follows. Dry and transfer to a weighed porcelain crucible, mix with a
little pure sulphur, and ignite at a red heat for 5 or 10 minutes in a
current of hydrogen. Allow to cool while the hydrogen is still passing.
Weigh. The subsulphide of copper thus obtained contains 79.85 per cent.
of copper; it is a greyish-black crystalline mass, which loses no weight
on ignition if air is excluded.
Copper may be separated from its solutions by means of sodium
hyposulphite. The solution is freed from hydrochloric and nitric acids
by evaporation with sulphuric acid; diluted to about a quarter of a
litre; heated nearly to boiling; and treated with a hot solution of
sodium hyposulphite (added a little at a time) until the precipitate
settles and leaves the solution free from colour. The solution contains
suspended sulphur. The precipitate is easily washed, and under the
proper conditions the separation is complete, but the separation with
sulphuretted hydrogen is more satisfactory, since the conditions as to
acidity, &c., need not be so exact.
Zinc or iron is sometimes used for separating copper from its solutions,
but they are not to be recommended.
ELECTROLYTIC ASSAY.
The separation of copper by means of a current of electricity is largely
made use of, and forms the basis of the most satisfactory method for the
determination of this metal. If the wire closing an electric circuit be
broken, and the two ends immersed in a beaker of acidulated water or
solution of any salt, the electricity will pass through the liquid,
bringing about some remarkable changes. Hydrogen and the metals will be
liberated around that part of the wire connected with the zinc end of
the battery, and oxygen, chlorine, and the acid radicals will be set
free around the other. Different metals are deposited in this way with
varying degrees of ease, and whether or not any particular metal will be
deposited depends--(1) on the conditions of the solution as regards acid
and other substances present, and (2) on the _intensity_ of the current
of electricity used. For analytical purposes the metal should be
deposited not only free from the other metals present, but also as a
firm coherent film, which may afterwards be manipulated without fear of
loss. This is, in the case of copper and many other metals, effected by
a simple control of the conditions. It is necessary that the electrodes,
or wires which bring the electricity into the solution, should be made
of a material to which the deposited metal will adhere, and which will
not be attacked by substances originally present or set free in the
solution. They are generally made of platinum. There are various
arrangements of apparatus used for this purpose, but the following plan
and method of working is simple and effective, and has been in daily use
with very satisfactory results for the last five or six years.
The battery used is made up of two Daniell cells, coupled up for
intensity as shown in fig. 49--that is, with the copper of one connected
with the zinc of the other. For eight or ten assays daily the quart size
should be used, but for four or five two pint cells will be sufficient.
[Illustration: FIG. 49.]
The outer pot of each cell is made of sheet copper, and must be clean
and free from solder on the inside. It is provided near the top with a
perforated copper shelf in the shape of a ring, into which the inner or
porous cell loosely fits. It is charged with a saturated solution of
copper sulphate, and crystals of this salt must be added, and always
kept in excess. When the battery is at work copper is being deposited on
the inner surface of this pot.
The inner or porous pot contains the zinc rod, and is charged with a
dilute acid, made by diluting one volume of sulphuric acid up to ten
with water. The object of the porous pot is to prevent the mixing of the
acid and copper sulphate solutions, without interrupting the flow of
electricity. The copper sulphate solution will last for months, but the
acid must be emptied out and recharged daily.
The zinc rods must be well amalgamated by rubbing with mercury under
dilute acid until they show a uniformly bright surface. They should not
produce a brisk effervescence when placed in the acid in the porous pot
before coupling up.
The battery when working is apt to become dirty from the "creeping" of
the copper and zinc sulphate solution. It must be kept away from the
working bench, and is best kept in a box on the floor.
[Illustration: FIG. 50.]
The connection of the battery with, and the fixing of, the electrodes
may be made by any suitable arrangement, but the following is a very
convenient plan. The wire from the zinc is connected by means of a
binding screw with a piece of stout copper wire, which, at a distance
sufficiently great to allow of easy coupling with the battery, is led
along the back of a piece of hard wood. This is fixed horizontally
about one foot above the working bench. The general arrangement is shown
in fig. 50, in which, however, for the sake of economy of space, the
battery is placed on the working bench instead of on the floor. The
piece of wood is one inch square and three or four feet long. It is
perforated from front to back at distances of six inches by a number of
small holes, in which are inserted screws like that shown in fig. 51.
These are known as "terminals," and may be obtained of any electrician.
The head of each screw is soldered to the wire mentioned above as
running along the back and as being connected with the zinc end of the
battery. These terminals serve to fix the electrodes on which the copper
is to be deposited. The wire from the copper end of the battery is
similarly connected by a connecting screw (fig. 52) with another wire (H
in fig. 53), which runs along the top of the rod and has soldered to it,
at distances of six inches, cylindrical spirals of copper wire. These
should project from the rod at points about half-way between the
terminals already described. They may be made by wrapping copper wire
around a black-lead pencil for a length of about three inches.
[Illustration: FIG. 51.]
[Illustration: FIG. 52.]
[Illustration: FIG. 53.]
The rod is perforated from top to bottom with a series of small holes,
one in advance of each terminal but as near it as possible. Into these
short pieces of glass tube are inserted to ensure insulation. These
receive the other electrodes, which are connected with the wire leading
to the copper end of the battery, through the spirals, with the help of
a binding screw. The figure will make this clear. (Fig. 53.)
[Illustration: FIG. 54]
~The electrodes~ consist of a platinum spiral and cylinder. The spiral
should have the shape shown in A, fig. 54. When in work it is passed
through one of the holes fitted with glass tubes and connected with the
copper end of the battery. The thickness of the wire of which it is made
is unimportant, provided it is stout enough to keep its form and does
not easily bend. The spiral will weigh about 8 grams. The cylinder (C,
fig. 54) will weigh about 12 grams. It should have the shape shown in
the figure. In working it is clamped to one of the terminals, and on it
the copper is deposited. A cylinder will serve for the deposition of
from 1 to 1.5 gram of copper. It is made by rivetting a square piece of
foil on to a stiff piece of wire, and then bending into shape over a
glass tube or piece of rounded wood. Each cylinder carries a distinctive
number, and is marked by impressing Roman numerals on the foil with the
blade of a knife. The weight of each is carefully taken and recorded.
They lose slightly in weight when in use, but the loss is uniform, and
averages half a milligram per month when in daily use. The cylinders are
cleaned from deposited copper by dissolving off with nitric acid and
washing with water; and from grease by igniting.
The ~beakers~, to contain the solution of copper to be electrolysed, are
ordinary tall beakers of about 200 c.c. capacity, and are marked off at
100 c.c. and 150 c.c. They are supported on movable stands, consisting
of wooden blocks about six inches high and three inches across. The bar
of wood which carries the connecting wires and electrodes is permanently
fixed over the working bench, at such a height that, with the beakers
resting on these blocks, the electrodes shall be in position for
working.
To fix the electrodes to the rod, remove the stand and beaker and pass
the long limb of the spiral up through one of the glass tubes. Connect
it with the free end of the copper spiral by means of a connecting screw
(fig. 52), and then draw out and bend the copper spiral so that the
platinum one may hang freely. Screw the wire of the cylinder to the
terminal, and, if necessary, bend it so that the cylinder itself may be
brought to encircle the rod of the spiral in the manner shown in fig.
53.
The ~general method of working~ is as follows:--The quantity of ore to
be taken for an assay varies with the richness of the ore, as is shown
in the following table:--
Percentage of Copper Quantity of Ore
in the Ore. to be taken.
1 to 5 5 grams
5 to 10 3 "
10 to 30 2 "
30 to 50 1.5 "
50 to 100 1 "
The weighed quantity of ore is dissolved by evaporating with nitric acid
and taking up with hydrochloric, as already described. Any coloured
residue which may be left is generally organic matter: it is filtered
off, calcined, and any copper it contains is estimated colorimetrically.
Nearly always, however, the residue is white and sandy. The copper is
separated from the solution as sulphide by means of a rapid current of
sulphuretted hydrogen. The liquid is decanted off through a filter, the
precipitate washed once with hot water and then rinsed back into the
flask (the filter paper being opened out) with a jet of water from a
wash bottle. Fifteen c.c. of nitric acid are added to the contents of
the flask, which are then briskly boiled until the bulk is reduced to
less than 10 c.c. The boiling down is carried out in a cupboard free
from cold draughts, so as to prevent the condensation of acid and steam
in the neck of the flask. Twenty c.c. of water are next added, and the
solution is warmed, and filtered into one of the beakers for
electrolysis. The filtrate and washings are diluted with water to the
100 c.c. mark, and the solution is then ready for the battery. It must
not contain more than 10 per cent. by volume of nitric acid.
The number and weight of the platinum cylinder having been recorded,
both electrodes are fixed in position and the wooden block removed from
under them. The beaker containing the copper solution is then brought up
into its place with one hand, and the block replaced with the other so
as to support it. All the assays having been got into position, the
connecting wires are joined to the battery. If everything is right
bubbles of oxygen at once stream off from the spiral, and the cylinder
becomes tarnished by a deposit of copper. If the oxygen comes off but no
copper is deposited, it is because the assay solution contains too much
nitric acid. If no action whatever takes place, it is because the
current is not passing. In this case examine the connections to see that
they are clean and secure, and the connecting wires to see that they are
not touching each other.
The action is allowed to go on for sixteen or seventeen hours, so that
it is best to let the current act overnight. In the morning the
solutions will appear colourless, and a slow stream of oxygen will still
be coming off from the spiral.
A wash-bottle with cold distilled water and two beakers, one with
distilled water and the other with alcohol, are got ready. The block is
then removed, the spiral loosened and lowered with the beaker. The
cylinder is next detached and washed with a stream of water from the
wash-bottle, the washings being added to the original solution. The
current from the battery is not stopped until all the cylinders are
washed. After being dipped in the beaker of water and once or twice in
that with the alcohol, it is dried in the water-oven for about three
minutes, and then weighed. The increase in weight is due to deposited
copper. This should be salmon-red in colour, satin-like or crystalline
in appearance, and in an even coherent deposit, not removed by rubbing.
It is permanent in air when dry, but sulphuretted hydrogen quickly
tarnishes it, producing coloured films. With ores containing even very
small proportions of bismuth, the deposited copper has a dark grey
colour, and when much of this metal is present the copper is coated with
a grey shaggy deposit.
It still remains to determine any copper left undeposited in the
solution. This does not generally exceed four or five milligrams, and
is estimated colorimetrically. Thirty c.c. of dilute ammonia (one of
strong ammonia mixed with one of water) are added to the electrolysed
solution, which is then diluted up to the 150 c.c. mark with water. It
is mixed, using the spiral as stirrer, and, after standing a few minutes
to allow the precipitate to settle, 100 c.c. of it are filtered off
through a dry filter for the colorimetric determination. Since only
two-thirds of the solution are taken for this, the quantity of copper
found must be increased by one-half to get the quantity actually
present.
[Illustration: FIG. 55.]
The ~colorimetric determination~ may be made in the manner described
under that head, but where a number of assays are being carried out it
is more convenient to have a series of standard phials containing known
amounts of copper in ammoniacal solution. By comparing the measured
volume of the assay solution with these, the amount of copper present is
determined at a glance. These standard bottles, however, can only be
economically used where a large number of assays are being made daily.
A convenient plan is to get a quantity of white glass four-ounce phials,
like that in fig. 55, and to label them so that they shall contain 100
c.c. when filled up to the bottom of the labels. The labels should be
rendered permanent by coating with wax, and be marked with numbers
indicating the milligrams of copper present. The bottles are stopped
with new clean corks, and contain, in addition to the specified quantity
of copper, 6 c.c. of nitric acid and 10 c.c. of strong ammonia, with
sufficient water to make up the bulk to 100 c.c. The copper is best
added by running in the requisite amount of a standard solution of
copper, each c.c. of which contains 0.001 gram of the metal.
The standard bottles should be refilled once every three or four months,
since their colorimetric value becomes slowly less on keeping. The
following determinations of a set which had been in use for three months
will illustrate this. The figures indicate milligrams of copper in 100
c.c.: the first row gives the nominal and the second row the actual
colorimetric value of the standards. The difference between the two
shows the deterioration.
1 2 3 4 6 8 10 12 14
1 2 3 3.7 5.5 7.5 9 11 13
The amount of copper in the assay is got by increasing that found
colorimetrically by one-half and adding to that found on the platinum
cylinder. The percentage is calculated in the usual way. The following
examples will illustrate this, as well as the method of recording the
work in the laboratory book:--
---------------------------------------------
Cylinder I. + Cu 9.5410
Cylinder I. 9.5170
------
0.0240
By colour 100 c.c. = 0.0015}
0.0007} 0.0022
------ ------
0.0022 0.0262
IX. Sample. Took 5 grams.
Copper = 0.52%
---------------------------------------------
Cylinder VI. + Cu 10.5705
Cylinder VI. 10.0437
-------
0.5268
By colour, 100 c.c. = 0.0070}
0.0035} 0.0105
------ ------
0.0105 0.5373
Matte, No. 1070. Took 1.5 gram.
Copper = 35.82%
---------------------------------------------
Cylinder XIII. + Cu 12.0352
Cylinder XIII. 11.0405
-------
0.9947
By colour 100 c.c. = 0.0005}
0.0002} 0.0007
------ ------
0.0007 0.9954
X. Sample, Cake copper. Took 1.0053 gram.
Copper = 99.00%
---------------------------------------------
In the electrolytic assay of metals, alloys, precipitates, and other
bodies rich in copper, the preliminary separation of the copper by
sulphuretted hydrogen is unnecessary. It is sufficient to dissolve the
weighed sample in 10 c.c. of nitric acid, boil off nitrous fumes, dilute
to 100 c.c. with water, and then electrolyse.
~General Considerations.~--In the preliminary work with the copper
sulphide there is a small loss owing to its imperfect removal in washing
the filter paper, and another small loss in dissolving in nitric acid
owing to the retention of particles in the fused globules of sulphur. To
determine its amount the filter-papers and sulphur were collected from
forty assays, and the copper in them determined. The average amount of
copper in each assay was 0.175 gram; that left on the filter paper was
0.00067 gram; and that retained by the sulphur 0.00003 gram; thus
showing an average loss from both sources of 0.00070 gram. The
determinations from another lot of forty-two similar assays gave on an
average
Copper left on filter paper 0.00070 gram
Copper retained by sulphur. 0.00004 "
The loss from these sources is trifling, and need only be considered
when great accuracy is required.
The deposition of the copper under the conditions given is satisfactory,
but, as already stated, if the solution contain more than 10 per cent.
of nitric acid it is not thrown down at all; or if a stronger current is
used, say that from three Bunsen cells, it will be precipitated in an
arborescent brittle form, ill adapted for weighing. It may be noted here
that increasing the size of the cells does not necessarily increase the
intensity of the current.
In two determinations on pure electrotype copper the following results
were obtained:--
Copper Taken. Copper Found.
0.8988 gram 0.8985 gram
0.8305 " 0.8303 "
The presence of salts of ammonia, &c., somewhat retards the deposition,
but has no other ill effect.
The organic matter generally present in copper ores interferes more
especially in the colorimetric determination of the residual copper. It
can be detected on dissolving the ore as a light black residue insoluble
in nitric acid. It is filtered off at once, or, if only present in small
amount, it is carried on in the ordinary process of the assay and
separated in the last filtration before electrolysis.
The following experiments were made to test the effect of the presence
of salts of foreign metals in the solution during the precipitation of
copper by electrolysis:--
--------------+----------------------------------------+---------------
Copper Taken. | Other Metal Added. | Copper Found.
--------------+----------------------------------------+---------------
0.1000 gram | 0.1000 gram of silver | 0.1800
0.1050 " | 0.1000 " " | 0.2000
0.1030 " | 0.1000 " mercury | 0.2010
0.1037 " | 0.1000 " " | 0.2015
0.1020 " | 0.1000 " lead | 0.1020
0.1030 " | 0.1000 " " | 0.1028
0.1010 " | 0.1000 " arsenic | 0.1010
0.1007 " | 0.1000 " " | 0.1022
0.1030 " | 0.1000 " antimony | 0.1050
0.1034 " | 0.1000 " " | 0.1057
0.0990 " | 0.1200 " tin | 0.0990
0.1014 " | 0.1000 " " | 0.1015
0.1000 " | 0.1000 " bismuth | 0.1662
0.1040 " | 0.1000 " cadmium | 0.1052
0.1009 " | 0.1300 " zinc | 0.1017
0.1014 " | 0.1000 " nickel | 0.1007
0.1079 " | 0.1200 " iron | 0.1089
0.1054 " | 0.1000 " chromium (Cr_{2}O_{3}) | 0.1035
0.1034 " | 0.1000 " " (K_{2}CrO_{4}) | 0.1010
0.1075 " | 0.1000 " aluminium | 0.1078
0.1010 " | 0.1000 " manganese | 0.0980
--------------+----------------------------------------+---------------
It will be seen from these that mercury, silver, and bismuth are the
only metals which are precipitable[52] along with the copper under the
conditions of the assay. Mercury, which if present would interfere, is
separated because of the insolubility of its sulphide in nitric acid.
Bismuth is precipitated only after the main portion of the copper is
thrown down. It renders the copper obviously unsuitable for weighing. It
darkens, or forms a greyish coating on, the copper; and this darkening
is a delicate test for bismuth. In assaying ores containing about three
and a half per cent. of copper, and known to contain bismuth in
quantities scarcely detectable in ordinary analysis, the metal deposited
was distinctly greyish in colour, and would not be mistaken for pure
copper. Ten grams of this impure copper were collected and analysed,
with the following results:--
Copper 99.46 per cent.
Bismuth 00.30 "
Iron 00.14 "
Arsenic 00.10 "
------
100.00
The quantity of copper got in each assay was 0.175 gram, and
consequently the bismuth averaged 0.00053 gram.
To separate the bismuth in such a case the deposit is dissolved off by
warming it in the original solution. The bismuth is precipitated by the
addition of ammonic carbonate, and the solution, after filtering and
acidifying with nitric acid, is re-electrolysed.
~Determination of Copper in Commercial Copper.~--Take from 1 to 1.5
gram, weigh carefully, and transfer to a beaker; add 20 c.c. of water
and 10 c.c. of nitric acid; cover with a clock glass, and allow to
dissolve with moderate action; boil off nitrous fumes, dilute to 100
c.c., and electrolyse. The cylinder must be carefully weighed, and the
electrolysis allowed to proceed for 24 hours. The weight found will be
that of the copper and silver. The silver in it must be determined[53]
and deducted.
~Determination of Copper in Brass, German Silver, or Bronze.~--Treat in
the same manner as commercial copper. If nickel is present, the few
milligrams of copper remaining in the electrolysed solution should be
separated with sulphuretted hydrogen, the precipitated sulphide
dissolved in nitric acid, and determined colorimetrically.
VOLUMETRIC PROCESSES.
There are two of these in use, one based on the decolorising effect of
potassic cyanide upon an ammoniacal copper solution, and the other upon
the measurement of the quantity of iodine liberated from potassic iodide
by the copper salt. The cyanide process is the more generally used, and
when carefully worked, "on certain understood and orthodox conditions,"
yields good results; but probably there is no method of assaying where a
slight deviation from these conditions so surely leads to error. An
operator has no difficulty in getting concordant results with duplicate
assays; yet different assayers, working, without bias, on the same
material, get results uniformly higher or lower; a difference evidently
due to variations in the mode of working. Where a large number of
results are wanted quickly it is a very convenient method. The iodide
process is very satisfactory when worked under the proper conditions.
CYANIDE METHOD.
The process is based upon the facts--(1) that when ammonia is added in
excess to a solution containing cupric salts, ammoniacal copper
compounds are formed which give to the solution a deep blue colour; and
(2) that when potassic cyanide is added in sufficient quantity to such a
solution the colour is removed, double cyanides of copper and potassium
or ammonium being formed.[54] In the explanation generally given the
formation of cuprous cyanide is supposed[55]; but in practice it is
found that one part of copper requires rather more than four parts of
cyanide, which agrees with the former, rather than the latter,
explanation.
Reliance on the accuracy of the process cannot rest upon the
supposition that the cyanide required for decoloration is proportional
to the copper present, for varying quantities of ammonia salts, ammonia
and water, and differences of temperature have an important effect. The
results are concordant and exact only when the cyanide is standardised
under the same conditions as it is used. It is best to have the assay
solution and that used for standardising as nearly as possible alike,
and to titrate the two solutions side by side. This demands an
approximate knowledge of the quantity of copper contained in the ore and
a separation of the bulk of the impurities.
For the titration there is required a standard solution of potassium
cyanide made by dissolving 42 grams of the salt, known to dealers as
Potassium Cyanide (Gold), in water and diluting to one litre: 100 c.c.
of this will be about equivalent to one gram of copper. For poor ores
the solution may conveniently be made half this strength.
The solution of the ore and the separation of the copper as sulphide are
effected in the same ways as have been already described for
electrolysis. Similarly, too, the sulphide is attacked with 15 c.c. of
nitric acid and the assay boiled down to 10 c.c. Add 20 c.c. of water
and warm, filter into a pint flask, wash well with water, and dilute to
about 150 c.c.; add 30 c.c. of dilute ammonia, and cool.
Prepare a standard by dissolving a quantity of electrotype copper
(judged to be about the same as that contained in the assay) in 20 c.c.
of water and 10 c.c. of nitric acid, boil off the nitrous fumes, and
dilute to 150 c.c.: add 30 c.c. of dilute ammonia and cool.
Fill a burette with the standard cyanide solution. The burette with
syphon arrangement, figured on page 52, is used. A number of titrations
can be carried on at the same time provided the quantity of copper
present in each is about the same. This is regulated in weighing up the
ore. The flasks must of course be marked, and should be arranged in
series on a bench in front of a good light and at such a height that the
liquid can be looked through without stooping. Supposing about 50 c.c.
of cyanide will be required, 30 c.c. should be run into each, and each
addition be recorded as soon as made; then run 15 c.c. into each. The
solutions will now probably show marked differences of tint: add 1 c.c.
of cyanide to the lighter ones and more to the darker, so as to bring
the colours to about the same depth of tint. They should all be of
nearly equal tint just before finishing. At the end add half a c.c. at a
time until the colours are completely discharged. A piece of damp filter
paper held between the light and the flask assists in judging the
colour when nearly finished. Overdone assays show a straw yellow colour
which deepens on standing.
The following will illustrate the notes recorded of five such assays and
one standard:--
(1) 30 c.c. 15 c.c. 5 c.c. 2 c.c. 1 c.c. 1/2 c.c. -- c.c. = 53-1/2 c.c.
(2) 30 " 15 " 1 " 1 " 1 " 1/2 " -- " = 48-1/2 "
(3) 30 " 15 " 3 " 1 " 1 " 1/2 " -- " = 50-1/2 "
(4) 30 " 15 " 5 " 2 " 1 " 1/2 " 1/2 " = 54 "
(5) 30 " 15 " 2 " 1 " 1 " 1/2 " -- " = 49-1/2 "
(6) 30 " 15 " 2 " 1 " 1 " 1/2 " 1/2 " = 50 standard
Three grams of ore were taken, and the standard contained 0.480 gram of
copper.
In this series the difference of half a c.c. means about 0.15 per cent.
on the ore; with a little practice it is easy to estimate whether the
whole or half of the last addition should be counted.
To get satisfactory results, the manner of finishing once adopted must
be adhered to.
The following experiments show the effect of variation in the conditions
of the assay:--Use _a solution of copper nitrate_, made by dissolving 10
grams of copper in 50 c.c. of water and 35 c.c. of nitric acid, and
diluting to a litre. 100 c.c. = 1 gram of copper.
~Effect of Varying Temperature.~--In these experiments 20 c.c. of copper
nitrate were used, with 10 c.c. of nitric acid, 30 c.c. of dilute
ammonia, and water to 200 c.c. The results were--
Temperature 15° 30° 70° 100°
Cyanide required 21.5 c.c. 20.8 c.c. 19.7 c.c. 18.8 c.c.
The temperature is that of the solution _before_ titrating. These show
the importance of always cooling before titrating, and of titrating the
assay and standard at the same temperature.
~Effect of Varying Bulk.~--The quantities of copper, acid, and ammonia
were the same as in the last-mentioned experiments. The results were:--
Bulk 100.0 c.c. 200.0 c.c. 300.0 c.c. 400.0 c.c.
Cyanide required 23.3 " 21.7 " 21.4 " 21.4 "
These show that large variations in bulk must be avoided.
~Effect of Varying Ammonia.~--The quantities of copper and acid were the
same as in the series of experiments last noticed. The bulk was 200 c.c.
The results were:--
Dilute ammonia 20.0 c.c. 30.0 c.c. 50.0 c.c. 100.0 c.c.
Cyanide required 20.9 " 21.7 " 22.3 " 24.6 "
~Effect of Varying Acid.~--The quantities of copper and water were the
same as in the last-noticed set of experiments: 30 c.c. of dilute
ammonia were used.
Nitric acid 5.0 c.c. 10.0 c.c. 15.0 c.c.
Cyanide required 21.6 " 21.7 " 21.5 "
On adding nitric acid to the solution it combines with a portion of the
ammonia to form ammonic nitrate; it will be seen from the last series of
experiments that the lessening of the amount of free ammonia will
decrease the quantity of cyanide required; but, on the other hand, the
ammonic nitrate which is at the same time formed will increase the
amount required; under the conditions of the assay these two effects
neutralise each other, and such differences in the quantity of acid as
are likely to occur are unimportant.
~Effect of Varying Ammonic Salts.~--The quantities of copper, water, and
ammonia were the same as in the last mentioned set of experiments, but
no nitric acid was used.
Ammonic nitrate added 1 gram 5 grams 10 grams 20 grams
Cyanide required 21.2 c.c. 22.1 c.c. 23.1 c.c. 24.1 c.c.
These show that combined ammonia seriously affects the titration, and
that the principle sometimes recommended of neutralising the acid with
ammonia, and then adding a constant quantity of ammonia, is not a good
one, because there is then an interference both by the ammonia and by
the variable quantity of ammonic salts.
The same quantity of combined ammonia has the same effect, whether it is
present as sulphate, nitrate, chloride, or acetate, as the following
experiments show. Four lots of 20 c.c. of "copper nitrate" were taken,
and 20 c.c. of dilute ammonia added to each. These were carefully
neutralised with the respective acids, rendered alkaline with 30 c.c.
more of ammonia, cooled, diluted to bulk, and titrated. The results
were:--
With sulphuric acid 22.5 c.c. of cyanide
" nitric acid 22.6 " "
" hydrochloric acid 22.6 " "
" acetic acid 22.5 " "
~Effect of Foreign Salts.~--Sulphates, nitrates and chlorides of sodium
or potassium have no action, whilst the hydrates, carbonates,
bicarbonates, sulphites, and nitrites have an important effect. The
interference of ammonic salts has already been shown.
Salts of silver, zinc, and nickel react with cyanide just as copper
does, and consequently interfere. Ferrous salts are sure to be absent,
and ferric salts yield ferric hydrate with the ammonia, which is not
acted on by the cyanide, but, owing to its bulkiness, it settles slowly;
this lengthens the time required for titration, and so modifies the
manner of working. _An assay should not be worked with ferric hydrate
present, unless the standard contains about the same amount of it._ On
mines it is often inconvenient to separate the copper by means of
sulphuretted hydrogen; hence it is customary to titrate without
previous separation. In this case, instead of standardising the cyanide
with electrotype copper, a standard ore should be used. This should be
an ore (of the same kind as those being assayed) in which the copper has
been carefully determined.
~Effect of Varying Copper.~--In these experiments 10 c.c. of nitric
acid, 30 c.c. of ammonia, and water to 200 c.c. were used.
Copper nitrate present 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.
Cyanide required 0.7 " 11.2 " 21.7 " 54.5 " 108.1 "
These results show that under the conditions laid down the various
causes of disturbance nearly neutralise one another, and the results
within a fair range are practically proportional.
~Determination of Copper in Copper Pyrites.~--Weigh up 2 grams of the
dried and powdered ore, and place in an evaporating dish about four
inches in diameter. Cover with 20 c.c. of nitric acid and put on a hot
plate. Evaporate to dryness without further handling. Allow to cool and
take up with 30 c.c. of hydrochloric acid, boil, dilute, and transfer to
a pint flask, filtering if necessary. Make up the bulk with the washings
to about 150 c.c. Precipitate with sulphuretted hydrogen, filter, and
wash back the precipitate into the flask. Add 15 c.c. of nitric acid,
and boil down rapidly to 10 c.c. Dilute, add 30 c.c. of dilute ammonia,
make up to 150 c.c., and cool. For the standard, weigh up 0.5 gram of
copper, more or less, according to the quantity judged to be present in
the assay. Dissolve in 20 c.c. of dilute nitric acid, boil off nitrous
fumes, add 30 c.c. of dilute ammonia, make up to the same bulk as that
of the assay, and cool. Titrate the two solutions side by side and as
nearly as possible in the same manner.
Since the assay solution is often turbid from the presence of small
quantities of lead and of iron from incomplete washing, and since this
slight precipitate is very slow in settling, the standard can hardly be
compared strictly with the assay. This can be counteracted by
precipitating in both solutions a mixture of ferric and aluminic
hydrates, which settles readily and leaves the supernatant liquor clear.
To effect this, boil the nitric acid solutions with 30 c.c. of a
solution containing 15 grams each of alum and ferrous sulphate to the
litre. In an actual determination 2 grams of the ore were taken and
compared with 0.5 gram of copper. The assay required 57.7 c.c. of
cyanide and the standard 52.5 c.c.
52.5 : 0.5 :: 57.7 : 0.5495
This on 2 grams of ore = 27.47 per cent.; the same sample by
electrolysis gave 27.60 per cent. of copper.
~Determination without Previous Separation.~--Dissolve up 2 grams as
before, but, instead of passing sulphuretted hydrogen, add 30 c.c. of
dilute ammonia, shake well, and cool. Prepare a standard by dissolving
0.5 gram of copper in 1 c.c. of nitric acid, add 0.6 gram of iron in the
form of ferric chloride and 20 c.c. of hydrochloric acid, dilute to
about 150 c.c., add 30 c.c. of dilute ammonia, and cool. Titrate the two
solutions side by side. In a determination on the sample last used, 58
c.c. were required for the assay and 53 c.c. for the standard, which
indicates 27.3 per cent. of copper.
This method of working is somewhat rough.
IODIDE METHOD.
This is based upon the fact that when potassic iodide in excess is added
to a strong solution of a cupric salt in a faintly acid solution,
cuprous iodide is formed and an equivalent of iodine liberated.[56] The
iodine is measured by titrating with a solution of sodium
hyposulphite,[57] using starch paste as indicator. The iodine is soluble
in the excess of potassium iodide, forming a deep brown solution; the
hyposulphite is added until this brown colour is almost removed. Starch
paste is then added, and strikes with the remaining iodine a dirty blue
colour. The addition of the "hypo" is continued until the blue colour is
discharged. The end reaction is sharp; a drop is sufficient to complete
it.
As regards the titration, the process leaves little to be desired; the
quantity of "hypo" required is strictly proportional to the copper
present, and ordinary variations in the conditions of working are
without effect. The presence of salts of bismuth masks the end reaction
because of the strong colour imparted to the solution by the iodide of
bismuth. Under certain conditions there is a return of the blue colour
in the assay solution after the finishing point has apparently been
reached, which is a heavy tax on the patience and confidence of the
operator. This is specially apt to occur when sodium acetate is present,
although it may also be due to excessive dilution.
~The standard "hypo" solution~ is made by dissolving 39.18 grams of the
crystallised salt (Na_{2}S_{2}O_{3}.5H_{2}O) in water and diluting to
one litre. One hundred c.c. will equal one gram of copper.
The starch solution is made by mixing 1 gram of starch into a thin paste
with cold water, pouring it into 200 c.c. of boiling water, and
continuing the boiling for a minute or so. The solution must be cold
before use, and about 2 c.c. is used for each assay. It should not be
added until the bulk of the iodine has been reduced.
To standardise the "hypo," weigh up 0.3 or 0.4 gram of pure copper,
dissolve in 5 c.c. of dilute nitric acid, boil off nitrous fumes, and
dilute with an equal bulk of cold water. Add "soda" solution until a
permanent precipitate is obtained, and then 1 c.c. of acetic acid. This
should yield a clear solution. Fill an ordinary burette with the "hypo."
Add 3 grams of potassium iodide crystals to the copper solution, and,
when these are dissolved, dilute to 100 c.c. with water. Run in the
"hypo" solution rather quickly until the brown colour is nearly
discharged--_i.e._, to within 3 or 4 c.c. of the finish. Add 2 c.c. of
the starch solution, and continue the addition of the "hypo" a few drops
at a time until the tint suddenly changes to a cream colour. The blue
colour must not return on standing three or four minutes. Calculate the
standard in the usual way.
In assaying ores, the copper is dissolved and separated with
sulphuretted hydrogen as in the other processes, but the sulphide should
be washed more completely to ensure the absence of iron salts.
The following experiments show the effect of variation in the conditions
of the assay. Use a solution of copper sulphate containing 39.38 grams
of copper sulphate crystals (CuSO_{4}.5H_{2}O) in the litre. 100 c.c.
equal 1.00 gram of copper.
~Effect of Varying Temperature.~--The assay after the addition of the
potassic iodide must be kept cold, else iodine may be volatilised.
~Effect of Varying Potassium Iodide.~--In various descriptions of the
process the amount of iodide required is variously stated at from "a few
crystals" to as much as 10 grams. The proportion required by theory for
1 gram of copper is a little over 5 grams: an excess, however, is
required to keep the liberated iodine in solution. On economic grounds
this excess should not be extravagant; if the student uses 10 parts of
the iodide for each part of copper in the assay he will have sufficient.
In the experiments there were used 20 c.c. of the copper sulphate, with
varying amounts of potassic iodide, and the following results were
got:--
Potassic iodide added 1.5 gram 3 grams 5 grams
"Hypo" required 20.0 c.c. 20.0 c.c. 20.0 c.c.
In these the iodide was added direct to the solution containing the
copper, which was afterwards diluted to 100 c.c. and titrated. In
another series the iodide was added after the dilution to 100 c.c., and
the results were:--
Potassic iodide added 1.5 gram 3 grams 5 grams 10 grams
"Hypo" required 20.0 c.c. 20.1 c.c. 20.0 c.c. 20.0 c.c.
~Effect of Varying Bulk.~--In these experiments, 20 c.c. of copper
sulphate were taken, 3 grams of potassic iodide added, and also water to
the required bulk.
Bulk 20.0 c.c. 100.0 c.c. 200.0 c.c. 500.0 c.c.
"Hypo" required 20.0 " 20.0 " 20.0 " 19.9 "
In the last of these experiments the colour was discharged at 18 c.c.,
but gradually returned until 19.9 c.c. had been run in. It will be seen
that considerable variation in bulk does not interfere.
~Effect of Acetic Acid.~--These experiments were like the last series
mentioned, but the bulk was 100 c.c., and varying amounts of acetic acid
were added.
Acetic acid added 0 c.c. 1.0 c.c. 5.0 c.c. 10.0 c.c. 20.0 c.c.
"Hypo" required 20.0 " 20.1 " 20.1 " 20.0 " 20.2 "
Acetic acid, then, does not interfere to any serious extent.
~Effect of Varying Sodium Acetate.~--These experiments were like those
last mentioned, but without acetic acid, and with varying amounts of
sodium acetate.
Sodium acetate added 0 gram 1 gram 2 grams 5 grams 10 grams
"Hypo" required 20.0 c.c. 20.0 c.c. 20.2 c.c. 19.3 c.c. 18.2 c.c.
In the 5 grams experiment, when the finishing point had been apparently
reached the colour slowly returned; but as the results generally on
titrating were not satisfactory a repetition of the experiment was made
with the addition of 5 c.c. of acetic acid, which gave an equally bad
result.
~Effect of Foreign Salts.~--The conditions of these experiments were the
same as before. The salts were added and dissolved before the addition
of the potassium iodide. Using 5 grams (or in the case of the acids, 5
c.c.), the results were as follows:--
dilute Acetic
Salt added -- H_{2}SO_{4} acid NaAc NaCl
"Hypo" required 20.0 c.c. 20.0 c.c. 20.1 c.c. 19.3 c.c. 20.1 c.c.
Salt added KNO_{3} Na_{2}SO_{4} AmCl Am_{2}SO_{4}
"Hypo" required 20.2 c.c. 18.7 c.c. 20.0 c.c. 19.9 c.c.
The low result with the sulphate of soda was evidently due to the
formation of a sparingly soluble double salt, which removed copper from
the solution; on adding a little acetic acid the full amount of "hypo"
was required. The effect of the presence of certain metals is important.
The method of determining it was to add the substance to the solution
containing the copper, and partly precipitate with soda solution; then
treating with 1 c.c. of acetic acid, adding the iodide, and proceeding
as before.
Substance Added. "Hypo" Required,
- 20. c.c
0.050 gram arsenic as As_{2}O_{5} 20.0 "
0.050 " antimony as SbCl_{5} 19.8 "
0.050 " lead as Pb(NO_{3})_{2} 20.1 "
A similar experiment with 0.050 gram of bismuth nitrate could not be
determined, the end-reaction being masked. Bismuth iodide is soluble in
potassic iodide, forming a brown solution, the colour of which is very
similar to that produced by iodine; and although it does not strike a
blue colour with starch, "hypo" has an action on it.
A similar experiment with 0.050 gram of iron as ferric chloride required
22.3 c.c. of "hypo," and the colour returned on standing. This shows
that ferric acetate liberates iodine under the conditions of the assay.
Trying to counteract this, by adding to a similar solution 0.5 gram of
phosphate of soda dissolved in a little water, 19.7 c.c. of "hypo" were
required instead of 20.0, but the assay showed signs of returning
colour.
In standardising, the same result was obtained, whether the copper was
present as nitrate or sulphate before neutralising.
~Effect of Varying Copper.~--With the same conditions as before, but
with varying amounts of copper and a proportionally increasing quantity
of iodide, the results were:--
Copper present 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.
"Hypo" required 1.0 " 10.0 " 20.0 " 50.0 " 100.0 "
showing the results to be exactly proportional.[58]
~Determination of Copper in Copper Pyrites.~--Take 2 grams of the dried
and powdered ore and treat in a porcelain dish with 20 c.c. of nitric
acid, and evaporate to dryness. Take up with 30 c.c. of hydrochloric
acid, dilute, and transfer to a pint flask; make up with water to 200
c.c., warm, and pass sulphuretted hydrogen to excess. Filter, and wash
the precipitate with water acidified with sulphuric acid. Wash the
precipitate back into the flask, and dissolve with 15 c.c. of nitric
acid. Evaporate almost to dryness; add 20 c.c. of water, and boil till
free from nitrous fumes; filter off the sulphur and gangue; neutralise
with soda, avoiding excess; add 1 or 2 c.c. of acetic acid, and shake
till clear. Add 5 grams of potassium iodide, dilute to 100 c.c., and
titrate. The following is an example:--
0.5 gram of copper required 50.5 c.c. "hypo."
The assay required 55.6 " "
which is equal to 27.5 per cent. of copper.
COLORIMETRIC PROCESS.
This is based on the blue coloration of ammoniacal copper solutions. The
quantity of copper in 100 c.c. of the assay solution should not be more
than 15 milligrams, or less than half a milligram. It is not so delicate
as most other colorimetric methods, but nevertheless is a very useful
one.
The manner of working is the same as that described under iron.
~Standard Copper Solution.~--Weigh up 0.5 gram of electrotype copper,
dissolve in 10 c.c. of nitric acid, boil off nitrous fumes, and dilute
to 1 litre. 1 c.c. = 0.5 milligram.
In nearly all cases it will be necessary to separate the copper with
sulphuretted hydrogen from a solution of about 5 grams of the material
to be assayed. The filter paper containing the sulphide (and, probably,
much sulphur) is dried and burnt. The ashes are dissolved in 5 c.c. of
dilute nitric acid, 10 c.c. of dilute ammonia added, and the solution
filtered through a coarse filter into a Nessler tube, washing the paper
with a little dilute ammonia.
The estimation of the colour and calculation of the result are made in
the way described on page 44.
The effect of varying conditions on the assay may be seen from the
following experiments.
~Effect of Varying Temperature.~--The effect of increased temperature is
to slightly decrease the colour, but this can only be observed when a
fair quantity of copper is present.
1.0 c.c. at 15° showed the colour of 1.0 c.c. at 70°
2.5 " " " " 2.7 "
5.0 " " " " 5.0 "
10.0 " " " " 9.0 "
~Effect of Varying Ammonia.~--The solution must, of course, contain free
ammonia; about 5 c.c. of dilute ammonia in 50 c.c. bulk is the quantity
to be used in the experiments. A larger quantity affects the results,
giving lower readings and altering the tint. With small quantities of
ammonia the colour approaches a violet; with larger, a sky-blue.
2.5 c.c. with 25 c.c. of strong ammonia read 2.2 c.c.
5.0 " " " " " 4.0 "
10.0 " " " " " 8.0 "
~Effect of Ammonic Salts.~--The following table shows the results after
addition of ammonic salts:--
--------------+-----------------+------------------+------------------
C.c. Present.| With 10 grams | With 10 grams | With 10 grams
| Ammonic Nitrate.| Ammonic Chloride.| Ammonic Sulphate.
--------------+-----------------+------------------+------------------
2.5 | 2.5 | 2.5 | 2.0
5.0 | 5.0 | 5.3 | 4.3
10.0 | 10.0 | 10.0 | 8.5
--------------+-----------------+------------------+------------------
These show that sulphates should be avoided, and either nitrate or
chloride solutions be used in the standard as well as in the assay.
~Determination of Copper in a Manganese Ore.~--Treat 3 grams of the ore
with 20 c.c. of hydrochloric acid, and evaporate to dryness. Take up
with 10 c.c. of hydrochloric acid; dilute to about 200 c.c., and pass
sulphuretted hydrogen until the solution smells of the gas; filter,
burn, take up with 5 c.c. of dilute nitric acid, add 10 c.c. of dilute
ammonia, and filter into the Nessler tube, and make up with the washings
to 50 c.c. Into the "standard" tube put 5 c.c. of dilute nitric acid and
10 c.c. of dilute ammonia. Make up to nearly 50 c.c. with water, and run
in the standard copper until the colours are equal. In a determination 4
c.c. (= 2.0 milligrams of copper) were required; this in 3 grams of ore
= 0.07 per cent.
~Determination of Copper in "Black Tin."~--Weigh up 3 grams of the dried
ore, boil with 10 c.c. of hydrochloric acid, and afterwards add 1 c.c.
of nitric; boil off nitrous fumes, evaporate to about 5 c.c., dilute to
50 c.c., add 20 c.c. of dilute ammonia; stir, and filter. If much iron
is present, dissolve the precipitate of ferric hydrate in acid, and
reprecipitate with ammonia. Mix the two filtrates, and dilute to 100
c.c. Take 50 c.c. for the test. A sample of 3 grams of an ore treated in
this way required 5.2 c.c. of standard copper to produce equality of
tint. This gives 0.35 per cent.
~Determination of Copper in Tin.~--Weigh up 1 gram of the sample,
transfer to an evaporating dish, and cover with 30 c.c. of aqua regia.
Warm until the metal has dissolved, then evaporate almost to dryness.
Take up with a few c.c. of hydrochloric acid and again evaporate.
Dissolve the residue in 10 c.c. of dilute hydrochloric acid and transfer
to a 100 c.c. flask. Add 10 c.c. of dilute ammonia and make up with
water to the containing mark.
Filter off 50 c.c. of the solution into a Nessler glass and determine
the copper in it colorimetrically.
EXAMINATION OF COMMERCIAL COPPER.
Very pure copper can be obtained in commerce, owing to the demand for
metal of "high conductivity" for electrical purposes, which practically
means for metal free from impurities.
Much of the metal sold contains as much as one per cent. of foreign
substances, of which arsenic is the most important. The other elements
to be looked for are bismuth, lead, antimony, silver, gold, iron,
nickel, cobalt, sulphur, and oxygen. In "blister copper" (which is the
unrefined metal), aluminium, silicon, and phosphorus may be met with.
~Oxygen.~--All commercial copper carries oxygen; most of it is present
as cuprous oxide, which is dissolved by molten copper. The estimation of
oxygen is often made "by difference." The copper and the other
impurities being determined, the rest is assumed to be oxygen. Probably
this is nearly correct, but the whole of the oxygen should not be
ascribed to cuprous oxide; for any arsenic the metal contained would be
present as cuprous arsenite, since arsenide of copper and cuprous oxide
could not exist together at the temperature of fusion without
interacting. In the report of the analysis, it is best to state the
proportion of oxygen thus:--
Oxygen ---- per cent. by difference.
There is a method of determination by fusing 5 or 10 grams in a brasqued
crucible, and counting the loss as oxygen; and another method for the
determination of cuprous oxide based on the reaction of this substance
with nitrate of silver.[59] About 2 grams of silver nitrate, dissolved
in 100 c.c. of water, is allowed to act upon 1 gram of the copper in the
cold. The precipitate is filtered off, washed thoroughly with water, and
the basic salt dissolved and determined colorimetrically.
One part of copper found represents 1.68 part of cuprous oxide, or 0.19
part of oxygen. Copper generally carries from 0.1 to 0.2 per cent. of
oxygen.
~Silver~ is found in most samples, but occurs in variable proportions;
when it amounts to 30 ounces per ton it has a commercial value. To
determine its amount, dissolve 10 grams of the copper in 35 c.c. of
nitric acid and 50 c.c. of water, boil off nitrous fumes, and dilute to
about 100 c.c. One or two c.c. of dilute hydrochloric acid (one to 100
of water) are added, stirred in, and the precipitate allowed to settle
for twenty-four hours. Filter through a double Swedish paper, dry, burn,
and cupel the ashes with one gram of sheet lead.
Ten grams of a sample of copper gave in this way 4.7 milligrams of
silver. Ten grams of the same copper, to which 24 milligrams of silver
had been added gave 28.2 milligrams.
~Gold.~--To determine it, dissolve 10, 20, or 50 grams of the sample in
35, 70, or 175 c.c. of nitric acid and an equal volume of water, boil
till free from nitrous fumes, and dilute to double its volume. Allow to
stand for some time, decant on to a filter, dry, burn, and cupel the
ashes with 1 gram of sheet lead. If silver is present, owing to traces
of chlorides in the re-agents used, "parting" will be necessary. (See
_Gold_.)
Working in this way on 20 grams of copper, to which 1.8 milligram of
gold had been added, a button weighing 2.0 milligrams was obtained.
~Antimony~ is not a frequent impurity of copper: it can be detected in
quantities over 0.1 per cent. by a white residue of Sb_{2}O_{4},
insoluble in nitric acid. With material containing only small quantities
of antimony the white oxide does not show itself for some time, but on
long-continued boiling it separates as a fine powder. It is best (when
looking for it) to evaporate the nitric acid solution to the
crystallising point, to add a little fresh nitric acid and water, and
then to filter off the precipitate. After weighing it should be examined
for arsenic and bismuth.
~Lead.~--Refined coppers are often free from lead, anything more than
traces being seldom found; in coarse coppers it is sometimes present in
considerable quantities.
Its presence may be detected in the estimation of the copper
electrolytically, the platinum spiral becoming coated with a brown or
black deposit of lead dioxide. The depth of colour varies with the lead
present, and obviously could be made the basis of an approximate
estimation. The colour shows itself within an hour or so, but is best
observed when all the copper has been deposited.
Electrolysing a solution of one gram of pure copper, to which 0.5
milligram of lead had been added, the deposit was dark brown; in a
similar solution with 1 milligram of lead it was much darker, and with 2
milligrams it was black. Under the conditions of the assay the dioxide
cannot be weighed, as it partly dissolves on breaking the current. When
lead has been found, its quantity may be estimated by evaporating to
dryness the nitric acid solution to which an excess of sulphuric acid
has been added, taking up with water, and filtering off and weighing the
lead sulphate.
The separation of traces of lead as chromate is a fairly good one.
Dissolve 5 grams of the copper in 17 c.c. of nitric acid and an equal
volume of water; boil off nitrous fumes, neutralise with soda, and
afterwards acidulate with acetic acid; and dilute to a litre. Add 20
grams of sodium acetate, warm, and precipitate the lead with a dilute
solution of potassium chromate. Copper chromate (yellow) may be at the
same time thrown down, but it is readily soluble on diluting. Filter off
the precipitate; wash it into a beaker and pass sulphuretted hydrogen;
oxidise the sulphide and weigh as lead sulphate. Treated in this way 5
grams of copper yielded sulphate of lead equal to 2.0 milligrams of
lead. Five grams of the same sample to which 10 milligrams of lead were
added gave 11.4 milligrams.
~Nickel and Cobalt.~--Nickel is always present in larger or smaller
quantities in commercial copper, and, perhaps, has an influence on the
properties of the metal. It is determined as follows:--Dissolve 10 grams
of the copper in 35 c.c. of nitric acid and an equal bulk of water, boil
off nitrous fumes and neutralise with soda, add 2 grams of carbonate of
soda dissolved in water, boil, and filter. Acidify the filtrate with 2
or 3 c.c. of dilute nitric acid and dilute to 1 or 1-1/2 litres. Pass
sulphuretted hydrogen through the cold solution till the copper is all
down and the liquid smells of the gas. Filter and evaporate the filtrate
to a small bulk, and determine the nickel by electrolysing the solution
rendered ammoniacal, or by precipitating as sulphide and weighing as
sulphate. (See under _Nickel_.) The precipitate, after weighing, should
be tested for cobalt. If present it is separated with potassium nitrite
as described under _Cobalt_. Ten grams of copper gave 6.0 milligrams of
nickel; and another lot of 10 grams of the same copper, to which 10.0
milligrams of nickel had been added, gave 17.2 milligrams.
~Sulphur.~--The amount of sulphur in refined copper is very small,
seldom exceeding 0.005 per cent. In coarse copper, as might be expected,
it is found in larger quantities.
In determining it, it is first converted into sulphuric acid, and then
precipitated and weighed as barium sulphate. The precipitation cannot be
effected from a nitric acid solution. Ten grams of copper are dissolved
in nitric acid, as for the other determinations, and then boiled with
excess of hydrochloric acid till the nitric acid is completely removed.
There is then added a few drops of a dilute solution of baric chloride,
and the solution is allowed to stand for some hours. The baric sulphate
is filtered off and weighed.
The necessity for precipitating from a hydrochloric acid solution is
seen from the following determinations. In each experiment 10 grams of
copper was used, and a known weight of sulphur, in the form of copper
sulphate, added.
--------------+---------------------------+---------------------
Sulphur added.| Sulphur found in | Sulphur found in
|Hydrochloric Acid Solution.|Nitric Acid Solution.
--------------+---------------------------+---------------------
5 milligrams | 8 milligrams | 0.03 milligrams
10 " | 11 " | 0.03 "
15 " | 17 " | 12.00 "
--------------+---------------------------+---------------------
~Bismuth.~--Nearly all samples of copper contain bismuth, but only in
small quantities. It is best determined colorimetrically as described
under _Bismuth_. The method of concentrating and preparing the solution
for colorimetric assay is as follows. Dissolve 10 grams of copper in
nitric acid, as in the other determinations; neutralise with soda; add 1
or 1.5 grams of bicarbonate of soda and boil for ten minutes; filter,
dissolve the precipitate in hot dilute sulphuric acid; add sulphurous
acid and potassium iodide in excess, and boil till free from iodine.
Filter and dilute to 500 c.c. Take 50 c.c. of the yellow solution for
the determination. A few c.c. of a dilute solution of sulphurous acid (1
in 100) will prevent the liberation of iodine. The following experiments
test the method of separation. Ten grams of copper were treated as above
and precipitated with 1.5 gram of "soda;" the precipitate contained 0.6
milligram of bismuth (= 0.006 per cent.). The filtrate treated with
another 1.5 gram of "soda" gave a precipitate which was free from
bismuth. To the filtrate from this was added 1.0 milligram of bismuth,
and another fraction was precipitated with 1.5 gram of "soda." In this
precipitate was found 1.0 milligram of bismuth. To the filtrate another
milligram of bismuth was added and the separation with "soda" repeated.
The bismuth was separated from this precipitate with ammonic carbonate
before determination, and 0.9 milligram was found.
~Arsenic.~--The proportion of arsenic in copper varies from 0.01 to 0.75
per cent. whilst in coarse copper it may amount to 2 or even 3 per cent.
To determine it, dissolve 5, 10, or 20 grams of the copper (according to
the amount of arsenic present) in 18 c.c., 35 c.c., or 70 c.c. of nitric
acid, and an equal volume of water. Boil off the nitrous fumes, dilute
to 100 c.c. and neutralise with soda; add 1.5 or 2 grams of carbonate of
soda dissolved in a little water, and boil. Filter (washing is
unnecessary) and dissolve back into the flask with a little dilute
hydrochloric acid; add 30 c.c. of dilute ammonia and 25 c.c. of
"magnesia mixture," and allow to stand overnight. The whole of the
arsenic is precipitated as ammonic-magnesic arsenate in one hour, but it
is advisable to leave it longer. The precipitate may be dried and
weighed, or, better, titrated with uranium acetate. (See _Arsenic_.) To
test this method of separation 10 grams of pure copper were taken and
0.200 gram of arsenic dissolved with it. The arsenic was determined by
titration with uranium acetate, and 0.200 gram was found. Two other
similar experiments with 0.080 and 0.010 gram of arsenic added, gave
0.079 and 0.012 gram respectively.
Antimony or bismuth may be present without interfering with the
titration. With 0.100 gram of antimony and 0.100 gram of arsenic, 0.100
gram of arsenic was found; and in another case, with 0.100 gram of
bismuth and 0.060 gram of arsenic, 0.060 gram was found. In these
experiments the antimony and bismuth were present in the assay solution
when titrated. For a gravimetric determination they would require to be
removed before precipitating with "magnesia mixture."
Phosphorus, if present, counts as arsenic in the proportion of 1 to 2.4;
but, except in the case of coarse coppers, it is always absent.
Iron, if present, interferes by forming a white flocculent precipitate
of ferric arsenate after the addition of the sodium acetate and
preliminary to the titration. Each milligram of iron abstracts, in this
way, 1.3 milligrams of arsenic.
~Iron.~--Refined coppers carry traces of iron, varying from 0.001 to
0.01 per cent. It is best determined during the arsenic estimation. The
precipitate of the ammonic-magnesic arsenate will contain the whole of
the iron as ferric hydrate. On dissolving in hydrochloric acid,
neutralising with ammonia, adding 5 c.c. of sodic acetate, diluting, and
boiling, it reappears as a white precipitate of ferric arsenate. It is
filtered off (the arsenic being estimated in the filtrate), dissolved in
warm hydrochloric acid, and determined colorimetrically as described
under _Iron_. A series of experiments testing the separation is there
given.
~Phosphorus.~--Refined coppers do not carry phosphorus, although it may
be present in "coarse copper" up to 1 per cent. or more. In such samples
the following method is adopted for the estimation of both phosphorus
and arsenic. Dissolve 10 grams of copper and 0.1, 0.2, or 0.3 gram of
iron wire (according to the amount of arsenic and phosphorus present) in
35 c.c. of nitric acid and an equal volume of water. Add soda till the
free acid is nearly neutralised. Next add a strong solution of sodium
acetate, until the solution ceases to darken on further addition, then
dilute with water to half a litre. The solution is best contained in a
large beaker; it is next heated to the boiling point, and at once
removed and allowed to settle. If the precipitate is light coloured it
is evidence that sufficient iron has not been added, or, if it is green,
from basic copper salts, it shows that the solution was not sufficiently
acid. In either case start afresh. Filter off the precipitate and wash
with hot water containing a little sodium acetate, dissolve it off the
filter with hot dilute hydrochloric acid, add ammonia in excess, and
pass sulphuretted hydrogen for five minutes. Warm at about 70° C. for a
quarter of an hour. Filter. The clear yellow filtrate contains the
arsenic and phosphorus. Add dilute sulphuric acid in excess; filter off
the yellow precipitate of sulphide of arsenic, dissolve it in nitric
acid, and titrate with uranium acetate, as described under _Arsenic_.
The filtrate from the sulphide of arsenic is rendered alkaline with
ammonia and "magnesia mixture" added. The solution is stirred, and
allowed to stand overnight. The precipitate of ammonic-magnesic
phosphate is filtered off, dissolved, and titrated with uranium acetate,
using the same standard solution as is used in the arsenic assay: 0.5
gram of arsenic equals 0.207 gram of phosphorus.
~Copper.~--The method of determining this has been described under
_Electrolytic Assay_.
In the method of concentration by fractional precipitation with sodic
carbonate (which is adopted in most of these determinations) the
precipitate will contain all the bismuth, iron, and alumina; the arsenic
and phosphorus as cupric arsenate and phosphate; and the greater part of
the lead, antimony, and silver. The nickel and cobalt, and the sulphur
as sulphuric acid, will remain in solution with the greater part of the
copper.
PRACTICAL EXERCISES.
1. According to a wet assay 2 grams of a certain ore contained 0.3650
gram of copper. What would you expect the dry assay produce to be?
2. A standard solution is made by dissolving 25 grams of potassic
cyanide and diluting to a litre. Assuming the salt to be 98 per cent.
real cyanide, what would 100 c.c. of the solution be equivalent to in
grams of copper?
3. How would you make a solution of "hypo" of such strength that 100
c.c. shall equal 0.633 gram of copper?
4. What weight of ore, containing 17.0 per cent. of copper, would you
take in order to get about 0.5 gram of copper in solution for
electrolysis?
5. The solution of copper in nitric acid is effected by the following
reaction:--
3Cu + 8HNO_{3} = 3Cu(NO_{3})_{2} + 4H_{2}O + 2NO.
What volume of nitric acid will be required to dissolve 1 gram of
copper?
LEAD.
The chief ore of lead is galena, a sulphide of lead, common in most
mining districts, and frequently associated with blende and
copper-pyrites. It always carries more or less silver; so that in the
assay of the ore a silver determination is always necessary. Carbonate
(cerussite), sulphate (anglesite), and phosphate (pyromorphite) of lead
also occur as ores, but in much smaller quantities.
Lead ores are easily concentrated (owing to their high specific gravity,
&c.) by mechanical operations, so that the mineral matter sent to the
smelter is comparatively pure.
Lead is readily soluble in dilute nitric acid. The addition of sulphuric
acid to this solution throws down heavy, white, and insoluble lead
sulphate.
Galena is soluble in hot hydrochloric acid, sulphuretted hydrogen being
evolved; but the action is retarded by the separation of the sparingly
soluble lead chloride. If a rod of zinc is placed in this solution,
metallic lead is precipitated on it as a spongy mass, the lead chloride
being decomposed as fast as it is formed. The opening up of the ore is
thus easily effected, the sulphur going off as sulphuretted hydrogen,
and the lead remaining in a form easily soluble in dilute nitric acid.
Galena itself is readily attacked by nitric acid, part of the lead going
into solution, and the rest remaining as insoluble lead sulphate. The
sulphate is due to the oxidation of the sulphur by nitric acid; its
amount will vary with the quantity and concentration of the acid used.
Sulphate of lead is soluble in solutions of ammonium or sodium acetate;
or it may be converted into carbonate by boiling with carbonate of soda.
The carbonate, after washing off the sulphate of soda, dissolves easily
in nitric acid. The precipitation of lead from acid solutions with
sulphuric acid, and the solubility of the precipitate in ammonium
acetate, distinguishes it from all other metals. The addition of
potassium chromate to the acetate solution reprecipitates the lead as a
yellow chromate.
DRY ASSAY.
The dry assay of lead is largely used, but it is only applicable to rich
or concentrated ores, and even with these only gives approximate
results. Both lead and lead sulphide are sensibly volatile at a
moderately-high temperature; hence it is necessary to obtain a slag
which is easily fusible. As a reducing agent iron is almost always used,
and this is added either in the form of an iron rod, or the crucible
itself is made of this metal. The flux used is carbonate of soda.
When a clay crucible is used, the method of working is as
follows:--Weigh up 25 grams of the dry and powdered ore, mix with an
equal weight of "soda" and 2 grams of tartar; place in a crucible (E.
Battersea round), and then insert a piece of iron rod about half an inch
in diameter, and of such a length that it will just allow the crucible
to be covered. The rod should be pushed down so as to touch the bottom
of the crucible, and the mixture should be covered with a sprinkling of
borax. Place in a furnace heated to, but not above, redness, and cover
the crucible. In about twenty minutes the charge will be fused: the
fusion is complete when bubbles of gas are no longer being evolved; and
then, but not till then, the iron is withdrawn, any adhering buttons of
lead being washed off by dipping the rod a few times in the slag. Cover
the crucible, leave it for a minute or two, and then pour. Detach the
slag, when cold, by hammering. The weight of the button multiplied by 4
gives the percentage. The commoner errors of students in working the
process are too high a temperature and too quick a withdrawal.
A sample of ore treated in this manner gave on duplicate assay 17.5 and
17.6 grams of lead, equalling 70.0 and 70.4 per cent. respectively. By
wet assay the sample gave 73.3 per cent. Using an iron crucible, the
results will be 1 per cent. or so higher. The crucible must be made of
wrought iron; and, if it has been previously used, should be cleaned by
heating to dull redness and scraping the scale off with a stirrer. Take
30 grams of the ore, mix with 30 grams of "soda" and 3 grams of tartar;
put the mixture in the crucible, and cover with a sprinkling of borax;
heat for about twenty minutes at not too high a temperature, and then
scrape down the slag adhering to the side with a stirrer. Leave in the
furnace till action has ceased. Before pouring, tap the pot gently, and
then tilt it so as to make the slag wash over the part of the crucible
along which the charge is to be poured. Pour; and, when cold, clean and
weigh the button of metal. A crucible may be used from ten to twenty
times.
These assays are for ores containing the lead chiefly as sulphide. For
oxidised ores, charcoal or tartar is employed as the reducing agent. The
student may practise on red lead as follows:--Take 30 grams of red lead;
mix with 10 grams each of borax and "soda" and about 1.5 gram of
powdered charcoal; place in a small clay crucible with a cover (C.
Battersea round), fuse at a gentle heat, and pour when action ceases.
This assay will only take a few minutes.
Where lead is present as phosphate (as in the case of pyromorphite), or
mixed with phosphates (as sometimes happens), carbonate of soda is a
suitable flux; but the phosphate of soda which is formed makes a thick
tenacious slag, which is very apt to be carried out of the pot by the
escaping gas. A wide-mouthed clay pot is taken and a little fluor spar
added. For the assay of pyromorphite the following charge may be
used:--Ore, 20 grams; "soda," 25 grams; tartar, 7 grams; and fluor spar,
5 grams; and 2 grams of borax as a cover. This will melt down in about
ten minutes, and should be poured as soon as tranquil.
WET ASSAY.
In the case of galena the best method of getting the lead into solution
is to treat with hydrochloric acid and zinc. Put 1 gram of the ore in an
evaporating dish 4 inches across, and cover with 10 c.c. of dilute
hydrochloric acid. Heat till the evolution of sulphuretted hydrogen
becomes sluggish, and then drop in a piece of zinc rod. If the solution
effervesces too strongly, dilute it. Continue the heating until the
sulphide is seen to be all dissolved; when the lead is all precipitated,
pour off the liquid and wash twice with cold water. Peel off the
precipitated lead with the help of a glass rod, and then clean the zinc.
Cover the lead with 20 c.c. of water and 5 c.c. of dilute nitric acid,
and heat gently till dissolved; all the lead will be in solution, and,
when filtered off from the gangue, will be ready for a gravimetric
determination. For volumetric work this filtering is unnecessary.
The chief objection to this method is that commercial zinc carries
considerable quantities of lead. Although this can be determined and
allowed for, the correction required is in most cases too large to be
satisfactory. The following method is applicable in all cases, but is
more troublesome:--Treat 1 gram of the ore with 10 c.c. of dilute nitric
acid in an evaporating dish covered with a clock-glass, and evaporate
till nearly dry. Take up with 50 c.c. of water, and add 10 c.c. of
dilute sulphuric acid. Filter. The residue contains the lead as
sulphate, together with the insoluble matter of the ore and globules of
sulphur. Warm with a solution of ammonium acetate, and filter. The lead
will be in the filtrate, and is recovered in a state fit for direct
gravimetric estimation by the addition of dilute sulphuric acid. If the
volumetric method is to be used, the lead sulphate should be dissolved
out with a solution of sodium acetate instead of with the ammonium salt
solution.
GRAVIMETRIC DETERMINATION.
The lead is separated and precipitated as sulphate, as already
described. The solution must be allowed to stand, and the clear liquid
be decanted through a filter. Transfer the precipitate, and wash with
very dilute sulphuric acid (1 or 2 c.c. in 100 c.c. of water). The acid
must be completely removed with one or two washes with cold water, and
then with alcohol. The volume of liquid required for washing is small,
as the precipitate is dense and easily cleaned; but the washing must be
carefully done, since if any acid remains it will, on drying, char the
paper, and render the subsequent work troublesome. Dry, transfer to a
watch-glass, and burn the filter paper, collecting its ash in a weighed
porcelain crucible. The filter paper must be freed as much as possible
from the lead sulphate before burning, and the ash treated with a drop
or two of nitric and sulphuric acids. Transfer the lead sulphate to the
crucible; ignite gently, keeping the temperature below redness; cool,
and weigh. The precipitate will contain 73.6 per cent. of lead oxide or
68.3 per cent. of lead.
~Determination of Lead in Commercial Zinc.~--Take 10 grams of zinc, and
treat (without heating) with 60 c.c. of dilute hydrochloric acid. When
the zinc is nearly all dissolved, decant off the clear liquid, and
dissolve the residue in 2 c.c. of dilute nitric acid. Evaporate till
most of the acid is removed; dilute to 20 or 30 c.c. with water, and add
10 c.c. of dilute sulphuric acid. Filter off, and weigh the lead
sulphate. Ten grams treated in this way gave--0.1610 gram of lead
sulphate, equivalent to 1.10 per cent. of lead.
VOLUMETRIC METHOD.
This is based upon the reaction between chromate of potash and soluble
lead salts in neutral solutions, whereby an insoluble yellow chromate of
lead is produced.[60] An excess of the chromate is required to complete
the reaction, so that the point at which an indicator shows the presence
of undecomposed chromate cannot be satisfactorily taken as the finish.
Therefore an excess of the standard chromate must be run in, and such
excess determined.
Chromate of lead is not precipitated from strong nitric acid solutions,
and only incompletely from dilute ones. Acids generally are detrimental
to the precipitation, and must be neutralised before titrating. If the
lead is present as sulphate in sodic acetate solution, it is well to
render it distinctly alkaline with ammonia.
Lead chromate precipitated in the cold is a lemon-yellow, light
precipitate, very difficult to filter: on heating to 40° C. the colour
becomes orange; at 60° C. it assumes a deeper hue, and becomes
flocculent; and at a boiling temperature it still further darkens and
settles readily. These changes in colour are not due to any chemical
change, as will be seen by testing the filtrate for chromium or lead:
this is an advantage to the assay, since it is only at the higher
temperature that the precipitate can be easily filtered. The lead is not
completely precipitated, but the amount remaining in solution is only 2
or 3 milligrams, which is just sufficient to give a dark coloration with
sulphuretted hydrogen.
_The standard chromate of potash solution_ is made by dissolving 7.13
grams of bichromate of potash and 2.0 grams of caustic soda in water,
and diluting to 1 litre; or 9.40 grams of the neutral chromate
(K_{2}CrO_{4}) may be dissolved and diluted to 1 litre: 100 c.c. will be
equivalent to 1.000 gram of lead.
_Standard Lead Solution._--16 grams of nitrate of lead (Pb(NO_{3})_{2})
are dissolved in water and diluted to 1 litre; 100 c.c. will contain
1.000 gram of lead.
_Acetate of Soda Solution._--250 grams of the crystallised salt
(NaAc.3H_{2}O) are dissolved, and diluted to 1 litre. Use 40 c.c. for
each assay.
In the titration the assay solution should measure 150 to 200 c.c., and
should be boiling or nearly so. It is best contained in a pint flask,
and the standard chromate solution used with an ordinary burette. Run in
the chromate solution in a steady stream until the whole of the lead has
been precipitated. The amount required for this may be calculated: for
example, 1 gram of an 80 per cent. ore would require 80 c.c. A little of
the assay may be filtered off, and if it does not show a yellow colour
in the filtrate run in 2 c.c. more of the standard solution and continue
this addition till a colour is shown. After this run in another c.c. to
ensure an excess, dilute to 250 c.c., and heat to boiling; allow to
settle for three or four minutes, filter off 50 c.c. into a Nessler
glass, and determine the excess of chromate colorimetrically. The excess
found in the 50 c.c. must, of course, be multiplied by five, and then be
deducted from the quantity of chromate originally run into the assay
solution. The quantity to be deducted should not exceed 3 c.c. Where a
number of determinations are made the colorimetric estimation is
facilitated by using a series of standard phials similar to those
described under the _Electrolytic Copper Assay_. The determination is
rendered sharper and less liable to error by the addition of a few drops
of acetic acid to convert the chromate into bichromate. The same
chromate solution must be used in this determination as was used in the
precipitation.
In standardising the chromate solution, the standard lead nitrate
solution is used. A quantity containing about as much lead as the assay
is supposed to contain is measured off, rendered alkaline with dilute
ammonia, and then neutralised with acetic acid, using a small piece of
litmus paper dropped into the solution as indicator. Then dilute, boil,
and titrate. When the lead in the assay has been separated as sulphate
and dissolved in sodic acetate, less chromate is apparently required,
and in this case it will be necessary to precipitate the lead in the
standard with an equivalent of sodic sulphate and redissolve in sodic
acetate just as in the assay. In these solutions (although there is
considerable chromate in excess) a further addition of 5 or 6 c.c. of
the chromate solution will cause a further precipitate. The following
experiments show the effect of variation in the conditions of the
assay:--
~Effect of Varying Temperature.~--Twenty c.c. of lead nitrate solution
and 10 grams of sodium acetate were used; diluted to 200 c.c., heated to
the desired temperature, and titrated. The results were:--
Temperature 15° 30° 50° 100°
"Chromate" required 19.8 c.c. 19.5 c.c. 19.3 c.c. 19.2 c.c.
The first two of these filtered badly, the precipitate coming through
the filter; the last was very satisfactory in the working.
~Effect of Varying Bulk.~--Using 20 c.c. of lead nitrate, and 10 grams
of sodium acetate as before, diluting to the required bulk, heating to
boiling, and titrating, the results were:--
Bulk 100.0 c.c. 200.0 c.c. 500.0 c.c. 1000.0 c.c.
"Chromate" required 19.6 " 19.3 " 19.4 " 19.4 "
~Effect of Varying Acetic Acid.~--Since the experiments are carried out
in the presence of sodic acetate, acetic acid is the only acid whose
effect need be considered. Working as before, but with 200 c.c. bulk and
varying amounts of the acid, the results were:--
Acid present -- 10.0 c.c. 20.0 c.c. 40.0 c.c.
"Chromate" required 19.7 c.c. 19.1 " 18.5 " 17.3 "
These experiments show that only slight quantities of acid are
admissible.
~Effect of Varying Sodium Acetate.~--With the same conditions as before,
but with varying weights of sodium acetate, the results were:--
Sodium acetate present -- 5 grams 10 grams 25 grams 50 grams
"Chromate" required 19.7 c.c. 19.6 c.c. 19.6 c.c. 18.8 c.c. 17.8 c.c.
These experiments show that excessive quantities of sodium acetate must
be avoided. Ammonium acetate interferes to a greater extent, and if
both acetic acid and this salt are present, each exerts its disturbing
influence. With 10 grams of ammonium acetate, 19.4 c.c. of the chromate
solution were required instead of 19.7 c.c. in the absence of this salt;
with 10 grams of the acetate and 10 c.c. of acetic acid, only 18.6 c.c.
were required.
~Effect of Foreign Salts.~--As already stated, sulphates interfere.
Twenty c.c. of the lead nitrate solution were taken, precipitated with
sulphate of soda, and the precipitate dissolved in 10 grams of sodium
acetate and titrated as before. Duplicate experiments required 18.6 c.c.
and 18.7 c.c. of the chromate solution. A similar experiment with 40
c.c. of lead nitrate required 37.4 c.c. of chromate. If the sulphate had
not been present, the results would have been 19.7 c.c. and 39.4 c.c.
respectively.
~Effect of Varying Lead.~--In these experiments the conditions were as
before, but with varying amounts of lead.
Lead nitrate solution present 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.
Chromate solution required. 9.4 " 19.7 " 48.8 " 98.2 "
~Determination of Lead in Galena.~--Weigh up 1 gram of the powdered and
dried ore, and boil in an evaporating dish with 10 c.c. of dilute
hydrochloric acid. When the action becomes sluggish, dilute with an
equal bulk of water, and add a weighed piece of zinc rod about 1 inch
long and quarter-inch across. Keep up a moderate action by warming till
the ore is seen to be completely attacked and the lead precipitated.
Decant off the solution, wash once, strip off the lead, wash and weigh
the remaining zinc. Dissolve the lead in 5 c.c. of dilute nitric acid,
and 5 c.c. of water with the aid of heat. Dilute and transfer to a pint
flask; add a slight excess of dilute ammonia, and render faintly acid
with acetic acid. Dilute to 150 c.c., heat to boiling, and run in the
standard chromate in slight excess, noting the amount required, and make
up to 250 c.c. with water. Boil the solution, allow to settle for a
minute or so, filter off 50 c.c., and determine the excess of chromate
colorimetrically. As an example, 1 gram of an impure galena was
precipitated with 75 c.c. of standard chromate (100 c.c. = 1.020 gram
lead). The excess found in 50 c.c. was 0.3 c.c., which, multiplied by 5,
gives 1.5 c.c. as the excess in the whole solution. The remaining 73.5
c.c. of "chromate" required by the assay, are equivalent to 0.7497 gram
of lead. The zinc used up weighed 1.5 grams, and contained 0.0165 gram
of lead. Thus we get--
Lead in the assay 0.7497 gram
Lead from the zinc 0.0165 "
------
.'. Lead in the galena 0.7332 "
Equivalent to 73.3 per cent.
Another sample, in which the galena was accompanied with a large
quantity of pyrites, gave the following results:--Three grams were
treated with 30 c.c. of dilute hydrochloric acid and a rod of zinc. The
zinc and lead were carefully transferred to another vessel, the zinc
cleaned, and the lead (dissolved in 5 c.c. of dilute nitric acid and 20
c.c. of water) treated as before.
14.5 c.c. of the chromate were required = 0.1479 gram lead
Lead in 2 grams of zinc = 0.0220 "
------
.'. Lead in 3 grams of the ore = 0.1259 "
Equivalent to 4.20 per cent.
The same ore gave by separation of the lead with sulphuretted hydrogen,
and conversion into sulphate, 4.16 per cent.
With fairly pure ores, free from sulphate, the assay may be made more
quickly as follows: Dissolve 1 gram of the finely powdered ore by
boiling gently with 40 c.c. of dilute hydrochloric acid for 15 minutes;
cool; add a few drops of permanganate; neutralise with ammonia, add
acetic acid and a little sodium acetate. Titrate with standard chromate.
COLORIMETRIC PROCESS.
This is based upon the brown coloration produced in very dilute
solutions of lead by the action of a solution of sulphuretted hydrogen.
The quantity of lead in the 50 c.c. of the assay solution must not much
exceed 0.5 milligram, nor be less than 0.01. The sulphuretted hydrogen
is used in the form of a solution, and is not bubbled through the assay.
The principle of working is the same as previously described.
_Standard Lead Solution._--Each c.c. of this should contain 0.1
milligram of lead. It is made by diluting 10 c.c. of the solution of
lead nitrate, described under the volumetric process, to 1 litre.
_Sulphuretted hydrogen water_ is made by passing a current of the washed
gas into water till the latter is saturated.
Five c.c. of the sulphuretted hydrogen water are put into a Nessler
tube, the measured portion of the assay solution added, and the whole
diluted with water to the 50 c.c. mark. Into the standard Nessler tube
the same amount of the sulphuretted hydrogen water is put, and diluted
to nearly 50 c.c. The standard lead solution is then run in till the
tints are equal. The assay solution must not contain much free acid, and
if the conditions will allow it, may with advantage be rendered alkaline
with ammonia. The chief cause of disturbance is the precipitation of
lead sulphide forming a black turbid solution instead of a brown clear
one. This may be caused by using hot solutions or an excess of acid.
Other metals precipitable by sulphuretted hydrogen must be absent as
well as strong oxidising agents.
~Effect of Varying Temperature.~--The effect of increased temperature is
to change the colour from brown to black, and to render the estimation
difficult.
1 c.c. at 15° C. showed the colour of 0.5 c.c. at 60° C.
2 " " " " 1.5 " at 60° C.
3 " " " " 5.0 " at 50° C.
~Effect of Varying Time.~--The colour becomes lighter on standing: 2
c.c. on standing 10, 20, and 40 minutes became equal in colour to 1.7
c.c.
~Effect of Acids and Ammonia.~--Two c.c. of the solution with 2 c.c. of
dilute hydrochloric acid became cloudy and equivalent to about 2.5 c.c.;
and a similar result was got with 2 c.c. of dilute sulphuric acid. With
2 c.c. of dilute ammonia the solution became somewhat darker, or equal
to 2.3 c.c.; but gave a very clear solution easy to compare.
~Determination of Lead in Commercial Zinc.~--Dissolve 0.1 gram of the
metal in 1 c.c. of dilute nitric acid evaporates till a solid separates
out, dilute to 100 c.c. with water, and take 20 c.c. for assay. A sample
treated in this way required 2.4 c.c.; this multiplied by 5 gives 12.0
c.c., equivalent to 1.2 milligram of lead, or 1.2 per cent. By
gravimetric assay the sample gave 1.10 per cent.
PRACTICAL EXERCISES.
1. Thirty grams of galena gave on dry assay 21 grams of lead; and this,
on cupellation, gave 15 milligrams of silver. Calculate the results in
per cents. of lead and in ounces of silver to the ton of ore.
2. How many ounces of silver to the ton would be contained in the lead
got from this ore if the loss in smelting is equal to that of the assay?
3. Having given you a sample of white lead freed from oil by washing
with ether, how would you proceed to determine the percentage of lead in
it?
THALLIUM.
Thallium is a rare metal, found in small quantities in some varieties of
iron and copper pyrites, and in some lithia micas. It resembles lead in
appearance. Its compounds resemble the salts of the alkalies in some
respects; and, in others, those of the heavy metals.
It is detected by the green colour which its salts impart to the flame.
This, when examined with the spectroscope, shows only one bright green
line.
It is separated and estimated by dissolving in aqua regia; converting
into sulphate by evaporation with sulphuric acid; separating the second
group of metals with sulphuretted hydrogen in the acid solution, boiling
off the excess of the gas; nearly neutralising with carbonate of soda;
and precipitating the thallium with an excess of potassic iodide. On
allowing the liquid to stand for some time a bright yellow precipitate
of thallous iodide separates out. This is collected on a weighed filter;
washed with cold water, finishing off with alcohol; dried at 100° C.,
and weighed. The precipitate is thallous iodide TlI, and contains 61.6
per cent. of thallium.
BISMUTH.
Bismuth is nearly always found in nature in the metallic state; but
occasionally it is met with as sulphide in bismuthine and as carbonate
in bismutite. It is also found in some comparatively rare minerals, such
as tetradymite, combined with tellurium, and associated with gold. In
minute quantities it is widely distributed: it is a common constituent
of most copper ores; hence it finds its way into refined copper, which
is seldom free from it. It is occasionally met with in silver in
sufficient quantity to interfere with the working qualities of that
metal.
Bismuth compounds are used in medicine and in the manufacture of alloys.
Bismuth possesses many useful properties. It has considerable commercial
value, and sells at a high price.
The metal is brittle, breaks with a highly crystalline fracture, and has
a characteristic reddish-yellow colour. It is almost insoluble in
hydrochloric, but readily dissolves in nitric, acid; and gives, if the
acid is in excess, a clear solution. Bismuth salts have a strong
tendency to separate out as insoluble basic compounds; this is more
especially true of the chloride which, on diluting with a large volume
of water, becomes milky; the whole of the bismuth separating out. The
nitrate, carbonate, and hydrate yield the oxide (Bi_{2}O_{3}) on
ignition. This oxide closely resembles litharge. It combines with
silica, forming fluid slags; and at a red heat is liquid enough to be
absorbed by a cupel; in fact, bismuth may take the place of lead in
cupellation. The metal itself is easily fusible, and may be separated
from its ores by liquation.
The assay of bismuth by wet methods presents little difficulty, and is
fairly accurate. The price of the metal is such that only methods which
yield good results should be adopted; and, since bismuth is volatile at
the temperature of the furnace, and is found mixed with ores not easy to
flux, as also with metals which are not easily separated by the dry
method, the dry assay can only be considered as having a qualitative
value.
DRY ASSAY.
~By Liquation.~--This is adapted to ores containing the bismuth as
metal. Take 20 grams of the powdered ore and place in a crucible with a
perforated bottom, put this crucible into another of about the same size
and lute the joint. Lute on a cover, place in the furnace and heat to
redness. The bismuth melts readily and drains into the lower crucible
from which, when cold, it is taken and weighed.
~By Fusion.~--For fairly pure ores the process is as follows:--Take 20
grams of the ore and mix with 20 grams of fusion mixture, 10 grams of
salt and 5 or 10 grams of potassium cyanide; place in a crucible, cover,
and fuse at a moderate temperature for about fifteen minutes; pour; when
cold detach the metal and weigh.
For coppery ores in which the metals are present as sulphides use the
fluxes just given with 2 grams of charcoal (instead of the cyanide) and
a little sulphur.
For coppery ores in which the metals are present as oxides, mix 20 grams
of the ore with 10 grams of fusion mixture, 4 grams of salt, 4 grams of
sulphur and 2 grams of charcoal; and fuse.
A considerable percentage of bismuth is lost in these assays; it is
stated as being nearly 8 per cent. of the metal present.
WET METHODS.
~Detection.~--Bismuth is detected by dissolving the substance in nitric
or hydrochloric acid and precipitating the diluted solution with
sulphuretted hydrogen. The precipitated sulphides, after digesting with
soda and washing, are dissolved in nitric acid and the solution boiled
with ammonium carbonate. The precipitate is washed and then warmed with
dilute sulphuric acid. The solution will contain the bismuth. Add a
solution of potassium iodide in excess, and boil; a yellow or dark brown
solution proves that bismuth is present. Another good test for small
quantities of bismuth is to add tartaric acid to the solution to be
tested, and then to make it alkaline with potash. Add a few c.c. of
Schneider's liquid,[61] and heat. A brownish-black colour is produced by
as little as one part of bismuth in 200,000 of solution. The test is not
applicable in the presence of mercury, copper, or manganese.
Compounds of bismuth fused with cyanide of potassium in a Berlin
crucible readily give a globule of bismuth which is recognised by its
appearance and fracture.
~Solution and Separation.~--The solution of bismuth compounds presents
no difficulty. They are soluble in nitric acid or aqua regia, and,
provided the solution is sufficiently acid, they remain dissolved. In
separating it from other metals the solution is made up to about 100
c.c. and treated with a current of sulphuretted hydrogen. The bismuth
comes down in a tolerably strong acid solution. The sulphide is decanted
on to a filter and washed. It is next digested with ammonic sulphide;
or, better (especially when other metals are present), dissolved in
nitric acid, and treated with an excess of ammonia and a current of
sulphuretted hydrogen. The precipitate is filtered off and evaporated to
dryness with nitric acid. It is taken up with a few drops of sulphuric
acid and a little water; and warmed and filtered, if necessary. The
filtrate is nearly neutralised with ammonia; ammonium carbonate added in
slight excess; and the liquid heated to boiling and filtered. The
bismuth will be contained in the precipitate with perhaps traces of
lead, antimony, tin, or sometimes iron from incomplete separation or
washing. When only traces of a precipitate are got it must be tested.
The bismuth precipitate is readily soluble in dilute nitric acid.
GRAVIMETRIC DETERMINATION.
The bismuth having been separated and dissolved in nitric acid[62] is
precipitated (after dilution) by the addition of carbonate of ammonium
in slight excess, and boiling. The precipitate is filtered off, washed
with hot water, dried, ignited, and weighed. The ignition should be
performed carefully at not above a low red heat. The oxide which is
formed has, at this temperature, a dark yellow or brown colour, and
becomes yellow on cooling. It is bismuthic oxide (Bi_{2}O_{3}) and
contains 89.65 per cent. of bismuth. Fusion with potassium cyanide at a
temperature just sufficient to melt the salt reduces it to the metal
which falls to the bottom and runs into a globule. The button of metal
may be weighed, but it often sticks tenaciously to the bottom of the
crucible. The precipitation with ammonic carbonate must not be made in a
sulphate or chloride solution; since basic compounds would then be
thrown down, and the result on weighing would either be too low (because
of the volatilisation of the chloride), or too high (because of the
retention of sulphuric acid).
Bismuth compounds in a nitric acid solution are readily decomposed by
the electric current, but the deposited bismuth is not coherent. It
comes down in shaggy tufts which are difficult to wash and easy to
oxidise.
VOLUMETRIC ASSAY.
There are two methods which have been proposed; one based on the
precipitation as chromate and the estimation of the chromic acid; and
the other on the precipitation as oxalate and subsequent titration with
permanganate of potash. These offer little advantage over the easy
gravimetric determination.
COLORIMETRIC METHOD.
Bismuth iodide dissolves in excess of potassium iodide, forming a
yellow-coloured solution, indistinguishable in colour from that given by
iodine. The colour, however, is not removed by boiling or by sulphurous
acid. Since none of the commoner metals give such a colour, and free
iodine is easily separated by boiling, this method is specially suited
for small determinations of bismuth.
It requires a _solution of bismuth_, made by dissolving 0.1 gram of
bismuth in a drop or so of nitric acid, evaporating with a little
sulphuric acid and diluting with water to 1 litre. 1 c.c. will contain
0.1 milligram of bismuth. And a _solution of sulphurous acid_, made by
diluting 10 c.c. of the commercial acid to 1 litre with water.
The determination is made in the usual way: 50 c.c. of the prepared
solution, which should not carry more than 0.75 milligram nor less than
0.01 milligram of bismuth, are placed in a Nessler tube and the colour
compared with that observed in a similar tube containing water and
potassium iodide on adding the standard solution of bismuth.
The assay solution is prepared by separating the bismuth with
sulphuretted hydrogen, boiling the precipitate with nitric acid, and
evaporating with sulphuric acid. Take up with water, add 10 or 20 c.c.
of solution of potassium iodide, boil off any iodine liberated, dilute,
filter, and make up to 100 c.c. According to the depth of colour take
10, 20, or 50 c.c. and transfer to the Nessler tube. Add a few c.c. of
the solution of sulphurous acid. Into the other Nessler tube put as much
potassium iodide solution as is contained in the assay tube, with
sulphurous acid and water to within a few c.c. of the bulk. Then add the
standard bismuth solution till the tints are equal.
The student must be careful not to confuse the colour of the bismuth
iodide with that of free iodine. If the yellow colour is removed by
boiling and returns on standing it is due altogether to iodine; if it is
lessened by the addition of a few drops of the dilute sulphurous acid,
it is in part due to it. Hence the necessity of having a little free
sulphurous acid in each tube. A strong solution must not be used, since
it liberates iodine from potassium iodide.
The following experiments illustrate the effect of variation in the
conditions of the assay:--
~Effect of Varying Temperature.~--At a higher temperature the colour is
somewhat lessened.
1.0 c.c. at 15° C. showed the colour of 0.8 c.c. at 70° C.
2.5 " " " " 2.0 "
5.0 " " " " 5.0 "
~Effect of Free Acid.~--
2.5 c.c. with 5 c.c. of nitric acid equalled 2.5 c.c.
5.0 " " sulphuric acid " 5.0 "
Hydrochloric acid almost completely removes the colour, which, however,
is restored by the addition of a few crystals of potassium iodide.
~Effect of Alkalies.~--Ammonia, soda, or potash destroys the colour, but
it is restored on acidifying with nitric or sulphuric acid.
~Effect of Ammonic Salts.~--The following table shows the results after
addition of ammonic salts:--
-------------+----------------+-----------------+-----------------
C.c. present.|With 10 grams |With 10 grams |With 10 grams
|Ammonic Nitrate.|Ammonic Sulphate.|Ammonic Chloride.
-------------+----------------+-----------------+-----------------
1.0 c.c. | 0.9 c.c. | 1.1 c.c. | --
2.5 " | 2.5 " | 2.7 " | --
5.0 " | 5.0 " | 5.5 " | --
-------------+----------------+-----------------+-----------------
Ammonic chloride, like hydrochloric acid, removes the colour, which may
be restored on the addition of more potassium iodide. Nitrates and
sulphates do not thus interfere.
~Effect of Foreign Salts.~--Sodic hyposulphite almost completely removes
the colour. Copper salts liberate iodine; but when this has been removed
by boiling and the cuprous iodide has been filtered off there is no
further interference. Dilute solutions of lead salts give no colour.
PRACTICAL EXERCISES.
1. A fusible alloy is made up of 8 parts of bismuth, 5 of lead, and 3 of
tin. What weight of oxide of bismuth, Bi_{2}O_{3}, would you get on the
analysis of 1 gram of it?
2. What weight of bismuth can be got from 2 grams of the subnitrate
BiONO_{3}.H_{2}O?
3. How would you detect and separate arsenic, lead, and copper in a
sample of bismuth?
ANTIMONY.
Antimony occurs in the native state, but is rare; its common ore is
antimonite, the sulphide (Sb_{2}S_{3}). Jamesonite and other sulphides
of lead and antimony are frequently met with. Sulphide of antimony is
also a constituent of fahlerz and of many silver ores.
Antimonite occurs generally in fibrous masses, has a lead-like metallic
lustre, is easily cut with a knife, and melts in the flame of a candle.
Antimony itself has a very crystalline fracture, is brittle, and has a
bluish-white colour. It is used in the preparation of alloys with lead
and tin for the manufacture of type-metal. It is readily fusible, and
imparts hardness and the property of taking a sharp cast to its alloys.
It is practically insoluble in hydrochloric acid. On boiling with strong
nitric acid it is converted into antimonic oxide (Sb_{2}O_{5}), which is
a powder almost insoluble in this acid or in water, but which may be got
into solution with difficulty by the prolonged action of hydrochloric
and tartaric acids. Antimonic oxide is converted on ignition into the
tetroxide (Sb_{2}O_{4}) with loss of oxygen. Antimony forms two series
of salts, antimonious and antimonic; and advantage is taken of this in
its determination volumetrically. Either sulphide of antimony yields
antimonious chloride on boiling with hydrochloric acid, sulphuretted
hydrogen being given off; and, in the case of antimonic sulphide,
sulphur is deposited. Antimonious is converted into antimonic chloride
by treatment with permanganate of potash in an acid solution. Antimonic
chloride and potassium iodide react, forming antimonious chloride and
free iodine. This latter may be got rid of by boiling. Sulphide of
antimony is separated from the ore by liquation; this regulus is met
with in commerce as "crude antimony."
DRY ASSAY.
An approximate determination of the amount of sulphide of antimony in an
ore may be made by fusing and liquating in a luted double crucible in
the manner described under bismuth. This is unsatisfactory. The
determination of metallic antimony in an ore is made either by fusion
with potassium cyanide or by fusion with iron, as in the galena assay.
Both methods yield poor results; and, where iron is used, it must be
added in quantity only sufficient for desulphurising; this amounts to
about 40 per cent. in pure ores. If the iron is in excess it alloys with
the reduced antimony. If, on the other hand, it is insufficient, the
metal will contain sulphur; or sulphide of antimony will be lost in the
slag.
The following note, for which we are indebted to Mr. Bedford McNeill,
A.R.S.M., gives a description of the method adopted in the commercial
valuation of a parcel of antimony ore:--
The antimony smelter, when he wishes to determine the value of any
parcel of ore--usually the sulphide--that may be offered for sale,
practically has recourse to the smelting operation. That is, a quantity
of 2 or 3 cwts. taken by his sampler having been obtained, he treats it
under the immediate supervision of the foreman smelter as if it formed
part of the ore in process of daily reduction at his works. He thus
determines by actual trial the output which it may fairly be anticipated
will be yielded by the bulk, and upon the result of this trial or assay,
and the knowledge gained of the actual behaviour of the ore under
treatment, he bases his tender, knowing that, should he secure the
parcel, he may confidently expect a similar return.
Briefly, the process consists of the three ordinary operations of--
(a) Singling or removing most of the antimony from the ore;
(b) Doubling;
(c) Refining or "starring."
But in the assay sufficient information is generally given by the first
two of these.
A new pot having been taken and made hot in the furnace, 40 or 45 lbs.
of the ore is weighed in (the mineral from the necessities of sampling
not exceeding walnut size); 1 to 3 lbs. of salt cake is now added to
render the separation of the resulting sulphide of iron more easy, as
also to assist in the fusion of the gangue; 20 to 25 lbs. of tin-plate
scrap, beaten more or less into ball shape, is weighed, placed on the
top of the ore and salt cake, and the whole brought to a state of
fusion. The foreman from time to time takes notice of the behaviour of
the ore under the working conditions. Ores that manifest a tendency to
"boil" or "froth " require the admixture of other more sluggish mineral
in order to render their reduction economically practicable.
After 1-1/4 to 1-1/2 hours (the time depending mainly on the
temperature), the contents of the crucible are usually in a state of
tranquil fusion. The pot is now lifted from the fire, and its contents
transferred to a conical iron mould, the empty pot being immediately put
back into the fire, and the latter "mended" with sufficient coke for
another run. The conical mould (when dealing with a "strange" ore, and
the possibility of insufficient iron being present to satisfy the
sulphur contents) is wiped inside with clay previous to pouring in the
molten charge. Otherwise the mould itself will be attacked, and the
contents after solidifying will require to be chiselled out piecemeal.
A further 40 lbs. of the ore is now charged into the crucible with iron
as above; but before this second charge is ready to be drawn an
inspection of the first may suggest the addition of either 3 or 5 lbs.
more iron, or 5 or 10 lbs. more ore.
It is a good fault rather to aim at an excess of iron as tending to
clean the ore from antimony, any of the latter that (from an
insufficiency of iron) may be left in the slag from the first process
being irretrievably lost; whereas, if the iron be in excess, that which
is combined with the crude antimony resulting from the first process is
easily got rid of by adding 3 to 5 lbs. or so of ore in the second
process.
This latter, as practised for the determination of the value of a parcel
of ore, consists in selecting two of the best quality singles, resulting
from perhaps four or five trials as above, and running them down with a
few pounds of salt cake, or a mixture of salt cake with American potash,
and (as is generally necessary) a small addition of ore.
Upon the final result (confirmed perhaps on another pair of singles,
and, judging from the total weight or output of the metal as calculated
from the ore used in "singling," plus any added in the "doubling," the
crystalline fracture and face of the metal, its colour, etc.) the price
to be offered for the parcel of ore is fixed.
WET METHODS.
~Detection.~--The antimony, if any, being got into solution by treating
the ore with hydrochloric acid or aqua regia may be detected by
evaporating with hydrochloric acid, diluting, and filtering into the
cover of a platinum crucible or (better) a platinum dish. A small lump
of zinc is then added, and, if antimony is present, _the dish_ will in a
minute or so be stained black with a deposit of metallic antimony. This
stain is removed by nitric, but not by hydrochloric, acid. The reaction
is delicate and characteristic; arsenic under like conditions is evolved
as arseniuretted hydrogen, and tin is deposited as metal _on the zinc_.
~Solution.~--Ores, &c., containing antimony are best opened up by
boiling with hydrochloric acid or aqua regia; treatment with nitric
acid should be avoided wherever possible, since it forms antimonic acid,
which is subsequently dissolved only with difficulty. Salts of antimony
in solution have a tendency to form insoluble basic salts; so that care
must be exercised in diluting. Compounds such as antimonite which are
soluble in hydrochloric should be dissolved at once in that acid.
~Separation.~--To the solution add potash in excess and a little free
sulphur, and pass a current of sulphuretted hydrogen for some minutes;
allow to digest for an hour or so on a hot plate; filter; and wash the
residue. Acidulate the filtrate with hydrochloric acid: the precipitate
will contain the antimony (as Sb_{2}S_{5}), and possibly arsenic or tin.
The precipitate is transferred to a beaker and boiled with hydrochloric
acid; the solution is filtered off and diluted. Add a few crystals of
tartaric acid, and pass a current of sulphuretted hydrogen for some
time. The first flocculent precipitate will become denser, and render
the filtering more easy. Transfer the precipitate (after washing free
from chlorides) to a Berlin dish, and treat cautiously with fuming
nitric acid. The action of this acid on the sulphide is very violent.
Evaporate and ignite, transfer to a silver dish, and fuse with four or
five times its weight of caustic soda, cool and extract with a little
water, then add an equal volume of alcohol, and allow to stand
overnight. Filter, wash with dilute alcohol. (The filtrate will contain
the tin.) The residue contains the antimony as antimonate of soda, and
is dissolved off the filter with hot dilute hydrochloric, with the help
of a little tartaric, acid. The filtrate is now ready for the
gravimetric determination.
GRAVIMETRIC ASSAY.
Pass a current of sulphuretted hydrogen through the solution containing
the antimony to which a little tartaric acid has been previously added.
Pass the gas till the precipitate becomes dense, and the antimony is all
down. The solution must not be too strongly acid. Filter off the
precipitate, wash with hot water, dry in the water oven, transfer to a
weighed porcelain dish, and cautiously treat with fuming nitric acid.
Continue the action on the water bath till the sulphur and antimony are
completely oxidised. Evaporate; ignite, gently at first, then strongly
over the blast; cool, and weigh. The residue is a white infusible
powder, and consists of antimony tetroxide, Sb_{2}O_{4}, containing
78.94 per cent. of the metal.
~Determination of Antimony as Bigallate.~--What appears to be a very
good method has been worked out by M.A. Guyard, and is described in
Crookes' _Select Methods_, p. 398.
The antimony must be in solution as antimonious chloride, and must not
be accompanied by an excess of hydrochloric acid. To ensure these
conditions, the solution is treated with potassium iodide until no more
iodine is evolved, and is then evaporated to remove the excess of
hydrochloric acid. To the concentrated, and nearly neutral, solution a
freshly-prepared solution of gallic acid is added in slight excess. A
bulky white precipitate is formed that settles rapidly. The solution is
diluted with hot water and washed by decantation. Then the precipitate
is collected on a weighed double filter, washed once or twice with hot
water, and dried at 100° C. The dried substance is antimony bigallate,
and contains 40.85 per cent. of antimony. It should be completely
soluble in ammonium sulphide. The solution in which the antimony is
precipitated need not be quite free from other metals.
VOLUMETRIC METHOD.
This is based on the reduction of antimonic chloride (SbCl_{5}) to
antimonious (SbCl_{3}) by the action of potassium iodide in strong
hydrochloric acid solution.[63] Iodine is at the same time liberated,
and the amount of antimony reduced is got at by titrating with sodium
hyposulphite, which measures the iodine set free.
The standard solution of sodium hyposulphite is made by dissolving 41.32
grams of the salt (Na_{2}S_{2}O_{3}.5H_{2}O) in water, and diluting to 1
litre. One hundred c.c. will be equivalent to about 1 gram of antimony.
It is standardised with the help of a solution of antimony made as
follows:--Weigh up 5 grams of powdered antimony, transfer to a flask,
and cover with 50 c.c. of hydrochloric acid; boil, and add nitric acid
(5 or 10 drops at a time) until the metal is dissolved. Allow the action
of the nitric acid to cease before adding more. Boil down to a small
bulk, add 250 c.c. of hydrochloric acid, and dilute to nearly 1 litre.
Warm until any precipitate which has formed is redissolved; allow to
cool slowly, and run in from a pipette a weak solution of permanganate
until a faint brown colour is produced. Dilute to exactly 1 litre; 100
c.c. contain 0.5 gram of antimony as antimonic chloride.
In standardising, take 50 c.c. of the antimony solution, and transfer to
a flask; add 2 grams of potassium iodide crystals, and when dissolved,
after standing a few minutes, run in the solution of "hypo" from an
ordinary burette until the greater part of the iodine has been reduced.
Add a few drops of starch solution, and continue the addition of the
"hypo" until the muddy-green colour changes to a clear brownish-yellow.
The solution must be shaken after each addition of the "hypo."
In determining antimony in ore, weigh up 0.5 to 1 gram, and dissolve in
hydrochloric acid with, if necessary, the help of chlorate of potash.
The antimony is separated as sulphide, redissolved in hydrochloric acid,
and oxidised with a crystal of chlorate of potash. Chlorine is boiled
off, and the solution diluted with an equal bulk of water. To the clear
cold solution potassium iodide is added, and after a few minutes the
liberated iodine is titrated with "hypo," as already described. The
method only yields satisfactory results when the standard and assay are
carried out alike.
FOOTNOTES:
[50] "Modern American Methods of Copper Smelting" (Dr. Peters).
[51] "Journal of the Society of Chemical Industry," vol. v. No. 2.
[52] Lead when present is precipitated on the _spiral_ in the form of a
dark powder of dioxide (PbO_{2}). Manganese is also thrown down on the
spiral as dioxide (MnO_{2}), the solution at the same time becomes
violet from the formation of permanganic acid.
[53] See the method given under _Examination of Commercial Copper_.
[54] CuSO_{4} + 4KCy = 2KCy.CuCy_{2} + K_{2}SO_{4}.
[55] 2CuSO_{4} + 3KCy + Am_{2}O = Cu_{2}Cy_{2} + Am_{2}SO_(4) +
K_{2}SO_{4} + KCyO.
[56] 2CuSO_{4} + 4KI = Cn_{2}I_{2} + 2I + 2K_{2}SO_{4}.
[57] 2Na_{2}S_{2}O_{3} + 2I = 2NaI + Na_{2}S_{4}O_{6}.
[58] For further information, see Appendix B., and a paper by J.W.
Westmoreland, _Journal of the Society of Chemical Industry_, vol. v.
p. 48.
[59] 3Cu_{2}O + 6AgNO_{3} + 3H_{2}O = 2Cu_{2}H_{3}O_{3}NO_{3} +
2Cu(NO_{3})_{2} + 6Ag. (Insoluble basic salt.)
[60] K_{2}CrO_{4} + Pb(NO_{3})_{2} = PbCrO_{4} + 2KNO_{3}
[61] Made by dissolving 12 grams of tartaric acid and 4 grams of
stannous chloride in water, and adding potash solution till it is
alkaline. The solution should remain clear on heating to 60° or 70° C.
[62] It must be remembered that arsenate of bismuth is completely
insoluble in this acid.
[63] SbCl_{5} + 2KI = I_{2} + SbCl_{3} + 2KCl.
CHAPTER XI.
IRON--NICKEL--COBALT--ZINC--CADMIUM.
IRON.
Iron rusts or oxidises very readily, and, consequently, is rarely found
in the metallic state in nature; such native iron as is found being
generally of meteoric origin or imbedded in basalt and other igneous
rocks. It chiefly occurs as oxide, as in magnetite, hæmatite, and in the
brown iron ores and ochres. Chalybite, which is carbonate of iron, is an
ore of great importance. Iron is found combined with sulphur in
pyrrhotine and pyrites, and together with arsenic in mispickel. It is a
common constituent of most rocks, imparting to them a green, black, or
brown colour; and is present, either as an essential part or as an
impurity, in most substances.
The chemistry of iron is somewhat complicated by the existence of two
oxides, each of which gives rise to a well-marked series of compounds.
Those derived from the lower oxide, known as ferrous salts, are
generally pale and greenish. Ferric salts are derived from the higher
oxide, and are generally red, brown, or yellow. The existence of these
two well-marked families of salts renders the assay of iron
comparatively easy, for the quantity of iron present in a solution can
be readily measured by the amount of oxidising or reducing agent
required to convert it from the one state into the other--that is, from
ferrous to ferric, or from ferric to ferrous, as the case may be.
In the red and brown iron ores and ochres ferric iron is present; in
chalybite the iron is in the ferrous state; and in magnetite it is
present in both forms. Traces of iron in the ferrous state may be found
(even in the presence of much ferric iron) by either of the following
tests:--
1. Ferricyanide of potassium gives a blue precipitate or green
coloration; with ferric salts a brown colour only is produced.
2. A solution of permanganate of potassium is decolorised by a
ferrous salt, but not by a ferric one.
Traces of ferric iron can be detected (even in the presence of much
ferrous iron) by the following tests:--
(1) By the brown or yellow colour of the solution, especially
when hot.
(2) By giving a pink or red coloration with sulphocyanide of
potassium.
Substances containing oxide of iron yield the whole of the iron as metal
when fused at a high temperature with charcoal and suitable fluxes. The
metal, however, will contain varying proportions of carbon and other
impurities, and its weight can only afford a rough knowledge of the
proportion of the metal in the ore. There are two or three methods of
dry assay for iron, but they are not only inexact, but more troublesome
than the wet methods, and need not be further considered. Chalybite and
the hydrated oxides dissolve very readily in hydrochloric acid; hæmatite
and magnetite dissolve with rather more difficulty. Iron itself, when
soft, is easily soluble in dilute hydrochloric, or sulphuric, acid.
Pyrites, mispickel, &c., are insoluble in hydrochloric acid, but they
are readily attacked by nitric acid. Certain minerals, such as chrome
iron ore, titaniferous iron ore, and some silicates containing iron,
remain in the residue insoluble in acids. Some of these yield their iron
when attacked with strong sulphuric acid, or when fused with the acid
sulphate of potash. Generally, however, it is better in such stubborn
cases to fuse with carbonate of soda, and then attack the "melt" with
hydrochloric acid.
When nitric acid, or the fusion method, has been used, the metal will be
in solution in the ferric state, no matter in what condition it existed
in the ore. But with dilute hydrochloric or sulphuric acid it will
retain its former degree of oxidation. Hydrochloric acid, for example,
with chalybite (ferrous carbonate) will give a solution of _ferrous_
chloride; with hæmatite (ferric oxide) it will yield _ferric_ chloride;
and with magnetite (ferrous and ferric oxides) a mixture of ferrous and
ferric chlorides. Metallic iron yields solutions of _ferrous_ salts. It
is convenient to speak of the iron in a ferrous salt as ferrous iron,
and when in the ferric state as ferric iron. Frequently it is required
to determine how much of the iron exists in an ore in each condition. In
such cases it is necessary to keep off the air whilst dissolving; the
operation should, therefore, be performed in an atmosphere of carbonic
acid.
~Separation.~--The separation of the iron from the other substances is
as follows:--Silica is removed by evaporating the acid solution, and
taking up with acid, as described under _Silica_; the whole of the iron
will be in solution. The metals of Groups I. and II. are removed by
passing sulphuretted hydrogen, and at the same time the iron will be
reduced to the ferrous state. The solution should be filtered into a 16
oz. flask, boiled to get rid of the gas, and treated (whilst boiling)
with a few drops of nitric acid, in order to convert the whole of the
iron into the ferric state. When this condition is arrived at, an
additional drop of nitric acid causes no dark coloration. The boiling
must be continued to remove nitrous fumes. Next add caustic soda
solution until the colour of the solution changes from yellow to red.
The solution must be free from a precipitate; if the soda be
incautiously added a permanent precipitate will be formed, in which case
it must be redissolved with hydrochloric acid, and soda again, but more
cautiously, added. After cooling, a solution of sodium acetate is added
until the colour of the solution is no longer darkened. The solution,
diluted to two-thirds of the flaskful with water, is heated to boiling.
Long-continued boiling must be avoided. The precipitate is filtered
quickly through a large filter, and washed with hot water containing a
little acetate of soda.
The precipitate will contain all the iron and may also contain alumina,
chromium, titanium, as well as phosphoric, and, perhaps, arsenic
acids.[64]
Dissolve the precipitate off the filter with dilute sulphuric acid,
avoiding excess, add tartaric acid and then ammonia in excess. Pass
sulphuretted hydrogen, warm, and allow the precipitate to settle. Filter
and wash with water containing a little ammonic sulphide.
GRAVIMETRIC METHOD.
Dissolve the precipitate in dilute hydrochloric acid; peroxidise with a
few drops of nitric acid and boil, dilute to about 200 c.c., add ammonia
(with constant stirring) till the liquid smells of it, and heat to
boiling. Wash as much as possible by decantation with hot water.
Transfer to the filter, and wash till the filtrate gives no indication
of soluble salts coming through. The filtrate must be colourless and
clear. The wet precipitate is very bulky, of a dark-brown colour and
readily soluble in dilute acids, but insoluble in ammonia and dilute
alkalies. When thrown down from a solution containing other metals it is
very apt to carry portions of these with it, even when they are by
themselves very soluble in ammoniacal solutions. It must be dried and
ignited, the filter paper being burnt separately and its ash added. When
further ignition ceases to cause a loss of weight, the residue is ferric
oxide (Fe_{2}O_{3}), which contains 70 per cent. of iron. The weight of
iron therefore can be calculated by multiplying the weight of oxide
obtained by 0.7.
The presence of ammonic chloride causes loss of iron during the
ignition, and organic matter causes an apparent loss by reducing the
iron to a lower state of oxidation. When the iron in the solution much
exceeds 0.2 gram the volumetric determination is generally adopted, as
the bulkiness of the precipitate of ferric hydrate makes the gravimetric
method very inconvenient.
VOLUMETRIC METHODS.
As already explained these are based on the measurement of the volume of
a reagent required to bring the whole of the iron from the ferrous to
the ferric state (oxidation), or from the ferric to the ferrous
(reduction). Ferrous compounds are converted into ferric by the action
of an oxidising agent in the presence of an acid. Either permanganate or
bichromate of potash is generally used for this purpose.[65]
Ferric compounds are reduced to ferrous by the action of:--
(1) Stannous chloride;
(2) Sulphuretted hydrogen;
(3) Sodium sulphite; or
(4) Zinc.[66]
The processes, then, may be divided into two kinds, one based on
oxidation and the other on reduction. In each case the titration must be
preceded by an exact preparation of the solution to be assayed in order
that the iron may be in the right state of oxidation.
~PERMANGANATE AND BICHROMATE METHODS.~
These consist of three operations:--
(1) Solution of the ore;
(2) Reduction of the iron to the ferrous state; and
(3) Titration.
~Solution.~--The only point to be noticed concerning the first operation
(in addition to those already mentioned) is that nitric acid must be
absent. If nitric acid has been used, evaporate to dryness, of course
without previous dilution; add hydrochloric or sulphuric acid, and boil
for five or ten minutes. Dilute with water to about 100 c.c., and warm
until solution is complete.
The reduction is performed by either of the following methods:--
1. _With Stannous Chloride._--Fill a burette with a solution of stannous
chloride,[67] and cautiously run the liquid into the hot assay solution
(in which the iron is present as _chloride_) until the colour is
discharged. A large excess of the stannous chloride must be avoided.
Then add 5 c.c. of a 2-1/2 per cent. solution of mercuric chloride, this
will cause a white precipitate (or a grey one if too large an excess of
the stannous chloride has been added). Boil till the solution clears,
cool, dilute, and titrate.
2. _With Sulphuretted Hydrogen._--Cool the solution and pass through it
a current of washed sulphuretted hydrogen till the liquid smells
strongly of the gas after withdrawal and shaking. A white precipitate of
sulphur will be formed, this will not interfere with the subsequent
titration provided it is precipitated in the cold. If, however, the
precipitate is coloured (showing the presence of the second group
metals), or if the precipitation has been carried out in a hot solution,
it should be filtered off. Boil the solution until the sulphuretted
hydrogen is driven off; this may be tested by holding a strip of filter
paper dipped in lead acetate solution in the steam issuing from the
flask. The presence of sulphuretted hydrogen should be looked for rather
than its absence. It is well to continue the boiling for a few minutes
after the gas has been driven off. Cool and titrate.
3. _With Sodium Sulphite._--Add ammonia (a few drops at a time) until
the precipitate first formed redissolves with difficulty. If a permanent
precipitate is formed, redissolve with a few drops of acid. To the warm
solution add from 2 to 3 grams of sodium sulphite crystals. The solution
will become strongly coloured, but the colour will fade away on standing
for a few minutes in a warm place. When the colour is quite removed, add
20 c.c. of dilute sulphuric acid, and boil until the steam is quite free
from the odour of sulphurous acid. Cool and titrate.
4. _With Zinc._--Add about 10 grams of granulated zinc; if the hydrogen
comes off violently add water; if, on the other hand, the action is very
slow, add sufficient dilute sulphuric acid to keep up a brisk
effervescence. The reduction is hastened by warming, and is complete
when the solution is quite colourless and a drop of the liquid tested
with sulphocyanate of potassium gives no reaction for ferric iron.
Filter through "glass wool" or quick filtering paper. The zinc should be
still giving off gas rapidly, indicating a freely acid solution; if not,
acid must be added. Wash with water rendered acid. Cool and titrate.
With regard to the relative advantages of the different methods they may
be roughly summed up as follows:--The stannous chloride method has the
advantage of immediately reducing the ferric iron whether in hot or cold
solution and under varied conditions in regard to acidity, but has the
disadvantage of similarly reducing salts of copper and antimony, which,
in a subsequent titration, count as iron. Moreover, there is no
convenient method of eliminating any large excess of the reagent that
may have been used; and, consequently, it either leaves too much to the
judgment of the operator, or entails as much care as a titration.
Students generally get good results by this method.
The sulphuretted hydrogen method also has the advantage of quick
reduction under varying conditions, and the further one of adding
nothing objectionable to the solution; in fact it removes certain
impurities. The disadvantages are the necessity for boiling off the
excess of the gas, and of filtering off the precipitated sulphur,
although this last is not necessary if precipitated cold. The tendency
with students is to get high results. The sodium sulphite method has the
advantages of being clean and neat, and of requiring no nitration. On
the other hand it requires practice in obtaining the best conditions for
complete reduction; and, as with sulphuretted hydrogen, there is the
necessity for boiling off the gas, while there is no simple and delicate
test for the residual sulphurous acid. In addition, if an excess of
sodium sulphite has been used and enough acid not subsequently added,
the excess will count as iron. Students generally get low results by
this method.
The advantages of the zinc method are, that it is easily worked and that
the excess of zinc is readily removed by simply filtering. The
disadvantages are the slowness[68] with which the last portions of
ferric iron are reduced, the danger of loss by effervescence, the
precipitation of basic salts, and, perhaps, of iron, and the loading of
the solution with salts of zinc, which in the titration with bichromate
have a prejudicial effect. The tendency in the hands of students is to
get variable results, sometimes low and sometimes high.
Generally speaking, the sulphuretted hydrogen and sodium sulphite
methods are to be preferred. Carefully worked each method will yield
good results.
The titration may be done with a standard solution of (1) permanganate
of potash, or (2) bichromate of potash.
1. _With Permanganate of Potash._--Prepare a standard solution by
dissolving 2.82 grams of the salt and diluting to one litre. The
strength of this should be 100 c.c. = 0.5 gram of iron, but it varies
slightly, and should be determined (and afterwards checked every two or
three weeks) by weighing up 0.2 gram of iron wire, dissolving in 10 c.c.
of dilute sulphuric acid, diluting to about 100 c.c., and titrating.