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.