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 1H and 13C 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. Keywords: Formaldehyde, formaldehyde analysis, chromotropic acid, chromogen, dibenzoxanthylium cation.

Die Desaktivierung von β‐Hydroxydecanoylthioesterdehydrase durch den Thioester (I) beruht, wie sich 13C‐NMR‐spektroskopisch zeigen läßt, auf einer Umlagerung von (I) zum entsprechenden Allen, der die Alkylierung des Histidinrestes am aktiven Zentrum des Enzyms zu einem Addukt vom Typ (II) folgt.

. J. Chem. 69, 1207 (1991).The 'H and I3C NMR spectra of chromotropic acid (CTA) (4,5-dihydroxy-2,7-naphthalenedisulphonic acid) have been unambiguously assigned. Proton NOED spectra were used to show the proximity of both H-3 and H-6 and the hydroxyl groups. Two-dimensional 'H-'~c NMR correlation spectra of CTA, of its corresponding diacetoxy derivative, and of 3-bromo-and 3,6-dibromo-CTA support the assignments. A regioselective deuterium exchange reaction of the C-3 and C-6 protons of CTA with deuterium oxide was observed during the NMR experiments. This latter finding is strongly indicative of the mode of formation, and of the nature of the chromogen formed in the reaction of CTA with formaldehyde in the well-known CTA-formaldehyde analytical reaction.

J. Chem. 69, 1207 (1991). The 'H and I3C NMR spectra of chromotropic acid (CTA) (4,5-dihydroxy-2,7-naphthalenedisulphonic acid) have been unambiguously assigned. Proton NOED spectra were used to show the proximity of both H-3 and H-6 and the hydroxyl groups. Two-dimensional 'H-'~c NMR correlation spectra of CTA, of its corresponding diacetoxy derivative, and of 3-bromo-and 3,6-dibromo-CTA support the assignments. A regioselective deuterium exchange reaction of the C-3 and C-6 protons of CTA with deuterium oxide was observed during the NMR experiments. This latter finding is strongly indicative of the mode of formation, and of the nature of the chromogen formed in the reaction of CTA with formaldehyde in the well-known CTA-formaldehyde analytical reaction.