Dissolution of mercury from dental amalgam at different pH values.

Dissolution of mercury from dental amalgam has been shown to be diminished by the formation of a tin oxide film on the surface of the mercury-rich gamma 1 phase (Marek, 1990b). Since tin oxides dissolve at low pH values (Deltombe et al., 1974), acidic conditions in the oral cavity may cause an increase in the mercury release. The purpose of this study was to determine the effect of acidity in the range of pH 1 to pH 8 on the rate of mercury dissolution in synthetic saliva from tin-free and tin-containing gamma 1 phase and two commercial dental amalgams. The tested hypothesis was that pH affects mercury dissolution only when a protective oxide film dissolves in an acidic environment. After exposures of the specimens for 2 hr or 24 hr in sealed glass bottles, the solutions were analyzed by flameless atomic absorption spectrophotometry for mercury and silver. The results have shown pH-independent mercury dissolution in the range of pH 3 to 8, and a much faster dissolution at pH 1. At all pH values, more mercury dissolved from the tin-free phase than from the tin-containing phase, and the rate of dissolution was lowest for the dental amalgams. The results were affected by the length of the test exposure. The pH independence in a wide range of pH values has been attributed to the atomic mechanism of mercury dissolution. The low rate of mercury dissolution from specimens containing tin has been explained by the formation of a barrier tin oxide film, which dissolved only at the lowest pH. Dissolution of silver at low pH values is believed to have accelerated dissolution of mercury from the tin-free gamma 1 phase. Variation of the dissolution rate with concentration of the dissolved species and kinetics of oxide film dissolution caused the effect of the exposure period.