The effect of the electrode potential on the release of mercury from dental amalgam

The effect of the electrode potential on the rate of mercury dissolution from the matrix phase (gamma 1) of dental amalgam was investigated. Specimens of the Ag-Hg phase, with and without dissolved tin, were exposed to synthetic saliva and maintained at various potentials by means of a potentiostat. The amount of dissolved mercury was determined after 24 h by cold-vapor Atomic Absorption Spectrophotometry. Anodic polarization curves for the gamma 1 specimens and pure silver and mercury were also recorded. The results for the Ag-Hg phase showed a potential-independent dissolution rate between -0.6 and -0.1 V (SCE), followed by a sharp increase. Mercury dissolution from the Ag-Hg-Sn phase was much slower than from the tin-free gamma 1 phase, and potential-independent to +0.05 V (SCE); a sharp increase in mercury dissolution was observed at +0.1 V (SCE). The anodic polarization curves for the Ag-Hg-Sn phase indicated passivity between -0.67 V and +0.1 V (SCE), but no clear passivation for the Ag-Hg specimen, silver, or mercury. The results show that in the potential-independent region, mercury dissolved in the atomic form from both the tin-free and tin-containing gamma 1 phase. The increase in dissolution in the upper range of potentials was tentatively attributed to selective anodic dissolution of silver and an onset of electrochemical dissolution. The results indicate that the rate of mercury release from the gamma 1 phase of dental amalgam was not affected by potential changes due to alloying or galvanic contacts unless the potential exceeded some critical value.