A New Method for the Colorimetric Determination of Hypophosphate Ions with the Molybdenum(V)-Molybdenum(VI) Reagent

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Abstract
It was found that hypophosphoric acid reacts with the molybdenum(V)-molybdenum(VI) reagent at room temperature as well as at 95°C to form a blue complex, the absorption spectrum of which has maximums at 590 and 670 mμ. Although orthophosphoric acid reacts with this reagent at 95°C to form a blue complex, it gives only a faint blue color at room temperature. Hypophosphorous and phosphorous acid do not give any colored products when treated with this reagent. Therefore, the reaction of hypophosphoric acid with this reagent at room temperature can be applied to the colorimetric determination of hypophos-phate ions in the presence of hypophosphite, phosphite, and orthophosphate ions. The differences in the absorption spectra and solvent extraction behavior between the two blue complexes due to hypophosphoric and ortho-phosphoric acid (“hypophosphoric heteropoly blue” and “orthophosphoric heteropoly blue”) have been described. The mole ratios of molybdenum to hypophosphoric and orthophosphoric acid in the blue complexes were found to be approximately 9 and 12 respectively.