Purge and trap in-tube colorimetric detection method for the determination of ethanol in alcoholic and non-alcoholic beverages

A simple and cost-effective analytical approach based on the combination of purge and trap technique coupling with smartphone-based image analysis was developed for the sensitive detection of ethanol. Herein, the dichromate solution was used as the detection reagent to determine the ethanol concentration in alcoholic beverages. The ethanol from the sample solution was purged by the airstream and then trapped into a 1-mL Eppendorf tube containing the detection reagent. During the process, ethanol is oxidized by dichromate reagent to form ethanoic acid, and the orange color of the dichromate ion is started to become darker after one minute. The color image of the Eppendorf tube was taken by smartphone in a controlled-light box and then analyzed by the laptop ImageJ software. Experimental parameters affecting the method sensitivity, such as extraction time, stirring rate, and dichromate concentration, were studied. Under the optimal conditions, the calibration curve was linear for the ethanol concentrations ranging from 0.15 and 3.0 %v/v (R² >0.993). The limit of detection of 0.05 %v/v was obtained. The intra- and inter-assay results (RSD%) were less than 1.0% and 2.0%, respectively. The developed approach has been successfully applied to determine ethanol percentage in beverage samples (alcoholic beer, brandy, and non-alcoholic beer), with relative recoveries ranging from 86-112%. Finally, the accuracy was assessed by comparison of the results obtained by our method and the UV-Vis spectrophotometric method, which agreed at the 95% confidence level.