Standard cell potentials, variation with

Since concentration variations have measurable effects on the cell voltage, a measured voltage cannot be interpreted unless the cell concentrations are specified. Because of this, chemists introduce the idea of standard-state. The standard state for gases is taken as a pressure of one atmosphere at 25°C the standard state for ions is taken as a concentration of 1 M and the standard state of pure substances is taken as the pure substances themselves as they exist at 25°C. The half-cell potential associated with a halfreaction taking place between substances in their standard states is called ° (the superscript zero means standard state). We can rewrite equation (37) to include the specifications of the standard states ... 

Standard electrode potentials of the Ag-AgI electrode were determined in the temperature range 5 °-35°C in 20-80 wt % ethylene glycol + diethylene glycol mixtures by emf measurements on the cell Pt-H2(g, 1 atm)/HOAC (mt), NaOAC (m2) KX (m3)/AgX-Ag in the solvent. The standard molal potentials Em°, in the various solvent mixtures have been expressed as a function of temperature. The various thermodynamic parameters for the transfer of hydrogen iodide from ethylene glycol to these media at 25° C are reported, and their variation with solvent composition is discussed. The transfer free energies of the proton and the iodide at 25°C, on the basis of the ferrocene reference method with ethylene glycol as the reference solvent, are also reported in the mixtures. 

A reference electrode [183-185] is a half-cell that defines a potential to which all other measurements are referred. The primary standard electrode is the standard hydrogen electrode (SHE), but as this electrode is inconvenient for practical work, other reference electrodes are used. Such reference electrodes must have a potential that changes very little and is known to within 1 mV or so. In some cases of controlled potential electrolysis, it is sufficient to know the potential of the working electrode during the electrolysis within 10-20 mV, because the potential variations between different points of the electrode are of this magnitude (see earlier), and less precise electrodes may be termed comparison electrodes. In principle, any electrode at the surface of which an electrochemical reaction with a large exchange current can take place may be used as a reference electrode. 

The emf of a cell depends on the concentrations of ions and on gas pressures. For that reason, cell emfs provide a way to measure ion concentrations. The pH meter, for example, depends on the variation of cell emf with hydrogen-ion concentration. You can relate cell emfs for various concentrations of ions and various gas pressures to standard electrode potentials by means of an equation first derived by the German chemist WalthCT Nanst (1864—1941). ... 

In order to overcome the difficulty due to the discrepancy of the values of activities and concentrations, which may be important, the concept of formal potentials E° has been devised. (The symbol E° of formal potentials is, unfortunately, the same as that used for standard biological potentials.) The formal potential is that which is experimentally observed with solutions containing both Ox and Red forms of the couple at the unit concentration and that also may contain other species whose concentrations are specified. They take into account the variations in activity coefficients with the ionic strength, the acid-base equilibria involving the Ox and/or Red form(s), and their possible complexations with the other solution species. They are experimentally determined using electrochemical cells of the classes described above, after measurements of their zero-current cell potentials. The formal potentials can be used only when the experimental conditions of the redox reaction under study are the same as those under which they have been determined. 

The potential in standard conditions ( °) of other electrochemical pairs can be obtained with respect to Eq. 3.4, permitting the compilation of a list of semireaction potentials (electrochemical series ). In this list, all the semi-reactions are written in such a way to evaluate the tendency of the oxidized forms to accept electrons and become reduced forms (positive potentials correspond to spontaneous reductions) [2]. These potentials can be correlated to thermodynamic quantities if the electrochemical system behaves in a reversible way from a thermodynamic point of view, i.e., when the electrochemical system is connected against an external cell with the same potential, no chemical reaction occurs, while any inhnitesimal variation of the external potential either to produce or to absorb current is exactly inverted when the opposite variation is applied (reversible or equilibrium potentials, Eeq)- When the equilibrium of the semi-reaction considered is established rapidly, its potential against the reference can be experimentally determined. 

The Variation of the Standard Potentials of Some Electrodes with the Temperature. In a number of cases the standard potentials of galvanic cells without liquid junctions have been determined over a range of temperatures. From these determinations it has been possible to prepare Table V, which gives the standard potentials of a number of electrodes at intervals of 12.5° from 0° to 50°. Some slight adjustments, of the order of 0.2 millivolt, of the original data have been necessary to bring the figures into accord with the Ho values at 25° adopted in this book. A more complete table of standard potentials of the elements at 25° will be found at the end of Chapter 14. 

The required attributes listed above effectively limit the range of primary buffers available to between pH 3 and 10 (at 25 °C). Calcium hydroxide and potassium tetraoxalate tire excluded because the contribution of hydroxide or hydrogen ions to the ionic strength is significant. Also excluded are the nitrogen bases of the type BH+ (such as tris(hydroxymethyl)aminomethane and piperazine phosphate) and the zwitterionic buffers (e.g. HEPES and MOPS (10)). These do not comply because either the Bates-Gu enheim convention is not applicable, or the liquid junction potentials are high. This means the choice of primary standards is restricted to buffers derived from oxy-carbon, -phosphorus, -boron and mono, di- and tri-protic carboxylic acids. The uncertainties (11) associated with Harned cell measurements are calculated (1) to be 0.004 in pH at NMIs, with typical variation between batches of primary standard buffers of 0.003. 

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