Product Code Database
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula . It is a white solid ionic compound consisting of sodium and hydroxide .
Sodium hydroxide is a highly corrosive base and alkali that decomposes and at ambient , and may cause severe at high concentrations. It is highly soluble in water, and readily absorbs moisture and carbon dioxide from the air. It forms a series of . The monohydrate crystallizes from water solutions between 12.3 and 61.8 °C. The commercially available "sodium hydroxide" is often this monohydrate, and published data may refer to it instead of the anhydrous compound.
As one of the simplest hydroxides, sodium hydroxide is frequently used alongside neutral water and acidic hydrochloric acid to demonstrate the pH scale to chemistry students.
Sodium hydroxide is used in many industries: in the making of wood pulp and paper, , drinking water, and , and as a drain cleaner. Worldwide production in 2022 was approximately 83 million tons.
Similar to the hydration of sulfuric acid, dissolution of solid sodium hydroxide in water is a highly exothermic reaction where a large amount of heat is liberated, posing a threat to safety through the possibility of splashing. The resulting solution is usually colorless and odorless. As with other alkaline solutions, it feels slippery with skin contact due to the process of saponification that occurs between and natural skin oils.
Early reports refer to hydrates with n = 0.5 or n = 2/3, but later careful investigations failed to confirm their existence.
The only hydrates with stable melting points are (65.10 °C) and (15.38 °C). The other hydrates, except the metastable ones and (β) can be crystallized from solutions of the proper composition, as listed above. However, solutions of NaOH can be easily supercooled by many degrees, which allows the formation of hydrates (including the metastable ones) from solutions with different concentrations.
For example, when a solution of NaOH and water with 1:2 mole ratio (52.6% NaOH by mass) is cooled, the monohydrate normally starts to crystallize (at about 22 °C) before the dihydrate. However, the solution can easily be supercooled down to −15 °C, at which point it may quickly crystallize as the dihydrate. When heated, the solid dihydrate might melt directly into a solution at 13.35 °C; however, once the temperature exceeds 12.58 °C it often decomposes into solid monohydrate and a liquid solution. Even the n = 3.5 hydrate is difficult to crystallize, because the solution supercools so much that other hydrates become more stable.
A hot water solution containing 73.1% (mass) of NaOH is a eutectic that solidifies at about 62.63 °C as an intimate mix of anhydrous and monohydrate crystals.
A second stable eutectic composition is 45.4% (mass) of NaOH, that solidifies at about 4.9 °C into a mixture of crystals of the dihydrate and of the 3.5-hydrate.
The third stable eutectic has 18.4% (mass) of NaOH. It solidifies at about −28.7 °C as a mixture of water ice and the heptahydrate .M. Conde Engineering: " Solid-Liquid Equilibrium (SLE) and Vapour-Liquid Equilibrium (VLE) of Aqueous NaOH ". Online report, accessed on 2017-04-29.
When solutions with less than 18.4% NaOH are cooled, water ice crystallizes first, leaving the NaOH in solution.
The α form of the tetrahydrate has density 1.33 g/cm3. It melts congruously at 7.55 °C into a liquid with 35.7% NaOH and density 1.392 g/cm3, and therefore floats on it like ice on water. However, at about 4.9 °C it may instead melt incongruously into a mixture of solid and a liquid solution.
The β form of the tetrahydrate is metastable, and often transforms spontaneously to the α form when cooled below −20 °C. Once initiated, the exothermic transformation is complete in a few minutes, with a 6.5% increase in volume of the solid. The β form can be crystallized from supercooled solutions at −26 °C, and melts partially at −1.83 °C.
The "sodium hydroxide" of commerce is often the monohydrate (density 1.829 g/cm3). Physical data in technical literature may refer to this form, rather than the anhydrous compound.
In general, such neutralization reactions are represented by one simple net ionic equation:
This type of reaction with a strong acid releases heat, and hence is exothermic. Such acid–base reactions can also be used for . However, sodium hydroxide is not used as a primary standard because it is hygroscopic and absorbs carbon dioxide from air.
A few , however, may react quite vigorously with sodium hydroxide under milder conditions.
In 1986, an aluminium tank truck in the UK was mistakenly used to transport 25% sodium hydroxide solution, causing pressurization of the contents and damage to tankers. The pressurization is due to the hydrogen gas which is produced in the reaction between sodium hydroxide and aluminium:
Zinc and lead salts dissolve in excess sodium hydroxide to give a clear solution of or .
Aluminium hydroxide is used as a gelatinous flocculant to filter out particulate matter in water treatment. Aluminium hydroxide is prepared at the treatment plant from aluminium sulfate by reacting it with sodium hydroxide or bicarbonate.
In 2022, world production was estimated at 83 million dry tonnes of sodium hydroxide, and demand was estimated at 51 million tonnes. In 1998, total world production was around 45 million . North America and Asia each contributed around 14 million tonnes, while Europe produced around 10 million tonnes. In the United States, the major producer of sodium hydroxide is Olin Corporation, which has annual production around 5.7 million from sites at Freeport, Texas; Plaquemine, Louisiana; St. Gabriel, Louisiana; McIntosh, Alabama; Charleston, Tennessee; Niagara Falls, New York; and Bécancour, Canada. Other major US producers include Oxychem, PPG Industries, Shintek, and Formosa. All of these companies use the chloralkali process. Kirk-Othmer Encyclopedia of Chemical Technology, 5th edition, John Wiley & Sons
Historically, sodium hydroxide was produced by treating sodium carbonate with calcium hydroxide (slaked lime) in a metathesis reaction which takes advantage of the fact that sodium hydroxide is soluble, while calcium carbonate is not. This process was called causticizing.
The sodium carbonate for this reaction was produced by the Leblanc process in the early 19th century, or the Solvay process in the late 19th century. The conversion of sodium carbonate to sodium hydroxide was superseded entirely by the chloralkali process, which produces sodium hydroxide in a single process.
Sodium hydroxide is also produced by combining pure sodium metal with water. The byproducts are hydrogen gas and heat, often resulting in a flame.
This reaction is commonly used for demonstrating the reactivity of alkali metals in academic environments; however, it is not used commercially aside from a reaction within the mercury cell chloralkali process where sodium amalgam is reacted with water.
Sodium hydroxide is used in many scenarios where it is desirable to increase the alkalinity of a mixture, or to neutralize acids. For example, in the petroleum industry, sodium hydroxide is used as an additive in drilling mud to increase alkalinity in bentonite mud systems, to increase the mud viscosity, and to neutralize any acid gas (such as hydrogen sulfide and carbon dioxide) which may be encountered in the geological formation as drilling progresses. Another use is in salt spray testing where pH needs to be regulated. Sodium hydroxide is used with hydrochloric acid to balance pH. The resultant salt, NaCl, is the corrosive agent used in the standard neutral pH salt spray test.
Poor quality crude oil can be treated with sodium hydroxide to remove impurities in a process known as caustic washing. Sodium hydroxide reacts with weak acids such as hydrogen sulfide and mercaptans to yield non-volatile sodium salts, which can be removed. The waste which is formed is toxic and difficult to deal with, and the process is banned in many countries because of this. In 2006, Trafigura used the process and then dumped the waste in Ivory Coast.
Other common uses of sodium hydroxide include:
Sodium hydroxide is frequently used in the process of decomposing roadkill dumped in landfills by animal disposal contractors. Due to its availability and low cost, it has been used by criminals to dispose of corpses. Italian serial killer Leonarda Cianciulli used this chemical to turn dead bodies into soap. In Mexico, a man who worked for drug cartels admitted disposing of over 300 bodies with it.
Sodium hydroxide is a dangerous chemical due to its ability to hydrolyze protein. If a dilute solution is spilled on the skin, burns may result if the area is not washed thoroughly and for several minutes with running water. Splashes in the eye can be more serious and can lead to blindness.
Sodium aluminate is an inorganic chemical that is used as an effective source of aluminium hydroxide for many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white crystalline solid having a formula variously given as , , , or . Formation of sodium tetrahydroxoaluminate(III) or hydrated sodium aluminate is given by:
This reaction can be useful in etching, removing anodizing, or converting a polished surface to a satin-like finish, but without further passivation such as anodizing or alodining the surface may become degraded, either under normal use or in severe atmospheric conditions.
In the Bayer process, sodium hydroxide is used in the refining of alumina containing ores (bauxite) to produce alumina (aluminium oxide) which is the raw material used to produce aluminium via the electrolysis Hall-Héroult process. Since the alumina is amphoteric, it dissolves in the sodium hydroxide, leaving impurities less soluble at high pH such as iron oxides behind in the form of a highly alkaline red mud.
Other amphoteric metals are zinc and lead which dissolve in concentrated sodium hydroxide solutions to give sodium zincate and sodium plumbate respectively.
For the manufacture of biodiesel, sodium hydroxide is used as a catalyst for the transesterification of methanol and triglycerides. This only works with anhydrous sodium hydroxide, because combined with water the fat would turn into soap, which would be tainted with methanol. NaOH is used more often than potassium hydroxide because it is cheaper and a smaller quantity is needed. Due to production costs, NaOH, which is produced using common salt is cheaper than potassium hydroxide.
Specific foods processed with sodium hydroxide include:
A common use of sodium hydroxide is in the production of parts washer . Parts washer detergents based on sodium hydroxide are some of the most aggressive parts washer cleaning chemicals. The sodium hydroxide-based detergents include surfactants, rust inhibitors and defoamers. A parts washer heats water and the detergent in a closed cabinet and then sprays the heated sodium hydroxide and hot water at pressure against dirty parts for degreasing applications. Sodium hydroxide used in this manner replaced many solvent-based systems in the early 1990s when trichloroethane was outlawed by the Montreal Protocol. Water and sodium hydroxide detergent-based parts washers are considered to be an environmental improvement over the solvent-based cleaning methods.
Sodium hydroxide is used in the home as a type of drain openers to unblock clogged drains, usually in the form of a dry crystal or as a thick liquid gel. The alkali reacts with fat to produce water soluble soap and glycerol. It also hydrolysis proteins, such as those found in hair, which may block waste water pipes. Dissolving sodium hydroxide in water is an exothermic reaction producing considerable quantities of heat which assists in speeding up the reactions with grease and other organic matter. Such alkaline drain cleaners and their acidic versions are highly corrosive and should be handled with great caution.
Moreover, solvation of sodium hydroxide is highly exothermic, and the resulting heat may cause heat burns or ignite flammables. It also produces heat when reacted with acids.
Sodium hydroxide is mildly corrosive to glass, which can cause damage to glazing or cause ground glass joints to bind. Sodium hydroxide is corrosive to several metals, like aluminium which reacts with the alkali to produce flammable hydrogen gas on contact.
Sodium hydroxide's toxicity level for fish etc. is around 20—200 mg/L and is associated with increased pH value. However as it is quickly neutralized and does not accumulate, its effect on the environment is usually easily handled.
Sodium hydroxide is often stored in bottles for small-scale laboratory use, within intermediate bulk containers (medium volume containers) for cargo handling and transport, or within large stationary storage tanks with volumes up to 100,000 gallons for manufacturing or waste water plants with extensive NaOH use. Common materials that are compatible with sodium hydroxide and often utilized for NaOH storage include: polyethylene (HDPE, usual, XLPE, less common), carbon steel, polyvinyl chloride (PVC), stainless steel, and fiberglass reinforced plastic (FRP, with a resistant liner).
Sodium hydroxide must be stored in airtight containers to preserve its normality as it will absorb water and carbon dioxide from the atmosphere.
The recipe called for passing water repeatedly through a mixture of alkali (Arabic: , where is ash from Glasswort plants, which are rich in sodium; hence alkali was impure sodium carbonate) See footnote 5 on p. 53. From p. 53: "5. Sodium carbonate. Qily is the ashes of certain plants, e.g. Salsola and Salicornia … , which grow near the sea, or in salty places … " and quicklime (calcium oxide, CaO), whereby a solution of sodium hydroxide was obtained. European soap makers also followed this recipe. When in 1791 the French chemist and surgeon Nicolas Leblanc (1742–1806) patented a Leblanc process, natural "soda ash" (impure sodium carbonate that was obtained from the ashes of plants that are rich in sodium) was replaced by this artificial version. However, by the 20th century, the electrolysis of sodium chloride had become the primary method for producing sodium hydroxide.O'Brien, Thomas F.; Bommaraju, Tilak V. and Hine, Fumio (2005) Handbook of Chlor-Alkali Technology, vol. 1. Berlin, Germany: Springer. Chapter 2: History of the Chlor-Alkali Industry, p. 34.
|
|
