Formaldehyde is present in the indoor air of many industrial and non-industrial environments. The procedure for its analysis which is commonly employed in North America is a NIOSH recommended one that uses chromotropic acid and concentrated sulphuric acid. The nature of the purple chromogen that is produced in the analytical procedure has not been fully understood until now. Using 1 H and 13 C nuclear magnetic resonance spectroscopic techniques and calibration line studies, evidence has been obtained to support the hypothesis that the chromogen has a mono-cationic dibenzoxanthylium structure and not a para,para-quinoidal one that is commonly cited.
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Georghiou et al. (1989) studied this question.