Abstract: Objective: To investigate the effect of different pH value of hydrogen peroxide solution on dentin color and transparency. Methods: 30 recently extracted human premolars were randomly divided into two groups, 15 cases in each group. Along the long axis, the teeth were cut, removed of pulp, and bleached with, two hydrogen peroxide solution using the pH=4.5 and pH=7.0. Changes of dentin color and transparency were recorded with Crystaleye computer colorimeter as well as the relationship and time. A preliminary analysis was performed to learn its mechanism. Results: One week after the pH7.0 hydrogen peroxide solution bleaching dentin became yellow, transparency decreased, the activity of matrix metalloproteinases decreased. One week after pH4.5 solution of hydrogen peroxide bleaching dentin became yellow, transparency reduced. Part of the dentin color restored after 3 weeks, and matrix metalloproteinase activity increased, with a significant difference between two groups (P<0.05). Conclusion: Alkaline hydrogen peroxide solution has strong oxidation resistance of dentin, great damage. Neutral hydrogen peroxide solution has slight dentin destruction. The mechanism is related to odontogenic matrix metalloproteinase activity.

Key words: pH, Hydrogen peroxide bleaching, Dentin