PHYSICAL CHEMISTRY OF ENAMEL DISSOLUTION '251 



ward diffusion, in spite of the high concentrations of calcium and 

 phosphate in the exterior solution. The dissociation, or association, 

 of these complexes under the different conditions existing in the 

 external solution would allow this process to continue. 



It should also be pointed out that the conditions of the solution 

 phase will determine the solid phase, which will either reprecipitate 

 and/or exist at anv equilibrium that is attained. 



Chemical Kinetics and Equilibrium 

 Kinetics of Dissolution 



In the presence of an acid maintained at a constant concentration, 

 the rate of enamel dissolution, as well as the various concentration 

 gradients just described, will shift onh' gradually ])ut continuously 

 as the mineral substrate breaks down. It is this pseudo steady state 

 that can be measured quantitatively and for which data have been 

 presented. If the acid supply increases and decreases, as would be 

 expected under oral conditions, there will occur a cycling of events 

 that is difficult to describe mathematicalK . Consequently, only the 

 condition of a single, continuous exposure will now be considered 

 and treated mathematically. 



It has been shown that the rate of incipient carious lesion forma- 

 tion is determined largely by the rate of reaction of acid with the 

 inorganic portion of enamel. The small effect of temperature on 

 rate of enamel dissolution during incipient carious lesion formation 

 supports the concept of diffusion control. As shown above, the rate 

 depends on total available acid concentration, but hydrogen ion 

 contributes little directly to the rate because of its low concentra- 

 tion. The undissociated acids, alwa\ s relatively high in concentration 

 as compared with hydrogen ions, provide the hydrogen ions for 

 reaction through diffusion to and dissociation at the apatite crystal 

 surface. The linear relationship of the enamel dissolution rate as a 

 function of total acid concentration shows that the rate of dissolution 

 is "first order" with respect to acid concentration (Figs. 32 and 38). 



The dissociated buffer anion appears to have no direct controlling 

 effect on the rate of dissolution during incipient carious lesion forma- 

 tion. The linear relation of the plot of Fig. 29 supports this conclu- 



