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Potassium hydroxide
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Potassium hydroxide is the inorganic compound with the formula KOH. Along with sodium hydroxide, 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. KOH is noteworthy as the precursor to most soft and liquid soaps as well as numerous potassium-containing chemicals.
ssium hydroxide is usually sold as translucent pellets, which will become tacky in air because KOH is hygroscopic.
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Encyclopedia
Potassium hydroxide is the inorganic compound with the formula KOH. Along with sodium hydroxide, 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. KOH is noteworthy as the precursor to most soft and liquid soaps as well as numerous potassium-containing chemicals.
Properties and structure
Potassium hydroxide is usually sold as translucent pellets, which will become tacky in air because KOH is hygroscopic. Consequently, KOH characteristically 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.
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 Ĺ (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 Ĺ, 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.
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. KOH sublimes unchanged at 400 °C; the gaseous species is dimeric. Even at high temperatures, dehydration does not occur. 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 alkali solutions in water and other polar solvents. These solutions are capable of deprotonating many acids, even weak ones. In analytical chemistry, titratons 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.
Reactions with inorganic compounds
Complementary to its reactivity toward acids, KOH attacks anhydrides, defined in the broadest sense. Thus, SiO2 and CO2 are attacked by KOH to give the silicates and bicarbonate, respectively:
- 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 was 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 modern 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.
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 radium 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. 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 saponification of the fats in vegetable oil. Glycerin from potassium hydroxide-processed biodiesel is useful as an inexpensive food supplement for livestock, once the toxic methanol is removed.
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.
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.
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. 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. It is often the main active ingredient in chemical "cuticle removers."
See also
External links
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