The effect of tin in the Ag-Hg phase of dental amalgam on dissolution of mercury

Abstract

Objectives. The aim of this study was to determine the relationship between the tin content in the γ 1 phase of dental amalgam and the kinetics of mercury dissolution. Methods. The tested materials were the same eleven commercial amalgams used by Mahler et al. (1994) to study mercury evaporation, which contained from 0.13% to 2.49% Sn in the γ 1. In one set of tests, specimens were stabilized in air before exposure to synthetic saliva for three 24 h sampling periods. In another set of tests, they were exposed for 2 h immediately after wet-grinding. Solutions were analyzed for mercury by atomic absorption spectrophotometry. The results were analyzed by ANOVA and Tukey tests (p < 0.05). Results. For air-stabilized specimens, the 24 h Hg loss was independent of the tin content in γ 1, for 0.92 to 2.49% Sn. The loss was higher for amalgams containing 0.13% and 0.32% Sn. For abraded specimens, the 2 h Hg loss decreased with increasing tin content in γ 1. It is proposed that tin oxide growth is initially controlled by diffusion of tin to the γ 1/tin oxide interface, the rate of which increases with increasing Sn content in γ 1. Later the growth becomes controlled by the electric field across the oxide. Mecury release rate, controlled by diffusion through the oxide, is inversely proportional to the oxide thickness and initially lower for high Sn content in γ 1. For a steady state oxide thickness, tin diffusion and tin content in γ 1 no longer affect mercury dissolution. When tin oxide film dissolves, a minimum tin content is required to maintain a barrier against mercury dissolution. Significance. The results show the importance of rapid formation of a surface oxide to minimize mercury dissolution. The theoretical analysis provides a quantitative model for explanation of the effects of tin content and time.