# Sodium silicates

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 .

## Occurrence

Sodium silicates occur in nature in the form of the minerals Ertixiite Na 2 Si 4 O 9 and Natrosilite Na 2 Si 2 O 5 .

## 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 ).

${\ displaystyle \ mathrm {Na_ {2} CO_ {3} + SiO_ {2} \ longrightarrow Na_ {2} SiO_ {3} + CO_ {2}}}$

## properties

Sodium silicates
Surname Sodium orthosilicate Disodium
Metasilicate Sodium Metasilicate
E 550
Disodium disilicate Disodium trisilicate
Structural formula
CAS number 13472-30-5 6834-92-0
13517-24-3 (nonahydrate)
10213-79-3 (pentahydrate)
13870-28-5 13870-30-9
1344-09-8 (unspecified)
ECHA InfoCard 100.033.387 100.027.193
100.106.270
100.122.400
100.034.189 100.034.191
100.014.262
PubChem 26051 23266 61699 23266
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
Physical state firmly
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)
GHS
labeling
 no GHS pictograms
danger
 no classification available
 no classification available
H and P phrases no H-phrases 290-314-335 see above see above
no EUH phrases no EUH phrases see above see above
no P-phrases 261-280-305 + 351 + 338-310 see above see above
Tox data 1150 mg kg −1 ( LD 50ratoral )

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.

## use

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.

## Risk assessment

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.