Page 40 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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of the cone, causing some overhang of the burner. The cone elongates as the gas flow is increased. If
this flow is increased so much that the gas velocity exceeds the burning velocity, the flame will lift off.
If the flow is decreased so that the reverse happens, the flame will strike back with possibly explosive
effect.

The primary reaction zone is so thin that thermodynamic equilibrium cannot possibly be established in
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it and the partially combusted gases and the flame radicals (e.g. OH , H , C CH and CN ), which
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propagate the flame pass into the interconal zone. Equilibrium is quickly established here as radicals
combine. It is usually regarded as the hottest part of the flame and the most favoured for analytical
spectrometry.
The hot, partly combusted gases then come into contact with oxygen from the air and the final flame
products are formed. This occurs in what is known as the secondary reaction zone or diffusion zone.
This discussion refers to premixed gas flames with laminar (i.e. non-turbulent) flow of the gas mixture
to the flame.

Q. What are the requirements of a flame in AAS?

Q. Why are long slot burners preferred for AAS?

Q. Describe and explain the shape of the primary reaction zone.

Q. How can flames be prevented from striking back?

Q. Why does a kettle boil faster when the tips of the blue cones of the Bunsen burner flame are
immediately below its base?

2.2.2.1 Flame Temperatures

Various approaches to measuring flame temperature are well described in Gaydon's book on flames (see
Appendix C). The best methods are spectroscopic rather than those which use thermocouples. The
sodium line reversal method is perhaps the easiest. Sodium is added to the flame and the sodium D
lines viewed against a bright continuum source (e.g. a hot carbon tube). When the flame is cooler than
the source the lines appear dark because of absorption. When the flame is hotter than the tube, the
bright lines stand out in emission. The current to the tube, which will have been precalibrated for
temperature readings by viewing the tube with an optical pyrometer, is adjusted until the lines cannot be
seen. At this reversal point, the flame and tube temperature should be equal.
Other methods, based upon two lines, may be used. Two-line methods may be used in absorption,
emission or fluorescence. The signal is