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is the inorganic compound
with the formula KOH
. KOH, as it is familiarly known, along with sodium hydroxide
, is a prototypical "strong base". It has many industrial as well as niche applications. Most applications exploit its reactivity toward acids and its corrosive nature. In 2005, an estimated 700 – 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. Samples of KOH 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
, leading to a temperature rise, sometimes up to boiling point. Concentrated aqueous solutions are called potassium lyes
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-
in size). At room temperature the OH-
groups are ordered and the environment about the K+
centers is distorted with K+
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·H2
O, the dihydrate KOH·2H2
O, and the tetrahydrate KOH·4H2
Solubility and desiccating properties
Approximately 121 g of KOH will dissolve in 100 mL of water (compared with 100 g of NaOH in the same volume). Lower alcohols
such as methanol
, 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 is a desiccant
. In the laboratory, it is particularly useful for drying basic solvents, especially amines and pyridines. Distillation of these basic liquids from a slurry
of KOH yields the anhydrous reagent.
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-casted as pellets or rods, forms that have low surface area and convenient handling properties.
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
are attacked by KOH to give the silicates and bicarbonate
- KOH + CO2 → KHCO3
Of historical significance is the old method of boiling a solution of potassium carbonate
(potash) with calcium hydroxide
). A metathesis reaction
occurs, precipitating calcium carbonate
, leaving potassium hydroxide in solution:
- Ca(OH)2 (s), (aq) + K2CO3 (aq) → CaCO3 (s) + 2 KOH (aq)
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 and slaked lime. Probably known since antiquity, 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 K+ (aq) + 2H2O (l) + 2e− → H2 (g) + 2 KOH (aq)
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:
- 2 Cl– — 2e− → Cl2 (g)
Separation of the anodic and cathodic spaces in the electrolysis cell is essential for this process.
KOH and NaOH are often used interchangeably, 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
, 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 soft soaps
with KOH is used to prepare the corresponding potassium soaps
. Such soaps tend to be more soluble and are found in liquid soaps. The more common sodium soaps are more easily solidified.
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.
KOH attracts numerous specialized applications, but virtually all capitalize 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 corrosivity is sometimes used in cleaning and disinfection
of resistant surfaces and materials. It is often the main active ingrediant in chemical "cuticle removers."