Quantitative Determination of Ethanol in Local Thai Alcoholic Beverages by Raman Spectroscopy

Abstract

This study focuses on various concentration of ethanol content produced locally in Thailand. The alcoholic beverage with ethanol content of 35% and ethanol content of 40%, named sample A and sample B, were quantitatively analyzed using Raman spectroscopy. The characteristic vibration of CH^- and OH band in the ethanol molecule (CH₃CH₂OH) were analyzed. To create the calibration curves (CH) and (OH), the band areas of stretching vibrations of the CH group within the wavenumber ranging from 2800-3000 cm^-1 and the OH group within the wavenumber ranging from 3100-3600 cm^-1 were plotted in comparison with the various ethanol concentrations in the ethanol-water binary standard solution (0 - 95%v/v with 5%v/v step). The calibration curve (CH) clearly demonstrated a linear relationship between the vibrational band area and the ethanol concentration with an R² value of 0.997. On the other hand, the calibration curve for OH exhibits poor linearity with a coefficient value (R² = 0.947). The calibration curve (CH) calculation yielded the exact acceptable values of ethanol concentration in samples A and B of about 33.88±0.03 and 39.47±0.02, respectively. The percentage difference between sample A and B (comparing with the Ebulliometer value) was about 2.69% and 4.22%, respectively. However, the calibration curve (OH) determined that the extreme percentage differences between samples A and B were 40.86% and 30.01%, respectively, due to strong vibration of H-bonding present in water which interrupts the OH stretching vibration in ethanol resulting in higher calculating error in the ethanol-based samples. Based on these findings, it can be stated that quantitative Raman measurement using a calibration curve (CH) performs well in estimating the ethanol content of locally produced alcoholic beverages in Thailand.