Detection of Molybdenum

Molybdenum most frequently occurs in the hexavalent state in derivatives of the trioxide MoO₃, this being the fully oxidised condition and its most stable form. The lower oxides and their salts can readily be oxidised to molybdates, and this is usually done before the metal is estimated.

When mixed with fusion mixture and heated on carbon in a reducing flame, molybdenum compounds yield a blue or dark red incrustation. The borax bead gives a brown colour when hot, which becomes green on cooling, but more distinct colours are obtained with a bead of microcosmic salt. Molybdenum compounds impart a faint yellowish-green colour to the Bunsen flame. Traces of molybdic acid and its compounds may be detected in a powder by adding a few particles of the latter to a drop of sulphuric acid on a platinum foil or on porcelain and heating until thick fumes are evolved; on cooling and moistening the residue with the breath, an intense blue colour appears, which vanishes on heating but reappears on cooling. The colour is permanently destroyed if water is added.

When hydrogen sulphide is passed through an acidified solution containing molybdenum, the trisulphide is thrown down. The precipitate dissolves in ammonium sulphide, and it is therefore in ordinary analysis separated with the Group IIb metals, namely, arsenic, antimony, tin, gold, and platinum. The last four metals may be precipitated by addition of metallic zinc, the arsenic expelled by evaporation, and, after taking to dryness with nitric acid, the molybdenum may be extracted from the residue with ammonia. The trisulphide may be reprecipitated directly by the addition of nitric acid to the solution in ammonium sulphide. A soluble sulphide added to a solution of ammonium molybdate gives a blue colour.

Various colour effects are obtained by the addition of reducing agents to solutions of molybdenum compounds. Zinc in the presence of acid, and stannous chloride, produce colours changing from blue to green, brown, and yellow. Sulphur dioxide produces a bluish-green coloration or precipitate according to the amount of molybdenum present. A blue precipitate is also obtained when sodium thiosulphate is added to a slightly acid solution of ammonium molybdate.

Precipitation reactions with ordinary reagents are not very characteristic. A neutral solution of molybdate gives with silver nitrate a white, curdy precipitate soluble in ammonia or nitric acid, and with barium chloride a white precipitate soluble in hydrochloric acid. Disodium hydrogen phosphate added to a solution of ammonium molybdate gives a yellow precipitate of ammonium 12-molybdophosphate insoluble in nitric acid, soluble in ammonia.

Numerous colour reactions for the detection of molybdenum have been described, some of which are extremely sensitive. Hydrogen peroxide gives a yellow colour with a molybdate in presence of sulphuric acid; if the solution is first evaporated and the residue treated with concentrated ammonia before adding the hydrogen peroxide, a cherry-red or pale rose coloration, due to the formation of ammonium permolybdate, is obtained. A delicate test for molybdenum in presence of mercury consists in precipitating mercurous iodide by means of potassium iodide and redissolving the precipitate in excess of the reagent, whereupon reduction to mercury takes place which in its turn reduces the molybdate and the supernatant liquid turns blue. Very dilute solutions of molybdates are not affected. The blue colour may be obtained with tungstates, although the test is of little value for their detection; in order to determine whether the blue colour is due to molybdenum or tungsten, potassium thiocyanate should be added, when, if the former is present, the blue colour turns to blood-red. In a similar manner reduction of a molybdate by means of zinc in presence of thiocyanate yields a crimson colour; if iron is present, the red colour first produced disappears as reduction proceeds and the crimson for molybdenum develops. When a solution of a molybdate containing an alkali thiocyanate is treated with concentrated hydrochloric or sulphuric acid, a purple colour is produced, while, if instead of the mineral acid acetic acid is used, the colour is yellow, and if the solution is concentrated, yellow acicular crystals, of composition R^•SCN.R^•₂O.4MoO₃.5H₂O (R^• = K, NH₄), separate. Reduction of a molybdate by means of stannous chloride in presence of a thiocyanate, and extraction of the red compound formed by means of ether, affords an extremely delicate test for traces of molybdenum, 1 part of molybdenum in 625,000 parts of water being readily detected.

Similar colour reactions to the above are based on the reducing action of certain organic compounds. For example, a solution of an alkali molybdate in acetic acid gives an orange colour with pyrogallol or pyrocatechol, and a deep blue colour with quinol or hydrazine, while with excess of phenylhydrazine a red colour is obtained. An alcoholic solution of diphenylcarbazide gives an indigo-violet colour when added to a slightly acid solution of a molybdate, a violet precipitate being formed with excess of the reagent; tungsten, vanadium, and titanium give no reaction. Tannin gives an orange coloration when added to a neutral solution of molybdic acid, which becomes cherry-red in concentrated, and yellow in dilute, solutions. The colour is not altered by boiling, but is destroyed by the addition of acids.

One of the most sensitive reagents for molybdenum is xanthic acid. It was first observed by Siewert that this reagent gave a characteristic red colour with molybdenum compounds. To a freshly prepared xanthate solution, made by shaking a solution of potassium hydroxide in absolute alcohol with excess of carbon disulphide until no more of the latter dissolves, a 30 per cent, solution of acetic acid is added until a slight yellow turbidity appears. This solution is added drop by drop to the solution to be tested, and if molybdenum is present a red colour develops, the intensity of which is proportional to the amount of molybdenum present. The test is capable of detecting 0.00000064 gram of molybdenum per c.c., and is not affected by the presence of vanadium, tungsten, titanium, or uranium, so that it may be used for detecting the element in iron and steel. Chromates give a dark colour with xanthic acid, and should therefore be first reduced if present.