Estimation of Molybdenum

In the analysis of molybdenum ores, a solution is best obtained by fusion with sodium carbonate and potassium nitrate and extraction with hydrochloric acid. In the case of iron and steel, the metal may be dissolved in a mixture of hydrochloric and nitric acids with gentle heating so that the iron is oxidised. Sufficient sodium hydroxide is then added to precipitate the iron and to dissolve the molybdic acid on heating. After cooling, the alkali is neutralised with hydrochloric acid, and the molybdenum may then be estimated by one of the methods to be described.

The metal is conveniently precipitated from solution as lead molybdate by means of lead acetate in presence of acetic acid. The precipitate is granular, and, after washing with hot water, is dried, ignited, and weighed as PbMoO₄. By this means molybdenum may be completely separated, if the solution is sufficiently acid, from copper, cadmium, mercury, arsenic, phosphorus, iron, aluminium, chromium, zinc, manganese, nickel, cobalt, calcium, barium, strontium, magnesium, or uranium, but vanadium or tungsten, if present, must first be removed. Molybdenum may also be precipitated as the trisulphide, and the precipitate ignited and weighed as the trioxide, MoO₃. The conversion of the sulphide to the trioxide is complete at about 400° C., and above 450° C. the trioxide sublimes; the temperature must therefore be maintained at 400° to 425° C., and this is best done by using the electric furnace. If the ore under examination has been fused with alkali carbonate and the alkali molybdate extracted, a suitable method of estimation is the precipitation of the molybdenum as mercurous molybdate by means of mercurous nitrate, the precipitate then being converted to the trioxide by gentle ignition. If chromium, vanadium, tungsten, arsenic, or phosphorus are present, these elements will also be precipitated, so that a separation should first be effected.

A method, suitable for the analysis of molybdates and molybdenum ores, consists in heating the substance at 400° to 560° C. in a stream of carbon tetrachloride vapour, when molybdic acid volatilises and may be collected in a receiver, evaporated with nitric acid, ignited, and weighed. If iron is present it also volatilises, and must be separated from the condensed product.

Molybdic acid and soluble molybdates, when boiled with potassium iodide in presence of hydrochloric acid, are reduced, while free iodine is liberated according to the equation

2MoO₃ + 4KI + 4HCl = 2MoO₂I + I₂ + 4KCl + 2H₂O.

The iodine may be estimated by means of a standard thiosulphate solution, or by shaking with specially prepared electrolytic silver in an atmosphere of hydrogen, and measuring the increase in weight of the silver; or, after removing the iodine by boiling, the molybdenum in the reduced solution may be directly estimated by reoxidation in alkaline solution by means of standard iodine or potassium permanganate.

Other methods are similarly based on the reduction of molybdic acid; for example, by means of zinc, magnesium, or sulphur dioxide, the resulting solution then being titrated with standard potassium permanganate. The Jones reductor is most generally used, the molybdic acid solution being passed through a column of amalgamated zinc into a solution of ferric alum. In the absence of titanium, tungsten, chromium, and vanadium, the reduced solution may be titrated with a standard solution of methylene blue. The solution of ferric alum may be replaced by hydrochloric acid containing iodine monochloride solution and chloroform, the liquid then being titrated with potassium iodate solution, when the reaction proceeds according to the equation

KIO₃ + Mo₂O₃ + 2HCl = KCl + Mo₂O₅ + ICl + H₂O.

Small quantities of molybdenum may be determined by reduction with excess of titanous chloride, and titrating back with ferric chloride solution, potassium thiocyanate being used as indicator; or the reduction may be followed electrometrically. If tungsten is present it must be eliminated by precipitation before the addition of the reducing agent.

Molybdenum may be accurately determined in solutions of molybdates by means of electrolysis. In presence of free sulphuric acid the metal is completely precipitated on the cathode as the hydrated sesquioxide. Hydrochloric acid may be used instead of sulphuric acid to accelerate the deposition. The precipitate after washing may be converted by gentle ignition to molybdenum trioxide, and weighed as such.