Abstract
The effect of impurities and its control in the colorimetric determination of phosphoric acid by vanadomolybdic acid method has been investigated, . The coloring agent is made as follows: (1) A solution containing NH4VO3 1.12g/l, (NH4)6Mo7O24·4H2O 27g/l and HNO3 (sp. gr. 1.38?1.40) 250cc/l is used for the samples containing no citric acid as in the case of analysis of total- or water-soluble phosphoric acid. (2) A solution containing NH4VO3 1.12g/l, (NH4)6Mo7O24·4H2O 50?55g/l and HNO3(sp. gr. 1.38?1.40) 150cc/l is used for the samples containing soluble- or citrate-soluble phosphoric acid such as the solution obtained by the extraction of 2cc Peterman's citrate solution (equivalent to 0.34g citric acid) or that containing 0.34g citric acid and 0.7N HNO3 added for controlling the effect of silicic acid. Addition of nitric acid serves to eliminate the disturbing effect of silicic acid in a sample solution containing less than 8mg P2O5. For the actual use of the reagent (1) or (2), 20cc of the reagent was added to 80cc of the sample solution. These coloring agents were used for determination of phosphoric acid in various, fertilizers and good results were obtained.
