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potassium hydroxide

Product name potassium hydroxide
Synonyms caustic potash
potash lye
potassa
GOST 9285-78
CAS 1310-58-3

Potassium hydroxide KOH is a colorless, odorless, corrosive, deliquescent crystalline solid. It readily absorbs water and carbon dioxide from air. KOH has a good solubility in water, 49.4% wt at 0°C, its solvation is highly exothermic. It also dissolves in methyl alcohol (35.5% wt at 28°C), ethanol (27.9% wt at 28°C). Potassium hydroxide forms stable mono-, di- and tetrahydrates (see table below):

CompoundKOH*H2O KOH*2H2O KOH*4H2O
Melting point, °C 150 33 -33.5
Solubility in water
g/100g ( °C)
147.0 (60°C)
311.5 (150°C)
117.4 (25°C) ----

Anhydrous potassium hydroxide exists in two crystalline modifications. Monoclinic modification is stable below 247°C, at higher temperature KOH has a NaCl-like cube lattice.

Potassium hydroxide is a "strong base", along with other alkalis such as sodium hydroxide, lithium hydroxide, calcium hydroxide. Solid KOH violently reacts with mineral acids (neutralization reaction) to form salts. It also readily combines with their anhydrides (acidic oxides SO2, CO2, NO2 and others) to give corresponding salts. Thus, interaction with carbon dioxide results in potassium hydrocarbonate:

KOH + CO2 → KHCO3

It also easily reacts with other "acidic" gases such as hydrogen chloride HCl, hydrogen bromide HBr, dihydrogen sulfide H2S to produce salts:

KOH + HCl → KCl + H2O

Interaction of KOH with hydrogen fluoride leads to a mixture of potassium fluoride KF, potassium bifluoride KF*HF and potassium trifluoride KF*2HF. It reacts with carbon monooxide to form potassium formate:

KOH + CO → HC(O)OK

Anhydrous KOH combines with chlorine or bromine only if temperature higher 600°C.

Molten potassium hydroxide reacts with aluminum, zinc, gallium, beryllium, tin, lead and antimony releasing dihydrogen H2 and to give oxometallates such as KGaO2, KSnO2 and so on. Same products may be obtained by the reaction of KOH with these metals oxides and hydroxides.

Potassium hydroxide is fully ionized (dissociated) in water solutions. Its water solution dissolves silicon, germanium, boron, their oxides and acids to give corresponding salts. It lixiviates silicate glasses converting SiO2 into potassium polysilicates. After this action glass acquires ulcer, rough surface and becomes fragile.

Potassium hydroxide forms compounds and solid solutions with other alkali and alkali-earth metal hydroxides and salts. Thus, addict with lithium hydroxide has a formula KOH*2LiOH (m.p. 313°C, decomposes). With NaOH it forms solid solutions and eutectic (m.p. 170°C, 50% mol of KOH). Continuous series of solid solutions is formed with rubidium hydroxide RbOH. With barium hydroxide KOH forms solid solutions and adduct KOH*4Ba(OH)2 (m.p. 390°C). It also forms eutectic with potassium carbonate K2CO3 (m.p. 360°C, 78% wt of KOH).

Potassium hydroxide reacts with a number of organic compounds. Thus, interaction with carboxylic acids, their anhydrides, amides results in corresponding potassium salts. Alkyl- and arylhalogenated derivatives being treated with KOH convert into alcohols. It also saponifies esters to produce carboxylic acid salt and alcohol:

R1C(O)OR2 + KOH → R1COOK + R2OH

Potassium hydroxide, either in anhydrous or dissolved form, can pose several risks. It may cause chemical burns of skin or mucous membranes, permanent injury or scarring, and blindness. It is able to permeate deep into animal tissue damaging adipose, lipids and proteins.

Production.

Almost commercial potassium hydroxide is manufactured by electrolysis of aqueous solution of potassium chloride KCl or potassium carbonate K2CO3:

2KCl + 2H2O → 2KOH + Cl2↑ + H2

There are three technological processes most commonly used for KCl electrolysis:

  • Mercury cell electrolysis. Titanium anodes are located above a liquid mercury cathode in the electrolysis bath with a solution of potassium chloride. When an electrical current is applied, chlorine is released at the titanium anodes and metal potassium dissolves into the mercury cathode forming an amalgam. Dissolved potassium is then reacted with water releasing hydrogen to produce KOH. There have been concerns about mercury releases, although modern technologies claim to be safe in this regard. This method consumes vast amounts of energy.
  • Diaphragm cell method. In this route a porous asbestos or polymer diaphragm is deposited on an iron grid cathode preventing the chlorine forming at the anode and the potassium hydroxide forming at the cathode from re-mixing. The potassium hydroxide must usually be concentrated and the salt excess removed. This is done using an evaporative process. Potassium hydroxide produced by this method is contaminated with potassium chloride;
  • Membrane cell method. - similar to the diaphragm cell process. The electrolysis cell is divided into two by a ion exchange membrane. Only potassium ions and a little water molecules pass through the membrane. This method is nearly as efficient as the diaphragm cell and produces very pure potassium hydroxide but requires very pure potassium chloride solution. Of the three processes, the membrane cell process requires the lowest consumption of electric energy and the amount of steam needed for concentration of the end product.
Aqueous potassium hydroxide produced by one of these processes then is dried in vacuum at 360-400°C to obtain solid product.

Uses.

  • as an electrolyte in alkali cells (e.g. Ni-Cd batteries and so on);
  • as a row material in potassium and complex fertilizers manufacturing;
  • interaction of KOH with stearic or palmitic acid results in liquid salts, which are used as liquid soap;
  • for the mercerization of wood cellulose. This process is used in viscose fibers and yarns production;
  • for the treatment of cotton textile to increase hygroscopicity;
  • as an absorbent for the "acidic" gases such as dihydrogen sulfide, sulfur dioxide, carbon dioxide and so on;
  • as a dehydrating agent for gases, which do not interact with KOH. (e.g. ammonia NH3, phosphine PH3, nitrous oxide N2O);
  • as a dehydrating agent for liquids in synthetic organic chemistry;
  • to measure the concentration of acids by titration;
  • as a component of cleaners and detergents;
  • as a defoaming agents in the manufacture of paper;
  • to diagnose fungal infections in human medicine;
  • as an anisotropic etchant of silicon, exposing octahedral planes. This method can create pyramids and regularly-shaped etch pits for applications such as micro electromechanical systems;

Manufacturer(s) Bratsk chlorine plant
CJSC Soda-Hlorat
Chemical structure of potassium hydroxide

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