Potassium hydroxide

Potassium hydroxide

Potassium hydroxide flakes
Names
IUPAC name Potassium hydroxide
Systematic IUPAC name Potassium hydroxide
Other names Caustic potash Lye Potash lye Potassia Potassium hydrate
Properties
Chemical formula	KOH
Molar mass	56.106 g/mol
Appearance	White solid, deliquescent
Odor	Odorless
Density	2.044 g/cm3 (20 °C) 2.12 g/cm3 (25 °C)
Melting point	360–380 °C (680–716 °F; 633–653 K)
Boiling point	1,327 °C (2,421 °F; 1,600 K)
Solubility in water	85 g/100 ml (-23.2 °C) 97 g/100 ml (0 °C) 121 g/100 ml (25 °C) 138.3 g/100 ml (50 °C) 162.9 g/100 ml (100 °C)
Solubility	Soluble in ethanol, glycerol, isopropanol, methanol Insoluble in anh. ammonia, diethyl ether
Solubility in isopropanol	14 g/100 g (28 °C)
Solubility in methanol	55 g/100 g (28 °C)
Vapor pressure	~0 mmHg
Thermochemistry
Std molar entropy (So298)	79.32 J·mol-1·K-1
Std enthalpy of formation (ΔfHo298)	-425.8 kJ/mol
Hazards
Safety data sheet	Sigma-Aldrich
Lethal dose or concentration (LD, LC):
LD50 (Median dose)	273 mg/kg (rat, oral)
Related compounds
Related alkali hydroxides	Lithium hydroxide Sodium hydroxide Rubidium hydroxide Caesium hydroxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Potassium hydroxide is a white solid with the formula KOH. It finds many applications in the lab as a general base, though sodium hydroxide is generally preferred as it is easier to find and slightly cheaper.

Properties

Chemical

Potassium hydroxide dissolves in water and fully dissociates to give a solution of potassium ions and hydroxide ions. Potassium hydroxide solutions will very readily absorb atmospheric carbon dioxide to form potassium carbonate, and must be protected from the atmosphere. Being a strong base, potassium hydroxide also has the ability to dissolve glass (particularly so at higher temperatures) so solution must be kept in a chemically resistant plastic containers, such as high density polyethylene (HDPE).

Potassium hydroxide can be reacted with the corresponding acids to yield potassium salts. Potassium hydroxide is used along with manganese dioxide and an oxidizer to form crude potassium manganate at very high temperatures.

Potassium hydroxide can also be used in a reaction with magnesium powder or shavings in an inert, high boiling solvent with a tertiary alcohol catalyst to yield clean spheres of potassium metal.

Physical

Potassium hydroxide is a waxy white solid which is extremely hygroscopic. It is very soluble in water and its dissolution is highly exothermic. One-hundred percent potassium hydroxide is very difficult to create due to the hygroscopic nature and commercial samples generally contain around 10% water.

Availability

Potassium hydroxide is available reasonably cheaply from soap making suppliers and biodiesel companies such as Duda Diesel. It is also available from Elemental Scientific at a slightly higher cost.

Certain barbecue cleaning products contain 25% KOH, as aqueous solution.

Preparation

Preparation of solid potassium hydroxide is difficult in an amateur setting due to the highly hygroscopic nature of potassium hydroxide.

Historically, potassium hydroxide was made by reacting potassium carbonate (potash) with calcium hydroxide (slaked lime).

Ca(OH)₂ + K₂CO₃ → CaCO₃ + 2 KOH

This method can be performed by the amateur chemist, though it requires corrosion resistant containers when drying or concentrating the potassium hydroxide. Another inconvenient represents the organic residues from the wood ash, which are difficult to remove. They can however be removed completely by roasting the potash in the kiln at very high temperatures in an excess of oxygen.

Heating potassium carbonate at over 891 °C will yield potassium oxide that will react quickly and extremely exothermic with water to form hydroxide. It will also react with any carbon dioxide immediately at lower temperatures. This method is not very practical, as it requires high temperatures.

Electrolysis of a saturated solution of KCl with an ion exchange membrane will also yield potassium hydroxide. This preparation method is similar to that for sodium hydroxide.

Another route, though expensive, involves the reaction of potassium metal or potassium oxide with water.

Projects

• Make potassium permanganate solution
• Make potassium manganate solid
• Produce clean potassium metal
• Make liquid soap
• Make an alkali battery
• Identify certain species of fungi
• Synthesize potassium ferrate

Handling

Safety

Potassium hydroxide is a strong base and is therefore highly caustic, but is not toxic in the sense that it shuts down chemical processes, like arsenic or cyanide. The potassium ion itself is nontoxic when taken orally, but injected into the blood stream it becomes highly toxic and may cause cardiac arrest. The primary danger is saponification of tissue on contact with the hydroxide ions.

Storage

Potassium hydroxide should be stored in closed thick plastic containers, in a dry and cool place, away from any corrosive vapors. Avoid storing it in glass bottles.

Disposal

Potassium hydroxide will turn to potassium carbonate if left in open air, which poses no toxicity to the environment and it's even used as a source of potassium for plants.

References

Relevant Sciencemadness threads

• Making KOH from wood ashes why is it yellow ?
• Storage of KOH solution