Effect of cycloamyloses on apparent dissociation constants of carboxylic acids and phenols: equilibrium analytical selectivity induced by complex formation.

Apparent dissociation constants of organic acids were determined by potentiometric titration in the presence of cyclohexaamylose or cycloheptaamylose. The quantity deltapKa' = pKa' (cycloamylose)-pKa (water) was positive or zero for all carboxylic acids studied and negative or zero for all phenols. The term delta pKa' can be related to the cycloamylose concentration, K11alpha, and K11b, where K11a and K11b are 1:1 stability constants for complexes of the acid and the anion, respectively. From the dependen1a and K11b can be obtained. If deltapKa' not equal to 0, then K11a not equal to K11b; for carboxylic acids, K11a greater than or equal to K11b; for phenols, K11b greater than or equal to K11a. Because of variable pKa' shifts, it is possible to carry out differentiating titrations of some acid mixtures in cycloamylose solutions, whereas the same acids cannot be differentiated in water. If an acid is weakened by cycloamylose, its conjugate base is strengthened, and some carboxylate salts can be readily titrated in the presence of a cycloamylose.