Physico-chemical Methods for the Identification of Microcristalline Basic Calcium 133 



48 hr, to prevent reactions with atmospheric-COo . pH was continuously measured 

 by a combined glass electrode and registered. 



Two kinds of curves were obtained: one by an excess of Ca^* or stoichiometric 

 mixture, the other by excess of PO4 . 



The first type of curve (Fig. 1) shows an initial and fast neutralization, which 

 appears after PO4 is added to the Ca(N03)2 + NH3 mixture. In one minute the 

 pH falls from 10.7 — 10.5 down to 8.8 — 8.5 and stabilises during a period of 4 — 8 hr 

 at an approximate value of 8. After this a new fall of pH occurs which lowers the 

 pH to 6.5 to 5.3. 



204 



2,06 

 2.09 

 208 

 2.13 



10 15 20 



Fig. 1. pH-curve of HA 



25 h 



The higher the excess of Ca"^^ is, the larger becomes the fall, and the sooner it 

 appears. After this second step the pH reaches its final value in about ten hours. This 

 end-pH depends directly upon the Ca/P-ratio; i. e. for an end-pH of 5.30 (200*'/o 

 Ca^^ excess) a Ca/P of 2.13 is obtained. 



X-ray diffraction patterns show the diagram of the hydroxyapatite of small 

 dimensions. Norelco X-ray diffraction gives a dimension along the c-axis of about 

 300 A. The specific area measured by the method of B.E.T. is 110 ±5 m^/g. Electron 

 micrograph show needles, which are mostly agglomerated to bigger and needlelike 

 aggregates with dimensions of 1200X120 A. Such aggregates show a calculated 

 specific area of about 100 m-/g, which is in good agreement with the measured B.E.T. 

 area. Single needles can sometimes be seen with similar dimensions as those given by 

 Norelco. The Ca/P (2.04 — 2.13) indicates that the preparations obtained in the 

 described way, can be attributed to HA. Indeed, the TG and the DTA confirm this. 

 The TG curve is really the one of the HA, with the characteristic points at 100 '^C, 

 300 °C and 500 °C, giving waterlosses of 1.5 moles, 1.0 moles, 0.5 moles and a total 

 waterloss of 2.0 moles at 1000 "C if we subtract 1.5 moles waterloss up to 100 ^C, 

 which can be attributed to adsorbed water. The DTA curve corresponds with the 

 TG curve showing an endothermic peak at 100 ''C and an exothermic one at 330 °C, 



