SiOH4

SiOH4, also written as Si(OH)4, is orthosilicic acid, the simplest orthosilicic acid species. It is the tetrahedral silicon center bound to four hydroxyl groups. In aqueous environments it can be considered the hydrated form of silicon dioxide; it can be formed by dissolving silica in water: SiO2 + 2 H2O ⇌ H4SiO4.

In solution, H4SiO4 is a weak acid with pKa values around 9.5 and 13.5. At near-neutral pH, H4SiO4 predominates; as pH rises, deprotonated silicate species such as H3SiO4− become more common. Silicic acid readily undergoes condensation reactions, leading to Si–O–Si linkages and water loss; this polymerization underlies the formation of silica gels, amorphous silica, and silicate minerals.

In the solid state, a discrete molecule of Si(OH)4 is not typically isolated; instead, orthosilicic acid condenses to polymeric silicate networks. In silicate minerals and glasses, silicon exists almost exclusively as SiO4 tetrahedra linked by oxygen bridges.

In soils and natural waters, H4SiO4 is the principal form of dissolved silicon and is a nutrient for some plants and organisms. Its concentration is limited by the solubility of silica and by polymerization tendencies. Preparation is generally by dissolution of silica in water under controlled conditions; handling mainly involves aqueous systems.