15
Experiment 2
Measurement of an Absorption Spectrum
The absorption spectrum of a substance helps identify it. In this experiment, you
will measure the absorbencies of Cr
3+
(chromium III ions) at a series of wavelengths
in the visible light region and draw its absorption spectrum.
The materials required for this experiment are 20 ml 0.5M Cr
3+
(aq) solution and two
round cuvettes.
Procedure:
1.
Place the wavelength dial at a position of 400 nm and adjust the second
order filter lever position.
2.
Fill one cuvette with de-ionized water and insert it into the sample
compartment. Blank the instrument according to the procedure in Basic
Operation. Then, remove the cuvette.
3.
Fill another cuvette (or the same cuvette rinsed thoroughly with the stock
solution) with the Cr+3 stock solution and inset it into the sample
compartment. Record the absorbance of the solution at this wavelength and
then remove the cuvette.
Repeat
Step 1
through
Step 3
for wavelengths from 400 to 650 nm at 20 nm
intervals. Make sure that the meter/display 0%T and 100%T readings are adjusted
at each new wavelength.
Calculations:
1.
Study the data and identify the points where the peaks are likely to be found.
Make measurements at smaller intervals (e.g. 5 nm) around these points to
locate the peaks more accurately.
2.
On regular graph paper, label the horizontal axis wavelength, and mark it
from 400 to 650 nm in 20 nm intervals. Label the vertical axis absorbance
and mark it in equal intervals from zero to a convenient round value above
your highest absorbance.
3.
Plot the absorbance for wavelength measured. Draw a smooth curve through
the data points.
Experiment 3
Study of Beer’s Law
Beer’s Law is the basis for the determination of the concentration of an unknown
solution. It states that there is a linear relationship between the absorbance and the
concentration of the absorbing substance.
In order to verify Beer’s law, we can measure the absorbance for different
concentrations of a substance at the same wavelength, plot them on a piece of
graph paper and see if the data points lie along a straight line.
Actually, the points probably will not form an ideal straight line, due to the
uncertainty in the measurements.
