Citric acid and Citrus juice in soap
What does it do in soap? Citric acid is the acid in the juice of citrus fruits, including lemons, limes, and oranges. It can also be purchased as a pure powder used to acidify tomatoes when home canning and to make "bath bombs" and other bath and body products.
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 it is easier to find and cheaper to buy citric acid. It is also easy to make citrate from citric acid and lye.
How much citric acid is in citrus juice? Lemons and lime juices have more citric acid than other citrus juices such as orange or grapefruit, but the actual amount of citric acid in citrus juices will vary. This research report provides some reasonable numbers to use for estimating the citric acid content:
"...[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.]..." (Link to source...)
The authors found the citric acid about 2% by weight in samples of fresh and reconstituted orange juice and about 2.7% by weight in reconstituted grapefruit juice.
These measurements were based on only a few samples of each type of juice. Note: My comments in the quote above are in brackets [ ].
Here is how to estimate the citric acid in citrus juice:
First choose an appropriate percentage for the citric acid in your juice:
Percent citric acid in fresh lemon or lime juice = 5% by weight
Percent citric acid in reconstituted lemon or lime juice = 3.9% by weight
Percent citric acid in reconstituted grapefruit juice = 2.7% by weight
Percent citric acid in orange juice (fresh or reconstituted) = 1.4% by weight
Estimate the citric acid weight:
Citric acid in juice, 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 in lime juice, grams = 120 X 3.9 / 100 = 4.7 grams
How much citric acid should I use? Typical dosage is 10 g to 30 g citric acid powder for every 1,000 g fats (1% to 3% ppo). Use more for hard water, less for soft.
The dosage range is 0.1% to 3% ppo. 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). The medium to higher dosages will reduce soap scum formation as well as protect against DOS.
Using citric acid above 2% 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 much lye does it neutralize? 10 g citric acid neutralizes 6.24 g NaOH. 10 g citric acid neutralizes 8.42 g KOH. When using citric acid in your recipe, add the appropriate extra weight of lye needed to react with the acid. If you do not add any extra lye, the acid will increase the superfat in your soap.
When making soap using NaOH (sodium hydroxide), calculate the extra NaOH needed this way:
Decide the percent of citric acid to use. If you are not sure, I suggest 2% ppo (20 grams citric acid per 1000 grams of fats).
Find how many grams of fat are in the soap recipe.
Citric acid weight, grams = Fat weight, grams X 2 / 100
If you are using a dosage other than 2% ppo, replace the "2" in the formula above with the percentage you are actually using.
NaOH for citric acid, grams = Citric acid weight, grams X 6.24 / 10
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
When making soap using KOH (potassium hydroxide), follow the method given above, but use the following formula to calculate the KOH needed:
KOH for citric acid, grams = Citric acid, grams X 8.42 / 10
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 it to my soap? Dissolve citric acid in about 2 times its weight of water. Stick blend the citric acid mixture into the oils. Use citrus juice as-is. If using fresh or reconstituted juice as a full replacement for water, use the juice to make the lye solution. If using juice as a partial replacement for the water, you can either add it to the water to make the lye solution or stick blend the juice into the oils.