Electrodes reference water-organic solvent mixtures

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

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

Gold electrodes coated with PP or poly(l-methylpyrrole) were used for the oxidation of hydroquinone and the reduction ofp-benzoquinone in aqueous acidic solutions as well as in a water/organic solvent mixture (and references cited therein). While with uncoated Au electrodes the anodic and cathodic peak separations are larger than 200 mV, the PP coating reduced the separation to 60 mV. This corresponds... 

The reduction is usually made in a multi-compartment electrochemical cell, where the reference electrode is isolated from the reaction solution. The solvent can be water, alcohol or their mixture. As organic solvent A,A-dimethyl form amide or acetonitrile is used. Mercury is often used as a cathode, but graphite or low hydrogen overpotential electrically conducting catalysts (e.g. Raney nickel, platinum and palladium black on carbon rod, and Devarda copper) are also applicable. 

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. 

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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