An experimental determination of absolute half-cell emf’s and single ion free energies of solvation

Abstract

The concept of absolute half-cell emf is discussed and defined as VMS-φM for the reaction M→M+(solution)+e−(M), where VMS is the electrostatic potential difference between metal electrode and solution and φM the work function of metal in contact with solution. It is shown that this quantity is equal to VRS-φR, where VRS is the electrostatic potential difference between a reference electrode in air above the solution and the solution, and φR the work function in air of this reference. The quantity VRS′, the potential difference between reference electrode and the solution surface, was found experimentally by the vibrating condenser method for a number of half-cells, and φR was determined photoelectrically. It is shown from the variation of VRS′ with electrolyte concentration that the potential difference betwen the bulk of pure H2O and its air interface is ∼0.05 V, the surface being negative relative to bulk, and that this potential is increasingly screened out as electrolyte concentration increases. From these results for several different half-cells the absolute value of the standard half-cell emf for H2→2H+ is found to be −4.73±0.05 V. This result permits the calculation of single ion free energies of solvation. It is shown that the simple Born model as used by Latimer, Pitzer, and Slansky works remarkably well for simple cations, including polyvalent ones, and for spherical anions, but breaks down for complex anions like OH−, NO−3, etc. Ions in which chemical bonding effects to the solvent play an important role show anomalously high solvation energies. The solvation energy of H+ is −10.98 eV in H2O and varies very little from this value in several different solvents, suggesting that free H+ may predominate in these solvents. This could result from the fact that the small effective radius of free H+ leads to a greater solvation energy than the combination of bond formation and solvation of the resultant much larger ion (H solvent)+. Similar arguments can be used to explain why for instance Fe3+ is not reduced by H2O despite the fact that the third ionization potential of Fe is 30 and that of water 12.6 eV. Other possible applications of the method used in these experiments are discussed.

Integral equation theory for the electrode-electrolyte interface with the central force water model. Results for an aqueous solution of sodium chloride

“…Our value µ ex N a + = −3.23 eV follows this prediction and is in good agreement with the value −3.93 eV determined experimentally for the free energy of solvation of a Na + -ion [37]. Around the Cl − -ion (cf.…”

“…As a result of the overpopulation of the anionic hydration shell, we obtain rather large values of U solv Cl − = −7.34 eV and µ ex Cl − = −6.25 eV compared with computer simulation results (cf. table IV) and the experimental value of −3.53 eV for the free energy of solvation [37]. With the RISM integral equation method Pettitt et al [26] calculated for the Cl − -ion a more negative ion-water contribution to the solvation energy than for the Na + -ion.…”

The Temperature Dependence of Salt−Protein Association Is Sequence Specific

“…a solvated environment (22)(23)(24)(25). This might reflect a deficiency of the current CHARMM force field for ions, and it would be of particular interest in the future to compare results to simulations using polarizable force fields (26,27).…”

Single-Ion Solvation Free Energies and the Normal Hydrogen Electrode Potential in Methanol, Acetonitrile, and Dimethyl Sulfoxide

“…18,[184][185][186][187][188][189][190][191][192][193] Unlike the solvation free energies obtained using the cluster pair or TATB approximations, the solvation free energies obtained from these types of experiments contain, in addition to the free energy required to couple the solute to the bulk solvent, a free energy term that depends on the surface potentials of the two phases that are brought into contact with one another. (As discussed in the first paragraph of the present article, the solvation free energies resulting from these types of experiments are usually referred to as real solvation free energies to distinguish them from absolute, or Gibbs, solvation free energies, which do not contain the contribution due to the potential of the phase.)…”