The Protoelectric Potential Map (PPM): The Two-Dimensional Plot of Absolute Redox Potentials and Absolute pH Values

We recently introduced a unified Brønsted acidity scale on the basis of the chemical potential of the proton in any medium allowing the comparison of pH values of different media and a precise definition of superacidic media. For this purpose we defined the reference state of maximum acidity as the standard chemical potential µ_abs°(H⁺, g) of the proton in the gas phase (ideal H⁺ gas at 1 bar and 298.15 K), which is arbitrarily set to 0 kJ mol^–1. The acidity and hence the chemical potential of the gaseous proton H⁺(g) is lowered by the interaction with any type of a medium, i.e. complexation of H⁺ in the gas phase, solution, or in the solid state. In this way the defined absolute pH value pH_abs depends in a simple manner on the absolute potential of the proton in the media of interest. [1]

Analogously to the definition of the pH­_abs value we define an absolute redox potential (pe_abs) scale on the basis of the absolute chemical potential of the electron with the ideal electron gas at 1 bar and 298.15 K as reference state (µ_abs°(e^–, g) = 0 kJ mol^–1). This scale establishes the possibility of direct comparison of the reduction power (which we term “reducity”) of any redox system in any medium. [2]

Above all, both scales are combined to the Protoelectric Potential Map (PPM, pe_abs vs. pH_abs, see fig.) as the Hydrogen Evolution Reaction (HER) correlates the pH scale to the redox scale. [2] The thermodynamics of any redox, acid-base or combined system in any medium can be represented and related to any other. For example, the pH-dependency of a redox system can be easily read off by the slope of the line representing the redox system in the whole pH range of the solvent under consideration. Thermodynamic stabilities of solvents can be clearly displayed and even predictions of reactions become possible at a glance, please compare with K. Pütz who presents that silver is a non-noble metal in the acidic IL HmimBr at this conference.

Figure: The Protoelectric Potential Map (PPM).

[1] a) D. Himmel, S.K. Goll, I. Leito, I. Krossing, Angew. Chem., Int. Ed. Engl. 2010, 49, 6885-6888; b) VIP-Paper: D. Himmel, S.K. Goll, I. Leito, I. Krossing, Chem. Eur. J. 2011, 17, 5808 – 5826.

[2] V. Radtke, D. Himmel, K. Pütz, S. Goll, I. Krossing, Chem. Eur. J., 2014, DOI: 10.1002/chem.201302473