The reaction between hexacyanomanganate(IV) and hydrogen peroxide in a
cidic medium is strongly catalysed by vanadium. The stoichiometry has
been proved to be Delta[Mn(IV)]/Delta[H2O2] = 1. S-shaped absorbance-t
ime curves are obtained for a wide range of conditions, which seems to
be due to the transient formation of hydroperoxyl radicals, HQ,, ther
efore, the reaction is autoinduced. The reaction kinetics is first ord
er on the hexacyanomanganate(IV) concentration. Complex dependence of
the initial rate on the H2O2 as well as H+ concentrations are observed
both for the uncatalysed and for the catalysed reactions, but the rat
e laws are given and reaction mechanisms are proposed. The catalytic e
ffect of vanadium has been used to determine traces of vanadium (the (
IV) and (V) oxidation states are determined together). By applying the
initial rate method a detection limit of 0.9 ng ml(-1) (18 nM), and a
linear range up to 50 ng ml(-1) were found. The relative standard dev
iations for the 4 and 25.5 ng ml(-1) levels were 5.2 and 2.5% respecti
vely. Positive kinetic interferences from Cr(VI), Hg(I) and Ag(I) have
been observed; other interferences are less severe and Cr(III), Fe(II
) and Fe(III), among many others, do not interfere. The Mn(IV) solutio
n must be prepared daily and its conservation needs some care (0 degre
es C) but the proposed method has been applied to the determination of
vanadium in a phosphate rock (reference material, BCR No. 32) without
previous separations. Recovery experiments have also been performed;
excellent results were obtained in both cases. (C) 1998 Elsevier Scien
ce B.V. All rights reserved.