Quantitative Analysis by Spectrophotometric Methods

2206 Words Sep 8th, 2008 9 Pages

In this experiment, the absorbance of KMnO4 was measured by spectrophotometric method to determine the molar concentration and the molar extinction coefficient of KMnO4. In part 1, in order to determine the maximum absorbance wavelength of KMnO4, we measured the absorbance of the sample solution which contains KMnO4 at the wavelengths between 330nm and 660nm, and plotted the λ and A points; the λmax was 530nm. In part 2, the effect of concentration on the absorbance was examined. We prepared five differently concentrated (but, same path length) solutions, and measured the absorbance of them at the λmax(530nm) discovered in part 1; According to the results, higher concentrated solution had higher absorbance value. The extinction
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In higher concentrated solution, more absorbing molecules are present in the path of the light, and the chance the light strikes the molecules will increase. So, more light will be absorbed by the sample; less light will be transmitted. In contrast, in lower concentrated solution, less light will be absorbed, and more light will be transmitted. The concentration is represented by the symbol C and is typically measured in mole/L.
Another factor that affects the amount of light a sample absorbs is the path length which is the length of sample that the light passes through. The path length is represented by the symbol l and is typically measured in centimeters. When the light travels through longer distance, it will strike more absorbing molecules, so more light will be absorbed, and less light will be transmitted. The relationship of two factors (path length & concentration) can be combined to yield a general equation called Beer's Law.
- log10T = A = εlC (3)
The quantity ε is the molar absorptivity; in older literature it is sometimes called the molar extinction coefficient. It is the measure of how strongly a substance absorbs light at a particular wavelength; a larger extinction coefficient means that substance absorbs more light. The units of ε are usually in M-1cm-1 or L mol-1cm-1. A is the absorbance of
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