Baking soda (sodium bicarbonate)
A gentle scrub. Soap Queen supplier or guide
Opens the calculator’s starter recipe, 1000 g of oils, with this additive at the dose it fills in, ready to change. The batch opens as hot process, because it usually goes in after the cook.
What it is
Baking soda is sodium bicarbonate, sold as a leavening for baking. It is a mild alkali, pH 8.3 in a 1 % solution, and free lye turns it into sodium carbonate.
USDA FoodData Central primary source, Arm & Hammer primary source, OpenStax primary source
How much
- 1 tsp per pound of soap in melt and pour or rebatch filled in Bramble Berry supplier or guide
- 4.7 % of the batch in a controlled test Crafter’s Choice supplier or guide
- 1 tbsp per pound of soap in hot process (from a published recipe) Grosso, The Everything Soapmaking Book, 3rd ed. primary source
- up to 43.5 % of the batch in rinse-off products, the highest reported use: not a recommendation CIR primary source
The calculator fills in 1 tsp per pound of soap: 14.6 g for the 1000 g of oils in its starter recipe.
When and how
- Usually: after the cook.
- Also used: at light trace.
- Stir it in as it is.
- Not before the cook: there it reacts with the lye and uses it up.
- Not recommended in cold process.
Crafter’s Choice supplier or guide, Bramble Berry supplier or guide, Grosso, The Everything Soapmaking Book, 3rd ed. primary source
What it does to the recipe
- Every 100 g takes 47.61 g of sodium hydroxide while there is free lye, and the calculator adds that to the lye. After the cook none is left, so it takes none.
OpenStax primary source, Simmons & Appleton primary source
Worth knowing
- Caution: In a test at 4.7 % of the batch, saponification went fine but the lather suffered. Use it after the cook, once no lye is left for it to take. Bramble Berry supplier or guide, Crafter’s Choice supplier or guide
- Too much can bloom on the surface as a white powder. Simmons & Appleton primary source
Spoons and drops
- 1 tsp ≈ 4.6 g (4.6–4.88 g)
- 1 tbsp ≈ 13.8 g (13.8–14.6 g)
- 1 cup ≈ 221 g (221–234 g)
USDA FoodData Central primary source, Arm & Hammer primary source
INCI name
Sodium Bicarbonate
- CAS number
- 144-55-8
- EC number
- 205-633-8
- In the finished bar
- with NaOH, Sodium Carbonate; with KOH, Sodium Carbonate, Potassium Carbonate
It changes only while there is free lye: stirred in after the cook, it stays as it went in.
Guides that discuss it
Sources
Soap Queen, "Sunday Night Spotlight: Baking Soda": it "adds gentle exfoliation to soap and scrubs"; in their tests "the baking soda and cold process soap did not work well", but "adding baking soda to rebatch worked wonderfully", where it "creates a scrubby bar that gives gentle exfoliation".
OpenStax, Chemistry 2e, section 18.6: a bicarbonate reacts with a hydroxide to give a carbonate and water. NaHCO₃ + NaOH → Na₂CO₃ + H₂O; 39.997 ÷ 84.007 = 0.4761 g NaOH per gram of baking soda.
W. H. Simmons and H. A. Appleton, The Handbook of Soap Manufacture (1908): bicarbonate "converts the caustic alkali into carbonate", and excess carbonate effloresces; the water must be "free from calcium and magnesium salts"; sugar "may caramelize if the temperature gets too high".
Crafter’s Choice, Kevin Dunn, "Baking soda: can it be used in cold process soap?": at 7 g, 4.7 % of the batch, "neither additive had an adverse effect on saponification ... lather performance suffers"; use it "after the cook (after the lye has been consumed)".
Bramble Berry, sodium bicarbonate: "1 teaspoon per pound of product. Not recommended for cold process soap."
Alicia Grosso, The Everything Soapmaking Book, 3rd edition (Adams Media, 2013). Chapter 8, hot process: recipes on a "2-pound batch recipe of your choice" that each "YIELDS 3 POUNDS", the addition stirred "into the neutral soap" after the cook: "3 tablespoons baking soda" and "2 tablespoons lemon zest" (Lemon/Baking Soda Kitchen Soap), "2 tablespoons yellow cornmeal", "1 tablespoon dried lavender flowers", "1 tablespoon mint, dried and ground", "1 tablespoon dried yarrow"; "You can add liquid glycerin to hot-process soap to get it to flow more easily from the mold. Use 1 tablespoon for each pound of oils in the batch." Chapter 9, transparent soap: 19.5 oz of oils with "10 ounces (plus 2 ounces more if needed for clarity) 90 percent ethanol (190 proof vodka)" (190 proof is 95 %) and "5 ounces sugar"; "Alcohol fumes are highly flammable." Chapter 11, Lavender Oat Float: "1 tablespoon dried lavender flowers" for "1 pound casting soap".
Cosmetic Ingredient Review, sodium carbonates (2024): sodium bicarbonate up to 43.5 % in bath soaps and detergents.
The highest concentration reported to CIR in industry surveys: what has been used, not a recommendation.
USDA FoodData Central, food 175040: Leavening agents, baking soda.
Arm & Hammer Performance Products, sodium bicarbonate safety data sheet (2025): density 62 lb/ft³ (0.99 g/cm³); pH 8.3 at 1 %.