=110. Water-Soluble Phosphoric Acid.=—Glaser has modified the volumetric method of Kalmann and Meissels for the volumetric estimation of water-soluble phosphoric acid so as to avoid the double titration required by the original method.[91] If methyl orange be used as an indicator in the original method, the determination does not at once lead to the tricalcium salt, but the liquid still contains, after neutralization, some monocalcium phosphate, which is determined by a further titration with phenolphthalein. In the modified method the total phosphoric acid is estimated in one operation as a tricalcium salt. This is secured, by adding, at the proper time, an excess of calcium chlorid. Two grams of the superphosphate are shaken with water several times, and, after settling, filtered, and the insoluble residue finally washed on the filter until the total volume of the filtrate is a quarter of a liter. Of this, fifty cubic centimeters are taken and titrated with tenth normal soda-lye, with addition of two drops of methyl orange, until the acid reaction has entirely disappeared. There is then added some neutral calcium chlorid solution in excess. If iron and alumina be present, a precipitate is produced of which no account need be made. The acid reaction is thus restored. Five drops of the phenolphthalein solution are added and the titration continued until the alkaline reaction is noted throughout the whole mass. Each cubic centimeter of the soda-lye corresponds, in the first titration, to 7.1, and in the second to 3.55 milligrams of phosphoric acid. =111. Estimation of Phosphoric Acid in the Presence of a Large Excess of Iron.=—The method given below, due to Emmerton, depends upon the precipitation of a phosphomolybdate, of constant composition, in the presence of a large excess of iron, as in the analysis of iron and steel and iron ores.[92] The molybdenum trioxid obtained is reduced by zinc to Mo₁₂O₁₉. The action of permanganate on this compound is shown in the following equation: 5Mo₁₂O₁₉ + 17(K₂OMn₂O₇) = 60MoO₃ + 17K₂O + 34MnO. Seventeen molecules of permanganate are equal to sixty molecules of molybdenum trioxid. The iron or steel is dissolved in nitric acid, evaporated to dryness, heated, and redissolved in hydrochloric acid, then treated again with nitric acid and evaporated until a clear and concentrated solution is obtained free from hydrochloric acid.