POTASSIUM HYDROXIDE

CAS No.: 1310-58-3
EC No.: 215-181-3

Synonyms:
POTASSIUM HYDROXIDE; Potassium hydroxide; Potasyum hidroksit; POTASYUM HİDROKSİT; CAUSTIC POTASH; KALII HYDROXIDUM; KOH; LYE; POTASH; POTASH CAUSTIC; POTASH LYE; POTASSIUM HYDRATE; POTASSIUM HYDROXIDE; POTASSIUM HYDROXIDE ETHANOLIC; POTASSIUM HYDROXIDE STANDARD; POTASSIUM OXIDE HYDRATE; Causicpotash; causticpotashsolutions; Hydroxyde de potassium; hydroxydedepotassium; hydroxydedepotassium(french); hydroxydedepotassium(solide); hydroxydedepotassium(solutions); Kaliumhydroxid; Caswell No. 693; Caustic potash; CCRIS 6569; Cyantek CC 723; EC 215-181-3; POTASSIUM HYDROXIDE; Potassium hydroxide; Potasyum hidroksit; POTASYUM HİDROKSİT; POTASSIUM HYDROXIDE; Potassium hydroxide; Potasyum hidroksit; POTASYUM HİDROKSİT; EINECS 215-181-3; EPA Pesticide Chemical Code 075602; HSDB 1234; Hydroxyde de potassium; Hydroxyde de potassium [French]; Potash; Potash lye; Potassa; Potasse caustique; Potasse caustique [French]; Potassio (idrossido di); Potassio (idrossido di) [Italian]; Potassium (hydroxyde de); Potassium (hydroxyde de) [French]; Potassium hydrate; Potassium hydroxide; UNII-WZH3C48M4T; Potassium hydroxide; POTASSIUM HYDROXIDE; Potassium hydroxide; Potasyum hidroksit; POTASYUM HİDROKSİT; POTASSIUM HYDROXIDE; Potassium hydroxide; Potasyum hidroksit; POTASYUM HİDROKSİT; Potassium hydroxide (K(OH)); Caustic potash; Caustic potash solution; Potash lye; Potassa; Potassium hydroxide; Potassium hydroxide solution; Potassium hydroxide, solid; Potassium hydroxide, solid [UN1813] [Corrosive]; Potassium hydroxide, solution; Potassium hydroxide, solution [UN1814] [Corrosive]; UN1813; UN1814; POTASSIUM HYDROXIDE; POTASSIUM HYDROXIDE; Potassium hydroxide; Potasyum hidroksit; POTASYUM HİDROKSİT; POTASSIUM HYDROXIDE; Potassium hydroxide; Potasyum hidroksit; POTASYUM HİDROKSİT; Caustic potash; 1310-58-3; Potash lye; Potassium hydrate; Hydroxyde de potassium; Potassium hydroxide (K(OH)); Potassa; Potasse caustique; Potassium hydroxide solution; Caswell No. 693; Cyantek CC 723; Caustic potash solution; 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POTASSIUM HYDROXIDE

Potassium hydroxide
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Potassium hydroxide
Crystal structure of KOH
Pellets of potassium hydroxide
Names
IUPAC name
Potassium hydroxide
Other names
Caustic potash, Lye, Potash lye, Potassia, Potassium hydrate, KOH
Identifiers
CAS Number    
1310-58-3 check
3D model (JSmol)    
Interactive image
ChEBI    
CHEBI:32035 check
ChemSpider    
14113 check
ECHA InfoCard    100.013.802 Edit this at Wikidata
EC Number    
215-181-3
E number    E525 (acidity regulators, ...)
PubChem CID    
14797
RTECS number    
TT2100000
UNII    
WZH3C48M4T check
UN number    1813
CompTox Dashboard (EPA)    
DTXSID5029633 Edit this at Wikidata
InChI[show]
SMILES[show]
Properties
Chemical formula    KOH
Molar mass    56.11 g mol−1
Appearance    white solid, deliquescent
Odor    odorless
Density    2.044 g/cm3 (20 °C)[1]
2.12 g/cm3 (25 °C)[2]
Melting point    360[3] °C (680 °F; 633 K)
Boiling point    1,327 °C (2,421 °F; 1,600 K)
Solubility in water    85 g/100 mL (-23.2 °C)
97 g/100 mL (0 °C)
121 g/100 mL (25 °C)
138.3 g/100 mL (50 °C)
162.9 g/100 mL (100 °C)[1][4]
Solubility    soluble in alcohol, glycerol
insoluble in ether, liquid ammonia
Solubility in methanol    55 g/100 g (28 °C)[2]
Solubility in isopropanol    ~14 g / 100 g (28 °C)
Basicity (pKb)    −0.7[5](KOH(aq) = K+ + OH–)
Magnetic susceptibility (χ)    −22.0·10−6 cm3/mol
Refractive index (nD)    1.409 (20 °C)
Structure
Crystal structure    rhombohedral
Thermochemistry
Heat capacity (C)    65.87 J/mol·K[2]
Std molar
entropy (So298)    79.32 J/mol·K[2][6]
Std enthalpy of
formation (ΔfH⦵298)    -425.8 kJ/mol[2][6]
Gibbs free energy (ΔfG˚)    -380.2 kJ/mol[2]
Hazards
Safety data sheet    ICSC 0357
GHS pictograms    GHS05: CorrosiveGHS07: Harmful[7]
GHS Signal word    Danger
GHS hazard statements    H302, H314[7]
GHS precautionary statements    P280, P305+351+338, P310[7]
NFPA 704 (fire diamond)    
NFPA 704 four-colored diamond
030ALK
Flash point    Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (median dose)    273 mg/kg (oral, rat)[9]
NIOSH (US health exposure limits):
PEL (Permissible)    none[8]
REL (Recommended)    C 2 mg/m3[8]
IDLH (Immediate danger)    N.D.[8]
Related compounds
Other anions    Potassium hydrosulfide
Potassium amide
Other cations    Lithium hydroxide
Sodium hydroxide
Rubidium hydroxide
Caesium hydroxide
Related compounds    Potassium oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references
Potassium hydroxide is an inorganic compound with the formula KOH, and is commonly called caustic potash.

Along with sodium hydroxide (NaOH), this colorless solid is a prototypical strong base. It has many industrial and niche applications, most of which exploit its caustic nature and its reactivity toward acids. An estimated 700,000 to 800,000 tonnes were produced in 2005. KOH is noteworthy as the precursor to most soft and liquid soaps, as well as numerous potassium-containing chemicals. It is a white solid that is dangerously corrosive. Most commercial samples are ca. 90% pure, the remainder being water and carbonates.[10]

Contents
1    Properties and structure
1.1    Structure
1.2    Thermal stability
2    Reactions
2.1    Basicity, solubility and desiccating properties
2.2    As a nucleophile in organic chemistry
2.3    Reactions with inorganic compounds
3    Manufacture
4    Uses
4.1    Precursor to other potassium compounds
4.2    Manufacture of soft soaps
4.3    As an electrolyte
4.4    Food industry
4.5    Niche applications
5    Safety
6    See also
7    References
8    External links
Properties and structure
Potassium hydroxide is usually sold as translucent pellets, which become tacky in air because KOH is hygroscopic. Consequently, KOH typically contains varying amounts of water (as well as carbonates - see below). Its dissolution in water is strongly exothermic. Concentrated aqueous solutions are sometimes called potassium lyes. Even at high temperatures, solid KOH does not dehydrate readily.[11]

Structure
At higher temperatures, solid KOH crystallizes in the NaCl crystal structure. The OH group is either rapidly or randomly disordered so that the  OH− group is effectively a spherical anion of radius 1.53 Å (between Cl−
 and F−
 in size). At room temperature, the  OH− groups are ordered and the environment about the K+
 centers is distorted, with K+
—OH−
 distances ranging from 2.69 to 3.15 Å, depending on the orientation of the OH group. KOH forms a series of crystalline hydrates, namely the monohydrate KOH • H2O, the dihydrate KOH • 2H2O and the tetrahydrate KOH • 4H2O.[12]

Thermal stability
Like NaOH, KOH exhibits high thermal stability. The gaseous species is dimeric. Because of its high stability and relatively low melting point, it is often melt-cast as pellets or rods, forms that have low surface area and convenient handling properties.

Reactions
Basicity, solubility and desiccating properties
About 121 g of KOH dissolve in 100 mL water at room temperature, which contrasts with 100 g/100 mL for NaOH. Thus on a molar basis, NaOH is slightly more soluble than KOH. Lower molecular-weight alcohols such as methanol, ethanol, and propanols are also excellent solvents. They participate in an acid-base equilibrium. In the case of methanol the potassium methoxide (methylate) forms: [13]

KOH + CH3OH {displaystyle {ce {<=>>}}}{displaystyle {ce {<=>>}}} CH3OK + H
2O
Because of its high affinity for water, KOH serves as a desiccant in the laboratory. It is often used to dry basic solvents, especially amines and pyridines.

As a nucleophile in organic chemistry
KOH, like NaOH, serves as a source of  OH−, a highly nucleophilic anion that attacks polar bonds in both inorganic and organic materials. Aqueous KOH saponifies esters:

KOH + RCOOR' → RCOOK + R'OH
When R is a long chain, the product is called a potassium soap. This reaction is manifested by the "greasy" feel that KOH gives when touched — fats on the skin are rapidly converted to soap and glycerol.

Molten KOH is used to displace halides and other leaving groups. The reaction is especially useful for aromatic reagents to give the corresponding phenols.[14]

Reactions with inorganic compounds
Complementary to its reactivity toward acids, KOH attacks oxides. Thus, SiO2 is attacked by KOH to give soluble potassium silicates. KOH reacts with carbon dioxide to give bicarbonate:

KOH + CO2 → KHCO3
Manufacture
Historically, KOH was made by adding potassium carbonate to a strong solution of calcium hydroxide (slaked lime) The salt metathesis reaction results in precipitation of solid calcium carbonate, leaving potassium hydroxide in solution:

Ca(OH)2 + K2CO3 → CaCO3 + 2 KOH
Filtering off the precipitated calcium carbonate and boiling down the solution gives potassium hydroxide ("calcinated or caustic potash"). This method of producing potassium hydroxide remained dominant until the late 19th century, when it was largely replaced by the current method of electrolysis of potassium chloride solutions.[10] The method is analogous to the manufacture of sodium hydroxide (see chloralkali process):

2 KCl + 2 H2O → 2 KOH + Cl2 + H2
Hydrogen gas forms as a byproduct on the cathode; concurrently, an anodic oxidation of the chloride ion takes place, forming chlorine gas as a byproduct. Separation of the anodic and cathodic spaces in the electrolysis cell is essential for this process.[15]

Uses
KOH and NaOH can be used interchangeably for a number of applications, although in industry, NaOH is preferred because of its lower cost.

Precursor to other potassium compounds
Many potassium salts are prepared by neutralization reactions involving KOH. The potassium salts of carbonate, cyanide, permanganate, phosphate, and various silicates are prepared by treating either the oxides or the acids with KOH.[10] The high solubility of potassium phosphate is desirable in fertilizers.

Manufacture of soft soaps
The saponification of fats with KOH is used to prepare the corresponding "potassium soaps", which are softer than the more common sodium hydroxide-derived soaps. Because of their softness and greater solubility, potassium soaps require less water to liquefy, and can thus contain more cleaning agent than liquefied sodium soaps.[16]

As an electrolyte
Potassium carbonate, formed from the hydroxide solution leaking from an alkaline battery
Potassium carbonate, formed from the hydroxide solution leaking from an alkaline battery
Aqueous potassium hydroxide is employed as the electrolyte in alkaline batteries based on nickel-cadmium, nickel-hydrogen, and manganese dioxide-zinc. Potassium hydroxide is preferred over sodium hydroxide because its solutions are more conductive.[17] The nickel–metal hydride batteries in the Toyota Prius use a mixture of potassium hydroxide and sodium hydroxide.[18] Nickel–iron batteries also use potassium hydroxide electrolyte.

Food industry
In food products, potassium hydroxide acts as a food thickener, pH control agent and food stabilizer. The FDA considers it (as a direct human food ingredient) as generally safe when combined with "good" manufacturing practice conditions of use.[19] It is known in the E number system as E525.

Niche applications
Like sodium hydroxide, potassium hydroxide attracts numerous specialized applications, virtually all of which rely on its properties as a strong chemical base with its consequent ability to degrade many materials. For example, in a process commonly referred to as "chemical cremation" or "resomation", potassium hydroxide hastens the decomposition of soft tissues, both animal and human, to leave behind only the bones and other hard tissues.[20] Entomologists wishing to study the fine structure of insect anatomy may use a 10% aqueous solution of KOH to apply this process.[21]

In chemical synthesis, the choice between the use of KOH and the use of NaOH is guided by the solubility or keeping quality of the resulting salt.

The corrosive properties of potassium hydroxide make it a useful ingredient in agents and preparations that clean and disinfect surfaces and materials that can themselves resist corrosion by KOH.[15]

KOH is also used for semiconductor chip fabrication. See also: anisotropic wet etching.

Potassium hydroxide is often the main active ingredient in chemical "cuticle removers" used in manicure treatments.

Because aggressive bases like KOH damage the cuticle of the hair shaft, potassium hydroxide is used to chemically assist the removal of hair from animal hides. The hides are soaked for several hours in a solution of KOH and water to prepare them for the unhairing stage of the tanning process. This same effect is also used to weaken human hair in preparation for shaving. Preshave products and some shave creams contain potassium hydroxide to force open the hair cuticle and to act as a hygroscopic agent to attract and force water into the hair shaft, causing further damage to the hair. In this weakened state, the hair is more easily cut by a razor blade.

Potassium hydroxide is used to identify some species of fungi. A 3–5% aqueous solution of KOH is applied to the flesh of a mushroom and the researcher notes whether or not the color of the flesh changes. Certain species of gilled mushrooms, boletes, polypores, and lichens[22] are identifiable based on this color-change reaction.[23]

Safety
Potassium hydroxide and its solutions are severe irritants to skin and other tissue.[24]

Sodium or potassium hydroxide, preferably the latter. The corresponding alkoxide also can be used, but prohibitively expensive. Best if it has ≥85 per cent potassium hydroxide. Even best grades of potassium hydroxide have 14–15 per cent water which cannot be removed. It should be low in carbonate, because the carbonate is not an efficient catalyst and may cause cloudiness in the final ester. Sodium hydroxide pellets have given very good results. Because quantity of catalyst used is quite less, good quality catalyst (in spite of high cost) can be used.

During the distillation of hexane from potassium hydroxide pellets, it was found that the 2-l flask had become perforated by alkaline attack, and that hexane was leaking through two pinholes, generating much flammable vapor. The high rate of corrosion of glass is probably associated with the fact that fresh pellets of potassium hydroxide already contain some 14% of water, and absorption of further water from solvents during drying over potash pellets leads to the formation of drops of very concentrated aqueous potassium hydroxide solution.