^36 



J. A. GRAY AND M. D. FRANCIS 



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40 80 120 



EXPOSURE TIME (HOURS) 



160 



Fig. 25. Enamel dissolution as a function of time during exposure to a me- 

 dium consisting of 0.05 M lactic acid plus 6 per cent hydroxyethyl cellulose 

 (with low buffer content) adjusted to pH 3.5. The amounts of enamel dissolved 

 as determined by either calcium or phosphate analysis agreed well. Tempera- 

 ture had little effect at 90 hours exposure. The enamel dissolution rate during 

 incipient carious lesion formation decreased with increasing exposure time. 



a single exposure period, thus simplifying the experimental measure- 

 ments. Ideally, the initial rate at time zero is used, but an atypical 

 condition exists during the initial time period (approximately 1 

 hour) while the various diffusion gradients are established. How- 

 ever, the dissolution or decalcification rate was close to being linear 

 during the first 4 to 6 days, so that the relative effect of the different 

 variables could be determined using a long time interval. A period of 

 to 96 hours was chosen in order to give the necessary latitude ex- 

 perimentally and yet approach the conditions of a true rate measure- 

 ment as a function of initial conditions. This requires that neither 

 the initial reaction or solution conditions have been altered enough 

 nor the reaction products accumulated sufficiently to alter signifi- 

 cantly the reaction rate. 



