EDTA (Ethylenediaminetetraacetic acid)

About: EDTA  is a chelating agent; taking the hardness out of tap water, so that added cleansers and preservatives work more effectively.

Uses: Chelating agents work by binding metal ions that are found in water, extracts or other materials so that they can't interfere with the product efficacy and more importantly the preservative efficacy. In some cases metal ions act as nutrients for microbes encouraging their growth. This could manifest in your product growing mould or becoming unstable sooner than the preservative instructions suggest. 

Usage:  0.1-0.4% is enough and it is added into the water phase as soon as manufacturing begins.