Metal ions in hard water

EDDS is a weaker chelant for Ca2+ and Mg2+ compared with most other chelants, but is much stronger for transition metal ions, giving it a unique selectivity for troublesome transition metal ions in hard water. Its main weakness is that it is generally more expensive (per kilogram) than other chelants however, this can be offset by its superior selectivity (Table 10.4), which allows the use of lower doses in some applications. EDDS (as the S,S-stereoisomer) is also readily biodegradable and has an excellent toxicological and ecotoxicological profile (Section 10.5.3.1). 

Soap works because the nonpolar end dissolves in dirt (oil), leaving the polar end outside the dirt. This combination is known as a micelle (see Figure 16-30). To the surrounding water molecules, the micelle appears as a very large ion. These ions are water-soluble and repel each other due to their like charges, a behavior that causes them to remain separated. Metal ions in hard water (Ca2+/Mg2+/Fe2+) cause a precipitate to form because they react with carboxylate ions to form an insoluble material (also known as the soap scum hanging around the bathtub or shower). 

Phosphates such as sodium tripolyphosphate, shown here, are used to complex or sequester metal ions in hard water so these ions cannot interfere with the action of soap or detergents (Section 18.6)... 

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