POLAROGRAPHIC MEASUREMENT OF RELATIVE STRENGTHS OF BRONSTED ACIDS IN PYRIDINE - DISPERSION AND SOLVATION EFFECTS IN ACID-BASE EQUILIBRIA . ANALYSIS OF BRONSTED ACID MIXTURES

A Bronsted acid in pyridine solution produces a le diffusion controlled polarographic wave per acidic function; E12 is controlled by equilibria involving the hydrogen-bonded nonionic pyridine-acid adduct, the corresponding ionized but undissociated ion pair, and the dissociated pyridinium and acid anion ions. When the background electrolyte consists of large univalent ions, separate but parallel linear correlations exist for each type of acid between E1/2 and aqueous pKa; a leveling effect is seen for acids of pKa less than ca. 3. Comparison with the potentiometric half-neutralization potentials indicates that the E1/2 values are directly related to the pKa values of the acids in pyridine. on this basis, relative enhancement of acid strength (pK) in pyridine, compared to that in water, corresponds to 2.5 and 3.9 orders of magnitude for phenols and purines, respectively, compared to carboxylic acids; these changes are probably due to the contribution of the dispersion effect to the medium effect on free energy in connection with specific solute-solvent interaction. Mixtures of acids, depending on background electrolytes used, can be analyzed for total acid content or, depending on type and pKa, individually. © 1969, American Chemical Society. All rights reserved.