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INCONNECTION with the experimental preparation of phosphate fertilizers from rock phosphate or limestone and phosphoric acid, it became evident that more complete data were needed on the equilibrium between phosphoric acid and its calcium salts. The published phase-equilibrium data (1, 2, 4, 5) concerning the calcium phosphates in phosphoric acid solutions extend only up to the concentrations of 51, 62, 86, and 66 per cent phosphoric acid at 25°, 40°, 50.7°, and 80°C ., respectively. This investigation was undertaken primarily to extend these data into the region of more concentrated phosphoric acid solutions and higher temperatures. The study completed is a phase-equilibrium investigation of the calcium phosphates in 2 to 98 per cent phosphoric acid solutions at 40°, 75°, and 100°C ., and in 27.5 to 98 per cent phosphoric acid solutions at 25°C.

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