Studies on the colorimetric determination of phosphate

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Abstract
The difficulties in perfecting the method for the colorimetric determination of phosphate (based on the reduction of phosphomolybdic acid) are due to inadequate knowledge of the chem. mechanism of the reaction; hence quantitative investigations were carried out on the reducibility of molybdic acid in the absence and presence of phosphate. All the reducing agents tested reduce molybdic acid in the absence of phosphate. The reduction velocity is greatest at about 0.2 N, and diminishes rapidly to 0 with increasing acidity. Phosphates accelerate the reduction of molybdic acid; this acceleration occurs over a wide range, but decreases with increasing acidity. The rate of reduction of molybdic acid, the accelerating effect of phosphate and the end points reached in these reactions all depend on the concn. of molybdate and on the concn. and nature of the reducing agent: the stronger the reducing agent, the greater the number of molecules of MoO3 reduced per atom of P. The theory that the reduction of molybdic acid is catalytically activated by phosphate was confirmed. The reaction does not go to completion. but reaches definite end points, because the product of the reduction inactivates the catalyst.

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