Studies on the colorimetric determination of phosphate by the molybdenum blue method

Abstract

In the quantitative determination of phosphate by the colorimetric method of molybdenum blue with the use of stannous chloride as a reducing agent, the conditions for this reaction have been investigated in detail and the following results were obtained.
(1) The temperature of reaction affected the intensity of coloration. The error was about 1%/1°C at around 25°C, and the determination became impossible at above 70°C due to the formation of a white precipitate.
(2) The concentration of acid and the concentration of ammonium molybdate showed considerable influence on the intensity of coloration. Many experiments were carried out on these problems, and the relationships among these three factors are shown by a graph. The results indicated that the optimum concentration of sulfuric acid for quantitative determination was 0.65 N and that of ammonium molybdate was 0.15%.
(3) A change in the amount of stannous chloride added had practically no effect on the intensity of coloration.