Potassium hydroxide

Potassium hydroxide
Potassium hydroxide
IUPAC name
Potassium hydroxide
Other names
Caustic potash
Potash lye
Potassia
Potassium hydrate
Identifiers
CAS number 1310-58-3 YesY
PubChem 6093213
ChemSpider 14113 YesY
UNII WZH3C48M4T YesY
EC number 215-181-3
UN number 1813
ChEBI CHEBI:32035 YesY
RTECS number TT2100000
Jmol-3D images Image 1
Properties
Molecular formula KOH
Molar mass 56.1056 g/mol
Appearance white solid, deliquescent
Density 2.044 g/cm3[1]
Melting point

406 °C[1]
Boiling point

1327 °C[1]
Solubility in water 1210 g/L (25 °C)
1780 g/L (100 °C)[1]
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 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
NFPA 704.svg
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
 YesY hydroxide (verify) (what is: YesY/N?)
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, 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 applications exploit its reactivity toward acids and its corrosive nature. In 2005, an estimated 700,000 to 800,000 tonnes were produced. Approximately 100 times more NaOH than KOH is produced annually.[2][3][4] KOH is noteworthy as the precursor to most soft and liquid soaps as well as numerous potassium-containing chemicals.

Contents

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.[5]

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.[6]

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.[7]

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.[8]

Potassium hydroxide solution leaking from an alkaline battery
Potassium hydroxide solution leaking from an alkaline battery

Uses

See also: Sodium hydroxide

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.[2] 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 transesterification of the triglycerides in vegetable oil. Glycerin from potassium hydroxide-processed biodiesel is useful as an inexpensive food supplement for livestock, once the toxic methanol is removed.[9]

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.[10]

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.[11] The Nickel Metal Hydride batteries in the Toyota Prius use a mixture of potassium hydroxide and sodium hydroxide.[12]

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.[citation needed] Entomologists wishing to study the fine structure of insect anatomy may use a 10% aqueous solution of KOH to apply this process.[13] An example of chemical cremation outside of a laboratory environment is the use of a potassium hydroxide solution as the principal agent in the resomation process, an alternative to the well-known, thermal cremation process for the disposal of human remains after death.

In chemical synthesis, the choice between the use of KOH and the use of sodium hydroxide, NaOH, is guided by the solubility 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.[14]

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. Pre-shave 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 certain kinds of mushrooms. 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 boletes, polypores, and many gilled mushrooms are identified based on this color-change reaction.[15]

See also

References

  1. ^ a b c d Lide, D. R., ed (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. p. 4-80. ISBN 0-8493-0486-5. 
  2. ^ a b H. Schultz, G. Bauer, E. Schachl, F. Hagedorn, P. Schmittinger “Potassium Compounds” in Ullmann’s Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a22_039
  3. ^ "Caustic Potash." Oxy.com Retrieved on July 26, 2010.
  4. ^ "Potassium Hydroxide." MSDS Retrieved on January 24
  5. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  6. ^ Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.
  7. ^ W. W. Hartman, "p-Cresol", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV1P0175 ; Coll. Vol. 1: 175 
  8. ^ Römpp Chemie-Lexikon, 9th Ed. (in german)
  9. ^ James K. Drackley Glycerin as a potential feed ingredient for dairy cattle
  10. ^ K. Schumann, K. Siekmann “Soaps” in Ullmann’s Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a24_247
  11. ^ D. Berndt, D. Spahrbier, "Batteries" in Ullmann’s Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a03_343
  12. ^ "Toyota Prius Hybrid 2010 Model Emergency Response Guide". Toyota Motor Corporation. 2009. Archived from the original on 2011-10-29. http://www.webcitation.org/62ngVS3FD. 
  13. ^ Thomas Eisner, For the Love of Insects, Harvard University Press 2003, p. 71
  14. ^ Römpp Chemie-Lexikon, 9th Ed. (in German)
  15. ^ Testing Chemical Reactions at MushroomExpert.com

External links