A Text-book of Assaying: For the Use of Those Connected with Mines Part 8

These can again be divided into those which are dissolved by dilute acids and those which are not.

3. _Not precipitated in an acid or alkaline solution, but thrown down on neutralising the latter._

Sulphides of V and W.

Sulphuretted hydrogen is a strong reducing agent. Ferric salts are thereby quickly reduced to ferrous; in hot solutions nitric acid is decomposed. These changes are marked by a precipitation of sulphur, and the student must be careful to pa.s.s the gas sufficiently long, and not be too hasty in concluding that no sulphide will form because it does not at once make its appearance. The best indication that it has been pa.s.sed long enough is the smell of the gas in the solution after shaking.

~Sulphurous Acid~, H_{2}SO_{3}.--The reagent used may be regarded as a saturated solution of sulphur dioxide in water. It may be purchased, and keeps for a long time. It may be made by heating copper with sulphuric acid and pa.s.sing the gas formed into water. The heat should be withdrawn when the gas is coming off freely. It is used as a reducing agent, and should not be diluted.

~Sulphuric Acid~, H_{2}SO_{4}. (Sp. gr. 1.84, containing 96 per cent. of real acid, H_{2}SO_{4}.)--This acid forms insoluble sulphates with salts of lead, strontium, and barium. It has a high boiling point, 290 C., and, when evaporated with salts of the more volatile acids, converts them into sulphates. When nitrates or chlorides are objectionable in a solution, evaporation with sulphuric acid removes them. In working with this acid caution is necessary, since, on mixing with water, great heat is evolved; and, if either the acid or water has been previously heated, a serious accident may result. In diluting the acid it should be poured into cold water. Gla.s.s vessels containing boiling sulphuric acid should be handled as little as possible, and should not be cooled under the tap. The action of diluted sulphuric acid on metals closely resembles that of dilute hydrochloric acid. Magnesium, aluminium, iron, zinc, nickel, cobalt, manganese, and cadmium dissolve, with evolution of hydrogen, in the cold acid, or when warmed. The action of hot and strong sulphuric acid is altogether different; it acts as an oxidising agent, and is itself reduced to sulphur dioxide or even to sulphur. The following metals are attacked in this way:--copper, bis.m.u.th, mercury, silver, antimony, tin, and lead. Gold, platinum, and a.r.s.enic are not affected. This property is made use of in parting silver from gold and platinum. Metallic sulphides are similarly attacked; but this method of opening up minerals has the disadvantage of giving rise to the formation of anhydrous sulphates of iron, &c., which are not readily dissolved when afterwards diluted. The use of sulphuric acid in a.s.saying is (for these reasons) to be avoided. Its chief use is as a drying agent, since it has a strong affinity for water. Air under a bell jar may be kept dry by means of a basin of sulphuric acid, and gases bubbled through it are freed from water-vapour.

~Dilute Sulphuric Acid.~--This is made by diluting 1 volume of the strong acid with 4 of water.

~Tartaric Acid~, H_{2}[=T] or C_{4}H_{6}O_{6}.--A crystallised organic acid, soluble in less than its own weight of water, or in less than three parts of alcohol. It is used for the same purposes as citric acid is. The solution is made when required.

BASES, SALTS, &c.

~Alcohol~, C_{2}H_{6}O. (Commercial alcohol of sp. gr. 0.838; it contains 84 per cent. by weight of alcohol.)--It should burn with a non-luminous flame and leave no residue. It is used for was.h.i.+ng precipitates where water is inapplicable, and for facilitating drying.

~Ammonia~, NH_{3}. (Commercial ammonia, a solution having a sp. gr. of 0.88 to 0.89, and containing about 33 per cent. of ammonia.)--It is used as an alkali (more commonly than soda or potash), since an excess of it is easily removed by boiling. The salts of ammonium formed by it may be removed by igniting, or by evaporating in a porcelain dish with an excess of nitric acid. It differs in a marked way from soda or potash in its solvent action on the oxides or hydrates of the metals. Salts of the following metals are soluble in an ammoniacal solution in the presence of ammonic chloride:--copper, cadmium, silver, nickel, cobalt, manganese, zinc, magnesium, sodium, pota.s.sium, and the alkaline earths.

~Dilute Ammonia~ is made by diluting 1 vol. of commercial ammonia with 2 of water. The dilute ammonia is always used; but in a.s.says for copper a stronger solution (1 of strong ammonia to 1 of water) is required.

~Ammonic Carbonate~ (Am_{2}CO_{3}) is prepared by dissolving one part of the commercial sesquicarbonate of ammonia in four parts of water, and adding one part of strong ammonia.

~Ammonic Bicarbonate~ (HAmCO_{3}) is prepared by saturating a solution of the sesquicarbonate of ammonia with carbon dioxide.

~Ammonic Chloride~, AmCl.--Use the commercial salt in a 20 per cent.

solution in water. The salt should leave no residue on ignition.

~Ammonic Molybdate.~--The solution is prepared as follows:--Dissolve 100 grams of the powdered commercial salt in 200 c.c. of dilute ammonia, and pour the solution in a slow stream into 750 c.c. of dilute nitric acid; make up to 1 litre, and allow the mixture to settle before using. It is used for the purpose of separating phosphoric oxide from bases and from other acids, and also as a test for phosphates and a.r.s.enates. In using this solution the substance must be dissolved in nitric acid, and a considerable excess of the reagent added (50 c.c. is sufficient to precipitate 0.1 gram P_{2}O_{5}); when the phosphate is in excess no precipitate will be got. The precipitate is phospho-molybdate of ammonia.

~Ammonic Nitrate~ (AmNO_{3}) is used in the separation of phosphoric oxide by the molybdate method, and occasionally for destroying organic matter. It is soluble in less than its own weight of water. The solution is made when wanted.

~Ammonic Oxalate~ (Am_{2}C_{2}O_{4}.2H_{2}O) is used chiefly for the separation of lime. The solution is made by dissolving 15 grams of the salt in 100 c.c. of water.

~Ammonic Sulphide~ may be purchased in the state of a strong solution.

It is yellow, and contains the disulphide, S_{2}Am_{2}. It serves the same purpose as is obtained by pa.s.sing a current of sulphuretted hydrogen through an ammoniacal solution; but has the disadvantage of loading the solution with sulphur, which is precipitated when the solution is subsequently acidified. It is useful for dissolving the lower sulphide of tin (SnS).

~Baric Carbonate~ (BaCO_{3}) is sometimes used for precipitating the weaker bases. It should be prepared when wanted by precipitating a solution of baric chloride with ammonic carbonate and was.h.i.+ng. The moist precipitate is used without drying.

~Baric Chloride~, BaCl_{2}.2H_{2}O.--A crystallised salt, soluble in 2-1/2 parts of water. It is used for the detection and separation of sulphates. Make a 10 per cent. solution.

"~Black Flux.~"--A mixture of finely divided carbon with carbonate of potash or with carbonates of potash and soda. It is prepared by heating tartar or "roch.e.l.le salt" until no more combustible gas is given off.

One gram will reduce about 2 grams of lead from litharge.

~Borax~, Na_{2}B_{4}O_{7}.10H_{2}O.--It is chiefly used as a flux in dry a.s.saying, as already described. It is also used in testing before the blowpipe; many metallic oxides impart a characteristic colour to a bead of borax in which they have been fused.

~Calcium Chloride.~--The crystallised salt is CaCl_{2}.6H_{2}O; dried at 200 C. it becomes CaCl_{2}.2H_{2}O, and when fused it becomes dehydrated. The fused salt, broken into small lumps, is used for drying gases. It combines with water, giving off much heat; and dissolves in a little more than its own weight of water. Strong solutions may be used in baths in which temperatures above the boiling-point of water are required. One part of the salt and 2 of water give a solution boiling at 112, and a solution of 2 parts of the salt in 1 of water boils at 158.

The salt is very little used as a reagent.

~Calcium Fluoride~ or "~Fluor Spar~," CaF_{2}.--The mineral is used as a flux in dry a.s.saying; it renders slags which are thick from the presence of phosphates, &c., very fluid. Mixed with hydrochloric acid it may sometimes be used instead of hydrofluoric acid.

~Calcium Carbonate~, CaCO_{3}.--It is precipitated in a pure state by ammonic carbonate from a solution of calcium chloride. It is used for standardising. In the impure state, as marble or limestone, it is used in the preparation of carbonic acid.

~Calcium Hydrate~ or ~"Lime Water."~--This is used in testing for carbon dioxide and in estimating the amount of that gas present in air. It may be made by slaking quicklime and digesting the slaked lime with water.

One hundred c.c. of water at 15 C. dissolves 0.1368 grams of the hydrate (CaH_{2}O_{2}), and hot water dissolves still less. "_Milk of lime_" is slaked lime suspended in water.

~Cobalt Nitrate~ (Co(NO_{3})_{2}.6H_{2}O) is used in a 10 per cent.

solution for the detection of oxides of zinc, aluminium, &c.; on ignition with which it forms characteristically coloured compounds.

~Copper~, Cu.--Pure copper, as obtained by electrolysis, can be purchased. This only should be used.

~Copper Oxide~, CuO.--It occurs as a black, heavy, and gritty power, and is used for the oxidation of carbon and hydrogen in organic substances.

It should be ignited and cooled out of contact with air just before using, since it is hygroscopic. Oxide of copper which has been used may be again utilised after calcination.

~Copper Sulphate~ (CuSO_{4}.5H_{2}O) contains 25.4 per cent. of copper.

It is used in the outer cell of a Daniell-battery. The commercial salt is used for this purpose. The re-crystallised and pure salt is used for preparing the anhydrous sulphate, which is used for detecting moisture in gases. For this purpose it is dried at 200 C. till no trace of green or blue colour remains. It must be prepared when wanted. It may be conveniently used in the form of pumice-stone, saturated with a solution of the salt and dried. Traces of moisture develop a green colour.

~Ferric Chloride~, Fe_{2}Cl_{6}. (When crystallised, Fe_{2}Cl_{6}.6H_{2}O.)--The solution is prepared as described under iron. The commercial salt contains a.r.s.enic, and, since the chief use of ferric chloride is for the determination of this substance, it must be purified (_see_ under a.r.s.eNIC).

~Ferric Sulphate~ (Fe_{2}(SO_{4})_{3}) is a yellowish white deliquescent salt. It is used as an indicator in volumetric silver a.s.saying, and for the separation of iodine from bromine. It may be purchased as iron alum, Am_{2}Fe_{2}(SO_{4})_{4}.24H_{2}O. But it is best prepared by adding strong sulphuric acid to ferric hydrate in equivalent proportions. Use it as a solution containing 2 or 3 per cent. of iron.

~Ferrous Sulphate~, FeSO_{4}.7H_{2}O.--The granulated form is best, and can be purchased pure. It is used for standardising. It keeps better in crystals than in solution. It is readily soluble in water, but the solution is best made with the help of a little free acid. As a re-agent use a 10 per cent. solution. The crystals should be clear bluish-green; if their colour is dark green, brown, or blue, they should be rejected.

~Ferrous Sulphide~ (FeS) is used for the preparation of sulphuretted hydrogen. It may be purchased and broken in small lumps, nut-size, for use.

"~Fusion Mixture~" (K_{2}CO_{3}.Na_{2}CO_{3}) is a mixture of pota.s.sic and sodic carbonates in the proportions of 13 of the former to 10 of the latter, by weight. It is hygroscopic. A mixture of the bicarbonates is better, being purer and less apt to get damp.

~Gallic Acid~ (C_{7}H_{6}O_{5}.H_{2}O) is an organic acid, occurring as a pale fawn-coloured crystalline powder, soluble in 100 parts of cold water, or in 3 parts of boiling water. It is used for the determination of antimony. A 10 per cent. solution in warm water is made when required.

~Hydrogen~ (H) is a gas. It is obtained by acting on zinc with dilute hydrochloric or sulphuric acid. It is used as a reducing agent, and for providing an atmosphere free from oxygen. It reduces metallic oxides at a high temperature. It must be freed from water; and special precautions should be taken to prevent an admixture with air. It is generally required in a current which can be continued for an hour or more without interruption. The preparation can be conveniently carried out in the apparatus shown (fig. 33). A quart bottle is half filled with sheet zinc, and connected with bulbs filled with sulphuric acid, and with a calcium chloride tube. The last is connected with the apparatus through which the gas has to be pa.s.sed. Dilute hydrochloric acid mixed with a few cubic centimetres (20 c.c. to 1 pint) of stannous chloride sol. to fix any dissolved oxygen, is placed in the funnel, and let into the bottle by opening the stopc.o.c.k when required. Care must be taken to let the hydrogen escape for some time before starting the reduction.

[Ill.u.s.tration: FIG. 33.]

~Gold~, Au.--Gold, obtained by cupelling and "parting," is for most purposes sufficiently pure. It is best kept in the shape of foil. When the purer metal is required, gold should be dissolved in aqua regia, the solution evaporated to a paste, diluted, allowed to stand, and filtered.

The filtered solution is acidified with hydrochloric acid, warmed, and precipitated with sodium sulphite. The precipitate is collected, washed, and fused on charcoal.

~Iron~, Fe.--The soft wire (thin) is used for standardising. Rods are used in dry a.s.says as a desulphurising agent. Steel must not be used, since it is not pure, and contains a variable amount of iron.

~Lead~, Pb.--Granulated lead or lead-foil is used in the dry a.s.say for silver and gold, and in the preparation of lead salts. It can be obtained very pure, but always contains more or less silver, 1 or 2 milligrams in 100 grams. The amount of silver it contains must be determined and recorded.

~Lead Acetate~ (Pb[=A=c]_{2}.3H_{2}O, or Pb(C_{2}H_{3}O_{2})_{2}.3H_{2}O) is used as a test, specially for the detection and estimation of sulphuretted hydrogen. Prepare a 10 per cent. solution for use.

~Lead Nitrate~ (Pb(NO_{3})_{2}) can be purchased pure. It is used for standardising.

~Lead Dioxide~ (PbO_{2}) occurs as a dark-brown powder. It is used as an oxidizing agent and for absorbing sulphurous oxide. It can be prepared by digesting red lead with warm dilute nitric acid; was.h.i.+ng and drying the residue.