Sulfonic Versus Sulfinic Acid Formula of Aldehyde-Schiff Compounds

The sulfinic acid formula of aldehdye-Schiff compounds, with an SO2 group sandwiched between Schiff's reagent and the aidehyde, was introduced by Wieland and Scheuing in 1921 and is widely used in histochemistry and histotechnology. However, Wieland and Scheuing did not provide any evidence for their sulfinic acid hypothesis and ignored the chemical literature. Their formula was not accepted by most chemists. The sulfonic acid structure of aldehyde-Schiff reaction products was established by Prud'homme in 1900 and has been confirmed repeatedly in chemical and spectroscopic investigations. These studies demonstrated direct reaction of aldehydes with an unsubstituted amino group of Schiff's reagent; the sulfonic acid group attaches itself to the carbon atom of the aldehyde group. As shown by Prud'homme 85 years ago, aldehyde-Schiff compounds have the same staining properties as sulfonated dyes. This can easily be confirmed in the laboratory by adding formaldehyde to Schiff's reagent and staining sections in the resulting deep bluish red solution. It is therefore suggested that the incorrect sulfinic acid formula be replaced by the chemically proven sulfonic acid structure of aldehyde-Schiff compounds.