Sodium silicate

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Sodium silicate
IUPAC name Sodium metasilicate
Other names Waterglass Liquid glass E550
Identifiers
CAS number	6834-92-0 Y, 10213-79-3 (pentahydrate) 13517-24-3 (nonahydrate)
PubChem	23266
EC number	229-912-9
RTECS number	VV9365000
Properties
Molecular formula	Na2SiO3
Molar mass	122.06 g/mol (anhydrous) 212.14 g/mol (pentahydrate)
Appearance	colorless solid
Density	2.4 g/cm3, solid
Melting point	1088 °C (anhydrous) 72.2 °C (pentahydrate)
Solubility in water	Soluble
Refractive index (nD)	1.52 (anhydrous) 1.456 (pentahydrate)
Thermochemistry
Std enthalpy of formation ΔfHo298	−1519 kJ/mol
Standard molar entropy So298	113.8 J K−1 mol−1
Hazards
MSDS	Mallinckrodt Baker, Inc.
EU Index	014-010-00-8
EU classification	Corrosive (C)
R-phrases	R34, R37
S-phrases	(S1/2), S13, S24/25, S36/37/39, S45
NFPA 704	0 2 0
Related compounds
Other anions	Sodium carbonate Sodium germanate Sodium stannate Sodium plumbate
Other cations	Potassium silicate
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Sodium silicate is the common name for a compound sodium metasilicate, Na₂SiO₃, also known as water glass or liquid glass. It is available in aqueous solution and in solid form and is used in cements, passive fire protection, refractories, textile and lumber processing, and automobiles. Sodium carbonate and silicon dioxide react when molten to form sodium silicate and carbon dioxide:^[1]

Na₂CO₃ + SiO₂ → Na₂SiO₃ + CO₂

Anhydrous sodium silicate contains a chain polymeric anion composed of corner shared {SiO₄} tetrahedra, and not a discrete SiO₃²⁻ ion.^[1] In addition to the anhydrous form, there are hydrates with the formulae Na₂SiO₃·nH₂O (where n = 5, 6, 8, 9) which contain the discrete, approximately tetrahedral anion SiO₂(OH)₂²⁻ with water of hydration. For example, the commercially available sodium silicate pentahydrate Na₂SiO₃·5H₂O is formulated as Na₂SiO₂(OH)₂·4H₂O and the nonahydrate Na₂SiO₃·9H₂O is formulated as Na₂SiO₂(OH)₂·8H₂O.^[2]

History

Water Glass was defined in Von Wagner's Manual of Chemical Technology (1892 translation) as any of the soluble alkaline silicates, first observed by Van Helmont in 1640 as a fluid substance made by melting sand with excess alkali. Glauber made what he termed "fluid silica" in 1648 from potash and silica. Von Fuchs, in 1825, obtained what is now known as water glass by treating silicic acid with an alkali, the result being soluble in water, "but not affected by atmospheric changes".^[3] Von Wagner distinguished soda, potash, double(soda and potash), and fixin as types of water glass. The fixing type was "a mixture of silica well saturated with potash water glass and a sodium silicate" used to stabilize inorganic water color pigments on cement work for outdoor signs and murals.

Properties

Sodium silicate is a white powder that is readily soluble in water, producing an alkaline solution. It is one of a number of related compounds which include sodium orthosilicate, Na₄SiO₄, sodium pyrosilicate, Na₆Si₂O₇, and others. All are glassy, colourless and dissolve in water.

Sodium silicate is stable in neutral and alkaline solutions. In acidic solutions, the silicate ion reacts with hydrogen ions to form silicic acid, which when heated and roasted forms silica gel, a hard, glassy substance.

CAS registry number and EINECS number

Each and every substance has its own unique CAS registry number and EINECS number. The CAS No. and EINECS No. of sodium silicate and other related substances are:^[4]

Uses

Metal repair

Sodium silicate is used, along with magnesium silicate, in muffler repair and fitting paste. When dissolved in water, both sodium silicate, and magnesium silicate form a thick paste that is easy to apply. When the exhaust system of an internal combustion engine heats up to its operating temperature, the heat drives out all of the excess water from the paste. The silicate compounds that are left over have glass-like properties, making a somewhat permanent, brittle repair.

Automotive repair

Sodium silicate can be used to seal leaks at the head gasket. A common use is when an alloy cylinder head engine is left sitting for extended periods or the coolant is not changed at proper intervals, electrolysis can "eat out" sections of the head causing the gasket to fail.

Rather than remove the cylinder head, "liquid glass" is poured into the radiator and allowed to circulate. The waterglass is injected via the radiator water into the hotspot at the engine. This technique works because at 210–220 °F the sodium silicate loses water molecules to form a very powerful sealant that will not re-melt below 1500 °F.

A sodium silicate repair of a leaking head gasket can hold for up to two years and even longer in some cases. The effect will be almost instant, and steam from the radiator water will stop coming out the exhaust within minutes of application. This repair only works with water-to-cylinder or water-to-air applications and where the sodium silicate reaches the "conversion" temperature of 210–220 °F.

Car engine disablement

Sodium silicate solution is used to inexpensively, quickly, and permanently disable automobile engines. Running an engine with two quarts of a sodium silicate solution instead of motor oil causes the solution to precipitate, catastrophically damaging the engine's bearings and pistons within a few minutes.^[5] In the United States, this procedure is required by the Car Allowance Rebate System (CARS) program.^[5][6]

Aquaculture

Sodium silicate gel is also used as a substrate algal growth in aquaculture hatcheries.

Food preservation

Sodium silicate was also used as an egg preservation agent in the early 20th century with large success. When fresh eggs are immersed in it, bacteria which cause the eggs to spoil are kept out and water is kept in. Eggs can be kept fresh using this method for up to nine months. When boiling eggs preserved this way, it is well advised to pin-prick the egg to allow steam to escape because the shell is no longer porous.^[7]

Timber treatment

The use of sodium silicate as a timber treatment for pressure-treated wood began some time in the 19th century. It is suggested that that more costly "silicate of potash" (potassium silicate) may also be used, in "Handy Farm Devices and How to Make Them" by Rolfe Cobleigh, published in 1910. Since 1998, scientists have researched methods for rendering sodium silicate insoluble once the lumber has been treated with it. With or without the additional process, treating wood with sodium silicate preserves wood from insects and possesses some flame-retardant properties.

Concrete and general masonry treatment

Concrete treated with a sodium silicate solution helps to significantly reduce porosity in most masonry products such as concrete, stucco, plasters. A chemical reaction occurs with the excess Ca(OH)₂ in the concrete that permanently binds the silicates with the surface making them far more wearable and water repellent. It is generally advised to apply only after initial cure has taken place (7 days or so depending on conditions). These coatings are known as silicate mineral paint.

Passive fire protection (PFP)

Expantrol proprietary sodium silicate suspended in 1/4" thick layer of red rubber, type 3M FS195, inserted into a metal pipe, then heated, to demonstrate hard char intumescence, strong enough to shut a melting plastic pipe

Palusol based intumescent plastic pipe device used for commercial firestopping.

Sodium silicates are inherently intumescent. They come in prill (solid beads) form, as well as the liquid, water glass. The solid sheet form (Palusol) must be waterproofed to ensure longterm passive fire protection.

Standard, solid, bead form sodium silicates have been used as aggregate within silicone rubber to manufacture plastic pipe firestop devices. The silicone rubber was insufficient waterproofing to preserve the intumescing function and the products had to be recalled, which is problematic for firestops that are concealed behind drywall in buildings.

Pastes for caulking purposes are similarly unstable. This too has resulted in recalls and even litigation. Only 3M's "Expantrol" version, which has an external heat treatment that helps to seal the outer surface, as part of its process standard, has achieved sufficient longevity to qualify for DIBt approvals in the US for use in firestopping.

Not unlike other intumescents, sodium silicate, both in bead form and in liquid form are inherently endothermic, due to liquid water in the water glass and hydrates in the prill form. The absence in the US of mandatory aging tests, whereby PFP systems are made to undergo system performance tests after the aging and humidity exposures, are at the root of the continued availability, in North America, of PFP products that can become inoperable within weeks of installation. Indiscriminate use of sodium silicates without proper waterproofing measures are contributors to the problems and risk. When sodium silicates are adequately protected, they function extremely well and reliably for long. Evidence of this can be seen in the many DIBt approvals for plastic pipe firestop devices using Palusol, which use waterproofed sodium silicate sheets.

Refractory use

Water glass is a useful binder of solids, such as vermiculite and perlite. When blended with the aforementioned lightweight aggregates, water glass can be used to make hard, high-temperature insulation boards used for refractories, passive fire protection and high temperature insulations, such as moulded pipe insulation applications. When mixed with finely divided mineral powders, such as vermiculite dust (which is common scrap from the exfoliation process), one can produce high temperature adhesives. The intumescence disappears in the presence of finely divided mineral dust, whereby the waterglass becomes a mere matrix. Waterglass is inexpensive and abundantly available, which makes its use popular in many refractory applications.

Water treatment

Water glass is used as a water treatment in waste water treatment plants. Waterglass will bind to heavier molecules and drag them out of the water.

Detergent auxiliaries

It also can be used in detergent auxiliaries like complex sodium disilicate and modified sodium disilicate.

Magic crystals

Water glass was used in the Magic rocks toys invented in 1940. When waterglass was combined with a selection of different metals in solution, the waterglass would cause the metals to precipitate. Each metal would precipitate separately causing a different color stalagmite.

An early mention of crystals of metallic salts forming a "chemical garden" in sodium silicate is found in the 1946 Modern Mechanix magazine.^[8] This results in very colorful gardens—much more than shown in the illustrations.

In Europe the ingredients for such chemical gardens were available already around the early 1930s.

Dye auxiliary

Sodium silicate solution is used as a fixative for hand dyeing with reactive dyes that require a high pH in order to react with the textile fiber. After the dye is applied to a cellulose-based fabric, such as cotton or rayon, or onto silk, it is allowed to dry, after which the sodium silicate is painted on to the dyed fabric, covered with plastic to retain moisture, and left to react for an hour at room temperature.^[9]

See also

• Fireproofing

References

External links

• Centre Européen d'Etudes des Silicates
• International Chemical Safety Card 1137
• Sodium silicate Physical and Chemical Properties
• sodium silicate