Where the samples contain no ingredient capable of attacking aluminum, they can be conveniently dried, in circular dishes of this metal about seven centimeters in diameter and one centimeter deep, to constant weight, at the temperature of boiling water.
18. Moisture in Monocalcium Phosphates.—In certain fertilizers, especially superphosphates, containing the monocalcium salt, the estimation of water is a matter of extreme difficulty on account of the presence of free acids and of progressive changes in the sample due to different degrees of heat.
Stoklasa has studied these changes and reaches the following results[10]:
A chemically pure monocalcium phosphate of the following composition, viz.,
| CaO | 22.36 | per | cent. |
| P₂O₅ | 56.67 | “ | “ |
| H₂O | 21.53 | “ | “ |
was subjected to progressive dryings. The loss of water after ten hours was 1.83 per cent; after twenty hours, 2.46 per cent; after thirty hours, 5.21 per cent; after forty hours, 6.32 per cent; after fifty hours, 6.43 per cent. This loss of water remained constant at 6.43 per cent. This loss represents one molecule of water as compared with the total molecular magnitude of the mass treated. A calcium phosphate, therefore, of the following composition, CaH₄(PO₄)₂·H₂O loses, after forty hours, drying at 100°, its water of crystallization. The calcium phosphate produced by this method forms opaque crystals which are not hygroscopic and which give, on analysis, the following numbers:
| CaO | 24.02 | per | cent. |
| P₂O₅ | 16.74 | “ | “ |
| H₂O | 15.09 | “ | “ |
The temperature can be raised to 105° without marked change. If the temperature be raised to 200° the decomposition of the molecule is hastened according to the following formula:
4 CaH₄(PO₄)₂ = Ca₂P₂O₇ + Ca(PO₃)₂ + CaH₂P₂O₇ + 2 H₃PO₄ + 4 H₂O.
The chemical changes during the drying of monocalcium phosphates can be represented as follows, temperature 200° for one hour: