Color Reaction with Phloroglucinol-Hydrochloric Acid

1 Color Reaction with Phloroglucinol-Hydrochloric Acid 

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The Wiesner Reaction. The reaction of lignified tissue and phloroglucinol—hydrochloric acid gives a visible absorption spectmm with a maximum at 550 nm. This has been attributed to coniferaldehyde units in lignin as the groups responsible for the color formation. 

A sampling of the colors produced in the reaction of various phenols and amines with lignin is presented in Table 2.1.1. Phenolic compounds are applied as acidic aqueous solutions and aromatic amines as aqueous solutions of the corresponding hydrochloride or sulfate in the presence of acids. Of the phenols, phloroglucinol is the most important, and its solution in hydrochloric acid is known as the Wiesner reagent. 

The presence of the aldehyde is shown by treating the product of the reaction with aniline, C6H5.NH2, and hydrochloric acid with these reagents the aldehyde produces an intense red dye. When warmed with hydrochloric acid and phloroglucinol (537) pentosans and pentoses produce a cherry-red coloration. 

The presence of lignin in wood, straw, bran, and so forth, can be shown by certain characteristic color reactions. Such substances give a yellow color when treated with aniline hydrochloride, and a crimson color with phloroglucinol and hydrochloric acid. Lignin belongs to the class of compounds known as pe ntosans. They are so-called because they yield a pentose on hydrolysis. 

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