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Concept
Sodium silicate
Résumé
Sodium silicate is a generic name for chemical compounds with the formula Na2xSiyO2y+x or (Na2O)x·(SiO2)y, such as sodium metasilicate Na2SiO3, sodium orthosilicate Na4SiO4, and sodium pyrosilicate Na6Si2O7. The anions are often polymeric. These compounds are generally colorless transparent solids or white powders, and soluble in water in various amounts.
Sodium silicate is also the technical and common name for a mixture of such compounds, chiefly the metasilicate, also called waterglass, water glass, or liquid glass. The product has a wide variety of uses, including the formulation of cements, passive fire protection, textile and lumber processing, manufacture of refractory ceramics, as adhesives, and in the production of silica gel. The commercial product, available in water solution or in solid form, is often greenish or blue owing to the presence of iron-containing impurities.
In industry, the various grades of sodium silicate are characterized by their SiO2:Na2O weight ratio (which can be converted to molar ratio by multiplication with 1.032). The ratio can vary between 1:2 and 3.75:1. Grades with ratio below 2.85:1 are termed alkaline. Those with a higher SiO2:Na2O ratio are described as neutral.
Soluble silicates of alkali metals (sodium or potassium) were observed by European alchemists already in the 1500s. Giambattista della Porta observed in 1567 that tartari salis (cream of tartar, potassium hydrogen tartrate) caused powdered crystallum (quartz) to melt at a lower temperature. Other possible early references to alkali silicates were made by Basil Valentine in 1520, and by Agricola in 1550. Around 1640, Jean Baptist van Helmont reported the formation of alkali silicates as a soluble substance made by melting sand with excess alkali, and observed that the silica could be precipitated quantitatively by adding acid to the solution.
In 1646, Glauber made potassium silicate, which he called liquor silicum, by melting potassium carbonate (obtained by calcinating cream of tartar) and sand in a crucible, and keeping it molten until it ceased to bubble (due to the release of carbon dioxide).
Source officielle
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