Potassium Hydroxide
Properties
| State | Solid (white, deliquescent pellets or flakes) |
| Color | White |
| Solubility | Highly soluble in water (1210 g/L at 25°C; exothermic dissolution) |
| Melting Point | 360°C |
| Boiling Point | 1327°C |
About Potassium Hydroxide
Potassium hydroxide, KOH at 56.105 g/mol, is sodium hydroxide's softer cousin: same strong-base behavior, completely dissociated in water, but the resulting potassium salts of fatty acids stay liquid where the sodium analogues set up hard. That single property explains why KOH is the alkali of choice for liquid hand soaps, shaving creams, and the green soap that runs through laboratory glassware washers. The 'soft soap' versus 'hard soap' distinction predates modern chemistry by centuries — colonial soap-makers leaching wood-ash potash got soft soaps, while those using sodium-rich barilla or kelp ash got bar soaps. Industrially, KOH is the electrolyte in alkaline manganese dioxide batteries (the gray Duracells in your remote), the catalyst in transesterification of triglycerides into biodiesel, and the digesting reagent in Kjeldahl nitrogen analysis. Biodiesel producers prefer KOH over NaOH because the leftover glycerol phase contains potassium soaps that can be sold as fertilizer. In the lab, KOH pellets dropped into a desiccator scrub CO2 out of the atmosphere — handy for storing CO2-sensitive standard NaOH solutions or alkaline reagents. Industrial KOH comes from chlor-alkali electrolysis of KCl brine using the same membrane-cell technology that produces NaOH from NaCl.
Where you'll encounter it
If you've ever made cold-process liquid soap at home, etched aluminum off a counterfeit-detection scratch, or replaced the 9V battery in a smoke alarm, you've encountered KOH. In a synthesis lab, KOH is what you reach for when you need a strong base that's slightly more soluble in alcohols than NaOH — saponifying methyl esters or hydrolyzing amides under reflux in ethanolic KOH is a standard undergraduate organic procedure. In a biodiesel cooperative, the standard recipe for transesterifying a liter of waste vegetable oil is around 4-7 g KOH dissolved in 200 mL methanol, dropped into the warmed oil, and stirred 60 minutes; the methyl ester layer floats off the glycerol-and-soap layer and is washed with water until the runoff is neutral.
Common Uses
- Saponification catalyst in cold-process and hot-process liquid soap manufacturing
- Electrolyte in alkaline manganese dioxide AA, AAA, C, and D primary cells
- Transesterification catalyst in small-scale biodiesel production from waste oils
- Digestion reagent in Kjeldahl nitrogen analysis for protein quantification
- CO2 scrubber in dive rebreathers and submarine atmosphere control via Sodasorb
- Caustic etchant for aluminum and brass in metalworking and printing plate prep
- pH adjustment in concrete admixtures and oilfield drilling muds
Safety Information
GHS: H290 corrosive to metals, H302 harmful if swallowed, H314 causes severe skin burns and eye damage Cat 1A. OSHA PEL is 2 mg/m³ ceiling for the dust. Concentrated solutions (>5%) cause deep saponification burns that progress for hours after exposure — wash with copious water for at least 15 minutes and seek medical attention for any splash above eyebrow level. Dissolving the pellets is strongly exothermic; always add KOH to water, never the reverse, and use a Pyrex or polypropylene container — thin glassware can crack from thermal shock. Reacts with aluminum, zinc, and tin to evolve hydrogen gas (LEL 4%). Store in tightly sealed HDPE; the pellets deliquesce rapidly and absorb CO2 to form K2CO3.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.