Polyprotic Acids in Solution: Is the Inversion of the Constants of Stepwise Dissociation Possible?

Abstract

The paper is devoted to the problem of the ratio of the constants of stepwise dissociation of polyprotic acids in solution. Special attention is paid to the problem of close dissociation constants and in particular to the possibility of the inversion of constants, for example, Ka1 < Ka2 (or pKa1 > pKa2) for equilibria H2A ⇄ HA– ⇄ A2–, H2A+ ⇄ HA ⇄ A– or H2A2+ ⇄ HA+ ⇄A. Equilibria of inorganic and organic acids are successively considered. Some cases of inversion are considered. In particular, such relations are sometimes observed for fluorescein and phthalein compounds, and for porphyrins. In the last case, as well as in some other systems the acid-base reactions proceed relatively slow.
 An overview of approaches to estimation of the Ka1/ Ka2 ratio is presented. Namely, these approaches use the concepts of the detailed equilibrium scheme, dissociation microconstants, statistical and electrostatic factors, and the influence of intra-molecular hydrogen bonds. The variety of reasons for additional stabilization of the forms H2A (or H2A+, or H2A2+) and A2– (or A–, or A) and destabilization of the form HA– (or HA, or HA+) is regarded, including salt effects and ionic association. Peculiarities of stepwise protolytic equilibria of macrocyclic compounds, such as calixarenes, porphyrins, and cryptands, are considered. Dispersed systems such as polyelectrolytes, micelles of functionalized surfactants, monolayers, silica and modified silica, other oxides, nanodiamonds decorated by carboxylic groups, represent a peculiar type of polyprotic acids. In this case, a “spectrum” of pKa values appears because of influence of ionized functional groups on the properties of un-dissociated ones.
 It was demonstrated that such unusual ratio of constants, Ka1 < Ka2, may be caused by a set of factors, among which most important are (i) the tautomerism in true solutions and (ii) selective binding of different equilibrium forms in organized solutions, i.e., in micellar solutions of surfactants, suspensions of phospholipid liposomes, and related systems.