652 G. N. JENKINS AND C. DAWES 



A Criticism of Experiments on tlie ''Critieal })ir 

 and Its Investigation 



Since saliva is saturated at neutialit) with calcium and phosphate, 

 tooth sulDstance would be expected to be insoluble. If the pH falls 

 steadily, it will eventually reach a value at which the calcium and 

 phosphate concentrations are no longer able to saturate the saliva, 

 and below this value tooth substance would be expected to dissolve. 

 After the addition of chelating agents, however, apatite would 

 dissolve even in saliva which, on an ionic basis, was previously 

 saturated. In other words, the concept of a critical pH would be 

 invalid. Experiments designed to test the h\"pothesis of a critical 

 pH have consisted of measuring the solubilitv of tooth substance 

 either in saliva at different pH values (Fosdick and Starke, 1939; 

 Ericsson, 1949; Hills and Sullivan, 1958) or in buffers saturated 

 with calcium phosphate oxer a pH range (Enright et ciL, 1932; 

 Hills and Sullivan, 1958). In the saliva experiments, toluene or 

 thymol was added to prevent bac^^erial activity from making the 

 pH unstable, but it was not realized that this would also prevent 

 the formation of chelators either from sali\'a constituents or from 

 enamel. 



It seemed to us desirable, therefore, to find out whether mineral 

 matter from enamel could dissolve in saliva incubated without car- 

 bohydrates or toluene. Such a saliva would become alkaline, and the 

 pH would be so much higher than the critical value that no apatite 

 could dissolve unless chelating agents were formed. Over fortv ex- 

 periments were carried out, and the results (Table II) showed that 



TAl^LE II. Calcium and Phosphate Concentrations (jug, ml) before and 

 AFTER Incubating Salivary Sediment with Teeth for 48 Hours 



