Inorganic free radicals, reduction potentials

D.M. Stanbury, Reduction potentials involving inorganic free radicals in aqueous solution, Adv. Inorg. Chem. 33 (1989) 69-138. 

Reduction Potentials Involving Inorganic Free Radicals in Aqueous Solution David M. Stanbury... 

REDUCTION POTENTIALS INVOLVING INORGANIC FREE RADICALS IN AQUEOUS SOLUTION... 

The two primary reference works on inorganic thermochemistry in aqueous solution are the National Bureau of Standards tables (323) and Bard, Parsons, and Jordan s revision (30) (referred to herein as Standard Potentials) of Latimer s Oxidation Potentials (195). These two works have rather little to say about free radicals. Most inorganic free radicals are transient species in aqueous solution. Assignment of thermodynamic properties to these species requires, nevertheless, that they have sufficient lifetimes to be vibrationally at equilibrium with the solvent. Such equilibration occurs rapidly enough that, on the time scale at which these species are usually observed (nanoseconds to milliseconds), it is appropriate to discuss their thermodynamics. The field is still in its infancy of the various thermodynamic parameters, experiments have primarily yielded free energies and reduction potentials. Enthalpies, entropies, molar volumes, and their derivative functions are available if at all in only a very small subset. 

Stanbury D.M., Reduction Potentials Involving Inorganic Free Radicals in Aqueous Solution, Adv. Inorg. Chem., 1989,33,6-138. (All values of ° quoted in the text are from this source). 

Stanbury, D. M. (1989) Reduction potentials involving inorganic free radicals in aqueous solution. Advances in Inorganic Chemistry 33, 69-138 Stanhill, G. (1982) The Montsouris series of carbon dioxide concentration measurements, 1877-1910. Climate Change 4, 221-237... 

A. J. Bard, R. Parsons and J. Jordan, Standard Potentials in Aqueous Solution, Dekker, New York, 1985. D. M. Stanbury, Reduction Potentials of Free Radicals, Advances Inorganic Chemistry, Academic, New York, 1989, Vol. 33, p. 69. 

The hydroxyl radical is a powerful oxidant, having a reduction potential of 2.7 V in acidic solution. In neutral solution, where the free energy of neutralization of OH by H3O is not available, the reduction potential decreases to 1.9 V (Table 2). Several inorganic anions and low-valency transition metal ions readily undergo one-electron oxidation by reaction with OH, which is often represented as a simple electron transfer ... 

A compilation of reduction potentials of one-electron couples involving free radicals in aqueous solution (mainly organic couples) includes an introductory outline of the quantitative basis for expressing redox properties by means of reduction potentials, which the reader may find useful [11]. Other compilations concentrate on inorganic couples [12] or include data on the kinetics of electron-transfer equilibria [13]. 

The connection between the standard reduction potential and the mechanism for biomethylation seems highly rational, because E describes the relative thermodynamic tendency for the metals involved to accept or donate electrons. The electrophilic mechanism occurs with inorganic compounds with a standard reduction potential (E°) of +0.85 volts or higher (type 1) while the free radical mechanism occurs with inorganic compounds having E° of +0.50 volts or lower (Type 2) (Table 1). 

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