626 STATE BOARD OP AGRICULTURE. 



process and returned to its tribasic form, might be indicative of its 

 behavior in the soil as a source of plant food. In other words, super- 

 phosphates contain, not only dicalcic or reverted phosphate but also 

 available tricalcic phosphates and unavailable raw phosphate, the two 

 last existing as separate and distinct substances. The availability of 

 a limited amount of the tricalcium phosphate, which has always been 

 recognized, is thus due to an actual difference in the character of two 

 portions of this fraction and not merely to the limited solubility of it as 

 a whole. 



It would also serve to invalidate the contention that an alkaline citrate 

 solution might convert some soluble dicalcium phosphate into the 

 insoluhle tricalcium form. 



In some of the previous work upon which some of the above mentioned 

 conclusions are based, the purity of the reagents is open to question 

 and so it was decided to reinvestigate the problem, paying special at- 

 tention to this feature. 



Dicalciumphosphate was prepared as follows: 



Pure calcium chloride (Kahlbaum's C. P.) was dissolved in water 

 and treated with a solution of pure disodium phosphate. According to 

 Cameron and Seidell (8) as long as the solution is acid, the solid phase 

 separating out is the dicalcium form. Consequently the addition was 

 stopped while the solution was still acid, the precipitate filtered off 

 and thoroughly washed until free from chlorides. It was analyzed for 

 calcium and PoO^ with the following results : 



Tlj 

 CaO 

 P.O. 



Tricalcium phosphate was prepared from pure phosphoric acid and 

 calcium chloride. Kahlbaum's C. P. pliospliorus pentoxide was dissolved 

 in water and boiled with a little nitric acid. This solution was then 

 treated with calcium chloride in excess. According to Cameron and 

 Seidell the pure tricalcium salt is apparently unobtainable, the solid 

 phase always containing too much lime. Our product, however, 

 analyzed as follows and ai)peai"cd to be the tricalcium salt: 



Theory Found 



CaO 54.21 53.98 



P2O5 45.79 45.G9 



The following mixtures were prepared using the above materials, 

 (which however contained some moisture and hence gave somewhat 

 lower conlents of PoO.J a sample of raw rock phos|)hate and pure 

 calcium sulphate. 



