Sodium carbonate (also known aswashing soda,soda ash,sal soda, andsoda crystals) is theinorganic compound with the formulaNa2CO3 and its varioushydrates. All forms are white, odorless, water-soluble salts that yield alkaline solutions in water. Historically, it was extracted from the ashes of plants grown in sodium-rich soils, and because the ashes of these sodium-rich plants were noticeably different from ashes of wood (once used to producepotash), sodium carbonate became known as "soda ash".[12] It is produced in large quantities fromsodium chloride andlimestone by theSolvay process, as well as by carbonating sodium hydroxide which is made using thechloralkali process.
Sodium carbonate is obtained as threehydrates and as the anhydrous salt:
sodium carbonate decahydrate (natron), Na2CO3·10H2O, which readilyeffloresces to form the monohydrate.
sodium carbonate heptahydrate (not known in mineral form), Na2CO3·7H2O.
sodium carbonate monohydrate (thermonatrite), Na2CO3·H2O. Also known ascrystal carbonate.
anhydrous sodium carbonate (natrite), also known as calcined soda, is formed by heating the hydrates. It is also formed whensodium hydrogencarbonate is heated (calcined) e.g. in the final step of theSolvay process.
The decahydrate is formed from water solutions crystallizing in the temperature range −2.1 to +32.0 °C, the heptahydrate in the narrow range 32.0 to 35.4 °C and above this temperature the monohydrate forms.[13] In dry air the decahydrate and heptahydrate lose water to give the monohydrate. Other hydrates have been reported, e.g. with 2.5 units of water per sodium carbonate unit ("Penta hemihydrate").[14]
Sodium carbonate decahydrate (Na2CO3·10H2O), also known as washing soda, is the most common hydrate of sodium carbonate containing 10 molecules ofwater of crystallization. Soda ash is dissolved in water and crystallized to get washing soda.
Na2CO3 + 10 H2O → Na2CO3·10H2O
It is one of the few metalcarbonates that is soluble in water.
Some common applications of sodium carbonate include:
As a cleansing agent for domestic purposes like washing clothes. Sodium carbonate is a component of many dry soap powders. It hasdetergent properties through the process ofsaponification, which converts fats and grease to water-solublesalts (specifically, soaps).[15]
Sodium carbonate serves as aflux forsilica (SiO2, melting point 1,713 °C), lowering the melting point of the mixture to something achievable without special materials. This "soda glass" is mildly water-soluble, so somecalcium carbonate is added to the melt mixture to make the glass insoluble. Bottle and window glass ("soda–lime glass" with transition temperature ~570 °C) is made by melting such mixtures of sodium carbonate, calcium carbonate, and silica sand (silicon dioxide (SiO2)). When these materials are heated, the carbonates release carbon dioxide. In this way, sodium carbonate is a source of sodium oxide. Soda–lime glass has been the most common form of glass for centuries. It is also a key input for tableware glass manufacturing.[15]
Hard water usually contains calcium or magnesium ions. Sodium carbonate is used for removing these ions and replacing them with sodium ions.[16]
Sodium carbonate is a water-soluble source of carbonate. The calcium and magnesium ions form insoluble solid precipitates upon treatment withcarbonate ions:
Ca2+ + CO2−3 → CaCO3 (s)
The water is softened because it no longer contains dissolved calcium ions and magnesium ions.[16]
Sodium carbonate has several uses in cuisine, largely because it is a stronger base than baking soda (sodium bicarbonate) but weaker thanlye (which may refer tosodium hydroxide or, less commonly,potassium hydroxide). Alkalinity affectsgluten production in kneaded doughs, and also improves browning by reducing the temperature at which theMaillard reaction occurs. To take advantage of the former effect, sodium carbonate is therefore one of the components ofkansui (かん水), a solution of alkaline salts used to giveJapaneseramen noodles their characteristic flavour and chewy texture; a similar solution is used inChinese cuisine to makelamian, for similar reasons.Cantonese bakers similarly use sodium carbonate as a substitute for lye-water to givemoon cakes their characteristic texture and improve browning. InGerman cuisine (and Central European cuisine more broadly), breads such aspretzels andlye rolls traditionally treated with lye to improve browning can be treated instead with sodium carbonate; sodium carbonate does not produce quite as strong a browning as lye, but is much safer and easier to work with.[18]
Sodium carbonate is used in the production ofsherbet powder. The cooling and fizzing sensation results from the endothermic reaction between sodium carbonate and a weak acid, commonlycitric acid, releasing carbon dioxide gas, which occurs when the sherbet is moistened by saliva.
Sodium carbonate is also used as a relatively strongbase in various fields. As a common alkali, it is preferred in many chemical processes because it is cheaper thansodium hydroxide and far safer to handle. Its mildness especially recommends its use in domestic applications.
For example, it is used as apH regulator to maintain stable alkaline conditions necessary for the action of the majority of photographicfilm developing agents. It is also a common additive inswimming pools andaquarium water to maintain a desired pH and carbonate hardness (KH). Indyeing with fiber-reactive dyes, sodium carbonate (often under a name such as soda ash fixative or soda ash activator) is used asmordant to ensure proper chemical bonding of the dye with cellulose (plant) fiber. It is also used in thefroth flotation process to maintain a favourablepH as a float conditioner besidesCaO and other mildly basic compounds.
Sodiumbicarbonate (NaHCO3) or baking soda, also a component in fire extinguishers, is often generated from sodium carbonate. Although NaHCO3 is itself an intermediate product of the Solvay process, the heating needed to remove the ammonia that contaminates it decomposes some NaHCO3, making it more economical to react finished Na2CO3 with CO2:
Na2CO3 + CO2 + H2O → 2NaHCO3
In a related reaction, sodium carbonate is used to makesodium bisulfite (NaHSO3), which is used for the "sulfite" method of separatinglignin from cellulose. This reaction is exploited for removingsulfur dioxide from flue gases in power stations:
Na2CO3 + SO2 + H2O → NaHCO3 + NaHSO3
This application has become more common, especially where stations have to meet stringent emission controls.
Sodium carbonate is used by the cotton industry to neutralize the sulfuric acid needed for acid delinting of fuzzy cottonseed.
It is also used to form carbonates of other metals by ion exchange, often with the other metals' sulphates.
Sodium carbonate is used by the brick industry as a wetting agent to reduce the amount of water needed to extrude the clay. In casting, it is referred to as "bonding agent" and is used to allow wetalginate to adhere to gelled alginate. Sodium carbonate is used in toothpastes, where it acts as a foaming agent and an abrasive, and to temporarily increase mouth pH.
Sodium carbonate is also used in the processing and tanning of animal hides to neutralize acids.[19]
Sodium carbonate is soluble in water, and can occur naturally in arid regions, especially in mineral deposits (evaporites) formed when seasonal lakes evaporate. Deposits of the mineralnatron have been mined from dry lake bottoms in Egypt since ancient times, when natron was used in the preparation ofmummies and in the early manufacture of glass.
The anhydrous mineral form of sodium carbonate is quite rare and called natrite. Sodium carbonate also erupts fromOl Doinyo Lengai, Tanzania's unique volcano, and it is presumed to have erupted from other volcanoes in the past, but due to these minerals' instability at the Earth's surface, are likely to be eroded. All three mineralogical forms of sodium carbonate, as well astrona, trisodium hydrogendi carbonate dihydrate, are also known from ultra-alkalinepegmatitic rocks, that occur for example in theKola Peninsula in Russia.
Extra terrestrially, known sodium carbonate is rare. Deposits have been identified as the source ofbright spots on Ceres, interior material that has been brought to the surface.[21] While there arecarbonates on Mars, and these are expected to include sodium carbonate,[22] deposits have yet to be confirmed, this absence is explained by some as being due to a global dominance of lowpH in previously aqueousMartian soil.[23]
Trona, also known astrisodium hydrogendicarbonate dihydrate (Na3HCO3CO3·2H2O), is mined in several areas of the US and provides nearly all the US consumption of sodium carbonate. Large natural deposits found in 1938, such as the one nearGreen River, Wyoming, have made mining more economical than industrial production in North America. There are important reserves of trona in Turkey;[24] two million tons of soda ash have been extracted from the reserves near Ankara.
Several "halophyte" (salt-tolerant) plant species and seaweed species can be processed to yield an impure form of sodium carbonate, and these sources predominated in Europe and elsewhere until the early 19th century. The land plants (typicallyglassworts orsaltworts) or the seaweed (typicallyFucus species) were harvested, dried, and burned. The ashes were then "lixivated" (washed with water) to form an alkali solution. This solution was boiled dry to create the final product, which was termed "soda ash"; this very old name derives from the Arabic wordsoda, in turn applied toSalsola soda, one of the many species of seashore plants harvested for production. "Barilla" is a commercial term applied to an impure form ofpotash obtained from coastal plants orkelp.[25]
The sodium carbonate concentration in soda ash varied very widely, from 2–3 percent for the seaweed-derived form ("kelp"), to 30 percent for the bestbarilla produced fromsaltwort plants in Spain. Plant and seaweed sources for soda ash, and also for the relatedalkali "potash", became increasingly inadequate by the end of the 18th century, and the search for commercially viable routes to synthesizing soda ash from salt and other chemicals intensified.[26]
The second stage is the reaction to produce sodium carbonate andcalcium sulfide:
Na2S + CaCO3 → Na2CO3 + CaS
This mixture is calledblack ash. The soda ash is extracted from the black ash with water. Evaporation of this extract yields solid sodium carbonate. This extraction process was termedlixiviating.
The hydrochloric acid produced by theLeblanc process was a major source of air pollution, and thecalcium sulfide byproduct also presented waste disposal issues. However, it remained the major production method for sodium carbonate until the late 1880s.[26][27]
The resulting sodium bicarbonate was then converted to sodium carbonate by heating it, releasing water and carbon dioxide:
2NaHCO3 → Na2CO3 + H2O + CO2
Meanwhile, the ammonia was regenerated from the ammonium chloride byproduct by treating it with the lime (calcium oxide) left over from carbon dioxide generation:
2NH4Cl + CaO → 2NH3 + CaCl2 + H2O
The Solvay process recycles its ammonia. It consumes only brine and limestone, andcalcium chloride is its only waste product. The process is substantially more economical than the Leblanc process, which generates two waste products,calcium sulfide andhydrogen chloride. The Solvay process quickly came to dominate sodium carbonate production worldwide. By 1900, 90% of sodium carbonate was produced by the Solvay process, and the last Leblanc process plant closed in the early 1920s.[15]
The second step of the Solvay process, heating sodium bicarbonate, is used on a small scale by home cooks and in restaurants to make sodium carbonate for culinary purposes (includingpretzels andalkali noodles). The method is appealing to such users because sodium bicarbonate is widely sold as baking soda, and the temperatures needed (250 °F (121 °C) to 300 °F (149 °C)) to convert baking soda to sodium carbonate are readily achieved in conventional kitchenovens.[18]
This process was developed by Chinese chemistHou Debang in the 1930s. The earliersteam reforming by-product carbon dioxide was pumped through a saturated solution ofsodium chloride and ammonia to produce sodium bicarbonate by these reactions:
The sodium bicarbonate was collected as a precipitate due to its low solubility and then heated up to approximately 80 °C (176 °F) or 95 °C (203 °F) to yield pure sodium carbonate similar to last step of the Solvay process. More sodium chloride is added to the remaining solution of ammonium and sodium chlorides; also, more ammonia is pumped at 30–40 °C to this solution. The solution temperature is then lowered to below 10 °C. Solubility of ammonium chloride is higher than that of sodium chloride at 30 °C and lower at 10 °C. Due to this temperature-dependent solubility difference and thecommon-ion effect, ammonium chloride is precipitated in a sodium chloride solution.
The Chinese name of Hou's process,lianhe zhijian fa (联合制碱法), means "coupled manufacturing alkali method": Hou's process is coupled to theHaber process and offers betteratom economy by eliminating the production of calcium chloride, since any ammonia generated gets used by the reaction. The by-product ammonium chloride can be sold as a fertilizer.
^abcHimmelblau, David M.; Riggs, James B. (2022).Basic principles and calculations in chemical engineering. International series in the physical and chemical engineering sciences (Ninth ed.). Boston: Pearson.ISBN978-0-13-732717-1.
