Sodium Citrate and Citric Acid in Cold Process Soap: Doses and Lye Adjustment

Short answer: in cold process soap, most makers use citric acid at 1-2% of oil weight (10-20 g per 1,000 g of oils) and add 0.624 g of extra sodium hydroxide for every gram of citric acid. Sodium citrate goes in at about 1.3-3.9% of oil weight and needs no extra lye.

Both ingredients end up doing the same job in the bar. Citric acid reacts with lye to form sodium citrate, and citrate is a chelator: it binds the calcium and magnesium in hard water that turn soap into sticky soap scum, and the trace metals linked to orange rancidity spots. The difference is the lye maths. This guide gives the doses, the adjustment formula and a worked 1,000 g example.

How much citric acid or sodium citrate per 1,000 g of oils?

Additive	Typical dose (% of oil weight)	Per 1,000 g of oils	Extra NaOH needed
Citric acid (anhydrous)	1-2% (range seen: 0.1-3%)	10-20 g	0.624 g per g of citric acid
Sodium citrate	1.3-3.9%	13-39 g	None
Potassium citrate (for comparison)	1.6-4.8%	16-48 g	None

Use the higher end of each range for hard water, the lower end for soft water. The lowest citric acid dose, around 0.1%, is the one cited for protection against rancidity alone; the 1-3% range targets soap scum as well. Above about 2% citric acid, some makers see a layer of fine white citrate crystals on the bars as they age. The crystals wash off, but they look untidy.

How much extra lye does citric acid consume?

Citric acid has three acid groups, so one molecule neutralizes three molecules of sodium hydroxide. With molecular weights of 192.12 g/mol for anhydrous citric acid and 40.00 g/mol for NaOH, the ratio is 3 × 40.00 ÷ 192.12 = 0.624 g of NaOH per gram of citric acid. For potassium hydroxide soap the figure is 0.842 g of KOH per gram.

If you skip the adjustment, the citric acid uses up lye meant for the oils, and your superfat rises by an amount you did not plan. The formula is simple:

• Extra NaOH (g) = citric acid (g) × 0.624
• Total NaOH (g) = NaOH from your soap calculator + extra NaOH

The 0.624 factor applies to anhydrous citric acid. The monohydrate form contains water, so it neutralizes a little less lye per gram (about 0.571 g NaOH per gram, from its 210.14 g/mol molecular weight).

Worked example: 1,000 g of oils

1. Run your recipe in a lye calculator. Say it calls for 140 g of NaOH at your chosen superfat (your own number will differ).
2. Choose 2% citric acid: 1,000 g × 2 ÷ 100 = 20 g of citric acid.
3. Extra lye: 20 g × 0.624 = 12.5 g of NaOH.
4. Total lye: 140 g + 12.5 g = 152.5 g of NaOH.

With sodium citrate instead, you would simply add 13-39 g to the batch and leave the lye at 140 g. Note that our sodium citrate is the dihydrate form; it carries water of crystallization, so it holds about 12% less citrate per gram than anhydrous sodium citrate (294.10 vs 258.07 g/mol). Dose toward the middle of the range and adjust from there.

What are the safety rules for working with lye?

Sodium hydroxide is corrosive. The Canadian Centre for Occupational Health and Safety (CCOHS) recommends:

• Chemical safety goggles, with a face shield over them when splashing is possible;
• Protective gloves (nitrile, neoprene, butyl or PVC of adequate thickness), long sleeves and an apron;
• Adding lye slowly to cold water, never water onto lye: contact with water produces heat and can spatter;
• No aluminum containers or tools: lye reacts with aluminum and releases hydrogen gas. Use stainless steel or suitable heavy plastic.

Work in a ventilated area, keep children and pets out of the room, and label every container.

How do you add citric acid or sodium citrate to the batch?

1. Put on goggles and gloves before weighing anything.
2. Weigh the citric acid (or sodium citrate) and the adjusted lye separately.
3. Dissolve the citric acid fully in the room-temperature water you will use for the lye solution.
4. Add the NaOH slowly to that solution while stirring constantly. The acid and base react: if the mixture foams or bubbles, stop, keep stirring and let it settle before adding more.
5. Alternative: dissolve the citric acid or sodium citrate in about twice its weight of water and stick-blend that solution into the oils before adding the lye solution. This is the usual method for sodium citrate.
6. Continue with your normal cold process steps, mould, and cure as usual.

Citric acid or sodium citrate: which should you choose?

• Citric acid is the more common pantry item and is also used for bath bombs and descaling, but it requires the lye adjustment every time.
• Sodium citrate skips the lye maths and the foaming step, so there is less room for error, at the cost of a slightly higher dose by weight.
• Lemon or lime juice also brings citric acid (fresh juice is roughly 5% citric acid by weight), but its content varies, so the powder is easier to dose precisely.

Sources

• Soapy Stuff (DeeAnna Rowe) — Citric acid and citrus juice in soap: dosage range, crystal threshold, NaOH and KOH neutralization factors, mixing methods.
• Soapy Stuff — Sodium citrate or potassium citrate in soap: citrate doses and why no extra lye is needed.
• CCOHS — Sodium hydroxide chemical profile: protective equipment, mixing with water, reaction with aluminum.
• PubChem — Citric acid: molecular weight used for the 0.624 factor.

LiquidShop supplies food-grade anhydrous citric acid and sodium citrate from Canadian stock, in formats from 1 kg. Formats and specifications are on the sodium citrate product page.