290 The Soluhilitu of Calcium Phosphates in Citric Acid 



100 grams mixture .■. contains 



I 37-59 grams CaO, 31-77 grams P.^Oj from the phosphate 



1 14-00 grams CaO 



from the carbonate 



51-59 grams CaO, 31-77 grams P2O5 

 Note that in this mixture there is an excess of 14 % of lime. 



Lime 



Phosphoric acid 



It will be noted that 86-8 % of the total lime and 84-4 % of the 

 total phosphoric acid have gone into solution in the first extraction of 

 30 minutes, and that the lime and phosphoric acid extracted are not 

 in the ratio of 3CaO : P2O5. The lime found is 13 grams more than 

 that necessary to form CagPgOg with the PgOg found, and this amount 

 is very nearly the same as that contained in the calcium carbonate, 

 viz. 14 grams. 



In the second extraction, the lime and phosphoric acid found are 

 very nearly or are practically exactly in the ratio of 3CaO : P2O5 ^^ ^^^^^ 

 apparently the salt dissolved is Ca3P.20g. 



In the third extraction, the amount of lime dissolved is nearly twice 

 as much as that necessary to combine with the P2O5 found to form 

 Ca3P208, indicating that lime which has gone into solution is derived 

 from calcium citrate, as well as from calcium phosphate. 



In the fourth extraction, the amount of lime dissolved is nearly 

 62 times as much as that necessary to form Qsl^^O^ with the P2O5 

 found. This appears to indicate that a larger proportion of the lime 

 which has gone into solution has done so as calcium citrate than as 

 calcium phosphate. 



It will be seen also that the so-called "citrate soluble" phosphoric 

 acid has fallen from 91 % to 84-4 % of the total phosphoric acid present. 



Experiment (c). Extraction of 5 grams of an intimate mixture 

 consisting of 50 % pure tricalcic phosphate and 50 % pure calcium 

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



