Chemistry, Third edition

INFRARED SPECTROSCOPY

Optical brighteners

Fluorescent molecules known as optical brightenersare used in washing powders

to make clothes ‘whiter than white’. Ageing fibres from white garments acquire a yel-

lowish tinge which cannot be removed by cleaning. The fluorescent molecules in the

washing powder attach themselves to the clothes during the washing cycle. The fluo-

rescent molecules absorb UV light from the sun and emit blue light, and such emis-

sions (when added to the reflected light of the fabric) cause the fabric to appear

whiter than it would otherwise be. This is sometimes called ‘blue whiteness’!

Infrared spectroscopy

Absorption of infrared radiation by compounds

To illustrate infrared absorption, we require a simple IR detector connected to

a voltmeter (a suitable detector is the ‘infrared sensor’, sold by the Philip Harris

Company). There is usually sufficient IR radiation reflected by benches and walls to

provide a substantial background laboratory source of infrared light (Fig. 20.13(a)).

20.7

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Predicting colour from spectra

Predict the colour of the dichromate(VI) ion from its spectrum (Fig. 20.12).

Exercise 20I

Fig. 20.12Ultraviolet–visible spectrum of

2.5 10 –4mol dm–3potassium dichromate

solution (1.0 cm quartz cell).

Sunscreens are creams

containing compounds which

absorb the ultraviolet radiation

emitted by the Sun. Skin

protection is also provided by

UV reflective materials (such

as zinc oxide paste).