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Online Chemistry Experiments
Experiment 7: Ultraviolet-Visible Absorption Spectroscopy & Colour in Chemical Solutions
Why are leaves green, why are carrots and some fizzy drinks orange, why are certain transition metal complexes highly coloured?
This introductory experiment answers these questions, teaching students the basics of UV-Vis absorption spectroscopy at the same time.
These exercises are aimed at post 16 students: in schools, colleges and universities (foundation level/year 1). Let your students have the opportunity to run their own spectra & operate a UV-Vis absorption spectrometer simulator!
We look at the relationship between absorbance spectra and the colour that is perceived of the sample solution by using the colour wheel.
Several examples will be studied in this experiment: firstly, the
absorption spectra of simple dyes and food dyes will be recorded to
gain experience in operating the absorption spectrometer simulator.
Students will predict the colour of the dye solution from its absorption spectrum.
Students then have practice at preparing solutions accurately, performing dilutions by using the virtual flask.
The students can then run spectra of their virtual samples using the UV-Vis absorbance spectrometer simulator:
Transition metal complexes will be examined with chromium (iii) as
the metal centre. The relationship between the colour of the sample and electronic structure will be investigated.
How the type of ligand can affect the colour of a complex is also covered.
Students will build up their own spectrochemical series by running spectra of various chromium (iii) solutions and making observations. They will calculate the Crystal Field Stabilisation Energy for each sample from it’s spectrum.
The Beer-Lambert Law is introduced in the final exercise which relates
the absorbance to the concentration of the sample.
Students will make up a series of solutions of chromium nitrate and measure the absorbance spectra. A Beer-Lambert plot will be constructed.
Supervisors can allocate a chromium nitrate solution of unknown concentration. The students can determine the concentration of the unknown sample from their Beer-Lambert plot.
A detailed experimental procedure guides the students through the tasks in the experiment in addition to providing instruction on how to use the spectrometer simulator and the virtual flask (extract shown below):
Students can work their way through the support notes describing the basics of UV-Vis absorption spectroscopy as they progress through the experiment (extract shown on left).
They also have the option to submit their experimental findings in a workbook (extract shown below):
Students gain experience of running absorption spectra by using the simulator to control the wavelength range, scanning speed and mode of operation (either absorbance or transmittance) – it’s just like using the real thing!