A Text-book of Assaying: For the Use of Those Connected with Mines Part 48

When, as is generally the case, the chromium exists altogether as chromate (phosphates and a.r.s.enates being absent) it is best to proceed as follows:--Render the solution acid with acetic acid, then add sodium acetate to the solution and heat nearly to boiling; next treat with a slight excess of acetate of lead, and boil. Allow to settle, and filter.

Wash the precipitate with hot water, dry in the water-oven or at a low temperature. Transfer the precipitate to a weighed Berlin crucible, burn the filter separately, ignite below redness, cool in the desiccator, and weigh. The substance is lead chromate, PbCrO_{4}, and contains 16.1 per cent. of chromium, or 23.53 per cent. of chromic oxide (Cr_{2}O_{3}).

VOLUMETRIC METHOD.

This is based on the oxidation of ferrous iron by the solution containing the chromium as chromate. A known weight of iron (0.5, 1, or 1.5 gram, according to the quant.i.ty of chromate) is dissolved in 50 c.c.

of dilute sulphuric acid. The solution containing the chromate is added, and the remaining ferrous iron t.i.trated with the permanganate or b.i.+.c.hromate of pota.s.sium solution, as described under _Iron_. The iron thus found is deducted from that taken, and the difference gives the iron oxidised by the chromate. This multiplied by 0.3101 gives the chromium, Cr, and when multiplied by 0.4529 gives the chromic oxide, Cr_{2}O_{3}.

COLORIMETRIC METHOD.

Small quant.i.ties of chromium may be determined, after conversion into chromate, colorimetrically. The solution, which should not contain more than a few milligrams in 100 c.c., is acidified with acetic acid and compared against an equal volume of water rendered acid with acetic acid and tinted with a standard b.i.+.c.hromate of pota.s.sium solution. This standard b.i.+.c.hromate is made by dissolving 2.827 grams of the salt in water and diluting to 1 litre. One c.c. will contain 1 milligram of chromium, Cr. The manner of working this a.s.say is the same as that adopted in the other colorimetric processes.

~Determination of Chromium in Steel.~[89]--Weigh up 2.4 grams, dissolve in hydrochloric acid, and evaporate to dryness. Fuse with sodium carbonate and nitre, extract with water, and make up to 301 c.c. Take 250 c.c. of the clear liquor, boil with hydrochloric acid, add sodium phosphate, and then ammonia in slight excess. Heat till clear. Filter off the precipitate, dissolve it in hydrochloric acid, and evaporate to dryness. Take up with a little acid, filter, and precipitate with a slight excess of ammonia. Wash, ignite, and weigh as chromium phosphate (3Cr_{2}O_{3},2P_{2}O_{5}), which contains 42.2 per cent. of chromium.

VANADIUM.

Vanadium occurs in certain rare minerals, such as vanadinite (3Pb_{3}(VO_{4})_{2}.PbCl_{2}), a vanadate of lead; mottramite, a vanadate of copper and lead; and dechenite, a vanadate of lead and zinc.

It is occasionally found in iron and copper ores and in the slags from them. In Spanish copper-precipitates it is found along with chromium, and is probably derived from the iron used for precipitating. The vanadates, like the chromates, are coloured compounds, generally yellow or red. On reduction, blue solutions are got. In their general reactions vanadates resemble phosphates.

Vanadium is detected by the red colouration produced by pa.s.sing sulphuretted hydrogen into ammoniacal solutions for some time. On adding an acid to the filtered solution a brown precipitate of the sulphide is produced. This gives with borax a colourless bead in the oxidising, and a green one in the reducing, flame.

It is separated by fusing the ore with pota.s.sic nitrate, extracting with water and precipitating with baric chloride. The precipitate is boiled with dilute sulphuric acid, filtered, neutralised with ammonia, and saturated with ammonic chloride. Ammonium vanadate separates out. It is filtered off, ignited, and weighed as vanadic oxide, V_{2}O_{5}, containing 56.18 per cent. of vanadium.

MOLYBDENUM.

Molybdenum occurs in nature chiefly as molybdenite (MoS_{2}); it also occurs in wulfenite, a molybdate of lead (PbMoO_{4}), and in molybdic ochre (MoO_{3}).

Molybdate of ammonia is an important reagent in the determination of phosphates, the manufacture of which compound is the chief purpose to which molybdenum is applied.

Iron and copper ores frequently contain molybdenum, sometimes in quant.i.ty; consequently it is met with in slags and pig-iron.

Molybdenum forms several series of salts. In those corresponding to the lower oxides it is basic; but the trioxide (MoO_{3}) is the acid oxide which forms a series of salts called the molybdates. All molybdenum compounds are converted into the trioxide by boiling with nitric acid.

The trioxide is a white powder readily dissolved by ammonia. It fuses at a red heat, and volatilises freely in contact with air. It is slightly soluble in water.

Molybdates are easily reduced, with the production of coloured solutions, by most reducing agents. Sulphuretted hydrogen first produces a blue tint, and then precipitates a brown sulphide. The precipitation as sulphide is only complete on prolonged treatment; a green colour indicates that some molybdenum still remains in solution. The precipitated sulphide is soluble in ammonium sulphide.

~Detection.~--Molybdenum is detected by its behaviour with sulphuretted hydrogen. Molybdenite can only be mistaken for graphite, from which it is easily distinguished by yielding sulphur dioxide on roasting, and by giving, on charcoal, a yellowish white incrustation, which becomes blue on touching it for a moment with the reducing flame. The borax-bead is colourless in the oxidising, and dark-brown in the reducing, flame.

GRAVIMETRIC DETERMINATION.

The solution containing the molybdate is neutralised and treated with an excess of mercurous nitrate. The precipitate is allowed to settle for some time, filtered, and washed with a dilute solution of mercurous nitrate. Then it is dried, transferred to a weighed Berlin crucible containing ignited oxide of lead, mixed, ignited, and weighed. The increase in weight gives the amount of trioxide, MoO_{3}. This contains 66.7 per cent. of molybdenum.

URANIUM.

Uranium occurs chiefly as pitchblende, which is an impure oxide (U_{3}O_{8}). It is also found as sulphate in uranochre, johannite, &c.; and as phosphate in the uranites, torbernite (hydrated phosphate of uranium and copper), and autunite (hydrated phosphate of uranium and lime). It also occurs in some rarer minerals.

The oxide is used for colouring gla.s.s; and the nitrate and acetate are used as reagents. "Uranium yellow," used for enamel painting, is sodium uranate. The uranates, in which the oxide of uranium acts as an acid, are mostly insoluble and of secondary importance.

Uranium forms two families of salts, uranous and uranic; corresponding to the oxides UO_{2} and UO_{3} respectively. The former are generally green and the latter yellow. Uranous salts are converted into uranic by boiling with nitric acid or other oxidising agents. Uranic salts, on the other hand, are easily reduced by sulphuretted hydrogen, stannous chloride or zinc. This property is made use of in determining the quant.i.ty of uranium in pure solutions by t.i.trating with permanganate of pota.s.sium solution as in the case with iron.

~Detection.~--The most characteristic reaction of the uranium compounds is their behaviour in the presence of alkaline carbonates in which they are freely soluble; even ammonium sulphide will not precipitate uranium from these solutions. On neutralising the carbonate with an acid a uranate of the alkali is precipitated. Ammonia or sodic hydrate (free from carbonates) give yellow precipitates, which are insoluble in excess of the reagent, but are soluble in acids. Ferrocyanide of pota.s.sium gives a reddish-brown precipitate. Uranium colours the borax-bead yellowish-green in the oxidising, and green in the reducing, flame.

~Solution and Separation.~--The compounds of uranium are soluble in acids. Powder the substance and evaporate with an excess of nitric acid.

Take up with hydrochloric acid, dilute, pa.s.s sulphuretted hydrogen, and filter. Peroxidise the filtrate with a little nitric acid, add an excess of ammonic carbonate and some ammonium sulphide, and filter. Render the solution acid, boil; and precipitate the uranium by means of ammonia.

Filter off, and wash it with dilute ammonic chloride. Ignite, and weigh as protosesqui-oxide, U_{3}O_{8}.

GRAVIMETRIC DETERMINATION.

The solution containing the uranium free from other metals is, if required, first peroxidised by boiling with nitric acid. Ammonia in slight excess is added to the nearly-boiling solution. A yellow precipitate is formed, which is filtered off hot and washed with a dilute solution of ammonium chloride. The precipitate is dried and ignited; and weighed as U_{3}O_{8}, which contains 84.8 per cent. of uranium.

VOLUMETRIC METHOD.

This is based on the precipitation of uranium as phosphate from acetic acid solutions and the recognition of complete precipitation by testing with pota.s.sic ferrocyanide; it is the converse of the process for the volumetric determination of phosphate.

_The standard solution of phosphate_ is prepared by dissolving 29.835 grams of hydric sodic phosphate (Na_{2}HPO_{4}.12H_{2}O) in water and diluting to 1 litre. 100 c.c. will be equivalent to 2 grams of uranium.

Take 1 gram of the sample (or, if poor in uranium, 2 grams) and separate the uranium as described. Dissolve the precipitate in nitric acid and evaporate to a small bulk, add 2 grams of sodium acetate, dilute with water to 100 c.c., and boil. t.i.trate the boiling solution with the sodium phosphate till it ceases to give a brown colouration with pota.s.sium ferrocyanide. Calculate the percentage in the usual way.

FOOTNOTES:

[78] MnO_{2} + 4HCl = MnCl_{2} + Cl_{2} + 2H_{2}O.

[79] Provided a sufficiency of ammonic chloride is present.

[80] With some silicates, &c., a preliminary fusion with sodium carbonate will be necessary.

[81] Instead of sodium acetate, ammonium succinate can be used.

[82] _Journ. Soc. Chem. Industry_, vol. x. p. 333.

[83] MnO_{2} + 2FeSO_{4} + 2H_{2}SO_{4} = Fe_{2}(SO_{4})_{3} + MnSO_{4} + 2H_{2}O.

[84] If the ore is very rich, a smaller quant.i.ty (0.75 or 1.5 gram) must be taken; otherwise the iron will be insufficient.

[85] MnO_{2} + 4HCl = MnCl_{2} + 2H_{2}O + Cl_{2}.

Cl_{2} + 2KI = 2KCl + I_{2}.

[86] Iodine probably lost by volatilisation.

[87] Obtained as a brown powder by digesting red lead with nitric acid and filtering.