Fuchsin-sulfurous acid reagent

The test for ethylene glycol was performed by oxidation of the sample with periodic acid and detection of the resulting formaldehyde with fuchsin-sulfurous acid reagent (9). 

For the reaction between a simple aldehyde and Schiff s reagent it has been postulated77 that a complex is formed between two molecules of the aldehyde and the fuchsin-sulfurous acid reagent and there is evidence to support this idea.78,79... 

Fuchsin — sulfurous acid (Schiff s reagent for aldehydes). 

The above substitutes for the iodometric determination with one or two exceptions have not been tested for specificity for free SO2. Mathers (1949) on the basis of the fact that free sulfur dioxide is removed early in the distillation of wine proposed that 10 ml. of the distillate from the alcohol determination be mixed with 0.5 ml. of 5% neutral lead acetate and the turbidity of the suspension formed be used to correct volatile acidity for sulfur dioxide. La Rosa (1950) assumed that the color formed on mixing the fuchsin-sulfuric acid-formaldehyde reagent with white wine was a measure of free SO2 but did not give any data to confirm this. The fuchsin procedure and the lead sulfite procedures are very sensitive, the former more so, and can be applied only to very small aliquots or to dilute solutions. 

In the same period, Schiff also concocted the so called Schiff s reagent as a test to identify aldehydes this reagent is used today to determine the sequence of DNA fragments. It is a mixture of a fuchsine dye (which contains some aromatic amines) with sulfurous acid or bisulfite sulfonation by the latter dismpts the dye s conjugated chain and renders it colorless. In the presence of an aldehyde, a complex series of reactions with various products takes place initially the fuchsine s aromatic amines react with the aldehyde to produce aldimines, which further react with bisulfite to produce resonance-stabilized products, restoring the red fuchsine color. 

Schiff s reagent is an aqueous solution formed by combining p-rosaniline hydrochloride (basic fuchsin), sodium bisulfite, and concentrated hydrochloric acid (Eq. 25.10). The reagent reacts with aldehydes to produce an imine, also called a Schiff base, that is magenta or purple in color (Eq. 25.11). The mechanism for forming the addition product presumably involves nucleophilic attack of sulfur on the aldehydic carbonyl group. 

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