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

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.