Separate the phosphoric acid in the form of the yellow precipitate and wash this latter in the usual way. Too high a heat should not be employed, nor should the solutions be allowed to stand too long lest excess of molybdic acid separate. Dissolve the phosphomolybdate in 100 cubic centimeters of cold two and five-tenths per cent ammonia and add as many cubic centimeters of the usual magnesia mixture (fifty-five grams magnesium chlorid and seventy grams ammonium chlorid dissolved in a liter of two and five-tenths per cent ammonia) as there are centigrams of phosphorus pentoxid present. Addition should not be made faster than ten cubic centimeters per minute. Stir during the addition. After the precipitation, stir briskly once more and then allow to stand at least three hours. Wash with two and five-tenths per cent ammonia till the chlorin reaction disappears, dry the filter, and introduce into a well-cleaned crucible which has been thoroughly ignited. Place the lid at an angle, carbonize the filter, and gradually raise the heat, though not higher than a medium red heat, till the pyrophosphate becomes completely white. When this happens bring the blast into action and ignite to constant weight. The weight finally accepted must not change even after half an hour’s ignition. Upon this requirement especial stress must be laid. Pure magnesium pyrophosphate does not suffer any loss even after several hours’ ignition nor does a good platinum crucible. To the weighed amount of pyrophosphate, add the correction given in the table. For example, if the weight be 250 milligrams, the correction to be added is four and two-tenths milligrams, and the correct weight is then 254.2 milligrams. Multiplication of the sum by sixty-four gives the amount of phosphorus pentoxid in the weight taken for analysis. CORRECTION FOR PHOSPHORIC ACID DETERMINATION. Found, Lost, Found, Lost, Mg₂P₂O₇ milligrams Mg₂P₂O₇ milligrams in grams. Mg₂P₂O₇. in grams. Mg₂P₂O₇. 0.10 0.6 0.24 4.0 0.12 0.8 0.25 4.2 0.14 1.2 0.26 4.6 0.15 1.4 0.27 5.0 0.16 1.6 0.28 5.5 0.17 2.4 0.29 6.1 0.18 2.6 0.30 6.8 0.19 3.2 0.31 7.6 0.20 3.5 0.32 8.6 0.21 3.6 0.33 9.6 0.22 3.8 0.34 10.6 When phosphoric acid is to be estimated as pyrophosphate it must always be first separated as molybdate, even when the original solution contains no bases capable of forming insoluble phosphates, as otherwise these corrections will not be applicable.