Sodium hydroxide

What it is and what it does

Sodium hydroxide (NaOH), sold as lye or caustic soda, is a white solid and one of the strongest common alkalis. In soapmaking it supplies the hydroxide that splits each fat molecule into three molecules of soap and one of glycerin. The sodium stays with the soap, and sodium soaps are what let a bar set hard.

The reaction runs in fixed proportions, which is why lye is weighed to the gram. An oil's saponification value says how much alkali it takes. Those values are published in milligrams of potassium hydroxide per gram of fat, so the calculator converts them for sodium hydroxide with the ratio of the two molar masses, 39.997 to 56.106.

Soapmakers weigh a little less lye than the oils could use. That margin is the superfat, or lye discount, and it makes the lye the ingredient that runs out first. At the calculator's default of 5 %, 1000 g of olive oil takes 128.7 g of sodium hydroxide: enough for 95 % of the oil, with the rest left in the bar as oil. Once the reaction is over there is no lye left without a partner, and what remains is soap, glycerin, the superfat and some water.

The reaction is fast at first and slow at the end. In Kevin Dunn's studies, with the soap kept warm for its first four hours, most of it was over within a day, and most of his cured bars held no free alkali at all when titrated. That is why fresh soap is handled in gloves for its first days and a cured, correctly calculated bar is not.

Forms, grades and purity

The solid comes as flakes, as small round beads (sold as pearls or microbeads) and as granules. NIOSH describes it as flakes, beads or granules, and the EU food-additive specification adds pellets, sticks and fused masses. The form changes how it pours and nothing about what it does. Whatever the form, weigh it: a recipe's lye is a weight, never a volume.

Food grade sodium hydroxide, the additive E 524, must be at least 98.0 % alkali as NaOH and may hold no more than 0.5 % sodium carbonate, under the EU specification in Regulation (EU) No 231/2012. The calculator's note gives 98 to 100 % for food and technical grades, and a manufacturer's certificate that Dunn showed to soapmakers read 99.9 %.

That is why the calculator assumes 100 %, and why the assumption is a safe one. If your lye is really 98 % and you leave the setting at 100 %, our everyday bar ends up at a 6.9 % superfat instead of 5 %: a little softer and milder, never caustic. The error to avoid is the other way round, a purity entered lower than the lye really has, because the calculator then weighs out more. Change the setting only when a label, a certificate of analysis or your own titration gives you a figure.

Drain cleaner is usable only if its label says 100 % sodium hydroxide, or 100 % lye, and nothing else, as Anne Watson puts it. Many drain cleaners are blends. Anne-Marie Faiola warns that some granular ones contain bits of aluminium, which lye attacks, giving off hydrogen, a flammable gas; others carry ingredients that, in Alicia Grosso's words, are not at all suitable for soapmaking. A blend brings its other chemicals into the soap and can react in the jug in ways plain lye does not, so never use one. Plain sodium hydroxide is white or nearly white; specks, a tint or anything metallic in the tub means it goes back.

Dissolving it

Always add the lye to the water, slowly, never the water to the lye; the International Chemical Safety Card says the same. Dissolving sodium hydroxide gives off 44.51 kJ per mole, about 1.1 kJ per gram, and the solution takes all of it. Fresh lye solution passes 90 °C within moments: David Fisher, Catherine Failor and Anne-Marie Faiola each give about 93 °C (200 °F). Water poured onto a heap of lye meets far more lye than it can carry the heat away from, so it boils where it lands and spits.

Start with cold water. Grosso warns that lye added to hot liquid takes it "dangerously close to or over the boiling point in no time", and NIOSH notes that the heat can be enough to set fire to combustible material nearby. For the first minute the jug gives off steam that carries a fine mist of lye. Susan Miller Cavitch found the fumes overwhelming after about ten seconds and stepped out of the room for two or three minutes; turning your face away and working by an open window or under a cooker hood does the same job. It takes very little of that mist to matter: the US workplace limit is 2 mg of sodium hydroxide in a cubic metre of air.

Ice and frozen liquids keep the heat down. One of Faiola's ways to cool lye is to weigh ice cubes as the recipe's liquid and dissolve the lye on them. Milk, tea and other liquids with sugars or proteins scorch in hot lye, so Grosso and Faiola freeze them to a slush and sprinkle the lye in a spoonful at a time. Whatever is frozen counts as the recipe's water. Ice dropped into a finished solution is extra water the recipe did not ask for; to cool a jug, stand it in a basin of cold or iced water, as Watson does.

Lye attacks aluminium, tin, lead and zinc, giving off hydrogen as it does, and OxyChem's handbook adds copper and its alloys, brass and bronze. So no aluminium pans or foil, no galvanised buckets and no brass spoons. Stainless steel is the metal to use: OxyChem rates 304L and 316L stainless steel for caustic soda up to 200 °F (93 °C).

Among plastics, polypropylene (PP, recycling code 5) is the jug to mix in: a labware maker rates it excellent against 50 % sodium hydroxide and usable to 135 °C. High-density polyethylene (HDPE, code 2) resists lye just as well but has a heat deflection temperature of 65 °C, so it suits dry lye and cooled solution better than a fresh, hot one. Polycarbonate and PETG, the clear "unbreakable" plastics, are damaged by strong lye at once, and a drinks bottle is never a lye container.

Heat-proof glass is where the books disagree. Cavitch mixed her lye in heat-resistant glass, Watson allows oven-safe glass and Fisher a sturdy glass jug, while Grosso stopped recommending it because heat-proof measuring jugs have shattered, and our lye safety guide calls glass better avoided. Both sides have a point. Borosilicate glass handles caustic solutions up to 30 % at room temperature without difficulty, its maker says, but corrosion rises with temperature, and fresh lye solution is both stronger and far hotter than that. Hot alkali slowly etches glass, and a hot glass jug stood in cold water to cool can crack. Polypropylene has neither problem.

Concentration and the water it needs

Water takes no part in the reaction. It dissolves the lye and carries it to the oils, and most of it leaves the bar as it cures. How much to use is a choice. The calculator lets you state it as a lye concentration (the lye's share of the lye solution), a water-to-lye ratio or water as a percentage of the oils, and shows all three at once. It starts at a 33 % solution and marks anything above 40 % as strong and below 25 % as dilute. The water guide covers what each step does to trace, firmness, cure and soda ash.

The floor is set by how much lye water can hold. Sodium hydroxide saturates at about 52 % by weight at 20 °C, some 108 g in 100 g of water (OECD). With less water than that, some lye cannot dissolve and ends up as caustic grains in the bars, and the calculator warns whenever the water weighed with the lye is less than the lye's weight divided by 1.08. In practice a 50 % solution, equal weights of lye and water, is as strong as anyone should go, and it begins to crystallise below about 12 °C (OxyChem). A master batch kept in a cold room can drop crystals, and the liquid above them is then weaker than its label.

A stronger solution runs hotter as it dissolves and, as Grosso notes, burns skin faster. Milk and other liquids count only for the water they carry, and a liquid held back until trace leaves the lye dissolved in less water than the recipe's figure suggests. The calculator shows the concentration as mixed and warns when it goes past the limit.

Storing it, and knowing when it has gone off

Keep sodium hydroxide in its original container, labelled, with the lid screwed down between scoops. The chemical safety card asks for it to be kept dry and well closed, separated from food, strong acids and metals. Store it out of reach of children and pets and apart from potassium hydroxide, and never decant it into a food tub or a drinks bottle.

Lye pulls water out of the air, and it takes up carbon dioxide too, which turns part of it into sodium carbonate; the EU food-additive specification spells out that reaction. Both leave less hydroxide in every gram you weigh. Water alone makes the soap lye-light and softer, as Dunn found when he simulated caustic soda that had drifted to 95, 90, 85 and 80 % purity. Carbonate cannot be taken out again, and it goes into the batch with the lye. Soap factories once added carbonate on purpose to stiffen cheap soap, and Simmons and Appleton warned that too much made the soap brittle and made it bloom white on the surface. The white film on a fresh loaf, soda ash, usually forms another way, from unreacted lye at the surface meeting the air, and the soda ash guide deals with it.

Fresh lye pours as separate, dry beads or flakes. Lye that has taken up water clumps and cakes; Cavitch, soaping in humid Memphis, bought hers in small containers so she never had to dig a measurement out of a solid lump. Carbonate is harder to see. Dunn's check is to dissolve a sample into a 50 % solution, goggles and gloves on: sodium carbonate does not dissolve in strong lye, so contaminated lye shows white specks floating on the surface. A few are unavoidable; a lot means the lye is unfit for soap. If a tub has been left open or has caked, measure its purity with the citric acid titration in the NaOH and KOH guide, or replace it.

Buying it, and the law

Buy it labelled as sodium hydroxide, as flakes or beads, from a soapmaking or chemical supplier, or as a food or technical grade. It is a dangerous good in transport, UN 1823, class 8 (corrosive), packing group II, and that classification decides how it may be packed and shipped.

In the EU a tub sold to the public carries the CLP label: the corrosion pictogram, the word Danger, H314 and precautionary statements. Because it is a category 1 skin corrosive, the pack must also have a child-resistant fastening and a tactile warning of danger for people with impaired sight (CLP Annex II, sections 3.1 and 3.2).

In England, Wales and Scotland, sodium hydroxide at more than 12 % by weight has been a "corrosive product" under the Offensive Weapons Act 2019 since 6 April 2022. It is an offence to sell it to anyone under 18, and a seller who sells it at a distance, online or by phone, commits an offence by delivering it, or arranging its delivery, to residential premises or a parcel locker. A seller there cannot lawfully send it to a home address.

In the United States, household products with 10 % or more of free sodium hydroxide must carry the signal word "POISON", a rule that goes back to the Federal Caustic Poison Act, and dry household products at that strength need child-resistant packaging (16 CFR 1500.129 and 1700.14). Other countries set their own rules. Buy from a supplier who can send it to you legally, and keep the label and the safety data sheet.

Spills and disposal

Sweep up dry beads with a dustpan and brush, gloves on, into a dry plastic container with a lid, as the chemical safety card advises, then wipe the spot with plenty of water. Beads bounce and cling by static, so check the floor, the scale and your sleeves before the gloves come off.

Soak up spilled solution with paper towels, bin them, and wipe the surface down with plenty of water. Neutralising is how industry finishes a clean-up, not how it starts one: OxyChem collects the spilled material first, neutralises only the residue with dilute acid and then flushes the area with water, and its handbook warns that caustic solutions give off considerable heat as they are neutralised. At home that means a wipe of diluted vinegar on a hard surface after the lye has gone, never vinegar poured onto a puddle of hot solution.

The easiest lye to dispose of is the lye you never mixed, so make only what the recipe needs. Leftover solution keeps for the next batch in a tightly closed polypropylene or HDPE bottle, labelled as lye with its strength and the date; the calculator's master batch setting weighs it out. Solution you will not use, and dry lye you no longer want, go to a household hazardous-waste collection, not down the sink and not into the bin: water companies ask for chemicals to go there, and some recycling centres take them only in their original, labelled container, so ask before you bring a jar of homemade solution. Raw soap batter is still caustic and can set in the pipe (Grosso), so scrape it into a spare mould or the bin instead.

Sodium hydroxide or potassium hydroxide

Both alkalis do the same work, molecule for molecule. The difference is the soap they leave. Sodium soaps of the saturated fatty acids crystallise at room temperature and set into bars; their potassium versions stay dissolved, which is why potassium hydroxide makes liquid and soft soap (McBain and Sierichs, 1948). A dual-lye recipe does part of the saponification with each: a small potassium share for a bar that lathers more readily, a larger one for shaving soap, and mostly potassium for cream soap.

They look almost the same and are not interchangeable by weight: 56.1 g of potassium hydroxide does what 40.0 g of sodium hydroxide does. Weigh sodium hydroxide to a potassium hydroxide figure and a bar gets about half as much lye again as its oils can use. For 1000 g of olive oil that is 200.6 g in place of 128.7 g, a superfat of about −48 %, and a caustic bar. Label both tubs, keep them apart, and read the name of the lye above the figure in the calculator before you weigh.

Common mistakes and myths

"Handmade soap still has lye in it." A correctly calculated and made bar has none left: the lye is weighed short of what the oils could take and is used up in the reaction, as Dunn's titrations of cured bars show. Every true soap is made with lye, melt-and-pour base included; the question is whether any is left. Soap standards measure exactly that. The East African toilet soap standard, for one, allows at most 0.1 % free caustic alkali and sets no pH limit, because every true soap reads alkaline, about pH 9 to 10.

"Touch it to your tongue." The zap test puts a possibly caustic bar in your mouth, catches only a gross excess, and can be fooled by scents such as cinnamon and clove, which set off the same burning-sensation receptor. Phenolphthalein, a calibrated pH meter or a laboratory titration answer the question without the risk; the soap pH guide explains how to read each.

The mistakes that cause real harm are few and well known: water poured onto lye; a jug of solution left unattended or unlabelled; aluminium anywhere near it; old, caked lye weighed as if it were fresh; the wrong alkali chosen in the calculator; a purity entered lower than the lye really has; and vinegar on a burn in place of running water.