Potassium hydroxide

What it is and why it makes soft soap

Potassium hydroxide (KOH), caustic potash, is a white solid that NIOSH describes as lumps, rods, flakes, sticks or pellets; soapmakers buy it as flakes. It saponifies fats exactly as sodium hydroxide does. Saponification values are defined in it, as milligrams of potassium hydroxide per gram of fat (AOCS Cd 3-25), so a potassium hydroxide recipe needs no conversion before its purity is allowed for.

What changes is the soap. In McBain and Sierichs's classic measurements, potassium laurate, myristate and oleate, and the potassium soap of coconut oil's mixed fatty acids, all dissolved at room temperature, while the sodium soaps of the saturated acids needed heat. So coconut and olive oil saponified with potassium hydroxide stay as a translucent paste that dilutes into a pourable soap, and Thomssen's 1922 manual defines liquid soap as nothing more than a potash soap dissolved in water. Potassium soaps of palmitic and stearic acid are the exception: they crystallise as the soap cools, which clouds a liquid soap, and potassium stearate is the backbone of shaving and cream soaps.

Older books use "potash" for potassium carbonate as well: Simmons and Appleton call pearl ash an impure carbonate of potash. The carbonate cannot saponify a neutral fat, so for soap you need the hydroxide.

Forms and purity

Potassium hydroxide is never sold pure. The OECD's assessment gives the industrial solid as about 91 % KOH and about 8 % water. The EU food additive, E 525, need only be 85.0 % potassium hydroxide, with up to 3.5 % potassium carbonate, and reagent-grade pellets are guaranteed to at least 85 %, with no more than 2 % carbonate. The rest of the weight is mostly water, with some carbonate.

The calculator assumes 90 % because that is where ordinary flake sits. Its note gives 85 to 95 % and asks you to check the label, and it warns if you set the purity to 99.5 % or more. It divides the pure requirement by the purity, so a recipe that needs 100.0 g of pure potassium hydroxide weighs 111.1 g of 90 % flake.

A wrong purity matters most in liquid soap, which is made close to zero superfat. Take our liquid soaps at a 3.5 % superfat. Flakes that are really 95 % but entered as 90 % deliver 5.6 % more hydroxide than intended and take the superfat to about −1.9 %: a paste with free alkali in it. Flakes that are really 85 % but entered as 90 % deliver 5.6 % less, a superfat of about 8.9 %, far more free oil than a clear soap can carry, so it clouds. Enter 90 % flake as 100 % and the paste is a tenth short of lye. In a dual-lye bar the error is diluted by the sodium share, but it still moves the superfat.

To set it, choose Potassium hydroxide or Dual in the Lye section and type the figure from the label or certificate into KOH purity. With no figure, or a tub that has been open for a while, measure it with the citric acid titration in the NaOH and KOH guide. The calculator works out the water from the flakes as weighed, so the water inside them is not subtracted, and the solution you mix is a little weaker in KOH than the figure on screen.

Dissolving it

Every rule for sodium hydroxide applies. Add the flakes slowly to cold water, never water to the flakes, which the chemical safety card for potassium hydroxide spells out in capitals. Wear goggles, gloves and sleeves, work where air moves, and use a polypropylene or stainless steel jug: potassium hydroxide corrodes aluminium, tin, lead and zinc, and gives off hydrogen as it does.

It is the livelier of the two. Dissolving pure potassium hydroxide releases 57.61 kJ per mole against 44.51 kJ for sodium hydroxide (CRC Handbook), so the same saponifying power gives off about 30 % more heat; commercial flake, which already holds some water, gives off a little less. Fisher describes the solution fizzing loudly, as though it were boiling. The liquid soap method lets it cool below 82 °C (180 °F) before it meets the oils.

It dissolves to about 110 g in 100 mL of water at 25 °C, so, like sodium hydroxide, it cannot be made much stronger than half lye. Liquid soap pastes are usually made far weaker than that: Fisher's uses three times the potassium hydroxide's weight in water, a 25 % solution.

Liquid soap: paste, then dilution

Liquid soap is made twice. First the oils and potassium hydroxide are brought to a stiff, taffy-like paste and cooked until a spoonful dissolves clear in boiling water; then the paste is weighed and diluted by weight. The trace does not behave like a bar's: the batter thickens, separates and comes back, sometimes several times, before it sets. Coconut-rich potash soap swells as it saponifies, so the pot should hold four to five times the batch.

Liquid soap is usually made at a superfat of 0 to 3 %, because free oil does not dissolve in it and shows as haze. Our recipes use 3.5 %, so that the spread of real saponification values cannot tip them into free lye, and that margin also makes "neutralising" with borax or citric acid unnecessary. Borax is prohibited in cosmetic products in the EU and Great Britain in any case.

The dilution calculator counts the oils plus the pure potassium hydroxide as soap, since saponification conserves mass, and everything else in the paste as water. A pump bottle suits 15 to 25 % soap. Dilute with hot distilled water, leave it overnight before adding more, and dilute only what you will use soon: the paste keeps, while diluted soap is mostly water and needs clean bottles and, if you sell it, a preservative that works at pH 9 to 10. The liquid soap guide gives the full method.

Dual-lye soaps

A dual-lye recipe does part of its saponification with potassium hydroxide and the rest with sodium hydroxide. The calculator's KOH share is a share of the saponification, not of the weight, and since potassium hydroxide is the heavier molecule and is sold less pure, its share of the weight is always larger. Our shaving soap, at a 40 % share, asks for 43.7 g of sodium hydroxide and 45.4 g of 90 % potassium hydroxide for 500 g of oils: two-fifths of the work, 51 % of the lye you weigh.

The share sets the character. Thomssen's 1922 shaving soaps did about 25 to 45 % of their saponification with potash, and our stearic shaving soap goes to 50 % so that a damp brush loads quickly. Fisher's basic cream soap works out at about three-quarters potassium, and Grosso's at about 89 % with 90 % flakes; the higher the share, the softer the cream stays. With a master batch, the stock solution holds the sodium hydroxide and the potassium hydroxide is weighed dry. The shaving soap and cream soap guides go into the ratios.

Storage and carbonation

Potassium hydroxide is deliquescent: it can pull enough water from the air to dissolve in it (OECD), and it readily takes up carbon dioxide as well. NIOSH notes that it heats up as it absorbs both. Flakes left open turn sticky, then clump, and every step leaves less hydroxide in each gram. Carbon dioxide turns part of it into potassium carbonate, which does no saponifying; even new food-grade material may carry 3.5 % of it.

Keep it in its original, well-closed container, dry and separated from food, strong acids and metals, as the chemical safety card asks, labelled, out of reach of children, and stored apart from sodium hydroxide so the two are never confused. Close the tub the moment you have weighed. Flakes that stick together or look wet have already changed; measure their purity before you trust a recipe to them, or replace them.

Buying it, and the law

Buy flakes labelled potassium hydroxide with a stated purity, from a soapmaking or chemical supplier. It travels as a dangerous good: UN 1813, class 8, packing group II.

In the EU the CLP label carries the corrosion and exclamation-mark pictograms, the word Danger, H302 and H314, and packs sold to the public need a child-resistant fastening and a tactile warning of danger. In the United States, household products with 10 % or more of free potassium hydroxide must carry the signal word "POISON", and dry ones at that strength need child-resistant packaging (16 CFR 1500.129 and 1700.14). In Great Britain the Offensive Weapons Act 2019 lists sodium hydroxide among its corrosive products but not potassium hydroxide, so its age limit and its rule against delivering to homes do not cover it.

Spills and disposal

Sweep dry flakes into a covered plastic container, gloves on, as the chemical safety card advises, and wipe the spot with plenty of water straight away: flakes left on a damp worktop draw water from the air and become a small pool of strong lye. Mop up solution with paper towels and wipe down with water; a final wipe of diluted vinegar on a hard surface is fine once the lye has gone, as for caustic soda. Leftover solution and unwanted flakes go the same way as sodium hydroxide: kept, labelled, for the next batch, or taken to a household hazardous-waste collection, not poured down the sink. Raw paste is caustic until it has cooked, so scrape tools into the bin before washing them.

Common mistakes

The commonest mistake is leaving the purity at 100 % for 90 % flakes, which gives a cloudy, greasy paste. Close behind come choosing the wrong alkali in the calculator or weighing from the wrong tub, and neutralising a paste by habit when the recipe had no spare lye, which turns soap back into fatty acid and clouds it. Diluting by volume, or by "one part paste", without weighing the paste after the cook gives a soap of unknown strength, and diluting with tap water adds scum to the bottle.

The last is thinking of potassium hydroxide as the milder alkali because it makes a gentle soap. It is just as corrosive, and it gets hotter as it dissolves.