A Practical and Instructive Approach to Purify Acetonitrile for a Wide Electrochemical Window
DOI:
https://doi.org/10.29356/jmcs.v67i4.2013Keywords:
Acetonitrile purification, electrolyte purification, electrochemical window, nonaqueous electrolytes, CH3CNAbstract
Abstract. Because of its large electrochemical window, acetonitrile (MeCN) is one of the most widely used solvents in electrochemistry. It is a suitable solvent for nonaqueous electrolytes that allows studies of cathodic and anodic processes, but electrolyte purification remains challenging. As received, the high-performance liquid chromatography (HPLC) grade is unsuitable for most electroanalytical applications. We present an approach to optimize the purification of HPLC-grade acetonitrile to yield a tetrabutylammonium perchlorate (TBAP)/MeCN electrolyte for experiments in nonaqueous media. We used cyclic voltammetry (CV) to show the background due to impurities and to guide the experimental design to a background current acceptable for CVs of a 1 mM typical concentration of a redox-active molecule. We use 3A molecular sieves, followed by distillation over CaH2 with a final treatment with Al2O3. The optimized procedure yields CH3CN with small background currents, increasing the signal-to-noise ratio and minimizing chemical complications over a wide potential window. Our approach includes discriminating between impurities in the solvent and electrolyte salts; for TBAP, we recrystallize from ethyl acetate and 95 % ethanol. The process and theoretical guidelines apply to other nonaqueous electrolytes dealing with electroactive impurities, including organic molecules, oxygen, and water.
Resumen. El acetonitrilo (MeCN) es uno de los disolventes más utilizados en electroquímica por su amplia ventana electroquímica. Es un disolvente adecuado para electrolitos no acuosos que permite estudiar procesos electroquímicos catódicos y anódicos. Sin embargo, para llevar a cabo estos estudios, es importante considerar la purificación del electrolito que sigue siendo un reto. Tal como se recibe, el acetonitrilo para cromatografía de alto rendimiento (HPLC, por las siglas en inglés) es inadecuado para la mayoría de las aplicaciones electroanalíticas. En este trabajo presentamos un método para optimizar la purificación del acetonitrilo grado HPLC y obtener un electrolito de perclorato de tetrabutilamonio, (TBAP)/MeCN. Utilizamos voltametría cíclica para determinar la corriente de fondo y diseñar una corriente residual aceptable para una concentración típica de 1 mM para una molécula con actividad electroquímica. Utilizamos tamices moleculares 3Å, seguidos de destilación sobre CaH2 y finalizamos con un tratamiento en Al2O3. El procedimiento optimizado produce CH3CN con pequeñas corrientes de fondo, aumentando la relación señal-ruido y minimizando las complicaciones químicas en una amplia ventana de potencial. Nuestro enfoque incluye la discriminación entre las impurezas del disolvente y en las sales del electrolito. Recristalizamos el TBAP con una mezcla de acetato de etilo y etanol al 95 %. El proceso y las directrices teóricas se pueden aplicar a otros electrolitos no acuosos con impurezas electroactivas, como moléculas orgánicas, oxígeno y agua.
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