PHYSICAL CHEMISTRY OF ENAMEL DISSOLUTION 



^243 



0.05 0.10 0.15 



TOTAL ACID CONCENTRATION (moles/liter) 



[H+] + [UNDISSOCIATED BUFFER] 



Fig. 32. The enamel dissolution rate during incipient carious lesion forma- 

 tion was a straight-line function of total acid concentration for a medium con- 

 taining lactic acid plus 6 per cent hydroxvethvl cellulose over a pH range of 3 

 to 6 at 37°C. A very good lesion was obtained even at pH 6. Therefore, the 

 rate can be expressed by a simple equation in terms of acid concentration (see 

 equation 2, page 252). 



radiograph (Fig. 35); the advancing front in this same sample can 

 be seen in an electron micrograph ( Fig. 36 ) . The appearance of this 

 defect in the electron micrograph was quite different from that of 

 an incipient carious lesion (Fig. 22). As the concentration of total 

 lactate was decreased at a constant pH value of 4.5 until only h> - 

 drogen ions were present (i.e., as HCl), the ratio of subsurface to 

 surface dissolution increased ( microradiograph, Fig. 37; photomicro- 

 graph, Fig. 3i), but surface dissolution was always present. The 

 addition of calcium and phosphate ions to the acid solutions, still in 

 the absence of organic pohiner, further increased the ratio of sub- 

 surface to surface dissolution and permitted a higher acidic buffer 

 concentration to be used (microradiograph. Fig. 38). The lesions 



