A qualitative and quantitative color reaction for pyridoxal with sulfuric acid

The carbonyl-phenol-acid reaction system yields color reactions with phenols, carbonyl compounds (aldehydes and ketones), and inorganic acids. 'I'o test for one of the components of this reaction system, the remaining two components constitute the specific reagents.The four related compounds, pyridoxine, pyridoxamine, pyridoxal, and pyridoxic acid each possess a phenolic hydroxyl. Pyridoxal possesses an aldehyde group in addition to the phenolic hydroxyl. Pyridoxal yields an intense yellow color on treatment with concentrated sulfuric acid. Pyridoxine, pyridoxamine and pyridoxic acid prove not to be chromogenic on treatment with concentrated sulfuric acid. Pyridoxal can therefore be differentiated by means of the sulfuric acid reaction from the other three compounds related to vitamin B₆.The colored product obtained by the interaction of pyridoxa1 and concentrated sulfuric acid yields a characteristic absorption spectrum and follows the Beer-Lambert law in the concentrations of pyridoxal tested (l0—100 μg).