Determination of ethanol in alcoholic beverages by gas-diffusion/FIA.

Rapid and selective determination of ethanol in alcoholic beverages has been achieved by using a potential buffer and a gas-diffusion unit equipped with a membrane permselective to alcohol. The membrane is a poly (subs tituted-acetylene)/poly siloxane graft copolymer, developed in Sagami Chemical Research Center. The sample solution (140 μl) injected into a carrier water stream was conducted through one compartment of the gas diffusion unit, which was kept at 70 °C in a water bath. Ethanol vapor permeated across the membrane, was absorbed into a carrier water stream, flowing through the adjacent compartment in the gas diffusion unit. This stream was merged with a stream of potassium dichromate solution, which oxidized flowing ethanol in the reaction tube at 70 °C. The stream of potassium dichromate solution was subsequently merged with a stream of Fe(II) solution. The amount of Fe(III) formed was determined by measuring the potential change, the peak potential, with an oxidation-reduction potential electrode detector, to obtain the ethanol content in the sample. A linear relationship between peak heights and ethanol concentrations was observed in the range of 5 to 30v/v% of ethanol. The relative standard deviation was 1.85% (n=6) for the analysis of 16v/v% ethanol at the sampling rate of ca. 20 samples/h. Coexisting sugars, amino acids and organic acids did not interfere with the analysis of ethanol. The proposed method was applied to the determination of ethanol in alcoholic beverages (sake, shochu and whisky) and provided results consistent with those observed by a GC method.