Vitreous silica hydroxyl concentrations

Vitreous silica transmission curves are shown in Figure 7. The hydroxyl concentration for each silica type is listed in Table 9. These curves represent only the general characteristics of the different silica types and should not be used for calculations of transmittance. 

Most of the other properties of vitreous silica are only slightly affected by the hydroxyl concentration. While it is possible to measure the effect of hydroxyl content on the density, refractive index, thermal expansion coefficient, dielectric constant, and other properties of vitreous silica, the effects are very small as compared to those for the flow and optical transmission behavior. The density of commercial vitreous silica ranges from 2.197 to 2.203 gcm while the refractive index is 1.457 0.003 for most products. Since the effects of hydroxyl on the properties of vitreous silica are of the same magnitude as the effects of changes in fictive temperature, it is very difficult to separate the effect of hydroxyl from those of thermal history. 

As for the effect of hydroxyl ion, it is not possible to see how it could catalyze the dissolution of stishovite in which silicon has already reached its maximum coordination number. No data seem to be available on the effect of pH on the dissolution rate of stishovite, but it is interesting that at pH 8.4 it dissolves about as fast as vitreous silica when compared on the basis of equal areas of surface being exposed to the solution. Furthermore, it continues to dissolve past the saturation level for vitreous or amorphous silica. The concentration of soluble silica can reach as high as 190 ppm, at which point colloidal particles are nucleated (139). It is likely that stishovite is hydrolytically unstable and would eventually decompose completely to amorphous silica. Whether or not" pH has an effect on the rate of hydrolysis is not known. 

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