(_b_) _Insoluble Phosphoric Add._—Moisten, in a porcelain dish, ten grams of the powdered sample with water; add fifty cubic centimeters of concentrated sulfuric acid, and heat for fifteen minutes so high that fumes of sulfuric acid will escape. When the mass has cooled, wash it into a half liter graduated flask, fill to the mark, and shake well. After filtration, the clear filtrate may, after some time, turn turbid by separation of calcium sulfate, but as the ammonium citrate, which is afterwards added, again brings the precipitate into solution, it is of no importance. Take fifty cubic centimeters of the solution, corresponding to one gram of the powdered sample, add twenty cubic centimeters of the citric acid solution, neutralize the mixture approximately, but not exactly, by ammonia; after cooling, add twenty-five cubic centimeters of magnesia mixture; stir the fluid by means of a feather, as described above, till no more precipitate is formed, and finally add thirty-three cubic centimeters of ammonia while stirring for a couple of minutes more; after half an hour the precipitate may be separated by filtration, washed, and ignited, as usually directed. The above process is essentially the one used with basic slags. When much organic matter is present, by continuing the heating with sulfuric acid for some time, it may be destroyed. =66. Methods Adopted by the Brussels Congress, 1894.=—The report of the committee on methods of analysis of phosphoric acid requires the molybdate method to be used in all cases where the quantity to be determined is very small. In other cases the citrate method may be employed.[58] (1) _Soluble Phosphoric Acid._—The soluble phosphoric acid is determined by the method adopted at Brussels in the following manner: Five grams of the sample are rubbed to a powder in a mortar, and then from fifty to sixty cubic centimeters of water added. After allowing to settle for a few minutes the liquid portion is decanted upon a filter. This operation is repeated three or four times. Finally the solid portions are washed upon the filter, and the washing with water is continued until the filtrate amounts to about three-quarters of a liter. A few drops of hydrochloric acid are added until the filtrate is perfectly clear, and the volume is then made up to one liter. Fifty cubic centimeters of the solution are then treated with thirty cubic centimeters of ammonium citrate solution and one-third as much ammonia. Afterwards thirty cubic centimeters of magnesia mixture are added, drop by drop, with constant stirring. For superphosphates containing more than eighteen per cent of phosphoric acid only one gram is taken, for ordinary superphosphates two grams, and for compound fertilizers four grams. The sample is first treated as above for soluble acid until the filtrate amounts to 200 cubic centimeters, then clarified with a drop of nitric acid, and made up to a quarter of a liter.