Fundamental Conditions and Reaction Mechanisms in Formation of Phosphomolybdenum Blue

Abstract

Abstract The effects of acidity and molybdate concentration on the development of phosphomolybdenum blue have been studied in detail and the relationships among these three factors shown by a graph, called a “phosphomolybdenum blue color field” by the author. A further investigation was undertaken in order to explain the mechanisms of color development and the fundamental properties of reaction products under the optimum conditions established by the author for the determination of phosphate. The acidity and molybdate concentration in the graph was divided into 4 fields, the fields denoting the concentration conditions given below. Field 1: All phosphate forms phosphomolybdate, and then forms phosphomolybdenum blue. No blue color due to the reagent blank is obtained. This field is suitable for the phosphomolybdenum blue method. Field 2: All phosphate forms phosphomolybdate, and then forms phosphomolybdenum blue. However, the reagent blank also becomes blue. This field is suitable for the phosphomolybdate method. Field 3: Only a part of the phosphate forms phosphomolybdate and then phosphomolybdenum blue. Heating promotes the reaction, making it suitable for the phosphomolybdenum blue method. Field 4: No phosphomolybdate is formed. This field is not applicable for determinations. When the reaction of phosphomolybdenum blue development is carried out in a solution containing 0.65 n sulfuric acid and 0.15% ammonium molybdate, which is optimum for the determination of phosphate, phosphate is completely converted to phosphomolybdate, which in turn is quantitatively reduced by stannous chloride to form phosphomolybdenum blue. At the first stage of the reduction, one phosphomolybdenum blue molecule is regarded as possessing seven molybdenum atoms that have been reduced to molybdenum (V). Reduction by the use of excess amounts of stannous chloride proceeds in the following order: (1) Phosphomolybdate → Phosphomolybdenum blue (2) Molybdate reagent → Molybdenum of low valency (3) Phosphomolybdenum blue → Phosphomolybdenum blue of a low valency The reduction product in reaction 2 is probably a molybdenum (III) compound. A large excess of stannous chloride fades the blue color of the solution by means of reaction 3. Reaction 3 is negligible compared with reactions 1 and 2 in solutions containing a slight excess of stannous chloride. Under the conditions established by the author for color development, no blue product is formed as a results of the reduction of the molybdate reagent. Molybdate is considered to have been reduced directly to reddish brown molybdenum (III). The knowledge obtained from these experiments is thought to be enough for the performance of the phosphomolybdenum blue method, although it is still insufficient for a complete explanation of the properties of the reactions and the reaction products.