Potassium hydroxide

Potassium hydroxide
IUPAC name Potassium hydroxide
Other names Caustic potash Potash lye Potassia Potassium hydrate
Identifiers
CAS number	1310-58-3 Y
PubChem	6093213
ChemSpider	14113
EC number	215-181-3
UN number	1813
RTECS number	TT2100000
Properties
Molecular formula	KOH
Molar mass	56.1056 g/mol
Appearance	white solid, deliquescent
Density	2.044 g/cm3
Melting point	420 °C
Boiling point	1327 °C
Solubility in water	110 g/100 mL (25 °C) 178 g/100 mL (100 °C)
Solubility	soluble in alcohol, glycerol insoluble in ether, liquid ammonia
Acidity (pKa)	13.5 (0.1 M)
Refractive index (nD)	1.409
Structure
Crystal structure	monoclinic
Coordination geometry	rhombohedral
Hazards
MSDS	ICSC 0357
EU Index	019-002-00-8
EU classification	Corrosive (C) Harmful (Xn)
R-phrases	R22, R35
S-phrases	(S1/2), S26, S36/37/39, S45
NFPA 704	0 3 1
Flash point	Non-flammable
LD50	273 mg/kg
Related compounds
Other anions	Potassium hydrosulfide Potassium amide
Other cations	Lithium hydroxide Sodium hydroxide Rubidium hydroxide Caesium hydroxide
Related compounds	Potassium oxide
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Potassium hydroxide is an inorganic compound with the formula KOH. Its common name is caustic potash. Along with sodium hydroxide (NaOH), this colourless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive nature. In 2005, an estimated 700,000 to 800,000 tons were produced. Approximately 100 times more NaOH than KOH is produced annually.^[1][2][3] KOH is noteworthy as the precursor to most soft and liquid soaps as well as numerous potassium-containing chemicals.

Contents 1 Properties and structure 1.1 Structure 1.2 Solubility and desiccating properties 1.3 Thermal stability 2 Reactions 2.1 As a base 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 biodiesel 4.3 Manufacture of soft soaps 4.4 As an electrolyte 4.5 Niche applications 5 See also 6 References 7 External links

Properties and structure

Potassium hydroxide can be found in pure form by reacting sodium hydroxide with impure potassium. Potassium hydroxide is usually sold as translucent pellets, which will 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, meaning the process gives off significant heat. Concentrated aqueous solutions are sometimes called potassium lyes. Even at high temperatures, solid KOH does not dehydrate readily.^[4]

Structure

At higher temperatures, solid KOH crystallizes in the NaCl motif. 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.^[5]

Solubility and desiccating properties

Approximately 121 g of KOH will dissolve in 100 mL of water at room temperature (compared with 100 g of NaOH in the same volume). Lower alcohols such as methanol, ethanol, and propanols are also excellent solvents. The solubility in ethanol is about 40 g KOH/100 mL.

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: distillation of these basic liquids from a slurry of KOH yields the anhydrous reagent.

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

As a base

KOH is highly basic, forming strongly alkaline solutions in water and other polar solvents. These solutions are capable of deprotonating many acids, even weak ones. In analytical chemistry, titrations using solutions of KOH are used to assay acids.

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. In perhaps its most well-known reaction, aqueous KOH saponifies esters:

KOH + RCO₂R' → RCO₂K + 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.^[6]

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 boiling a solution of potassium carbonate (potash) with calcium hydroxide (slaked lime), leading to a metathesis reaction which caused calcium carbonate to precipitate, 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 used potash extracted from wood ashes using slaked lime. It was the most important method of producing potassium hydroxide until the late 19th century, when it was largely replaced by the current method of electrolysis of potassium chloride solutions, analogous to the method of manufacturing sodium hydroxide (see chloralkali process):

2 KCl + 2 H₂O → 2 KOH + Cl₂ + H₂

Hydrogen gas forms as a by-product 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.^[7]

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.^[1] The high solubility of potassium phosphate is desirable in fertilizers.

Manufacture of biodiesel

Although more expensive than using sodium hydroxide, KOH works well in the manufacture of biodiesel by esterification of the free fatty acids in vegetable oil.^[8] Glycerin from potassium hydroxide-processed biodiesel is useful as an inexpensive food supplement for livestock, once the toxic methanol is removed.^[9][10]

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.^[11]

As an electrolyte

Aqueous potassium hydroxide is employed as the electrolyte in alkaline batteries based on nickel-cadmium and manganese dioxide-zinc. Potassium hydroxide is preferred over sodium hydroxide because its solutions are more conductive.^[12]

Niche applications

KOH attracts numerous specialized applications, which virtually all rely on its basic or degradative properties. KOH is widely used in the laboratory for the same purposes. KOH is also used in a process commonly referred to as "chemical cremation" which hastens the decomposition process of soft tissues (both animal and human), leaving behind only bones. It is used in this manner, in solutions of 10% concentration, by entomologists wishing to study the fine structure of insect anatomy. ^[13]

In chemical synthesis, the selection of KOH vs. NaOH is guided by the solubility for the resulting salt. Its corrosive properties make it useful as an ingredient in cleaning and disinfection of resistant surfaces and materials.^[14]

It is often the main active ingredient in chemical "cuticle removers."

KOH is also widely used as a way to remove hair from animal hides, leaving the hides in a solution of KOH and water for a few hours. It is used in resomation to dissolve human remains.

Aggressive bases will damage the cuticle of the hair shaft, and thus is useful for weakening the hair in preparation for shaving. Pre-shave products and shave creams such as Proraso contain Potassium Hydroxide in order to force the cuticle open and act as a hygroscopic agent to attract and force water into the shaft, causing further damage to the hair. In this state, the hair is more easily cut by razor blade.

KOH is used in the identification of certain mushrooms. A 3-5% aqueous solution is applied to flesh of the mushroom and the researcher notes whether or not a change in the color of the flesh takes place. Certain species of boletes, polypores, and many gilled mushrooms are identified based on the reaction that takes place with this chemical.^[15]

See also

• Soda lime

References

External links

• Newscientist article dn10104
• MSDS from JTBaker

Potassium compounds KBr · KBrO3 · KCN · KCNO · KCl · KClO3 · KClO4 · KF · KH · KHCO2 · KHCO3 · KHF2 · KHSO3 · KHSO4 · KH2AsO4 · KI · KIO3 · KIO4 · KMnO4 · KNO2 · KNO3 · KOCN · KOH · KO2 · KPF6 · KSCN · K2CO3 · K2CrO4 · K2Cr2O7 · K2FeO4 · K2MnO4 · K2O · K2O2 · K2PtCl4 · K2PtCl6 · K2S · K2SO3 · K2SO4 · K2SO5 · K2S2O5 · K2S2O7 · K2S2O8 · K2SiO3 · K3[Fe(CN)6] · K3[Fe(C2O4)3] · K4[Fe(CN)6] · K3PO4 · K4MnO4 · K4Mo2Cl8