A. A. Ramsay 295 



When calcium carbonate is dissolved in citric acid solution the addition 

 of phosphoric acid causes no precipitate. 



Calcium phosphates dissolve in citric acid solutions, the solution 

 remaining clear. These solutions will then dissolve large quantities of 

 calcium carbonate and still remain clear. On continuing the addition 

 of calcium carbonate, a critical point is reached at which a faint cloudi- 

 ness or precipitate appears. This is Si mixture of calcium phosphate 

 and calcium citrate, but immediately after the cloudiness has become 

 apparent the solution becomes like a magma and calcium citrate is 

 precipitated which contains only traces of phosphoric acid. Apparently 

 then in a mixture of calcium phosphate and citrate lime salts precipitate 

 calcium citrate. Again calcium citrate in aqueous solutions of calcium 

 phosphate renders both lime and phosphoric acid soluble. 



The following was tried : 



A, Calcium phosphate + 75 c.c. water. B, Calcium phosphate (as 

 in A), calcium citrate, 75 c.c. water. Both were allowed to stand over- 

 night. On filtering aliquots of A and B it was found that filtrate A 

 contained both lime and phosphoric acid while filtrate B contained 

 about 10 times as much hme as filtrate A with only about 3| times as 

 much phosphoric acid as in filtrate A. 



Summary. 



The results of these investigations show : 



1. The substances sold as "Phosphate of lime" and "Calcii Phos- 

 phas B.P." are not pure tricalcic phosphate but are mixtures of di- and 

 tricalcic phosphates. 



2. Sodium phosphate (Na2HP04) added to ammoniacal calcium 

 chloride and the resulting precipitate washed with water yields a 

 mixture of di- and tricalcic phosphate and calcium hydrate. 



3. Bone ash dissolved in hydrochloric acid and precipitated with 

 ammonia, the precipitate being well washed, yields also a mixture of 

 di- and tricalcium phosphate and calcium hydrate. 



4. When three equivalents of Ume (3CaO) are made to act on one 

 equivalent of phosphoric acid (P2O5) and the resulting precipitate 

 removed with little delay pure tricalcium phosphate is obtained. 



5. When two equivalents of lime (2CaO) are made to act on one 

 equivalent of phosphoric acid (P2O5) the product is not dicalcic 

 phosphate but is a mixture of di- and tricalcium phosphate. 



6. In the case of pure tricalcium phosphate, about 91 % of the 

 total phosphoric acid is soluble in the prescribed 2 % citric acid solution 



• 20—2 



