A colorimetric method for determining phosphorus is described, using perchloric acid instead of sulfuric acid for the oxidation of organic matter. The method involves the reduction of phosphomolybdic acid to a blue color, which is proportional to the phosphate concentration. The reducing agent used is 1:2:4-aminonaphtholsulphonic acid, which produces a full blue color in 5 minutes and provides good color proportionality over a wide range of phosphate concentrations. This agent is less affected by interfering substances such as ammonium and iron salts, nitrites, nitrates, chlorides, and silicates compared to quinol. The method combines the procedures of Martland and Robison and Fiske and Subbarow, retaining high acidity and keeping acid and molybdate separate. Perchloric acid is preferred for total phosphorus determination due to its superior oxidizing properties, especially at high temperatures, and its ability to dissolve most of its salts. It is also less likely to cause superheating or etching in glassware compared to sulfuric acid. The method is suitable for both free and total phosphorus determinations, and the use of perchloric acid eliminates the need to remove barium from barium salts. The procedure involves measuring the sample, adding perchloric acid, molybdate, and reducing agent, and reading the color after 5 minutes. The method is effective in the presence of various interfering substances, as demonstrated by the results in Table I. The method is also applicable to the determination of inorganic phosphate in trichloroacetic acid filtrates of blood, urine, and other solutions where barium is not present. The use of sulfuric acid in such cases is also described. The method has been found to be effective for the determination of total phosphorus in barium salts of phosphoric acid esters, as shown in Table II.A colorimetric method for determining phosphorus is described, using perchloric acid instead of sulfuric acid for the oxidation of organic matter. The method involves the reduction of phosphomolybdic acid to a blue color, which is proportional to the phosphate concentration. The reducing agent used is 1:2:4-aminonaphtholsulphonic acid, which produces a full blue color in 5 minutes and provides good color proportionality over a wide range of phosphate concentrations. This agent is less affected by interfering substances such as ammonium and iron salts, nitrites, nitrates, chlorides, and silicates compared to quinol. The method combines the procedures of Martland and Robison and Fiske and Subbarow, retaining high acidity and keeping acid and molybdate separate. Perchloric acid is preferred for total phosphorus determination due to its superior oxidizing properties, especially at high temperatures, and its ability to dissolve most of its salts. It is also less likely to cause superheating or etching in glassware compared to sulfuric acid. The method is suitable for both free and total phosphorus determinations, and the use of perchloric acid eliminates the need to remove barium from barium salts. The procedure involves measuring the sample, adding perchloric acid, molybdate, and reducing agent, and reading the color after 5 minutes. The method is effective in the presence of various interfering substances, as demonstrated by the results in Table I. The method is also applicable to the determination of inorganic phosphate in trichloroacetic acid filtrates of blood, urine, and other solutions where barium is not present. The use of sulfuric acid in such cases is also described. The method has been found to be effective for the determination of total phosphorus in barium salts of phosphoric acid esters, as shown in Table II.