Potassium hydroxide

The chemical compound potassium hydroxide (KOH), sometimes known as caustic potash, potassa, potash lye, and potassium hydrate, is a metallic base. It is very alkaline and is a "strong base", along with sodium hydroxide, lithium hydroxide, calcium hydroxide, barium hydroxide and strontium hydroxide.

Properties

Pure potassium hydroxide is a colorless, highly hygroscopic, solid crystalline compound, having density of about 2.04 g/cm³, readily soluble in water (1 g KOH dissolves in 0.5 g water) and lower alcohols (methanol, ethanol, propanols; solubility in ethanol being about 1 g KOH/2.5 mL EtOH), as well as other polar solvents. The dissolution in water is strongly exothermic, producing substantial amounts of energy in form of heat, leading to temperature rise, sometimes up to boiling point and over; concentrated aqueous solutions are called potassium lyes. Potassium hydroxide forms solid hydrates, namely the monohydrate KOH·H₂O, the dihydrate KOH·2H₂O, and the tetrahydrate KOH·4H₂O; it is used therefore as a highly intensive desiccant agent, e.g. for drying liquid amines or their solutions in indifferent, nonpolar solvents (such as hydrocarbons). It is a highly basic compound, forming strongly alkali solutions in water and other polar solvents, capable of deprotonating many acids, even weak ones, and decompose some inorganic as well as organic materials (e.g. dissolution and hydrolysis of broken glass or fats in concentrated KOH solutions). It is an important industrial chemical, used to manufacture many industrial as well as commercial goods and products, for example, most potassium salts; it's also an important laboratory chemical and reagent, again, used to prepare potassium salts, to neutralize acids, further as basic standard in analytics and many more applications. As a very strong base/alkali, potassium hydroxide is strongly corrosive, both towards inorganic as well as organic materials, including living tissues; care must be therefore taken, when handling the substance and its solutions. Its corrosivity is sometimes used in cleaning and disinfection of resistant surfaces and materials.

Manufacture

Of historical relevance is the old method of boiling a solution of potassium carbonate (potash) with calcium hydroxide (slaked lime); the ion transfer reaction takes place, because forming calcium carbonate is less soluble than lime, precipitating out of the reaction mixture:

Ca(OH)_{2 (s), (aq)} + K₂CO_{3 (aq)} → ↓CaCO_{3 (s)} + 2 KOH _(aq)

precipitated calcium carbonate was than filtered out and the resulting KOH solution boiled down, leaving solid potassium hydroxide ("calcinated or caustic potash"). This method, using potash extracted from wood ashes and slaked lime was known probably already in antique times and 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:

2 K⁺ _(aq) + 2H₂O _(l) + 2e⁻ → ↑H_{2 (g)} + 2 KOH _(aq)

Hydrogen gas forms as by-product on the cathode; concurrently, an anodic oxidation of the chloride ion takes place:

2 Cl^– — 2e⁻ → ↑Cl_{2 (g)},

forming chlorine gas as another byproduct. Separation of the anodic and cathodic spaces in the electrolysis cell is essential for this process. For details, see chloralkali process.

Uses

Agriculture

• Correction of the pH of acidic soils
• As a fungicide
• As a herbicide

Biological and medical tests and diagnostics

• KOH test (to diagnose fungal infections).
• Whiff test (to diagnose bacterial vaginosis).
• In botany and mycology, an aqueous solution (typically 10% or less) of KOH is used to rehydrate dried herbarium material in preparation for microscopy.
• In palynology, an aqueous solution of KOH is used to deflocculate soil particles aggregated onto pollen grains.

Medical therapeutics (human and veterinary)

• wart and cuticle solvent.
• (veterinary medicine) disbudding calves horns; dissolving scales and hair.

Food preparation

Food uses of KOH include:

• Washing or chemical peeling of fruits and vegetables
• Chocolate and cocoa processing
• Caramel color production
• Poultry scalding
• Soft drink processing
• Thickening ice cream
• Soaking of olives to soften them and neutralize natural acids
• Glazing of pretzels and German lye rolls before baking to make them crisp.
• Soaking of cod to make the Scandinavian delicacy known as lutefisk (from lutfisk, "lye fish"). Cod is soaked in KOH to a jelly-like consistency, then served with bacon fat, potatoes, brown cheese (brunost) sauce and mushy peas.
• Making of hominy, which is dried maize (corn) kernels reconstituted by soaking in KOH.

Manufacturing and commercial chemical processes

KOH is a major industrial chemical because it is used as a base in a wide variety of chemical processes. Some uses of KOH include:

• Acrylate ester copolymer coating
• Anti-foaming agent used in the manufacture of paper
• saponifying oils for liquid soap
• Formulation aid for food
• pH control agent (raises pH by neutralizing acids)
• Making polyethylene resins
• Textile processing
• As a catalyst in reactions like the production of biodiesel.
• Manufacture of many cleaning products, such as:
  • Washing powders
  • Cleansers (for tubs, tile, etc)
  • Some denture cleaners
  • Non-phosphate detergents
  • Drain or pipe cleaners (clog dissolvers)
  • Widely used in carpet cleaning degreasers
• Manufacture of alkaline batteries, which use an aqueous solution of KOH as an electrolyte. Thus, potassium hydroxide helps to power flashlights, smoke detectors, and other battery-powered household items.
• As an anisotropic etchant of silicon, exposing octahedral planes. This technique can create pyramids and regularly-shaped etch pits for uses such as microelectromechanical systems.
• An anisotropic etchant of GaAs, exposing Ga and As {111} planes. This technique can create rectangular etch pits by etching As (111) plane faster.

See also

• Soda lime

External links

• Newscientist article dn10104
• MSDS from JTBaker
• Reheis [1]
• Making lye from ashes