Theoretical models of mercury dissolution from dental amalgams in neutral and acidic flows

This article reports an experimental and theoretical investigation of mercu ry dissolution from dental amalgams immersed in neutral (noncorrosive) and acidic (corrosive) flows. Atomic absorption spectrophotometric measurements of Hg loss indicate that in neutral flow, surface oxide films formed in ai r prior to immersion persist and effectively suppress significant mercury r elease. In acidic (pH 1) flows, by contrast, oxide films are unstable and d issolve; depending on the amalgam's material composition, particularly its copper content, two distinct mercury release mechanisms are initiated. In l ow copper amalgam, high initial mercury release rates are observed and appe ar to reflect preferential mercury dissolution from unstable Sn8Hg (gamma ( 2)) grains within the amalgam matrix. In high copper amalgam, mercury relea se rates are initially low, but increase with time. Microscopic examination suggests that this feature reflects corrosion of copper from grains of Cu6 Sn5 (eta') and consequent exposure of Ag2Hg3 (gamma (1)) grains; the latter serve as internal mercury release sites and become more numerous as corros ion proceeds. Three theoretical models are proposed in order to explain obs erved dissolution characteristics. Model I, applicable to high and low copp er amalgams in neutral flow, assumes that mercury dissolution is mediated b y solid diffusion within the amalgam, and that a thin oxide film persists o n the amalgam's surface and lumps diffusive in-film transport into an effec tive convective boundary condition. Model II, applicable to low copper amal gam in acidic flow, assumes that the amalgam's external oxide film dissolve s on a short time scale relative to the experimental observation period; it neglects corrosive suppression of mercury transport. Model III, applicable to high copper amalgam in acidic flow, assumes that internal mercury relea se sites are created by corrosion of copper in eta' grains and that corrosi on proceeds via an oxidation-reduction reaction involving bound copper and diffusing hydrogen ions. The models appear to capture the correct time depe ndence of each dissolution mechanism and to provide reasonable fits to the experimental data.