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 tiniest traces of metallic contamination in soap (or other bath and body products) will cause the soap to quickly become rancid. It is nearly impossible to eliminate all metallic contamination, no matter how careful you are, so a chelator is a valuable ingredient to consider adding to your soap.
Soap that is rancid will smell "off" or musty. The soap will either turn entirely orange (all-over rancidity) or it may develop the unsightly orange dots or splotches that soapers call "DOS" or "Dreaded Orange Spots."
Another benefit of using a chelator is it also reduces the sticky, irritating soap scum created when lye-based soap is used in hard water. The calcium and magnesium ions in hard water are the 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 "claws" that can attract and trap an electrically charged metal ion. Each of these claws by itself is not very strong, so 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 reduce metallic contamination, but you can never 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 effective 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.
Large amounts of EDTA are used in food, medicine, and industry because it is such an effective chelator and it is not acutely (quickly) toxic to humans and other animals. When released into the natural environment, EDTA biodegrades very slowly and continues to immobilize metals in soil and water. Living organisms require trace amounts of metals for growth and good health. If EDTA removes too much of these necessary metals from soil or water, living organisms may not thrive. When EDTA finally does degrade, it can quickly release these metals back into the environment in large, toxic amounts -- a damaging boomerang effect.
Other chelators, such as sodium gluconate and glutamic diacetic acid (GLDA), biodegrade quickly, so they are not as likely to create a boomerang effect in the natural environment. Some of these chelators are as effective as EDTA, so I expect them to become widely used and more easily available to us small-scale makers.
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, but they will not be found in the typical kitchen pantry.