Page 77 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon
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(iii) Thermal dissociation of the salt or oxide. This will be related to the temperature, pressure and
other species present, For dissociation of an oxide, the relevant equations are
and
where G is the free energy of the reaction, a is the degree of dissociation of the oxide, Kp is the
equilibrium constant for the dissociation and Po2 is the partial pressure of oxygen. The partial pressure
of oxygen is effectively controlled by the equilibria
(iv) Reduction of the metal oxide. Data concerning carbon reduction of metal oxides are readily
available in forms such as Ellingham diagrams used by chemists and metallurgists. The essential
reactions to consider are
Such a thermodynamic approach can be extended to consider problems such as carbide formation:
The weakness of this approach is that it deals with equilibrium criteria, whereas the situation in a
furnace and certainly on a filament is highly dynamic. It must also assume some dissociation at all
temperatures, and thus the appearance temperature becomes that at which the free metal is first
detectable; hence the parameter should be dependent upon the detection limit and concentration. Useful
insights have been afforded by the application of thermodynamics, but clearly kinetic factors must also
play a role.
3.4.2 Kinetic Considerations
As L'Vov first pointed out, to achieve analytically useful sensitivity, the rate of formation of the free
atoms must be equal to or greater than their rate of
