The rate of change of pH of lactic acid exposed to glass-ionomer dental cements

The rate of change of pH of aqueous lactic acid at pH 4.2–4.5 (i.e. a little below that of active caries in vivo) in contact with disks of various commercial glass-ionomer cements has been determined in two configurations. In the first of them, a thin film set-up, 20 μl of solution was spread across the surface of a cement disk (diameter: 13 mm), and its pH determined by pressing a flat-ended electrode against the film at varying time intervals. In the second, a similar disk was immersed in 1.5 ml of solution, removed after varying time intervals, after which the pH of the solution was measured using a round-ended electrode. The latter measurement was more reliable, in that the pH electrode had time to equilibrate, whereas the former was more realistic because the film was approximately the same thickness as that of saliva on a tooth surface. Both series of experiments showed measurable differences in pH after only 30 s, with the thin-film configuration showing a range of pH changes of 0.5–1.2 units depending on the cement and the small volume configuration showing a range of 0.1–0.5 units, also depending on the cement. After 10 min, in the small volume experiments, the pH had generally increased further. The extent and speed of the change in pH led to the conclusion that ability of glass-ionomers to increase pH is likely to be an important mechanism of caries protection under clinical conditions.