Acid-base equilibria in organic solvents. Part 3. An Absolute pH Scale from Proton Basicity Evaluated by Cyclic Voltammetry

The relative basicity of some organic solvents of electroanalytical interest (propylene carbonate, sulfolane, 2-methyltetrahydrofuran, methyl formate and morpholine) is evaluated by cyclic voltammetry. Anhydrous perchloric acid is electrogenerated in these solvents and the half-wave potentials of the redox couple H+/H2 are recorded by referring them to a reference system, here bis(biphenyl)chromium (I)/bis(biphenyl)chromium(0), which is independent of the nature of the solvent. The data obtained, together with those found previously for other solvents by following the same procedure, are used to draw an absolute pH scale covering a wide range of basicities from nitromethane to morpholine. The same approach is also utilized to evaluate and compare, on the basis of this absolute pH scale, the strength in the solvents studied of some nitrogen bases which are suitable as titrants in nonaqueous media. The proposed proton basicity scale is compared with other basicity series defined in the literature. © 1988.