ALL the known colorimetric methods for the determination of phosphate are liable to errors arising from (a) slight changes in acidity and concentration of reagents, and (b) the presence of interfering substances (e.g. fluorides, nitrites etc.). The following method is almost entirely independent of these disturbing factors, and possesses the added advantage of being more sensitive than those previously described. EXPERIMENTAL Principle of the methodThe method is based on the ready solubility of the reducible phosphomolybdic acid in isobutyl alcohol. It consists essentially of the reduction of phosphomolybdic acid to the blue complex by shaking the alcoholic extract with an acidified aqueous solution of stannous chloride. Since the extraction occurs readily over a wide acid range (0.05-1.5 N H2SO4), and is not affected by the presence of excess of molybdic acid, considerable variation in the concentrations of these reagents is permissible without any danger of interference with the ultimate colour production. The concentration of reducing agent may also vary over a wide range, while the effects of interfering substances are prevented by the use of high concentrations of molybdate and reducing agent. Solutions required(1) 10 N sulphuric acid (approx.): 28 % conc. H2SO4. (3) 5 % ammonium molybdate (Kahlbaum "for analysis"). This solution must be kept in a bottle, coated inside with paraffin wax, to prevent the slow formation of a silicon compound of molybdic acid which yields a blue colour on reduction.(4) Stock solution of stannous chloride:. 10 g. stannous chloride is dissolved in 25 ml. conc. HCl (kept in a brown glass-stoppered bottle.

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