Abstract

OSMOND [1887] proposed the estimation of P by causing the formation of phosphomolybdic acid and its subsequent reduction to a blue com'pound which could be measured colorimetrically. Of the many variants of this procedure, that of Fiske & Subbarow [1925] has perhaps been the most widely adopted.2 They carried out the reduction in strongly acid solution (0.5N H2SO4) and used as reducing reagent a solution of I-amino-2-naphthol-4-sulphonic acid in sul- phite-bisulphite mixture. The principal advantages claimed for the method were (1) rapidity of colour development; (2) proportionality between colour density and amount of P over a wide range; (3) relative freedom from interference by such substances as ammonium and iron salts, nitrates, nitrites, silicates and chlorides.