Sequestrants

Abstract

Sequestrants 3 3.1 Defi nitionSequestrants are chemical agents forming water -soluble complexes with metals. The binding process sequesters or inactivates the metal ions and prevents them from reacting with anions that give a low solubility product, such as carbonate, sulfate, and phosphate. Competition is the most important feature of the sequestering reaction. Two types of anions compete for polyvalent cations. The fi rst type (precipitants) cause insolubility, and the second (sequestrants) endeavor to pull the metal out of the precipitated material and keep it soluble. As already mentioned, the result of the competition depends on several variables which depend on the metals involved: concentration, pH, temperature, stability constant, and ionic strength. For this reason, a signifi cant number of molecules with functional groups suitable for forming coordination bonds have been synthesized in addition to the traditional polyphosphates, EDTA, NTA, citrate, and gluconate. The molecular engineers continue to investigate new combinations of functional groups in new structures, aiming at fi nding out new categories of sequestrants and making the already existing classes perform better.The words ' ligand ' and ' chelant ' are used to describe sequestrants, and ' chelation ' and ' complexation ' are adopted as synonyms of sequestration. However, ligand denotes the type of functional group able to form bonds with metals. The term ligand should be used to identify the single coordinating group instead of the whole sequestering molecule. In the same way, chelant should refer to the stoichiometric sequestrants. Sequestrant is intended as a general name defi ning molecules suitable for forming coordination complexes.The primary distinction among sequestrants concerns stoichiometric and threshold behavior.A sequestrant is defi ned to be stoichiometric when, excluding the coordination bonds, it is able to react with a number of positive charges (metals) exactly equal to the sum of its negative valences. When the amount of metal causes this number to be exceeded, all the complexes are inactivated and the sequestrant itself precipitates and contributes to scale. Sanitation: Cleaning and Disinfection in the Food Industry. Mario Stanga