UV/ VIS Spectrometry

(a) State the principles on which ultraviolet/ visible spectroscopy is based.
(5 marks)
(b) An experiment is carried out to determine the concentration of iron (II)
(Fe 2+ ) found in water samples taken from a well. A measured volume of
the water sample is treated with a buffer and the reducing agent
hydroxylamine, followed by the reagent 1,10 phenanthroline, shown
below. The resultant solution is then topped up to a volume of 50cm 3 .
The solution is subsequently analysed in a cuvette, of side 1cm 3 at a
wavelength of 510nm in the UV/VIS spectrophotometer.

(i) What would be observed on the addition of the 1,10
phenanthroline? (1 mark)
(ii) What properties of the organic reagent make it suitable for this
analysis? (3 marks)
(iii) Suggest a reason for selecting a wavelength of 510nm for analysis.

Fe2+ ions in solution are a very pale green (low molar absorptivity). If Phenanthroline is added, they form a complex with it that is a very dark orange colour (high molar absorptivity).

If you tried to use colorimetry to measure Fe2+ conc without phenanthroline it would not work very well unless you had high concs of Fe2+. Addin the phenanthroline makes the colour much stronger- makes it easier to measure conc by colorimetry accurately.

ii. Phenantroline has two N atome each with a lone pair of electrons. these form dative (coordinate) bonds to the Fe2+ ion, forming a colured complex. The lone pairs on the electronegative N atoms are essential for phenanthroline to be able to form a complex with fe 2+ ions

Why use 510nm for analysis- this is because this is the wavelength of light that the fe2+/ phenanthroline complex absorbs at the most

sorry i didnt catch the last part where it said 'absorbs the most'