CYANAMID — MANUFACTURE, CHEMISTRY AND USES QI 



MIXTURES WITH ACID PHOSPHATE. 



In ordinary acid phosphate analyzing i6 per cent, available 

 phosphoric acid, there is usually found about 5 per cent, as 

 free phosphoric acid, 9 per cent, as mono-calcium phosphate, 

 and 2 per cent, as di-calcium phosphate. When such a phos- 

 phate is mixed with Cyanamid there is obviously a neutraliza- 

 tion of free acid, and of acid hydrogen of the mono- and di- 

 calcium phosphate, the extent of the reaction depending upon 

 the amount of active lime introduced by the Cyanamid. The 

 neutralization is, of course, attended by evolution of heat, 

 and this heat is the cause of the unfavorable results of mixing 

 large quantities of Cyanamid with acid phosphate. 



In America, phosphates are sold on the basis of their con- 

 tent of phosphoric acid soluble in ammonium citrate solution 

 of standard strength, since it has been shown that there is no 

 appreciable difference in the agricultural value of the water- 

 soluble and the citrate soluble part of the phosphate. It is to 

 the interest of mixers of commercial fertilizers to prevent the 

 neutralization of the acid phosphate beyond the di-calcium 

 or citrate soluble stage. With increasing quantities of CaO 

 the following reactions should take place successively, but 

 with relatively decreased velocity: 



(a) HgP^Og -f CaO — CaH,P,0, + H,0, 

 Phos. Acid. Water Sol. 



(b) CaH.PA -h CaO -- Ca.H^P.O^ + H,0, 

 Water Sol. Citrate Sol. 



(c) Ca,H,P,03 + CaO — Ca3P,03 + H,0. 



The last reaction would require a vast excess of CaO, since 

 CaaHaPaOg is practically insoluble in water, and is practically 

 undissociated. This reaction does not apply in the practical 

 mixing of Cyanamid and acid phosphate. There is, however, 

 a further reaction, tha^ may take place with prejudicial results. 



(d) 2Ca,H,P,03 + Heat — Ca^Vfi, + CaH^P^^, 

 Citrate Sol. Cit. Insol. Water Sol. 



It has been found that with a constant quantity of lime. 



