We recently advanced non-resonant second order nonlinear spectroscopy to probe specific ion interactions at fused silica: aqueous interfaces via the SHG phase and amplitude. The approach avoids the strong absorber problem of liquid water and provides the total potential as well as the second order nonlinear susceptibility of buried across interfaces in contact with ions of various chemical identity. We now employ this method to quantify the total number of absorbed cations Al 3+ , Mg 2+ , and Na + cations at fused silica surfaces maintained at pH 4 and during exposure to varying concentrations of the metal cation chlorides. Unlike in our recent work, here we work at pH 4, which simplifies the cation speciation so that hydroxides, important for alkaline earths and trivalent cations at elevated pH, are not prevalent.