Silicic acid
[CAS# 1343-98-2]

Identification

• Classification: Inorganic chemical industry >> Inorganic salt >> Silicide and silicate
• Name: Silicic acid
• Synonyms: Orthosilicic acid
• Molecular Formula: H₄SiO₃
• Molecular Weight: 96.11
• CAS Registry Number: 1343-98-2
• EC Number: 215-683-2
• SMILES: O[Si](O)(O)O

Properties

• Density: 1.8±0.1 g/cm³ Calc.^*
• Index of refraction: 1.535 (Calc.)^*

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS05;GHS07 Danger
• Hazard Statements: H314-H318-H335
• Precautionary Statements: P260-P261-P264-P264+P265-P271-P280-P301+P330+P331-P302+P361+P354-P304+P340-P305+P354+P338-P316-P317-P319-P321-P363-P403+P233-P405-P501

Discovory and Applicatios

Silicic acid refers to a group of compounds containing silicon, oxygen, and hydrogen, typically represented by the general formula H₄SiO₄ or its polymeric forms. It is not a single defined compound but rather a family of weak acids that are formed when silicates are dissolved in water. These compounds are significant in both natural systems, such as biological and geological environments, and industrial applications, particularly in materials science and chemistry.

The concept of silicic acid originated in the 19th century when chemists investigated the reactions of silicates with acids. Upon acidification of aqueous sodium silicate, a gel-like substance was observed, which was identified as a hydrated form of silica and referred to as silicic acid. Although pure monomeric silicic acid (orthosilicic acid, H₄SiO₄) is difficult to isolate due to its rapid polymerization, its existence in dilute aqueous solutions has been confirmed. It is the simplest form of soluble silica and is commonly found in natural waters, such as seawater and groundwater, usually at concentrations below saturation.

In aqueous solution, silicic acid tends to undergo condensation reactions, forming oligomeric and polymeric species, eventually leading to the formation of colloidal silica or amorphous silica gel. This process is influenced by pH, temperature, concentration, and the presence of other ions. In neutral or slightly acidic solutions, silicic acid remains in its monomeric or low-polymer forms. At higher concentrations or under changing environmental conditions, it polymerizes and precipitates as hydrated silicon dioxide (SiO₂·nH₂O), which is used in a variety of applications.

One of the most widespread applications of silicic acid and its derivatives is in the production of silica gel. Silica gel is manufactured by the acidification of sodium silicate solution, which leads to the formation of a gelatinous precipitate of hydrated silica. This material is then washed, dried, and used as a desiccant due to its high surface area and porosity, which allows it to adsorb moisture efficiently. Silica gel is commonly found in packaging to protect products from humidity, in laboratory drying agents, and in chromatographic media.

Silicic acid also plays a crucial role in the formation of zeolites, which are microporous aluminosilicate minerals used as molecular sieves and catalysts. In the synthesis of zeolites, silicic acid or soluble silica serves as the silicon source, which, together with alumina and structure-directing agents, forms the crystalline framework of the zeolite under hydrothermal conditions.

In the field of materials science, silicic acid derivatives are used to prepare silica nanoparticles and coatings. The sol-gel process, which involves the transition of a system from a liquid "sol" into a solid "gel" phase, uses alkoxysilanes or silicic acid precursors to form silica materials with controlled porosity and structure. These materials are used in applications such as optical coatings, insulating films, and as supports for catalysts.

In environmental science, silicic acid is a key component of the biogeochemical cycle of silicon. Diatoms, a major group of microalgae in marine and freshwater ecosystems, require silicic acid to build their siliceous cell walls, known as frustules. The uptake of silicic acid by diatoms and its recycling through the food web and sedimentation processes influence the global carbon and silica cycles.

In biological systems, there is evidence that silicic acid, especially in its monomeric form, may have a role in human and animal physiology. It has been associated with bone mineralization, connective tissue health, and possibly the regulation of aluminum bioavailability. Some dietary supplements contain stabilized orthosilicic acid, marketed for supporting skin, hair, nail, and bone health, although the exact mechanisms and efficacy are still under investigation.

Despite its weak acidity and limited stability in solution, silicic acid remains an important intermediate in various industrial processes and natural systems. Its chemistry is closely related to that of silica and silicates, and it serves as a crucial link between soluble silicon species and solid silicon-containing materials in both nature and technology. The study and utilization of silicic acid continue to be a field of active research, particularly in understanding its polymerization behavior and improving its applications in materials design and environmental science.

References

1987 Silicate minerals and the interferon system. Environmental Research, 43(2).
DOI: 10.1016/s0013-9351(87)80040-3

2008 Arsenic in Rice (Oryza sativa L.) Related to Dynamics of Arsenic and Silicic Acid in Paddy Soils. Environmental Science & Technology, 42(20).
DOI: 10.1021/es801194q

2016 A tightly regulated expression system for E. coli using supersaturated silicic acid. Biotechnology Letters, 38(8).
DOI: 10.1007/s10529-016-2118-z

2024 Silicon (Ca, K & Mg) Induced Resistance in Two Contrasting Rice Genotypes Against Phloem Feeding Pest, Brown Planthopper, Nilaparvata lugens (Stal.) through Modulation of Defense Responses. Silicon.
DOI: 10.1007/s12633-024-03203-5