PHYSICAL CHEMISTRY OF ENAMEL DISSOLUTION 



241 



10 



Average of 15 values 

 (Range 4.6-6.5 



-etLl^L^:^!^ LACTATE 



0.01 0.02 0.03 0.04 



ADDED CaCl2 CONCENTRATION (moles/liter) 



Fig. 30. Enamel dissolution rate as a function of CaClo concentration dur- 

 ing exposure at 37°C to a medium containing lactic acid and CaCL plus 6 per 

 cent hydroxyethyl cellulose. Addition of CaCL decreased the rate at two dif- 

 ferent pH levels and lactate concentrations. 



face dissolution (e.g., at pH 3.5, 0.61 m lactate, 0.03 m CaClo). The 

 maximum rate of enamel dissolution before surface damage became 

 detectable by visual or microscopic observation was about 20 mg 

 of enamel per cm- of surface per 96 hours exposure. Although not 

 all the levels of composition variables investigated gave ideal in- 

 cipient carious lesions, some measurements outside of the conditions 

 for producing a typical lesion were necessary in order to assess the 

 effects within the preferred region. 



The importance of undissociated acid in addition to the hydrogen 

 ion for controlling the rate of enamel dissolution during incipient 

 carious lesion formation has been suggested ( Gray et ah, 1962 ) . The 

 recent results mentioned earlier (Kapur et al., 1962) support this 

 hypothesis. The data from the acid-organic polymer systems studied 

 in this presentation were plotted versus total acid concentration 

 (Figs. 32 and 33) to test this concept further. The undissociated 

 lactic acid was calculated from the dissociation constant (Gray, 



