296 The Solubility of Calcium Phosphates in Citric Acid 



in 30 minutes when following the method adopted for the determination 

 of "citrate soluble" phosphoric acid. 



7. By the simple addition of calcium carbonate to a pure tricalcic 

 phosphate, the "citrate solubihty" of the phosphoric acid is reduced 

 from 91 to 84%. 



8. The availabihty of the phosphoric acid as judged by the ex- 

 traction with 2 % citric acid solution is inexact, since the availabihty 

 of the phosphoric acid in pure tricalcium phosphate is reduced from 

 91 % to 84-5 % by the addition of 14 % of hme as carbonate, and to 

 84-3 % by the further addition of 14 % lime as carbonate. 



9. The prescribed 2 % citric acid solution is more correctly a 

 solvent for lime than for phosphoric acid, since broadly speaking, the 

 whole of the excess of lime beyond that present as tricalcic phosphate 

 goes into solution in the first 30 minutes' extraction. 



10. Pure tricalcic phosphate is largely soluble in 2 % citric acid 

 solution, as are also the so-called tricalcic phosphates produced by the 

 addition of ammonia to acid solutions of tricalcic phosphate, or by the 

 mixing of disodium hydrogen phosphate with ammoniacal calcium 

 chloride. (These are mixtures of di- and tricalcium phosphate in 

 varying amounts.) 



11. Since tricalcic phosphate and dicalcic phosphate are both 

 soluble in the prescribed 2 % citric acid solution the statement that 

 dicalcic phosphate can be differentiated from tricalcic phosphate, by 

 means of the selective action of this solvent, is untenable. 



It follows that the manurial value of phosphates cannot be deter- 

 mined by a 2 % citric acid solvent in the method prescribed, and it 

 therefore is a matter for consideration whether or not the further use 

 of this method should be continued. 



APPENDIX. 



A paper by R. Warington^, published in the Journal of the Chemical 

 Society, 1873, has been quoted extensively by authors of text-books on 

 chemistry as the authority on the decomposition of CagPaOg on boiling 

 this with water, and the conclusion formed is that uCagPgOg . CaOHgO 

 agrees best with the basic salt produced. 



^ R. Warington, "On the decomposition of Tricalcic Phosphate by Water," Journal 

 Chemical Society (Entire Series), Vol. xxvj, p. 083. 



