Tetrasodium EDTA in soap
Why is it used? EDTA (ethylene-diamine-tetra-acetic acid) offers two benefits when used in soap. It increases the shelf life of your soap by chelating (binding up) certain metals that can cause DOS (dreaded orange spots, also known as rancidity). EDTA also reduces the amount of sticky soap scum formed when lye soap is used in hard water.
EDTA and citrate do similar jobs, so if one does not appeal, then consider using the other. EDTA is arguably not as "crunchy" as citrate, so it is not the best choice for a "natural" soap. Both will function well to increase shelf life, but the consensus from informal discussions with other soapers is EDTA is more effective than citrate in reducing soap scum.
What kind to use? There are two kinds of EDTA generally available to home crafters, so be sure to buy the correct type.
TETRAsodium EDTA should be used in soap and other alkaline products. This is the type of EDTA that I am talking about in this article.
DIsodium EDTA is the other kind of EDTA; it should only be used in neutral pH to slightly acidic products. Disodium EDTA can be converted to the tetrasodium form if needed. But to keep soaping as simple as possible, I recommend buying the tetrasodium form.
Tetrasodium EDTA is available commercially in two forms: powder that is about 85% pure EDTA and a liquid solution of 39% EDTA in water. Either form can be used. My dosage suggestions are based on the powder form.
How much to use? Typical dosage for bar (NaOH) soap or liquid (KOH) soap is 0.5% of EDTA powder based on total batch weight. This is 5 g EDTA powder per 1000 g of total batch weight. Total batch weight is the sum of the oil weight + water weight + alkali weight.
The range of dosages suggested by various sources is very wide -- ranging from 0.05% to 4% of EDTA powder based on total batch weight. Here are some tips to help you decide what amount to use:
To prevent rancidity (DOS) in soap, dosages range from 0.05% to 0.25% (0.5 g to 2.5 g EDTA powder per 1000 g total batch weight).
Kevin Dunn, author of the book Scientific Soapmaking, recommends 0.5 g EDTA per 1000 g oils to effectively control DOS. (Note his use of oil weight, not total batch weight.)
Dunn found a blend of 1.0 g rosemary oleoresin (ROE) plus 0.5 g EDTA powder added per 1000 g oils may be more effective at preventing DOS than either EDTA or ROE alone.
To also reduce hard water scum, a slightly higher dose of EDTA appears to be needed. Handcrafted soapers typically use 0.25% to 0.5% (2.5 g to 5 g powder per 1000 g batch weight) and get good results.
Use the lower percentage for soft water. Use the higher amount for hard water or if you are not sure about the hardness of the water.
How to measure it? The amount of EDTA powder needed is very small, so I think it is easier to use a homemade solution of EDTA and distilled water. Some suppliers sell a ready-to-use 39% EDTA solution.
You can make an EDTA solution in any concentration up to about 50% by weight (50 grams EDTA and 50 grams of distilled water). Measure the desired amount of EDTA powder and mix with distilled water until the EDTA is fully dissolved. Store in a labeled, airtight container.
If you know how much EDTA powder you want in a batch of soap, here is how to calculate the amount of 50% EDTA solution to use:
50% EDTA solution weight = 100 / 50 X (EDTA powder weight)
A simplified version of this formula for 50% EDTA solution:
50% EDTA solution weight = 2 X EDTA powder wt
If you are using an EDTA solution that is not 50%, then replace the "50" in first equation above with the correct percentage.
How much lye does it neutralize? None! Do not adjust your lye weight for EDTA.
How should I add it to my soap? If using an EDTA solution, blend it into your fats along with any other additives you want to add to the fats, add the lye solution, and soap as usual. If using EDTA powder, mix the powder into the water or water-based liquid in your recipe until the powder is dissolved and blend this mixture into your fats.
CAUTION: Do not add EDTA directly to lye if the lye concentration is above about 35%. If you add EDTA solution to such lye, the EDTA will precipitate (turn into solid particles). If you add EDTA powder to this lye, the powder may not fully dissolve. This has two downsides -- The mixture can become a thickened white "pudding" that is difficult to work with, and the EDTA will be less effective as a chelator if it does not fully dissolve. If you have this problem, add water to the mixture to bring the lye concentration below 35% and stir until the EDTA is fully dissolved.
You might be wondering why EDTA is sold as a 39% concentration. Why not just make a 50% mixture?
EDTA is used in industry to remove metal ions from water for steam boilers and other processes that use steam or hot water. It turns out 1 gram of 39% EDTA solution will chelate 1 millimole of metal ions.
When you are frequently adjusting the chelation dosage in an industrial process, using an easy-to-remember number like "1 gram EDTA per 1 millimole of metal ions" makes the calculations simpler and reduces the chance of error.
As soap makers, we don't do our chemistry in quite the same way. For us, a 39% solution is a pain in the patooty! A 50% concentration makes more sense.