Detection test for molybdenum

Journal

of the Less-Common

Metals,

85 (1982)

L5

L5 - L6

Letter

Detection test for molybdenum

B. V. RAO Defence (Received

Metallurgical March

Research

Laboratory,

Kanchanbagh,

Hyderabad

500258

(India)

20, 1982)

During investigations [l] of the influence of various elements on the determination of cobalt(II), which is based on its oxidation in a l,lOphenanthroline or 2,2’-bipyridine medium at pH 2 - 6.5 to the corresponding cobalt(II1) complex with gold(II1) chloride, it was found that the reaction is affected by molybdenum(V1) which is reduced and precipitated as a blue molybdenum complex by cobalt(I1). This reduction proceeds slowly at pH 3 - 2 and rapidly at pH 1. A simple specific detection test has been developed for the presence of molybdenum on the basis of this reduction. In solutions with l,lO-phenanthroline or 2,2’-bipyridine, cobalt(I1) reduces dilute molybdenum(V1) ( low3 M) to a blue molybdenum complex at pH 1 and room temperature. A positive response is obtained with 5 pg of molybdenum, indicating the attainable limit of identification. 0.05 cm3 of 0.2 M l,lO-phenanthroline or 2,2’-bipyridine was placed in a 1 cm3 beaker together with 0.05 cm3 of 0.025 M cobalt(I1) nitrate solution and 0.05 cm3 of molybdenum(V1) solution containing 5 pg, 10 pg or 20 pg of molybdenum. The solution was shaken well, kept for some time and then carefully poured onto a filter paper strip. The presence of molybdenum was indicated by a blue ring on the paper. The reaction was complete in 0.5 min, 1 min and 2 min for 20 pg, 10 pg and 5 pg of molybdenum respectively. The detection of 10 pg of molybdenum is feasible in the presence of 5 pg of copper( iron(II1) or vanadium(V), 10 1.18of zirconium(IV) or selenium(IV), 50 pg of nickel(II), magnesium(I1) or chromium(III), 100 pg of lead(II), manganese(II), zinc(II), cadmium(II), calcium(II), arsenic(III), antimony(III), bismuth(II1) or tungsten(VI), and 200 pg of aluminium(II1) or tellurium(IV). Molybdenum metal is becoming increasingly useful in the nuclear and metallurgical industries. Its identification and differentiation from other common metals is therefore of interest. In the test based on the redox reaction described above one drop of a 1 :1:2 HCl-HN03-Ha0 mixture is introduced onto the clean metal surface. When the reaction is complete, a piece of filter paper of dimensions 0.25 cm X 0.25 cm is dipped into the drop until it is thoroughly moist and is placed in the depression on a spot 0022-5088/82/0000-0000/$02.75

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plate. 0.05 ml of 0.2 M l,lO-phenanthroline or 2,2’-bipyridine and 0.05 ml of 0.025 M cobalt(I1) nitrate solution are added to the centre of the damp spot. A blue coloration indicates molybdenum. To establish the selectivity of this test for molybdenum, several other metals including tungsten were tested but none gave a positive result. The test requires about 1 min and does not spoil the material. Detailed investigations of the possible utilization of the redox reaction between molybdenum(V1) and cobalt(I1) in a phenanthroline or bipyridine medium for a possible quantitative evaluation of molybdenum are in progress and will be reported later.

1 B. V. Rao, S. V. Athavale, T. H. Rao, S. L. N. Acharyulu and R. V. Tamhankar, Anal. Chim. Ada, 70 (1974) 169.