1909-10.] A New Hydrate of Orthophosphoric Acid. 
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V. — A New Hydrate of Orthophosphoric Acid. By Alexander 
Smith and Alan W. C. Menzies. 
[Abstract.^ 
Our purpose was, by a systematic study of the solubilities concerned, to 
ascertain whether any hydrates of orthophosphoric acid, other than a 
semi-hydrate, 2H 3 P0 4 ,H 2 0 (Joly’s hydrate), exist. The solubilities of 
Temperature. 
Fig. 1. 
phosphoric acid and Joly’s hydrate at various temperatures had not 
previously been measured. Crystalline, anhydrous phosphoric acid and 
Joly’s hydrate were prepared in pure form. The solid was stirred in a 
thermostat with water until equilibrium was reached at each temperature, 
and the concentration of the solution was determined. The diagram 
(fig. 1) shows the results. AB is the curve of solubilities of Joly’s hydrate 
from — 16° to 29'35°, its melting point. BC contains the melting points 
of Joly’s hydrate, depressed by phosphoric acid. CD is the solubility 
curve of a new hydrate, 10H 3 PO 4 ,H 2 O. DE is the solubility curve of 
anhydrous phosphoric acid, and E is the melting point of the latter (42‘30°). 
