Colorimetric determination of alcohols in spirit drinks using a reversible solid sensor

A sensor for the colorimetric estimation of the alcoholic content has been developed based on the entrapment of the reagent (4-N,N-dioctylamino)-4'-trifluoroacetylazobenzene (ETHT 4001) into polydimethylsiloxane (PDMS) modified with tretraethylorthosilicate (TEOS). When exposed to an alcoholic media, the sensor delivers ETHT 4001 so that it reacts with alcohols, resulting in a change in the solution color. The diffusion of the entrapped ETHT 4001 from the polymeric sensor to the working solution depends on the hydrophobicity of both the sensor matrix and the media, while the time of exposure determines whether the reagent reacts with methanol, or with methanol and ethanol. Thus, the sensors are a versatile tool for the estimation of the methanol content in ethanol or in ethanol/water mixtures, and for the measurement of the total alcoholic content (methanol plus ethanol) in an hydroalcoholic media. The proposed sensors are very easy to prepare, and show good precision and long-term stability. Moreover, the reagent delivery is reversible which allows its recovery once the absorbance measurements have been made simply by adding water, so the sensors can be reutilized and the generation of wastes is reduced. The potential utility of the proposed sensors has been demonstrated by measuring the alcoholic strength of commercial spirit drinks, and the methanol content in spiked hydroalcoholic samples.