NoteConversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25°C
Introduction
The number of publications devoted to the redox chemistry of transition metal complexes in nonaqueous solutions has increased markedly in the last few years. This is especially true for those intended as models for the active sites of metalloproteins. These properties are consequently discussed in monographs [1], [2] and reviews [3], [4], [5]. The redox potential of such species (normally denoted Ef or E1/2 for MLn+/ML(n−1)+) is a key parameter that reflects not only the relative thermodynamic stabilities of the central metals’ oxidation states, but also determines the chemical reactivity of such a complex in redox reactions.
However, use and comparison of these redox potential values is complicated by the fact that recalculation is necessary when different reference electrodes have been used for measurements. The choice of the reference electrode is frequently determined by personal biases of the person carrying out the experiment, as well as the kinds of solvent, supporting electrolyte and complex. The saturated calomel electrode (SCE)1 has been historically one of the most frequently used reference electrodes in laboratory practice, because of the inconvenience of the standard hydrogen electrode (SHE). However, its application in nonaqueous solutions is expected to be limited by: (a) water leakage into the nonaqueous phase and (b) the frequent incompatibility of KCl with these media, particularly when they contain perchlorate ion [70]. Thus, various nonaqueous half-cells based on silver salts have been introduced as useful reference electrodes [6], which also have the benefit of eliminating that part of the junction potential due to the phase boundary. Use of the ferrocenium/ferrocene redox couple as an internal standard has also become widespread during the last decade [7], [8], [9].
Nonetheless, the collation of potentials measured versus different reference electrodes continues to be a significant annoyance in reporting and discussing literature data. The E1/2 for Fc+/Fc in CH3CN appears most commonly stated as being +400 mV versus the normal hydrogen electrode (NHE, vide infra) [7], [8] and most authors adopt this in order to recalculate data standardised against Fc+/Fc. This requires [7], [9], [10] the (actually false [11], [12]) extrathermodynamic assumption that the redox potential of this couple is invariant with solvent2. Furthermore, even recent publications give conflicting statements about the Fc+/Fc potential in acetonitrile (either +450 or +400 mV versus SCE [4], [13]). A logical inference from these statements could be that the redox potentials of couples measured against either of these two (SCE and NHE) electrodes could be reported as being the same, were it not for the fact that the potential of the SCE is actually +244 mV versus the SHE at 25°C [14]. The situation is worsened by the still-frequent usage of ‘NHE’ (Ef=−6 mV3) when ‘SHE’ is probably usually intended — an occurrence common despite its inappropriateness, as clearly pointed out by Ramette [15].
Redox potentials reported versus the Fc+/Fc couple in some cases are also of poor reliability, because the cited E1/2 for Fc+/Fc has not actually been measured under the same experimental conditions [15], [16], [17], [18], and/or has been substituted by a dogmatic reliance [19], [20]4 on one of the values (usually +400 mV versus SCE) iterated in the literature. However, even a brief review (see Table 3) shows that this potential changes in an amazing manner as a function of geography and time, making it extremely difficult to correctly compare various E1/2 values measured against an ‘Fc+/Fc standard’ without appropriate conversion.
Nonaqueous silver electrodes using silver nitrate or perchlorate are reliable reference electrodes for CH3CN and other nonaqueous solutions [11]. However, the exact conversion scales for potentials measured against these electrodes are not always apparent, because the actual Ag+-concentration or salt anion in the Ag+/Ag electrode is not reported in most papers. Also, some publications [5] report potentials against such a reference electrode without any indication of what would be involved in conversion of such potentials to a more fundamental scale. Such logical absences and discrepancies in the recent literature lead to confusion when one needs to recalculate E1/2 values measured versus different electrodes. In some accelerated communication journals, such as J. Chem. Soc., Chem. Commun. and Angew. Chem., the rules for reporting redox potential values are lax and generally potentials reported in these journals are useless for external numerical comparison.
Therefore, we performed direct measurements of these reference electrodes versus each other in order to confirm the relationships amongst their E1/2 values.
Section snippets
Experimental
Potentiometry measurements in acetonitrile solutions (thermostatted at 25.0±0.3°C) were performed using a PAR-173 potentiostat and a Radio Shack 22-163 digital multimeter. The SCEs were from Fisher Scientific and Broadley-James Inc., the saturated sodium chloride calomel electrode (SSCE) was itself prepared from a commercial SCE. Silver was 99.99% wire (Aldrich), CH3CN was distilled off P4O10 under N2, ferrocene (Sigma) was vacuum-sublimed, the supporting electrolytes tetraethylammonium
Results and discussion
The potentials obtained for the various electrodes referenced to the SCE were:APE: +296±2 mV ANE1: +343±2 mV ANE2: +298±2 mV ANE3: +246±2 mV SSCE: −4±2 mV
Adding the well-known relationship between the SCE and the SHE [14], the various electrode potential difference values permit us to propose the correct
Acknowledgements
We thank M.J. Prushan and J.A. Murray for their assistance and Drexel University for support. We offer an apology to those authors whose data are not included in Table 3. V.V.P. personally appreciates the persistence of H.R.C.
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