288 The Solubility of Calcium Phosphates in Citric Acid 



The water of crystalUsation present appears to indicate that the 

 dicalcic phosphate present corresponds to (CaO)2H20P2052H20 or 



100-000 



It appears, therefore, that the product formed by the interaction of 

 3 equivalents of hme (CaO) on 1 equivalent of phosphoric acid {P2O5) 

 yields pure tricalcic phosphate when the precipitate formed is removed 

 from the fluid portion within an hour of the commencement of the 

 reaction. The tricalcic phosphate formed contains water of crystalli- 

 sation. The amount of this is intermediate between that occurring 

 in the mineral ornithite Ca3P2082H20, and collophanite Ca3P208H20. 

 When however 2 equivalents of lime (CaO) are added to 1 equivalent 

 of phosphoric acid (P2O5) the resultant product is not dicalcic phosphate, 

 but is a mixture of di- and tricalcic phosphates. 



Having now obtained a pure tricalcium phosphate (preparation 

 No. 9), experiments were carried out to ascertain what would be the 

 result of treating (a) pure tricalcic phosphate, (6) tricalcic phosphate 

 plus 25 % pure calcium carbonate, and (c) tricalcic phosphate plus 

 50 % pure calcium carbonate, with a 2 % citric acid solution in the 

 manner prescribed for determining citrate soluble phosphoric acid^. 

 Briefly stated, the method is to shake for 30 minutes in a rotary shaker 

 making 30 to 40 revolutions per minute 5 grams of the substance and 

 500 c.c. of a 2 % citric acid solution. 



In these experiments about to be described, the first extraction was 

 made in the prescribed manner. The unexhausted phosphate remain- 

 ing after the first extraction, was extracted a second time with a further 

 quantity of 500 c.c. of a 2 % citric acid solution for a further period 

 of 30 minutes. This formed the "second" extraction. A third and a 

 fourth extraction was similarly made. At this stage the whole of the 

 Hme and the whole of the phosphoric acid had gone into solution. 



Experiment (a). Extraction of 5 grams pure tricalcic phosphate 

 containing 50-12 % CaO, 42-36 % PgOg, 7-52 % H2O with 500 c.c. of 



1 Fertilisers and Feeding Stuffs Regulations, 1908. Board of Agriculture. Fisheries 

 Leaflet 18, p. 17. 



