284 The Soluhility of Calcium Phosphates in Citric Acid 



present in the hydrated product previously discussed (No. 5) we would 

 have : 



CaO P2O5 



Dicalcic phosphate ... 5-46 6-92 =12-38 



Tricalcic phosphate... 4.3-69 3G-93 = 80-62 



Lime (free) 7-00 — = 7-00 



56-15 43-85 =100-00 



When time permits I hope to investigate the question of the estima- 

 tion of the free lime actually present in such a mixture as the above, 

 but it is evident that the product obtained by this method is not tri- 

 calcic phosphate as is usually stated and as it is assumed to be. 



Experiments were next made to prepare tri- and dicalcic phosphates 

 from pure phosphoric acid (H3PO4) by the addition of three equivalents 

 and of two equivalents respectively of lime (CaO). The solution of 

 phosphoric acid used was prepared by taking about 32 grams pure 

 phosphorus pentoxide, dissolving in water and making up to 500 c.c. 

 Gravimetric analysis showed that the 500 c.c. solution contained 

 31-66 grams phosphoric acid (P2O5). 



Tricalcium phosphate preparation No. 7. 



For the preparation of this 243 c.c. of the above mentioned solution 

 of phosphoric acid containing 15-386 grams P2O5 were taken and to this 

 was added 18-203 grams pure freshly ignited Hme (CaO). A further 

 small quantity of lime was added in the form of lime-water, the idea 

 being to have a faint excess of lime— which excess could be easily 

 washed out by water. Owing to an absence from Sydney, the pre- 

 cipitate formed was not separated from the liquid portion for about 

 a fortnight after the addition of the lime to the phosphoric acid. 



This precipitate when dried weighed 36-22 grams, and was marked 

 preparation No. 7. The filtrate and washings contained hme but no 

 phosphoric acid and measured 1000 c.c. On evaporation, the filtrate 

 and washings yielded 0-53 gram lime (CaO). 



Dicalcium phosphate preparation No. 8. 



To 243 c.c. of the phosphoric acid solution mentioned above con- 

 taining 15-386 grams P2O5, 12-135 grams pure lime (CaO) (equivalent 

 to 2 molecules of lime) were added. 



This mixture also stood for a fortnight before filtering. 19-63 grams 

 dried preparation (No. 8) were obtained. 



The filtrate (without the washings) which measured about 500 c.c. 

 was evaporated. When boiling commenced, the clear fluid became 

 milky, and a precipitate separated out. Evaporation was continued 



