Standard reduction potentials in aqueous solutions

Table 16.1 Standard Reduction Potentials in Aqueous Solution at 25 °C...

Practically in every general chemistry textbook, one can find a table presenting the Standard (Reduction) Potentials in aqueous solution at 25 °C, sometimes in two parts, indicating the reaction condition acidic solution and basic solution. In most cases, there is another table titled Standard Chemical Thermodynamic Properties (or Selected Thermodynamic Values). The former table is referred to in a chapter devoted to Electrochemistry (or Oxidation - Reduction Reactions), while a reference to the latter one can be found in a chapter dealing with Chemical Thermodynamics (or Chemical Equilibria). It is seldom indicated that the two types of tables contain redundant information since the standard potential values of a cell reaction ( n) can be calculated from the standard molar free (Gibbs) energy change (AG" for the same reaction with a simple relationship... 

TABLE 1.1 STANDARD REDUCTION POTENTIALS IN AQUEOUS SOLUTION AT 25°C... 

Of the Group 10 elements, nickel, palladium and platinum, only the +2 states of Ni and Pd are well characterized in aqueous acid solutions. Their + 2/0 standard reduction potentials in acid solution are given in the Latimer diagram ... 

TABLE 4.2 Some Standard Electrode (Reduction) Potentials in Aqueous Solution at 25°C... 

Before we discuss standard electrode potential, we will talk about electromotive force (emf). The electromotive force of a cell is the potential difference between the two electrodes. This can be measured using a voltmeter. The maximum voltage of a cell can be calculated using experimentally determined values called standard electrode potentials. By convention, the standard electrode potentials are usually represented in terms of reduction half-reactions for 1 molar solute concentration. The standard electrode potential values are set under ideal and standard-state conditions (latm pressure and 25°C temperature). From the MCAT point of view, you can assume that the conditions are standard, unless stated otherwise. Table 12-1 shows a list of standard electrode potentials (in aqueous solution) at 25°C. 

Source A. J. Bard, R. Parsons, and J. Jordan (eds.), Standard Potentials in Aqueous Solution (prepared under the auspices of the International Union of Pure and Applied Chemistry), Marcel Dekker, New York, 1985 G. Chariot etal. .Selected Constants Oxidation-Reduction Potentials of Inorganic Substances in Aqueous Solution, Butterworths, London, 1971. 

Table A5.6 gives selected values for standard reduction potentials at T = 298.15 K. The values were taken from W. M. Latimer, The Oxidation States of the Elements and their Potentials in Aqueous Solutions, Second Edition, Prentice-Hall, Inc., Engelwood Cliffs, N.J. (1952).

A few elements—C, N, O, S, Fe, Mn—are predominant participants in aquatic redox processes. Tables 8.6a and 8.6b present equilibrium constants for several couples pertinent to consideration of redox relationships in natural waters and their sediments. Data are taken principally from the second edition of Stability Constants of Metal-lon Complexes and Standard Potentials in Aqueous Solution (Bard et al., 1985). A subsidiary symbol pe (W) is convenient for considering redox situations in natural waters. pe°(W) is analogous to pe except that H" and OH in the redox equilibrium equations are assigned their activities in neutral water. Values for pe°(W) for 25 °C thus apply to unit activities of oxidant and reductant at pH = 7.00. pe°(W) is defined by... 

We are looking for the standard reduction potential in basic aqueous solution, so the potential of standard hydrogen electrode has to be modified for the pH of the solution (pH = 14). 

The other diad system, consisting of sensitizer 1 deposited on Cab-O-Sil and electron donor 3 in solution, shows more interesting photochemistry. Referring to Figure 5, the redox potential of 3 allows for the reductive quenching of excited-state Ru(II) via reaction 2. The formal potential of 3 is +0.50 V versus SCE (standard calomel electrode) in aqueous solution (0.1 M KCl, Pt working electrode) by way of comparison, the formal potential of the Ru-(II)-Ru(I) couple is ca. +0.6 V (97, 98). 

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