Voltammetric and Potentiometric Characterization of Model Redox Couples for Intercomparing Scales of Potentials and pH in Mixed Aqueous-Organic Solvents

pH measurements, which are so important in electrochemistry, electroanalysis, biology and corrosion studies, are subjected to many problems and misunderstandings. Most frequent errors are: (i) to calibrate the measure electrode for solvent Z against an aqueous pH standard and using an aqueous reference electrode; (ii) ignoring the difference of width of pH scales in water (W) and in the non aqueous solvent (Z); and (iii) ignoring the relative position of the zero point of the pH scale in Z with respect to W. The last point amounts to evaluate the so called Primary Medium Effect (PME), i.e the standard Gibbs energy change for the transfer of the H+ ion from standard state in W to standard state in Z, which is given by: (1) wherein the term (2) is exactly the position of the zero point of pH scale of Z vs W. Clearly, knowledge of the difference of standard potentials is essential. To this purpose the availability of a reference electrode of potential invariant with varying solvents is necessary: to this aim we reassessed Strehlow’s ferrocinium/ferrocene redox couple in the improved form of the decamethylferrocinium/decamethylferrocene couple, which proved its much better level of invariancy of potential, from coordinated cyclovoltammetric and potentiometric measurements. In this context we have : a) determined comparatively the standard potential of the redox couples Fc+ | Fc and Me10Fc+ | Me10Fc in three series of mixed solvents (Methanol+Water, Ethanol+Water and Acetonitrile+Water) in which we had previously determined primary pH-metric standards according to the IUPAC-endorsed procedure. The underlying cell was: -Pt | Hg |Hg2Cl2|KCl satd. in W|| pH buffer, Fc|Fc+ (o Me10Fc| Me10Fc+) in Z |Glassy Carbon | Pt+ b) determined the standard potential of the quinidrone (QY) electrode in the same series of solvents and the same series of pH-metric standards used as supporting electrolytes. The underlying cell was: -Pt | Hg |Hg2Cl2|KCl satd. in W|| pH buffer, QY in Z | Glassy Carbon | Pt + On the basis of the assumption of invariancy of the potential of the Me10Fc+ | Me10Fc couple the intersolvental liquid junction potentials were cancelled by definition in the difference of emf of the two above cells. From this difference we found , i.e the sought PME was evaluated, to be used to compare the acid ends of pH scales in W and Z. It is worth to remain that equation 2 gives the correction to be applied to a pH measured in Z to get it comparable to a pH measured in W.