Sodium hydroxide

Sodium, Na   Oxygen, O   Hydrogen, H
Names
IUPAC name Sodium hydroxide[3]
Other names Ascarite Caustic soda Lye[1][2] Soda lye Sodium hydrate White caustic[3]
Identifiers
CAS Number	1310-73-2 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:32145 Y
ChemSpider	14114 Y
ECHA InfoCard	100.013.805
EC Number	215-185-5
E number	E524 (acidity regulators, ...)
Gmelin Reference	68430
KEGG	D01169 Y
MeSH	Sodium+Hydroxide
PubChem CID	14798
RTECS number	WB4900000
UNII	55X04QC32I Y
UN number	1823 (solid) 1824 (solution)
CompTox Dashboard (EPA)	DTXSID0029634
InChI InChI=1S/Na.H2O/h;1H2/q+1;/p-1 Y Key: HEMHJVSKTPXQMS-UHFFFAOYSA-M Y InChI=1/Na.H2O/h;1H2/q+1;/p-1 Key: HEMHJVSKTPXQMS-REWHXWOFAM
SMILES .
Properties
Chemical formula	NaOH
Molar mass	39.9971 g/mol
Appearance	White, opaque crystals
Odor	odorless
Density	2.13 g/cm3[4]
Melting point	323 °C (613 °F; 596 K)[4]
Boiling point	1,388 °C (2,530 °F; 1,661 K)[4]
Solubility in water	418 g/L (0 °C) 1000 g/L (25 °C)[4] 3370 g/L (100 °C)
Solubility	soluble in glycerol, negligible in ammonia, insoluble in ether, slowly soluble in propylene glycol
Solubility in methanol	238 g/L
Solubility in ethanol	<<139 g/L
Vapor pressure	<2.4 kPa (20 °C) 0.1 kPa (700 °C)
Acidity (pKa)	15.7
Magnetic susceptibility (χ)	−15.8·10−6 cm3/mol (aq.)[5]
Refractive index (nD)	1.3576
Structure[6]
Crystal structure	Orthorhombic, oS8
Space group	Cmcm, No. 63
Lattice constant	a = 0.34013 nm, b = 1.1378 nm, c = 0.33984 nm
Formula units (Z)	4
Thermochemistry[7]
Heat capacity (C)	59.5 J/(mol·K)
Std molar entropy (S⦵298)	64.4 J/(mol·K)
Std enthalpy of formation (ΔfH⦵298)	−425.8 kJ/mol
Gibbs free energy (ΔfG⦵)	-379.7 kJ/mol
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H290, H302, H314
Precautionary statements	P280, P305+P351+P338, P310
NFPA 704 (fire diamond)	3 0 1 ALK
Lethal dose or concentration (LD, LC):
LD50 (median dose)	40 mg/kg (mouse, intraperitoneal)[9] 140 - 340 mg/kg (rat, oral) 1350 mg/kg (rabbit, dermal)
LDLo (lowest published)	500 mg/kg (rabbit, oral)[10]
NIOSH (US health exposure limits):
PEL (Permissible)	TWA 2 mg/m3[8]
REL (Recommended)	C 2 mg/m3[8]
IDLH (Immediate danger)	10 mg/m3[8]
Safety data sheet (SDS)	External SDS
Related compounds
Other anions	Sodium hydrosulfide Sodium hydride Sodium oxide
Other cations	Lithium hydroxide Potassium hydroxide Rubidium hydroxide Caesium hydroxide Francium hydroxide
Related compounds	Sodium deuteroxide Sodium chloroxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). Y verify (what is YN ?) Infobox references

Sodium hydroxide, also known as lye and caustic soda,^[1][2] is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations Na⁺ and hydroxide anions OH⁻.

Sodium hydroxide is a highly corrosive base and alkali that decomposes lipids and proteins at ambient temperatures and may 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·nH₂O.^[11] The monohydrate NaOH·H₂O 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.^[12]

Sodium hydroxide is used in many industries: in the making of wood pulp and paper, textiles, drinking water, soaps and detergents, and as a drain cleaner. Worldwide production in 2004 was approximately 60 million tons, while demand was 51 million tons.^[13]

Properties

Physical properties

Pure sodium hydroxide is a colorless crystalline solid that melts at 318 °C (604 °F) without decomposition and boils at 1,388 °C (2,530 °F). It is highly soluble in water, with a lower solubility in polar solvents such as ethanol and methanol.^[14] Sodium hydroxide is insoluble in ether and other non-polar solvents.

Similar to the hydration of sulfuric acid, dissolution of solid sodium hydroxide in water is a highly exothermic reaction^[15] 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 NaOH and natural skin oils.

Viscosity

Concentrated (50%) aqueous solutions of sodium hydroxide have a characteristic viscosity, 78 mPa·s, that is much greater than that of water (1.0 mPa·s) and near that of olive oil (85 mPa·s) at room temperature. The viscosity of aqueous NaOH, as with any liquid chemical, is inversely related to its temperature, i.e., its viscosity decreases as temperature increases, and vice versa. The viscosity of sodium hydroxide solutions plays a direct role in its application as well as its storage.^[14]

Hydrates

Sodium hydroxide can form several hydrates NaOH·nH₂O, which result in a complex solubility diagram that was described in detail by Spencer Umfreville Pickering in 1893.^[16] The known hydrates and the approximate ranges of temperature and concentration (mass percent of NaOH) of their saturated water solutions are:^[11]

• Heptahydrate, NaOH·7H₂O: from −28 °C (18.8%) to −24 °C (22.2%).^[16]
• Pentahydrate, NaOH·5H₂O: from −24 °C (22.2%) to −17.7 °C (24.8%).^[16]
• Tetrahydrate, NaOH·4H₂O, α form: from −17.7 °C (24.8%) to 5.4 °C (32.5%).^[16][17]
• Tetrahydrate, NaOH·4H₂O, β form: metastable.^[16][17]
• Trihemihydrate, NaOH·3.5H₂O: from 5.4 °C (32.5%) to 15.38 °C (38.8%) and then to 5.0 °C (45.7%).^[16][11]
• Trihydrate, NaOH·3H₂O: metastable.^[16]
• Dihydrate, NaOH·2H₂O: from 5.0 °C (45.7%) to 12.3 °C (51%).^[16][11]
• Monohydrate, NaOH·H₂O: from 12.3 °C (51%) to 65.10 °C (69%) then to 62.63 °C (73.1%).^[16][18]

Early reports refer to hydrates with n = 0.5 or n = 2/3, but later careful investigations failed to confirm their existence.^[18]

The only hydrates with stable melting points are NaOH·H₂O (65.10 °C) and NaOH·3.5H₂O (15.38 °C). The other hydrates, except the metastable ones NaOH·3H₂O and NaOH·4H₂O (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.^[11][18]

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.^[11]

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.^[19][18]

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.^[11]

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 NaOH·7H₂O.^[16][20]

When solutions with less than 18.4% NaOH are cooled, water ice crystallizes first, leaving the NaOH in solution.^[16]

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 NaOH·3.5H₂O and a liquid solution.^[17]

The β form of the tetrahydrate is metastable, and often transforms spontaneously to the α form when cooled below −20 °C.^[17] 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.^[17]

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.

Crystal structure

NaOH and its monohydrate form orthorhombic crystals with the space groups Cmcm (oS8) and Pbca (oP24), respectively. The monohydrate cell dimensions are a = 1.1825, b = 0.6213, c = 0.6069 nm. The atoms are arranged in a hydrargillite-like layer structure, with each sodium atom surrounded by six oxygen atoms, three each from hydroxide ions and three from water molecules. The hydrogen atoms of the hydroxyls form strong bonds with oxygen atoms within each O layer. Adjacent O layers are held together by hydrogen bonds between water molecules.^[21]

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) + H₂O(l)

In general, such neutralization reactions are represented by one simple net ionic equation:

OH⁻(aq) + H⁺(aq) → H₂O(l)

This type of reaction with a strong acid releases heat, and hence is exothermic. Such acid–base reactions can also be used for titrations. However, sodium hydroxide is not used as a primary standard because it is hygroscopic and absorbs carbon dioxide from air. Zdroj:https://en.wikipedia.org?pojem=Caustic_soda
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