BEER'S BEER
Learning Objectives:
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Understand the benefits of Raman and IR spectroscopies
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Differentiate similar compounds with vibration-based spectroscopy
Background:
Raman spectroscopy is commonly used for identification of compounds, but it can also be used for qualitative measurements. In this experiment, students will determine the volume fraction of alcohol in a light beer.
According to Beer's law, the absorptive capacity of a dissolved substance is directly proportional to its concentration in a solution.
A Raman Spectrophotometer determines the relationship between the number of photons being scattered and the difference in their energy before and after scattering. In the context of absorption, the more photons that are scattering in a solution, the less of them are being absorbed. Therefore, the photon count or 'Raman Intensity' of each peak is directly related to concentration and Beer’s Law can be employed.
Beer has a matrix effect on incident light since it contains many compounds other than ethanol, each with its own optical density. This effect increases the intensity of the signals. To account for this, students will use a technique called 'analysis by standard addition'. This technique entails using the same quantity of 'analyte' or sample being tested (i.e. beer) in each measurement so that all of the samples will be affected by the matrix the same way. In this experiment, successive small standard additions of ethanol will be poured in and a reading will be taken each time. The spectral data obtained in each reading will be used to generate a plot of signal intensity as a function of ethanol concentration. The linear equation corresponding to that plot will then be used to ascertain the concentration of alcohol in the original beer sample.
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