Some Applications of the Colorimetric Phosphate Method

Two years ago, a modification (1) of the Bell-Doisy phosphate method was published and at that time technique ‘was given for its application to inorganic phosphorus in blood and urine. Since then its use has been extended to a variety of analytical procedures, some of which are given below. In principle the method consists of the formation of phosphomolybdic acid and its subsequent reduction by hydroquinone and sulfur dioxide with the production of a stable blue color, proportional to the inorganic phosphorus present. The excess molybdic acid is not reduced. Reagents.-The following reagents are used in all of the procedures to be given. Molybdate Solution.-This contains 5 per cent ammonium molybdate in 5 N HzS04, and is prepared as follows: Dissolve 25 gm. of ammonium molybdate in 300 cc. of water and to this add 75 cc. of concentrated H&SO4 diluted with 125 cc. of water. Hydroquinone Solution.-This is a 1 per cent solution to which a drop of concentrated HzS04 is added to retard oxidation. Sulfite Solution.-A 20 per cent solution of sodium sulfite is used. The strength of sulfite solutions deteriorates by oxidation, but the amount used in color development is sufficient provided an easily detectable odor of SOZ is evolved. Stock KH2PO4 Solution.-This is prepared from pure KHzPOd which has been pulverized and dried for several days over concentrated HzS04. 4.394 gm. are dissolved in a liter of distilled water and then 5 cc. of CHCb added as preservative. 1 cc. = 1 mg. of phosphorus.