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Sodium silicate is a generic name forchemical compounds with the formulaNa
2xSi
yO
2y+x or(Na
2O)
x·(SiO
2)
y, such assodium metasilicate (Na
2SiO
3),sodium orthosilicate (Na
4SiO
4), andsodium pyrosilicate (Na
6Si
2O
7). The anions are oftenpolymeric. 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 calledwaterglass,water glass, orliquid glass. The product has a wide variety of uses, including the formulation ofcements, coatings,passive fire protection, textile and lumber processing, manufacture ofrefractory ceramics, asadhesives, and in the production ofsilica gel. The commercial product, available inwater 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.[1] Grades with ratio below 2.85:1 are termed alkaline. Those with a higher SiO2:Na2O ratio are described as neutral.
Soluble silicates ofalkali metals (sodium orpotassium) were observed by Europeanalchemists in the 16th century.Giambattista della Porta observed in 1567 thattartari salis (cream of tartar,potassium bitartrate) caused powderedcrystallum (quartz) to melt at a lower temperature.[2] Other possible early references to alkali silicates were made byBasil Valentine in 1520,[3] and byAgricola in 1550. Around 1640,Jan 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 thesolution.[4]
In 1646,Glauber madepotassium silicate, which he calledliquor silicum, by meltingpotassium carbonate (obtained bycalcinating cream of tartar) and sand in a crucible, and keeping it molten until it ceased to bubble (due to the release ofcarbon dioxide). The mixture was allowed to cool and then was ground to a fine powder.[5] When the powder was exposed to moist air, it gradually formed a viscous liquid, which Glauber called "Oleum oder Liquor Silicum, Arenæ, vel Crystallorum" (i.e., oil or solution of silica, sand or quartz crystal).[6]
However, it was later claimed that the substances prepared by those alchemists were not waterglass as it is understood today.[7] That would have been prepared in 1818 byJohann Nepomuk von Fuchs, by treatingsilicic acid with an alkali; the result being soluble in water, "but not affected by atmospheric changes".[8]
The terms "water glass" and "soluble glass" were used byLeopold Wolff in 1846,[9] byÉmile Kopp in 1857,[10] and byHermann Krätzer in 1887.[11]
In 1892, Rudolf Von Wagner distinguishedsoda,potash, double (soda and potash), and fixing (i.e., stabilizing) 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.[12][13][14][15]
Sodium silicates are colorless glassy or crystalline solids, or white powders. Except for the most silicon-rich ones, they are readily soluble in water, producingalkaline solutions.[citation needed] When dried up it still can be rehydrated in water.[16]
Sodium silicates are stable inneutral andalkaline solutions. Inacidic solutions, thesilicate ions react with hydrogen ions to formsilicic acids, which tend to decompose intohydrated silicon dioxidegel.[citation needed] Heated to drive off the water, the result is a hard translucent substance calledsilica gel, widely used as adesiccant. It can withstand temperatures up to 1100 °C.[citation needed]
Solutions of sodium silicates can be produced by treating a mixture ofsilica (usually asquartz sand),caustic soda, and water, with hot steam in areactor. The overall reaction is
Sodium silicates can also be obtained by dissolvingsilicaSiO
2 (whosemelting point is 1713 °C) in moltensodium carbonate (that melts with decomposition at 851 °C):[17]
The material can be obtained also fromsodium sulfate (melting point 884 °C) withcarbon as a reducing agent:
In 1990, 4 milliontons of alkali metal silicates were produced.[1]
Sodium silicate may be produced as a part ofhydrogen production by dissolving ferrosilicon in anaqueoussodium hydroxide (NaOH·H2O) solution:[18]
Though unprofitable, Na2SiO3 is a byproduct ofBayer process which is often converted tocalcium silicate (Ca2SiO4).
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The main applications of sodium silicates are in detergents, paper industry (as adeinking agent), water treatment, and construction materials.[1]
The adhesive properties of sodium silicate were noted as early as the 1850s[19] and have been widely used at least since theFirst World War.[20] The largest application of sodium silicate solutions is a cement for producingcardboard.[1] When used as a paper cement, the sodium silicate joint tends to crack within a few years, at which point it no longer holds the paper surfaces cemented together.
Sodium silicate solutions can also be used as a spin-on adhesive layer to bond glass to glass[21] or a silicon dioxide–covered silicon wafer to one another.[22] Sodium silicate glass-to-glass bonding has the advantage that it is a low-temperature bonding technique, as opposed to fusion bonding.[21] It also requires less processing than glass-to-glass anodic bonding,[23] which requires an intermediate layer such assilicon nitride (SiN) to act as a diffusion barrier for sodium ions.[23] The deposition of such a layer requires a low-pressure chemical vapor deposition step.[23] A disadvantage of sodium silicate bonding, however, is that it is very difficult to eliminate air bubbles.[22] This is in part because the technique does not require a vacuum and also does not use field assistance[clarification needed] as in anodic bonding.[24] This lack of field assistance can sometimes be beneficial, because field assistance can provide such high attraction between wafers as to bend a thinner wafer and collapse[24] onto nanofluidic cavity or MEMS elements.
Sodium silicate may be used for various paints and coatings, such as those used onwelding rods. Such coatings can be cured in two ways. One method is to heat a thin layer of sodium silicate into a gel and then into a hard film. To make the coating water-resistant, high temperatures of 100 °C (212 °F; 373 K) are needed.[16] The temperature is slowly raised to 150 °C (302 °F; 423 K) to dehydrate the film and avoid steaming and blistering. The process must be relatively slow, but infrared lamps may be used at first.[16] In the other method, when high temperatures are not practical, the water resistance may be achieved by chemicals (oresters), such asboric acid,phosphoric acid,sodium fluorosilicate, andaluminium phosphate.[16] Before application, an aqueous solution of sodium silicate is mixed with a curing agent.[16]
It is used in detergent auxiliaries such as complex sodium disilicate and modified sodium disilicate. The detergent granules gain their ruggedness from a coating of silicates.[1]
Sodium silicate is used as an alum coagulant and an ironflocculant inwastewater treatment plants. Sodium silicate binds tocolloidal molecules, creating largeraggregates that sink to the bottom of the water column. The microscopic negatively charged particles suspended in water interact with sodium silicate. Theirelectrical double layer collapses due to the increase ofionic strength caused by the addition of sodium silicate (doubly negatively charged anion accompanied by two sodium cations) and they subsequently aggregate. This process is calledcoagulation.[1]
It is used as a binder of the sand when doingsand casting of all common metals. It allows for the rapid production of a strong mold or core by three main methods.[citation needed]
Since the sodium silicate does not burn during casting (it can actually melt at pouring temperatures above 1800 °F), it is common to add organic materials to provide for enhanced sand breakdown after casting. The additives include sugar, starch, carbons, wood flour and phenolic resins.
Water glass is a useful binder for solids, such asvermiculite andperlite. When blended with the latter lightweight fraction, water glass can be used to make hard, high-temperature insulation boards used for refractories,passive fire protection, and high-temperature insulations, such as in 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[clarification needed] 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.
Sodium silicate is used as adeflocculant in casting slips helping reduceviscosity and the need for large amounts of water to liquidize the clay body. It is also used to create a crackle effect in pottery, usually wheel-thrown. A vase or bottle is thrown on the wheel, fairly narrow and with thick walls. Sodium silicate is brushed on a section of the piece. After five minutes, the wall of the piece is stretched outward with a rib or hand. The result is a wrinkled or cracked look.
It is also the main agent in "magic water", which is used when joining clay pieces, especially if the moisture level of the two differs.[25]
Sodium silicate solution is used as a fixative for hand dyeing withreactive dyes that require a high pH 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.[26]
Sodium silicate is used, along withmagnesium silicate, inmuffler repair and fitting paste. Magnesium silicate can be mixed with a solution of sodium silicate to form a thick paste that is easy to apply. When theexhaust system of an internal combustion engine heats up to itsoperating temperature, the heat drives out all of the excess water from the paste. Thesilicate compounds that are left over have glass-like properties, making a temporary, brittle repair that can be reinforced with glass fibre.[citation needed]
Sodium silicate can be used to fill gaps in thehead gasket of an engine. This is especially useful foraluminium alloycylinder heads, which are sensitive to thermally induced surface deflection. Sodium silicate is added to the cooling system through theradiator and allowed to circulate. When the sodium silicate reaches its "conversion" temperature of 100–105 °C (212–221 °F), it loses water molecules and forms a glass seal with a re-melting temperature above 810 °C (1,490 °F). This repair can last two years or longer, and symptoms disappear instantly. However, this repair works only when the sodium silicate reaches its "conversion" temperature. Also, sodium silicate (glass particulate) contamination of lubricants is detrimental to their function, and contamination of engine oil is a serious possibility in situations in which a coolant-to-oil leak is present.
Sodium silicate solution is used to inexpensively, quickly, and permanently disable automobile engines. Running an engine with half a U.S. gallon (or about two liters) of a sodium silicate solution instead ofmotor oil causes the solution toprecipitate, catastrophically damaging the engine's bearings and pistons within a few minutes.[27] In the United States, this procedure was used to comply with requirements of theCar Allowance Rebate System (CARS) program.[27][28]
A mixture of sodium silicate andsawdust has been used in between the double skin of certainsafes. This not only makes them morefire resistant, but also makes cutting them open with anoxyacetylene torch extremely difficult due to thesmoke emitted.
Sodium silicate is frequently used indrilling fluids to stabilize and avoid the collapse ofborehole walls. It is particularly useful when drill holes pass throughargillaceousformations containing swellingclay minerals such assmectite ormontmorillonite.
Concrete treated with a sodium silicate solution helps to reduceporosity in mostmasonry products such asconcrete,stucco, andplasters. This effect aids in reducing water penetration, but has no known effect on reducing water vapor transmission and emission.[29] A chemical reaction occurs with the excess Ca(OH)2 (portlandite) present in the concrete that permanently binds the silicates with the surface, making them far more durable and water repellent. This treatment generally is applied only after the initialcure has taken place (approximately seven days depending on conditions). Thesecoatings are known assilicate mineral paint. An example of the reaction of sodium silicate with the calcium hydroxide found in concrete to formcalcium silicate hydrate (CSH) gel, the main product in hydrated Portland cement, follows.[30]
When crystals of a number of metallic salts are dropped into a solution of water glass, simple or branchingstalagmites of colored metal silicates are formed. This phenomenon has been used by manufacturers of toys and chemistry sets to provide instructive enjoyment to many generations of children from the early 20th century until the present. An early mention of crystals of metallic salts forming a "chemical garden" in sodium silicate is found in the 1946Modern Mechanix magazine.[31] Metal salts used included the sulfates and/or chlorides of copper, cobalt, iron, nickel, and manganese.
Sodium silicate with additives was injected into the ground to harden it and thereby to prevent further leakage of highly radioactive water from theFukushima Daiichi nuclear power plant in Japan in April, 2011.[32] The residual heat carried by the water used for cooling the damaged reactors accelerated the setting of the injected mixture.
On June 3, 1958, theUSSNautilus, the world's first nuclear submarine, visited Everett and Seattle. In Seattle, crewmen dressed in civilian clothing were sent in to secretly buy 140 quarts (160 liters) of an automotive product containing sodium silicate (originally identified as Stop Leak) to repair a leaking condenser system. TheNautilus was en route to the North Pole on a top secret mission to cross the North Pole submerged.[33]
A historical use of the adhesive properties of sodium silicates is the production ofpaper cartridges for black powder revolvers produced byColt's Manufacturing Company between 1851 and 1873, especially during theAmerican Civil War. Sodium silicate was used to seal combustible nitrated paper together to form a conical paper cartridge to hold the black powder, as well as to cement the lead ball or conical bullet into the open end of the paper cartridge. Such sodium silicate cemented paper cartridges were inserted into the cylinders of revolvers, thereby speeding the reloading of cap-and-ball black powder revolvers. This use largely ended with the introduction of Colt revolvers employing brass-cased cartridges starting in 1873.[34][35] Similarly, sodium silicate was also used to cement the top wad into brassshotgun shells, thereby eliminating any need for a crimp at the top of the brass shotgun shell to hold a shotgun shell together. Reloading brass shotgun shells was widely practiced by self-reliant American farmers during the 1870s, using the same waterglass material that was also used to preserve eggs. The cementing of the top wad on a shotgun shell consisted of applying from three to five drops of waterglass on the top wad to secure it to the brass hull. Brass hulls for shotgun shells were superseded by paper hulls starting around 1877. The newer paper-hulled shotgun shells used a roll crimp in place of a waterglass-cemented joint to hold the top wad in the shell. However, whereas brass shotshells with top wads cemented with waterglass could be reloaded nearly indefinitely (given powder, wad, and shot, of course), the paper hulls that replaced the brass hulls could be reloaded only a few times.
Sodium silicate and other silicates are the primary components in "instant" wrinkle remover creams, which temporarily tighten the skin to minimize the appearance of wrinkles and under-eye bags. These creams, when applied as a thin film and allowed to dry for a few minutes, can present dramatic results. This effect is not permanent, lasting from a few minutes up to a couple of hours. It works like water cement, once the muscle starts to move, it cracks and leaves white residues on the skin.
Waterglass has been used as an egg preservative with large success, primarily when refrigeration is not available. Fresh-laid eggs are immersed in a solution of sodium silicate (waterglass). After being immersed in the solution, they are removed and allowed to dry. A permanent air tight coating remains on the eggs. If they are then stored in appropriate environment, the majority of bacteria which would otherwise cause them to spoil are kept out and their moisture is kept in. According to the cited source, treated eggs can be kept fresh using this method for up to five months. When boiling eggs preserved that way, the shell is no longer permeable to air, and the egg will tend to crack unless a hole in the shell is made (e.g., with a pin) in order to allow steam to escape.[36]
Sodium silicate'sflocculant properties are also used to clarify wine and beer by precipitating colloidal particles. As a clearing agent, though, sodium silicate is sometimes confused withisinglass which is prepared fromcollagen extracted from the driedswim bladders ofsturgeon and other fishes. Eggs can be preserved in a bucket of waterglass gel, and their shells are sometimes also used (baked and crushed) to clear wine.[37]
Sodium silicate gel is also used as a substrate for algal growth inaquaculture hatcheries.[38]
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