Electrochemical NMR spectroscopy: Electrode construction and magnetic sample stirring

Abstract

The use of nuclear magnetic resonance (NMR) spectroscopy to study electrochemical reactions in situ (EC-NMR), in solution, was demonstrated more than four decades ago. Although, the immense potential of EC-NMR has been demonstrated in several applications, this method has been limited to a few specialized groups since there are no commercial EC-NMR cells or cells that can be easily assembled for routine measurements. In most of the proposed EC-NMR cells the electrodes are placed inside the NMR coil, which deteriorates the magnetic field homogeneity and reduces the signal-to-noise ratio. To minimize these interferences, electrochemical cells have been constructed as flow cells, using ultra thin metallic films as electrodes, which are too complex to be produced by non-experts, or use non-metallic electrodes that have limited electrochemical applications. In this paper we report a very simple and efficient way to construct an EC-NMR cell using standard Pt and Ag wires as electrodes. These wires are assembled as coils on capillaries and are inserted into 5 mm NMR tubes and placed approximately 0.5 mm above the NMR coil. During the in situ EC-NMR reaction the magnetohydrodynamic effect stirs the solution, homogenizing the concentration of the reagent and the product in the NMR detection region. This effect allows the concentration of the analyte to be measured in real time, even with the electrodes outside the NMR detection region. The electrode materials are not limited to Pt or Ag wires and can be replaced by other materials according to the studied reaction. Details showing how to make the electrodes are presented in a video in the supplementary material. The application and performance of this cell were demonstrated with an in situ EC-NMR study of the electro-oxidation of ascorbic acid in solution. In this particular case the magnetic field increased the reaction rate by a factor of approximately two.