A disposable voltammetric cell for determining the titratable acidity in wood-vinegars

Abstract

A disposable voltammetric cell using three pencil leads as working, reference, and counter electrodes was developed for determining titratable acidity, i.e. the acid content in wood-vinegar. The materials of the pencil leads were graphite-reinforcement carbons (GRCs). The voltammetric detection of acids was based on the electrochemical reduction of quinone, 3,5-di-tert-butyl 1,2-benzoquinone (DBBQ), in an unbuffered solution. Acetic acid in an unbuffered ethanol-water (3 : 2, v/v) solution of DBBQ gave rise to a new peak (termed the prepeak) on a voltammogram at more positive potentials than that corresponding to the normal reduction peak of DBBQ. The prepeak current was found to be proportional to the acetic acid concentration from 0.05 to 2.7 mM with a correlation coefficient of 0.999. The cell-to-cell reproducibility for 1 mM acetic acid was evaluated with five individual disposable cells. The RSD of the prepeak current and potential were 2.0% and 1.5%, respectively. The titratable acidity in six wood-vinegar samples was determined by voltammetry using disposable cells, and was compared with that of the titratable acidity determined by the conventional potentiometric titration method. We then observed the results by both methods, and found a correlation coefficient of 0.999. As such, the voltammetry using disposable-cell required only two thousandth of the volume of a wood-vinegar sample for the titration method. The present method was superior to the potentiometric titration method in terms of facility, environment-friendly, and economy, and thus a sensor using the present cell would be useful for routine work in the quality control of wood-vinegar.