The hydrolysis of bridging oxygen bonds in glass network tetrahedra is known to be promoted by dissolved hydroxide. Several mechanisms of hydrolysis of bridging oxygen bonds have been proposed in the literature. Two potential mechanisms for network hydrolysis were evaluated against their consistency with an empirical model of Product Consistency Tests (PCT) boron release as a function of glass composition. One mechanism, the associative SN_2 mechanism, was found to be inconsistent with constituent coefficients in the empirical PCT model. This was evident from the increased PCT boron release coefficient with increasing electronegativity of the network-forming ions. The opposite relationship would be expected if the associative SN_2 mechanism were the rate controlling mechanism. A second mechanism was found to be consistent with the PCT boron release coefficients. In this mechanism, the rate-determining step for the hydrolysis of network tetrahedra is the deprotonation of surface hydroxyl groups. Both the PCT boron release constituent coefficients and the labiality of surface hydroxyl protons were found to be a function of the optical-basicity of network tetrahedra.
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