Sodium silicates are chemical compounds from the group of sodium compounds and silicates that occur in several molar ratios (monosilicate, metasilicate, polysilicate, ...). Depending on how they are made and the degree of purity , they are more or less hydrated or soluble. In addition, the sodium salt Na 4 SiO 4 of the orthosilicic acid Si (OH) 4 is also referred to as sodium orthosilicate .
Extraction and presentation
Sodium silicates in amorphous form are produced by melting together silicon dioxide and sodium oxide (or sodium carbonate with evolution of carbon dioxide ) in molar ratios between 1: 1 to 4: 1. The solids obtained in this way are dissolved in water at about 150 ° C. and 5 bar steam pressure in order to obtain a solution in water (soda waterglass ).
Metasilicate Sodium Metasilicate
|Disodium disilicate||Disodium trisilicate|
|Molecular formula||Na 4 O 4 Si||Na 2 O 3 Si||Na 2 O 5 Si 2||Na 2 O 7 Si 3|
|Molar mass||184.04 g mol −1||122.06 g mol −1||182.15 g mol −1||242.23 g mol −1|
|Brief description||as a pure substance colorless and odorless solid|
|Melting point||1018 ° C||1098 ° C
48 ° (nonahydrate)
72 ° C (pentahydrate)
|density||1.39 g cm −3||2.61 g cm −3|
|solubility||soluble in water (175… 350 g l −1 at 20 ° C)|
|H and P phrases||no H-phrases||--||see above||see above|
|no EUH phrases||no EUH phrases||see above||see above|
|no P-phrases||---||see above||see above|
|Tox data||1150 mg kg −1 ( LD 50 , rat , oral )|
Sodium silicates are crystalline or, as a mixture of different silicates, vitreous (amorphous) solids or solutions that are viscous in water. As a pure substance they are colorless, technical products appear slightly blue to green or yellow to brown due to impurities. Soda water glass has an alkaline reaction due to partial hydrolysis . The solution is gradually neutralized by the carbon dioxide in the air, with precipitation products of the silica being formed depending on the concentration . Sodium metasilicate also occurs in several hydrate forms , with the nonahydrate melting at 48 ° C and the pentahydrate at 72 ° C.
Silicates have such a broad field of application that only a few important uses are listed. These are mainly used in the form of so-called water glass (water-soluble silicates of alkali metals , primarily sodium and potassium ). Sodium silicate is used as binder , fire protection material, as well as a buffer and a stabilizer for chemicals (z. B. H 2 O 2 ), for the briquetting of coal , glass, bronze , in the paper-recycling, in the coating used paper, and metal degreasing. It is also found in detergents , bleach , soaps , detergents , cements , mortars , ceramic products, paints and cosmetic preparations.
In 2015, disodium metasilicate was included by the EU in the Community's rolling action plan ( CoRAP ) in accordance with Regulation (EC) No. 1907/2006 (REACH) as part of substance evaluation . The effects of the substance on human health and the environment are re-evaluated and, if necessary, follow-up measures are initiated. The reasons for the uptake of disodium metasilicate were concerns about consumer use , high (aggregated) tonnage and high risk characterization ratio (RCR) as well as the possible danger from reproductive toxicity . The re-evaluation started in 2015 and was carried out by Latvia . A final report was then published.
- Sodium silicates (KEMI) ( Memento from January 17, 2012 in the Internet Archive )
- Patent EP0860399 : Method of preparing crystalline sodium silicates. Published on April 4, 2001 , inventors: Josef Holz, Günther Schimmel, Alexander Tapper, Volker Thewes.
- OECD : Screening Information Dataset (SIDS) Initial Assessment Report (SIAR) for Disodium silicate, pentahydrate
- Werner Wieland: A contribution to the knowledge of silica . Zurich 1930, doi : 10.3929 / ethz-a-000144203 (dissertation, ETH Zurich).
- minerals (mineral atlas)
- Tetrasodium orthosilicate data sheet (PDF) from GuideChem, accessed on October 27, 2014.
- Sodium metasilicate data sheet from Sigma-Aldrich , accessed on May 4, 2011 ( PDF ).
- European Chemicals Agency (ECHA): Substance Evaluation Conclusion and Evaluation Report .
- Community rolling action plan ( CoRAP ) of the European Chemicals Agency (ECHA): disodium metasilicate , accessed on May 20, 2019.