 It is a weak acid with p"K"a1 = 9.84, p"K"a2 = 13.2 at 25 °C.It is usually written as H4SiO4, but the formula Si(OH)4 is generally accepted.Other silicic acids such as "metasilicic acid" (H2SiO3), "disilicic acid" (H2Si2O5), and "pyrosilicic acid" (H6Si2O7) have been characterized.