Since about 100 years, the quest on the determination of Gibbs energies of transfer of single-ions exists. While some consider that the chemical potential of single-ions is physically senseless, other determined Gibbs solvation energies or Gibbs transfer energies of single-ions with the help of extra-thermodynamic assumptions (e.g. TATB-assumption, ferrocene-assumption and others), thereby introducing an error of the respective quantity of insufficiently assessable magnitude.^[1]
We suggest a procedure for the thermodynamically rigorous, experimental determination of the Gibbs energy of transfer of single-ions Δ_trG°(i, S1--> S2). The method is based on potential difference measurements between two electrochemical half-cells with different solvents (solvation cells) connected by an ideal ionic liquid salt bridge (ILSB).^[2] We point out the specific requirements for the IL with regard to the procedure, thus ensuring that those parts of liquid junction potentials (LJP) at both ends of the ILSB cancel which depend on the redox system used. The remaining parts of the LJPs can be determined by separate electromotive force measurements, the knowledge of the ionic transference numbers and Gibbs energies of transfer of the used salts is mandatory. No extra-thermodynamic assumptions are necessary for this procedure. The accuracy of the target quantity is an inherent part of the measurements and depends, amongst others, on the ideality of the IL used. We present results concerning the crucial steps a) and b) of the methodology exemplarily for Ag⁺ and Cl⁻ with S1 being water and S2 being acetonitrile as an important intermediate goal to evaluate our concept for the assumption-free determination of single-ion Gibbs transfer energies Δ_trG°(i, S1--> S2).[3] The used IL, [N₂₂₂₅]⁺[NTf₂]⁻, shows nearly ideal behavior, that is, nearly identical diffusion of anion and cation, in the pure IL, but also in water and acetonitrile solution. Electromotive force measurements of solvation cells between S1 and S2 demonstrate Nernstian behavior for Ag⁺ concentration cells and constant like cell potentials for solutions with five tested Ag+ counterions.

The Gibbs transfer energies can be utilized to collect data for the unified scales of acidity pH_abs and reducity and pe_abs and for their two dimensional plot, the Protoeletric Potential Map (PPM), enabling the possibility to easily compare acidities and redox potentials between different media.^[4]

Literature:
[1] P. Hünenberger, M. Reif, Single-Ion Solvation. Experimental and Theoretical Approaches to Elusive Thermodynamic Quantities, The Royal Society of Chemistry, Cambridge, 2011.
[2] V. Radtke, A. Ermantraut, D. Himmel, T. Koslowski, I. Leito, I. Krossing, Angew. Chem. Int. Ed. 2018, 57, 2344.
[3] A. Ermantraut, V. Radtke, N. Gebel, D. Himmel, T. Koslowski, I. Leito, I. Krossing, Angew. Chem. Int. Ed. 2018, 57, 2348.
[4] D. Himmel, V. Radtke, B. Butschke, I. Krossing, Angew. Chem. Int. Ed., 2018, doi:10.1002/anie.201709057.