Citric acid is the acid in the juice of citrus fruits, including lemons, limes, and oranges. It can also be purchased as a white powder that is often used to acidify tomatoes when home canning. Citric acid powder is also used to make "bath bombs" and other bath and body products.
What does citric acid do in soap?
Citric acid reacts with lye to make sodium citrate or potassium citrate, depending on whether your lye is NaOH or KOH. Either kind of citrate reduces the amount of sticky soap scum created when lye soap is used in water by chelating (binding up) the metals that create the scum and reduces the likelihood of rancidity (DOS) in the soap.
Citric acid is often used rather than citrate because citric acid is often easier to find and cheaper to buy. It is also easy to make citrate from citric acid and lye.
How much citric acid powder can be used?
The amounts I have seen can range from 1 g to 30 g citric acid powder for every 1,000 g fats (0.1% to 3% of total oil weight).
The lowest dosage is recommended by Kevin Dunn, author of the book Scientific Soapmaking, specifically for protecting the soap from DOS (dreaded orange spots, also known as rancidity).
Higher dosages -- from 1 to 3% -- will reduce soap scum formation as well as protect against DOS. Use more for hard water, less for soft.
How much citric acid powder is typically used?
Many soap makers use 10 g to 20 g citric acid powder for every 1,000 g fats (1% to 2% based on total fat weight).
Using more than about 2% citric acid may cause a layer of tiny white crystals to form on the outside of your soap as it ages. These citrate crystals will easily wash off and are harmless, but look unsightly. In informal discussions, it appears that a dosage of 1.5% to 2% or less does not cause this issue.
How to calculate the weight of citric acid powder to add to soap?
1. Decide the percent of citric acid to use. I suggest starting with 2% of the total fat weight (20 grams citric acid per 1000 grams of fats).
2. Calculate the weight of citric acid powder
Citric acid weight, grams = Fat weight, grams X 2 / 100
If you are using a dosage other than 2% of total oil weight, replace the "2" in the formula above with the percentage you are actually using.
How much citric acid is in citrus juice?
An alternative to using citric acid powder is to use citrus juice. Lemons and lime juices have more citric acid than citrus juices such as orange or grapefruit. Here are some estimates to use:
"...[Fresh] [l]emon juice and lime juice are rich sources of citric acid, containing 1.44 and 1.38 g/oz, respectively. [About 5% by weight.]
"Lemon and lime juice [reconstituted from] concentrates contain 1.10 and 1.06 g/oz, respectively. [About 3.9% by weight.]..." (1)
The authors of this study also found fresh and reconstituted orange juice contain about 2% citric acid by weight and reconstituted grapefruit juice contained about 2.7% by weight.
These measurements were based on only a few samples of each type of juice. Note: My comments in the quote above are in brackets [ ].
How to calculate the weight of citric acid in juice?
1. Estimate the percentage of citric acid in the juice:
Citric acid in fresh lemon or lime juice = 5% by weight
Citric acid in reconstituted lemon or lime juice = 3.9% by weight
Citric acid in reconstituted grapefruit juice = 2.7% by weight
Citric acid in orange juice (fresh or reconstituted) = 1.4% by weight
2. Decide the total weight of juice you want to add to your recipe.
3. Calculate the weight of citric acid in this juice:
Citric acid, grams = (Weight of juice, grams) X (Percent citric acid in juice) / 100
Example: I want to add 120 grams of reconstituted lime juice to my soap recipe. I estimate the citric acid in this juice is about 3.9% by weight. About how much citric acid is in this juice? Calculate the answer this way:
Citric acid, grams = 120 X 3.9 / 100 = 4.7 grams
How much lye does citric acid neutralize?
10 g citric acid neutralizes 6.24 g NaOH. 10 g citric acid neutralizes 8.42 g KOH.
How to calculate the extra lye to react with citric acid?
Now that you know how much citric acid you want to add to the soap, you need to decide how much extra lye to add react with this acid. If you do not add any extra lye, the citric acid will increase the superfat in your soap.
Sodium hydroxide, NaOH
1. NaOH for citric acid, grams = (Citric acid weight, grams) X 6.24 / 10
2. Total NaOH, grams = NaOH for citric acid, grams + NaOH for saponification, grams
Example: From the previous example, I estimate the reconstituted lime juice I want to use will add 4.7 grams of citric acid to my soap. If I am making soap with NaOH (sodium hydroxide), the extra NaOH needed to react with the citric acid will be:
NaOH for citric acid, grams = 4.7 X 6.24 / 10 = 2.9 grams
Potassium hydroxide, KOH
1. KOH for citric acid, grams = Citric acid, grams X 8.42 / 10
3. Total KOH, grams = KOH for citric acid, grams + KOH for saponification, grams
Example: As before, I estimate the reconstituted lime juice I want to use will add 4.7 grams of citric acid to my soap. If I am making soap with KOH (potassium hydroxide), the extra KOH needed to react with the citric acid will be:
KOH for citric acid, grams = 4.7 X 8.42 / 10 = 4 grams
Making a dual-lye recipe?
For recipes that use both NaOH and KOH as well as citric acid, please see my tips here....
How should I add citric acid to my soap?
Method 1: Dissolve citric acid powder in about 2 times its weight of room temperature water. Stick blend the citric acid solution into the oils.
Method 2: Dissolve the citric acid powder in the ROOM TEMPERATURE water you will be using to make the lye solution. When the powder is fully dissolved, SLOWLY add the NaOH to the water while STIRRING CONSTANTLY. If the mixture starts to foam or bubble, stop adding NaOH, keep stirring, and let the mixture settle down. Then SLOWLY add more NaOH while stirring.
CAUTION: GO SLOW when directly mixing NaOH and citric acid. If you mix these ingredients together too quickly, the mixture may bubble up and overflow the container or spatter into the air.
Method 3: If using citrus juice as a full replacement for water, use the refrigerator-cold or room temperature juice to make the lye solution. Make the lye solution per Method 2.
Method 4: If using citrus juice as a partial replacement for some of the water, mix the juice with the water and then use the juice-water mixture to make the lye solution per Method 2.
References
(1) Kristina L. Penniston, et al. Quantitative Assessment of Citric Acid in Lemon Juice, Lime Juice, and Commercially-Available Fruit Juice Products. Journal of Endourology. 2008. Vol 22, issue 3, pages 567–570. Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2637791/
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