140 The Citric Sofiibilit!/ of Mineral P/iosp/idtea 



Vril. DICALCIUM PHOSPHATE EXPERIMENTS. 



The observed amount of phosphate dissolved by citric acid per cent, 

 of the theoretical amount for a completed reaction found in these experi- 

 ments may be contrasted with corresponding figures obtained from the 

 interaction between a pure substance likedicalcium phosphate and hydro- 

 chloric acid. A series of experiments was conducted with this end in 

 view. In each experiment a constant ([uantity (25 grams) of precipitated 

 dicalcium phospliate (Calir042H20) was shaken for half an hour in a 

 constant volume (500 c.c.) of water. The proportion of hydrochloric 

 acid was varied as shown in Ta])k' X. Tlie following results were obtained : 



Table X. 



These results show the decreasing percentage of phosphate di.s.solved 

 with decreasing acid concentration as usually obtained in such experi- 

 ments. The result, however, to be contrasted with the corresponding 

 previous results, is the relative completeness of the reaction as seen in 

 cols. I and II of the above table (Table X). Since there is excess of 

 dicalcium phosphate the undernoted equation may be taken to represent 

 the reaction namely, 



2 CaH PO4 ! 2 1 1( '1 1; ( 'aH4(P04)2 + CaCl^ (2) 



In this equation two molecules of hydrochloric acid are used up to 

 produce one molecule of monocalcium phosphate and one molecule of 

 calcium chloride. Hence if we start with w gram-molecules of hydro- 

 chloric acid we shall have, for a completed reaction, liv gram-molecules 

 of monocalcium phosphate, the substance determined. If iv = molecular 

 concentration of iiydrochloric^ acid per litre at the beginning of the 

 experiment and u -^ molecular concentration of phosphate expressed as 

 Ca3(F04).2 per litre at the end of 30 minutes shaking when equilibrium 

 may be presumed in this case to be established, then according to the 



