Acidity of the poly(acrylamidoxime) adsorbent in aqueous solution: Determination of the proton affinity distribution via potentiometric titrations

Poly(acrylamidoxime) fibers are the current state of the art adsorbent for mining uranium from seawater. While the acid dissociation constants, pK_a, of characteristic amidoxime and carboxylate ligands have been reported in the literature, the proton affinity distribution of the poly(acrylamidoxime) fiber is yet to be established. Herein, we report the poly(acrylamidoxime) proton affinity distribution between pH 2 and pH 10 via the stable numerical solution of the adsorption integral equation using splines (SAIUS) algorithm. Two peaks in the proton affinity distribution of poly(acrylamidoxime) were observed: the neutral to anionic dissociation of the carboxylate monomer between pH 3.2 and pH 4.4 (pK_a 4.0) and the protonated to neutral dissociation of the acyclic amidoxime monomer between pH 5.6 and pH 6.8 (pK_a 6.1). The acidity constants obtained for the carboxylate and amidoximate monomers vary from the acidity constants of acetic acid and acetamidoxime, respectively. These variations in acidity can be attributed to charge interactions between the carboxylate (pK_a 4.76) and amidoxime (pK_a 5.78) monomers. This is a first step to resolving the metal cation affinity distribution of the poly(acrylamidoxime) fibers, which can aid in improving the selectivity of subsequent generations of chelating polymers used to mine uranium from seawater.