Continuous-flow determination of serum inorganic phosphate with a single reagent--the vanadomolybdate method re-evaluated.

The merits of the vanadomolybdate method in the determination of inorganic phosphate are highly underestimated with regard to the simplicity of the method and the stability of both the reagents and the color that is formed. The absorption curve of the phosphovanadomolybdate complex, with its maximum at 335 nm, extends into the visible range of the spectrum. This permits measurements with inexpensive tungsten-lamp colorimeters. On-stream dialysis is best done in a nitric acid medium, 0.15 mol/liter. Paralleled by the change in the H2PO4-/H3PO4 ratio, appreciable protein binding and poor dialysis efficiency are seen at lower acid concentrations (pH greater than 1.0). Optimum reaction-mixture concentrations of vanadium and molybdenum appeared to be respectively 0.2 and 5 mmol/liter up to 3 mmol of phosphate per liter, in a final acid medium of 0.2 mol/liter, concentrations considerably lower than those used in some studies published earlier. Color development with the stable combined reagent is complete after only 20 s at room temperature and the color is stable for at least 2 h. Figures on precision and accuracy demonstrate the reliability of the method.