A spectrophotometric study of phosphomolybdenum blue formed by the reaction of phosphate with a mixture of molybdenum(V) (Mo2O42+) and molybdenum(VI) and application to the spectrophotometric determination of small amounts of phosphates

The spectrophotometric determination of trace amounts of phosphates, based on the color development of phosphomolybdenum blue formed by the reaction of phosphate with a mixture of molybdenum(V) (Mo2O42+) and molybdenum(VI), is described. The rates of formation of phosphomolybdenum blue were measured at 250C in a perchloric acid medium. The maximum constant color development was obtained in the perchloric-acid range from 0.20 to 0.41 M under given conditions by warming the solution for 10 min at 800C. The method obeys Beer's law in the phosphate-concentration range from 0.08 to 1.16 μg/ml. The molar absorptivity at 840 nm was calculated to be 2.4 x 104 l mol-1 cm-1. The rate is inverse fifth-order with respect to the perchloric-acid concentration in the range from 0.20 to 0.41 M and is first-order with respect to the molybdenum(V) concentration. The formation rates increase with the phosphate and molybdenum(VI) concentrations, although the rate dependence on the concentrations of these reactants is complicated. The ratio of P: Mo(V) for phosphomolybdenum blue was determined to be 1:1 using the molar-ratio method. (auth.)