A kinetic method for determination of free vanadium(IV) and (V) at trace level concentrations

A kinetic method based on alkaline phosphatase has been developed to measure free trace levels of vanadium(IV) and (V). The method involves measuring the rate of the alkaline phosphatase-catalyzed hydrolysis of p-nitrophenyl phosphate with (Vi) and without (Vo) a competitive inhibitor in the assay. Michaelis-Menten kinetics for a competitive inhibitor was used to express the relationship between Vo/Vi and the inhibitor concentration. Measuring both Vo and Vi thus yields a Vo/Vi ratio that allows calculation of the competitive inhibitor concentration. Determination of free vanadium in complex fluids can be accomplished by comparing the ratio of rates of p-nitrophenyl phosphate hydrolysis with and without a sequestering agent to the ratios of rates measured on addition of a known vanadium concentration. Free vanadium(V) can conveniently be measured from 10(-7) to 10(-5) M and free vanadium(IV) can be measured at 10(-8) M and above. The error limits on the vanadium determinations range from +/- 3 to +/- 12% of the concentration under investigation depending on the conditions under which the assay was conducted.