A KINETIC METHOD FOR DETERMINATION OF FREE VANADIUM(IV) AND VANADIUM(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 (V1) and without (V0) a competitive inhibitor in the assay. Michaelis-Menten kinetics for a competitive inhibitor was used to express the relationship between V0 V1 and the inhibitor concentration. Measuring both V0 and V1 thus yields a V0 V1 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. © 1990.