The binding of free calcium ions in aqueous solution using chelating agents, phosphates and poly(acrylic acid)

Abstract

Organic chelating agents, phosphates, and polyelectrolytes were studied to determine their calcium ion binding power (CBP) and binding mechanism. A calcium ion electrode was used to measure the equilibrium calcium ion concentrations from which stability constants were calculated. Except for EDTA and NTA, the binding power did not correspond to the formation of complexes with a simple stoichiometry. In most cases, more than one equilibrium is involved. Increase in the level of binding agent decreased CBP, especially for those agents which formed complexes with low stability constants. CBP calculated on the basis of weight should be used to measure the relative cost effectiveness of commercial calcium binding agents. At an amount lower than required to form stable stoichiometric complexes, poly(acrylic acid) had a higher CBP than traditional organic chelating agents. The electrostatic effect accounts for the excellent CBP of poly(acrylic acid) at low concentrations. To get maximum efficiency in detergent and water treatment applications, use of low levels is recommended. aPresent address: The Clorox Co. Technical Center, Pleasanton, CA 94566.