PHYSICAL CHEMISTRY (W ENAMEL DISSOLUTKIX 



"247 



Fig. 37. A microradiograph of a section of enamel after a 15-day exposure 

 at room temperature to an acjueous solution of hydrochloric acid at pH 4.5 

 shows that decalcification had occurred near the surface but had not penetrated 

 very far. Examination of the surface with a metallurgical microscope showed 

 that light pitting had occurred. ( X 200. ) 



Fig. 38. A microradiograph of a section of enamel after a 15-day exposure 

 at room temperature to an aqueous solution of 0.0005 m CaClo and 0.0005 m 

 NaH^POj, adjusted to pH 4.5, shows that extensive decalcification had occurred. 

 The subsurface dissolution penetrated quite far but was fairly uniform through- 

 out the damaged region. The surface was much pitted by the exposure, but this 

 could be seen only with a metallurgical microscope. ( X 200. ) 



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ENAMEL SURFACE AREA 

 Fig. 39. The enamel dissolution rate was measured in a medium at 37°C 

 consisting of 0.05 m lactic acid and 0.002 m CaCU plus 6 per cent hydroxyethyl 

 cellulose adjusted to pH 4.5, using enamel samples with different surface areas. 

 The smaller the surface area, the faster was the absolute rate of dissolution. 

 This effect is a result of the geometry of the experimental set-up and is probably 

 related to diffusion conditions. 



