Absorption Spectra and the Beer-Lambert Law

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LS1120 Practical 3
ABSORPTION SPECTRA AND THE BEER-LAMBERT LAW

The purpose of this practical was to measure the amount of a chemical substance present in a sample. Primarily, the aim of Experiment 1 was to measure the absorption spectrum of a particular coloured substance (in this case Bromophenol Blue and Methyl Orange) at varying wavelengths of light. For Experiment 2, the process of the experiment focuses on the substance and records its absorption levels at different concentrations. The absorption levels (A) that are obtained when samples of any given substance exposed to a source of visible light are measured by: The length (l) of the light path (i.e. the distance over which the light had to travel), and the concentration (c) of that substance. This, essentially, is the Beer-Lambert law. Therefore, a molecule can only absorb a specific amount of visible light because of the size of its container (the length from one side to the other) and the concentration. Absorption levels vary across compounds. To measure the absorbance, a reference sample has to be used along with the actual dye sample being tested. The intensity of the light passing through the reference cell is measured for each wavelength of light used. (Chemguide, 2007) Method

Experiments 1 and 2 were carried out as specified in the LS1120 Practical Schedules booklet. In doing so, a problem was encountered. While measuring the absorbance using Bromophenol Blue, it was discovered that the liquids leaked out into the compartment. After this was cleaned up, it became evident that the problem arose due to a zeroing of the colorimeter that was too forceful. This error was corrected and the experiment then continued smoothly. Discussion

A very important step in the course of both experiments was to zero the spectrophotometer using distilled water. This was done in order to improve the accuracy of the results, because water has been found to be an optimum standard for comparison due to the fact that...
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