long as the test drop

shows signs of a precipitate. When only a coloration is produced run in

cautiously a few drops at a time so long as two drops of the assay gives

with the test a colour which is even faintly greener than two drops of

the assay solution placed alongside. The finishing point is decided and

practically permanent, although it demands a little practice to

recognise it. The titration with permanganate of potassium has the

advantage of a more distinct finishing point and easier mode of working;

its application, however, is somewhat limited by the disturbing effects

of hydrochloric acid. The bichromate method has the advantage of a

standard solution which does not alter in strength, and the further one

of being but little affected by altering conditions of assay.

Hydrochloric acid has practically no effect on it. Both methods give

accurate results and are good examples of volumetric methods.

The following results illustrate the extent to which the methods may be

relied on; and the influence which the various conditions of experiment

have on the assay.

Solutions of ferrous sulphate and of ferrous chloride were made

containing 0.5 gram of iron in each 100 c.c., thus corresponding to the

standard solutions of permanganate and bichromate of potassium. These

last were prepared in the way already described. The solution of ferrous

sulphate was made by dissolving 5.01 grams of iron wire in 100 c.c. of

dilute sulphuric acid and diluting to 1 litre. A similar solution may be

made by dissolving 24.82 grams of pure ferrous sulphate crystals in

water, adding 100 c.c. of dilute sulphuric acid, and diluting to 1

litre.

~Rate of Oxidation by Exposure to Air.~--This is an important

consideration, and if the rate were at all rapid would have a serious

influence on the manner of working, since exclusion of air in the

various operations would be troublesome. 20 c.c. of the solution of

ferrous sulphate were taken in each experiment, acidified with 10 c.c.

of dilute sulphuric acid, and diluted to 100 c.c. The solution was

exposed, cold, in an open beaker for varying lengths of time, and

titrated with permanganate of potassium.

Time exposed 1 hour 1 day 2 days 3 days

c.c. required 19.2 19.1 19.0 19.0

These results show that the atmospheric oxidation in cold solutions is

unimportant. With boiling solutions the results are somewhat different;

a solution which at the outset required 20 c.c. of permanganate of

potassium, after boiling for an hour in an open beaker (without any

precautions to prevent oxidation), water being added from time to time

to replace that lost by evaporation, required 19.2 c.c. If the solution

be evaporated to dryness the oxidising power of concentrated sulphuric

acid comes into play, so that very little ferrous iron will be left. A

solution evaporated in this way required only 2.2 c.c. of permanganate

of potassium.

~Effect of Varying Temperature.~--In these experiments the bulk was in

each case 100 c.c., and 10 c.c. of dilute sulphuric acid were present.

The permanganate required by

1 c.c. of ferrous sulphate was, at 15° 1.0 c.c., and at 70° 1.1 c.c.

10 " " " 9.7 " 9.8 "

100 " " " 97.7 " 96.8 "

The lower result with the 100 c.c. may be due to oxidation from

exposure.

~Effect of Varying Bulk.~--The following experiments show that

considerable variations in bulk have no practical effect. In each case

20 c.c. of ferrous sulphate solution and 10 c.c. of dilute acid were

used.

Bulk of assay 30 c.c. 100 c.c. 500 c.c. 1000 c.c.

Permanganate required 20.0 " 20.0 " 20.2 " 20.5 "

~Effect of Free Sulphuric Acid.~--Free acid is necessary for these

assays; if there is an insufficiency, the assay solution, instead of

immediately decolorising the permanganate, assumes a brown colour. The

addition of 10 c.c. of dilute sulphuric acid suffices to meet

requirements and keep the assay clear throughout. The following

experiments show that a considerable excess of acid may be used without

in the least affecting the results. In each case 20 c.c. of ferrous

sulphate were used.

Dilute sulphuric

acid 1.0 c.c. 5.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

Permanganate

required 19.3 " 19.3 " 19.3 " 19.3 " 19.3 " 19.3 "

~Effect of Foreign Salts.~--When the assay has been reduced with zinc

varying quantities of salts of this metal pass into solution, the amount

depending on the quantity of acid and iron present. Salts of sodium or

ammonium may similarly be introduced. It is essential to know by

experiment that these salts do not exert any effect on the titration.

The following series of experiments show that as much as 50 grams of

zinc sulphate may be present without interfering.

Zinc sulphate present 0 gram 1 gram 10 grams 50 grams

Permanganate required 19.3 c.c. 19.3 c.c. 19.3 c.c. 19.3 c.c.

Magnesium, sodium, and ammonium salts, are equally without effect.

Ammonic sulphate present 0 gram 1 gram 10 grams

Permanganate required 19.3 c.c. 19.2 c.c. 19.3 c.c.

Sodic sulphate present 0 gram 1 gram 10 grams

Permanganate required 19.3 c.c. 19.3 c.c. 19.3 c.c.

Magnesic sulphate present 0 gram 1 gram 10 grams

Permanganate required 19.3 c.c. 19.3 c.c. 19.3 c.c.

~Effect of Varying Amounts of Iron.~--It is important to know within

what limits the quantity of iron in an assay may safely vary from that

used in standardising. In the following experiments the conditions as to

bulk, acidity, and mode of working were the same as before:--

Ferrous sulphate solution taken 1 c.c. 10 c.c. 20 c.c. 50 c.c. 100 c.c.

Permanganate required 1.0 " 9.7 " 19.6 " 48.9 " 97.7 "

The ferrous sulphate solution is here a little weaker than that of the

permanganate of potassium, but the results show that the permanganate

required is proportional to the iron present.

~Titrations in Hydrochloric Solutions.~--These are less satisfactory

than those in sulphuric solutions, since an excess of hydrochloric acid

decomposes permanganate of potassium, evolving chlorine, and since the

finishing point is indicated, not by the persistence of the pink colour

of the permanganate, but by a brown coloration probably due to

perchloride of manganese. Nevertheless, if the solution contains only

from 5 to 10 per cent. of free hydrochloric acid (sp. g. 1.16) the

results are the same as those obtained in a sulphuric acid solution.

Equal weights (0.1 gram) of the same iron wire required exactly the same

quantity of the permanganate of potassium solution (20 c.c.) whether the

iron was dissolved in dilute sulphuric or dilute hydrochloric acid. The

following series of experiments are on the same plan as those given

above with sulphuric acid solutions. A solution of ferrous chloride was

made by dissolving 5.01 grams of iron wire in 50 c.c. of dilute

hydrochloric acid and diluting to 1 litre. The dilute hydrochloric acid

was made by mixing equal volumes of the acid (sp. g. 1.16) and water.

~Rate of Atmospheric Oxidation.~--20 c.c. of the ferrous chloride

solution were acidified with 10 c.c. of the dilute hydrochloric acid and

diluted to 100 c.c. This solution was exposed cold in open beakers.

Time exposed -- 1 hour 1 day 2 days 3 days

Permanganate required 19.6 c.c. 19.6 c.c. 19.5 c.c. 19.4 c.c. 19.5 c.c.

Similar solutions boiled required, before boiling, 20 c.c.; after

boiling for one hour, replacing the water as it evaporated, 19.3 c.c.;

and after evaporation to a paste and redissolving, 17.0 c.c.

~Effect of Varying Temperature.~--Solutions similar to the last were

titrated and gave the following results:--

Temperature 15° 30° 50° 70°

Permanganate required 19.8 c.c. 19.6 c.c. 19.5 c.c. 19.4 c.c.

~Effect of Varying Bulk.~--As before, 20 c.c. of the iron solution, and

10 c.c. of the dilute acid were diluted to the required volumes and

titrated.

Bulk 30 c.c. 100 c.c. 500 c.c. 1000 c.c.

Permanganate required 20.4 " 20.3 " 20.8 " 21.5 "

The variation due to difference in bulk here, although only equal to an

excess of 0.7 milligram of iron for each 100 c.c. of dilution, are about

three times as great as those observed in a sulphuric acid solution.

~Effect of Free Hydrochloric Acid.~--In these experiments 20 c.c. of the

ferrous chloride solution were used with varying quantities of acid, the

bulk of the assay in each case being 100 c.c.

Dilute acid present 5 c.c. 10 c.c. 50 c.c. 100 c.c.

Permanganate required 20.2 " 20.2 " 20.5 " 21.0 "

The last had a very indistinct finishing point, the brown coloration

being very evanescent. The effect of the acid is modified by the

presence of alkaline and other sulphates, but not by sulphuric acid.

Repeating the last experiment we got--

Without further addition 21.0 c.c.

With 100 c.c. of dilute sulphuric acid 22.0 "

" 10 grams ammonic sulphate 20.5 "

" 10 " sodic sulphate 20.0 "

" 10 " magnesium sulphate 20.4 "

" 10 " manganese sulphate 20.2 "

The results with these salts, in counteracting the interference of the

acid, however, were not a complete success, since the end-reactions were

all indistinct, with the exception, perhaps, of that with the manganese

sulphate.

~Effect of Varying Amounts of Iron.~--In these experiments the bulk of

the assay was 100 c.c., and 10 c.c. of acid were present.

Ferrous chloride used 1 c.c. 10 c.c. 20 c.c. 50 c.c. 100 c.c.

Permanganate required 1.1 " 10.3 " 20.3 " 50.4 " 100.1 "

In making himself familiar with the permanganate of potassium titration,

the student should practise by working out a series of experiments

similar to the above, varying his conditions one at a time so as to be

certain of the cause of any variation in his results. He may then

proceed to experiment on the various methods of reduction.

_A solution of ferric chloride_ is made by dissolving 5.01 grams of iron

wire in 50 c.c. of hydrochloric acid (sp. g. 1.16), and running from a

burette nitric acid diluted with an equal volume of water into the

boiling iron solution, until the liquid changes from a black to a

reddish-yellow. About 4.5 c.c. of the nitric acid will be required, and

the finishing point is marked by a brisk effervescence. The solution of

iron should be contained in an evaporating dish, and boiled briskly,

with constant stirring. There should be no excess of nitric acid. Boil

down to about half its bulk; then cool, and dilute to one litre with

water. Twenty c.c. of this solution diluted to 100 c.c. with water, and

acidified with 10 c.c. of dilute hydrochloric acid, should not

decolorise any of the permanganate of potassium solution; this shows the

absence of ferrous salts. And 20 c.c. of the same solution, boiled with

20 c.c. of the ferrous sulphate solution, should not decrease the

quantity of "permanganate" required for the titration of the ferrous

sulphate added. In a series of experiments on the various methods of

reduction, the following results were got. The modes of working were

those already described.

(1) _With Stannous Chloride._--Twenty c.c. of the ferric chloride

solution required, after reduction with stannous chloride, 20 c.c. of

"permanganate." Fifty c.c. of a solution of ferrous chloride, which

required on titration 49.8 c.c. of "permanganate," required for

re-titration (after subsequent reduction with stannous chloride) 50 c.c.

of the permanganate solution.

(2) _With Sulphuretted Hydrogen._--Two experiments with this gas, using

in each 20 c.c. of the ferric chloride solution, and 10 c.c. of

hydrochloric acid, required (after reduction) 20.2 c.c. and 20.1 c.c. of

"permanganate." Repeating the experiments by passing the gas through a

nearly boiling solution, but in other respects working in the same way,

21.3 c.c. and 21.6 c.c. of the permanganate solution were required. The

sulphur was not filtered off in any of these. In another experiment, in

which 50 c.c. of the ferrous sulphate solution were titrated with

"permanganate," 48 c.c. of the latter were required. The titrated

solution was next reduced with sulphuretted hydrogen, brought to the

same bulk as before, and again titrated; 47.9 c.c. of the permanganate

of potassium solution were required.

(3) _With Sodium Sulphite._--Twenty c.c. of the ferric chloride

solution, reduced with sodium sulphite, required 19.9 c.c. of

"permanganate." In one experiment 50 c.c. of the ferrous sulphate

solution were titrated with "permanganate"; 49.3 c.c. of the

last-mentioned solution were required. The titrated solution was reduced

with sodium sulphite, and again titrated; it required 49.2 c.c. of the

permanganate of potassium solution.

(4) _With Zinc._--Twenty c.c. of the ferric chloride solution, reduced

with zinc and titrated, required 20.8 c.c. of "permanganate." Fifty c.c.

of a solution of ferrous sulphate which required 49.7 c.c. of

"permanganate," required for re-titration, after reduction with zinc,

49.7 c.c.

The student should next practise the titration with bichromate, which is

more especially valuable in the estimation of hydrochloric acid

solutions. The following experiments are on the same plan as those

already given. In each experiment (except when otherwise stated) there

were present 20 c.c. of the ferrous chloride solution, and 10 c.c. of

dilute hydrochloric acid, and the bulk was 300 c.c.

~Effect of Varying Temperature.~--The quantities of the bichromate of

potassium solution required were as follows:--

Temperature 15° 30° 70° 100°

Bichromate required 20.2 c.c. 20.3 c.c. 20.3 c.c. 20.4 c.c.

~Effect of Varying Bulk.~--

Bulk 50 c.c. 100 c.c. 200 c.c. 500 c.c. 1000 c.c.

Bichromate required 20.4 " 20.4 " 20.4 " 20.5 " 20.8 "

~Effect of Varying Acid.~--In these, variable quantities of dilute

hydrochloric acid were used.

Acid present 10 c.c. 50 c.c. 100 c.c.

Bichromate required 20.3 " 20.3 " 20.2 "

~Effect of Foreign Salts.~--The effect of the addition of 10 grams of

crystallized zinc sulphate was to decrease the quantity of "bichromate"

required from 20.3 c.c. to 20.1 c.c., but the colour produced with the

test-drop was very slight at 18.5 c.c., and with incautious work the

finishing point might have been taken anywhere between these extremes.

Zinc should not be used as a reducing agent preliminary to a

"bichromate" titration. Ten grams of ammonic sulphate had the effect of

rendering the finishing point faint for about 0.5 c.c. before the

titration was finished, but there was no doubt about the finishing point

when allowed to stand for a minute. The student should note that a

titration is not completed if a colour is developed on standing for five

or ten minutes. Ten grams of sodic sulphate had no effect; 20.3 c.c.

were required.

~Effect of Varying Iron.~--The results are proportional, as will be seen

from the following details:--

Ferrous chloride

present 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

Bichromate required 1.0 " 10.2 " 20.3 " 51.0 " 102.3 "

The student may now apply these titrations to actual assays of minerals.

The following examples will illustrate the mode of working and of

calculating the results:--

~Determination of Iron in Chalybite.~--Weigh up 1 gram of the dry

powdered ore, and dissolve in 10 c.c. of dilute sulphuric acid and an

equal volume of water with the aid of heat. Avoid evaporating to

dryness. Dilute and titrate. The result will give the percentage of iron

existing in the ore in the ferrous state. Some ferric iron may be

present. If it is wished to determine this also, add (in dissolving

another portion) 10 c.c. of dilute hydrochloric acid to the sulphuric

acid already ordered, and reduce the resulting solution before

titrating. By dissolving and titrating (without previous reduction) one

has a measure of the ferrous iron present; by dissolving, reducing, and

then titrating, one can measure the total iron; and as the iron exists

in only two conditions, the total iron, less the ferrous iron, is the

measure of the ferric iron.

~Determination of Iron in Brown or Red Ores or Magnetite.~--Weigh up 0.5

gram of the ore (powdered and dried at 100° C.), and dissolve in from 10

to 20 c.c. of strong hydrochloric acid, boiling until all is dissolved,

or until no coloured particles are left. Dilute, reduce, and titrate.

~Determination of Iron in Pyrites.~--Weigh up 1 gram of the dry powdered

ore, and place in a beaker. Cover with 10 c.c. of strong sulphuric acid,

mix well by shaking, and place on the hot plate without further handling

for an hour or so until the action has ceased. _Allow to cool_, and

dilute to 100 c.c. Warm until solution is complete. Reduce and titrate.

~Determination of Iron in Substances Insoluble in Acids.~--Weigh up 1

gram of the ore, mix with 5 or 6 grams of carbonate of soda and 0.5 gram

of nitre by rubbing in a small mortar, and transfer to a platinum

crucible. Clean out the mortar by rubbing up another gram or so of soda,

and add this to the contents of the crucible as a cover. Fuse till

tranquil. Cool. Extract with water. If the ore carries much silica,

evaporate to dryness with hydrochloric acid to separate it. Re-dissolve

in hydrochloric acid, and separate the iron by precipitating with

ammonia and filtering. If only a small quantity of silica is present,

the aqueous extract of the "melt" must be filtered, and the insoluble

residue washed and dissolved in dilute hydrochloric acid. Reduce and

titrate.

A convenient method of at once separating iron from a solution and

reducing it, is to add ammonia, pass sulphuretted hydrogen through it,

filter, and dissolve the precipitate in dilute sulphuric acid. The

solution, when boiled free from sulphuretted hydrogen, is ready for

titrating.

STANNOUS CHLORIDE PROCESS.

The colour imparted to hot hydrochloric acid solutions by a trace of a

ferric compound is so strong, and the reducing action of stannous

chloride is so rapid, that a method of titration is based upon the

quantity of a standard solution of stannous chloride required to

completely decolorise a solution containing ferric iron. This method is

more especially adapted for the assay of liquors containing much ferric

iron and of those oxidised ores which are completely soluble in

hydrochloric acid. It must be remembered, however, that it only measures

the ferric iron present, and when (as is generally the case) the total

iron is wanted, it is well to calcine the weighed portion of ore

previous to solution in order to get the whole of the iron into the

higher state of oxidation, since many ores which are generally supposed

to contain only ferric iron carry a considerable percentage of ferrous.

_The stannous chloride solution_ is made by dissolving 20 grams of the

commercial salt (SnCl_{2}.2H_{2}O) in 100 c.c. of water with the help of

20 c.c. of dilute hydrochloric acid, and diluting to a litre. The

solution may be slightly opalescent, but should show no signs of a

precipitate. The strength of this is about equivalent to 1 gram of iron

for each 100 c.c. of the solution, but it is apt to lessen on standing,

taking up oxygen from the air, forming stannic chloride. A larger

proportion of hydrochloric acid than is ordered above would remove the

opalescence, but at the same time increase this tendency to atmospheric

oxidation, as the following experiments show. The stannous chloride

solution (20 c.c.) was mixed with varying amounts of strong hydrochloric

acid (sp. g. 1.16), diluted to 100 c.c., and exposed in open beakers for

varying lengths of time; and the residual stannous chloride measured by

titration with permanganate. The quantities required were as follows:--

Time Exposed. 50 per cent. Acid. 10 per cent. Acid. 1 per cent. Acid.

1 hour 33.2 c.c. 34.4 c.c. 34.5 c.c.

1 day 5.0 " 24.0 " 27.6 "

2 days 3.0 " 14.5 " 21.3 "

These indicate very clearly the increased susceptibility to oxidation in

strongly acid solutions.

_A standard solution of ferric chloride_ is prepared in the same manner

as that described under the experiments on the methods of reduction; but

it should be of twice the strength, so that 100 c.c. may contain 1 gram

of iron. This solution is used for standardising the stannous chloride

when required; and must be carefully prepared; and tested for the

presence of nitric acid.

The titration is more limited in its application than either of the

oxidising processes because of the restrictions as to bulk, quality and

quantity of free acid present, and other conditions of the solution to

be assayed. The following experiments show the conditions necessary for

a successful titration.

~Effect of Varying Temperature.~--Twenty c.c. of ferric chloride

solution with 20 c.c. of strong hydrochloric acid, diluted to 50 c.c.,

gave the following results when titrated:--

Temperature 15° 30° 70° 100°

Stannous chloride required 22.8 c.c. 22.0 c.c. 22.1 c.c. 22.0 c.c.

The finishing point, however, is more distinct the hotter the solution;

so that it is best in all cases to run the standard into the boiling

solution.

~Effect of Varying Bulk.~--Solutions containing the same quantity of

iron and acid as the last, but diluted to various bulks, and titrated

while boiling, gave the following results:--

Bulk 30 c.c. 100 c.c. 500 c.c.

Stannous chloride required 21.5 " 21.7 " 24.3 "

~Effect of Varying Quantities of Hydrochloric Acid.~--In these

experiments the bulk before titration was 50 c.c. except in the last, in

which it was 70 c.c. With less than 5 c.c. of strong hydrochloric acid

the finishing point is indistinct and prolonged.

Strong hydrochloric

acid present 5 c.c. 10 c.c. 20 c.c. 30 c.c. 50 c.c.

Stannous chloride

required 21.1 " 21.1 " 21.2 " 21.8 " 22.2 "

~Effect of Free Sulphuric Acid.~--In these experiments 20 c.c. of

hydrochloric acid were present, and the bulk was 50 c.c.

Strong sulphuric acid

present -- c.c. 3 c.c. 5 c.c. 10 c.c.

Stannous chloride

required 21.6 " 22.3 " 22.9 " 23.1 "

This interference of strong sulphuric acid may be completely

counteracted by somewhat modifying the mode of working. Another

experiment, like the last of this series, required 21.6 c.c.

~Effect of Foreign Salts.~--Experiments in which 10 grams of various

salts were added showed them to be without effect. The results were as

follows:--

Salt present -- AmCl Am_{2}SO_{4} MgCl_{2}

Stannous chloride required 21.6 c.c. 21.6 c.c. 21.6 c.c. 21.6 c.c.

Salt present CaCl_{2} FeCl_{2} Al_{2}Cl_{6}

Stannous chloride required 21.8 c.c. 21.6 c.c. 21.6 c.c.

~Effect of Varying Iron.~--Titrating a solution (with 20 c.c. of

hydrochloric acid) measuring 50 c.c., and kept boiling, the quantity of

stannous chloride solution required is practically proportional to the

iron present.

Ferric chloride

added 1 c.c. 10 c.c. 20 c.c. 50 c.c. 100 c.c.

Stannous chloride

required 1.1 " 10.5 " 20.6 " 51.4 " 102.6 "

The student, having practised some of the above experiments, may proceed

to the assay of an iron ore.

~Determination of Iron in Brown Iron Ore.~--Weigh up 1 gram of the dried

and powdered ore, calcine in the cover of a platinum crucible, and

dissolve up in an evaporating dish[69] with 20 c.c. of strong

hydrochloric acid. When solution is complete, dilute to 50 c.c. after

replacing any acid that may have been evaporated. Boil, and run in the

stannous chloride solution until the colour is faintly yellow; boil

again, and continue the addition of the stannous chloride solution,

stirring continuously until the solution appears colourless. Note the

quantity of the stannous chloride solution required. Suppose this to be

59 c.c. Take 60 c.c. of the standard ferric chloride solution, add 20

c.c. of hydrochloric acid, boil and titrate in the same way as before.

Suppose this to require 61 c.c. Then as 61 is equivalent to 60 of the

iron solution, 59 is equivalent to 58.13.[70] This gives the percentage.

It is not necessary to standardise the stannous chloride solution in

this way with each sample assayed, the ratio 61: 60 would serve for a

whole batch of samples; but the standardising should be repeated at

least once each day.

COLORIMETRIC METHOD.

This method is valuable for the determination of small quantities of

iron present as impurities in other metals or ores. It is based on the

red coloration developed by the action of potassic sulphocyanate on acid

solutions of ferric salts.

_Standard Ferric Chloride Solution._--Take 1 c.c. of the ferric chloride

solution used for standardising the stannous chloride solution, add 2

c.c. of dilute hydrochloric acid, and dilute to 1 litre with water. 1

c.c. = 0.01 milligram.

_Solution of Potassic Sulphocyanate._--Dissolve 60 grams of the salt in

water, and dilute to a litre. It should be colourless. Use 10 c.c. for

each test.

The quantity of the substance to be weighed for the assay should not

contain more than a milligram of iron; consequently, if the ore contain

more than 0.1 per cent. of that metal, less than a gram of it must be

taken.

The method is as follows:--Weigh up 1 gram of the substance and dissolve

in a suitable acid; dilute; and add permanganate of potash solution

until tinted. Boil for some time and dilute to 100 c.c. Take a couple of

Nessler tubes, holding over 100 c.c., but marked at 50 c.c.; label them

"1" and "2"; and into each put 10 c.c. of the potassic sulphocyanate

solution and 2 c.c. of dilute hydrochloric acid. The solutions should be

colourless. To "1" add 10 c.c. of the assay solution, and dilute to the

50 c.c. mark. To the other add water, but only to within 5 or 10 c.c. of

this mark. Now run in the standard ferric chloride solution from a small

burette, 1 c.c. at a time, stirring after each addition till the colour

is nearly equal to that of the assay (No. 1). At this stage bring the

solution to the same level by diluting, and make a further addition of

the standard ferric chloride solution till the colours correspond. The

amount of iron will be the same in each tube; that in the standard may

be known by reading off the volume from the burette and multiplying by

0.01 milligram.

If the 10 c.c. of the assay solution gave a colour requiring more than 5

or 6 c.c. of the standard ferric chloride solution, repeat the

determination, taking a smaller proportion.

The effect of varying conditions on the assay will be seen from the

following experiments:--

~Effect of Varying Temperature.~--The effect of increase of temperature

is to lessen the colour; in fact, by boiling, the colour can be entirely

removed. All assays are best carried out in the cold.

1 c.c. at 15° would only show the colour of 0.75 c.c. at 45°

2 " " " " 1.75 "

5 " " " " 4.0 "

~Effect of Time.~--The effect of increase of time is to increase the

colour, as will be seen from the following experiments:--

2 c.c. on standing 10 minutes became equal to 2.25 c.c.

2 " 20 " " " 2.75 "

2 " 40 " " " 3.00 "

~Effect of Free Acid.~--If no acid at all be present, the sulphocyanate

of potassium solution removes the colour it first produces, so that a

certain amount of acid is necessary to develop the colour. The use of a

large excess has a tendency to increase the colour produced.

5 c.c. nitric acid (sp. g. 1.4) read 3.7 c.c. instead of 2 c.c. with the

dilute acid.

5 c.c. sulphuric acid (sp. g. 1.32) read 2.2 c.c. instead of 2 c.c. with

the dilute acid.

5 c.c. hydrochloric acid (sp. g. 1.16) read 2.5 c.c. instead of 2 c.c.

with the dilute acid.

~Effect of Foreign Metals.~--Lead, mercury, cadmium, bismuth, arsenic,

tin, antimony, nickel, cobalt, manganese, aluminium, zinc, strontium,

barium, calcium, magnesium, sodium, or potassium, when separately

present in quantities of from 100 to 200 times the weight of iron

present, do not interfere if they have previously been brought to their

highest oxidised condition by boiling with nitric acid or by treating

with permanganate. Arsenic and phosphoric acids interfere unless an

excess of free hydrochloric or other acid is present. Oxalic acid (but

not tartaric acid) in minute quantities destroys the colour. Nitrous

acid strikes a red colour with the sulphocyanate of potassium;

consequently, when nitric acid has been used in excess, high results may

be obtained. Copper and some other metals interfere, so that in most

cases it is advisable to concentrate the iron before estimating it. A

blank experiment should always be made with the reagents used in order

to determine the iron, if any, introduced during the solution, &c., of

the substance assayed.

~Determination of Iron in Metallic Copper.~--This may be most

conveniently done during the estimation of the arsenic. The small

quantity of white flocculent precipitate which may be observed in the

acetic acid solution before titrating, contains the whole of the iron as

ferric arsenate. It should be filtered off, dissolved in 10 c.c. of

dilute hydrochloric acid, and diluted to 100 c.c.; 10 c.c. of this may

be taken for the estimation. For example: 10 grams of copper were taken,

and the iron estimated; 3.0 c.c. of standard ferric chloride solution

were used, equivalent to 0.03 milligram of iron; this multiplied by 10

(because only 1/10th of the sample was taken) gives 0.3 milligram as the

iron in 10 grams of copper. This equals 0.003 per cent.

In a series of experiments with this method working on 10-gram lots of

copper, to which known quantities of iron had been added, the following

were the results:--

Iron present 0.015% 0.070% 0.100% 0.495%

Iron found 0.015" 0.061" 0.087" 0.522"

When no arsenic is present in the copper, the iron can be separated by

fractionally precipitating with sodic carbonate, dissolving in ammonia,

and filtering off the ferric hydrate. Coppers generally carry more iron

the less arsenic they contain.

~Determination of Iron in Metallic Zinc.~--Dissolve 1 gram of zinc in 10

c.c. of dilute hydrochloric acid, adding a drop or two of nitric acid

towards the end to effect complete solution. Boil, dilute, and tint with

the permanganate of potassium solution; boil till colourless, and dilute

to 100 c.c. Take 10 c.c. for the determination. Make a blank experiment

by boiling 10 c.c. of dilute hydrochloric acid with a drop or two of

nitric acid; add a similar quantity of the permanganate of potassium

solution, boiling, &c., as before. The quantity of iron in zinc varies

from less than 0.005 to more than 2.0 per cent. When 1 gram is taken

and worked as above, each c.c. of ferric chloride solution required

indicates 0.01 per cent. of iron.

~Determination of Iron in Metallic Tin.~--Cover 1 gram of tin with 5

c.c. of hydrochloric acid, add 1 c.c. of nitric acid, and evaporate to

dryness. Take up with 2 c.c. of dilute hydrochloric acid, add 10 c.c. of

the potassic sulphocyanate solution, and make up to 50 c.c. Probably the

colour developed will be brown instead of red owing to the presence of

copper; in this case, add to the standard as much copper as the assay is

known to contain (which must have previously been determined; see

_Copper_); the titration is then carried out in the usual way.

Or the iron may be separated from the copper in the tin by the following

process:--Dissolve 5 grams of metal in 30 c.c. of hydrochloric acid and

5 c.c. of nitric acid, and evaporate to dryness. Take up with 5 c.c. of

dilute hydrochloric acid, add 10 grams of potash dissolved in 30 c.c. of

water, and warm till the tin is dissolved. Pass sulphuretted hydrogen,

boil, cool, and filter. The iron and copper will be in the precipitate.

They are separated in the ordinary manner.

PRACTICAL EXERCISES.

1. Calculate from the following determinations the percentages of

ferrous, ferric, and total iron in the sample of ore used.

1 gram of ore dissolved and titrated required 26.7 c.c. of bichromate of

potassium solution.

1 gram of ore dissolved, reduced, and titrated required 43.5 c.c. of

bichromate of potassium solution.

Standard = 1.014.

2. One gram of an ore contained 0.307 gram of ferrous iron and 0.655

gram of total iron. The iron existing as oxide, what are the percentages

of ferrous oxide (FeO) and ferric oxide (Fe_{2}O_{3}) in the ore?

3. One gram of brown iron ore dissolved in hydrochloric acid required

59.2 c.c. of stannous chloride (standard = 0.930). Another gram

dissolved in acid and titrated with "permanganate" required 8.2 c.c.

(standard = 0.4951). Calculate the percentages of ferrous, ferric, and

total iron.

4. Another gram of the same ore, roasted, dissolved and titrated with

stannous chloride, required 63.5 c.c. To what extent does this result

confirm the others?

5. Two grams of a metal were dissolved and diluted to 100 c.c. Five c.c.

were taken for a colorimetric determination, and required 4.5 c.c. of

the standard ferric chloride solution. What is the percentage of iron in

the metal?

NICKEL.

Nickel and cobalt are closely related in their chemical properties, and

may best be considered together. Nickel is the commoner of the two, and

is met with in commerce alloyed with copper and zinc as German silver;

as also in the coinage of the United States and on the Continent. It is

used for plating polished iron and steel goods, forming a coating little

liable to rust and taking a good polish. The ores of nickel are not very

common. Kupfernickel and chloanthite are arsenides of nickel with,

generally, more or less iron and cobalt. Noumeite and garnierite are

hydrated silicates of nickel and magnesia. The chief sources of nickel

are these silicates, which are found in large quantity in New Caledonia;

and a pyrites found in Norway, containing three or four per cent. of the

metal. In smaller quantities it is more widely distributed, being

frequently met with in copper ores; consequently, commercial copper is

rarely free from it.

Nickel is readily soluble in moderately concentrated nitric acid. Its

salts are mostly green, and soluble in excess of ammonia, forming blue

solutions; in these respects it resembles copper. The acid solutions,

however, are not precipitated by sulphuretted hydrogen, although in

alkaline solutions a black sulphide is formed which is insoluble in

dilute hydrochloric acid. If the sulphide is formed in a solution

containing much free ammonia, the precipitation is incomplete, some

sulphide remaining in the solution and colouring it dark brown. These

reactions serve to distinguish and separate nickel from other metals,

except cobalt. If the separated sulphide be heated in a borax bead, the

colour obtained will be a sherry brown in the outer flame, and grey or

colourless in the inner flame if nickel only is present. In the presence

of cobalt these colours are masked by the intense and characteristic

blue yielded in both flames by that metal.

DRY ASSAY.

The dry assay of nickel (cobalt being at the same time determined) is

based on the formation of a speise which will carry the cobalt, nickel,

copper, and some of the iron of the ore in combination with arsenic. A

speise of this kind, fused and exposed at a red heat to air, first loses

arsenide of iron by oxidation. It is only when the iron has been

oxidised that the arsenide of cobalt begins to be attacked; and when the

removal of the cobalt is complete, the nickel commences to pass into the

slag, the copper being left till last. The changes are rendered evident

by fusion in contact with borax. The process is as follows:--Weigh up 5

grams of the ore, and calcine thoroughly on a roasting dish in the

muffle. Rub up with some anthracite, and re-roast. Mix intimately with

from 3 to 5 grams of metallic arsenic, and heat in a small covered clay

crucible at dull redness in a muffle until no more fumes of arsenic come

off (about 15 minutes). Take out the crucible, and inject a mixture of

20 grams of carbonate of soda, 5 grams of flour, and 2 grams of fused

borax. Place in the wind furnace, and raise the temperature gradually

until the charge is in a state of tranquil fusion. Pour; when cold,

detach the button of speise, and weigh.

Weigh out carefully a portion of about 1 gram of it. Place a shallow

clay dish in the muffle, and heat it to bright redness; then add about

1.5 gram of borax glass wrapped in a piece of tissue paper; when this

has fused, drop the piece of speise into it. Close the muffle until the

speise has melted, which should be almost at once. The arsenide of iron

will oxidise first, and when this has ceased the surface of the button

brightens. Remove it from the muffle, and quench in water as soon as the

button has solidified. The borax should be coloured slightly blue.

Weigh: the loss is the arsenide of iron. Repeat the operation with the

weighed button on another dish, using rather less borax. Continue the

scorification until a film, green when cold, floating on the surface of

the button shows that the nickel is beginning to oxidise. Cool,

separate, and weigh the button as before. The loss is the arsenide of

cobalt.

If copper is absent, the speise is now arsenide of nickel.

The weight of nickel corresponding to the arsenide got is calculated by

multiplying by 0.607; and, similarly, the weight of the cobalt is

ascertained by multiplying the loss in the last scorification by

0.615.[71] It must be remembered that the nickel and cobalt so obtained

are derived from a fraction only of the speise yielded by the ore taken,

so that the results must be multiplied by the weight of the whole of the

speise, and divided by the weight of the fragment used in the

determination. As an example, suppose 5 grams of ore gave 3.3 grams of

speise, and 1.1 gram of this gave 0.8 gram of nickel arsenide. Then--

0.8×0.607 = 0.4856 gram of nickel

0.4856×3.3/1.1 = 1.456 gram of nickel

And this being obtained from 5 grams of ore is equivalent to 29.12 per

cent.

When copper is also present, weigh up accurately about 0.5 gram of gold,

and place it on the scorifier with the button of nickel and copper

arsenide, using borax as before. Scorify until the button shows the

bluish-green colour of a fused gold-copper alloy. Then cool, and weigh

the button of copper and gold. The increase in weight of the gold button

gives the copper as metal. The weight of the copper multiplied by 1.395

is the weight of the copper arsenide (Cu_{3}As) present. The difference

will be the nickel arsenide.

The student should enter the weighings in his book as follows:

Ore taken -- grams

Speise got -- "

Speise taken -- grams

Arsenides of cobalt, nickel, and copper -- "

" nickel and copper -- "

Gold added -- "

Gold and copper got -- "

Showing Cobalt -- per cent.

Nickel -- "

Copper -- "

WET METHODS.

~Solution and Separation.~--Two or three grams of a rich ore, or 5 to 10

grams if poor, are taken for the assay. If much arsenic is present (as

is usually the case), the ore must be calcined before attacking with

acids. Transfer to a flask; and boil, first with hydrochloric acid until

the oxides are dissolved, and then with the help of nitric acid, until

nothing metalliferous is left. Dilute, nearly neutralise with soda, and

separate the iron as basic acetate,[72] as described in page 233.

Through the filtrate pass sulphuretted hydrogen till saturated. Allow to

settle (best overnight), filter, and wash. Transfer the precipitate to a

beaker, and dissolve in nitric acid. Dilute with water, pass

sulphuretted hydrogen, and filter off the precipitate, if any. Boil off

the gas, add ammonia until a precipitate is formed, and then acidify

somewhat strongly with acetic acid. Pass sulphuretted hydrogen in a slow

stream until any white precipitate of zinc sulphide, there may be,

begins to darken. Filter; to the filtrate add ammonia, and pass

sulphuretted hydrogen. The precipitate will contain the nickel and

cobalt as sulphides.

Where small quantities of nickel and cobalt are present, and an

approximate determination is sufficient, they can be concentrated as

follows:--Remove the copper, &c., by passing sulphuretted hydrogen

through the acid solution and filtering; add ammonia to the filtrate,

and again pass sulphuretted hydrogen; then heat nearly to boiling, and

filter. Dissolve the precipitate off the filter with dilute hydrochloric

acid; the residue will contain nearly all the nickel and cobalt as

sulphides.

~Separation of Nickel and Cobalt.~--Dissolve the sulphides separated as

above in nitric acid; render alkaline with a solution of potash, then

acidify with acetic acid; add a concentrated solution of _nitrite_ of

potash. The liquid after this addition must have an acid reaction. Allow

to stand for 24 hours in a warm place. Filter off the yellow precipitate

of nitrite of potash and cobalt, and wash with a 10 per cent. solution

of acetate of potash. The cobalt is determined in the precipitate in the

way described under _Cobalt_. The nickel is separated from the solution

by boiling with sodic hydrate, filtering, and dissolving the precipitate

in nitric acid. The solution will contain the nickel.

GRAVIMETRIC DETERMINATION.

The solution, which contains the nickel free from other metals, is

heated, and a solution of sodic hydrate added in slight excess. The

precipitate is filtered off, washed with boiling water, dried, ignited

at a red heat, and weighed when cold. The ignited substance is nickel

oxide (NiO), and contains 78.67 per cent. of nickel. The oxide is a

green powder, readily and completely soluble in hydrochloric acid, and

without action on litmus paper. It is very easily reduced by ignition in

hydrogen to metallic nickel.

[Illustration: FIG. 56.]

Nickel is also determined by electrolysis, as follows:--The nitric acid

solution is rendered strongly ammoniacal, and placed under the

electrolytic apparatus used for the copper assay. Three cells (fig. 56),

however, must be used, coupled up for intensity, that is, with the zinc

of one connected with the copper of the next. The electrolysis is

allowed to go on overnight, and in the morning the nickel will be

deposited as a bright and coherent film. A portion of the solution is

drawn off with a pipette; if it smells of ammonia, has no blue colour,

and gives no precipitate with ammonic sulphide, the separation is

complete. Wash the cylinder containing the deposited metal, first with

water and then with alcohol, as in the copper assay. Dry in the water

oven, and weigh. The increase in weight is metallic nickel.

As an example:--There was taken 1 gram of a nickel alloy used for

coinage. It was dissolved in 10 c.c. of nitric acid, and diluted to 100

c.c. with water. The copper was then precipitated by electrolysis. It

weighed 0.734 gram. The solution, after electrolysis, was treated with

sulphuretted hydrogen, and the remaining copper was thrown down as

sulphide, and estimated colorimetrically. This amounted to 3-1/2

milligrams. The filtrate was evaporated, treated with ammonia, warmed,

and filtered. The ferric hydrate was dissolved in dilute acid, and

reprecipitated, dried, ignited, and weighed. Its weight was 0.0310 gram.

The two filtrates were mixed, and reduced in bulk to about 50 c.c.; a

considerable excess of ammonia was added, and the nickel precipitated by

electrolysis. It weighed 0.2434 gram. These quantities are equivalent

to:

Copper 73.75 per cent.

Nickel 24.34 "

Iron 2.17 "

------

100.26

VOLUMETRIC DETERMINATION.

An alkaline solution of potassium cyanide, to which a little potassium

iodide has been added, can be assayed for its strength in cyanide by

titrating with a standard solution of silver nitrate. Nickel interferes

with this assay, doing the work of its equivalent of silver; and the

quantity of nickel present can be calculated from the amount of its

interference in the titration. A volumetric assay for nickel is based on

this. It has the disadvantage of all indirect titrations in that it

requires two standard solutions. On the other hand it gives good results

even under unfavourable conditions, and is applicable in the presence of

much zinc. Small quantities of cobalt will count as so much nickel, but

larger quantities make the assay unworkable. Some of the other

metals--lead for example--have no appreciable effect; but practically

the solution demands a preliminary treatment which would result in their

removal. Nevertheless it is a very satisfactory method and makes the

determination of nickel quick and comparatively easy in most cases.

_The standard solution of silver nitrate_ is made by dissolving 14.48

grams of recrystallised silver nitrate in distilled water and diluting

to 1 litre: 100 c.c. of this solution are equivalent to 0.25 gram of

nickel.[73]

_The standard solution of potassium cyanide_ should be made so as to be

exactly equal to the silver nitrate solution. This can be done as

follows: Weigh up 12 grams of good potassium cyanide (95 per cent.),

dissolve in water, add 50 c.c. of a 10 per cent. solution of sodium

hydrate and dilute to 1 litre. Fill one burette with this and another

with the solution of silver nitrate. Run 50 c.c. of the cyanide into a

flask; add a few drops of potassium iodide solution and titrate with the

standard silver nitrate until there is a distinct permanent yellowish

turbidity. The titration is more fully described under _Cyanide_, p.

165. The cyanide solution will be found rather stronger than the silver

nitrate; dilute it so as to get the two solutions of equal value. For

example, 51.3 c.c. of silver nitrate may have been required: then add

1.3 c.c. of water to each 50 c.c. of the cyanide solution remaining. If

the full 950 c.c. are available, then add to them 24.7 c.c. of water.

After mixing, take another 50 c.c. and titrate with the silver nitrate;

the two solutions should now be exactly equal. The cyanide solution,

being strongly alkaline with soda, keeps very well; but its strength

should be checked from time to time by titrating with silver nitrate;

should there be any slight inequality in the strengths of the two

solutions it is easily allowed for in the calculations.

~The titration.~--The solution, containing not much more than 0.1 gram

of nickel, and free from the interfering metals, must be cooled. It is

next neutralised and then made strongly alkaline with a solution of soda

(NaHO); an excess of 20 or 30 c.c. suffices. This will produce a

precipitate. The cyanide solution is now run in from a burette until the

solution clears, after which an excess of about 20 c.c. is added. It is

well to use some round number of c.c. to simplify the calculation. Add a

few drops of potassium iodide solution, and run in the standard solution

of silver nitrate from a burette. This should be done a little at a

time, though somewhat rapidly, and with constant shaking, till a

permanent yellow precipitate appears. If the addition of the cyanide did

not result in a perfectly clear solution, this is because something

besides nickel is present. The residue may be filtered off, though with

a little practice the finishing-point may be detected with certainty in

the presence of a small precipitate. If the student has the slightest

doubt about a finish he should run in another 5 c.c. of the cyanide and

again finish with silver nitrate. The second result will be the same as

the first. For example, if 40 c.c. of cyanide and 30 c.c. of silver

nitrate were required at the first titration, then the 45 c.c. of

cyanide in the second titration will require 35 c.c. of silver nitrate.

The difference between the quantities of the two solutions used in each

case will be 10 c.c. It is this difference in the readings of the two

burettes which measures the quantity of nickel present. Each c.c. of the

difference is equal to .0025 gram of nickel. But if the cyanide solution

is not exactly equal in strength to the silver nitrate, the quantity of

cyanide used should be calculated to its equivalent in silver nitrate

before making the subtraction.

The following experimental results illustrate the accuracy of the assay

and the effect upon it of varying conditions. A solution containing 1

gram of nickel sulphate (NiSO_{4}.6H_{2}O) in 100 c.c. was used. By a

separate assay the sulphate was found to contain 22.25 per cent. of

nickel. For the sake of simplicity the results of the experiments are

stated in weights of nickel in grams.

~Effect of varying excess of Cyanide Solution.~--In each experiment

there was 20 c.c. of the nickel solution, equal to .0445 gram of nickel.

There were also 10 c.c. of soda solution, 3 or 4 drops of potassium

iodide and sufficient water to bring the bulk to 100 c.c. before

titrating.

Cyanide in excess 6 c.c. 4 c.c. 8 c.c. 12 c.c. 25 c.c.

Nickel found .0434 .0436 .0440 .0442 .0444

Although the difference between the highest and lowest of these results

is only 1 milligram, their meaning is quite obvious. The excess of

cyanide should not be less than 20 c.c.

~Effect of varying the quantity of Soda.~--There were two series of

experiments, one with 2 c.c. of nickel solution (= .0044 gram of

nickel), the other with 20 c.c. The conditions were as before, except

that the quantity of soda was varied.

Soda added 5 c.c. 15 c.c. 30 c.c.

Nickel found, 1st series .0037 .0042 .0045

" " 2nd series .0444 .0444 .0442

These show that the presence of much soda, though it has only a small

effect, is beneficial rather than otherwise. Ammonia has a bad effect,

if present in anything like the same quantities.

~Effect of varying the Nickel.~--In experiments with 10, 20, and 40 c.c.

of the nickel solution, the results were:--

Nickel present .0222 .0445 .0890

Nickel found .0220 .0442 .0884

~Effect of Zinc.~--In these experiments 20 c.c. of nickel solution (=

.0445 gram of nickel), 10 c.c. of soda, 6 drops of potassium iodide and

water to 100 c.c. were used. The excess of cyanide was purposely kept at

from 10 to 15 c.c., which is hardly sufficient.

Zinc added 0 .25 gram. .5 gram.

Nickel found .0442 .0440 .0407

On increasing the excess of cyanide to over 20 c.c. and doubling the

quantity of soda, the experiment with 0.5 gram of zinc gave 0.441 gram

of nickel. Hence the titration is satisfactory in the presence of zinc

provided that not fewer than 20 or 30 c.c. of soda are used, and that

the excess of cyanide is such that not fewer than 20 or 30 c.c. of

silver nitrate are required in the titration. Moreover, these

precautions should be taken whether zinc is present or not.

~Effect of other Metals.~--If metals of the first and second groups are

present they should be removed by passing sulphuretted hydrogen and

filtering. If _iron_ is present it must be removed, since ferrous salts

use up much cyanide, forming ferrocyanides, and ferric salts yield

ferric hydrate, which obscures the end reaction. Hence the sulphuretted

hydrogen must be boiled off and the iron removed as basic ferric acetate

by the method described on p. 233. If the precipitate is bulky it should

be dissolved in a little dilute acid, neutralised and again precipitated

as basic acetate. The nickel will be in the two filtrates. In the

absence of manganese and cobalt the titration may be made without

further separation.

_Manganese_ does not directly interfere, but the precipitated hydrate,

which rapidly darkens through atmospheric oxidation, obscures the end

reaction. It may be removed by passing sulphuretted hydrogen through the

filtrate from the acetate separation: sulphides of nickel, cobalt and

zinc will be precipitated, whilst manganese remains in solution: the

addition of more sodium acetate may assist the precipitation. The

precipitate must be filtered off and dissolved in nitric acid: the

solution should be evaporated to dryness. The filtrate may retain a

little nickel; if so, add ammonia till alkaline, then acidify with

acetic acid and again filter; any small precipitate obtained here should

be added to that first obtained.

It is only when _cobalt_ is present that any further separation is

required. Cobalt hydrate takes up oxygen from the air, and on adding

potassium cyanide some may refuse to dissolve; and the solution itself

acquires a brown colour, which becomes deeper on standing. At this stage

the cobalt is easily separated. The solution containing the nickel and

cobalt with no great excess of acid, is made alkaline by adding 20 c.c.

of soda exactly as in preparing for a titration. So, too, the solution

of cyanide is added so as to have an excess of 20 or 30 c.c.; the

solution may have a brown colour, but if it is not quite clear it _must_

be filtered. Then warm (boiling is not needed) and add from 50 to 100

c.c. of bromine water. This throws down all the nickel as black

peroxide in a condition easy to filter. Filter it off and wash with

water. The precipitate can be dissolved off the filter with the greatest

ease by a little warm sulphurous acid. The filtrate and washings, boiled

till free from sulphurous acid, yield the nickel as sulphate in a clean

condition.

~Determination of Nickel in Nickel Sulphate Crystals.~--Take 0.5 gram of

the salt, dissolve in 50 c.c. of water and add 25 c.c. of solution of

soda. Run in from a burette, say, 60 c.c. "cyanide." Add a few drops of

potassium iodide and titrate back with "silver nitrate." Suppose 15.5

c.c. of the latter is required. Then 15.5 c.c. subtracted from 60 c.c.

leaves 44.5 c.c., and since 100 c.c. = 0.25 gram of nickel, 44.5 c.c.

will equal 0.11125 gram of nickel. This in 0.5 gram of the salt equals

22.25 per cent.

~Determination of Nickel in German Silver.~--Weigh up 0.5 gram of the

alloy, and dissolve in a dish with 5 or 10 c.c. of dilute nitric acid.

Add 5 c.c. of dilute sulphuric acid and evaporate till all the nitric

acid is removed. Cool, take up with 50 c.c. of water, and when dissolved

pass sulphuretted hydrogen through the solution. Filter off the

precipitate and wash with water containing sulphuretted hydrogen and

dilute sulphuric acid. Boil down the filtrate and washings to get rid of

the excess of the gas; add some nitric acid and continue the boiling.

Cool, neutralise the excess of acid with soda, add 1 gram of sodium

acetate and boil. Filter off the precipitate which contains the iron.

The filtrate, cooled and rendered alkaline with soda, is ready for the

titration.

COBALT

Occurs less abundantly than nickel. Its chief ores are smaltite and

cobaltite, which are arsenides of cobalt, with more or less iron,

nickel, and copper. It also occurs as arseniate in erythrine, and as

oxide in asbolan or earthy cobalt, which is essentially a wad carrying

cobalt.

It is mainly used in the manufacture of smalts for imparting a blue

colour to glass and enamels. The oxide of cobalt forms coloured

compounds with many other metallic oxides. With oxide of zinc it forms

"Rinman's green"; with aluminia, a blue; with magnesia, a pink. This

property is taken advantage of in the detection of substances before the

blow-pipe.

The compounds of cobalt in most of their properties closely resemble

those of nickel, and the remarks as to solution and separation given for

the latter metal apply here. Solutions of cobalt are pink, whilst those

of nickel are green.

The detection of cobalt, even in very small quantity, is rendered easy

by the strong blue colour which it gives to the borax bead, both in the

oxidising and in the reducing flame. It is concentrated from the ore in

the same way as nickel, and should be separated from that metal by means

of potassic nitrite in the way described. The dry assay of cobalt has

been given under _Nickel_.

GRAVIMETRIC METHOD.

The yellow precipitate from the potassium nitrite, after being washed

with the acetate of potash, is washed with alcohol, dried, transferred

to a weighed porcelain crucible, and cautiously ignited with an excess

of strong sulphuric acid. The heat must not be sufficient to decompose

the sulphate of cobalt, which decomposition is indicated by a blackening

of the substance at the edges. The salt bears a low red heat without

breaking up. If blackening has occurred, moisten with sulphuric acid,

and ignite again. Cool and weigh. The substance is a mixture of the

sulphates of cobalt and potash (2CoSO_{4} + 3K_{2}SO_{4}), and contains

14.17 per cent. of cobalt.

Cobalt is also gravimetrically determined, like nickel, by electrolysis,

or by precipitation with sodic hydrate. In the latter case, the ignited

oxide will be somewhat uncertain in composition, owing to its containing

an excess of oxygen. Consequently, it is better to reduce it by igniting

at a red heat in a current of hydrogen and to weigh it as metallic

cobalt.

PRACTICAL EXERCISES.

1. In the dry assay of an ore containing cobalt, nickel, and copper, the

following results were obtained. Calculate the percentages. Ore taken, 5

grams. Speise formed, 0.99 gram. Speise taken. 0.99 gram. Arsenides of

cobalt, nickel, and copper got, 0.75 gram. Arsenide of nickel and copper

got, 0.54 gram. Gold added, 0.5 gram. Gold and copper got, 0.61 gram.

2. Calculate the percentage composition of the following compounds:

Co_{2}As, Ni_{2}As, and Cu_{2}As.

3. A sample of mispickel contains 7 per cent. cobalt. What weight of the

mixed sulphates of potash and cobalt will be obtained in a gravimetric

determination on 1 gram of the ore?

4. 0.3157 gram of metal was deposited by the electrolysis of a nickel

and cobalt solution. On dissolving in nitric acid and determining the

cobalt 0.2563 gram of potassium and cobalt sulphates were got. Find the

weights of cobalt and nickel present in the deposit.

5. What should be the percentage composition of pure cobaltite, its

formula being CoAsS?

ZINC.

Zinc occurs in nature most commonly as sulphide (blende); it also occurs

as carbonate (calamine) and silicate (smithsonite). Each of these is

sufficiently abundant to be a source of the metal.

The metal is known in commerce as "spelter" when in ingots, and as sheet

zinc when rolled. It is chiefly used in the form of alloys with copper,

which are known as brasses. It is also used in the form of a thin film,

to protect iron goods from rusting--galvanised iron.

Ores of zinc, more especially blende, are met with in most lead, copper,

gold, and silver mines, in larger or small quantities scattered through

the lodes. Those ores which generally come under the notice of the

assayer are fairly rich in zinc; but alloys and metallurgical products

contain it in very varying proportions.

Zinc itself is readily soluble in dilute acids; any residue which is

left after boiling with dilute hydrochloric or sulphuric acid consists

simply of the impurities of the metal; this is generally lead.

All zinc compounds are either soluble in, or are decomposed by, boiling

with acids, the zinc going into solution. Zinc forms only one series of

salts, and these are colourless. Their chief characteristic is

solubility in an alkaline solution, from which sulphuretted hydrogen

produces a white precipitate of zinc sulphide. Zinc is detected by

dissolving the substance in hydrochloric or nitric acid, boiling, and

adding sodic hydrate in excess, filtering, and adding ammonic sulphide

to the filtrate. The precipitate contains the zinc, which can be

dissolved out by boiling with dilute sulphuric acid, and detected by the

formation of a white precipitate on the addition of potassic

ferrocyanide.

The dry assay of zinc can only be made indirectly, and is

unsatisfactory. Zinc is volatile, and at the temperature of its

reduction is a gas. It is impracticable to condense the vapour so as to

weigh the metal, consequently its amount is determined by loss. The

following method gives approximate results: Take 10 grams of the dried

and powdered ore and roast, first at a low temperature and afterwards at

a higher one, with the help of carbonate of ammonia to decompose the

sulphates formed; cool and weigh. The metals will be present as oxides.

Mix with 2 grams of powdered charcoal and charge into a black-lead

crucible heated to whiteness, cover loosely, and leave in the furnace

for about a quarter of an hour. Uncover and calcine the residue, cool

and weigh. The loss in weight multiplied by 8.03 gives the percentage of

zinc in the ore.

WET METHODS.

Solution and separation may be effected as follows: Treat 1 or 3 grams

of the substance with 10 or 30 c.c. of hydrochloric acid or aqua regia;

evaporate to dryness; take up with 10 c.c. of hydrochloric acid and

dilute to 100 c.c.; heat nearly to boiling; saturate with sulphuretted

hydrogen; filter, and wash with water acidulated with hydrochloric acid.

Boil off the sulphuretted hydrogen and peroxidise with a few drops of

nitric acid. Cool; add caustic soda till nearly, but not quite,

neutralised, and separate the iron as basic acetate by the method

described under _Iron_. To the filtrate add ammonia till alkaline, and

pass sulphuretted hydrogen. Allow to settle and decant on to a filter.

Dissolve off the precipitate from the filter with hot dilute

hydrochloric acid. The solution will contain the zinc, together with any

manganese the ore contained, and, perhaps, traces of nickel and cobalt.

If the zinc is to be determined volumetrically, and manganese is

present, this latter is separated with carbonate of ammonia, as

described further on; but if a gravimetric method is used, and only

small quantities of manganese are present, it is better to proceed as if

it were absent, and to subsequently determine its amount, which should

be deducted.

GRAVIMETRIC DETERMINATION.

The solution containing the zinc is contained in an evaporating dish,

and freed from sulphuretted hydrogen by boiling, and, if necessary, from

an excess of acid by evaporation. The evaporating dish must be a large

one. Cautiously add sodium carbonate to the hot, moderately dilute

solution, until the liquid is distinctly alkaline, and boil. Allow the

precipitate to settle, decant on to a filter, and wash with hot water.

Dry, transfer to a porcelain crucible (cleaning the paper as much as

possible), add the ash, ignite, and weigh. The substance weighed is

oxide of zinc, which contains 80.26 per cent. of the metal. It is a

white powder, becoming yellow when heated. It must not show an alkaline

reaction when moistened. If it contains manganese this metal will be

present as sesquioxide (Mn_{2}O_{3}). Its amount can be determined by

dissolving in dilute acid and boiling with an excess of sodic hydrate.

The oxide of manganese will be precipitated, and can be ignited and

weighed. Its weight multiplied by 1.035 must be deducted from the weight

of oxide of zinc previously obtained. The results yielded by the

gravimetric determination are likely to be high, since the basic

carbonate of zinc frequently carries down with it more or less soda

which is difficult to wash off.

VOLUMETRIC DETERMINATION

This method is based on the facts that zinc salts in an acid solution

decompose potassium ferrocyanide, forming a white insoluble zinc

compound; and that an excess of the ferrocyanide can be detected by the

brown coloration it strikes with uranium acetate. The method resembles

in its working the bichromate iron assay. The standard solution of

potassium ferrocyanide is run into a hot hydrochloric acid solution of

the zinc until a drop of the latter brought in contact with a drop of

the indicator (uranium acetate) on a white plate strikes a brown colour.

The quantity of zinc in the solution must be approximately known; run in

a little less of the ferrocyanide than is expected will be necessary;

test a drop or two of the assay, and then run in, one or two c.c. at a

time, until the brown colour is obtained. Add 5 c.c. of a standard zinc

solution, equivalent in strength to the standard "ferrocyanide,"

re-titrate, and finish off cautiously. Of course 5 c.c. must be deducted

from the reading on the burette. The precipitate of zinc ferrocyanide

formed in the assay solution is white; but if traces of iron are

present, it becomes bluish. If the quantity of ferrocyanide required is

known within a few c.c., the finishing point is exactly determined in

the first titration without any addition of the standard zinc solution.

Unfortunately this titration serves simply to replace the gravimetric

determination, and does not, as many volumetric processes do, lessen the

necessity for a complete separation of any other metals which are

present. Most metals give precipitates with ferrocyanide of potassium in

acid solutions. If the conditions are held to, the titration is a fairly

good one, and differences in the results of an assay will be due to

error in the separation. Ferric hydrate precipitated in a fairly strong

solution of zinc will carry with it perceptible quantities of that

metal. Similarly, large quantities of copper precipitated as sulphide by

means of sulphuretted hydrogen will carry zinc with it, except under

certain nicely drawn conditions. When much copper is present it is best

separated in a nitric acid solution by electrolysis. The titration of

the zinc takes less time, and, with ordinary working, is more

trustworthy than the gravimetric method.

_The standard ferrocyanide solution_ is made by dissolving 43.2 grams of

potassium ferrocyanide (K_{4}FeCy_{6}.3H_{2}O) in water, and diluting to

a litre. One hundred c.c. are equal to 1 gram of zinc.

_The standard zinc solution_ is made by dissolving 10 grams of pure zinc

in 50 c.c. of hydrochloric acid and 100 or 200 c.c. of water, and

diluting to 1 litre, or by dissolving 44.15 grams of zinc sulphate

(ZnSO_{4}.7H_{2}O) in water with 30 c.c. of hydrochloric acid, and

diluting to 1 litre. One hundred c.c. will contain 1 gram of zinc.

_The uranium acetate solution_ is made by dissolving 0.2 gram of the

salt in 100 c.c. of water.

To standardise the "ferrocyanide" measure off 50 c.c. of the standard

zinc solution into a 10 oz. beaker, dilute to 100 c.c., and heat to

about 50° C. (not to boiling). Run in 47 or 48 c.c. of the

"ferrocyanide" solution from an ordinary burette, and finish off

cautiously. Fifty divided by the quantity of "ferrocyanide" solution

required gives the standard.

In assaying ores, &c., take such quantity as shall contain from 0.1 to 1

gram of zinc, separate the zinc as sulphide, as already directed.

Dissolve the sulphide off the filter with hot dilute hydrochloric acid,

which is best done by a stream from a wash bottle. Evaporate the

filtrate to a paste, add 5 c.c. of dilute hydrochloric acid, dilute to

100 c.c. or 150 c.c., heat to about 50° C., and titrate. Manganese, if

present, counts as so much zinc, and must be specially separated, since

it is not removed by the method already given. The following method will

effect its removal. To the hydrochloric acid solution of the zinc and

manganese add sodium acetate in large excess and pass sulphuretted

hydrogen freely. Allow to settle, filter off the zinc sulphide and wash

with sulphuretted hydrogen water. The precipitate, freed from manganese,

is then dissolved in hydrochloric acid and titrated.

The following experiments show the effect of variation in the conditions

of the assay:--

~Effect of Varying Temperature.~--Using 20 c.c. of the standard zinc

solution, 5 c.c. of dilute hydrochloric acid, and diluting to 100 c.c.

Temperature 15° C. 30° C. 70° C. 100° C.

"Ferrocyanide" required 20.6 c.c. 20.3 c.c. 20.3 c.c. 20.3 c.c.

The solution can be heated to boiling before titrating without

interfering with the result; but it is more convenient to work with the

solution at about 50° C. Cold solutions must not be used.

~Effect of Varying Bulk.~--These were all titrated at about 50° C., and

were like the last, but with varying bulk.

Bulk 25.0 c.c. 50.0 c.c. 100.0 c.c. 200.0 c.c.

"Ferrocyanide" required 20.2 " 20.4 " 20.3 " 20.4 "

Any ordinary variation in bulk has no effect.

~Effect of Varying Hydrochloric Acid.~-- With 100 c.c. bulk and varying

dilute hydrochloric acid the results were:--

Acid added 0.0 c.c. 1.0 c.c. 5.0 c.c. 10.0 c.c. 20.0 c.c.

"Ferrocyanide"

required 24.4 " 20.2 " 20.3 " 20.3 " 20.7 "

~Effect of Foreign Salts.~--The experiments were carried out under the

same conditions as the others. Five grams each of the following salts

were added:--

Salt added { Ammonic Ammonic Sodium Sodium

{ chloride. sulphate. chloride. sulphate.

"Ferrocyanide"

required 20.3 c.c. 20.5 c.c. 20.6 c.c. 20.4 c.c.

Salt added { Potassium Magnesium Nil.

{ Nitrate. sulphate.

"Ferrocyanide"

required 20.2 c.c. 20.4 c.c. 20.4 c.c.

In a series of experiments in which foreign metals were present to the

extent of 0.050 gram in each, with 20 c.c. of zinc solution and 5 c.c.

of dilute hydrochloric acid, those in which copper sulphate, ferrous

sulphate, and ferric chloride were used, gave (as might be expected) so

strongly coloured precipitates that the end reaction could not be

recognised. The other results were:--

"Ferrocyanide"

required.

With nothing added. 20.3 c.c.

" 0.050 gram lead (as chloride) 20.9 "

" 0.050 " manganese (as sulphate) 25.5 "

" 0.050 " cadmium (as sulphate) 23.5 "

" 0.050 " nickel (as sulphate) 26.2 "

~Effect of Varying Zinc.~--These were titrated under the usual

conditions, and gave the following results:--

Zinc added 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

"Ferrocyanide"

required 1.1 " 10.2 " 20.3 " 50.6 " 101.0 "

~Determination of Zinc in a Sample of Brass.~--Take the solution from

which the copper has been separated by electrolysis and pass

sulphuretted hydrogen until the remaining traces of copper and the lead

are precipitated, filter, boil the solution free from sulphuretted

hydrogen, put in a piece of litmus paper, and add sodic hydrate solution

in slight excess; add 10 c.c. of dilute hydrochloric acid (which should

render the solution acid and clear); warm, and titrate.

A sample of 0.5 gram of brass treated in this manner required 16.4 c.c.

of "ferrocyanide" (standard 100 c.c. = 0.9909 zinc), which equals 0.1625

gram of zinc or 32.5 per cent.

~Determination of Zinc in Blende.~--Dissolve 1 gram of the dried and

powdered sample in 25 c.c. of nitric acid with the help of two or three

grams of potassium chlorate dissolved in the acid. Evaporate to complete

dryness, taking care to avoid spirting. Add 7 grams of powdered ammonium

chloride, 15 c.c. of strong ammonia and 25 c.c. of boiling water; boil

for one minute and see that the residue is all softened. Filter through

a small filter, and wash thoroughly with small quantities of a hot one

per cent. solution of ammonium chloride. Add 25 c.c. of hydrochloric

acid to the filtrate. Place in the solution some clean lead foil, say 10

or 20 square inches. Boil gently until the solution has been colourless

for three or four minutes. Filter, wash with a little hot water; and

titrate with standard ferrocyanide.

~Determination of Zinc in Silver Precipitate.~--This precipitate

contains lead sulphate, silver, copper, iron, zinc, lime, &c. Weigh up 5

grams of the sample, and extract with 30 c.c. of dilute sulphuric acid

with the aid of heat. Separate the copper with sulphuretted hydrogen,

peroxidise the iron with a drop or two of nitric acid, and separate as

acetate. Render the filtrate ammoniacal, pass sulphuretted hydrogen;

warm, and filter. Dissolve the precipitated zinc sulphide in dilute

hydrochloric acid, evaporate, dilute, and titrate. Silver precipitates

carry about 2.5 per cent. of zinc.

GASOMETRIC METHOD.

Metallic zinc is readily soluble in dilute hydrochloric or sulphuric

acid, hydrogen being at the same time evolved.[74] The volume of the

hydrogen evolved is obviously a measure of the amount of zinc present in

the metallic state. The speed with which the reaction goes on (even in

the cold) and the insolubility of hydrogen renders this method of assay

a convenient one. It is especially applicable to the determination of

the proportion of zinc in zinc dust. The apparatus described in the

chapter on gasometric method is used. The method of working is as

follows: Fill the two burettes with cold water to a little above the

zero mark, place in the bottle about 0.25 gram of the substance to be

determined, and in the inner phial or test tube 5 c.c. of dilute

sulphuric acid; cork the apparatus tightly and allow to stand for a few

minutes; then bring the water to the same level in the two burettes by

running out through the clip at the bottom. Read off the level of the

liquid in the graduated burette. Turn the bottle over sufficiently to

spill the acid over the zinc, and then run water out of the apparatus so

as to keep the liquid in the two burettes at the same level, taking care

not to run it out more quickly than the hydrogen is being generated.

When the volume of gas ceases to increase, read off the level of the

liquid, deduct the reading which was started with; the difference gives

the volume of hydrogen evolved. At the same time read off the volume of

air in the "volume corrector," which must be fixed alongside the gas

burettes. Make the correction. For example: A piece of zinc weighing

0.2835 gram was found to give 99.9 c.c. of gas at a time when the

corrector read 104 c.c.[75] Then the corrected volume is

104 : 100 :: 99.9 : _x_.

_x_ = 96.0 c.c.

100 c.c. of hydrogen at 0° C. and 760 mm. is equivalent to 0.2912 gram

of zinc; therefore the quantity of zinc found is

100 : 96 :: 0.2912 : _x_.

_x_ = 0.2795 gram of zinc.

This being contained in 0.2835 gram of metal is equivalent to 98.5 per

cent.

As an example of a determination in which reducing the volume of

liberated hydrogen to 0° C. and 760 mm. is avoided, the following may be

taken:--

0.2315 gram of pure zinc gave 82.1 c.c. of gas;

and the volume of air in the corrector was 103.6 c.c.

0.2835 gram of the assay gave 99.9 c.c. of gas;

and the volume of air in the corrector was 104.0 c.c.;

104 : 103.6 :: 99.9 : _x_.

_x_ = 99.5 c.c.

This is the volume of gas got in the assay if measured under the same

conditions as the standard,

82.1 : 99.5 :: 0.2315 : _x_.

_x_ = 0.2806.

Then 0.2835 : 0.2806 :: 100: _x_.

_x_ = 98.9 per cent.

As these assays can be made quickly, it is well for the sake of greater

accuracy to make them in duplicate, and to take the mean of the

readings. One set of standardisings will do for any number of assays.

The student must carefully avoid unnecessary handling of the bottle in

which the zinc is dissolved.

~Colorimetric Method.~--Zinc salts being colourless, there is no

colorimetric determination.

EXAMINATION OF COMMERCIAL ZINC.

Take 20 grams of zinc, and dissolve them in dilute nitric acid; boil,

allow to settle; filter; wash, dry; ignite the precipitate, if any, and

weigh as oxide of tin. Examine this for arsenic.

~Lead.~--Add ammonia and carbonate of ammonia to the liquid, and boil,

filter off the precipitate, wash with hot water. Digest the precipitate

with dilute sulphuric acid; filter, wash, and weigh the sulphate of

lead.

~Iron.~--To the filtrate from the sulphate of lead add ammonia, and pass

sulphuretted hydrogen; digest, and filter. (Save the filtrate.) Dissolve

the precipitate in hydrochloric acid, oxidise with nitric acid, and

precipitate with ammonia. Wash, ignite, and weigh as ferric oxide.

Calculate to iron.

~Arsenic.~--To the filtrate from the sulphide of iron add hydrochloric

acid in slight excess; filter off, and wash the precipitate. Rinse it

back into the beaker, dissolve in nitric acid, filter from the sulphur,

and add ammonia, in excess, and magnesia mixture. Filter off the

ammonic-magnesic arsenate, and wash with dilute ammonia. Dry, ignite

with nitric acid, and weigh as magnesic pyrarsenate. Calculate to

arsenic, and add to that found with the tin.

~Copper.~--To the filtrate from the ammonia and ammonic carbonate add

sulphuric acid in small excess, and pass sulphuretted hydrogen. Allow to

settle, filter, and wash. Rinse the precipitate into a beaker, boil with

dilute sulphuric acid, and filter. (Save the filtrate.) Dry, burn the

paper with the precipitate, treat with a drop or two of nitric acid,

ignite, and weigh as copper oxide. Calculate to copper.

~Cadmium.~--To the filtrate from the sulphide of copper add ammonia, so

as to nearly neutralise the excess of acid, and pass sulphuretted

hydrogen. Collect and weigh the precipitate as cadmium sulphide, as

described under _Cadmium_.

PRACTICAL EXERCISES.

1. What weight of hydrogen will be evolved in dissolving 1 gram of zinc

in dilute sulphuric acid?

2. How many c.c. would this quantity of hydrogen measure at 0° C. and

760 m.m.? (1 litre weighs 0.0896 gram).

3. 0.23 gram of zinc are found to give 77.9 c.c. of hydrogen. In another

experiment under the same conditions 80.2 c.c. are got. What weight of

zinc was used for the second experiment?

4. A sample of blende is found to contain 55 per cent. of zinc. What

percentage of zinc sulphide did the sample contain?

5. How much metallic lead would be precipitated from a solution of lead

acetate by 1 gram of zinc?

CADMIUM.

Cadmium occurs in nature as cadmium sulphide in greenockite, CdS, which

is very rare. It is widely diffused in calamine, blende, and other zinc

ores, forming, in some cases, as much as 2 or 3 per cent. of the ore.

Oxide of cadmium forms the "brown blaze" of the zinc smelters.

Sulphide of cadmium is used as a pigment (cadmium yellow); and the metal

and some of its salts are useful reagents.

The salts of cadmium closely resemble those of zinc. The hydrate,

however, is insoluble in excess of potash, and the sulphide is insoluble

in dilute acids. It forms only one series of salts.

Cadmium is detected by giving with sulphuretted hydrogen in solutions,

not too strongly acid, a yellow precipitate, which is insoluble in

solutions of the alkalies, alkaline sulphides, or cyanide of potassium.

~Solution and Separation.~--Substances containing cadmium are soluble in

acids. The solution is evaporated to dryness (to render any silica that

may be present insoluble) and taken up with 10 c.c. of dilute

hydrochloric acid. Dilute to 100 c.c., and pass sulphuretted hydrogen.

Filter, digest the precipitate with soda, wash, and boil with dilute

sulphuric acid. Filter; the filtrate contains the cadmium and, possibly,

a small quantity of zinc, from which it is best separated by

reprecipitating with sulphuretted hydrogen.

GRAVIMETRIC DETERMINATION.

The solution containing the cadmium freed from the other metals is

precipitated with sulphuretted hydrogen in a moderately-acid solution.

The precipitate is collected on a weighed filter, and washed, first with

an acid solution of sulphuretted hydrogen, and afterwards with water. It

is dried at 100° C. and weighed. If free sulphur is suspected to be

present, extract with bisulphide of carbon, and again weigh. The residue

is cadmium sulphide, which contains 77.78 per cent. of cadmium. It is a

yellow powder insoluble in solutions of the alkalies, alkaline

sulphides, or cyanide of potassium. It dissolves readily in acid. It

cannot be ignited in a current of hydrogen without loss.

VOLUMETRIC METHOD.

The solution containing the cadmium is concentrated by evaporation, and

mixed with an excess of oxalic acid and alcohol. The precipitate is

filtered, washed with alcohol, dissolved in hot hydrochloric acid, and

titrated with permanganate of potassium.

FOOTNOTES:

[64] When chromium is present some of the iron may escape precipitation

but it can be recovered from the solution by means of ammonic sulphide.

[65]

(1) 10FeSO_{4} + 2KMnO_{4} + 8H_{2}SO_{4} = 5Fe_{2}(SO_{4})_{3} +

2MnSO_{4} + K_{2}SO_{4} + 8H_{2}O.

(2) 6FeCl_{2} + K_{2}Cr_{2}O_{7} + 14HCl = 3Fe_{2}Cl_{6} + Cr_{2}Cl_{6}

+ 2KCl + 7H_{2}O.

[66] (1) Fe_{2}Cl_{6} + SnCl_{2} = 2FeCl_{2} + SnCl_{4}.

(2) Fe_{2}Cl_{6} + SH_{2} = 2FeCl_{2} + 2HCl + S.

(3) Fe_{2}Cl_{6} + Na_{2}SO_{3} + H_{2}O = 2FeCl_{2}

+ Na_{2}SO_{4} + 2HCl.

(4) Fe_{2}Cl_{6} + Zn = 2FeCl_{2} + ZnCl_{2}.

[67] 20 grams of stannous chloride and 20 c.c. of dilute hydrochloric

acid are diluted to one litre.

[68] The maximum reducing effect of zinc is obtained by exposing as

large a surface as possible of the metal in a hot concentrated solution

containing but little free acid (Thorpe).

[69] About 5 inches in diameter.

[70] 61: 60:: 59: 58.13.

The iron in the ore is, then, the same in amount as that in 58.13 c.c.

of the ferric chloride solution; and since 100 c.c. of the latter

contain 1 gram of iron, 58.13 c.c. of the same contains 0.5813 gram of

iron; and, further, if 1 gram of ore carries this amount of iron, 100

grams of ore will obviously give 58.13 grams of iron.

[71] These compounds are Ni_{2}As and Co_{2}As.

[72] With large quantities of iron the ferric precipitate should be

re-dissolved and re-precipitated. The filtrate must be added to the

original filtrate.

[73] 4KCy + NiSO_{4} = K_{2}NiCy_{4} + K_{2}SO_{4} 2KCy + AgNO_{3}

= KAgCy_{2} + KNO_{3}

.'. 2AgNO_{3} = Ni

[74] Zn + H_{2}SO_{4} = H_{2} + ZnSO_{4}.

[75] These 104 c.c. are equivalent to 100 c.c. of dry air at 0° C. and

760 mm.

CHAPTER XII.

TIN--TUNGSTEN--TITANIUM.

TIN.

Tin occurs in nature as cassiterite (containing from 90 to 95 per cent.

of oxide of tin), which mineral is the source from which the whole of

the tin of commerce is derived. Tin also occurs as sulphide combined

with sulphides of copper and iron in the mineral stannine or bell-metal

ore. It is a constituent of certain rare minerals, such as tantalite.

The methods of assaying tin in actual use are remarkable when compared

with those of other metals. The more strictly chemical methods are

rendered troublesome by the oxide being insoluble in acids, resembling

in this respect the gangue with which it is associated. Moreover, it is

not readily decomposed by fusion with alkalies. The oxide has first to

be reduced to metal before the tin can be dissolved. The reduction may

be performed by fusing with potassic cyanide, by heating to moderate

redness in a current of hydrogen or coal gas, or by heating to a higher

temperature with carbon. The reduced metal is only slowly dissolved by

hydrochloric acid, and although it is readily soluble in aqua regia, the

solution cannot be evaporated or freed from the excess of acids, by

boiling, without loss of tin, because of the volatility of stannic

chloride. There has long been a difficulty in getting a quick wet

method.

The process of assaying tin ores adopted in the mines of Cornwall is a

mechanical one known as "vanning," the object of which is to find the

percentage of "black tin," which, it is well to remember, is not pure

cassiterite, much less pure oxide of tin. Tin ore, as taken from the

lode, contains from 2 to 5 per cent. of cassiterite, and is mainly made

up of quartz, felspar, chlorite, schorl, and other stony minerals,

together with more or less mispickel, iron and copper pyrites, oxide of

iron, and wolfram. The cassiterite has a specific gravity (6.4 to 7.1)

considerably higher than that of the vein-stuff (2.5 to 3.0), and is

concentrated by a series of washings till it is free from the lighter

material. Those minerals which have a specific gravity approaching that

of the cassiterite are not completely removed. The mispickel and copper

and iron pyrites are converted into oxides by roasting, and are in great

part removed by a subsequent washing. The concentrated product is known

as "black tin," and in this condition is sold to the smelter. The chief

foreign matters in the black tin are silica, oxides of iron and copper,

and wolfram, with traces of manganese and niobic acid; and in certain

stream ores there may be as much as 6 or 7 per cent. of titaniferous

iron. The black tin from the mines contains from 5 to 12 per cent. of

water, and is sold and assayed wet. A series of typical samples of black

tin ranged as follows:--

--------------------------+---------------------+-----------------

Source of Material. | Percentage of Metal |Specific Gravity.

| in Dry Ore. |

--------------------------+---------------------+-----------------

Good mine ore | 72.0 | 6.39

Inferior do. | 71.5 | 6.64

Titaniferous stream ore | 67.0 | 6.39

Mine ore with wolfram | 64.5 | 6.67

Ore from stream works | 58.5 | 5.99

--------------------------+---------------------+-----------------

It will be seen from these figures that black tin is a very variable

substance; and that the specific gravity is largely influenced by the

impurities; hence, it is only an indication of the percentage of metal

when the same kind of ore is dealt with.

As already pointed out, the object of vanning is to determine the

proportion of black tin in the lode stuff. The relation between the

actual content in oxide of tin and the produce got by vanning has been

tested on several occasions with results which show a fair degree of

approximation.

The following are some published results of assays of the same batch of

ore. The vanning results were obtained by a Cornish vanner of recognised

ability, and the wet assays by two London firms of the highest

standing:--

Vanning results:

(Average) 91 lbs. of "black tin."

Wet Assay results:

A 83.7 lbs. of stannic oxide.

B 79.7 lbs. "

The vanner reported his black tin as containing 70 per cent. of tin.

This will bring his result, if calculated as stannic oxide, to 80.9 lbs.

to the ton; which agrees with the others.

According to our experience the "van" assay agrees fairly well with the

"wet" one, if the black tin is assumed to contain 92.5 per cent. of

stannic oxide (SnO_{2}).

Vanners are, as a rule, skilful men, and show remarkable dexterity in

separating the black tin, with the help of their apparatus, which

consists simply of a shovel and a kieve of water. An account of the

process is given below. But different vanners, all good men, will get

different results working on material new to them. The black tin weighed

by the vanner is supposed to correspond in quality with the black tin

returned from the floors of the mine for which he is assaying, but this

differs materially in different mines with the nature of the gangue. The

process leaves too much to the judgment of the vanner. It is more than

probable that in practice the returns from the dressing-floors check the

assayer, instead of, as should properly be the case, the assayer

checking the returns. It is only when this last is done that any control

is had over the system of dressing. A correct assay of this ore is a

matter of some importance, because of the high price of the metal.

The method of assaying the black tin is a dry one, and consists of

mixing it with "culm," and submitting it in a black-lead crucible to the

highest temperature of a wind furnace. The sample is taken wet as it

arrives at the smelting house, and is assayed direct. The product of the

assay is examined, and a deduction of a considerable percentage is very

properly made for impurities, since the assay really determines the

percentage, not merely of tin, but of the bodies present which are

reducible at a white heat. The judgment as to how much is to be deducted

is assisted partly by an examination of the metal got from the assay,

and partly by the experience acquired in smelting similar ores. The

produce, which is that of the impure tin, is stated in parts in twenty;

thus a produce of 14 is equivalent to 70 per cent., or to 14 cwt. per

ton.

[Illustration: FIG. 57.]

MECHANICAL SEPARATION.--VANNING.

This process, which has already been referred to, is carried out as

follows:--After sampling the ore in the ordinary way, a quantity

(varying with its richness) is weighed out. Special weights are

generally used. The standard weight, marked 200, weighs about an ounce;

with poor ores this quantity is taken for an assay, but with richer ores

100 or even 50 is sufficient. The unit of weight has no special name,

but the parts in 200 are spoken of as the produce; thus, if 200 of ore

were taken and 9.5 of black tin were separated, the produce would be

9-1/2: obviously half the "produce" will give the percentage. The

weighed portion of the ore is placed on the vanning shovel. The vanner

stands in front of a tub of water (kieve) and allows 30 or 40 c.c. of

water to flow on to the ore. He then raises the shovel a little above

the surface of the water, and, holding it nearly horizontal, briskly

rotates the water by imparting to the shovel a slight circular motion,

passing into an elliptical one (front to back). This causes the finer

mud to be suspended in the liquid, which is then run off, leaving the

body of the ore in the centre of the shovel. This is repeated until the

water after standing a moment is fairly clear. About half as much water

as before is brought on; then, with a motion which is similar to the

previous one, but with a jerk added in one direction, the heavier

minerals are thrown up, and the stony matter brought back. The jerk is

produced just as the wave of water is returning. The descending wave of

water draws with it the bulkier and lighter particles of the ore, whilst

the heavier matter lying on the bottom is scarcely affected by it. The

jerky motion, however, carries it to the front of the shovel. The

lighter stuff is washed off, and the residue dried by holding the shovel

over the furnace. It now corresponds, more or less, to the stuff which

on the mine is sent to the calciner. It is swept from the shovel into a

scoop, and transferred to a hot crucible; in which it is calcined until

free from sulphur. Some vanners calcine their samples before commencing

to van. The calcined ore is shaken out of the crucible on to the shovel;

rubbed up with a hammer; and washed (as at first) to get rid of the

finer and lighter "waste." The separating motions are again gone

through; and the "head" of the best of the black tin is thrown well up

on one side of the shovel in the form of a crescent, so as to leave room

on the shovel to work with the "tailings." The quantity of water used is

kept low, to prevent this "crop" tin from being washed back again. The

tailings are then crushed to free the tin from adherent oxide of iron;

and again washed to throw up the remaining tin ore. As this tin is

finely divided, it is more difficult to bring it up, so that a vigorous

and rapid motion is required. The tailings are now washed off, and the

whole of the black tin is brought into the centre of the shovel. It

requires two or three washings more to free it from the waste it

contains. Very small quantities of water are used. The purity of the

black tin can be seen by its appearance on the shovel. The cleaned ore

is dried as before, freed from particles of iron with the aid of a

magnet, and weighed. The weighings are carried to 1/8th of the unit

used. The following example illustrates the method of calculation

adopted on the mine. A parcel of 1 ton 2 cwt. 3 qrs. of tin ore with a

produce of 45 (equal to 22-1/2 per cent.) contains 5 cwt. 0 qrs. 12 lbs.

of black tin. This result is obtained as follows:--

ton cwt. qrs.

1 2 3

9 }

----------------- }

10 4 3 } equivalent to multiplying by 45.

5 }

---------------- }

5.1 3 3 strike off the first figure to the right.

4 multiply by 4 to reduce to quarters.

---------

4 12

3

---------

4 15

28 multiply by 28 to reduce to pounds.

-----

112

15

-----

12.7 strike off the first figure to the right.

Similarly, a parcel of 20 tons 10 cwt. with a produce of 9-1/2 contains

19 cwt. 1 qr. 25 lbs. of black tin. For the following information, as

well as for much of that already given about vanning, we are indebted to

Captain Reynolds, of Cook's Kitchen Mine. "To have a complete set of

tools for all vanning purposes, it will be necessary to get the

following:--A vanning shovel 14 inches long and 13 inches wide, weighing

not over 2-3/4 pounds. It is made of hammered sheet iron of the shape

shown in fig. 57. It must have a light wooden handle (preferably of

deal) 3 feet long. A bruising hammer, weighing 2-1/2 pounds, with a

handle 1 foot long. A pair of tongs (furnace) 2-1/2 feet long, made of

1/2-inch round iron. And a set of ordinary clay crucibles for calcining.

There ought to be two sets of scales and weights: the first should be

confined to weighing the powdered tin stuff, and the second ought to be

a much higher class one, for weighing the black tin obtained. The

furnace for roasting the sample should be 10 inches square and 12 inches

deep, with the fire-bars at the bottom three-quarters of an inch apart.

The water-box for vanning in should be at least 4 feet long, 2 feet 6

inches wide, and 8 inches deep."

DRY METHODS.

For the following description of the process adopted in Cornwall we are

indebted to Mr. A.K. Barnett, F.G.S., of Chyandour.

~Cornish Method.~--_Tin Ore Assay._--The ore to be smelted or assayed

should be concentrated to say not less than 50 per cent. of metallic

tin; though to obtain satisfactory results it should be brought nearer

70 per cent., as with ore containing less than 40 to 50 per cent. of

metal there will be a considerable loss both in the assaying and in the

smelting. If the ore to be operated on does not contain this quantity of

metal, then the sample (if coarse) must be reduced to a fine state, the

gangue being removed by vanning, and the ore saved for the fire assay.

The method adopted for the determination of tin in the ore is as

follows:--About 2-1/2 ounces troy (1200 grains, or about 80 grams) of

the ore to be assayed is weighed out and mixed on a flat copper pan

(shaped with a long lip) with one-fifth of its weight (240 grains, or

15.5 grams) of powdered culm (anthracite). The mixture of ore and culm

is either transferred to a black-lead crucible before the latter is put

into the furnace, or, as some prefer, it is carefully swept into a

crucible which has been imbedded in the fire. Some assayers cover their

pots with a flat cover placed loosely on, while others leave the mixture

in the open pot. The furnace, which has been previously fired to a

strong heat, is then covered, and the sample is subjected to a sharp

fire for a period of from twelve to twenty minutes. No definite time can

be stated, as, besides the strength of the fire, the quality and

condition of the ore, and the impurities associated with it, greatly

affects the time required for the complete reduction of the ore. As soon

as the mixture in the crucible has settled down to a uniform white heat,

and any very slight ebullition which may have taken place has subsided,

the crucible is gently shaken, removed from the fire (the culm-ash or

slag which covers the metal being carefully drawn aside with an iron

scraper), and the metal is poured quickly into an iron ingot-mould,

which is usually placed on a copper pan to save the culm-slag and the

adherent metal which comes out with it. The crucible is then carefully

scraped, and the scrapings, together with the contents of the mould and

pan, are transferred to a mortar. There the ingot of tin is freed from

slag and then taken to the scales. The rest, after being finely

powdered, is passed through a sieve. The flattened particles of tin

which remain on the sieve are weighed with the ingot (the _lump_, as it

is called); whilst the siftings are vanned on a shovel, and (the slag

being washed off) the fine tin is collected, dried, and weighed with the

rest: the whole gives the produce or percentage of metal in the ore.

The results of the assays are expressed in cwts. of metal in the ton of

ore. The percentage is rarely given and never used in Cornwall.

Thus--"13-1/2 Produce" would mean that the assay yielded results at the

rate of 13-1/2 cwts. of metal for one ton of the ore. Some assayers use

a little powdered fluor-spar to assist the fusion of refractory slags. A

small quantity of borax will also occasionally be of service for ores

containing silica in excess of any iron that may be present. The borax

renders the slag more fusible, and assists the formation of a larger

lump (with less fine tin in the slag) than would be obtained by the use

of culm alone.

The quality and the percentage of _pure tin_ in the metal will vary

considerably, according to the impurities that are associated with the

ore to be assayed.

The crude lump is then remelted in a small iron ladle at as low a

temperature as possible, and the fused metal is poured into a shallow

trench about 4 inches long by 3/4 of an inch wide cut in a block of

white marble. The metal will be silvery-white if the temperature

employed be correct; if too hot, the surface will show a yellow, red, or

blue colour (according to the heat employed); in such case the metal

should be remelted at a lower temperature. If the metal on cooling

remains perfectly clear and bright, then it may be assumed that the tin

is of good quality and commercially pure. A crystallised or frosted

appearance of the metal indicates the presence of some alloy, say of

iron, copper, zinc, lead, antimony, &c. The assayer who has had much

practice can readily distinguish the metal or metals that are associated

with the ore by noting the appearance of the tin on cooling; and can

fairly judge the quantity of impurity present by the amount of the

crystallisation or stain.

Whilst the foregoing method of assaying cannot lay claim to scientific

accuracy, it is by no means so imperfect as some writers would have us

believe, who state that a loss of 5 to 10 per cent. arises in the

operation. It is certainly the most ready and expeditious mode of

determining the commercial value of a parcel of tin ore, which, after

all, is the main object of all assaying operations.

The difficulty which beginners find in obtaining satisfactory results,

and any loss of metal which those not accustomed to the process may

incur, will invariably occur in the vanning of the powdered slag for the

fine tin, the rest of the operations being easy of execution, and

requiring only the ordinary care necessary for all metallurgical work.

There is no doubt that if low percentage ores containing silica are

assayed in this manner, low results are obtained, as it is impossible

to reduce the whole of the tin in the presence of free silica; with this

class of ores, care should be taken to remove some of the silica by

preliminary vanning, or some flux should be added which will combine

with the silica, and so prevent its entering into combination with the

tin. Low quality tin ores containing iron, copper, lead, zinc, antimony,

etc., combined with arsenic, sulphur, or oxygen, will give very much

higher results than the actual percentage of tin in the sample. The

other metals (being readily reduced in the presence of tin) alloy with

it, and give a hard lump difficult to fuse in the iron ladle; where the

quantity of foreign metals is large, the metal can only be melted to a

stiff pasty mass; so that (in determining the value of a ton of tin ore,

or even reporting on the percentage of tin it contains) not only must

the weight of the assay be the basis for calculation, but the quality

and character of the metal obtained must also be considered. Thus two

ores of tin might be assayed both yielding a similar _produce_, say

13-1/2 (67-1/2 per cent.), and yet one might contain 5 per cent. less

tin than the other.

If it be required to obtain the pure metal from tin ores containing the

ores of other metals associated with them, the latter must be removed by

digesting in strong hydrochloric acid, and washing. The assay may then

be conducted in the usual way, and a fairly pure lump will be obtained.

If wolfram be present in any appreciable quantity in the ore, it

considerably reduces the proportion of lump, and at the same time it

increases the fine tin (or _prillion_, as it is termed) in the assay.

This may be got rid of by boiling in aqua regia, and dissolving out the

tungstic acid which has been liberated by means of ammonia.

It will be seen that this method of assaying tin has its advantages and

its drawbacks. It is quickly performed; with ores of good quality it

gives results not to be excelled by any other process; and it gives the

smelter the actual alloy and quality of metal he may expect to get in

the smelting of the ore, which no other mode of assaying will do:

against which may be set the skill required to obtain accurate results

with the vanning shovel; the loss of metal in poor ores containing an

excess of silica; and the high results from ores containing a large

quantity of metallic impurities.

~Cyanide Method.~--Weigh up 20 grams of the ore and dry it on a scoop

over the Bunsen flame. When dry, weigh, and calculate the percentage of

water from the loss in weight. Transfer the dried ore to an evaporating

dish, and cover with 30 c.c. of hydrochloric acid; boil for 10 or 12

minutes, and then add 5 c.c. of nitric acid and boil again. Dilute with

water, and filter. Transfer the filter and its contents to an E

Battersea crucible, and calcine it for a few minutes. Cool, and weigh

the residue. The loss equals the oxides soluble in acid. Transfer the

residue to the crucible and mix it with its own weight of cyanide of

potassium; add a similar amount of "cyanide" as a cover. Place in the

furnace, and when the charge has attained the temperature of the furnace

(in from 3 to 6 minutes), remove it at once; tap the pot _vigorously_

several times, and then pour its contents quietly into a mould. Dissolve

the slag in water, clean, dry, and weigh the button of tin.

WET METHODS.

~Detection.~--Tin ore is detected by its insolubility in acids, high

specific gravity, and characteristic appearance in water. The powder is

separated from the lighter gangue by washing. It is fused in a Berlin

crucible with five times its weight of potassic cyanide at a moderately

high temperature in a muffle, or over the blowpipe. The slag is washed

off with water, and the metallic buttons or residue treated with

hydrochloric acid (not aqua regia), for some time. One portion of the

solution strikes a purple colour with chloride of gold, another portion

gives a white or grey precipitate or cloudiness with mercuric chloride.

These reactions are characteristic of tin as stannous chloride.

Metallic tin treated with nitric acid becomes converted into a white

insoluble powder (metastannic acid). Aqua regia dissolves tin readily,

forming stannic chloride, and in this solution the metal is detected by

precipitation with sulphuretted hydrogen, which gives a yellow

precipitate. Tin in solution as stannic or stannous chloride is

precipitated as metal by means of zinc.

The fact that tin forms two well-defined series of compounds is taken

advantage of in assaying (just as in the case of iron), by determining

how much of an oxidising agent is required to convert it from the

stannous into the stannic state. For example, on the addition of a

solution of permanganate of potash to a solution of stannous chloride

the oxidation goes on rapidly, and the finishing point is sharp and

distinct; but acid solutions of stannous chloride quickly take up oxygen

from that dissolved in the water used and from the air. Unfortunately,

there is no obvious sign that such oxidation has taken place, except

that (fatal to the assay) a smaller volume of the permanganate is

required. Great care is required with such solutions, both before and

during titration. The addition of an excess of ferric chloride to the

stannous solution, as soon as the whole of the tin has been dissolved,

will lessen this liability to oxidation.

~Separation.~--If the tin is present in an alloy, the substance is

boiled in an evaporating dish with dilute nitric acid until the whole of

the material is attacked. Evaporate nearly to dryness, dilute, boil for

a few minutes, and filter off the white insoluble residue. Under certain

circumstances this residue will be nearly free from other metals, in

which case it is ignited and weighed. If not known to be pure it must be

ignited, reduced in a current of hydrogen, and treated as subsequently

described.

When the tin is present as insoluble oxide in an ore, the substance is

finely powdered, and from 1 to 5 grams of it (according to its richness)

boiled with 30 c.c. of hydrochloric acid in an evaporating dish till the

oxide of iron is seen to be dissolved. Then add 1 c.c. of nitric acid

(or more if much pyrites, &c., is present) and continue the boiling till

these are decomposed; dilute and filter off, washing first with dilute

acid and afterwards with a little dilute ammonia, dry, ignite, and place

in a combustion tube (together with the filter-ash) and heat to redness

for about thirty minutes in a current of dried hydrogen.

[Illustration: FIG. 58.]

The oxide of tin is placed in a porcelain boat (fig. 58), which is then

introduced into a piece of combustion tube. The latter, wrapped in a

piece of wire gauze, is supported on a couple of iron rings, and heated

by one or two Bunsen burners in a furnace fitted up with loose

fire-brick tiles, as shown in fig. 59.

[Illustration: FIG. 59.]

When the reduction is complete the tube is allowed to cool; the boat is

removed and the tin dissolved. Add a rod of zinc to the freely-acid hot

solution, and in a few minutes decant through a filter and wash with

water, after having removed the zinc. Wash the precipitated metal back

into the beaker, and dissolve in 10 c.c. of dilute nitric acid,

evaporate off the excess of acid; dilute, boil, and filter. Wash, dry,

ignite strongly in a porcelain crucible, and weigh.

In the absence of antimony the above separation works very well, but if

this metal is present in quantity the metals precipitated on the zinc

must be covered with hydrochloric acid and treated with a few drops of

nitric. It is then warmed with iron wire until no more of the latter

dissolves. The antimony is precipitated as metal, and the tin remains in

solution as stannous chloride. The antimony is filtered off, and may be

washed with alcohol, and weighed, whilst the tin in the filtrate is

precipitated with zinc, and treated as already described.

GRAVIMETRIC METHOD.

If the tin is not already in the metallic state it is reduced to this

condition by the method given (precipitation by zinc). Treat the

finely-divided metal (washed free from chlorides) in a four-inch

evaporating dish with 10 c.c. of dilute nitric acid, cover with a

clock-glass, and apply a gentle heat until the precipitate appears of a

white colour and the metal is completely attacked. Evaporate nearly to

dryness on a water-bath; then add 50 c.c. of water, heat to boiling, and

filter. Wash with hot water, dry, transfer to a weighed porcelain

crucible, add the filter-ash, ignite strongly, and weigh. The

precipitate after ignition is stannic oxide (SnO_{2}). It is a

yellowish-white powder (darker whilst hot), insoluble in acids, and

contains 78.67 per cent. of tin. Cold dilute nitric acid dissolves tin

to a clear solution, which becomes a white enamel-like jelly on heating;

this (filtered off, washed, and dried) forms an opal-like substance,

which is converted on ignition into stannic oxide with evolution of

nitrous fumes. Stannic oxide when ignited with chlorides is more or less

completely converted into stannic chloride, which volatilises. The

presence of chlorides during the evaporation with nitric acid causes a

similar loss.

~Determination of Tin in an Alloy.~--(_Bronze._)--Take 2 grams, and

attack with 20 c.c. of dilute nitric acid in a covered beaker with the

aid of heat. Boil till the bulk is reduced by one-half, dilute with 50

c.c. of water, allow to settle for a few minutes, and filter; wash well

first with water acidulated with a little nitric acid, and afterwards

with water; dry, ignite, and weigh as stannic oxide.

~Determination of Tin in Tin Ore.~--Treat 5 grams of the dried and

finely-powdered ore with 30 c.c. of hydrochloric acid in a four-inch

evaporating dish. After the soluble oxides have been dissolved add 1 or

2 c.c. of nitric acid, boil off nitrous fumes, dilute, and filter. Dry

the filter, transfer the cleaned ore to a piece of combustion tube ten

or twelve inches long and narrowed at one end. Pass a current of

hydrogen through the tube and heat to redness for 30 minutes; cool

whilst the gas is still passing. Dissolve in 20 c.c. of dilute

hydrochloric acid and keep the solution tinted with permanganate of

potassium. When the colour of the permanganate becomes permanent dilute

to a bulk of 50 c.c. with water, filter, and wash. Heat; add a rod of

zinc weighing about 3 grams; allow to stand for a few minutes; decant

through a filter; and wash, removing the remaining zinc and returning

the tin to the beaker. Treat with 5 c.c. of dilute nitric acid, boil for

some time, take up with water, filter, wash, dry, ignite, and weigh as

stannic oxide.

VOLUMETRIC METHOD.

~Titration with Solution of Permanganate of Potassium.~--This titration

may be made either directly on the solution of stannous chloride

(prepared by dissolving the precipitated metal in hydrochloric acid), or

indirectly, on a solution of ferrous chloride (produced by the reducing

action of the precipitated metal on ferric chloride). The standard

solution of permanganate of potassium is made by dissolving 5.356 grams

of the salt in water and diluting to one litre. 100 c.c. are equivalent

to 1.00 gram of tin.

The precipitated tin is transferred to a flask; and dissolved in 10 c.c.

hydrochloric acid, with the aid of heat and in an atmosphere of carbonic

acid. The acid and metal are placed in the flask; which is then filled

with the gas, and stopped with a cork provided with a rubber valve. When

solution is complete the flask is again filled with carbonic acid. Fifty

c.c. of water freed from air and saturated with carbonic acid are then

added. This water is made by adding a gram of bicarbonate of soda and 2

c.c. of hydrochloric acid to 100 c.c. of water: the effervescence sweeps

out the dissolved oxygen. The permanganate of potassium solution is then

run in from a stop-cock burette in the usual way until a faint pink

tinge is obtained.

The following experiments show the effect of variations in the

conditions of the assay. A solution of stannous chloride equivalent in

strength to the "permanganate" was made by dissolving 19.06 grams of the

crystallised salt (SnCl_{2}.2H_{2}O.) in 50 c.c. of water and 10 c.c. of

hydrochloric acid and diluting to 1 litre with water freed from

dissolved oxygen. 100 c.c. contain 1 gram of tin. In the first

experiments tap water was used and no precautions were taken for

excluding air. Except when otherwise stated, 20 c.c. of the stannous

chloride were used in each experiment with 10 c.c. of hydrochloric acid,

and were diluted to 100 c.c. with water before titration.

~Effect of Varying Hydrochloric Acid.~

Acid added 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c.

"Permanganate" required 18.8 " 18.9 " 18.8 " 18.8 "

The only effect of the increase in quantity of acid was to give the

brown of perchloride of manganese instead of the pink of permanganic

acid to mark the finishing point.

~Effect of Varying Temperature.~

Temperature 15° C. 50° C. 70° C. 100° C.

"Permanganate" required 18.8 c.c. 18.7 c.c. 18.6 c.c. 18.4 c.c.

~Rate of Atmospheric Oxidation.~--Solutions ready for titration were

exposed to air at the ordinary temperature for varying lengths of time

and then titrated.

Time exposed 0 min. 5 min. 10 min. 20 min. 60 min.

"Permanganate"

required 18.8 c.c. 18.8 c.c. 18.8 c.c. 18.8 c.c. 18.6 c.c.

It is best to titrate at once, although the loss by oxidation is only

small after one hour's exposure.

~Effect of Varying Tin.~

Stannous chloride

added 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

"Permanganate"

required 0.7 " 8.8 " 18.0 " 47.4 " 95.4 "

~Effect of Varying Bulk.~

Bulk 50.0 c.c. 100.0 c.c. 200.0 c.c. 500.0 c.c.

"Permanganate" required. 9.0 " 18.3 " 17.4 " 15.1 "

The two last series show an interference, which is due to the oxygen

dissolved in the water, as may be seen from the following similar

experiments, which were, however, performed with water freed from oxygen

and in which the titrations were effected in an atmosphere of carbonic

acid.

~Effect of Varying Tin.~--A new solution of stannous chloride was used.

Stannous chloride

added 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

"Permanganate"

required 1.0 " 10.0 " 19.8 " 49.6 " 99.3 "

~Effect of Varying Bulk.~

Bulk 30.0 c.c. 50.0 c.c. 100.0 c.c. 200.0 c.c. 500.0 c.c.

"Permanganate"

required 19.8 " 19.8 " 19.8 " 19.8 " 19.8 "

It will be seen that in working under these conditions the results are

proportional and the method satisfactory.

~Examination of Tin Phosphide.~--(_Phosphor Tin_.)--This substance is

used in the manufacture of "phosphor bronze" and similar alloys. It is a

crystalline, imperfectly-malleable, metallic substance. It is soluble in

hydrochloric acid with effervescence; phosphoretted hydrogen, which

inflames on the addition of a drop or two of nitric acid, being evolved.

It is attacked by nitric acid, yielding a white powder of stannic

phosphate; this is not easily decomposed by ammonium sulphide or readily

soluble in hydrochloric acid.

"Phosphor-tin" is made up only of tin and phosphorus. For the estimation

weigh up 1 gram. Place in a weighed Berlin dish; and cover with 10 c.c.

of nitric acid and 3 or 4 c.c. of water. Let the reaction proceed (under

a clock-glass) on the water-bath till complete. Remove the glass;

evaporate to dryness, and ignite, at first gently over a Bunsen burner,

and afterwards in the muffle at a red heat. Cool in the desiccator, and

weigh as quickly as possible when cold. The substance contains the tin

as stannic oxide, SnO_{2}, and the phosphorus as phosphoric oxide,

P_{2}O_{5}. The increase in weight on the gram of substance taken gives

the weight of the oxygen taken up by the phosphorus and tin, and since 1

gram of tin takes up only 0.271 gram of oxygen, and 1 gram of phosphorus

takes up 1.29 gram, the proportion of tin to phosphorus can be

calculated from the increase in weight. For example, 1 gram of a sample

gave 1.3410 gram of mixed oxides, which is 0.070 gram in excess of that

which would be got with pure tin. If the substance was all phosphorus

the excess would be 1.0190 gram; consequently the proportion of

phosphorus in the substance is 0.070 / 1.019, or 6.87 per cent. The tin

is calculated by difference, 93.13 per cent.

Another method of separating and determining the phosphorus is as

follows:--Take 1 gram of the substance and add to it 15 c.c. of hot aqua

regia. Boil till dissolved, dilute, and precipitate the tin with

sulphuretted hydrogen. To the filtrate add ammonia and "magnesia

mixture." Filter; wash the precipitate with dilute ammonia; dry, ignite,

and weigh as magnesic pyrophosphate. Calculate the phosphorus, and take

the tin by difference.

A sample of phosphor tin gave--

Tin 93.1 per cent. (by difference)

Phosphorus 6.9 "

-----

100.0

~Tin Arsenide.~--This is met with in tin-smelting; it closely resembles

the phosphide, but the crystals have a duller grey appearance. It

contains simply tin and arsenic. The determination is made by treating 1

gram of the substance with nitric acid and weighing the mixed oxides of

tin and arsenic in the same manner as in the case of the phosphide. One

gram of arsenic will give 1.533 gram of arsenic oxide, As_{2}O_{5};

consequently the excess of weight of the mixed oxides over 1.271 gram

must be divided by 0.262; the result multiplied by 100 gives the

percentage of arsenic. In consequence of the higher atomic weight of

arsenic the results by this method are not so close as with the

phosphide. Each milligram of excess weight (over 1.271) represents 0.38

per cent. of arsenic, As. Both in this and in the corresponding

phosphide determination care must be taken to avoid absorption of

moisture, by allowing the oxides to cool in a desiccator and weighing

quickly.

The percentage of arsenic is better determined as follows:--Weigh up 1

gram of the substance, dissolve in aqua regia, dilute, and pass

sulphuretted hydrogen. Render alkaline with ammonia, and add ammonium

sulphide till the precipitate is dissolved. Add "magnesia mixture."

Filter off the precipitate, wash with dilute ammonia, ignite with a few

drops of nitric acid, and weigh as magnesic pyrarsenate. Calculate the

arsenic and take the tin by difference. A sample treated in this way

gave--

Tin 96.8 per cent. by difference

Arsenic 3.2 "

-----

100.0

~Examination of Black Tin.~--Dry the ore, and reduce it to a fine

powder. Weigh up 2 grams, and boil with 20 c.c. of hydrochloric acid and

2 c.c. of nitric for ten or fifteen minutes. Filter, and reserve the

filtrate.

~Tungstic Acid.~--Digest the residue with about 50 c.c. of water and a

few c.c. of dilute ammonia for a few minutes, and filter; collect the

filtrate in a weighed porcelain dish, evaporate to dryness, ignite, and

weigh as tungstic acid, WO_{3}.

~Stannic Oxide.~--Dry, ignite, and weigh the insoluble residue. Transfer

to a porcelain boat, and reduce in a current of hydrogen at a red heat

for half an hour. Allow to cool whilst the hydrogen is still passing.

Transfer the boat to a beaker, and dissolve up the tin in 10 c.c. of

hydrochloric acid and a c.c. or so of nitric. Wash out the combustion

tube with some acid and add the washing to the contents of the beaker.

Warm gently, dilute with water, and filter. Collect, dry, ignite, and

weigh the insoluble residue. Through the filtrate pass a rapid current

of sulphuretted hydrogen, allow to settle, and filter. Wash the

precipitate with hot water, dry, calcine gently; ignite with ammonium

carbonate, and weigh as stannic oxide, SnO_{2}. The insoluble residue

will in most cases retain some tin. Fuse it with fusion mixture, take up

with hydrochloric acid, filter, pass sulphuretted hydrogen through the

filtrate, collect and wash the sulphide of tin. Ignite and weigh as

stannic oxide, and add it to that previously obtained.

~Copper.~--Pass sulphuretted hydrogen through the acid filtrate obtained

in the first cleaning of the ore, collect the precipitate, and wash

first with soda solution and then with hot water. Dry, ignite, and weigh

as cupric oxide, CuO. Mix the filtrate with that from the main portion

of the sulphide of tin.

~Ferric Oxide.~--Boil off the sulphuretted hydrogen from the mixed

filtrates and peroxidise with nitric acid. Add ammonia in slight excess,

boil, filter, dry, ignite, and weigh the precipitate as ferric oxide.

This will be practically pure, but the iron in it must be determined by

dissolving and titrating. The filtrate from the iron may contain zinc,

lime, and magnesia, but rarely in quantities sufficient to be

determined.

~Silica, &c.~---The silica may be calculated from the weight of the

residue insoluble in acid, after the reduction of the tin in hydrogen,

by deducting from it the weight of the oxide of tin subsequently found.

Or it may be determined as follows:--The insoluble portion is fused with

fusion mixture, and taken up with hydrochloric acid, as already

described. On filtering, the filter will retain a portion of the silica.

The rest is recovered, after the removal of the stannous sulphide, by

evaporating to dryness, taking up with hydrochloric acid, and filtering

through the same filter. It is washed, dried, ignited, and weighed as

silica. The filtrate from the silica is boiled with a little nitric acid

and precipitated with ammonia. The precipitate is collected, washed,

ignited, and weighed as ferric oxide and alumina (but it frequently

contains oxide of titanium). When the last is present it is determined

by fusing with bisulphate of potash and extracting with cold water. The

solution is nearly neutralised with ammonia, charged with sulphurous

acid, and boiled. The precipitate is collected, washed, dried, ignited,

and weighed as oxide of titanium, TiO_{2}. The difference between this

weight and that of the combined oxides gives the ferric oxide and

alumina. The filtrate from the mixed oxides is examined for lime and

magnesia.

~Sulphur.~--Rub up 5 grams of the ore with 5 grams of nitre, transfer to

a porcelain dish, and fuse over a Bunsen burner for fifteen minutes.

When cold, extract with water, and determine the sulphur volumetrically

with standard barium chloride. The sulphur may be present as sulphide or

sulphate.

~Arsenic.~--Take 5 grams, and evaporate with nitric acid; dilute, add

ammonia, pass sulphuretted hydrogen, and filter.

To the filtrate add "magnesia mixture." Collect the precipitate, ignite

with nitric acid, and weigh as magnesic pyrarsenate.

The following may be taken as an example of the composition of an impure

black tin:--

Tungstic acid 1.8 %

Stannic oxide 79.0

Silica 2.6

Titanic oxide 0.8

Copper oxide 0.9

Ferric oxide 13.4

Sulphur 0.4

Arsenic 0.3

----

99.2

~Examination of Hardhead.~--In the smelting of tin ores a quantity of

speise, known as "hardhead," is produced. It is essentially an arsenide

of iron, carrying a considerable quantity of tin. Much of this last is

present in the form of small buttons of metal distributed through the

mass. The buttons can be seen on careful inspection, and become evident

on powdering.

In assaying the substance, a variation in the usual method of sampling

is required, because of the quantity of metal present which cannot be

powdered. After powdering as finely as possible, the coarse particles

are sifted off and weighed. The weight of the powder is also taken. The

method of working is best illustrated by an example. A sample of

hardhead weighed 155.1 grams, and gave 21.0 grams of coarse particles,

equivalent to 13.5 per cent. of the whole. The fine portion weighed 134

grams, which is equivalent to 86.5 per cent.

Thirteen and a half grams of the coarse material were dissolved in aqua

regia, and diluted with water to 1 litre. Ten c.c. of this contain 0.135

gram of the metallic portion, which is the amount contained in 1 gram of

the original hardhead. If, in a determination, 1 gram of the substance

is wanted, weigh up 0.865 gram of the powdered portion, and add to it 10

c.c. of the solution. It will be seen that these together make up 1 gram

of the original sample. The solution of the metallic portion must be

saved until the analysis is finished.

~Tin and Copper.~--Weigh up the portion of the powdered stuff equivalent

to 1 gram of the sample. Transfer to a flask, and cover with 10 c.c. of

the solution of the metallic portion and 10 c.c. of aqua regia. Boil

gently till oxidation is complete and the nitric acid for the greater

part driven off. Dilute to 100 c.c. with water, and pass sulphuretted

hydrogen for some time. Filter, wash with hot water, and rinse through

the funnel back into the flask. Digest with yellow sodium sulphide until

only a light, flocculent, black precipitate is left. Filter this off,

wash with hot water, dry, calcine, treat with a little nitric acid,

ignite, and weigh as copper oxide, CuO. The weight multiplied by 0.7983

gives the weight of copper.

The filtrate containing the tin is rendered acid with hydrochloric acid,

and filtered. The precipitate is rinsed into a half-pint beaker, covered

with 20 c.c. of hydrochloric acid, and boiled down to about 20 c.c. The

solution is filtered off from the sulphur and sulphide of arsenic,

which, after washing with hot water, is transferred to a flask labelled

"arsenic." A strip of sheet zinc (2 in. by 1 in.) is placed in the

solution. The evolution of hydrogen should be brisk. In five or ten

minutes decant off a few c.c. of the liquid, and test with sulphuretted

hydrogen for tin. If no yellowish precipitate is formed, decant off the

rest of the liquid, and wash the precipitated metal with hot water two

or three times by decantation. The metal should be in a lump; if there

are any floating particles they must be made to sink by compression with

a glass rod. Transfer the washed metal to an evaporating dish 3 or 4 in.

across, and cover with a few c.c. of hot water. Add nitric acid drop by

drop till the tin is completely attacked. Evaporate nearly to dryness,

and add a drop or two more of nitric acid and 20 c.c. of water. Boil and

filter. Wash with hot water, dry, ignite, and weigh as stannic oxide,

SnO_{2}. Calculate to metallic tin by multiplying by 0.7867.[76]

The filtrate from the first treatment with sulphuretted hydrogen will

probably no longer smell of the gas. Warm and pass the gas for a few

minutes longer. Filter off any precipitate of sulphide of arsenic, and

transfer it to the flask for "arsenic." Boil the filtrate (ignoring any

signs of a further precipitation of arsenic) with a few c.c. of nitric

acid, and separate the iron as basic acetate. Wash; reserve the filtrate

for cobalt.

~Iron.~--Rinse back the "basic acetate," precipitate into the flask, add

ammonia, dilute with water to about 100 c.c., and pass sulphuretted

hydrogen for a few minutes. Filter, and wash with hot water. Collect the

filtrate in the flask labelled "arsenic." Boil the precipitate with

dilute sulphuric acid, filter, and titrate the filtrate with the

permanganate of potassium solution after boiling off the sulphuretted

hydrogen. Report the result as iron. The sulphuric acid will not effect

complete solution, a light black residue will remain, chiefly sulphur;

this must be rinsed into the filtrate from the acetate separation. It

contains cobalt.

~Cobalt.~--The filtrate from the acetate separation will have a pink

colour. Render it ammoniacal and pass sulphuretted hydrogen. Collect

the precipitate on a filter, dry, and ignite. Dissolve in hydrochloric

acid, and evaporate nearly to dryness with an excess of nitric acid.

Dilute with 10 or 20 c.c. of water and add potash solution in slight

excess. Add acetic acid until the solution is acid and the precipitate

is quite dissolved. Add 20 or 30 c.c. of a strong solution of potassium

nitrite, and determine the cobalt, as described on pp. 254, 256. Boil

the filtrate from the cobalt, precipitate with hydrochloric acid, render

ammoniacal, and test for zinc, nickel, and manganese.

_The remainder of the tin_ will be contained in the flask labelled

"arsenic." Acidify with hydrochloric acid and filter. Rinse into a

beaker, and evaporate to a small bulk with 10 c.c. of nitric acid.

Dilute and filter. Dry the precipitate, consisting of stannic arsenate

(2SnO_{2}.As_{2}O_{5}), ignite, and weigh. Calculate the tin it contains

by multiplying by 0.4453, and add to that already found.