Chemistry, Third edition

MORE ABOUT ULTRAVIOLET AND VISIBLE SPECTRA

Use of spectra

Spectra are useful for two reasons:

1.They may be used as a ‘fingerprint’ of a compound or element. This is useful in

chemical identification (qualitative analysis). Infrared and NMR absorption

spectra are widely used in the laboratory for this purpose.

2.The absorbance of a substance at a particular wavelength is proportional to the

concentration of that substance – a relationship called the Beer–Lambert law (see

p. 392). Similarly, the intensity of light emission by a substance increases with the

concentration of emitter. These relationships are useful in finding out the con-

centration of compounds or elements in mixtures (quantitative analysis).

For example, in the steel industry steel is analysed by subjecting it to a powerful

electric spark. This causes the atoms in the steel to emit certain wavelengths of light

(‘spectral lines’) which are used by the operators to deduce the elements (e.g. Mn, Cr),

in addition to iron, which are present in the steel. The intensity of the lines depends

upon the percentage of the element present in the sample of steel.

The remainder of this chapter discusses types of spectra (and their applications)

in greater detail.

More about ultraviolet and visible

spectra

Lyman emission series of the hydrogen atom

The simplest type of electronic spectra result from transitions within the simplest

atom, hydrogen. The emission spectrum of the hydrogen atom at ultraviolet wave-

lengths consists of a series of emission peaks (or in a photographic emulsion, dark

lines) called the Lyman series.

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In an atomic absorption spectrometer, a

sample is turned into atoms (atomized)

using a hot flame. UV or visible radiation at

a suitable wavelength is passed through

the atomized sample. The concentration of

absorbing atoms in the sample is

proportional to the observed absorbance at

that wavelength. The technique allows

accurate measurements to be made of the

concentrations of trace metals (e.g. Cu,

Pb, Cr or Mn) even where the

concentrations are below 1 mg per dm^3.