FOOTNOTES:

Temperature15° C.30° C.70° C.100° C."Uranium" required18.0 c.c.19.2 c.c.19.0 c.c.18.9 c.c.

Effect of Varying Bulk.—

Bulk50.0 c.c.100.0 c.c.200.0 c.c.300.0 c.c."Uranium" required18.8 "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 solution0.0 c.c.1.0 c.c.5.0 c.c.10.0 c.c.20.0 c.c."Uranium" required18.9 "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 chloride18.8 c.c."Uranium" solutionWith magnesium sulphate19.0 ""Without foreign salts18.9 ""

Effect of Varying Phosphate.—

"Phosphate" solution added10.0 c.c.20.0 c.c.50.0 c.c.100.0 c.c."Uranium" required9.8 "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 P2O5, 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.

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 3Ca3(PO4)2.CaClF would require 50 c.c. of standard uranium solution (100 c.c. equal to 0.5 gram of P2O5)?

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 P2O5was 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 occurs in nature in the free state, and forms about four-fifths of the atmosphere. In combination, as nitrate, it is found in nitre (KNO3), and Chili saltpetre (NaNO3), 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 (N2O5). 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. Itcombines 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.

It follows from what has been said that there is no direct gravimetric determination. The percentage of nitrogen pentoxide (N2O5) 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 sand26.6485 gramsNitre taken1.0000 "————27.6485 "Weight after ignition27.1160 "————Loss on ignition0.5325 "

This is equal to 53.25 per cent. of nitrogen pentoxide.

This is based on the oxidising action of nitric acid, or of nitrates in acid solutions on ferrous salts. The pentoxide (N2O5) 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 (K2Cr2O7) 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.

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:—

0.627milligramof 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.

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]Na3AsO3+ H2O + 2I = Na3AsO4+ 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.P2O5.Magnesic pyro-phosphate 2MgO.P2O5.Magnesic ortho-phosphate 3MgO.P2O5.[109]The composition of which is—MoO290.74,P2O53.14,(NH4)2O 3.57,H2O 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 P2O5, 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]N2O5+ 6FeO = 3Fe2O3+ 2NO.

[103]Na3AsO3+ H2O + 2I = Na3AsO4+ 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.P2O5.Magnesic pyro-phosphate 2MgO.P2O5.Magnesic ortho-phosphate 3MgO.P2O5.

[108]The constitution of these phosphates may be thus illustrated—

Magnesic meta-phosphate MgO.P2O5.Magnesic pyro-phosphate 2MgO.P2O5.Magnesic ortho-phosphate 3MgO.P2O5.

[109]The composition of which is—MoO290.74,P2O53.14,(NH4)2O 3.57,H2O 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 P2O5, 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]N2O5+ 6FeO = 3Fe2O3+ 2NO.

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 (SiO2) 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.

Silica, SiO2.Alumina, Al2O3.Ferric Oxide, Fe2O3.Ferrous Oxide, FeO.Lime, CaO.Magnesia, MgO.Potash, K2O.Soda, Na2O.Fluorine, Water &c.Potash-felspar65.218.20.2———14.71.5Soda-felspar67.019.2—0.31.21.82.27.2Lime-felspar43.335.4—1.317.40.350.50.9Potash-mica45.733.73.1——1.17.52.8F (0.8)H2O (4.9)Magnesia-mica39.115.47.1——23.67.52.6F (0.7)Hornblende40.614.35.87.212.514.01.51.6Augite50.03.72.46.622.813.5——MnO (0.1)Almandine (Garnet)39.719.7—39.7————MnO (1.8)Chlorite (Peach)32.118.5———36.7——H2O (12.1)Schorl37.033.19.36.20.52.60.71.4B2O3(7.7)F (1.5)China-clay46.739.6——————H2O (13.4)Talc61.7——1.7—31.7——H2O (3.8)Serpentine42.9——3.8—40.5——H2O (12.6)Olivine39.3——14.8—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 ofsilicon. 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 (SiO2), 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, andevaporating. 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.

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 theBulletin 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 againevaporated 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 treatedwith 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 (TiO2). 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 (P2O5), 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, washedout with hot water and filtered on to a small filter, dried, ignited, and weighed as Mn3O4. 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 apaste, 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.PtCl4). Its weight, multiplied by 0.1941, will give the weight of potash (K2O).

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 (Na2O).

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 (P2O5).—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 SiO2. This, multiplied by 0.4673, gives the weight of the silicon. The percentage is calculated in the usual way.

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.SiO2. 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 (CaCO3) would you take to make 30 grams of a slag having the formula CaO.SiO2?

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 ofOrganic 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.

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