ELECTROCHEMICAL BEHAVIOUR OF METHYLENE BLUE IN NON-AQUEOUS SOLVENTS

• The dye is electro-reduced in two separated monoelectronic charge transfers.
• Solvent/supporting electrolyte/acid/base modifies the electrochemical parameters.
• A dissociation equilibrium of the dye in non-aqueous solvent is proposed.
• The electro-generated and stable dye-radical is also chemically produced in EDA or KOH/DMF.
• A new species is reversibly formed in KOH/EtOH or ACN.

The electrochemical behaviour of methylene blue in solution of non-aqueous solvents with different supporting electrolytes was studied by cyclic voltammetry. Dye electro-reduction presents two well-defined processes of monoelectronic charge transfer yielding a free radical in the first process and an anion in the second electron transfer. Free radical and anion are long living species in some of the studied media. Effects of supporting electrolyte and solvent on the peak potentials, the peak current functions and the reversibility of the charge transfer processes are reported. A dissociation equilibrium of the dye in solution of non-aqueous solvents and the acid or base added determine markedly the electrochemical responses. In the particular cases of KOH/DMF or EDA basic media the chemical formation of the stable methylene blue radical was detected and it was characterized by EPR spectroscopy. A general reaction scheme is proposed.

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