So what is a chelator anyway?
A chelator (or chelant) is a molecule that has the unusual ability to attract and securely hold onto certain types of metal ions. Even the smallest traces of metal contamination in your soap can cause the soap to become rancid quickly. Soap that is rancid will smell "off" or musty and will have unisightly orange spots or splotches that soapers call DOS or Dreaded Orange Spots.
Adding a chelator to your soap prevents any metallic contamination from triggering rancidity. A chelator also helps to reduce the sticky, irritating soap scum created when lye-based soap is used in hard water. The calcium and magnesium in hard water are metals responsible for making soap scum.
A chelator has this unusual abililty to immobilize metals because its molecular structure has two or more electrically charged spots that can attract and trap an electrically charged metal ion. Each of these spots by itself is not very strong, so a molecule with only one "claw" cannot be a chelator. A molecule must have multiple claws to successfully function as a chelator.
Where does metal contamination come from?
Traces of metallic contamination in soap come from many sources -- the machinery needed for harvesting and processing natural ingredients, tap water, your utensils, dust and pollution from the air, contamination on your hands, ingredients that naturally contain metallic compounds, and so on. You can minimize some of this contamination (see below), but you cannot entirely eliminate it.
What metals can chelators remove?
Only metals that can become "multivalent ions," meaning metals with two or more electrical charges, can be trapped. Multiple electrical charges give the chelator multiple spots to grab and hold on. Lead, calcium, chromium, manganese, magnesium, iron, and copper are all examples of metals that can become multivalent ions. Sodium and potassium are examples of monovalent metals (metallic ions with only one charge); these metals cannot be trapped by a chelator.
What are common chelators for soap?
Citrate -- This is the ion that is created from sodium citrate or potassium citrate. Citrate has three "claws" in its structure to attract and catch a metal ion. For the chemistry geeks, the claws are negatively charged organic acid groups (COOH). The three O- symbols (below) represent citrate's claws.
EDTA -- This is the ion created from tetrasodium EDTA. It is a chelator because it has six claws in its structure that can trap and hold a metal ion. Again for the geeks, these are four organic acid groups and two electron-rich amine (nitrogen) groups. The O- and N symbols (below) represent EDTA's claws. The red M symbol is a metal ion trapped within the EDTA structure.
Can other chemicals be chelators?
People sometimes ask if household chemicals, such as table salt (sodium chloride), sodium acetate made from vinegar, sodium lactate, acetic acid in vinegar, or lactic acid in yogurt and other fermented dairy, can be effective chelators for soap. These chemicals are not chelators, because they do not have the chemical structure needed to effectively trap metal ions. There are other effective chelators that can be used in soap, but they will not be found in the typical kitchen pantry.
What are other ways to avoid rancidity?
It is nearly impossible to eliminate all metal contamination in your soaping ingredients, no matter how careful you are. Using a chelator combats rancidity triggered by this unavoidable contamination.
Using an antioxidant as well as a chelator can do an evenbetter job of preventing rancidity in soap than using either chemical alone. According to Kevin Dunn in his book Scientific Soapmaking, EDTA pairs well with antioxidant rosemary oleoresin (ROE) and citrate pairs well with the antioxidant BHT.
It is also a good idea to prevent unnecessary contamination, because a chelator and/or antioxidant can only do so much. Here are some ideas to consider --