Counterion Effect on the Titration Behavior of Poly(maleic acid)

The potentiometric titration of poly(maleic acid) (PMaA) in aqueous solutions containing alkali metal salts and a tetramethylammonium salt was carried out at 25 °C, and the effect of the counterions on the titration behavior of PMaA was examined in detail. The acidity of the primary carboxylic groups of PMaA increases in the order of (CH3)4N+<Li+<Na+<K+, whereas that of the secondary carboxylic groups increases with the decrease in the crystallographical radius of the counterions, in agreement with the results for ordinary polycarboxylic acids. The order of K+>Na+>Li+ for the binding strength to the primary carboxylate groups of PMaA is interpreted in terms of the radius of the hydrated cations; it can be attributed to the stable ring structure of the monoanion of the maleic acid residue, resulting from the hydrogen bonding between two adjacent carboxylate groups.