134 



P. Lerch, C. Vuilleumier 



followed by a slow exothermic rise corresponding to a slow dehydration of the 

 thermogravimetric curve. X-ray analyses at the mentioned temperatures show that the 

 structure of the product does not change except the reflexes becoming sharper. 



Fig. 



15 20 



pH-curve of TCPH 



25 



182 

 192 



195 

 1.97 



The pH curves obtained by a mixture containing an excess of PO4 show a 

 totally different behaviour. One observes a steady decrease of the pH towards its 

 end-value (Fig. 2). The curves show a fast and initial neutralization, which is fol- 

 lowed by a slow decrease, ending more quickly as the excess of P04""~ becomes 

 larger. As in the former curves there is a direct relation between the end-pH and the 

 Ca/P. The preparations obtained in this way show similar X-ray diffraction patterns 

 as the former products, similar dimensions and similar electron micrographic appear- 

 ance. The Ca/P indicates that the preparations are TCPH. So do the TG and DTA 

 diagrams. We have again characteristic points at 100 '^C, 300 "^'C and 500 °C corre- 

 sponding to waterlosses of about 1.5, 1.0, 0.5 moles and an entire new step between 

 700 "^C and 740 °C, corresponding to a waterloss of 0.5 moles. The total waterless is 

 2.0 moles up to 1000 °C. The DTA-curve also gives a new endothermic peak at 

 700 C, which corresponds with the 0.5 moles waterloss and which can easily be 

 attributed to the transformation of the calcium phosphate to /5-TCP. This trans- 

 formation can be shown by X-ray analysis at the mentioned temperatures. 



Formation of HA or TCPH depends essentially on the end-pH which is given by 

 the concentration of the reactives. The following equation was found between Ca/P 

 and end-pH: 



Ca/P = 2.65-P" (1) 



This equation shows that in physiological media we can expect a TCPH with a Ca/P 

 of 1.92 according to several authors. 



We have identified the product wich appears during the intermediary step of the 

 synthesis of HA. The isolated product shows an amorphous X-ray diffraction pattern 

 and an amorphous electron micrographic appearance which are similar to the amorphous 

 parts of the OCP obtained by the method of Watson and Robinson (1953). The 



