Potassium Hydroxide — Frequently Asked Questions

Solid KOH holds on to water much more strongly than caustic soda, and removing the last of it takes extra energy and cost. Commercial flakes therefore contain around 10% water of hydration, with only small amounts of true impurities such as carbonate and chloride. The practical effect is on dosing: calculate additions from the KOH assay on the certificate of analysis, not from the flake weight.

Use KOH when the potassium salt you form must be soft or highly soluble. Potassium soaps give liquid and soft soaps that sodium soaps cannot, and potassium-based detergent concentrates stay clear at strengths where sodium salts would crystallise. KOH is also used where the potassium itself is wanted, as in fertiliser salts, or where a process has been validated on it, as in some biodiesel plants. For general pH control, caustic soda is cheaper.

On a molar basis, 56.1 g of KOH does the work of 40.0 g of NaOH, so pure KOH is needed at about 1.40 times the weight of pure NaOH. With 90% KOH flakes and 99% caustic soda, the factor is about 1.55. In soap making, use the potassium saponification value of each oil and correct for the assay on the certificate of analysis.

Yes. KOH is widely used as a homogeneous catalyst for transesterifying oils with methanol, often dissolved in the methanol first as potassium methoxide. It dissolves faster in methanol than caustic soda, and the potassium soaps and glycerine phase that form are easier to separate. Water and carbonate in the catalyst reduce activity and increase soap formation, so specify them and keep the flakes sealed.

Keep them in the original sealed packaging in a cool, dry, covered store. KOH absorbs moisture so readily that open flakes become wet and eventually liquefy, and absorbed carbon dioxide converts part of the product to carbonate. Store away from acids, ammonium salts and aluminium, zinc or tin. Use polyethylene, polypropylene or stainless steel for handling equipment, and reseal part-used containers straight away.

Potassium hydroxide is CAS 1310-58-3 and EC 215-181-3, and it classifies under HS 2815.20 whether solid or in solution. For transport, the solid is UN1813 and solutions are UN1814, both Class 8 corrosives. Packing group depends on the form and concentration, so take it from section 14 of the safety data sheet and keep the same identifiers on the invoice, packing list and bill of lading.