A Colorimetric Method for the Determination of Inorganic Phosphate in Blood Serum

The methods most generally used for the determination of phosphates in blood are based upon the reduction of phosphomolybdic acid to the blue molybdic oxide, the color of which is compared to a solution containing a known amount of phosphate. The first method based upon this principle was introduced by Taylor and Miller (1) and was later modified by Bell and Doisy (2), Brig& (3), Benedict and Theis (4), and Fiske and Subbarow (5). However, the conditions necessary to obtain quantitative results have to be very carefully adjusted. Thus Roe, Irish, and Boyd (6) found the following conditions to influence greatly the production of the blue color: the concentration of the molybdic acid; the concentration of the reducing agent; the time allowed for the completion of the reaction; the hydrogen ion concentration; the presence of salts; and the amount of phosphate in the sample to be analyzed. It is evident that a method in which most of these obstacles could be eliminated would be worth while. Such a method is made possible by using a soluble uranium salt for isolating phosphate from solution, since phosphate combines quantitatively with uranium to produce an insoluble salt. A volumetric procedure based upon this principle for the determination of phosphate in urine has been in vogue for many years. Gibson and Estes (7) have modified the method for urine into a calorimetric procedure by adding a solution of uranium acetate of known strength in excess of the amount necessary to precipitate the phosphate completely, and after removing the insoluble uranium phosphate by filtration, determining the excess uranium acetate calorimetrically. Sato (8) applied this technique to feces and urine, but instead of deter-