Voltammetric determination of the total acid content in ethanol-water mixtures. Application to distilled beverages

The voltammetric behavior for the reduction of acetic and perchloric acid w as studied with a platinum microdisk electrode in synthetic ethanol-water m ixtures with an ethanol content of 40 and 50 vol.%. The results demonstrate d that the reduction of acetic acid proceeds through a fast chemical reacti on preceding the electrode transfer involving hydrogen evolution, as in wat er solutions, the dissociation constant of the acetic acid being greater th an about 1 x 10(-6). This allowed us to derive an analytical expression rel ating the steady-state limiting current and analytical concentration of the weak acid. Depending on the concentration level, either standard addition procedures or an absolute equation proved valid to quantify the acid conten t of the synthetic: mixtures. The detection limits evaluated in the synthet ic 40 and 50 vol.% ethanol-water mixtures, provided 1.92 x 10(-5) and 2.15 x 10(-5) M, respectively. The voltammetric procedure was applied to the det ermination of the total acid content in distilled beverages such as whisky, rum, vodka and grappa, having an ethanol content ranging from 38 to 50%. T he results obtained with the proposed method were compared with the AOAC of ficial method which requires a dilution of the sample in water, and a good agreement was found. Comparative measurements carried out directly in the r eal samples by acid-base titrations using either phenolpthalein dye or pote ntiometry with a glass electrode for end-point detection, gave unreliable r esults.