potassium hydroxide

potassium hydroxide

potassium hydroxide, chemical compound with formula KOH. Pure potassium hydroxide forms white, deliquescent crystals. For commercial and laboratory use it is usually in the form of white pellets. A strong base, it dissolves readily in water, giving off much heat and forming a strongly alkaline, caustic solution (see acids and bases). It is commonly called caustic potash. It closely resembles sodium hydroxide in its chemical properties and has similar uses, e.g., in making soap, in bleaching, and in manufacturing chemicals, but is less widely used because of its higher cost. It is prepared chiefly by electrolysis of potassium chloride; commercial grades of it sometimes contain the chloride as well as other impurities.
| Section8 = }} Potassium hydroxide 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.

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.

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, 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 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.

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-casted 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

Of historical significance is the old method of boiling a solution of potassium carbonate (potash) with calcium hydroxide (slaked lime). 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.

Uses

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, 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 soft soaps

The saponification of fats 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.

Niche applications

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."

See also

References

External links

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