Note that the pKa of 6.694 for the net reaction in table 1B is much lower than values for the autoionization of H4SiO4 reported by and similarly by . However, the pKa value is in close agreement to that derived from amorphous silica titration experiments by von Schindler and Kamber (1968; pKa1=6.8) and near the calculated values of -OH groups on the surface of quartz . These relationships need further investigation especially since different sites in silica appear to have different acidities . The dipolar complex may be another silica species in dilute solutions whose concentration is three orders more than the fully ionized H3SiO4 -. Observe that the pK of the formation of the zwitterion (pK=7.776) is within these ranges and that therefore the concentration of zwitterion is only about one order of magnitude less than dipolar complex I at these concentrations of silica.