PHYSICAL, CHEMISTRY OF ENAMEL DISSOLUTION 



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pH5 pH4 



[H+] X 10^ (moles/liter) 



Fig. 28. Enamel dissolution rate as a function of hvdrogen ion concentra- 

 tion during exposure at 37°C to a medium containing 0.05 m lactic acid plus 

 6 per cent hydroxyethvl cellulose with and without 0.03 m CaCL. In either 

 case, the enamel solubility rate increased with increasing hydrogen ion concen- 

 tration, but with a continuously decreasing slope. This latter effect is related to 

 the amount of undissociated acid formed from the buffers. CaCL caused a de- 

 crease in the rate of enamel dissolution. 



Also, the rate did not fall to zero at zero lactate concentration, re- 

 flecting the buffer capacity of the hydroxyethvl cellulose sample. 



The rate of enamel dissolution decreased with increasing calcium 

 concentration, but not linearly (Fig. 30). However, replotting the 

 data in logarithmic form resulted in a straight-line relationship ( Fig. 

 31). The effect of calcium chloride on the dissolution rate was meas- 

 ured at two different pH levels and lactate concentrations. A logarith- 

 mic plot is a typical procedure for determining the order of reaction 

 with respect to the variable under study. In this case, because of 

 mathematical requirements, the logarithm of the difference in rate 

 with and without calcium present is plotted versus the logarithm of 

 the calcium chloride concentration. The slope of this plot gives the 



