HSiO43

HSiO4^3-, often written as HSiO43-, is the tri-deprotonated form of silicic acid and a silicate anion. It consists of a silicon atom tetrahedrally coordinated to four oxygens, three of which carry negative charges and one forms a neutral silanol group (OH). This arrangement gives the anion an overall charge of −3.

Structure and bonding: The silicon center is four-coordinate with a tetrahedral geometry. Three oxygen atoms are deprotonated (O−) and one oxygen bears a proton (OH), resulting in a silicate framework with delocalized negative charge spread over the oxygens. The Si–O bonds exhibit characteristics typical of silicate species, with resonance contributing to bond orders between single and partial double bonds.

Formation and occurrence: In aqueous solution, silicic acid H4SiO4 dissociates stepwise as the pH increases. HSiO4^3- forms under strongly basic conditions after removal of three protons. It is not typically isolated as a solid; rather, it is detected in highly alkaline solutions and contributes to the overall speciation of silica in water. In natural waters, HSiO4^3- is a minor component compared with less-deprotonated silicate species at neutral to mildly basic pH.

Reactions and properties: As a highly charged anion, HSiO4^3- can act as a ligand toward metal cations, forming various silicate complexes. It participates in silicate polymerization and condensation processes that build larger silicate networks, especially under alkaline conditions. Its existence reflects the rich speciation of silica in aqueous environments and the tendency of silica to form multiple deprotonated forms depending on pH.

See also: silicic acid, silicate, orthosilicate, silicate chemistry.