Sequestering agents

Transition metal ions in the water supply can pose difficult problems in a dyehouse. Firstly, many of these ions catalyse the decomposition of hydrogen peroxide in bleaching baths. In addition, transition metals often give insoluble salts with dyes, or form complexes that are invariably duller and even different in shade. If the amounts are excessive, the water may be unusable without treatment. On adequate aeration of the water at pH values around 7, iron precipitates as insoluble Fe(OH)3 and can be removed. There is always concern about the possibility of iron from corrosion inside the water pipes in a textile plant. Addition of a sequestering agent to the water avoids many problems from relatively low concentrations of undesirable metal ions. Sequestering agents react with the metal ions to form very stable complex ions. Examples of such chemicals used in textile processing include EDTA, and related aminocarboxylic acids, as well as polyphosphates such as sodium tetrametaphosphate Na4P4O12. EDTA is a most effective sequestering agent, particularly in neutral or weakly alkaline solution. It forms such stable complexes with metal ions that it often removes the metal ion from a metal– complex dyestuff molecule, to give the EDTA – metal complex and uncomplexed dye. Polyphosphates can bind alkaline earth metal ions and thus decrease the effects of water hardness. The product Calgon, sodium hexametaphosphate Na6P6O18, is widely used for this purpose. Calcium ions replace sodium in the hexametaphosphate, forming a stable complex. The free calcium ion concentration in the water is then so low that calcium soaps do not precipitate. Polyphosphates are frequently present in domestic washing powders for the same reason.