612 



The author believes tliat the differences in the liygroscopic action of 

 superphosphates may often be explained by the differences in the 

 amount of free phosphoric acid they contain, and he further states that 

 superphosphates made with sulphuric acid of 50° Baum6 contain four 

 or five times as mncli free phosphoric acid as those made with 00° acid. 



Various opinions have been expressed as to the solubility and the 

 decomposition of monocalcic phosphate in water,* To farther study 

 this question, weighed quantities of distilled water and the salt in dif- 

 ferent proportions were shaken together in a liask at 150° 0. for 15 

 minutes; the solution was filtered, total and free phos[)horic acid and 

 lime were determined in the filtrate, and the contents of the filter were 

 dried and weighed. 



In the proportion of 1 : 1 (20 grams monocalcic phosphate to 20 c. c. 

 water) 20.34 per cent of the salt was decomposed, the [)rodacts being 

 free acid (7.51 per cent) and crystallized dicalcic phosphate ; the latter 

 remained upon the filter and was equal to 14.53 per cent. The rate of 

 decomposition diminished as the proi)ortion of water to monocalcic phos- 

 pliate was increased. Thus, with 5 grams of salt to 500 c. c. of water 

 (1: 100), 0.95 per cent of the salt ^as decomposed, 0.50 per cent of free 

 acid being formed, and 0.77 per cent of dicalcic phosphate separating^ 

 out. At 1 : 200 no free acid could be recognized ; the solution remained 

 clear (no dicalcic phosphate separated out), and consequently there was 

 no evidence of a decomposition of the salt. The author suspects, 

 however, that decomposition took place to a slight extent. 



Various experiments, in whicli the products of decomposition of the 

 salt were allowed to stand for a longer or a shorter time, with frequent 

 shaking, indicated that no monocalcic phosphate was reformed from the 

 freed phosphoric acid and dicalcic phosphate; but when the solution 

 made by shaking monocalcic phosphate with water was diluted, mono- 

 calcic phosi)hate was reformed from the dicalcic salt and the free acid 

 and no trace of free acid <;ould be found. 



To recapitulate brieily, monocalcic phosphate is not decomposed by 

 lying in the air, even when it absorbs water from the air in consider- 

 able quantities. Monocalcic phosphate is decomposed by water at 

 ordinary temperature; in a concentrated solution the decomposition is 

 energetic; in the proportion 1 : 200 it is so slight that it can not be rec- 

 ognized. The solubility of monocalcic phosi)hate is dependent on the 

 amount of free phosphoric acid present, and this salt is dissolved in 

 water v/ithout decomposition only when it contains an amount of free 

 phosphoric acid corresi)on(ling to the amount which would be formed 

 b}' the decomposition of pure monocalcic i)liospliate. The properties 

 of monocalcic phosphate containing free phosphoric acid are of par- 

 ticular interest in the study of superphosphates and of the changes 



* E. Erleumayer, Verhandlnnji^en d. math. pbys. Klasse d. k. bayer. Akademie, 

 1872, p. 2f)9; Ber. d. d, chein. Gcsdl., 1-^76, p. 1839; H. Watteuberg, Ber. d. d. cbem. 

 Gesell., 9, p.1839; Jouru. f. Laudw.,1879; H. Otto, Zeitscbr. f. augew. Chem., 20 

 (1887), p. 208, eto. 



