A Text-book of Assaying: For the Use of Those Connected with Mines Part 34

It is standardised with the help of a solution of antimony made as follows:--Weigh up 5 grams of powdered antimony, transfer to a flask, and cover with 50 c.c. of hydrochloric acid; boil, and add nitric acid (5 or 10 drops at a time) until the metal is dissolved. Allow the action of the nitric acid to cease before adding more. Boil down to a small bulk, add 250 c.c. of hydrochloric acid, and dilute to nearly 1 litre.

Warm until any precipitate which has formed is redissolved; allow to cool slowly, and run in from a pipette a weak solution of permanganate until a faint brown colour is produced. Dilute to exactly 1 litre; 100 c.c. contain 0.5 gram of antimony as antimonic chloride.

In standardising, take 50 c.c. of the antimony solution, and transfer to a flask; add 2 grams of pota.s.sium iodide crystals, and when dissolved, after standing a few minutes, run in the solution of "hypo" from an ordinary burette until the greater part of the iodine has been reduced.

Add a few drops of starch solution, and continue the addition of the "hypo" until the muddy-green colour changes to a clear brownish-yellow.

The solution must be shaken after each addition of the "hypo."

In determining antimony in ore, weigh up 0.5 to 1 gram, and dissolve in hydrochloric acid with, if necessary, the help of chlorate of potash.

The antimony is separated as sulphide, redissolved in hydrochloric acid, and oxidised with a crystal of chlorate of potash. Chlorine is boiled off, and the solution diluted with an equal bulk of water. To the clear cold solution pota.s.sium iodide is added, and after a few minutes the liberated iodine is t.i.trated with "hypo," as already described. The method only yields satisfactory results when the standard and a.s.say are carried out alike.

FOOTNOTES:

[50] "Modern American Methods of Copper Smelting" (Dr. Peters).

[51] "Journal of the Society of Chemical Industry," vol. v. No. 2.

[52] Lead when present is precipitated on the _spiral_ in the form of a dark powder of dioxide (PbO_{2}). Manganese is also thrown down on the spiral as dioxide (MnO_{2}), the solution at the same time becomes violet from the formation of permanganic acid.

[53] See the method given under _Examination of Commercial Copper_.

[54] CuSO_{4} + 4KCy = 2KCy.CuCy_{2} + K_{2}SO_{4}.

[55] 2CuSO_{4} + 3KCy + Am_{2}O = Cu_{2}Cy_{2} + Am_{2}SO_(4) + K_{2}SO_{4} + KCyO.

[56] 2CuSO_{4} + 4KI = Cn_{2}I_{2} + 2I + 2K_{2}SO_{4}.

[57] 2Na_{2}S_{2}O_{3} + 2I = 2NaI + Na_{2}S_{4}O_{6}.

[58] For further information, see Appendix B., and a paper by J.W.

Westmoreland, _Journal of the Society of Chemical Industry_, vol. v.

p. 48.

[59] 3Cu_{2}O + 6AgNO_{3} + 3H_{2}O = 2Cu_{2}H_{3}O_{3}NO_{3} + 2Cu(NO_{3})_{2} + 6Ag. (Insoluble basic salt.)

[60] K_{2}CrO_{4} + Pb(NO_{3})_{2} = PbCrO_{4} + 2KNO_{3}

[61] Made by dissolving 12 grams of tartaric acid and 4 grams of stannous chloride in water, and adding potash solution till it is alkaline. The solution should remain clear on heating to 60 or 70 C.

[62] It must be remembered that a.r.s.enate of bis.m.u.th is completely insoluble in this acid.

[63] SbCl_{5} + 2KI = I_{2} + SbCl_{3} + 2KCl.

CHAPTER XI.

IRON--NICKEL--COBALT--ZINC--CADMIUM.

IRON.

Iron rusts or oxidises very readily, and, consequently, is rarely found in the metallic state in nature; such native iron as is found being generally of meteoric origin or imbedded in basalt and other igneous rocks. It chiefly occurs as oxide, as in magnet.i.te, haemat.i.te, and in the brown iron ores and ochres. Chalybite, which is carbonate of iron, is an ore of great importance. Iron is found combined with sulphur in pyrrhotine and pyrites, and together with a.r.s.enic in mispickel. It is a common const.i.tuent of most rocks, imparting to them a green, black, or brown colour; and is present, either as an essential part or as an impurity, in most substances.

The chemistry of iron is somewhat complicated by the existence of two oxides, each of which gives rise to a well-marked series of compounds.

Those derived from the lower oxide, known as ferrous salts, are generally pale and greenish. Ferric salts are derived from the higher oxide, and are generally red, brown, or yellow. The existence of these two well-marked families of salts renders the a.s.say of iron comparatively easy, for the quant.i.ty of iron present in a solution can be readily measured by the amount of oxidising or reducing agent required to convert it from the one state into the other--that is, from ferrous to ferric, or from ferric to ferrous, as the case may be.

In the red and brown iron ores and ochres ferric iron is present; in chalybite the iron is in the ferrous state; and in magnet.i.te it is present in both forms. Traces of iron in the ferrous state may be found (even in the presence of much ferric iron) by either of the following tests:--

1. Ferricyanide of pota.s.sium gives a blue precipitate or green coloration; with ferric salts a brown colour only is produced.

2. A solution of permanganate of pota.s.sium is decolorised by a ferrous salt, but not by a ferric one.

Traces of ferric iron can be detected (even in the presence of much ferrous iron) by the following tests:--

(1) By the brown or yellow colour of the solution, especially when hot.

(2) By giving a pink or red coloration with sulphocyanide of pota.s.sium.

Substances containing oxide of iron yield the whole of the iron as metal when fused at a high temperature with charcoal and suitable fluxes. The metal, however, will contain varying proportions of carbon and other impurities, and its weight can only afford a rough knowledge of the proportion of the metal in the ore. There are two or three methods of dry a.s.say for iron, but they are not only inexact, but more troublesome than the wet methods, and need not be further considered. Chalybite and the hydrated oxides dissolve very readily in hydrochloric acid; haemat.i.te and magnet.i.te dissolve with rather more difficulty. Iron itself, when soft, is easily soluble in dilute hydrochloric, or sulphuric, acid.

Pyrites, mispickel, &c., are insoluble in hydrochloric acid, but they are readily attacked by nitric acid. Certain minerals, such as chrome iron ore, t.i.taniferous iron ore, and some silicates containing iron, remain in the residue insoluble in acids. Some of these yield their iron when attacked with strong sulphuric acid, or when fused with the acid sulphate of potash. Generally, however, it is better in such stubborn cases to fuse with carbonate of soda, and then attack the "melt" with hydrochloric acid.

When nitric acid, or the fusion method, has been used, the metal will be in solution in the ferric state, no matter in what condition it existed in the ore. But with dilute hydrochloric or sulphuric acid it will retain its former degree of oxidation. Hydrochloric acid, for example, with chalybite (ferrous carbonate) will give a solution of _ferrous_ chloride; with haemat.i.te (ferric oxide) it will yield _ferric_ chloride; and with magnet.i.te (ferrous and ferric oxides) a mixture of ferrous and ferric chlorides. Metallic iron yields solutions of _ferrous_ salts. It is convenient to speak of the iron in a ferrous salt as ferrous iron, and when in the ferric state as ferric iron. Frequently it is required to determine how much of the iron exists in an ore in each condition. In such cases it is necessary to keep off the air whilst dissolving; the operation should, therefore, be performed in an atmosphere of carbonic acid.

~Separation.~--The separation of the iron from the other substances is as follows:--Silica is removed by evaporating the acid solution, and taking up with acid, as described under _Silica_; the whole of the iron will be in solution. The metals of Groups I. and II. are removed by pa.s.sing sulphuretted hydrogen, and at the same time the iron will be reduced to the ferrous state. The solution should be filtered into a 16 oz. flask, boiled to get rid of the gas, and treated (whilst boiling) with a few drops of nitric acid, in order to convert the whole of the iron into the ferric state. When this condition is arrived at, an additional drop of nitric acid causes no dark coloration. The boiling must be continued to remove nitrous fumes. Next add caustic soda solution until the colour of the solution changes from yellow to red.

The solution must be free from a precipitate; if the soda be incautiously added a permanent precipitate will be formed, in which case it must be redissolved with hydrochloric acid, and soda again, but more cautiously, added. After cooling, a solution of sodium acetate is added until the colour of the solution is no longer darkened. The solution, diluted to two-thirds of the flaskful with water, is heated to boiling.

Long-continued boiling must be avoided. The precipitate is filtered quickly through a large filter, and washed with hot water containing a little acetate of soda.

The precipitate will contain all the iron and may also contain alumina, chromium, t.i.tanium, as well as phosphoric, and, perhaps, a.r.s.enic acids.[64]

Dissolve the precipitate off the filter with dilute sulphuric acid, avoiding excess, add tartaric acid and then ammonia in excess. Pa.s.s sulphuretted hydrogen, warm, and allow the precipitate to settle. Filter and wash with water containing a little ammonic sulphide.

GRAVIMETRIC METHOD.

Dissolve the precipitate in dilute hydrochloric acid; peroxidise with a few drops of nitric acid and boil, dilute to about 200 c.c., add ammonia (with constant stirring) till the liquid smells of it, and heat to boiling. Wash as much as possible by decantation with hot water.

Transfer to the filter, and wash till the filtrate gives no indication of soluble salts coming through. The filtrate must be colourless and clear. The wet precipitate is very bulky, of a dark-brown colour and readily soluble in dilute acids, but insoluble in ammonia and dilute alkalies. When thrown down from a solution containing other metals it is very apt to carry portions of these with it, even when they are by themselves very soluble in ammoniacal solutions. It must be dried and ignited, the filter paper being burnt separately and its ash added. When further ignition ceases to cause a loss of weight, the residue is ferric oxide (Fe_{2}O_{3}), which contains 70 per cent. of iron. The weight of iron therefore can be calculated by multiplying the weight of oxide obtained by 0.7.

The presence of ammonic chloride causes loss of iron during the ignition, and organic matter causes an apparent loss by reducing the iron to a lower state of oxidation. When the iron in the solution much exceeds 0.2 gram the volumetric determination is generally adopted, as the bulkiness of the precipitate of ferric hydrate makes the gravimetric method very inconvenient.

VOLUMETRIC METHODS.

As already explained these are based on the measurement of the volume of a reagent required to bring the whole of the iron from the ferrous to the ferric state (oxidation), or from the ferric to the ferrous (reduction). Ferrous compounds are converted into ferric by the action of an oxidising agent in the presence of an acid. Either permanganate or b.i.+.c.hromate of potash is generally used for this purpose.[65]

Ferric compounds are reduced to ferrous by the action of:--

(1) Stannous chloride; (2) Sulphuretted hydrogen; (3) Sodium sulphite; or (4) Zinc.[66]