What Zany said. A chelator, whether EDTA or other chemical, will lengthen the shelf life of soap (whether liquid or solid). It does this by immobilizing metallic contamination, specifically divalent metallic ions. Metals catalyze the oxidation of fatty acids in soap or fats. Oxidation of fatty acids will eventually lead to rancidity. More about chelators, including dosages, in my Soapy Stuff articles.
edit: Use tetrasodium EDTA in alkaline mixtures, including soap. Use disodium EDTA in neutral to acidic materials, such as lotion. You can convert di to tetra if you have di and want to use it in soap, but it's simpler to just start with tetra.