The determination of serum phosphate by the molybdivanadate method.

One of the commonly used methods for the calorimetric determination of phosphorus is based upon the reduction of molybdic acid to molybdenum blue and subsequent estimation of the intensity of the blue color formed.. An excellent discussion of the molybdenum blue reaction and its limitations is given by Woods and Mellon (1). In using this reaction over a period of years the authors have been increasingly aware of its shortcomings. Acid stannous chloride was used as the reducing agent and the procedures of Kuttner and Cohen (2), Woods and Mellon (l), and Fontaine (3) were carefully followed. In addition, modifications in concentration and type of both acid and molybdate have been studied. The results obtained were in accord with those of Woods and Mellon (1) who found that the relative amounts of acid and molybdate were important since, if the ratio of acid to molybdate was too low, some of the molybdate was reduced along with the heteropoly acid, and if too high, there was a marked decrease in the intensity of the color. In general, the difficulties encountered in our laboratory with the use of chlorostannous acid as the reducing agent were (1) control of concentration of acid and molybdate, (2) instability of the color after a short time interval, (3) non-conformity with Beer’s law when the concentration of phosphorus was above 4 parts per million, (4) rapid deterioration of the stannous chloride solution, particularly of the dilute reagent, and (5) inability to obtain duplicate results from day to day when standard solutions of phosphate were used. Three recent methods for the determination of phosphate have made use of ammonium vanadate with subsequent calorimetric estimation of the yellow molybdivanadophosphoric acid formed. Willard and Center (4) described a method for the determination of phosphorus in iron ore, Koenig and Johnson (5) one for the determination of phosphorus in foods and biological material, and Kitson and Mellon (6) a method used for the determination of phosphorus in carbon and low alloy steels. Since the procedure seemed to offer distinct advantages from the standpoint of stability of the final color and simplicity, the application of the method of Kitson and Mellon to the determination of serum phosphate has been investigated.