Citric acid (monohydrate)
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 monohydrate is citric acid crystallised with one molecule of water: 210.14 g a mole against 192.12 g for the anhydrous acid. Water is 7.5–9 % of what you weigh, so a gram of it carries a little less acid.
JECFA primary source, Scharlab primary source, Stoichiometry 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 57.1 g of sodium hydroxide, which the calculator adds to the lye.
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 ≈ 4.44 g (3.94–4.93 g)
- 1 tbsp ≈ 13.3 g (11.8–14.8 g)
- 1 cup ≈ 213 g (189–237 g)
Scharlab 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
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.
Scharlab, Citric acid monohydrate, technical data sheet AC0720: bulk density about 800–1000 kg/m³; water (Karl Fischer) 7.5–9.0 %.