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sodium silicate

sodium silicate, any one of several compounds containing sodium oxide, Na2O, and silica, Si2O, or a mixture of sodium silicates. Sodium orthosilicate is Na4SiO4 (or 2Na2O·SiO2); sodium metasilicate is Na2SiO3 (or Na2O·SiO2); sodium disilicate is Na2Si2O5 (or Na2O·2SiO2); sodium tetrasilicate is Na2Si4O9 (or Na2O·4SiO2). All these compounds are transparent, glassy or crystalline solids that have high melting points (above 800°C) and are water soluble. They are produced chiefly by fusing sand and sodium carbonate in various proportions. The product is commonly known as water glass. The greatest single use of sodium silicates is as a raw material for making silica gel.

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water-glass

water‐glass Sodium silicate; used at one time to preserve eggs, by forming a layer of insoluble calcium silicate around the shell, and so sealing the pores.

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Sodium Silicate

Sodium Silicate

KEY FACTS

OTHER NAMES:

Sodium metasilicate; soluble glass; water glass

FORMULA:

Varies; often represented as Na2SiO3; see Overview

ELEMENTS:

Sodium, silicon, oxygen

COMPOUND TYPE:

Salt (inorganic)

STATE:

Ranges from solid to liquid

MOLECULAR WEIGHT:

Varies; sodium metasilicate: 122.06 g/mol

MELTING POINT:

Varies; sodium metasilicate: 1089°C (1992°F)

BOILING POINT:

Not applicable

SOLUBILITY:

Soluble in cold water; reacts chemically with hot water

OVERVIEW

Sodium silicate (SO-dee-um SILL-uh-kate) is a generic term that refers to a series of compounds with varying proportions of sodium oxide (Na2O) and silicon dioxide (SiO2). The term is sometimes used synonymously with a specific form of the compound called sodium metasilicate, with the chemical formula Na2SiO3. Other forms typically range from Na2O•3.75SiO2 to 2Na2O•SiO2 with one or more molecules of water of hydration. Sodium silicate is a white amorphous (without crystalline structure) solid with the physical properties listed above. Other forms of sodium silicate range from white powders with varying degrees of solubility in water, to greenish, glass-like solids, to liquids with varying degrees of viscosity. Viscosities vary over a range of nearly a million, from about 0.5 poise to more than 600,000 poise. Poise is the unit of measurement for viscosities. Other physical properties of the sodium silicates also vary considerably depending on the relative proportions of sodium oxide and silicon dioxide present. The sodium silicates are sometimes referred to as the simplest form of glass.

HOW IT IS MADE

Sodium silicates are made by fusing (melting) sand (silicon dioxide) and soda ash (sodium carbonate) or sodium hydroxide in a gas-fired open hearth furnace, somewhat similar to the furnaces used in the manufacture of steel. The products of this reaction are lumps of sodium silicate that are broken apart and dissolved in a stream of hot steam. The proportions of sand and soda ash used, the temperature of the reaction, and the amount of water that remains in the final product all determine the physical properties of the final product.

COMMON USES AND POTENTIAL HAZARDS

About 1.1 million metric tons (1.3 million short tons) of sodium silicate was produced in the United States in 2004. The primary application for the compound is in the manufacture of soaps and detergents. It improves the cleaning ability of these products and is less damaging to metal components of dishwashers and washing machines than other ingredients of soaps and detergents. Sodium silicate is also used as a water softener, used by itself or as an ingredient in detergents.

Interesting Facts

  • One of the early uses of sodium silicate was as a preservative for eggs. Eggs were soaked in solutions of sodium silicate, or the compound was painted on the egg shells. The sodium silicate filled the pore in the egg shell, preventing bacteria from entering the egg and causing it to spoil. The process was very effective and preserved eggs for up to nine months. More efficient and less expensive methods of preserving eggs are now available.

The next most important applications of sodium silicate are as catalysts and in the pulp and paper industry. Catalysts are materials that increase the rate of a chemical reaction without undergoing any change in their own chemical structure. In the pulp and paper industry, sodium silicate is used to bleach raw pulp and help remove ink from scrap paper being reprocessed. The compound is also used in sealants and adhesives. Some other applications of sodium silicate include:

  • For the purification of water in municipal and industrial water treatment plants;
  • For the fireproofing of fabrics;
  • As an anti-caking agent in food products;
  • As an additive in cements, where it helps the cement set more quickly;
  • In the manufacture of other compounds of silicon by the chemical industry;
  • In fluids used to lubricate drilling instruments;
  • As a liner for chemical and industrial equipment, such as furnaces used to make steel;
  • In the processing of ores from which metals are extracted; and
  • As a binder on grindstones and abrasive wheels.

Sodium silicates are strong irritants to the skin, eyes, and respiratory system. Prolonged exposure to sodium silicate dust, powder, or liquid may cause inflammation of the skin, eyes, nose, and throat. More serious symptoms may include difficulty in swallowing, burns inside the stomach, damage to the mucous membranes, rapid heartbeat, hypertension, shock, severe damage to the lining of the gastrointestinal tract, various types of cancer, and death. These hazards are of concern primarily to workers who come into contact with sodium silicate in solid or liquid form in the workplace.

Words to Know

MUCOUS MEMBRANES
The soft tissues that line the breathing and digestive passages.
VISCOUS
Describing a syrupy liquid that flows slowly.

FOR FURTHER INFORMATION

Higgins, Kevin T. "Simplified Food-Oil Refining." Food Engineering (February 1, 2003). Also available online at http://www.foodengineeringmag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,6330,94941,00.html (accessed on November 10, 2005).

"Practical Uses for Sodium Silicate." The Chemistry Store.com. http://www.chemistrystore.com/sodium_silicate_uses.htm (accessed on November 10, 2005).

"Silicate Chemistry." PQ Corporation. http://www.pqcorp.com/technicalservice/understanding_silicatesolchem.asp (accessed on November 10, 2005).

"Sodium Metasilicate." International Programme on Chemical Safety. http://www.inchem.org/documents/pims/chemical/pim500.htm#SectionTitle:1.3%20%20Synonyms (accessed on November 10, 2005).

"Sodium Silicates." Chemical Land 21. http://www.chemicalland21.com/arokorhi/industrialchem/inorganic/SODIUM%20SILICATE.htm (accessed on November 10, 2005).

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