Sodium hydroxide
sodium hydroxide (NaOH), sodium hydroxide or sodium hydrate, also known as caustic soda (in Spain and Mexico) or caustic soda (in almost all of Latin America), is a caustic hydroxide used in industry (mainly as a (chemical) base) in the manufacture of paper, fabrics, and detergents. In addition, it is used in the oil industry in the preparation of water-based drilling muds. At the domestic level, its utilities are recognized for unblocking kitchen and bathroom drain pipes, making homemade soap, among others.
At room temperature, sodium hydroxide is an odorless white crystalline solid that absorbs moisture from the air (hygroscopic). It is a manufactured substance. When dissolved in water or neutralized with an acid, it releases a large amount of heat that may be enough to ignite combustible materials. Sodium hydroxide is very corrosive. It is generally used in solid form or as a 50% solution.
Sodium hydroxide is a highly corrosive base that breaks down proteins at normal room temperatures and can cause severe chemical burns. It is highly soluble in water and readily absorbs moisture and carbon dioxide from the air. It forms a series of hydrates NaOH-nH
2O. The monohydrate NaOH-H
2O crystallizes from aqueous solutions between 12.3 and 61.8 ° c. "Sodium hydroxide" Commercially available is usually this monohydrate, and published data may refer to it instead of the anhydrous compound.
As one of the simpler hydroxides, sodium hydroxide is often used in conjunction with neutral water and hydrochloric acid to demonstrate the pH scale to chemistry students.
Sodium hydroxide is used in many industries: in pulp and paper manufacturing, textiles, drinking water, soaps and detergents, and as a drain cleaner. World production in 2004 was approximately 60 million tons, while demand was 51 million tons.
Properties
Physical properties
Pure sodium hydroxide is a colorless crystalline solid that melts at 604.4°F (318°C) without decomposing, and has a boiling point of 2530.4°F (1388°C). It is highly soluble in water, with less solubility in polar solvents such as ethanol and methanol. NaOH is insoluble in ether and other nonpolar solvents.
Similar to the hydration of sulfuric acid, the dissolution of solid sodium hydroxide in water is a highly exothermic reaction in which a large amount of heat is released, posing a safety threat due to the possibility of splashes. The resulting solution is usually colorless and odorless. As with other alkaline solutions, it feels slippery on the skin due to the saponification process that occurs between the NaOH and the skin's natural oils.
Viscosity
Concentrated (50%) aqueous solutions of sodium hydroxide have a characteristic viscosity, 78 mPa-s, which is much higher than that of water (1.0 mPa-s) and close to that of olive oil (85 mPa-s) at room temperature. The viscosity of aqueous NaOH, like that of any liquid chemical, is inversely related to its service temperature, that is, its viscosity decreases with increasing temperature, and vice versa. The viscosity of sodium hydroxide solutions plays a direct role in their application, as well as in their storage.
Carbs
Sodium hydroxide can form various hydrates NaOH-nH
2O, giving rise to a complex "solubility diagram" that was described in detail by Spencer Umfreville Pickering in 1893. The known hydrates and the approximate ranges of temperature and concentration (mass percent NaOH) of their solutions of saturated water are:
- Heptahydrated, NaOH-7H
2O: from -28 °C (18.8%) to -24 °C (22.2%). - Pentahydrate, NaOH-5H
2O: from -24 °C (22.2%) to -17.7 (24.8%). - Tetrahydrate, NaOH-4H
2O, form α: from -17.7 (24.8%) to +5.4 °C (32.5%). - Tetrahydrate, NaOH-4H
2O, form β: metaestable. - Trihemihydrate, NaOH-3.5H
2O: from +5.4 °C (32.5%) to +15.38 °C (38.8%) and then to +5.0 °C (45.7%). - Trihydrate, NaOH-3H
2O: meta-stable. - Dihydrate, NaOH-2H
2O: from +5.0 °C (45.7%) to +12.3 °C (51%). - Monohydrate, NaOH-H
2O: from +12.3 °C (51%) to 65.10 °C (69%) and then to 62.63 °C (73.1%).
Early reports refer to hydrates with n = 0.5 or n = 2/3, but further careful investigation failed to confirm their existence.
The only hydrates with stable melting points are NaOH-H
2O (65,10 °C) and NaOH-3.5H
2O (15.38 °C). Other hydrates, except the metastable NaOH-3H
2O and NaOH-4H
2O (β) can be crystallized from solutions of the appropriate composition, as indicated above. However, NaOH solutions can easily be supercooled by many degrees, allowing the formation of hydrates (including metastables) from solutions with different concentrations.
For example, when a 1:2 mole ratio solution of NaOH and water (52.6% NaOH by mass) is cooled, the monohydrate usually starts to crystallize (at around 22°C) before the monohydrate. dihydrate. However, the solution can easily be supercooled to -15 °C, at which point it can rapidly crystallize as a dihydrate. When heated, the solid dihydrate can melt directly into a 13.35°C solution; however, once the temperature exceeds 12.58 °C. it usually decomposes into a solid monohydrate and a liquid solution. Even the hydrate n = 3.5 is difficult to crystallize, because the solution supercools so much that other hydrates become more stable.
A hot water solution containing 73.1% (mass) NaOH is a eutectic that solidifies at about 62.63 °C as an intimate mixture of anhydrous crystals and monohydrates.
A second stable eutectic composition is 45.4% (mass) NaOH, which solidifies at about 4.9 °C into a mixture of dihydrate and 3,5-hydrate crystals.
The third stable eutectic is 18.4% (mass) NaOH. It solidifies at about -28.7 °C as a mixture of water ice and the heptahydrate NaOH-7H
2 Or.
When solutions with less than 18.4% NaOH are cooled, ice water crystallizes first, leaving the NaOH in solution.
The α form of the tetrahydrate has a density of 1.33 g/cm3. It melts congruently at 7.55 °C in a liquid with 35.7% NaOH and density 1.392 g/cm3, so it floats in it like ice in water. However, at around 4.9 °C it can instead melt incongruously in a mixture of NaOH-3.5H
2O solid and a liquid solution.
The β form of the tetrahydrate is metastable, often transforming spontaneously to the α form when cooled below -20 °C. Once initiated, the exothermic transformation is complete within a few minutes, with an increase of 6 0.5% of the volume of the solid. The β-form can crystallize from supercooled solutions at -26 °C, and partially melts at -1.83 °C.
"Sodium hydroxide" of commerce is usually the monohydrate (density 1.829 g/cm3). Physical data from the technical literature may refer to this form, instead of the anhydrous compound.
Structure of crystals
NaOH and its monohydrate form orthorhombic crystals with the space groups Cmcm (oS8) and Pbca (oP24), respectively. The cell dimensions of the monohydrate are a = 1.1825, b = 0.6213, c = 0.6069 nm. The atoms are arranged in a layered structure similar to hydrargillite/O NaO O NaO/... Each sodium atom is surrounded by six oxygen atoms, three of them from hydroxyl anions HO−
and three water molecules. The hydrogen atoms of the hydroxyls form strong bonds with the oxygen atoms within each O shell. Adjacent O shells are held together by hydrogen bonds between the water molecules.
Chemical Properties
Reaction with acids
Sodium hydroxide reacts with protic acids to produce water and the corresponding salts. For example, when sodium hydroxide reacts with hydrochloric acid, sodium chloride is formed:
- NaOH(aq) + HCl(aq) → NaCl(aq) +H
2O(l)
In general, these types of neutralization reactions are represented by a simple net ionic equation:
- OH−
(aq) + H+
(aq) → H
2O(l)
This type of reaction with a strong acid releases heat, and is therefore exothermic. This type of acid-base reaction can also be used for titration. However, sodium hydroxide is not used as the primary standard because it is hygroscopic and absorbs carbon dioxide from the air.
Reaction with acid oxides
Sodium hydroxide also reacts with acidic oxides, such as sulfur dioxide. These types of reactions are often used to "scrub" harmful acid gases (such as SO2 and H2S) produced in the combustion of coal and thus prevent their release into the atmosphere. For example,
- 2 NaOH + SO
2 → Na
2SO
3 + H
2O
Reaction with metals and oxides
Glass reacts slowly with aqueous solutions of sodium hydroxide at room temperature to form soluble silicates. Therefore, glass gaskets and stopcocks exposed to sodium hydroxide have a tendency to 'freeze'. [Laboratory flasks and glass-lined chemical reactors are damaged by prolonged exposure to hot sodium hydroxide, which also freezes glass. Sodium hydroxide does not attack iron at room temperature, since iron does not have amphoteric properties (ie it only dissolves in acid, not base). However, at high temperatures (for example, above 500 °C), iron can react endothermically with sodium hydroxide to form iron(III) oxide, sodium metal, and hydrogen gas. This is due to the lower enthalpy of formation of iron(III) oxide (-824.2 kJ/mol) compared to sodium hydroxide (-500 kJ/mol) and the positive entropy change of the reaction, implying spontaneity at high temperatures (ΔST>ΔH, ΔG<0) and no spontaneity at low temperatures (ΔST<ΔH, ΔG>0). Consider the following reaction between molten sodium hydroxide and finely divided iron filings:
- 4 Fe + 6 NaOH → 2 Fe
2O
3 + 6 Na + 3 H
2
However, some transition metals can react vigorously with sodium hydroxide under milder conditions.
In 1986, a UK aluminum tanker was mistakenly used to transport a 25% sodium hydroxide solution, causing pressurization of the contents and damage to the tanker. The pressurization was due to hydrogen gas produced in the reaction between sodium hydroxide and aluminum:
- 2 Al + 2 NaOH + 6 H
2O → 2 NaAl(OH)
4 + 3 H
2
Production
Sodium hydroxide is produced industrially as a 50/50 solution by variations of the electrolytic chloralkali process. Chlorine is also produced in this process. Solid sodium hydroxide is obtained from this solution by evaporating water. Solid sodium hydroxide is most commonly sold in the form of flakes, prills, and cast blocks.
In 2004, world production was estimated at 60 million dry tons of sodium hydroxide and demand at 51 million tons. In 1998, total world production was about 45 million tons. North America and Asia each contributed about 14 million tons, while Europe produced about 10 million tons. In the United States, the largest producer of sodium hydroxide is Olin, which has annual production of about 5.7 million tons at facilities in Freeport, Texas, and Plaquemine, Louisiana, St Gabriel, Louisiana, McIntosh, Alabama, Charleston, Tennessee, Niagara Falls, New York, and Becancour, Canada. Other big US producers are Oxychem, Westlake, Shintek and Formosa. All these companies use the chloralkali process.
Historically, sodium hydroxide was produced by treating sodium carbonate with calcium hydroxide in a metathesis reaction that takes advantage of the fact that sodium hydroxide is soluble, while calcium carbonate is not. This process was called causticization.
- Ca(OH)
2(aq) + Na
2CO
3(s) → CaCO
3(s) + 2 NaOH(aq)
This process was replaced by the Solvay process at the end of the 19th century, which in turn was supplanted by the chlor-alkali process used today.
Sodium hydroxide is also produced by combining pure metallic sodium with water. The byproducts are hydrogen gas and heat, which often results in a flame.
- 2 Na + 2 H
2O → 2 NaOH + H
2
This reaction is commonly used to demonstrate the reactivity of alkali metals in academic settings; however, it is not commercially viable, as isolation of metallic sodium is typically done by reduction or electrolysis.
Applications and uses
Sodium hydroxide is used to make soaps, crayons, paper, explosives, paints, and petroleum products. It is also used in cotton textile processing, laundry and bleaching, oxide coating, electroplating, and electrowinning. It is commonly found in drain openers and oven cleaners. It is also used as a paint remover and by cabinetmakers to remove old paint from wooden furniture.
It is used in the traditional elaboration of table olive stew, especially in olive varieties such as manzanilla and gordal.
Its use in obtaining aluminum from bauxite in the Bayer process is also important.
Most of the sodium hydroxide is synthesized by the causticization method, that is, combining another hydroxide with sodium oxosalts.
The example reaction shows calcium hydroxide or slaked lime (from calcium oxide or quicklime and water) combining with sodium carbonate (from carbonic acid and sodium) to form sodium hydroxide and calcium carbonate.
- Ca(OH)2 (aq) + Na2CO3 (aq) → 2 NaOH
(aq) + CaCO3 (s)
Although nowadays it is manufactured by electrolysis in the chloralkali process of an aqueous solution of sodium chloride or brine, it is also a by-product that results from the process used to produce chlorine.
- Note: 2Cl−
→ Cl
2 (gas) + 2e- - Category: 2H
2O + 2e- → H
2 + 2OH−
As the electrolysis progresses, the chloride anions are released, and they are replaced by hydroxide ions that, combined with the sodium cations present in the solution, form sodium hydroxide. Sodium cations are not reduced to metallic sodium, due to their very low potential.
A solution of a small portion of soda diluted in water is used in the traditional method to produce common margarine, a pretzel, and also to make lutefisk, a traditional food from the Nordic countries based on fish.