dissolving silica at high pH

[nappy]

Please explain the theory and molecular interactions which cause the silica to dissolve at high pH in HPLC columns (RP & SEC). I read about hydroxide attach but how does this work? Are there any other mechanisms at higher pH? Also if silica is water soluble, how can it survive gradients 5-100% B (organic). Thank you in advance

[Serg]

Si-O-Si hydrolysis, I guess.

Regards.

[Mark Tracy]

The first equilibrium SiO2 + 2H2O <==> Si(OH)4 strongly favors the left side of the equation. The second equilibrium Si(OH)4 + OH- <==> Si(OH)3O- + H2O favors the right side. Solubility is pH dependent.

The recommended pH range for most columns is 2 - 8. The upper limit is based on the solubility of silica.

In practical bonded-phase columns, the C18 phase discourages the dissolution by keeping hydroxide away from the surface. Eventually, the hydroxide undermines the bonded layer, and the column fails. New special surface bondings are able to protect the underlying silica at elevated pH.

[nappy]

Thanks for your help. The the explanations and information was very good. I also appreciate Mark's equations and expertise. Thanks for the lesson!