Fixative for colours, texture and fragrance
Tetrasodium etidronate is the same as EDTA, which is widely used in colour cosmetics, soaps, shampoos and toiletries to prevent changes in colour, texture and fragrance.
EDTA is a chelating agent formed of four carboxylic acid groups attached to two nitrogen atoms, which means that EDTA latches onto minerals and keeps them in place.
In certain conditions the compound develops negative charges on four of the oxygen atoms; this allows EDTA to sequester (grab onto) free metal ions or minerals. The analogy that we like best is to describe EDTA like a lobster, with claws closing around minerals as they float past in the water!
Removing free metal ions or minerals has several benefits. Firstly, it adds to the stability of the material by acting as an antioxidant, this means that soaps stay brightly coloured and smelling beautiful.
A further benefit of removing the metal ion is that the hardness of water is reduced, providing a soft feel to the skin.
The final benefit of removing metal ions or minerals is that it helps reduce the scum that some soaps naturally develop when used in the bath.