Measuring Single Ion Gibbs Transfer Energies without Extrathermodynamic Assumptions to Validate the Unified Acidity and Redox Scales within the Protoelectric Potential Map PPM

• Applicant: Professor Dr. Ingo Krossing
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 420771175

pH values and redox potentials are common knowledge and of utmost importance, e.g. to analytical chemistry, energy storage and conversion (electrolysis, fuel cells, batteries), material sciences (corrosion, metallization) or life sciences (proton pump, proton coupled electron transfer, photosystem). Their definitions, e.g. pH = −log a(H+), base on thermodynamic quantities of single ions. Although used worldwide, single ion quantities are fundamentally questioned for electroneutrality reasons. A major missing link in the existing theoretical framework of solution thermodynamics was the comparability of these thermodynamic quantities in and between all media. This is now possible by using the unified acidity and redox scales pHabs and peabs introduced by our group. However, now the experimental determination of single ion Gibbs transfer energies of corresponding redox/acid-base systems is the unresolved problem. So far, heavily discussed extrathermodynamic assumptions have to be used for this purpose, which are associated with uncertainties of at least 1 – 2 pH- / pe-units (60 – 120 mV, in some cases up to 1 V).Our core objective, therefore, is the determination of single ion Gibbs transfer energies in a strict thermodynamic sense by means of potential measurements between two half cells containing different solvents. They are connected – and this is new and crucial – by an “ideal” salt bridge, filled with a specific ionic liquid with identical diffusion coefficients of anion and cation. In this set up, the largest parts of the liquid junction potential occurring at interfaces between two different media cancel. The remaining parts can then be determined without extrathermodynamic assumptions and the measured cell potential can be directly assigned to the Gibbs energy of transfer of single ions. This would provide the last missing link for solution thermodynamics, including acceptance of single ion quantities.

• DFG Programme: Research Grants