The critical pH is the pH at which saliva and plaque fluid cease to be saturated with calcium and phosphate, thereby permitting the hydroxyapatite in dental enamel to dissolve. It is the highest pH at which there is a net loss of enamel from the teeth, which is generally accepted to be about 5.5 for enamel.6 The solubility of acid varies with pH but it is also complicated by the fact that teeth are bathed in saliva, which is constantly replenished and supersaturated with apatite, whose composition varies. By increasing the concentration of calcium and/or phosphate, it is possible to reduce the effective critical pH so that teeth are able to withstand lower pH values before demineralizing.17
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