Reaction between nitrite and vanadium(V) on determination of vanadium, and modification of diphenylamine sulfonate method

Abstract

The procedure of diphenylamine sulfonate colorimetry for determining vanadium has based on the following four reactions, i. e., (1) oxidation of vanadium(IV) to vanadium(V) by permanganate, (2) reduction of excessive permanganate by nitrite, (3) decomposition of excessive nitrite by urea, and (4) coloration of diphenylamine sulfonate by vanadium(V).
The reaction between nitrite and vanadium(V), which might cause an error for determination on above process (2) was investigated in this paper. Vanadium(V) was reduced to vanadium(IV) by nitrite more easily in sulfuric-phosphoric acid solution than in sulfuric acid solution. When no urea was used, remarkable decreases of vanadium(V) concentration were observed by the addition of 88 μg of sodium nitrite in sulfuric-phosphoric acid solution and by 350 μg in sulfuric acid solution. When urea was added initially to the solution before the nitrite treatment, a considerable error for determination was caused by the addition of 1 mg of sodium nitrite in sulfuric-phosphoric acid solution, while 1.5 mg of sodium nitrite did not affect in sulfuric acid solution.
The improved procedure recommended by the authors is as follows. (1) Relatively lower concentrations of 0.3% are proper for both of potassium permanganate and sodium nitrite solution. (2) Phosphoric acid required to control the affect of iron(III) should be added after the reduction process of permanganate by nitrite.