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

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
 
Final Report Year 2025

Final Report Abstract

Certain physico-chemical quantities of single ions are considered elusive. For example, the pH value is probably the best known and most widely used quantity in chemistry, but the pH definition, pH = −lg a(H+), is merely notional. The reason for this is the macroscopic electroneutrality condition. It prevents the preparation and analysis of solutions containing only one type of ion: An electrically oppositely charged ion inevitably accompanies the object of investigation. Sørensen defined the pH value for aqueous solution (pHH2O), but there is no reason not to apply it to other solvents S (pHS). However, since the pHH2O values and the pHS values contain relative activities a(H+), i.e. relative to a reference state in the respective solvent, they cannot be compared with each other. To enable comparability, the Gibbs transfer energies of the proton from water to the solvent S, ∆trG°(H+, H2O→ S), must be known. This quantity is also elusive and its determination has so far been based on so-called extra-thermodynamic assumptions, which are reasonable but cannot be verified within the framework of thermodynamic formalism. The above explanations for the acidity of solutions can also be transferred to the reductivity, i.e. to reduction potentials, as at least one part of a redox system is an ion. This project deals with the determination of Gibbs transfer energies of single ions without extrathermodynamic assumptions. The targeted method (ILSB method) consists of measuring the electrical potential difference between two electrochemical half-cells filled with different solvents but the same redox systems. The half-cells are connected via a salt bridge filled with a pure ionic liquid (IL). The two liquid-liquid potentials (LJP) occurring at the half-cell/salt bridge interfaces were then to be investigated using methods described in the literature. For one of these methods however, it turned out that it cannot be applied when ionic liquids are involved. Nevertheless, for these cases we were able to approximate the sum of both LJPs in such a way that it is only described by the ions of the IL in the salt bridge. Empirically, it could be shown that the data found are very precise, i.e. they are consistent, and our approximations are justified. If the Gibbs transfer energy of the cation and the anion of the IL is the same between the solvents, then the measured cell potential directly reflects the Gibbs transfer energy of the redox system. The comparison with literature data obtained with other assumptions shows a so far unobserved accuracy of agreement. This leads to the conclusion that we can assume with high probability that the ILSB method developed in this project is suitable to determine the value of the transfer energies accurately to about 1 kJ mol−1. It is therefore the most accurate method to date, even if it is not thermodynamically verifiable. The measurement results were transferred to inter-solvent scales for the pHabs and Eabs values. The comparability of acidity and reductivity in the solvents investigated in the project has thus been established. Further measurements using this method are currently being carried out for additional solvents.

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