294 The Sohibillty of Calcium Phosphates in Citric Acid 



CagPaOg + 6HC1 = SCaCla + 2H3PO4. The addition of sodium hydrate 

 would be represented thus : 



3CaCl2 + 2H3P04 + 4NaOH = 3CaCl, + 2Na2HPO,+4H20 (1), 



and then 3CaCl2 + 2Na2HP04 + 4H,0 = Ca3P208+4Naa+4H20 +2HC1 (2). 



Were it possible to produce Na3P04 in the reaction we would have : 



SCaCla + 2H3PO4 + 6NaOH = SCaCla + 2H3PO4 + 6H0O ( 1 ) , 



and subsequently 



' 3CaCl2 + 2Na3P04+6H20=Ca3P20g + 6NaCl + 6H,0 (2), 



but as I know of no similar reaction in which Na3P04 is produced but 

 invariably Na2HP04, no other conclusion seems possible but that free 

 hydrochloric acid is produced which would react at once with the tri- 

 calcium phosphate present. In the case of ammonia being the pre- 

 cipitating alkali the following equations might represent what occurs : 



3CaCU + 2H3PO4 + GNH4HO = (a) 2 (NH4)3P04 + GH^O (which is very improbable) \ 

 or " {h) 2(NH4),HP04 + 2NH3 + 6H20 (1). 



(or 2NH4HO+4H2O) J 



and then (0) and {b) yielding CajPaOg + 6NH4Ci + 6H,0 (2), 



or we might have with less ammonia : 



3CaCU + 2H3PO4 + 4NH4HO =2 (NH4)2HP04 +4H2O +3CaCl2 (1), 



and subsequently 



2(NH4)2HP04+4H20+3CaCl2=Ca3P208 + 4NH4Cl + 4H„0 + 2HCl (2), 



if the latter alternative obtains, we would again have free hydrochloric 

 acid, as is apparently the case when sodium hydrate is used, and which 

 would immediately react with the tricalcic phosphate present. 



From analogy one might expect the primary action of citric acid on 

 calcium phosphates to be similar to that of hydrochloric acid, and that 

 calcium citrate and free phosphoric acid are produced at least primarily. 



Thus : 



CajP^Og + 2H3(CeH50,) =Ca3(CeH50,), + 2H3PO4 



1 : 1-0239 



3CaHP04 + 2H3(C6H50,) = Ca3(C6H50,)o + 3H3P( )4 . 

 1 : 0-931 



It will be noted that theoretically 5 grams of calcium phosphate 

 would require about 5 grams citric acid for the above reaction whereas 

 in the prescribed method of dissolving calcium phosphates in citric acid 

 solution 10 grams of citric acid are prescribed. 



A secondary reaction undoubtedly occurs since the filtrate from the 

 reaction after 30 minutes contains both lime and phosphoric acid. The 

 following equations are suggestions as to what may occur: 



Ca3(CoH50,)2 + 2H3PO4 = 2H3(CeH50,) + CaaP^Os 

 Ca3(CeHA)2 + 3H3P04 = 2H3(CeH50,)+3CaHP04 

 Ca3(CeH60-)2 + 6H3PO4 =2H3(CeH50,) + 3CaH4P20g . 



