Electron free energy levels in oxidic solutions: relating oxidation potentials in aqueous and non-aqueous systems

We provide background to the problem of describing the state of redox couples in different types of solvent media ranging from acidic aqueous solutions to high temperature molten silicates, pointing out the essential similarity between these solvent media in Lewis acid–base terms. We review the adaptation of the Gurney proton energy level diagram approach to the case of electron transfer processes. Using data from various spectroscopic and analytical chemistry sources, we review the construction of electron free energy level diagrams for redox couples in aqueous and non-aqueous systems using, as a common reference, the potential of the oxygen gas (1 atm)/ oxide ion couple in the solution of interest. We emphasize the anomalous effect of “oxide ion activity” (mean ionic activity of alkali oxide) on the state of equilibrium and interpret this in terms of oxide ion transfers that accompany electron transfers. After showing the essential agreement between recent direct electrochemical assessments of the energy levels and those deduced in our original analysis of oxidic melts of different glass formers, we provide an interpretation of the apparent “oxide ion transfer” in terms of the differential medium polarization by the two redox species involved in the equilibrium. We anticipate the extension of these ideas to redox chemistry in the currently burgeoning field of “ionic liquids” in its recent ambient temperature liquid incarnation. Introduction John Bockris pioneered the application of modern ideas about ionic substances to the case of silicate liquids and was the first to use the term “ionic liquids” for their description. His papers with Lowe [1], McKenzie [2], Kitchener [2, 3], and Tomlinson [3, 4] on this subject are now classics. In rather separate studies, he made great contributions to the electrochemical sciences, pioneering with Parsons and Conway [5] the strategies for electrochemical purifications, among a great many other developments. These latter works were focused on applications involving aqueous solutions, though they relate equally to purification of molten salts, polymer electrolytes, and liquid silicates. The electrochemistry of molten salts, if not liquid silicates, is a subject of great current importance in relation to the technology of nuclear fuel reprocessing [6]. In liquid silicates, redox chemistry has always been of great importance in the winning of metals from ores, but the electrochemical conditions of importance have been determined more by oxygen pressure variations controlled by carbon chemistry than by direct electrochemical interrogation. Electrochemical studies of redox potentials have been more recent. Most of the redox potentials in non-aqueous solutions discussed by us long ago [7] and now reproduced in this paper were gleaned from chemical equilibrium studies (at fixed oxygen pressure established by bubbling gases), e.g., [9–12], rather than the inverse process where the chemical concentrations are fixed by the experimentalist and the potentials then directly measured [13, 14]. The latter approach is best illustrated by the recent work of Rüssel and co-workers using square wave voltammetry [15–18], to be discussed further below. We became interested in the redox chemistry of species in silicate, borate, and phosphate liquids through the J Solid State Electrochem DOI 10.1007/s10008-008-0775-0 Dedicated to the 85th birthday of John O’M. Bockris. C. A. Angell (*) Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 8528, USA e-mail: [email protected] absorption spectra of transition metal ions that may be dissolved in them [19, 20], some of which (Ni and Co) provided useful probes for the likely local structures around important liquid silicate cations like Mg and Zn. The attractive feature of transition metal ion spectroscopy lies in the precision of the structural information that the spectra convey [21] and the richness of structural variation that can then be deduced from the changes in electronic absorption lines that accompany changes in silicate melt structure. These changes can themselves be controlled by chemical composition, specifically by the oxide oxygen-to-Si ratio, which in turn determines the activity of oxide ions (more precisely, the mean ionic activity of the basic oxide, such as Na2O, that is being used to donate oxide ions to the disruption of the initially polymeric silica network). As our work in this area proceeded, particularly the aspect driven by the need to produce a coherent body of work under the title Glass: structure by spectroscopy [7] (the book coauthored with friend and colleague Joe Wong), we became impressed by the essential similarity of redox equilibria witnessed in high temperature liquid silicates via absorption spectroscopy and those seen by electrochemical measurements in aqueous solutions. This similarity [7] has been given little attention in the literature since that time but has recently been noticed also by Duffy and Ingram [22]. However, the relevance of the Pourbaix type diagram for redox equilibria, developed in such detail for aqueous solutions and adopted by us for the case of silicate melts, seems to have been completely neglected in subsequent work. Given the current success of direct electrochemical measurements on oxide melts [15, 18], the failure to utilize such a useful and economic graphical summary of the redox equilibria characteristic of a given solvent as the Pourbaix diagram leaves a serious gap in the field, as we will review. A key insight from our earlier study of this problem concerned the reasons why highly basic environments in melts tend to favor the higher oxidation states in any given system. This was given particular emphasis in [23], which, isolated in an obscure symposium volume, has received little attention. While the more recent literature contains abundant confirmation of this trend [18, 24], there has been little concomitant development beyond, or even commentary on, our essentially simple interpretation of this trend. Since this insight seems to be missing from other publications in the solutions area, we give it some emphasis in this paper, reproducing much directly from [23]. The present paper then represents a revisiting of this phenomenology with a commentary on the progress that has been made during the 40 years of research since the publication of Glass: structure by spectroscopy. We begin with some observations intended to make molten silicate and aqueous solutions seem less different than they might appear at first sight. The familiar “basic” solution of aqueous solution chemistry is produced by reacting the strong Lewis base, Na2O, with the weak Lewis acid, H2O, initially to produce, by proton transfer, the “neutral” compound NaOH. To this product is then added a large excess of the acid component (55.5:1 molar ratio for a one molar solution). In an exactly analogous fashion, an ordinary sodium silicate glass is produced by reacting the same Lewis base Na2O with the Lewis acid SiO2, initially to form the neutral compound, Na4SiO6, to which is then added a large excess of the acid compound. It is not surprising then that the chemistry of the two types of acid–base systems should have common features: Both are characterized by very low values of the mean oxide activity a2 . At ambient temperatures, of course, one of the systems is a solid glass, hence out of equilibrium, while the other is mobile equilibrated solution (see footnote 1). However, the equilibria in the silicate systems were established at high temperatures in equilibrium with an air atmosphere, at pO2 1⁄4 0:2 atm (which requires a gas bubbler, to be reliable), and when the liquid is quenched, the redox equilibria are trapped in for subsequent ambient temperature study [the slowness of equilibration with stagnant atmospheres has not always been recognized, and where discrepancies between direct electrochemical and spectrophotometric (or chemical analysis-based results occur), this is a likely cause]. We note that, while fundamental silicate solution studies have mostly been carried out with sodium silicate solutions, practical glasses, e.g., “window” glass, require a certain amount of the weaker base CaO to be included for the very practical reason that the pure sodium silicate solution is rather soluble in water. Because it is not a common consideration in aqueous chemistry, it is useful to point out in this study that the oxide activity sensed by the redox species in oxidic solutions is very much dependent on the nature of the cations that accompany the basic oxide that reacts with the Lewis acid to generate the solution. In the field of geochemistry and also in the science of zero expansivity materials, the aluminum cation is important. A number of recent studies permit us to view the effect of these cations on the “oxide activity” of melts through their effect on the potentials of various redox couples [18]. These are generally consistent with the conclusions to be drawn from the well-known work of Duffy and Ingram [25–29] in which a spectroscopic method of assessing the activity of the oxide ion in melts and glasses is used. Duffy and Ingram used the ultraviolet spectra of ions such as Tl, Pb, and Bi to probe the availability of electrons in the 1 Interestingly enough, NaOH solutions of the same acid (H2O) to (Na2O) ratio as the most-studied Na2O–SiO2 solution, also have glassforming ability and have been much used in low-temperature radiation chemistry studies. J Solid State Electrochem