Tetrasodium etidronate is added to oils and caustic soda during saponification to prevent soaps from becoming mouldy and producing scum.
Tetrasodium etidronate is one of the salts of etidronic acid, a chelating agent. Chelating agents can sequester (grab onto) free metal ions or minerals. The analogy we like the most is to describe this material as a lobster, with claws closing around the minerals as they float past in the water.
Removing free metal ions or minerals has several benefits. It ensures successful saponification and stabilises formulas by acting as an antioxidant and therefore preventing changes in colour, texture, and fragrance. It also reduces the hardness of water, providing a soft feel to the skin.