described under _Arsenic_.[111] Use 5 c.c. for each assay.
The following experiments show the effect of variation in the conditions
of the titration:--
~Effect of Varying Temperature.~--The solution should be titrated while
boiling. This is especially necessary for the last few c.c. in order to
get a decided and fixed finishing point.
Temperature 15° C. 30° C. 70° C. 100° C.
"Uranium" required 18.0 c.c. 19.2 c.c. 19.0 c.c. 18.9 c.c.
~Effect of Varying Bulk.~--
Bulk 50.0 c.c. 100.0 c.c. 200.0 c.c. 300.0 c.c.
"Uranium" required 18.8 " 18.9 " 19.0 " 19.3 "
Variation in bulk affects the results; therefore, a constant bulk should
be adhered to.
~Effect of Varying Sodium Acetate and Acetic Acid Solution.~--
Sodium acetate
and acetic
acid solution 0.0 c.c. 1.0 c.c. 5.0 c.c. 10.0 c.c. 20.0 c.c.
"Uranium" required 18.9 " 18.9 " 19.0 " 18.8 " 17.5 "
As in the titration with arsenates, an excess is dangerous to the assay;
a definite quantity (5 c.c.) should, therefore, be used.
~Effect of Foreign Salts.~--Besides the sodium acetate, &c., added, the
only salts likely to be present are those of ammonia and magnesia. In
three experiments, in one of which no foreign salts were introduced,
while in the other two 5 grams of ammonic chloride and of magnesium
sulphate respectively were added, there were required:--
With ammonic chloride 18.8 c.c. "Uranium" solution
With magnesium sulphate 19.0 " "
Without foreign salts 18.9 " "
~Effect of Varying Phosphate.~--
"Phosphate" solution added 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.
"Uranium" required 9.8 " 18.9 " 47.6 " 94.5 "
The quantity of phosphoric oxide in the assay solution for the
conditions of titration should not be much less than 0.05 gram. For
smaller quantities the uranium solution should be diluted to half its
strength, and the assay solution concentrated by reducing its bulk to 50
c.c. and using 2.5 c.c. of the sodium acetate and acetic acid solution.
~Determination of Phosphoric Oxide in Apatite.~--Weigh up 0.5 gram of
the dried and powdered sample, and dissolve it in 5 c.c. of hydrochloric
acid. Evaporate to a paste, add 5 c.c. of the sodic acetate and acetic
acid solution, dilute to 100 c.c. with water, boil, and titrate with
uranium acetate solution.
In an example, 0.5 gram of apatite required 37.4 c.c. of uranium acetate
solution (standard equal to 0.5291 gram of phosphoric oxide). The sample
therefore contained 0.1979 gram of P_{2}O_{5}, equal to 39.58 per cent.
~Determination of Phosphoric Oxide in an Iron Ore.~--Take 10 grams, boil
with 50 c.c. of hydrochloric acid, and evaporate to a paste; take up
with 10 c.c. of dilute hydrochloric acid, and dilute with water to 400
c.c. Pass sulphuretted hydrogen for nearly a quarter of an hour; warm,
and filter. Boil off the excess of gas; cool, add ammonia till nearly
neutral, and then a few drops of ferric chloride solution, and 4 or 5
grams of sodium acetate, with a drop or two of acetic acid. Boil and
filter. Dissolve the precipitate in hot dilute hydrochloric acid, and
add citro-magnesia mixture and ammonia; allow to stand overnight;
filter, ignite, and weigh.
In an example, 10 grams of ore gave 28.5 milligrams of magnesic
pyrophosphate, which is equivalent to 0.18 per cent. of phosphoric
oxide.
~Determination of Phosphorus in Iron.~--Take from 2 to 10 grams
(according to the amount of phosphorus present), and dissolve in aqua
regia, keeping the nitric acid in excess; evaporate to dryness and take
up with hydrochloric acid, boil, dilute, and filter. Add 10 c.c. of
nitric acid, nearly neutralise with ammonia, render acid with 3 or 4
c.c. of nitric acid, and add 10 or 20 c.c. of ammonic molybdate
solution. Heat for some time, allow to settle, filter, and wash the
precipitate with a solution of ammonic nitrate. Dissolve the precipitate
in dilute ammonia, nearly neutralise with dilute hydrochloric acid, and
add first "magnesia mixture," and then ammonia; allow to stand
overnight; filter, wash with dilute ammonia, dry, ignite, and weigh as
magnesic pyrophosphate. Calculate to phosphorus.
PRACTICAL EXERCISES.
1. Ten grams of an iron yielded 12 milligrams of pyrophosphate of
magnesia. What percentage of phosphorus did the metal contain?
2. Ten grams of an iron ore gave 12 milligrams of pyrophosphate. What
percentage of phosphoric oxide did it contain?
3. What weight of apatite 3Ca_{3}(PO_{4})_{2}.CaClF would require 50
c.c. of standard uranium solution (100 c.c. equal to 0.5 gram of
P_{2}O_{5})?
4. You have reason to believe that a precipitate which has been weighed
as magnetic pyrophosphate contains some arsenate. How would you
determine the amount of phosphate really present?
5. Twenty c.c. of a solution of sodic phosphate containing 0.100 gram of
P_{2}O_{5} was found to require a solution containing 0.700 gram of
hydrated uranium acetate in a titration. The precipitate contains 80.09
per cent. uranium oxide and 19.91 per cent. of phosphoric oxide. What
percentage of uranium oxide was contained in the uranic acetate?
NITROGEN AND NITRATES.
Nitrogen occurs in nature in the free state, and forms about four-fifths
of the atmosphere. In combination, as nitrate, it is found in nitre
(KNO_{3}), and Chili saltpetre (NaNO_{3}), minerals which have a
commercial importance. The latter occurs in beds, and is extensively
worked for use as a manure and in the preparation of nitric acid.
Nitrogen is mainly characterised by negative properties, although many
of its compounds are very energetic bodies. It is a gas, present
everywhere, but so inactive that the assayer can always afford to ignore
its presence, and, except in testing furnace gases, &c., he is never
called on to determine its quantity.
The nitrates are an important class of salts, and may be looked on as
compounds of the bases with nitric pentoxide (N_{2}O_{5}). They are,
with the exception of a few basic compounds, soluble in water, and are
remarkable for the ease with which they give up their oxygen. The
alkaline nitrates fuse readily, and lose oxygen with effervescence
forming nitrites; while at a higher temperature they yield more oxygen
and lose their nitrogen, either as a lower oxide or as nitrogen. The
nitrates of the metals, on heating, leave the oxide of the metal. It is
as yielders of oxygen that nitrates are so largely used in the
manufacture of explosives. Gunpowder contains from 65 to 75 per cent. of
potassium nitrate (nitre).
Nitrates are best detected and determined by their yielding nitric oxide
when treated with sulphuric acid and a suitable reducing agent, such as
ferrous sulphate, mercury, or copper. Nitric oxide is a colourless gas
very slightly soluble in water. It combines at once with oxygen, on
mixing with the air, to form brown "nitrous fumes," and dissolves in a
solution of ferrous sulphate, producing a characteristic blackish-brown
colour. It is this colour which affords the best and most easily-applied
test for nitrates. The substance suspected to contain nitrates is
dissolved in about 1 c.c. of water, and treated with an equal volume of
strong sulphuric acid. After cooling, a solution of ferrous sulphate is
poured on its surface, so as to form a layer resting on it. On standing,
a brown or black ring is developed where the liquids join, if any
nitrate or nitrite is present. Nitrites are distinguished from nitrates
by effervescing and yielding brown fumes when treated with a little
dilute sulphuric acid.
The separation of nitrates is in many cases difficult. Generally, on
treating the substance with water, the nitrate will be in the solution,
and is filtered off from any insoluble matter. In the exceptional cases
it is got into solution by treating with a boiling solution of sodium
carbonate; the nitrate will contain it as an alkaline nitrate.
Since, however, in their determination, nitrates are never separated and
weighed as such, the difficulty of separating them has little
importance. Usually, the determination can be made on the original
aqueous solution, and it is never necessary to do more than remove any
special substance which has a bad effect; and this is easily done by the
usual reagents.
GRAVIMETRIC DETERMINATION.
It follows from what has been said that there is no direct gravimetric
determination. The percentage of nitrogen pentoxide (N_{2}O_{5}) in a
comparatively pure nitrate is sometimes determined indirectly in the
following way:--Place in a platinum-crucible 4 or 5 grams of powdered
and cleaned quartz. Ignite, cool in a desiccator, and weigh with the
cover. Mix 1 gram of the dried and powdered salt with the quartz in the
crucible by stirring with a stout platinum-wire. Cover the crucible, and
heat in a Bunsen-burner flame at scarcely visible redness for
half-an-hour. Cool and weigh. The loss in weight gives the amount of
nitrogen pentoxide. Sulphates and chlorides in moderate quantity do not
interfere. The following is an example of the process:--
Crucible and sand 26.6485 grams
Nitre taken 1.0000 "
-------
27.6485 "
Weight after ignition 27.1160 "
-------
Loss on ignition 0.5325 "
This is equal to 53.25 per cent. of nitrogen pentoxide.
VOLUMETRIC DETERMINATION.
This is based on the oxidising action of nitric acid, or of nitrates in
acid solutions on ferrous salts. The pentoxide (N_{2}O_{5}) of the
nitrate is reduced to nitric oxide (NO), so that 336 parts of iron
peroxidised represent 108 parts of nitric pentoxide as oxidising
agent.[112] The quantity of iron peroxidised is determined by taking a
known quantity of ferrous salt, oxidizing with a weighed sample of
nitrate, and then determining the residual ferrous iron by titration
with bichromate or permanganate of potassium solution. The difference
between the ferrous iron taken and that found, gives the amount oxidized
by the nitrate. The speed with which nitric oxide takes up oxygen from
the air, and thus becomes capable of oxidising more iron, renders some
precautions necessary; ferrous chloride should, therefore, be used,
since it is easier to expel nitric oxide (by boiling) from solutions of
a chloride than it is from those of a sulphate. The process is as
follows:--Dissolve 2 grams of thin soft iron wire in 50 c.c. of
hydrochloric acid in a flask provided with an arrangement for
maintaining an atmosphere of carbon dioxide. When the iron has
dissolved, allow the solution to cool, and add 0.5 gram of the nitrate.
Heat gently for a few minutes, and then boil until the nitric oxide is
expelled. An atmosphere of carbon dioxide must be kept up. Dilute with
water, and titrate the residual iron with standard solution of
bichromate of potassium. The standard "bichromate" is made by dissolving
17.5 grams of the salt (K_{2}Cr_{2}O_{7}) in water, and diluting to 1
litre: 100 c.c. equal 2 grams of iron. Deduct the weight of iron found
from the 2 grams originally taken, and multiply by 0.3214. This gives
the weight of the pentoxide in the sample. In an example, 0.5 gram of
nitre was taken, and 59.4 c.c. of the "bichromate" solution were
required. The 59.4 c.c. thus used are equivalent to 1.198 gram of iron.
This leaves 0.822 gram as the quantity oxidised by the nitre, which,
multiplied by 0.3214, gives 0.2642 gram for the nitrogen pentoxide, or
52.8 per cent.
GASOMETRIC METHOD.
This is based upon the measurement of the nitric oxide evolved on
shaking up a weighed quantity of the nitrate with sulphuric acid over
mercury in a nitrometer. Each c.c. of nitric oxide obtained, when
reduced to normal temperature and pressure, is equivalent to:--
0.627 milligram of nitrogen.
1.343 " of nitric oxide.
2.418 " of nitric pentoxide.
2.820 " of nitric acid.
3.805 " of sodium nitrate.
4.523 " of potassium nitrate.
In working on substances not rich in nitrates, an ordinary nitrometer
(fig. 69) is used; but in the assay of sodium nitrate, nitroglycerine,
&c., an instrument provided with a bulb having a capacity of 100 c.c. is
employed.
[Illustration: FIG. 69.]
The plan of working is as follows:--The "measuring tube" is filled with
mercury until it reaches up into the tap, and the levelling-tube is
placed so that it contains an inch or two of mercury. If the nitrate is
in solution, 2 or 3 c.c. of the liquid (dilute liquids are brought to
this bulk by evaporation) are measured into the cup. The levelling-tube
is lowered a little, and the tap cautiously opened until all but the
last drop of the liquid has run in. The cup is then rinsed with 2 or 3
c.c. of sulphuric acid, which is run in in the same way, and the
operation is repeated with another lot of acid. The measuring-tube is
now taken from the clamp, and shaken for two or three minutes, until no
more gas is given off. It is replaced, and the mercury-level in the two
tubes adjusted. Then it is allowed to stand until the froth has
subsided, and the gas has cooled to the temperature of the room. The
volume of the gas is then read off. In adjusting the level, account must
be taken of the sulphuric acid in the measuring-tube; this is allowed
for by having the mercury higher in the other tube by, say, 1 mm. for
each 6.5 mm. of sulphuric acid, or it is counterpoised by an equal
height of sulphuric acid in the levelling-tube, in which case the two
mercury-levels are made to correspond. On opening the tap after reading
off the volume, there should be no change in the level of the mercury.
If it should rise or fall a little, a slight increase or decrease (say
0.1 c.c.) is made to the volume previously read off.
In working with nitrate of soda, &c., in the bulb nitrometer, it is
necessary to take a quantity of the substance which will yield more than
100 and less than 150 c.c. of the gas.
FOOTNOTES:
[103] Na_{3}AsO_{3} + H_{2}O + 2I = Na_{3}AsO_{4} + 2HI. The acid is at
once neutralised.
[104] Mr. Thomas Gibb is the originator of this ingenious process.
[105] By taking hold of the water present, it may prevent the
dissociation of arsenious chloride.
[106] It is difficult to get ferric chloride free from arsenic; but the
following treatment will remove 80 or 90 per cent. of the arsenic
contained in the commercial material:--Dissolve 2 or 3 lbs. of ferric
chloride with the smallest amount of water that will effect solution
with the addition of 100 c.c. of hydrochloric acid; add a solution of
sulphurous acid in quantity sufficient to reduce 2 or 3 per cent. of the
iron to the ferrous state; allow to stand a week; and then boil, to
remove the hydrochloric acid added. Nitric acid, which is prejudicial,
is also removed by this treatment.
[107] When the amount of arsenic to be estimated is small (as in refined
coppers), it is better to use a weaker solution of iodine. This is made
by diluting 200 c.c. of the standard solution with water to 1 litre.
Each c.c. will equal 0.1 per cent., if 1 gram of the metal has been
taken for the assay.
[108] The constitution of these phosphates may be thus illustrated--
Magnesic meta-phosphate MgO.P_{2}O_{5}.
Magnesic pyro-phosphate 2MgO.P_{2}O_{5}.
Magnesic ortho-phosphate 3MgO.P_{2}O_{5}.
[109] The composition of which is--
MoO_{2} 90.74,
P_{2}O_{5} 3.14,
(NH_{4})_{2}O 3.57,
H_{2}O 2.55 = 100.00.
[110] This is made by adding 27 grams of magnesium carbonate (a little
at a time) to a solution of 270 grams of citric acid in 350 c.c. of warm
water; and, when dissolved, adding 400 c.c. of dilute ammonia, and
making up the bulk to 1 litre; 20 c.c. of the solution is sufficient for
0.1 gram of P_{2}O_{5}, although more will be required if much iron or
alumina is present.
[111] For the details of the titration, the student is referred to the
same place.
[112] N_{2}O_{5} + 6FeO = 3Fe_{2}O_{3} + 2NO.
CHAPTER XVIII.
SILICON, CARBON, BORON.
SILICON AND SILICATES.
In assaying, more especially products direct from the mine, there is
always found, when the rock is siliceous, a quantity of white
sandy-looking substance, insoluble in acids, which is sometimes
accompanied by a light gelatinous material very difficult to filter.
This is variously described as "insoluble," "sand," "insoluble
silicates," "gangue," or "rocky matter." It may be pure quartz; but
oftener it is mixed with silicates from the rock containing the mineral.
Some silicates, but not many, are completely decomposed by boiling with
hydrochloric acid or aqua regia; and others are partly so, they yield a
gelatinous precipitate of silica which greatly interferes with the
filtering. It is a common practice with assayers to carry the first
attack of the sample with acids to dryness, and to take up with a fresh
portion of acid. By this means the separated silica becomes granular and
insoluble, and capable of being filtered off and washed with comparative
ease.
This residue may be ignited and weighed; and be reported as so much per
cent. of "silica and silicates insoluble in acids." Unless specially
wanted, a determination of its constituents need not be made. When
required, the analysis is best made on the ignited residue, and
separately reported as "analysis of the insoluble portion."
Silicon only occurs in nature in the oxidised state; but the oxide
generally known as silica (SiO_{2}) is common, being represented by the
abundant minerals--quartz, flint, &c. Silica, combined with alumina,
lime, oxide of iron, magnesia and the alkalies, forms a large number of
rock-forming minerals. Most rock masses, other than limestones, contain
over 50 per cent. of silica. The following are analyses of some of the
commoner silicates; but it must be noted that these minerals often show
great variation in composition. This is more especially true of
chlorite, schorl, hornblende and augite.
[Table has been split into two because of its width--Transcriber]
------------------+--------+------------+------------+-------+----------------
| | | Ferric |Ferrous|
| Silica | Alumina | Oxide, | Oxide,| Fluorine,
|SiO_{2}.|Al_{2}O_{3}.|Fe_{2}O_{3}.| FeO. | Water &c.
------------------+--------+------------+------------+-------+----------------
Potash-felspar | 65.2 | 18.2 | 0.2 | -- |
Soda-felspar | 67.0 | 19.2 | -- | 0.3 |
Lime-felspar | 43.3 | 35.4 | -- | 1.3 |
Potash-mica | 45.7 | 33.7 | 3.1 | -- |F (0.8)
| | | | | H_{2}O (4.9)
Magnesia-mica | 39.1 | 15.4 | 7.1 | -- |F (0.7)
Hornblende | 40.6 | 14.3 | 5.8 | 7.2 |
Augite | 50.0 | 3.7 | 2.4 | 6.6 |MnO (0.1)
Almandine (Garnet)| 39.7 | 19.7 | -- | 39.7 |MnO (1.8)
Chlorite (Peach) | 32.1 | 18.5 | -- | -- |H_{2}O (12.1)
Schorl | 37.0 | 33.1 | 9.3 | 6.2 |B_{2}O_{3} (7.7)
| | | | | F (1.5)
China-clay | 46.7 | 39.6 | -- | -- |H_{2}O (13.4)
Talc | 61.7 | -- | -- | 1.7 |H_{2}O (3.8)
Serpentine | 42.9 | -- | -- | 3.8 |H_{2}O (12.6)
Olivine | 39.3 | -- | -- | 14.8 |
------------------+--------+------------+------------+-------+----------------
------------------+-----+--------+-------+--------+------------------------
| | | | |
|Lime,|Magnesia|Potash | Soda, | Fluorine,
|CaO. | MgO. |K_{2}O.|Na_{2}O.| Water, &c.
------------------+-----+--------+-------+--------+------------------------
Potash-felspar | -- | -- | 14.7 | 1.5 |
Soda-felspar | 1.2 | 1.8 | 2.2 | 7.2 |
Lime-felspar |17.4 | 0.35 | 0.5 | 0.9 |
Potash-mica | -- | 1.1 | 7.5 | 2.8 |F (0.8) H_{2}O (4.9)
| | | | |
Magnesia-mica | -- | 23.6 | 7.5 | 2.6 |F (0.7)
Hornblende |12.5 | 14.0 | 1.5 | 1.6 |
Augite |22.8 | 13.5 | -- | -- |MnO (0.1)
Almandine (Garnet)| -- | -- | -- | -- |MnO (1.8)
Chlorite (Peach) | -- | 36.7 | -- | -- |H_{2}O (12.1)
Schorl | 0.5 | 2.6 | 0.7 | 1.4 |B_{2}O_{3} (7.7) F (1.5)
| | | | |
China-clay | -- | -- | -- | -- |H_{2}O (13.4)
Talc | -- | 31.7 | -- | -- |H_{2}O (3.8)
Serpentine | -- | 40.5 | -- | -- |H_{2}O (12.6)
Olivine | -- | 45.8 | -- | -- |
------------------+-----+--------+-------+--------+------------------------
Silicon, from a chemical point of view, is an interesting body. It
combines with iron to form a silicide; and is present in this condition
in cast iron. Only in the case of the analysis of this and similar
substances is the assayer called on to report the percentage of
_silicon_. Silicon is readily converted into silica by the action of
oxidizing agents. Silica forms only one series of salts--the
silicates--which have in many cases a complex constitution; thus there
are a large number of double silicates, which vary among themselves, not
only in the relation of base to acid (which is the essential
difference), but also in the ratio of the bases between themselves
(which varies with almost every specimen).
Silica is detected by heating the substance with a fluoride and
sulphuric acid in a platinum-crucible. On holding a rod, moistened with
a drop of water, over the escaping fumes, the white crust of silica
formed on the drop of water shows its presence. The insolubility of a
fragment of the mineral in a bead of microcosmic salt, is also a very
good test; the fragment, on prolonged heating, does not lose its angular
form.
There is no dry assay for this substance, nor volumetric method; when
the determination is required, it is carried out gravimetrically and,
generally, by the following plan.
If the sample contains oxides, sulphides, &c., in any quantity, these
are first dissolved out by treatment with acid, evaporated to dryness,
taken up with hydrochloric acid, and filtered. The dried residue is
treated in the same way as the silicates. Some silicates are completely
decomposed by such treatment; but it saves time (unless one is sure that
no undecomposable silicate is present) to treat these in the same way as
the others. On the other hand, there are some silicates which are only
attacked with difficulty even by fusion with alkaline carbonates;
consequently, it is always well to have the substance reduced to the
finest state of division by careful powdering, as this greatly assists
the subsequent action. With very hard silicates, the grinding away of
the mortar in this operation will be perceptible; the foreign matter
thus introduced must not be ignored. Previously igniting the substance
sometimes assists the powdering; but it is best to use a steel mortar.
The particles of steel can be removed by a magnet, or, where the nature
of the substance will allow it, by boiling with a little dilute
hydrochloric acid.
The dried and powdered material is intimately mixed with four times its
weight of "fusion mixture" in a platinum-crucible or dish. It is then
moderately heated over a Bunsen burner, and afterwards more strongly
fused over a blast, or enclosed in a clay crucible in the wind-furnace.
The action is continued until the fused mass is perfectly tranquil. With
very refractory substances, the action must be long continued at a high
temperature. When sufficiently cold, the crucible is examined to see
that no particles of foreign matter are adhering to its outer surface.
It is then transferred to a five- or six-inch evaporating-dish, where
its contents are acted upon with warm water for some time. The "melt"
will slowly dissolve, but the solution should be hastened by keeping the
liquid moderately acid with hydrochloric acid. When the "melt" has
dissolved, clean and remove the platinum-dish, and evaporate the
solution to a paste. Continue the evaporation to dryness on a water-bath
(not on the hot plate), and whilst drying stir with a glass rod, feeling
at the bottom of the dish for any unfused particles, which, if present,
can be detected by their grittiness. If there is much grit, it will be
necessary to repeat the assay; but with a small quantity it will only be
necessary to refuse the grit and silica after ignition.
During solution of the "melt" and evaporation (which may be carried on
together), a clear solution will not be obtained, a flocculent silica
will separate out, and towards the end of the evaporation the mass will
get gelatinous. The drying of the jelly must be finished on the
water-bath; first, because at this temperature the silica is rendered
insoluble in hydrochloric acid, whilst the solubility of the alumina,
iron, &c., is unaffected, which would not be the case at a much higher
temperature; and second, because the gelatinous residue requires very
cautious drying to prevent loss from spirting.
When dry, the substance is moistened, and heated with strong
hydrochloric acid, and the sides of the dish are washed down with water.
The silica is washed by decantation two or three times with hydrochloric
acid and hot water, before being thrown on to the filter. The filtrate
is again evaporated to dryness, taken up with a little hydrochloric acid
and water and again filtered. The residue on the filter is silica. The
two lots of silica are washed free from chlorides with hot water, dried
on an air-bath, transferred to a platinum-crucible, ignited gently at
first, at last strongly over the blast or in a muffle, cooled in a
desiccator, and weighed.
The white powdery precipitate is silica (SiO_{2}), and its weight,
multiplied by 100, and divided by the weight of ore taken, gives the
percentage of silica in the sample. Where the percentage of silicon is
wanted, which is very rarely the case, it is got by multiplying this
result by 0.4667. It is always necessary to examine the purity of the
body weighed as silica. This is done by re-fusing the material weighed,
and re-determining the silica in it; or, better, by mixing a weighed
portion in a platinum-dish with a little strong sulphuric acid, covering
with hydrofluoric acid, and evaporating. In the latter case, the silica
will be converted into fluoride, which will be driven off, and the
impurities will be left behind as sulphates of barium, phosphate and
oxide of tin, titanium, &c. This must be weighed and deducted from the
weight of the silica. In a complete examination of a silicate it should
be treated with the precipitate containing alumina, ferric oxide, &c.
EXAMINATION OF SILICATES.
The student interested in the analysis of rocks and rock-forming
minerals is advised to consult a valuable paper by Dr. W.F. Hillebrand
in the _Bulletin of the United States Geological Survey_, _No._ 148, to
which I am very largely indebted in the revision of the following pages.
~Moisture.~--Five grams of the powdered sample is dried between
watch-glasses in the water-oven for two hours, or till its weight is
constant; and the loss is reported as water lost at 100° C. The rest of
the determinations are made on this dried mineral.
~Combined Water, &c.~--Weigh up 1 gram of the substance, and ignite
over the blowpipe for some time in a platinum-crucible, cool in a
desiccator, and weigh. Record the loss as "loss on ignition," not as
"combined water."
~Silica.~--The ignition should have been performed in an oxidising
atmosphere in a muffle or over a slanting blowpipe flame; this will
ensure the oxidation of any pyrites or other sulphide present, which if
unoxidised would injure the crucible in the next operation. The ignited
residue is mixed with 6 or 7 grams of anhydrous sodium carbonate. This
reagent should be the purest obtainable, but its purity should be
checked, or rather its impurities should be determined by running a
"check" or "blank" assay with 10 grams of it through the stages of the
analysis; the impurities will be chiefly silica, alumina and lime, and
altogether they ought not to exceed 1 milligram. The crucible with the
mixture is heated at first gently over a Bunsen and afterwards more
strongly in an oxidising atmosphere in a muffle or over the blowpipe.
The fused mass is allowed to cool in the crucible, and is then dissolved
out in a basin with water and a small excess of hydrochloric acid. After
the removal and cleaning of the crucible, the liquor is evaporated
almost to dryness. Dr. Hillebrand advises stopping short of complete
dryness. The residue is taken up with a little hydrochloric acid and
water and filtered and washed. The liquor, including the washings, is
again evaporated and taken up with water and a little acid. Usually
about 1 per cent. of silica will be thus recovered. It is to be filtered
off and washed and added to the main silica. The filtrate is reserved.
The silica, thoroughly washed, is dried and ignited at a high
temperature for twenty or thirty minutes. It is then weighed in a
platinum crucible. After weighing it is treated with hydrofluoric acid
and a little sulphuric, carefully evaporated and ignited strongly. The
residue, which in extreme cases may amount to 2 or 3 per cent. of the
rock, is weighed and deducted from the weight of the impure silica. It
is retained in the crucible.
~Alumina, &c.~--The filtrate from silica is treated by the basic acetate
method. That is, it is first treated by a cautious addition of a
solution of soda, almost to the point of producing a precipitate, in
order to neutralise the excess of acid; 2 or 3 grams of sodium acetate
are added, and the whole boiled for a minute or so. The precipitate is
filtered off and washed only slightly. Save the filtrate. The
precipitate is dissolved in hydrochloric, or, perhaps better, in nitric
acid; and is reprecipitated by adding an excess of ammonia and boiling.
The precipitate is filtered and washed with water containing 2 per cent.
of ammonium nitrate. Both filtrates are evaporated separately to a small
bulk, a drop or two of ammonia being added to the second towards the
finish. They are next filtered into a 6 or 8-ounce flask through a small
filter, the second filtrate coming after, and serving in a manner as
wash water for the first[113]. The two washed alumina precipitates are
dried and placed in the platinum crucible containing the residue from
silica after treatment with hydrofluoric acid. They are then ignited in
an oxidising atmosphere at a high temperature for about 10 minutes. The
weight, including that of the residue from the silica, is noted as that
of "alumina, &c."
The weighed oxides are next fused with bisulphate of potash for some
hours. The bisulphate should have been first fused, apart, until the
effervescence from the escape of steam has stopped. The melt is
dissolved out with cold water and dilute sulphuric acid, and any
insoluble residue is filtered off, washed, ignited and weighed. The
filtrate is reserved for determinations of iron and titanium. The
residue, after weighing, may be treated with hydrofluoric and sulphuric
acids for any silica,[114] which would be determined by loss. It may be
tested for barium sulphate by treatment with hot strong sulphuric acid;
in which this salt dissolves, but is again insoluble (and so comes out
as a white precipitate) on diluting with cold water; the acid also must
be cold before adding the water. The filtrate containing the iron is
reduced with sulphuretted hydrogen, boiled till free from that gas,
filtered and titrated with a standard solution of permanganate of
potassium. The iron found is calculated to ferric oxide by dividing by
.7. The iron solution after titration serves for the determination of
titanium oxide (TiO_{2}). This is done colorimetrically, by adding
peroxide of hydrogen free from hydrofluoric acid, and comparing the
brown colour produced with that produced by the addition of a standard
solution of titanium to an equal volume of water containing sulphuric
acid.[115] The alumina is determined by difference. From the weight of
the combined precipitate which has been recorded as "Alumina, &c.,"
deduct (1) the residue, insoluble, after fusion with bisulphate; (2) the
ferric oxide; (3) the titanium oxide; and (4) the phosphoric oxide
(P_{2}O_{5}), the amount of which is subsequently determined in a
separate portion. This gives the alumina.
~Manganous oxide, &c.~--The filtrate from the "alumina, &c." contained
in a 6 or 8-ounce flask, which it nearly fills, is made slightly
alkaline with ammonia and treated with a small excess of ammonium
sulphide; the flask is then corked and placed on one side for some time
(a day or so) so that the manganese sulphide may separate. The
precipitate is filtered off and washed with water containing ammonium
chloride and a few drops of ammonium sulphide. The filtrate is reserved
for lime, &c. The precipitate is digested with sulphuretted hydrogen
water, to which one-fifth of its volume of strong hydrochloric acid has
been added; this dissolves the sulphides of zinc and manganese; any
black residue should be tested for copper and perhaps nickel. The
solution is evaporated to dryness, taken up with a little water and
treated with a small excess of solution of carbonate of soda. It is
boiled and again evaporated, washed out with hot water and filtered on
to a small filter, dried, ignited, and weighed as Mn_{3}O_{4}. It is
calculated to MnO. It may contain, and should be tested for oxide of
zinc, which, if present, must be deducted. If the dish becomes stained
during evaporation, take up with a few drops of hydrochloric and
sulphurous acids, evaporate, and then treat with carbonate of soda.
~Lime, &c.~--The filtrate from the manganese sulphide is boiled, and
without cooling, treated with ammonium oxalate in solution, which also
should be heated to boiling. The liquid is filtered off and reserved for
magnesia. The precipitate is dissolved in very little hydrochloric acid
and reprecipitated by adding ammonium oxalate and ammonia to the boiling
solution. The filtrate and washings from this are reserved for magnesia.
The precipitate is either dissolved in dilute sulphuric and titrated
with permanganate of potash as described under Lime (p. 322); or it is
ignited and weighed as oxide. In this last case it may be examined for
barium and strontium, the former of which will rarely be present.
~Magnesia.~--The filtrate from the first lime precipitate is treated
with sodium phosphate and ammonia, and allowed to stand overnight. It is
then filtered. The precipitate is dissolved in hydrochloric acid; the
solution is filtered into the beaker containing the solution from the
second lime precipitate. Ammonia and sodium phosphate are again added,
and the precipitate, after standing, is filtered off, washed with water
containing ammonia; it is then dried, ignited and weighed as magnesium
pyrophosphate. This is calculated into magnesia.
~Potash and Soda.~--Weigh out .5 gram of the dried ore, and mix with an
equal quantity of ammonic chloride; and to the mixture add gradually 4
grams of calcium carbonate ("precipitated"). Introduce into a
platinum-crucible and cover loosely. Heat, at first, gently; and then at
a red heat for from forty to sixty minutes. Transfer to a porcelain
dish, and digest with 60 or 80 c.c. of water; warm and filter: to the
filtrate add ammonic carbonate and ammonia, and filter; evaporate the
filtrate to dryness, adding a few drops more of ammonic carbonate
towards the end; when dry, heat gently, and then raise the temperature
to a little below redness. Dissolve in a small quantity of water, add a
drop of ammonic carbonate, and filter through a small filter into a
weighed platinum dish. Evaporate, ignite gently, and weigh. The residue
contains the soda and potash of the mineral as chlorides.
To determine the proportion of potassium, dissolve this residue in a
little water, add platinum chloride in excess, evaporate to a paste,
extract with alcohol, decant through a small weighed filter, wash with
alcohol, and dry at 100° C. Weigh. The substance is potassium platinic
chloride (2KCl.PtCl_{4}). Its weight, multiplied by 0.1941, will give
the weight of potash (K_{2}O).
To find the proportion of soda, multiply the weight of the potassium
platinic chloride by 0.306; this gives the weight of potassium chloride.
Deduct this from the weight of the mixed chlorides first got; the
difference will be the sodium chloride, which weight, multiplied by
0.53, will give the weight of soda (Na_{2}O).
~Ferrous Oxide.~--When a qualitative test shows both ferric and ferrous
oxide to be present, the proportion of the ferrous oxide must be
separately determined. The finely ground mineral mixed with dilute
sulphuric acid is treated on a water bath with hydrofluoric acid.
Solution is best effected in an atmosphere of carbonic acid. In about an
hour the decomposition is complete, and the solution is diluted with
cold water, and titrated with the solution of bichromate or of
permanganate of potassium. The iron found is multiplied by 1.286, and
reported as ferrous oxide. To find the proportion of ferric oxide, the
ferrous iron found is multiplied by 1.428, and this is deducted from the
weight of ferric oxide obtained by precipitation with ammonia. The
ammonia precipitate contains the whole of the iron as ferric oxide;
hence the necessity for calculating the ferrous oxide as ferric, and
deducting it.
~Phosphoric Oxide (P_{2}O_{5}).~--Weigh up 5 grams of the finely-divided
and dry sample, and digest with 10 or 20 c.c. of nitric acid; evaporate
to dryness on the water-bath; take up with a little dilute nitric acid;
dilute with water; and filter. Add a few grams of ammonic nitrate and 10
c.c. of ammonium molybdate solution, heat nearly to boiling, and allow
to settle; filter off, and wash the yellow precipitate. Dissolve with
dilute ammonia, add "magnesia mixture," and allow to stand overnight.
Filter, wash with dilute ammonia, dry, ignite, and weigh as
pyrophosphate of magnesia. The weight, multiplied by 0.6396, gives the
weight of phosphoric oxide.
~Soluble Silica.~--Some silicates are acted on by hydrochloric acid, and
leave on evaporation a residue; which, when the soluble salts have been
washed out, consists generally of the separated silica with perhaps
quartz and unattacked silicates. It should be ignited, weighed and
boiled with a solution containing less than 10 per cent. of caustic
soda: this dissolves the separated silica. The liquor is diluted,
rendered faintly acid, and filtered. The residue is washed, ignited and
weighed. The loss gives the soluble silica.
~Estimation of Silica in Slags~ (Ferrous silicates).--Take 1 gram of the
powdered slag, treat with aqua regia, evaporate to dryness, extract with
hydrochloric acid, filter, dry, ignite, and fuse the ignited residue
with "fusion mixture," then separate and weigh the silica in the usual
way. Slags are for the most part decomposed by boiling with aqua regia,
but it will be found more convenient and accurate to first extract with
acids and then to treat the residue as an insoluble silicate.
~Estimation of "Silica and Insoluble Silicates" in an Ore.~--Take 2
grams of the powdered mineral, evaporate with nitric acid (if sulphides
are present), treat the dried residue (or the original substance if
sulphides are absent) with 10 or 20 c.c. of hydrochloric acid; again
evaporate to dryness, take up with dilute hydrochloric acid, filter,
wash, ignite, and weigh.
~Estimation of Silicon in Iron.~--Place 2 grams of the metal (borings or
filings) in a four-inch evaporating dish, and dissolve (with aid of
heat) in 25 c.c. of dilute nitric acid. Evaporate to complete dryness,
take up with 20 c.c. of hydrochloric acid, and allow to digest for one
hour. Boil down to a small bulk, dilute with a 5 per cent. solution of
hydrochloric acid, boil, and filter. Wash with acid and water, dry,
ignite in a platinum crucible, and weigh the SiO_{2}. This, multiplied
by 0.4673, gives the weight of the silicon. The percentage is calculated
in the usual way.
PRACTICAL EXERCISES.
1. A certain rock is a mixture of 70 per cent. of quartz, 25 per cent.
of potash-felspar, and 5 per cent. of potash-mica. What per cent. of
silica will it contain?
2. Two grams of a mixture of silica and cassiterite left, after
reduction in hydrogen, 1.78 grams. Assuming all the oxide of tin to have
been reduced, what will be the percentage of silica?
3. The formula of a compound is 2FeO.SiO_{2}. What percentage of silica
will it contain?
4. Two grams of a sample of cast-iron gave 0.025 gram of silica. Find
the percentage of silicon in the metal.
5. What weights of quartz and marble (CaCO_{3}) would you take to make
30 grams of a slag having the formula CaO.SiO_{2}?
CARBON AND CARBONATES.
Carbon compounds enter so largely into the structure of organised bodies
that their chemistry is generally considered apart from that of the
other elements under the head of _Organic Chemistry_. Carbon occurs,
however, among minerals not only in the oxidised state (as carbonates),
but also in the elementary form (as in diamond and graphite), and
combined with hydrogen, oxygen, &c. (as in petroleums, bitumens,
lignites, shales, and coals). In small quantities "organic matter" is
widely diffused in minerals and rocks. In shales and clays it may amount
to as much as 10 or 20 per cent. (mainly as bituminous and coaly
matters).
The assayer has only to take account of the organic matter when it is of
commercial importance, so that in assays it is generally included under
"loss on ignition."
In coals, shales, lignites, &c., the carbon compounds are, on heating,
split up into oils and similar compounds. The products of distillation
may be classified as water, gas, tars, coke, and ash. The assay of these
bodies generally resolves itself into a distillation, and, in the case
of the shales, an examination of the distillates for the useful oils,
paraffin, creosote, &c., contained in them.
Elementary carbon is found in nature in three different forms, but these
all re-act chemically in the same way. They combine with oxygen to form
the dioxide.[116] The weight of oxygen required to burn a given weight
of any form of carbon is the same, and the resulting product from all
three has the same characteristic properties. Carbon dioxide is the
common oxide of carbon. A lower oxide exists, but on burning it is
converted into the dioxide. Wherever the oxidation of carbon takes
place, if there is sufficient oxygen, carbon dioxide (carbonic acid) is
formed; this re-action is the one used for the determination of carbon
in bodies generally. The dioxide has acid properties, and combines with
lime and other bases forming a series of salts called carbonates.
The carbon-compounds (other than carbonates, which will be subsequently
considered) occurring in minerals are generally characterised by their
sparing solubility in acids. The diamond is distinguished from other
crystals by its hardness, lustre, and specific gravity. It may be
subjected to a red heat without being apparently affected, but at a
higher temperature it slowly burns away. Graphite, also, burns slowly,
but at a lower temperature. The other bodies (coals, shales, &c.) differ
considerably among themselves in the temperature at which they commence
to burn. Some, such as anthracite, burn with little or no flame, but
most give off gases, which burn with a luminous flame. They deflagrate
when sprinkled on fused nitre, forming carbonate of potash. In making
this test the student must remember that sulphur and, in fact, all
oxidisable bodies similarly deflagrate, but it is only in the case of
carbon compounds that carbonate of potash is formed. Carbon unites with
iron and some of the metals to form carbides; combined carbon of this
kind is detected by the odour of the carburetted hydrogen evolved when
the metal is treated with hydrochloric acid; for example, on dissolving
steel in acid.
The natural carbon compounds, although, speaking generally, insoluble in
hydrochloric or nitric acids, are more or less attacked by aqua regia.
The assayer seldom requires these compounds to be in solution. The
presence of "organic matter"[117] interferes with most of the reactions
which are used for the determination of the metals. Consequently, in
such cases, it should be removed by calcination unless it is known that
its presence will not interfere. When calcination is not admissible it
may be destroyed by heating with strong sulphuric acid and bichromate or
permanganate of potash or by fusion with nitre.
Carbon may be separated from other substances by conversion into carbon
dioxide by burning. In most cases substances soluble in acids are first
removed, and the insoluble residue dried, weighed, and then calcined or
burned in a current of air. The quantity of "organic matter" may be
determined indirectly by the loss the substance undergoes, but it is
better to determine the "organic carbon" by confining the calcination in
a tube, and collecting and weighing the carbon dioxide formed. Each gram
of carbon dioxide is equivalent to 0.2727 gram of carbon.
[Illustration: FIG. 70.]
[Illustration: FIG. 71.]
Instead of a current of oxygen or air, oxide of copper may be more
conveniently used. The operation is as follows:--Take a clean and dry
piece of combustion tube drawn out and closed at one end, as shown in
the figure (fig. 70), and about eighteen inches long. Fit it with a
perforated cork connected with a ~U~-tube (containing freshly-fused
calcium chloride in coarse grains) and a set of potash bulbs (fig. 71)
(containing a strong solution of potash), the exit of which last is
provided with a small tube containing calcium chloride or a stick of
potash. Both the ~U~-tube and bulbs should have a loop of fine wire, by
which they may be suspended on the hook of the balance for convenience
in weighing. They must both be weighed before the combustion is
commenced; to prevent absorption of moisture during weighing, &c., the
ends are plugged with pieces of tube and glass rod.
Fill the combustion tube to a depth of about eight inches with some
copper oxide, which has been recently ignited and cooled in a close
vessel. Put in the weighed portion for assay and a little fresh copper
oxide, and mix in the tube by means of an iron wire shaped at the end
after the manner of a corkscrew. Put in some more oxide of copper, and
clean the stirrer in it. Close loosely with a plug of recently ignited
asbestos, place in the furnace, and connect the ~U~-tube and bulbs in
the way shown in the sketch (fig. 72).
[Illustration: FIG. 72.]
See that the joints are tight, and then commence the combustion by
lighting the burners nearest the ~U~-tube; make the first three or four
inches red hot, and gradually extend the heat backwards the length of
the tube, but avoid too rapid a disengagement of gas. When gas ceases to
come off, open the pointed end of the tube and draw a current of dried
air through the apparatus.
The carbon dioxide is absorbed in the potash bulbs, and their increase
in weight multiplied 0.2727 gives the amount of carbon in the substance
taken.
The increase in weight in the calcium chloride tube will be due to the
water formed by the oxidation of the combined hydrogen. If this last is
required the increase in weight multiplied by 0.111 gives its amount.
COALS.
The determination of the actual carbon in coals and shales is seldom
called for; if required, it would be performed in the way just
described.[118] The ordinary assay of a sample of coal involves the
following determinations--moisture, volatile matter, fixed carbon, ash,
and sulphur. These are thus carried out:--
~Determination of Moisture.~--Take 3 grams of the powdered sample and
dry in a water-bath for an hour or so. The loss is reported as moisture.
Coals carry from 1 to 2 per cent. If the drying is carried too far,
coals gain a little in weight owing to oxidation, so that it is not
advisable to extend it over more than one or two hours.
~Determination of Volatile Matter.~--This determination is an
approximate one, and it is only when working under the same conditions
with regard to time, amount of coal taken, and degree of heat used, that
concordant results can be arrived at. It is a matter of importance
whether the coal has been previously dried before heating or not, since
a difference of 2 per cent. may be got by working on the dried or
undried sample. Take 2 grams of the powdered, but undried, sample of
coal, place in a weighed platinum crucible, and support this over a good
Bunsen burner by means of a thin platinum-wire triangle. The heat is
continued until no further quantity of gas comes off and burns at the
mouth. This takes only a few minutes. The cover is tightly fitted on,
and when cold the crucible is weighed. The loss in weight, after
deducting the moisture, gives the "volatile matter," and the residue
consists of "fixed carbon" and "ash."
~Determination of Ash.~--The coke produced in the last operation is
turned out into a porcelain dish and ignited over a Bunsen burner till
the residue is free from particles of carbon. Calcination is hastened by
stirring with a platinum wire. The operation may be done in a muffle,
but this gives results a few tenths of a per cent. too low. The dish is
cooled in a dessicator, and weighed. The increase in weight gives the
amount of "ash," and the difference between this and the weight of the
coke gives the "fixed carbon."
The assay is reported as follows:--
Moisture at 100° C. ---- per cent.
Volatile matter ---- "
Fixed carbon ---- "
Ash ---- "
contains sulphur ---- per cent.
~Determination of Sulphur.~--The sulphur exists in the coal partly in
organic combination, partly as metallic sulphide (iron pyrites,
marcasite, &c.), and, perhaps, as sulphate. So that the sulphur
determination must be separately reported, since a portion will go off
with the volatile matter, and the remainder would be retained and
weighed with the coke.
The sulphur is thus determined:--Take 1 gram of the coal and mix with
1.5 gram of a mixture of 2 parts of calcined magnesia and 1 part of
carbonate of soda, and heat in a platinum crucible for one hour or until
oxidation is complete. Turn out the mass and extract it with water and
bromine, filter, acidulate with hydrochloric acid, boil off the bromine,
and precipitate with baric chloride (estimating gravimetrically as given
under _Sulphur_). Another method is as follows:--Take 1 gram of the coal
and drop it gradually from a sheet of note paper on to 5 grams of fused
nitre contained in a platinum dish. Extract with water, acidify with
acetic acid, and estimate volumetrically as described under _Sulphur_.
[Illustration: FIG. 73.]
~Calorific Effect of Coals.~--The heat-giving value of a coal is best
expressed in the number of pounds of water, previously heated to the
boiling point, which it will convert into steam. This is generally
termed its evaporative-power. It may be determined by means of the
calorimeter (fig. 73). This consists of a glass cylinder marked to hold
29.010 grains of water. The instrument consists of a perforated copper
stand, provided with a socket and three springs. The socket holds a
copper cylinder which is charged with 30 grains of the dried coal mixed
with 300 grains of a mixture of 3 parts of potassium chlorate and 1 part
of nitre. The charge is well packed in the cylinder and provided with a
small fuse of cotton saturated with nitre. Fill the glass cylinder to
its mark with water and take the temperature with a thermometer marked
in degrees Fahrenheit. Ignite the fuse and immediately cover with the
outer copper cylinder (extinguisher-fashion), which will be held in its
place by the springs. The stop-cock should be closed before this is
done. Place the apparatus quickly in the cylinder of water. When the
action is over open the stop-cock and agitate the water by raising and
lowering the instrument a few times. Again take the temperature. The
rise in temperature, plus 10 per cent. for the heat used in warming the
apparatus and lost by radiation, gives the evaporative-power.
The following is an example:--
Temperature before experiment 67.0° F.
Temperature after " 79.0° "
--------
Rise 12.0° "
+ 1/10th 1.2° "
--------
Gives 13.2° "
One pound of the coal will evaporate 13.2 pounds of water.
SHALES, ETC.
The assay of these is carried out in the same way as that of coals, but
the volatile matters are separately examined, and, in consequence, a
larger quantity of material must be used. For the moisture, volatile
matter, fixed carbon and ash, the determinations are the same, but a
special distillation must be made to obtain a sufficient quantity of the
volatile products for subsequent examination. Take 500 or 1000 grams of
the well-sampled and powdered shale, and introduce into a cast-iron
retort as shown in fig. 74. Lute the joint with fire-clay, place the
cover on, and bolt it down. The bolts should have a covering of
fire-clay to protect them from the action of the fire. Place the retort
in a wind furnace, supporting it on a brick, and pack well around with
coke. Build up the furnace around and over the retort with loose
fire-bricks, and heat gradually.
[Illustration: FIG. 74.]
As soon as water begins to drip, the tube of the retort is cooled by
wrapping a wet cloth around it, and keeping wet with water. The water is
kept from running into the receiver by a ring of damp fire-clay. A
quantity of gas first comes over and will be lost, afterwards water and
oily matters. The retort must be red hot at the close of the
distillation, and when nothing more distils off, which occurs in about
two or three hours, the wet cloth is removed, and the tube heated with a
Bunsen burner to drive forward the matter condensed in it into the
receiver, and thus to clean the tube. It can be seen when the tube is
clean by looking up through it into the red-hot retort. The receiver is
then removed, and the retort, taken from the furnace, is allowed to
cool. When cold it is opened, and the fixed carbon and ash weighed, as a
check on the smaller assay.
The distillate of water and oil is warmed, and will separate into two
layers, the upper one of which is oil, and the lower water. These are
measured, and if the specific gravity of the oil is taken, its weight
may be calculated. If the two liquids do not separate well, the water
may be filtered off, after cooling, through a damped filter. The
separation is, however, best effected in a separator (fig. 75). The
liquids are poured into this, allowed to settle, and the lower layer
drained off. The volume of the water is measured and its weight
calculated in per cents. on the amount of shale taken.
[Illustration: FIG. 75.]
~Examination of the Oil.~--A sufficient quantity of the oil must be got,
so that if one distillation does not yield enough, the requisite
quantity must be obtained by making two or more distillations. The oils
are mixed, and the mixture, after having had its volume and specific
gravity ascertained, is placed in a copper retort, and re-distilled with
the aid of a current of steam. The residue in the retort is coke.
The distillate is separated from the water by means of the separator,
and shaken for ten minutes with one-twentieth of its bulk of sulphuric
acid (sp. g. 1.70). The temperature should not be allowed to rise above
40°. Allow to stand, and run off the "acid tar."
The oil is now shaken up with from 10 c.c. to 20 c.c. of sodic hydrate
solution (sp. g. 1.3), allowed to stand, warmed for half-an-hour, and
the "soda-tar" run off.
On mixing this soda-tar with dilute acid, the "crude shale oil creosote"
separates, and is measured off.
The purified oil is next re-distilled in fractions, which come over in
the following order:--"Naphtha," "light oil," "heavy oil," and "still
bottoms." For the first product, which is only got from certain shales,
the receiver is changed when the distillate has a specific gravity of
0.78. For the second product the process is continued till a drop of the
distillate, caught as it falls from the neck of the retort on a cold
spatula, shows signs of solidifying. This is "crude light oil."
The receiver is changed, and the "heavy oil" comes over; towards the end
a thick brown or yellow viscid product is got. The receiver is again
changed, and the distillation carried to dryness.
The "crude light oil" is washed cold with 2 per cent. of sulphuric acid
(concentrated), and afterwards with excess of soda. Thus purified it is
again distilled to dryness, three fractions being collected as before.
Naphtha, which is added to the main portion, and measured; "light oil,"
which is also measured; and "heavy oil," which is added to that got in
the first distillation. This last is poured into a flat-bottom capsule,
and allowed to cool slowly. The temperature may with advantage be
carried below freezing-point. The cooled cake is pressed between folds
of linen, and the paraffin scale detached and weighed.
The results may be reported thus:--
Naphtha, sp. g. ----
Light oil, sp. g. ----
Heavy oil, sp. g. ----
Paraffin scale ----
Coke, &c. ----
The results are calculated in per cents. on the oil taken. Some workers
take their fractions at each rise of 50° C. The composition of average
shale, as given by Mills, is as follows:--Specific gravity, 1.877;
moisture, 2.54.
Gas }
Volatile matter, water, ammonia } 23.53
Oil }
Fixed carbon 12.69
Ash 63.74
_____
99.96
The ash is made up of silica, 55.6; ferric oxide, 12.2; alumina, 22.14;
lime, 1.5; sulphur, 0.9; soluble salts (containing 0.92 per cent.
sulphuric oxide), 8.3.
Total sulphur in shale 1.8 per cent.
" " in ash 1.3 "
For further information on these assays, and for the assay of
petroleums, bitumens, &c., the student is referred to Allen's
"Commercial Organic Analysis," Vol. II.
~Determination of Organic Carbon in a Limestone.~--Take 1 or 2 grams and
dissolve with a very slight excess of dilute hydrochloric acid,
evaporate to dryness, and determine the carbon in the residue by
combustion with copper oxide.
~Estimation of Carbon in a Sample of Graphite (Black-lead).~--Weigh up 1
or 2 grams in a dish and calcine in the muffle till the carbon is burnt
off. Weigh the residue, and calculate the carbon by difference.
[Illustration: FIG. 76.]
[Illustration: FIG. 77.]
[Illustration: FIG. 78.]
~Determination of Carbon in Iron.~--The carbon exists in two
states--free (graphite) and combined. The following process estimates
the total carbon:--The carbon existing as graphite may be separately
estimated in another portion by the same process, but using hydrochloric
acid to dissolve the iron instead of the copper solution:--Weigh up 2
grams of the iron (or a larger quantity if very poor in carbon), and
attack it with 30 grams of ammonic-cupric chloride[119] dissolved in 100
c.c. of water. Let the reaction proceed for a quarter-of-an-hour, and
then warm until the copper is dissolved. Allow to settle, and filter
through a filtering-tube. This is a piece of combustion tube drawn out
and narrowed at one end, as shown in fig. 76. The narrow part is blocked
with a pea of baked clay, and on this is placed half-an-inch of silica
sand (previously calcined to remove organic matter), then a small plug
of asbestos, and then a quarter-of-an-inch of sand. The tube is
connected with a pump working at a gentle pressure, and the solution is
filtered through the tube with the aid of a small funnel (fig. 77). The
residue is washed, first with dilute hydrochloric acid, and then with
distilled water. The tube is dried by aspirating air through it, and
gently warming with a Bunsen burner. The tube is then placed in a small
combustion-furnace, and connected with calcium chloride and potash
bulbs, as shown in fig. 78. The potash bulb to the right of the figure
must be weighed. A slow stream of air is drawn through the apparatus,
and the heat gradually raised; in from thirty minutes to one hour the
combustion will be complete. The potash bulbs are then disconnected and
weighed, and the increase multiplied by 0.2727 gives the weight of
carbon.
CARBONATES.
Carbon dioxide, which is formed by the complete oxidation of carbon, is
a gas with a sweetish odour and taste, having a strong affinity for
alkalies, and forming a series of compounds termed carbonates. The gas
itself occurs in nature, and is sometimes met with in quantity in
mining. The carbonates occur largely in nature, forming mountain masses
of limestone, &c. Carbonates of many of the metals, such as carbonate of
lead (cerussite), carbonate of iron (chalybite), carbonates of copper
(malachite and chessylite), and carbonate of magnesia (magnesite), are
common.
All the carbonates (those of the alkalies and alkaline earths excepted)
are completely decomposed on ignition into the oxide of the metal and
carbon dioxide; but the temperature required for this decomposition
varies with the nature of the base. All carbonates are soluble with
effervescence in dilute acids; some, such as chalybite and magnesite,
require the aid of heat. The alkaline carbonates are soluble in water;
the rest, with the exception of the bicarbonates, are insoluble therein.
Carbonates are recognised by their effervescence with acids--a stream of
bubbles of gas are given off which collect in the tube, and possess the
property of extinguishing a lighted match. The most characteristic test
for the gas is a white precipitate, which is produced by passing it into
lime or baryta-water, or into a solution of subacetate of lead.
The expulsion of carbon dioxide by the stronger acids serves for the
separation of this body from the other acids and bases.
~Dry Assay.~--There is no dry assay in use. Any method which may be
adopted will necessarily be applicable only to special compounds.
WET METHODS.
There are several methods in use which leave little to be desired either
in speed or accuracy. We will give (1) a gravimetric method in which the
estimation may be made directly by weighing the carbonic acid, or,
indirectly, by estimating the carbon dioxide from the loss; (2) a
volumetric one, by which an indirect determination is made of the gas;
and (3) a gasometric method, in which the volume of carbon dioxide given
off is measured, and its weight deducted.
[Illustration: FIG. 79.]
[Illustration: FIG. 80.]
~Direct Gravimetric Method.~--Fit up the apparatus shown in the diagram
(fig. 79). The various tubes are supported by a fixed rod with nails and
wire loops, and connected by short lengths of rubber-tubing. The first
tube contains soda-lime. The small flask is fitted with a rubber-stopper
perforated with two holes, through one of which passes the tube of a
pipette holding 25 or 30 c.c. This pipette is to contain the acid. The
substance to be determined is weighed out into the flask. The second
tube contains strong sulphuric acid; the third, pumice stone, saturated
with copper sulphate solution, and dried until nearly white (at 200° C.);
the fourth contains recently fused calcium chloride; and the fifth,
which is the weighed tube in which the carbonic acid is absorbed,
contains calcium chloride and soda-lime,[120] as shown in fig. 80. The
sixth also contains calcium chloride and soda-lime; its object is to
prevent the access of moisture and carbonic acid to the weighed tube
from this direction; it is connected with an aspirator.
Having weighed the ~U~-tube and got the apparatus in order, weigh up 1,
2, or 5 grams of the substance and place in the flask. Fill the pipette
with dilute acid, close the clamp, and cork the flask. Then see that the
apparatus is tight. Open the clamp and allow from 10 to 20 c.c. of the
acid to run on to the assay. Carbonic acid will be evolved and will be
driven through the tubes. The gas should bubble through the sulphuric
acid in a moderate and regular stream. When the effervescence slackens
the clamp is opened and the greater part of the remaining acid run in.
When the effervescence has ceased the clamp is opened to its full extent
and a current of air drawn through with an aspirator. A gentle heat is
applied to the flask; but it should not be prolonged or carried to
boiling. After the removal of the heat a gentle current of air is drawn
through the apparatus for 30 or 40 minutes. The weighed ~U~-tube, which
in the early part of the operation will have become warm if much
carbonic acid was present, will by this time be cold. It is
disconnected, plugged, and weighed. The increase in weight is due to the
carbon dioxide of the sample.
_Example._--Ore taken 1 gram.
Weight of tube, before 42.6525 grams
" " after 43.0940 "
-------
Increase equals CO_{2} 0.4415 "
[Illustration: FIG. 81.]
~Indirect Gravimetric, or Determination by Loss.~--Take a Geissler's
carbonic-acid apparatus (fig. 81) and place in the double bulb some
strong sulphuric acid. Put into the other bulb, the stopcock being
closed, 3 or 4 c.c. of nitric acid diluted with water. Leave the
apparatus in the balance-box for a few minutes and weigh. Introduce into
the flask (through A) about 1 gram of the powdered substance and again
weigh to find the exact amount added. Allow the acid to run gradually on
to the carbonate, and when solution is complete, heat and aspirate. Cool
and again weigh; the loss in weight is the carbonic acid.
For _Example_:--
Weight of apparatus and acids 85.494 grams
" " marble 86.879 "
------
Equal to marble taken 1.385 "
Weight of apparatus and marble 86.879 grams
" " minus carbonic acid 86.2692 "
-------
Equal to carbonic acid 0.6098 "
1.385 : 100 :: 0.6098 : _x_
_x_ = 44.03 per cent.
The substance contains 44.03 per cent. of carbonic acid; a duplicate
experiment gave 43.73 per cent.
This method is quicker, but less exact, than the direct gravimetric
determination.
VOLUMETRIC METHOD.
This, which is of somewhat limited application, is based upon the
determination of the quantity of acid required to decompose the
carbonate. It consists in adding to a weighed quantity of the mineral a
known amount of standard solution of acid which is in excess of that
required to effect the decomposition. The quantity of residual acid is
then determined by titrating with standard solution of alkali. This
method has been described under _Lime_.
GASOMETRIC METHOD.
This method is the quickest of all, and the least troublesome after the
apparatus has been once prepared. It yields fairly accurate results when
worked in the manner described below; but if greater precautions are
taken the results are exact. It depends on the measurement of the volume
of gas given off on treating the weighed sample with acid. The apparatus
described, page 52, is used. Weigh out a portion of the mineral which
shall contain not more than 0.15 gram of carbonic acid (or 0.4 gram of
carbonate of lime) and put it in the bottle. Put in the inner tube 10
c.c. of dilute hydrochloric acid (1--1), cork tightly, and read off the
level of the liquid in the burette after adjusting the pressure. Turn
the acid over on to the mineral. Run out the water so as to keep the
level in the two burettes the same. When effervescence has ceased,
rotate the contents of the bottle; finally, adjust the level in the
burettes and read off the volume. The increase in volume is due to the
evolved carbon dioxide. At the same time read off the "volume
corrector."
Some of the carbon dioxide remains dissolved in the acid in the
generating bottle, and the quantity thus dissolved will depend on the
amount of carbonate as well as on the amount of acid present.
Consequently, a measured quantity of acid should be used in each assay
and a comparative experiment made with a known weight of pure carbonate
of lime which will yield about the same volume of gas. The number of
c.c. of gas got in the assay multiplied by 4.7 will give the number of
milligrams of pure carbonate of lime that must be taken for the
standard. With ordinary work the error rarely exceeds half a c.c.
The following example will illustrate the calculations:--
One gram of a mineral was taken, and yielded 49.0 c.c. of gas. The
"volume corrector" reading was 100.4 c.c.
0.2405 gram of pure carbonate of lime was then taken, and treated in the
same way; 50.5 c.c. of gas were got. The volume corrector still read
100.4 c.c.
0.2405 gram of carbonate of lime is equivalent to 0.1058 gram of carbon
dioxide; then,
50.5 : 49.0 :: 0.1058 : _x_
_x_ = 10.26 per cent.
~Estimation of Carbonic Acid in the Air of Mines.~--According to a
series of analyses by Angus Smith, the proportion of carbonic acid in
the air of underground workings varied from 0.04 to 2.7 per cent. by
volume. In places where men are working the proportion ought not to
reach 0.25 per cent.
A simple method of determining whether a sample of air reaches this
limit (0.25 per cent.) is described by Dr. C. Le Neve Foster in the
"Proceedings of the Mining Association and Institute of Cornwall" for
1888. The apparatus used is an ordinary corked 8-ounce medicine bottle.
This is filled with the air to be examined by sucking out its contents
with a piece of rubber-tube. Half-an-ounce of dilute lime-water[121]
(tinted with phenolphthalein) is poured in. If, on corking the bottle
and shaking, the colour is not discharged, the air contains less than
0.25 per cent. of carbon dioxide. "If the colour fades slowly, and does
not finally vanish till after a great deal of shaking, it may be assumed
that the percentage of carbon dioxide does not greatly exceed one
quarter; whereas, if the disappearance is rapid after a very few shakes,
the contrary, of course, is the case." The dilute lime-water is measured
out and carried in ordinary half-ounce phials. This method does not
pretend to great accuracy, but as a method of distinguishing between
good and bad air it is very convenient, and will be found useful.
For determining the actual proportion in the air the following plan is
adopted:--Take a bottle which will hold about 50 ounces, and measure its
capacity; fill the bottle with the air to be examined, pour in 100 c.c.
of lime-water, and shake up for some time; add phenolphthalein, and
titrate the remaining calcium hydrate with standard solution of oxalic
acid.
The solution of oxalic acid is made by dissolving 2.25 grams of
re-crystallised oxalic acid (H_{2}C_{2}O_{4}.2H_{2}O) in water and
diluting to 1 litre. One c.c. = 0.001 gram of lime (CaO), or 0.0007857
gram of carbon dioxide.
Take 100 c.c. of the same lime-water, to which add the same amount of
phenolphthalein as before. Titrate. The difference between the two
readings gives the amount of "acid" equivalent to the lime-water
neutralised by the carbon dioxide. The number of c.c. thus used up, when
multiplied by 0.3989, gives the number of c.c. of carbon dioxide (at
0° C. and 760 mm.) in the volume of air taken. This volume, which is that
of the bottle less 100 c.c., must in accurate work be reduced to the
normal temperature and pressure.[122] The percentage by volume can then
be calculated.
PRACTICAL EXERCISES.
1. In a gasometric determination 71.3 c.c. of gas were obtained from
0.2055 gram of mineral. The "volume corrector" reading was 102.2 c.c.
0.3445 gram of pure carbonate of lime gave 74.1 c.c. The "volume
corrector" reading was 100.6. What is the percentage of carbon dioxide
in the substance?
2. What volume of dry gas at 0° C. and 760 m.m. pressure should be
obtained from 0.3445 gram of carbonate of lime? 1 c.c. of CO_{2} under
these conditions weighs 1.97 milligrams.
3. A sample of coal is reported on as follows:--
Specific gravity 1.315
Moisture 1.001
Volatile matter 35.484
Fixed carbon 50.172
Ash 12.028
-------
100.000
What is there about this requiring explanation?
4. Calculate the percentage of carbonic acid in a mineral from the
following data:--
Weight of apparatus and acids 87.0888 grams
" " " plus mineral 88.8858 "
" " " after loss of carbonic acid 88.1000 "
5. A sample of pig iron contains 1.43 per cent. of "combined" and 2.02
per cent. of "free" carbon. Taking 2 grams of it for each determination,
what weight of CO_{2} will be got on burning the residue from solution
in ammonium cupric chloride, and what from the residue after solution in
hydrochloric acid?
BORON AND BORATES.
Boron occurs in nature as boric acid or sassoline (H_{3}BO_{3}); borax
or tincal (Na_{2}B_{4}O_{7}.10H_{2}O); ulexite or boronatrocalcite
(2CaB_{4}O_{7}.Na_{2}B_{4}O_{7}); borocalcite (CaB_{4}O_{7}.4H_{2}O);
boracite, 2Mg_{3}B_{8}O_{15}.MgCl_{2}, and some other minerals. Boric
acid is also a constituent of certain silicates, such as tourmaline,
axinite, and datholite.
The natural borates are used in the preparation of borax, which is
largely employed as a preservative agent, for fluxing, and for other
purposes.
There is only one series of boron compounds which have any importance.
These are the borates in which the trioxide (B_{2}O_{3}) acts the part
of a weak acid. The addition of any acid liberates boric acid, which
separates out in cold solutions as a crystalline precipitate. Boric acid
is soluble in alcohol and in hot water. On evaporating these solutions
it is volatilised, although the anhydrous oxide is "fixed" at a red
heat. The borates are mostly fusible compounds, and are soluble in acids
and in solutions of ammonic salts.
~Detection.~--Boron in small quantities will escape detection unless
specially looked for, but there is no difficulty in detecting its
presence. Heated in the Bunsen-burner flame with "Turner's test," it
gives an evanescent yellowish-green colour, due to fluoride of boron
(BF_{3}). "Turner's test" is a mixture of 5 parts of bisulphate of
potash and 1 part of fluor spar. Boric acid itself imparts a
characteristic green colour to the flame, which gives a spectrum made up
of four well-marked and equidistant lines, three in the green and one in
the blue. Solutions of boric acid give with "turmeric paper," which has
been dipped into it and dried, a characteristic red tint. This is a very
delicate test, but in trying it a blank experiment should be carried out
alongside with a solution made up of the same re-agents which have been
used in liberating the boric acid in the sample.
~Solution and Separation.~--The solution presents no difficulty, but the
separation is troublesome. The best method is that of Gooch; who, if
necessary, first fuses with carbonate of soda, and after the removal of
chlorides and fluorides (by nitrate of silver or a lime salt),
evaporates the aqueous extract with nitric or acetic acid to dryness in
a retort and, subsequently, with repeated doses of 10 c.c. each of
methyl alcohol. The distillate contains the boron as boric acid. Half a
gram of the trioxide (B_{2}O_{3}) is completely carried over by two
evaporations, each with 10 c.c. of the alcohol; but if water or foreign
salts are present, more than this is required. In ordinary cases six
such evaporations are sufficient for 0.2 gram of the oxide.[123]
GRAVIMETRIC DETERMINATION.
Before the introduction of Gooch's process it was usual to determine the
boron trioxide "by difference." If the alcoholic distillate containing
the boric acid is digested with about 1 gram (a known weight) of lime
for ten or fifteen minutes, the alcohol can be evaporated off without
danger of loss. Either calcium nitrate or acetate (which will be formed
at the same time) yields lime upon subsequent ignition. Consequently,
the increase in weight, after ignition, upon that of the lime taken
gives the amount of boron trioxide present. The trioxide contains 31.4
per cent. of boron (B). Since magnesia does not form a soluble hydrate
it cannot satisfactorily be used instead of lime.
[Illustration: FIG. 82.]
The apparatus required is shown in fig. 82. It consists of a small
retort or evaporating vessel made out of a pipette of 200 c.c. capacity.
This is heated by means of a paraffin-bath at 130° or 140° C. It is
connected with an upright condenser, at the lower end of which is a
small flask which serves as a receiver.
The quantity of the borate taken should contain not more than 0.2 gram
of the trioxide. Insoluble compounds are "dissolved in nitric acid at
once, or, if necessary, first fused with sodium carbonate." With soluble
and alkaline borates sufficient nitric acid is added to render it
faintly acid. The solution is then introduced into the retort.
"The lime, to retain the boric acid in the distillate, is ignited in the
crucible in which the evaporation of the distillate is to be made
subsequently." It is then cooled in the desiccator for ten minutes, and
weighed. The lime is transferred to the receiving flask and slaked with
a little water. The retort is lowered into the bath so that "only the
rear dips below the surface." The evaporation is carried to dryness,
the retort being lowered further into the bath as the evaporation
proceeds. Ten c.c. of methyl alcohol are introduced upon the residue,
and the evaporation again started. Six such portions of alcohol are thus
distilled and 2 c.c. of water are introduced and evaporated between the
second and third, as also between the fourth and fifth distillations. If
acetic acid is used instead of nitric in the first instance this
addition of water is unnecessary.
The distillate is evaporated in the crucible ignited over the blowpipe,
cooled in the desiccator for ten minutes and weighed. The increase in
weight gives the boron trioxide. The results tend to be from 1 to 2
milligrams too high.
VOLUMETRIC METHOD.
This method is applicable to the indirect determination of boric acid in
borax and similar compounds. It is based on the measurement of the
quantity of normal solution of acid required to replace the boric acid,
and, consequently, is rather a measure of the soda present. The process
is an alkalimetric one, and is carried out as follows:--Weigh up 3 grams
of the sample and dissolve in water. Tint with methyl orange, and run in
from an ordinary burette normal solution of sulphuric acid until a pink
tint is got. 100 c.c. of the normal solution of acid are equal to 7.0
grams of boron trioxide (B_{2}O_{3}), or 10.1 grams of anhydrous borax
(Na_{2}B_{4}O_{7}).
~Examination of Borax.~--In addition to the determination just given,
the following determinations are also required:--
~Water.~--Take about 2 grams and heat to tranquil fusion in a platinum
crucible. Count the loss in weight as water.
~Sulphuric Oxide.~--Take 2 grams, dissolve in water, acidify with
hydrochloric acid, filter, and precipitate with barium chloride. Wash
the precipitate, ignite, and weigh as barium sulphate (see _Sulphur_).
~Chlorine.~--Take 2 grams, dissolve in water, acidify with nitric acid,
filter, and add silver nitrate. Collect, wash, and weigh the precipitate
as silver chloride.
~Alumina.~--Take 5 or 10 grams, dissolve in water, boil, add ammonia in
slight excess, and filter off the precipitate when it has settled. Wash
with hot water, ignite, and weigh as alumina (Al_{2}O_{3}).
FOOTNOTES:
[113] If the dishes show a manganese stain, wash them out with a few
drops of hydrochloric and sulphurous acids. Pass the acid liquor through
the same small filter but collect the liquor apart. Make ammoniacal and
again pass through the filter, this time collecting the liquid with the
main filtrate.
[114] This rarely amounts to more than 1 milligram.
[115] To make this, dissolve 1 gram of titanium oxide by fusing for some
time with an excess of bisulphate of potash and dissolve out with cold
water and sulphuric acid. Dilute to 1 litre, having previously added not
less than 50 c.c. of strong sulphuric acid: 1 c.c. will contain .01 gram
of TiO_{2}. For the assay take 10 c.c. of this, add 2 c.c. of peroxide
of hydrogen and dilute to 100 c.c. Run this from a burette into the
flask until the colour equals that of the assay. Each c.c. equals 1
milligram of TiO_{2}. Fluorides must be absent.
[116] C + O_{2} = CO_{2}
[117] For example, soluble organic acids formed by partial oxidation
with nitric acid.
[118] For coals, and other bodies containing sulphur, chromate of lead
should be used instead of oxide of copper; and the temperature should be
limited to dull redness.
[119] This may be prepared by dissolving 534 grams of ammonium chloride
and 854 grams of crystallized cupric chloride (CuCl_{2}.2H_{2}O) in hot
water and crystallizing.
[120] Soda-lime is made by dissolving 100 grams of "soda" in water, and
carefully slaking 200 grams of lime with it. Evaporate to dryness in an
iron dish and ignite at a low red heat in a crucible. Use the small
lumps.
[121] Made by diluting 1 part by measure of saturated lime-water up to
10 with recently boiled distilled water.
[122] See under _Gasometric Assays_.
[123] See "A Method for the Separation and Estimation of Boric Acid," by
F.A. Gooch, _Chemical News_, January 7, 1887.
APPENDIX A.
TABLE OF ATOMIC WEIGHTS AND OTHER CONSTANTS.
---------+------------+----------+----------+---------
| | | |
Symbols.| Names. | Atomic | Specific | Melting
| | Weights. | Gravity. | Points.
---------+------------+----------+----------+---------
| | | | C.
Ag | Silver | 107.9 | 10.5 | 1000°
Al | Aluminium | 27.0 | 2.7 | 700°
As | Arsenic | 75.0 | 5.9 |
Au | Gold | 197.3 | 19.2 | 1200°
B | Boron | 11.0 | 2.7 |
Ba | Barium | 137.0 | 4.0 |
Be | Beryllium | 9.0 | 2.1 |
Bi | Bismuth | 208.9 | 9.8 | 270°
Br | Bromine | 80.0 | 3.2 | -25°
C | Carbon | 12.0 | |
Ca | Calcium | 40.0 | 1.6 |
Cd | Cadmium | 112.0 | 8.6 | 315°
Ce | Cerium | 140.2 | 6.7 |
Cl | Chlorine | 35.5 | |
Co | Cobalt | 59.0 | 8.5 |
Cr | Chromium | 52.1 | 7.3 |
Cs | Caesium | 132.9 | 1.9 | 25°
Cu | Copper | 63.4 | 8.9 | 1090°
Di | Didymium | 142.3 | 6.5 |
Er | Erbium | 166.3 | |
F | Fluorine | 19.0 | |
Fe | Iron | 56.0 | 7.8 |
Ga | Gallium | 69.0 | 5.9 | 30°
Ge | Germanium | 72.3 | |
H | Hydrogen | 1.0 | |
Hg | Mercury | 200.0 | 13.6 | -40°
I | Iodine | 126.8 | 4.9 | 106°
In | Indium | 113.7 | 7.4 | 175°
Ir | Iridium | 193.1 | 22.4 |
K | Potassium | 39.1 | 0.86 | 62.5°
La | Lanthanum | 138.2 | 6.1 |
Li | Lithium | 7.0 | 0.59 | 180°
Mg | Magnesium | 24.3 | 1.7 |
Mn | Manganese | 55.0 | 8.0 |
Mo | Molybdenum | 96.0 | 8.6 |
N | Nitrogen | 14.0 | |
Na | Sodium | 23.0 | 0.97 | 95.6°
Nb | Niobium | 94.0 | 4.1 |
Ni | Nickel | 58.7 | 8.9 |
O | Oxygen | 16.0 | |
Os | Osmium | 191.7 | 22.4 |
P | Phosphorus | 31.0 | 1.8 | 44°
Pb | Lead | 206.9 | 11.4 | 334°
Pd | Palladium | 106.6 | 11.4 | 1350°
Pt | Platinum | 195.0 | 21.5 | 2000°
Rb | Rubidium | 85.5 | 1.5 | 38.5°
Rh | Rhodium | 103.5 | 12.1 |
Ru | Ruthenium | 101.6 | 11.4 |
S | Sulphur | 32.0 | 2.0 | 115°
Sb | Antimony | 120.0 | 6.7 | 425°
Se | Selenium | 79.0 | 4.8 | 100°
Si | Silicon | 28.4 | 2.0 |
Sn | Tin | 119.0 | 7.3 | 235°
Sr | Strontium | 87.6 | 2.5 |
Ta | Tantalum | 182.6 | |
Te | Tellurium | 125.0 | 6.2 | 480°
Th | Thorium | 232.6 | 7.8 |
Ti | Titanium | 48.0 | 5.3 |
Tl | Thallium | 204.2 | 11.9 | 294°
U | Uranium | 239.6 | 18.4 |
V | Vanadium | 51.4 | 5.5 |
W | Tungsten | 184.0 | 19.1 |
Y | Yttrium | 89.1 | |
Yb | Ytterbium | 173.0 | |
Zn | Zinc | 65.3 | 6.9 | 423°
Zr | Zirconium | 90.6 | 4.1 |
_________|____________|__________|__________|_________
The atomic weights in this table are in accord with the numbers given by
F.W. Clarke (Dec. 6, 1890), chief chemist of the United States
Geological Survey.
[Illustration: _Table for Converting Degrees of the Centigrade
Thermometer into Degrees of Fahrenheit's Scale._]
Nitric Acid.
_Table showing the percentage, by Weight, of Real Acid_ (HNO_{3}) _in
Aqueous Solutions of Nitric Acid of different Specific Gravities.
Temperature_, 15° C.
-------+-------++-------+-------++-------+-------
1.530 | 100.0 || 1.405 | 66.0 || 1.205 | 33.0
1.527 | 99.0 || 1.400 | 65.0 || 1.198 | 32.0
1.524 | 98.0 || 1.395 | 64.0 || 1.192 | 31.0
1.520 | 97.0 || 1.390 | 63.0 || 1.185 | 30.0
1.516 | 96.0 || 1.386 | 62.0 || 1.179 | 29.0
1.513 | 95.0 || 1.380 | 61.0 || 1.172 | 28.0
1.509 | 94.0 || 1.374 | 60.0 || 1.166 | 27.0
1.506 | 93.0 || 1.368 | 59.0 || 1.159 | 26.0
1.503 | 92.0 || 1.363 | 58.0 || 1.152 | 25.0
1.499 | 91.0 || 1.358 | 57.0 || 1.145 | 24.0
1.495 | 90.0 || 1.353 | 56.0 || 1.138 | 23.0
1.492 | 89.0 || 1.346 | 55.0 || 1.132 | 22.0
1.488 | 88.0 || 1.341 | 54.0 || 1.126 | 21.0
1.485 | 87.0 || 1.335 | 53.0 || 1.120 | 20.0
1.482 | 86.0 || 1.329 | 52.0 || 1.114 | 19.0
1.478 | 85.0 || 1.323 | 51.0 || 1.108 | 18.0
1.474 | 84.0 || 1.317 | 50.0 || 1.102 | 17.0
1.470 | 83.0 || 1.311 | 49.0 || 1.096 | 16.0
1.467 | 82.0 || 1.304 | 48.0 || 1.089 | 15.0
1.463 | 81.0 || 1.298 | 47.0 || 1.083 | 14.0
1.460 | 80.0 || 1.291 | 46.0 || 1.077 | 13.0
1.456 | 79.0 || 1.284 | 45.0 || 1.071 | 12.0
1.452 | 78.0 || 1.277 | 44.0 || 1.065 | 11.0
1.449 | 77.0 || 1.270 | 43.0 || 1.060 | 10.0
1.445 | 76.0 || 1.264 | 42.0 || 1.053 | 9.0
1.442 | 75.0 || 1.257 | 41.0 || 1.047 | 8.0
1.438 | 74.0 || 1.251 | 40.0 || 1.041 | 7.0
1.435 | 73.0 || 1.244 | 39.0 || 1.034 | 6.0
1.431 | 72.0 || 1.238 | 38.0 || 1.028 | 5.0
1.427 | 71.0 || 1.232 | 37.0 || 1.022 | 4.0
1.423 | 70.0 || 1.225 | 36.0 || 1.016 | 3.0
1.418 | 69.0 || 1.218 | 35.0 || 1.010 | 2.0
1.414 | 68.0 || 1.212 | 34.0 || 1.004 | 1.0
1.410 | 67.0 || | || |
-------+-------++-------+-------++-------+--------
HYDROCHLORIC ACID.
_Table showing the percentage, by Weight, of Real Acid_ (HCl) _in
Aqueous Solutions of Hydrochloric Acid of different Specific Gravities.
Temperature_, 15° C.
-----------+---------++----------+---------++----------+---------
| 1.2000 | 40.78 || 1.1410 | 28.54 || 1.0798 | 16.31 |
| 1.1982 | 40.37 || 1.1389 | 28.13 || 1.0778 | 15.90 |
| 1.1964 | 39.96 || 1.1369 | 27.72 || 1.0758 | 15.49 |
| 1.1946 | 39.55 || 1.1349 | 27.32 || 1.0738 | 15.08 |
| 1.1928 | 39.14 || 1.1328 | 26.91 || 1.0718 | 14.68 |
| 1.1910 | 38.74 || 1.1308 | 26.50 || 1.0697 | 14.27 |
| 1.1893 | 38.33 || 1.1287 | 26.10 || 1.0677 | 13.86 |
| 1.1875 | 37.92 || 1.1267 | 25.69 || 1.0657 | 13.45 |
| 1.1857 | 37.51 || 1.1247 | 25.28 || 1.0637 | 13.05 |
| 1.1846 | 37.11 || 1.1226 | 24.87 || 1.0617 | 12.64 |
| 1.1822 | 36.70 || 1.1206 | 24.46 || 1.0597 | 12.23 |
| 1.1802 | 36.29 || 1.1185 | 24.06 || 1.0577 | 11.82 |
| 1.1782 | 35.88 || 1.1164 | 23.65 || 1.0557 | 11.41 |
| 1.1762 | 35.47 || 1.1143 | 23.24 || 1.0537 | 11.01 |
| 1.1741 | 35.07 || 1.1123 | 22.83 || 1.0517 | 10.60 |
| 1.1721 | 34.66 || 1.1102 | 22.43 || 1.0497 | 10.19 |
| 1.1701 | 34.25 || 1.1082 | 22.02 || 1.0477 | 9.78 |
| 1.1681 | 33.84 || 1.1061 | 21.61 || 1.0457 | 9.38 |
| 1.1661 | 33.43 || 1.1041 | 21.20 || 1.0437 | 8.97 |
| 1.1641 | 33.03 || 1.1020 | 20.79 || 1.0417 | 8.56 |
| 1.1620 | 32.62 || 1.1000 | 20.39 || 1.0397 | 8.15 |
| 1.1599 | 32.21 || 1.0980 | 19.98 || 1.0377 | 7.75 |
| 1.1578 | 31.80 || 1.0960 | 19.57 || 1.0357 | 7.34 |
| 1.1557 | 31.40 || 1.0939 | 19.16 || 1.0337 | 6.93 |
| 1.1536 | 30.99 || 1.0919 | 18.76 || 1.0318 | 6.52 |
| 1.1515 | 30.58 || 1.0899 | 18.35 || 1.0298 | 6.11 |
| 1.1494 | 30.17 || 1.0879 | 17.94 || 1.0279 | 5.51 |
| 1.1473 | 29.76 || 1.0859 | 17.53 || 1.0259 | 5.30 |
| 1.1452 | 29.36 || 1.0838 | 17.12 || 1.0239 | 4.89 |
| 1.1431 | 28.95 || 1.0818 | 16.72 || 1.0200 | 4.01 |
-----------+---------++----------+---------++----------+---------
AMMONIA.
_Table showing the percentage, by Weight, of Real Ammonia_ (NH_{3}) _in
Aqueous Solutions of Ammonia of different Specific Gravities.
Temperature_, 14° C.
----------+--------++----------+--------++----------+--------
0.8844 | 36.0 || 0.9145 | 23.6 || 0.9534 | 11.6
0.8852 | 35.6 || 0.9156 | 23.2 || 0.9549 | 11.2
0.8860 | 35.2 || 0.9168 | 22.8 || 0.9563 | 10.8
0.8868 | 34.8 || 0.9180 | 22.4 || 0.9578 | 10.4
0.8877 | 34.4 || 0.9191 | 22.0 || 0.9593 | 10.0
0.8885 | 34.0 || 0.9203 | 21.6 || 0.9608 | 9.6
0.8894 | 33.6 || 0.9215 | 21.2 || 0.9623 | 9.2
0.8903 | 33.2 || 0.9227 | 20.8 || 0.9639 | 8.8
0.8911 | 32.8 || 0.9239 | 20.4 || 0.9654 | 8.4
0.8920 | 32.4 || 0.9251 | 20.0 || 0.9670 | 8.0
0.8929 | 32.0 || 0.9264 | 19.6 || 0.9685 | 7.6
0.8938 | 31.6 || 0.9277 | 19.2 || 0.9701 | 7.2
0.8948 | 31.2 || 0.9289 | 18.8 || 0.9717 | 6.8
0.8957 | 30.8 || 0.9302 | 18.4 || 0.9733 | 6.4
0.8967 | 30.4 || 0.9314 | 18.0 || 0.9749 | 6.0
0.8976 | 30.0 || 0.9327 | 17.6 || 0.9765 | 5.6
0.8986 | 29.6 || 0.9340 | 17.2 || 0.9781 | 5.2
0.8996 | 29.2 || 0.9353 | 16.8 || 0.9790 | 4.8
0.9006 | 28.8 || 0.9366 | 16.4 || 0.9807 | 4.6
0.9016 | 28.4 || 0.9380 | 16.0 || 0.9823 | 4.2
0.9026 | 28.0 || 0.9393 | 15.6 || 0.9839 | 3.8
0.9036 | 27.6 || 0.9407 | 15.2 || 0.9855 | 3.4
0.9047 | 27.2 || 0.9420 | 14.8 || 0.9873 | 3.0
0.9057 | 26.8 || 0.9434 | 14.4 || 0.9890 | 2.6
0.9068 | 26.4 || 0.9449 | 14.0 || 0.9907 | 2.2
0.9078 | 26.0 || 0.9463 | 13.6 || 0.9924 | 1.8
0.9089 | 25.6 || 0.9477 | 13.2 || 0.9941 | 1.4
0.9100 | 25.2 || 0.9491 | 12.8 || 0.9959 | 1.0
0.9111 | 24.8 || 0.9505 | 12.4 || 0.9975 | 0.6
0.9122 | 24.4 || 0.9520 | 12.0 || 0.9991 | 0.2
0.9133 | 24.0 || | || |
----------+--------++----------+--------++----------+--------
SULPHURIC ACID.
_Table showing the percentage, by Weight, of Real Acid_ (H_{2}SO_{4})
_in Aqueous Solutions of Sulphuric Acid of varying Specific Gravity.
Temperature_, 15° C.
--------+--------++--------+--------++--------+--------
1.838 | 100.0 || 1.568 | 66.0 || 1.247 | 33.0
1.840 | 99.0 || 1.557 | 65.0 || 1.239 | 32.0
1.841 | 98.0 || 1.545 | 64.0 || 1.231 | 31.0
1.841 | 97.0 || 1.534 | 63.0 || 1.223 | 30.0
1.840 | 96.0 || 1.523 | 62.0 || 1.215 | 29.0
1.838 | 95.0 || 1.512 | 61.0 || 1.206 | 28.0
1.836 | 94.0 || 1.501 | 60.0 || 1.198 | 27.0
1.834 | 93.0 || 1.490 | 59.0 || 1.190 | 26.0
1.831 | 92.0 || 1.480 | 58.0 || 1.182 | 25.0
1.827 | 91.0 || 1.469 | 57.0 || 1.174 | 24.0
1.822 | 90.0 || 1.458 | 56.0 || 1.167 | 23.0
1.816 | 89.0 || 1.448 | 55.0 || 1.159 | 22.0
1.809 | 88.0 || 1.438 | 54.0 || 1.151 | 21.0
1.802 | 87.0 || 1.428 | 53.0 || 1.144 | 20.0
1.794 | 86.0 || 1.418 | 52.0 || 1.136 | 19.0
1.786 | 85.0 || 1.408 | 51.0 || 1.129 | 18.0
1.777 | 84.0 || 1.398 | 50.0 || 1.121 | 17.0
1.767 | 83.0 || 1.388 | 49.0 || 1.113 | 16.0
1.756 | 82.0 || 1.379 | 48.0 || 1.106 | 15.0
1.745 | 81.0 || 1.370 | 47.0 || 1.098 | 14.0
1.734 | 80.0 || 1.361 | 46.0 || 1.091 | 13.0
1.722 | 79.0 || 1.351 | 45.0 || 1.083 | 12.0
1.710 | 78.0 || 1.342 | 44.0 || 1.075 | 11.0
1.698 | 77.0 || 1.333 | 43.0 || 1.068 | 10.0
1.686 | 76.0 || 1.324 | 42.0 || 1.061 | 9.0
1.675 | 75.0 || 1.315 | 41.0 || 1.053 | 8.0
1.663 | 74.0 || 1.306 | 40.0 || 1.046 | 7.0
1.651 | 73.0 || 1.297 | 39.0 || 1.039 | 6.0
1.639 | 72.0 || 1.289 | 38.0 || 1.032 | 5.0
1.627 | 71.0 || 1.281 | 37.0 || 1.025 | 4.0
1.615 | 70.0 || 1.272 | 36.0 || 1.019 | 3.0
1.604 | 69.0 || 1.264 | 35.0 || 1.013 | 2.0
1.592 | 68.0 || 1.256 | 34.0 || 1.006 | 1.0
1.580 | 67.0 || | || |
--------+--------++--------+--------++--------+--------
APPENDIX B.
ESTIMATION OF SMALL QUANTITIES OF GOLD.[124]
In the case of small buttons of gold the weight can be determined more
easily and accurately by measuring with the help of a microscope than by
the actual use of a balance. Moreover, the method of measurement is
applicable to the determination of quantities of gold too minute to
affect even the most delicate balance.
For quantities of gold of from .5 to .005 milligram a microscope with
1/2 inch objective and B eyepiece is suitable. The measurements are made
with the help of a scale engraved (or, better, photographed) on a
circular piece of glass which rests on the diaphragm of the eyepiece.
This scale and the object upon the stage can be easily brought into
focus at the same time. The button of gold obtained by cupelling is
loosened from the cupel by gently touching with the moistened point of a
knife; it generally adheres to the knife, and is then transferred to a
glass slide. The slide is placed on the stage of the microscope,
illuminated from below; and the button is brought into focus, and so
placed that it apparently coincides with the scale. The diameters in two
or three directions (avoiding the flattened surface) are then read off:
the different directions being got by rotating the eyepiece. The mean
diameter is taken. The weight of the button is arrived at by comparing
with the mean diameter of a _standard prill_ of gold of known weight.
The weights are in the proportion of the cubes of the diameters. For
example, suppose a prill has been obtained which measures 12.5 divisions
of the scale, and that a standard prill weighing 0.1 milligram measures
11.1 divisions. The weight will be calculated as follows:
11.1^{3} : 12.5^{3} :: 0.1 : _x_
0.1×12.5×12.5×12.5
_x_ = -------------------- = 0.143 milligram.
11.1×11.1×11.1
The calculations are simplified by the use of a table of cubes. The
standard prills used in the comparison should not differ much in size
from the prills to be determined. They are prepared by alloying known
weights of gold and lead, so as to get an alloy of known composition,
say one per cent. gold. Portions of the alloy containing the weight of
gold required (say 0.1 milligram) are then weighed off and cupelled on
small smooth cupels, made with the finest bone-ash. Care must be taken
to remove the cupels as soon as cupellation has finished. Several
standard prills of the same size should be made at the same time, and
their mean diameter calculated. The lead for making the gold-lead alloy
is prepared from litharge purified by reducing from it about 10 per
cent. of its lead by fusion with a suitable proportion of flour; the
purified litharge is powdered, mixed with sufficient flour and reduced
to metal.
In determining the gold contained in small buttons of silver-gold alloy
obtained in assaying (and in which the silver is almost sure to be in
excess of that required for parting), transfer the button from the cupel
to a small clean porcelain crucible; pour on it a drop or two of nitric
acid (diluted with half its bulk of water), and heat gently and
cautiously until action has ceased. If the residual gold is broken up,
move the crucible so as to bring the particles together, so that they
may cohere. Wash three or four times with distilled water, about half
filling the crucible each time and decanting off against the finger. Dry
the crucible in a warm place; and when dry, but whilst still black, take
the gold up on a small piece of pure lead. Half a grain of lead is
sufficient, and it is best to hold it on the point of a blunt penknife,
and press it on the gold in the crucible. The latter generally adheres.
Transfer to a small smooth cupel and place in the muffle. When the
cupellation has finished, the button of gold is measured as already
described.
PRACTICAL NOTES ON THE IODIDE PROCESS OF COPPER ASSAYING.
For the following remarks and experiments we are indebted to Mr. J.W.
Westmoreland, who has had considerable experience with the process.
Having dissolved the ore he converts the metals into sulphates by
evaporating with sulphuric acid. The copper is then separated as
subsulphide by means of hyposulphite of soda, and the precipitate is
washed, dried, and calcined. The resulting oxide of copper is then
dissolved in nitric acid; and to the concentrated solution, a saturated
solution of carbonate of soda is added in sufficient quantity to throw
down a considerable proportion of the copper. Acetic acid is added to
dissolve the precipitate, and when this is effected more of the acid is
poured on so as to render the solution strongly acid. To this potassium
iodide crystals are added in the proportion of ten parts of iodide to
each one part of copper supposed to be present. The solution is then
titrated with "hypo" as usual.
For the examination of technical products experiments made in sulphuric
acid solutions have no value, since arsenic acid, which is generally
present to a greater or less extent, affects the end reaction. In such
solutions bismuth may also interfere.
The solution best suited for the assay is one containing acetate of soda
and free acetic acid. The presence of acetate of soda counteracts the
interference of arsenic and of bismuth.
The return of the blue colour after titration is due to the excessive
dilution of the assay, or to an insufficiency of potassium iodide, or to
the presence of nitrous fumes. The interference of an excess of sodium
acetate is avoided by adding more iodide crystals to the extent of
doubling the usual amount.
The interference of lead can be avoided by the addition of sulphuric
acid or of phosphate of soda to the acid solution containing the copper,
and before neutralising with carbonate of soda. The end reaction is,
however, with care distinguishable without this addition. The following
experiments, each containing .0648 gram of lead, were made by him in
illustration:
---------------+-------------------+---------------+---------------------
Copper taken. | Reagent added. | Copper found. | End reaction.
---------------+-------------------+---------------+---------------------
.2092 gram | -- | .2077 gram | fairly satisfactory
.2101 " | -- | .2092 " | "
.2167 " | sulphuric acid | .2152 " | "
.2117 " | " | .2108 " | "
.2109 " | phosphate of soda | .2092 " | good, colourless
.2205 " | " | .2174 " | rather yellow
---------------+-------------------+---------------+---------------------
_Effect of Sodium Acetate._--Each solution contained .3343 gram of
copper.
a.b.c. d. e. f. g.
grams. grams. grams. grams. grams.
"Acetate" added -- 16.2 16.2 16.2 16.2
"Iodide" added 3.5 3.5 7.0 3.5 7.0
Copper found .3343 .3324 .3351 .3269 .3356
In these experiments, except with the excessive quantities of acetate of
soda and the insufficiency of potassium iodide in the cases of c and
f, there was no difficulty with the after-blueing.
METHOD OF SEPARATING COBALT AND NICKEL.
The following method of separating and estimating cobalt and nickel has
been described by Mr. James Hope,[125] with whom it has been in daily
use for several years with completely satisfactory results.
The quantity of ore taken should contain about .5 gram of the mixed
metals. It is dissolved in hydrochloric acid or aqua regia, and the
solution evaporated to dryness. The residue is taken up with dilute
hydrochloric acid and hot water. The solution is filtered off from the
silica, freed from second group metals by treatment with sulphuretted
hydrogen and filtered, and after oxidation with nitric acid is separated
from iron and alumina by the basic acetate method (page 233). The
precipitate is redissolved in a little hydrochloric acid, and again
precipitated by sodium acetate. The two filtrates are mixed and treated
with a little acetic acid, and the cobalt and nickel are then
precipitated as sulphides by a current of sulphuretted hydrogen. The
precipitate is filtered off, washed, dried, and calcined, and the
resulting oxides are weighed to get an idea as to the quantity of the
two metals present.
The calcined precipitate is dissolved in a small covered beaker in aqua
regia with the help of a few drops of bromine to remove any separated
sulphur, and the solution evaporated to dryness with a few drops of
sulphuric acid. The residue is dissolved in hot water, diluted to about
50 c.c., and heated to boiling. About 2 grams (four times the quantity
of mixed metals present) of ammonium phosphate (AmH_{2}PO_{4}) are
weighed off, dissolved in the smallest possible quantity of water, and
boiled for a minute or two with a few c.c. of dilute sulphuric acid.
This is added to the boiling-hot solution of cobalt and nickel, which is
then treated cautiously with dilute ammonia until the precipitate
partially dissolves. The addition of the ammonia is continued drop by
drop with constant stirring, until the cobalt comes down as a pink
precipitate of ammonium cobalt phosphate (AmCoPO_{4}). The beaker is
placed on the top of a water bath with occasional stirring for five or
ten minutes. The blue liquid containing the nickel is decanted through a
small filter and the precipitate is dissolved with a few drops of dilute
sulphuric acid. The resulting solution is treated with a small excess of
ammonium phosphate and the cobalt again precipitated by the cautious
addition of ammonia exactly as before. The precipitate containing the
whole of the cobalt is filtered off and washed with small quantities of
hot water. The filtrate is added to the previous one containing the
greater part of the nickel.
The ammonium cobalt phosphate is dried, transferred to a platinum
crucible, and ignited over a Bunsen flame for fifteen or twenty minutes.
A purple coloured cobalt pyrophosphate (Co_{2}P_{2}O_{7}) is thus
formed, and is weighed. It contains 40.3 per cent. of cobalt.
The mixed filtrates containing the nickel are placed in a tall beaker,
and dilated if necessary to about 200 c.c. Ten c.c. of strong ammonia
are added, and the solution, heated to 70° C., is ready for
electrolysis. A battery of two 1-1/2 pint Bunsen cells is used. This is
found capable of depositing from .15 to .20 gram of nickel per hour, and
from two to three hours is generally sufficient for the electrolysis.
The electrode with the deposited nickel is washed with distilled water,
afterwards with alcohol as described under copper, and is then dried and
weighed.
The following results obtained with this method by Mr. Hope illustrate
the accuracy of the method. They were obtained by working on solutions
containing known weights of the two metals:
-------------------------+-------------------------
Taken. | Found.
------------+------------+------------+------------
Cobalt. | Nickel. | Cobalt. | Nickel.
------------+------------+------------+------------
.1236 gram | .1155 gram | .1242 gram | .1155 gram
.1236 " | .0577 " | .1232 " | .0575 "
.2472 " | .0577 " | .2449 " | .0585 "
.3708 " | .0577 " | .3701 " | .0580 "
.0618 " | .3465 " | .0619 " | .3454 "
.0618 " | .2310 " | .0625 " | .2295 "
.0618 " | .1155 " | .0621 " | .1155 "
------------+------------+------------+------------
FOOTNOTES:
[124] For fuller information see a paper on "The Estimation of Minute
Quantities of Gold," by Dr. George Tate; read before the Liverpool
Polytechnic Society, Nov. 1889.
[125] _Journal of the Society of Chemical Industry_, No 4, vol. ix.
April 30, 1890.
APPENDIX C.
A LECTURE ON THE THEORY OF SAMPLING.
The problem of the sampler is essentially the same as that of the
student of statistics. One aims at getting a small parcel of ore, the
other a number of data, but each hopes to obtain what shall represent a
true average applicable to a much larger mass of material. Ignoring the
mechanical part of the problems, the sampling errors of the one and the
deviations from the average of the other are the same thing.
It may be doubted whether many not specially trained in the study of
statistics could answer such a question as the following:--Seven hundred
thousand men being employed, there are, in a given year, one thousand
deaths from accident. Assuming the conditions to remain unaltered,
within what limits could one foretell the number of deaths by accident
in any other year?
On the other hand, there is a widespread belief in the efficacy of what
is called the law of averages. Even the ordinary newspaper reader is
accustomed to look on the national death-rate or birth-rate as a thing
capable of being stated with accuracy to one or two places of decimals,
and he knows that the annual number of suicides is practically constant.
If a man played whist often and kept a record of the number of trumps n
each hand, he would find fortune treated him quite fairly; in a year's
play the average number would deviate very little from the theoretical
average, _i.e._, one-quarter of thirteen. And a knowledge of this truth
is useful, and that not merely in keeping ejaculations in due restraint.
But every good player knows more than this: he has a sense of what
variations in the number of trumps may reasonably be expected. For
example, he will be prepared to risk something on neither of his
opponents having more than five trumps, and will accept it as a
practical certainty that no one has more than eight. Much of what is
known as good judgment is based on a proper estimate of deviations from
the average. The question has an important bearing on sampling, as may
be seen from the fact that shuffling and dealing at cards are but
modifications of the well-known mixing and quartering of the sampler.
Because of this bearing on sampling and for other reasons, I became many
years ago much interested in the question, and gave to its solution
perhaps more labour than it was worth. In books on Medical Statistics
the answer to the question is stated in a mathematical formula, called
Poisson's formula, which, in a modified form, I shall give further on.
But this did not satisfy me, because I wanted to learn what a reasonably
safe _limit of error_ actually meant, and this could be best learnt by
experiment; so with the help of some friends I went in for a thorough
course of penny-tossing.
Tossing a penny twenty times, an average result would be ten heads and
ten tails. To find the deviations from this, we tossed two hundred
twenties, _i.e._, four thousand times. Of the two hundred, thirty-three
gave the exact average, viz.:--10 heads; sixty-four gave an error of
one, viz.:--9 or 11 heads; forty-nine, an error of two; twenty-six, an
error of three; twenty, an error of four; eight gave an error of five,
and this limit was not exceeded. From these we may say that six is a
reasonably safe limit of error. Ninety-seven cases, say one-half, gave
an error not exceeding one; and the mean error is 1.8.
In other words, in twenty tosses you will not get more than 16 nor less
than 4 heads; you are as likely as not to get 9, 10, or 11 heads; and
lastly, if you lost in twenty throws all heads or tails over 10 your
average loss would be 1.8 penny, or say roughly 2d. on the twenty
throws.
It was necessary to compare these with another series containing a
larger average, say that of 100 heads in 200 throws. I confess the
labour of tossing pennies two hundred at a time was little to our taste.
So from a bag of pennies borrowed from the bank, we weighed out samples
containing two hundred, and for an evening we were busy counting heads
and tails in these. The heads in sixty samples ranged from 80 to 114.
One hundred heads occurred seven times. The extent and frequency of the
errors is shown in the table.
------+-------+------+-------+------+-------
Error.|No. of |Error.|No. of |Error.|No. of
| Times.| | Times.| | Times.
------+-------+------+-------+------+-------
1 | 8 | 6 | 3 | 11 | 1
2 | 5 | 7 | 3 | 14 | 3
3 | 6 | 8 | 3 | 15 | 1
4 | 3 | 9 | 7 | 18 | 2
5 | 6 | 10 | 1 | 20 | 1
--------------------------------------------
We may call the limit of error 21. Twenty-nine results out of sixty, say
one-half, had an error not exceeding 4; and the mean error is 5.6. In
comparing these with the series 10 in 20 we must, working by rule,
divide not by 10 but by 3.16, the square root of 10; for if we multiply
an average by any number[126] the error is also multiplied but only by
the square root of the number. The error varies as the square root of
the number. Now
21/3.16 = 6.6 = limit of error for 10 in 20.
5.6/3.16 = 1.8 = mean error " " "
4/3.16 = 1.2 = probable error " " "
It will be seen that these calculated results agree fairly well with
those actually obtained. The rule by which these calculations are made
is important and will bear further illustration. To calculate the
number of heads in 3200 throws, we have to find the limit of error on a
true average of 1600 in 3200. This being 16 times the average of 100 in
200, the corresponding errors must be multiplied by 4. This gives
21×4 = 84 = limit of error.
5.6×4 = 22.4 = mean error.
4×4 = 16 = probable error.
The results I have actually obtained with these large numbers are hardly
enough to base much on, but have a value by way of confirmation.
Expecting 1600 heads, the actual numbers were 1560, 1596, 1643, 1557,
1591, 1605, 1615, 1545.
It will be seen that exactly half are within the probable error; but
this, considering the small number of results, must be more or less of
an accident; it is more to the point they are all well within the limits
of error.
I have a large number of other results which with a single exception are
all in accord with those given; and this exception only just overstepped
the limits. It was like a case of nine trumps, which though in a sense
possible, is very unlikely to happen in any one's experience.
But even now we are not quite in a position to answer the question with
which we started. If you refer to it you will see that we are face to
face with this problem: the limit of variation on the 1000 who died
would be say 70,[127] ignoring decimals. But if we calculate on the
number who did not die, viz.--699,000,[128] we shall get a variation 26
times as great as this. But it is evident the variation must be the same
in each case. I submitted this kind of problem also to the test of
experiment, the results of which gave me great faith in Poisson's
formula.
Imagine two hundred pennies in a bag all heads up. Any shaking will
spoil this arrangement and give a certain proportion of tails. And,
further, the probable effect of shaking and turning will be to reduce
the preponderance of heads or tails whichever may be in excess. This of
course is the reason why we are so unlikely to get more than 120 of them
in either position.
But if the two hundred pennies are increased to 20,000 by adding pennies
which have tails on both sides, then the shaking or mixing would be less
effective. We should still expect as an average result to get the 100
heads but in 20,000 instead of 200. The variation will be 28 or 29 on
the 100 instead of 20. And this is a better limit in such cases. _Taking
28 as the limit of error on 100 instances_ and proportionally increasing
the others so that _the mean error becomes 7.8 and the probable error
5.6_, we may now calculate the answer without gross mistake.
The probable variation on the 1000 deaths by accident will be 18, the
mean variation will be 24.6, and the limits of variation 88.5. One such
table showing in five years a mean number of deaths of about 1120 per
annum gives an annual deviation of about 50 up or down of this. It will
be seen at once that an improvement of 30 or 40 in any one year would be
without meaning, but that an improvement of from 100 to 200 would
indicate some change for the better in the circumstances of the
industry. Before applying these principles to the elucidation of some of
the problems of sampling it will be well to give Poisson's formula (in a
modified form) and to illustrate its working.
Let _x_ equal the number of cases of one sort, _y_ the cases of the
other sort, and _z_ the total. In the example, _z_ will be the 700,000
engaged in the industry; _x_ will be the 1000 killed by accidents, and
_y_ will be the 699,000 who did not so die. The limit of deviation or
error calculated by Poisson's formula will be the square root of
8_xy_/_z_. Replacing _x_, _y_ and _z_ by the figures of the example we
get the square root of (8×1000×699000)/700,000, which works out to the
square root of 7988.57, or 89.3. Which means that we may reasonably
expect the number of deaths not to vary from 1000 by more than 89,
_i.e._, they will be between 1090 and 910. It will be seen that this
number is in very satisfactory agreement with 88.5 given by the rougher
calculation based on my own experiments.
To come to the question of sampling. Consider a powder of uniform
fineness and fine enough to pass through an 80 sieve. For purposes of
calculation this may be assumed to be made up of particles of about
one-eighth of a millimetre across (say roughly 1/200 of an inch); cubed,
this gives the content as about 1/500 (strictly 1/512) of a cubic m.m.
Now one cubic m.m. of water weighs 1 milligram; therefore 500 such
particles if they have the specific gravity of water weigh 1 milligram,
and otherwise weigh 1 milligram multiplied by the sp. gr.: 500 particles
of ruby silver (Pyrargyrite)[129] will weigh 5.8 milligrams and will
contain nearly 3.5 milligrams of silver.
Now suppose a portion of 3.2667 grams (1/10 Assay Ton) of silver ore to
contain 500 such particles of ruby silver and no other material carrying
silver: such an ore would contain 35 ozs. of silver to the ton. But the
limits of variation on 500 particles would be 28[130] multiplied by the
square root of 5, or 62 particles. Thus the limit of sampling error
would amount to just one-eighth of the silver present, or say to rather
more than 4 ozs. to the ton; the mean sampling error would be rather
more than a quarter of this, or say about 1.3 ozs. to the ton.
On the other hand, if one took for the assay a charge six times greater
(say about 20 grams), the number of particles would be 3000 and the
limits of variation would be 28 multiplied by the square root of 30, or
153 particles, which is very closely 1/20 of the silver present, or say
1.75 ozs. to the ton, whilst the mean error would amount to about .5
ozs. to the ton.
To work these examples by Poisson's formula let us assume the gangue to
have a mean sp. gr. of 3. Then 500 particles would weigh 3 milligrams;
and 3.2609[131] grams would contain 543,500 particles. There would be
then 500 of ruby silver and 543,500 of gangue, together 544,000, and the
formula gives the square root of (8×500×543500)/544000, which works out
to 63 particles as against 62 by the other method.
A practical conclusion from this is of course that either the ore must
be powdered more finely or a larger portion than 3 grams must be taken
for the assay. Moreover, it is evident that on such an ore no small
sample must be taken containing less than several million particles.
Consider now a copper ore of the same uniform fineness containing
particles of copper pyrites (sp. gr. 4) of which 1000 particles will
weigh 8 milligrams, mixed with gangue of which 1000 particles weigh 6
milligrams.
If one gram of such ore contain .5 gram of copper pyrites (= .1725 gram
copper) and .5 gram of gangue, these will contain 62,500 and say 83,500
particles respectively. Altogether 146,000 particles. With Poisson's
formula this gives the limit of sampling error as the square root of
(8×62500×83500)/146000 or 521 particles. But a variation of 521 on
62,500 is a variation of .83 per cent. The percentage of copper in the
ore is 17.25 per cent., and .83 per cent. of this is .14 per cent. The
limits of sampling error, therefore, are 17.11 per cent. and 17.39 per
cent. Again, it must be remembered that the mean sampling error would be
a little over one-quarter of this, or say from 17.2 per cent. to 17.3
per cent. The practical conclusion is that a powder of this degree of
fineness is not fine enough. In the last place let us consider a similar
iron ore containing 90 per cent. of hæmatite (sp. gr. 5) and 10 per
cent. of gangue (sp. gr. 3), 1 gram of such ore will contain 90,000
particles of hæmatite weighing .9 gram and containing .63 gram of iron
with say 16,500 particles of gangue weighing .1 gram. Altogether 106,500
particles.
Poisson's formula then gives the limits of variation as the square root
of (8×90000×16500)/106500 or 334 particles. But 334 on 90,000 is 0.23 on
63.0, which is the percentage of iron present. The limits of sampling
error then are 62.77 per cent. and 63.23 per cent. and the mean
variation is from 62.94 per cent. to 63.06 per cent.
These examples are worthy of careful consideration, and it must be
remembered that the calculations are made on the assumption that the ore
is made up of uniform particles of mineral of such fineness as would
pass easily through an 80 sieve, but which does not pretend to represent
with great exactness the fineness of the powdered ore customary in
practice. They show that having passed through such a sieve is no proof
of sufficient powdering, not that all ores powdered and so sifted are
unfit for assaying. This last would be an absurd and illogical
conclusion.
If an ore be powdered to a fairly fine sand and then be passed through a
series of sieves, say a 40, 60, and 80, in such a state that little or
none remains on the first, but the others retain a large proportion;
then of that which comes through the 80 sieve, perhaps two-thirds by
weight may be even coarser than the powder I have used in the example.
Of the rest most may be of about half this diameter; the weight of the
really fine powder may be quite inconsiderable. On the other hand, if
the grinding be continued until, on sifting, little or nothing that is
powderable remains on the sieves; then in the sifted product the
proportions will be very different. This last, of course, is the only
right way of powdering. Also it is evident that so much depends on the
manner of powdering that nothing precise can be stated as to the average
coarseness of the powder. Suppose, however, by good powdering a product
is obtained which may be represented by a uniform powder with particles
1/20th of a millimetre in diameter (say roughly 1/500 inch). Compared
with the previous powder, the diameter has been divided by 2.5; their
number, therefore, in any given weight has been increased by the cube of
2.5, which is 15.6. But the value of a sample varies as the square root
of the number of particles. Hence the reduction in size and consequent
increase in number has made the sample nearly four times better than
before; and it will be seen that this brings the sampling error within
tolerable limits.
There are one or two words of warning which should be given. In the
first place, using a 90 sieve instead of an 80 must not be too much
relied on; the powder I took in the example would pass through it. It is
a question of good powdering rather than of fine sifting. In the second
place, a set of, say half-a-dozen, assays concordant within 1 oz. where
the theory gives 4 ozs. as the limit of error does not upset the theory:
the theory itself states this as likely. It is the error you _may_ get
in one or two assays out of a hundred, not the error you are _likely_ to
get in any one assay, which is considered under the heading "limit of
error."
Accepting the result just arrived at that a portion of 1 gram may be
safely taken for an assay if the particles are 1-20th of a millimetre in
diameter, the further question remains as to what weight of the original
sample must be reduced to this degree of fineness. This may be answered
on the principle that the same degree of excellence should be aimed at
in each of a series of samplings. This principle is illustrated in the
table on page 2.
A fine sand, such as would pass a 40 sieve but be retained on a 60
sieve, would be fairly represented by particles one-quarter of a
millimetre in diameter. This being five times coarser, to contain the
same number of particles must be 125 times (the cube of 5) as heavy;
therefore 125 grams of it can be taken with the same degree of safety as
1 gram of the finer powder. Of such a sand about this weight should be
taken and reduced to the finer powder. If the ore were in coarse sand,
say in particles 1 millimetre in diameter, this would be four times as
coarse as that last considered, and we should have to take 64 times as
much of it: 64 times 125 grams is 8 kilos, or say roughly from 15 to 20
lbs. This should be crushed to the finer size and mixed; then from 100
to 150 grams should be taken and ground to the finest powder.
There is, however, a reason why, on the coarser stuff, a smaller
proportion may safely be used. This becomes more evident if we consider
a still coarser sample. A heap of ore in stones about 2 inches across
would be 50 times coarser than the sand, and an equivalent sample would
need to be 125,000 times heavier; this would amount to about 1000 tons.
Experienced samplers would say that under such conditions so large a
sample was hardly necessary.
This is because I have assumed in the calculations that the grains of
copper pyrites, for example, were all copper pyrites and the particles
of gangue were free from copper. This would be true or nearly so for the
very fine powder, but far from true in the case of the ore heap. In the
heap probably few of the stones would be pure ore and still fewer would
be free from copper. The stones would differ among themselves in their
copper contents only within certain comparatively narrow limits. And it
is evident that, if replacing one stone by another, instead of resulting
in the gain or loss of all the copper one or other contained, merely
affected the result to one-tenth of this amount, then a sample of
1-100th of the weight (say 10 tons) would be equally safe.
It should be remembered, however, that while the man who samples on a
large scale can safely and properly reduce the size of his samples on
this account, yet the principle is one which counts less and less as the
stuff becomes more finely divided, and ought to be ignored in the
working down of the smaller samples which come to the assayer.
FOOTNOTES:
[126] The 10 in 20 multiplied by 10 = 100 in 200.
[127] Multiply the errors for 100 by the square root of 10.
[128] Multiply the errors for 100 by the square root of 6990.
[129] Sp. Gr. 5.8. Silver 60 per cent.
[130] Taking 28 as the limit of variation on 100.
[131] The weight of the ore less the weight of ruby silver in it.
INDEX.
Acid measures, 49
Acidimetry, 323
Acidity of ores, 168
Acids, 54
strength of, 54, 75, 436
Air of mines, carbonic acid in, 428
Alkalies, 330
determination of, 331
Lawrence Smith's method for, 333, 412
separation of, 332
Alkalimetry, 323
Alkaline earths, 320
Alumina, 314
determination of, 315
in mineral phosphates, 316
separation of, 314, 316
Amalgamation, 126
Ammonia, detection of, 341
determination of, 342
in natural waters, 353
Antimony, 225
detection of, 227
dry assay for, 226
gravimetric assay, 228
separation of, 228
volumetric assay, 229
Arsenic, 381
detection of, 381
dry assay for, 382
gravimetric assay, 383
in brimstone, 393
in crude arsenic, 388, 393
in mispickel, 125, 392
iodine, assay for, 386
separation by distilling, 384
uranium acetate, assay for, 389
Volhard's method applied to, 124
Assay book, 11
note, 12
results, 7
tons, 13, 131
Assaying, 1
methods, 15
Assays, check, 154
preliminary, 147
Atomic weights, 69
table of, 433
Barium, 326
Baryta, 326
Barytes, sulphur in, 378
Base bullion, sampling of, 157
Basic acetate separation, 233
Baumé's hydrometer, 77
Beryllia, 319