The apparatus has proven extremely satisfactory in the precipitation of ammonium magnesium phosphate. The precipitate is very crystalline, and where the stirring is continued for some minutes, after the magnesia solution has all been added, no amorphous precipitate is observed on longer standing.
132. The Citrate Method Applied to Samples with Small Content of Phosphoric Acid.—It is well established that the citrate method does not give satisfactory results when applied to samples containing small percentages of phosphoric acid, especially when these are of an organic nature, as for instance, cottonseed cake-meal. In this laboratory attempts have been made to remedy this defect in the process so as to render the use of the method possible even in such cases.[120] Satisfactory results have been obtained by adding to the solution of the cake-meal a definite volume of a phosphate solution of known strength. Solutions of ordinary mineral phosphates are preferred for this purpose. The following example will show the application of the modified method:
In a sample of cake-meal, (cottonseed cake and castor pomace) the content of phosphoric acid obtained by the molybdate method, was 2.52 per cent.
Determined directly by the citrate method, the following data were obtained:
Allowing to stand thirty hours after adding magnesia mixture, 1.08 and 1.53 per cent in duplicates.
Allowing to stand seventy-two hours after adding magnesia mixture, 2.17 and 2.30 per cent in duplicates.
In each case fifty cubic centimeters of the solution were taken, representing half a gram of the sample.
In another series of determinations twenty-five cubic centimeters of the sample were mixed with an equal volume of a mineral phosphate solution, the value of which had been previously determined by both the molybdic and citrate methods. The fifty cubic centimeters thus obtained represented a quarter of a gram each of the cake-meal and mineral phosphates. The filtration followed eighteen hours after adding the magnesia mixture. The following data show the results of the determinations:
| Per cent P₂O₅ mineral phosphate. | Per cent P₂O₅ in organic sample. | Per cent P₂O₅ found in mixture × 2. | Per cent P₂O₅ in organic sample. | |
|---|---|---|---|---|
| 1 | 15.37 | 2.52 | 17.90 | 2.53 |
| 2 | 29.16 | 2.52 | 31.68 | 2.52 |
| 3 | 31.37 | 2.52 | 33.83 | 2.45 |
| 4 | 31.58 | 2.52 | 34.20 | 2.62 |
| Mean content of P₂O₅ in organic sample | 2.53 | |||
It is thus demonstrated that the citrate method can be applied with safety even to the determination of the phosphoric acid in organic compounds where the quantity present is less than three per cent. It is further shown that solutions of mineral phosphates varying in content of phosphoric acid from fifteen to thirty-two per cent may be safely used for increasing the content of that acid to the proper degree for complete precipitation. In cases where organic matters are present they should be destroyed by moist combustion with sulfuric acid as in the determination of nitrogen to be described in the next part.