Citric acid (anhydrous)
Binds hard-water minerals, for less soap scum.
Opens the calculator’s starter recipe, 1000 g of oils, with this additive at the dose it fills in, ready to change.
What it is
Citric acid (C6H8O7) is an acid sold dry, as granules or a finer granular powder, and graded as a food additive under a JECFA specification. This is the anhydrous form, with no water of crystallisation; the monohydrate is the same acid carrying 7.5–8.8 % water.
JECFA primary source, Cargill primary source, USDA FoodData Central primary source
How much
- 1–2 % of the oils, range 0.1–3 filled in Classic Bells supplier or guide
- up to 10 % of the batch in rinse-off products, the highest reported use: not a recommendation CIR primary source
The calculator fills in 1 % of the oils: 10 g for the 1000 g of oils in its starter recipe.
When and how
- Usually: in the water, before the lye.
- Also used: into the oils.
- Weigh it dry and dissolve it in the recipe’s own water: no extra water.
Classic Bells supplier or guide
What it does to the recipe
- Every 100 g takes 62.45 g of sodium hydroxide, which the calculator adds to the lye.
Dunn, Scientific Soapmaking primary source, JECFA primary source, Stoichiometry primary source
Worth knowing
- Caution: Add the lye slowly: the acid fizzes and heats where the two meet, and can bubble over. Classic Bells supplier or guide
- More than about 2 % can leave a layer of tiny white crystals on the soap. Classic Bells supplier or guide
Spoons and drops
- 1 tsp ≈ 4 g (4–4.42 g)
- 1 tbsp ≈ 12 g (12–13.3 g)
- 1 cup ≈ 192 g (192–212 g)
USDA FoodData Central primary source, Cargill primary source
INCI name
Citric Acid
- CAS number
- 77-92-9, 5949-29-1
- EC number
- 201-069-1
- In the finished bar
- with NaOH, Sodium Citrate; with KOH, Potassium Citrate
Guides that discuss it
Sources
Kevin M. Dunn, Scientific Soapmaking (Clavicula Press, 2010): "666 g of sodium hydroxide to react with 1000 g of acetic acid" (p. 165); vinegar is about 5 % acetic acid (p. 175); 10 g of NaOH neutralise 16.01 g of citric acid (pp. 354–355); "The most potent preservative was edta ... when added to the lye at a rate of as little as 0.3 ppt" (p. 285).
JECFA, Citric acid, FAO JECFA Monographs 16 (2014): "Each ml of 1 N sodium hydroxide is equivalent to 64.04 mg of C6H8O7"; monohydrate 210.14 g/mol with 7.5–8.8 % water.
Neutralisation stoichiometry with IUPAC 2013 molar masses: grams of NaOH per gram = protons × 39.997 ÷ molar mass. Disodium EDTA dihydrate (372.24 g/mol) takes 2; sodium bicarbonate (84.007 g/mol) 1; magnesium sulfate heptahydrate (246.47 g/mol) 2; borax decahydrate (381.37 g/mol) 2, to sodium metaborate; malic acid (134.087 g/mol) 2; citric acid (192.124 g/mol) 3. An acidity given in milliequivalents per litre takes that many millimoles of NaOH per litre.
Classic Bells (DeeAnna), "Citric acid": "1 g to 30 g citric acid powder for every 1,000 g fats (0.1% to 3%...)"; "Many soap makers use 10 g to 20 g"; dissolve it in the room-temperature water, then "SLOWLY add the NaOH", or "the mixture may bubble up and overflow"; above about 2 % it can leave "tiny white crystals".
Cosmetic Ingredient Review, citric acid and its salts (2012): up to 10 % in rinse-off products.
The highest concentration reported to CIR in industry surveys: what has been used, not a recommendation.
USDA FoodData Central, food 628368: a branded citric acid label (1.0 g = 1/4 tsp).
Cargill, citric acid anhydrous technical data sheet: bulk density 56 lb/ft³ granular and 54 lb/ft³ fine granular (0.87–0.90 g/mL).