3. Absorption Spectroscopy
Through observing simple absorption spectroscopy, pupils
can develop an appreciation for one of the underlying principles of
spectroscopy, that coloured objects absorb certain wavelengths of the visible spectrum
and transmit or reflect the others. In this example the spectrum on the upper
left shows that of white light transmitted through a sample of pure water, and
on the lower right a solution of aqueous potassium permanganate. The latter shows
characteristic absorption of the green light.
The
absorption of a sample is defined by the absorbance
of a sample at a particular wavelength.
The absorbance, A, is defined as: A = log10 (I0/It),
where I0 & It refer to the intensity of light
incident on the sample and transmitted by the sample respectively. The absorbance of a sample is related to
the concentration of the absorbing species and the pathlength of the sample by
the Beer –Lambert Law; A = e c l, where e is the molar extinction coefficient (mol-1
dm3 cm-1), c is the concentration (mol dm-3)
and l is the pathlength (cm).
Using
the RedTide spectrograph we can determine the
absorption spectrum of a sample, plotting the absorbance of the sample as a
function of wavelength. The RedTide is a combined spectrograph-CCD camera, and reads
out the intensity of light entering via the fibre optic as a function of
wavelength. To record an absorbance
spectrum we need to record the spectrum of a blank sample, that is a cell
containing pure solvent, and a spectrum of the sample itself. Calculation of absorbance as a function
of wavelength is then carried out by the PC and spectrum plotted on the
screen. A more detailed explanation
of the procedure provided in the Spectroscopy
in a Suitcase PowerPoint presentation.
A. Beeby, 04/09/08