The most abundant organic acid in lemons is citric acid. Lemons and lemon juice are one of the most concentrated sources of natural citric acid. A whole lemon contains about 3 grams of citric acid. Around 140 grams of lemon juice will contain roughly 6 grams of citric acid.
Citric Acid in Soap Making ~ What's the Point?
When citric acid and sodium hydroxide lye (NaOH) are combined, sodium citrate is created.
Sodium citrate works as a chelator in soap. A chelator traps metal ions such as calcium and magnesium that can be found in tap water. These metal contaminants can cause rancidity and soap scum in handmade soap. A chelator binds to these metals so that they can’t bind to the soap molecules. This way the soap won’t react with them and the soap's lather improves as well as increasing the shelf of your soap.
Further, citric acid consumes some of the lye and increases the superfat. ** See below regarding compensating for the lye.
Using citric acid in soap making reduces the problem of soap scum (particularly if you have hard water issues), increases soap lather, and aids to prevent oxidation in oils, which also extends the shelf life of the soap.
How to use Citric Acid
The citric acid used in soap making is generally the crystal or powder form.
How much Citric Acid?
Use between 1 to 3 percent of citric acid of your total oil weight in a soap recipe. If you live in a region where your water is hard, up to 3 percent is recommended. Use 1-2% in a soft water region.
** Because citric acid displaces some of the lye (NaOH) it is advised to add 6g extra lye for every 10g of citric acid added to a soap recipe if you want to keep the superfat % at the same level. To be exact - every 1g of Citric Acid consumes 0.624g of sodium hydroxide.
Example: if you are using 1000g total oils in your recipe and wish to add 2% citric acid (1000 x 0.02) = 20g citric acid. Therefore, 20g citric acid (2% of oils). 20g x 0.624 = 12.48g of extra sodium hydroxide is needed.
I usually use LyeCalc.com to prepare my soap recipes and calculates the exact amount of sodium hydroxide needed (1%, 2% or 3%) in addition to the already calculated amount in your recipe. All you need to do then is add the extra sodium hydroxide amount indicated under the % option to compensate for the citric acid % used.
How to use Citric Acid
It is recommended that you add citric acid to the distilled water and dissolve it completely before adding the lye.
Once the lye solution is completely dissolved and cooled, add it to the oils.
Using Fresh Lemon Juice
Use lemon juice as a water replacement by straining all lemon pieces from the juice and freezing it in ice-block containers.
The juice can then be used straight from the freezer for your lye solution. Don't forget to add the extra lye as stated above and stir the solution until completely dissolved prior to adding to your oils.