Acid/base and metal complex solution chemistry of sulfonated polyacrylate copolymer versus temperature and ionic strength

Abstract

Sulfonated polyacrylate copolymer (also known as vinyl sulfonated co-polymer; VS-Co) have been widely used in mineral scale control. However, the speciation chemistry of such polymers in solution is not fully understood. In this paper, the acid-base and calcium complex solution chemistry of a VS-Co has been determined from 0.01 to 2 m ionic strength and from 25 to 90 °C by combining electrostatic theory with potentiometric titrations. To process the titration data, VS-Co is treated as a hypothetical, averaged monoacid, HA, with the same concentration of the active carboxylic functional groups of VS-Co. The acid and calcium dissociation reactions of this acid are simply considered as 1:1 type hypothetical reactions, namely HA↔H+ + A− and Ca(A⋯A)↔Ca2++ (A⋯A)2−, with (A⋯A)2− as an arbitrary combinations of two dissociated A− units. The microconstants of the acid dissociation reaction are fitted with a linear electrostatic model with pKa=pKa,intr+belecθA−, where pKa,intr is an intrinsic constant referring to the condition of zero dissociation, θA− is the deprotonated fraction of carboxylic functional groups and belec is an electrostatic factor based on polyelectrolyte theory. Similarly, the microconstants of the Ca2+ dissociation reaction are fitted with pKCaA2=pKCaA2,intr+2belecθA− with doubled electrostatic effect since the complexation of Ca2+ with the polymer is considered to neutralize two negative charges. To quantitatively describe the speciation of VS-Co at various conditions, the pKa,intr, pKCaA2,intr, and belec values are fitted empirically as a function of temperature (T, K), and ionic strength (I, m):pKa,intr=5.068−128.398T−0.924×I1/2+0.311×IpKCaA2,intr=5.754−888.644T−5.749×I1/2+2.221×Ibelec=3.234−1.321×I1/2+0.399×I. Fitted results suggests no significant effect of temperature on the electrostatic factorbelec, which is consistent with electrostatic theory. Results from this study can be used to predict the equilibria of VS-Co in a solution, at various temperature, ionic strength, pH, and metal concentrations.