Spectrophotometry Essay

Words: 2299
Pages: 10

Herman, Harmon Chris T.

1Prof. Meynard Austria, of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology, Chm171L/A1, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology, Experiment # 4

The objectives of this experiment are to examine the components of a simple spectrophotometer- the Jenway 6100 & Perkin Elmer Lambda 40. As well as to determine the absorption spectrum of a solution and to prepare a Beer’s Law Plot. In the spectrometer used, the light source is imaged upon the sample. A fraction of the light is transmitted or reflected from the sample. The light from the sample is imaged upon the entrance slit of the
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To use a spectrophotometer it is necessary to establish a known series of dilutions containing known quantities of a solute. One of these will contain no solute and is known as the blank . It is used to adjust the instrument to read 100% transmittance or 0 absorbance. In use, a 0% transmittance value (infinite absorbance) is established by placing a curtain between the light source and the photocell. Electronic control is then exerted so that the meter will read 0% Transmittance on its scale. The blank sample (containing no solute or dye) is inserted, the curtain opened and the meter readjusted to read 100% transmittance. All other measures are then made by merely inserting the samples into the light path and measuring the % transmittance. Most spectrophotometers have a built in means of direct conversion of this reading to absorbance.
After recording the absorbance for a series of standards, a plot is made of the absorbance value (y axis) vs the concentration (x axis). The slope of the line is the extinction coefficient.
Note that this may be computed directly by rearrangement of the Beer-Lambert law to [pic] = A/C EQUATION G.2
This value can be calculated for each reading and the average taken as the value of variant. Remember that this value is a constant. Thus, once calculated, it can subsequently be used to determine an unknown concentration by one more rearrangement of the Beer-Lambert law C =