Organic solvent-water mixtures, reference electrode potentials

TABLE 8.21 Potentials of Reference Electrodes (in Volts) at 25°C for Water-Organic Solvent Mixtures Electrolyte solution of M HCl. 

Reference electrodes are divided Into two groups. One comprises the saturated calomel electrode, its variants (such as the "lithium S.C.E.", Hg/Hg2Cl2(s.), LiCl( s)> and others of the same ilk), and the normal hydrogen electrode. These are almost Invariably prepared with water, so that their use with a non-aqueous solution entails a liquid-junction potential between the non-aqueous solution of the compound being studied and the aqueous solution In the reference electrode. Some workers have sought to circumvent this by preparing similar electrodes In the same solvents or solvent mixtures that contain the compounds they study when this has been done, the symbol "(o)" (for "organic") follows the abbreviation that would denote the ordinary aqueous form of the reference electrode. 

TABLE 8.9 Potentials of Reference Electrodes (in volts) at 25°C for water-organic solvent mixtures Electrolyte solution oflMHCl... 

For the study of the solvent effect, comparable equilibrium constants have to be determined in water and in solution made with non-aqueous solvents or solvent mixtures. Potentiometric (usually pH-metric) equilibrium measurements are used for this purpose in polyfunctional systems. The solvent effect makes the application of potentiometry somewhat difficult. The substitution of water by organic solvents results in changes of the autoprotolysis constant of the solvent changing the pH scale. The lower relative permittivity of the system favours association processes which have to be considered, e.g., in the determination of the ionic strength of the solution. Diffusion potentials at the liquid junctions connecting the galvanic cell with the reference electrode may falsify the measured data. 

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