What Is Sodium Silicate?
Sodium Silicate is the generic name for the sodium salts of silicic acid. There are various compositions such as Na4SiO4, Na2Si2O5, and Na2Si4O9, but in general, sodium silicate is often referred to as sodium metasilicate, which is Na2SiO3.
Sodium Silicate is a colorless crystalline solid at room temperature and soluble in water. Its concentrated aqueous solution is irritating and is called aqueous glass. When hydrochloric acid is added to aqueous glass, silicic acid is obtained.
Uses of Sodium Silicate
Sodium Silicate is used in a variety of applications, including adhesives and ceramics. It is used as an adhesive in the manufacturing process of various paper products, such as toilet paper cores and cardboard, because of its high safety and low environmental impact.
In ceramics, coating cup handles and other parts with sodium silicate before attaching them to cups can prevent damage due to foaming.
Sodium Silicate is also used as a rust inhibitor for metals. It reacts with metal to produce a layer of silicic acid and metal oxides on the surface of the metal, preventing further erosion.
Other uses include additives in soaps and detergents, building materials, and foundries.
Derivatives of sodium silicate include silica gel, zeolite, and silicic anhydride.
Properties of Sodium Silicate
Aqueous solutions of sodium silicate are weakly alkaline, and are stable at pH 10.2 or higher. Adding acid lowers the pH and increases the viscosity due to siloxane bond formation.
Aqueous solutions of sodium silicate react with heavy metals and other substances to form precipitates. This property is widely used in sewage treatment and water treatment.
When alcohol, phenol, or aldehyde is added to sodium silicate, it dehydrates and releases silica gel.
Types of Sodium Silicate
Sodium Silicate is a silicate of sodium, generally expressed as Na2O, nSiO2, and mH2O. Depending on the ratio of the three components, there are various types of sodium silicate with different physical and chemical properties in glassy, crystalline, and aqueous solution forms. Industrially, it is manufactured with SiO2/Na2O ranging from 0.5 to 4.
Crystalline sodium silicates include sodium orthosilicate (2Na2O-SiO2.xH2O), sodium sesquisilicate (3Na2O-2SiO2.xH2O), and sodium metasilicate (Na2O-SiO2.xH2O).
Aqueous solutions are also available with different ratios of Na2O and SiO2, such as No. 1 and No. 2.
Structure of Sodium Silicate
The structure of sodium silicate varies depending on its type, but in general, it consists of silicate tetrahedra bonded via covalent oxygen atoms to form a three-dimensional network of silicon and oxygen atoms. Sodium cations are incorporated into the network, filling the spaces between the sodium silicate tetrahedra and balancing the negative charge of the oxygen atoms.
In aqueous solution, they exist as chained metasilicate ions rather than orthosilicate ions. As the concentration of sodium ions decreases, branching and cross-linking of the silicate ions occurs, giving rise to a network structure.
Other Information on Sodium Silicate
1. Production Process of Sodium Silicate
Sodium Silicate can be produced by either the dry process or the wet process.
In the dry process, silica sand is mixed with sodium hydroxide or sodium carbonate and melted. The resulting cullet and water are dissolved in an autoclave, filtered through a filter press, and the molar ratio is adjusted to obtain sodium silicate.
In the wet process, sodium hydroxide and silica sand are reacted in an autoclave with pressurized steam. Sodium silicate is then filtered and concentrated.
2. History of Sodium Silicate
Sodium Silicate was first scientifically recognized and studied in earnest around the 1820s. After 1920, industrial production began in the U.S. and Europe, and the use of sodium silicate began to expand.