UV-Vis Experiment 7: Ultraviolet-Visible Absorption Spectroscopy & Colour in Chemical Solutions

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Why are leaves green, why are carrots and some fizzy drinks orange, why are
certain transition metal complexes highly coloured?

This 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!

The Colour Wheel

The Colour Wheel

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.

Sim4Web Virtual Flask

 

Students then have practice at preparing solutions accurately, performing dilutions by using the virtual flask. Samples to be prepared include copper sulphate.

 

 

 

Sim4Web: Absorption Spectrometer Simulator

Sim4Web: Absorption Spectrometer Simulator

 

 

The students can then run spectra of their virtual samples using the UV-Vis
absorbance spectrometer simulator.
Oct Complex

 

 

 

 

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.

Beer-Lambert Measurement

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):

Procedure expt 7 scrren shot 4

Procedure expt 7 scrren shot 1

Procedure expt 7 scrren shot 2

Procedure expt 7 scrren shot 3

Sim Pack 6 screen shot

 

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).

 

 

 

They also have the option to submit their experimental findings in a workbook
(extract shown):
workbook screen capture 1workbook screen capture 2

spectrometer control 2Students 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!

Any questions? Please contact our experts who will be happy to help.