Quantification of the adsorption of methyl viologen on pyrex glass using a renewable perfluorosulfonated ionomer carbon paste electrode

Square wave voltammograms of methyl viologen (MV²⁺) were recorded at a renewable Nafion-loaded carbon paste electrode immersed in 20 mL of phosphate buffer solution (pH 7.4) contained in a beaker-type electrochemical cell (Pyrex glass; contact area = 33 cm²). From the calibration plot obtained in the concentration range 0.01–2.4 μM, it could be concluded that the maximum surface excess of MV²⁺ adsorbed on Pyrex glass (0.24 nmol cm⁻²) was reached at concentrations above 1.4 μM. The adsorption isotherm presented a sigmoidal shape characteristic of attractive interaction. From this plot, it was evaluated that the maximum depletion of MV²⁺ in solution (28%) did not occur at the lowest concentrations, but in nearly micromolar solutions. This result emphasizes the unavoidable and random errors which are involved when MV²⁺ solutions are purposely diluted in Pyrex glassware. The MV²⁺ adsorption proceeds to a less extent on Duran borosilicate glass and/or in acid or alkaline medium.