Schiff's reagent

The Schiff test invented ^[1] and named after Hugo Schiff is a chemical test for the detection of aldehydes.^[2] An unknown sample is added to the decolorized Schiff reagent and when aldehyde is present a characteristic magenta or purple color develops. The Schiff reagent is the reaction product of Fuchsine or the closely related Pararosaniline (lacks a methyl group) and sodium bisulfite. Human skin also contains aldehydes and gets stained as well.

Schiff reagents are used for various staining methods, eg. Feulgen stain and periodic acid-Schiff stain.

Mechanism

Fuchsin itself is colored due to its quinoid structure (see also for example viologen) but is discolorized by sulfonation at the central carbon atom which disrupts the favorable resonance in the molecule. More bisulphite (depending on stoichiometry ) reacts with available amine groups to so-called N-sulfinic acid groups.

The reaction of the Schiff reagent with aldehydes is complex with several research groups reporting multiple reaction products with model compounds. Two different mechanisms appear in the literature ^[3] The first mechanism explaining its action with aldehydes was proposed by H. Wieland in 1935. In it the aldehyde groups react with the sulfinic acid groups forming an sulfonamide.

In the second mechanism, the pararosanilin, sulfurous acid and aldehyde combine to form an alkyl sulfonic acid with sulfur bonded to carbon rather than to nitrogen. This mechanism was first proposed by P. Rumpf in 1935 and experimental evidence was obtained in 1964 by Hardonk and van Duijn ^[4]. They noted that when the aldehyde is reacted first with sulfurous acid and then with the Schiff compound the UV spectrum is identical to one obtained in a standard test. This particular logic was disputed by Stoward in 1966 although on the whole he agreed with the new mechanism ^[5]. In 1980 NMR spectroscopy (Robins, Abrams, Pincock) provided further support for the sulfonic acid mechanism ^[6].

External links

• What is Schiff's reagent?

References