Potassium hydroxide (KOH) is produced through two main methods: electrolysis of potassium chloride solution (the modern industrial approach) and leaching wood ash followed by a chemical reaction with lime (the traditional method). Both routes yield the same caustic, water-soluble compound used in soapmaking, biodiesel production, and electrochemistry. The method you choose depends on the scale you need and the equipment you have access to.
The Industrial Method: Electrolysis
Nearly all commercial KOH is made by passing an electric current through a solution of potassium chloride (KCl) dissolved in water. This is called the chloralkali process. A concentrated brine containing 250 to 350 grams of KCl per liter is fed into the anode side of an electrolytic cell. When current flows, the potassium chloride breaks apart: chlorine gas forms at the positive electrode, hydrogen gas forms at the negative electrode, and potassium ions migrate through a special membrane to the cathode side, where they combine with hydroxide ions to form KOH in solution.
The membrane is the key to purity. It allows potassium ions and water to pass through while blocking chloride ions and chlorine gas bubbles. Industrial cells run at current densities of 2 to 4 kiloamperes per square meter of electrode surface and produce KOH solutions concentrated to roughly 410 to 480 grams per liter. At that concentration, the solution is extremely caustic and ready for further processing into pellets or flakes.
Replicating this at home is technically possible on a small scale with a DC power supply, graphite or platinum electrodes, and a membrane separator, but the process generates chlorine gas, which is toxic. Without proper ventilation and gas-handling equipment, small-scale electrolysis of KCl is genuinely dangerous.
The Traditional Method: Wood Ash and Lime
Before industrial chemistry, soapmakers produced KOH by leaching hardwood ash in water, then reacting the resulting liquid with slaked lime. This two-step process has been in use since at least the 1700s and still works today for small batches.
Step 1: Leaching the Ash
Burning hardwood (oak, hickory, maple, or fruit woods) produces ash rich in potassium carbonate. Softwoods contain less potassium and yield weaker lye. The ash is placed in a container with a drain at the bottom, and water is slowly poured over it, allowed to soak, and then collected as it drips through. This liquid, called potash water, contains dissolved potassium carbonate along with smaller amounts of sodium carbonate and other minerals. Multiple passes of water through the same ash, or running the collected liquid through fresh ash a second time, increases the concentration.
Step 2: Causticizing With Lime
The potash water at this stage is a carbonate solution, not a hydroxide. To convert it, you mix the liquid with slaked lime (calcium hydroxide). The calcium hydroxide reacts with the dissolved potassium carbonate: the calcium grabs the carbonate to form solid calcium carbonate (ordinary chalk or limestone), and the potassium pairs with the hydroxide from the lime. The result is a solution of potassium hydroxide (mixed with some sodium hydroxide) floating above a sludge of calcium carbonate and other settled impurities.
This reaction works even though slaked lime barely dissolves in plain water. As the small amount that does dissolve gets consumed by the reaction, more solid lime dissolves to take its place, keeping the process going until the carbonates are used up. You pour off or filter the clear liquid from the sludge, and you have a usable KOH solution.
How to Check Concentration
The strength of a KOH solution is directly tied to its density. A 10% solution by weight has a density of about 1.09 grams per cubic centimeter. At 30%, the density climbs to roughly 1.29, and a 50% solution reaches 1.51. A simple hydrometer, the glass tube with a weighted bulb that floats at different levels in different liquids, lets you measure this density and estimate the KOH percentage without any chemistry equipment.
For soapmaking, most recipes call for a specific concentration. If your solution is too weak, you can boil off water gently (outdoors or in a well-ventilated space) to concentrate it. If it’s too strong, add distilled water in small amounts and recheck with the hydrometer.
Safe Handling and Storage
KOH is intensely corrosive. It dissolves fats and proteins on contact, which means it will cause deep chemical burns to skin and eyes far more quickly than most acids. If it contacts your skin, flush immediately with large amounts of water. If it reaches your eyes, irrigate continuously and seek medical attention. The dissolving process itself is strongly exothermic: adding KOH pellets to water generates significant heat, so always add KOH to water (not the reverse) and use a heat-resistant container.
For protective gear, neoprene, natural rubber, butyl rubber, and nitrile gloves all rate “very good” against potassium hydroxide. Chemical splash goggles that meet ANSI Z87.1 standards are the minimum for eye protection. A face shield adds an extra margin of safety when working with concentrated solutions. Long sleeves and closed-toe shoes are common sense additions.
What to Store It In
KOH will attack aluminum, zinc, tin, and lead, generating hydrogen gas in the process. Even stainless steel corrodes over time: 316L stainless steel exposed to a 50 gram per liter KOH solution loses about 5.6 micrometers of thickness per year, and corrosion accelerates at higher concentrations. For long-term storage, high-density polyethylene (HDPE) containers are the standard choice. They resist caustic solutions well, are widely available, and won’t shatter if dropped. Glass works chemically but poses a breakage risk, and KOH solutions can etch glass stoppers and create a seal that’s nearly impossible to open.
Keep containers tightly sealed. KOH is hygroscopic, meaning it pulls moisture from the air, and it also absorbs carbon dioxide, gradually converting back to potassium carbonate and losing its strength. Dry pellets stored in a sealed HDPE container in a cool, dry location will last for years.
Disposing of Excess KOH
Potassium hydroxide solutions with a pH of 12.5 or higher meet the federal definition of a corrosive hazardous waste under EPA regulations. Before pouring any KOH down a drain, you need to neutralize it. The simplest approach is to slowly add a weak acid, like white vinegar, while stirring and monitoring the pH. Once you bring the solution to a pH between 6 and 9, it’s no longer classified as corrosive and can typically go down a household drain in small quantities. Local regulations vary, so check your municipality’s rules if you’re working with more than a few liters.