A critical investigation of the liebermann colour test: The formation and behaviour of phenolindophenol in strong acid media

Abstract

The Liebermann test has been investigated by spectroscopic and Chromatographic methods. Phenols in concentrated sulphuric acid reacted with nitrite to give green to blue solutions which turned to red or violet, yellow or brown, respectively, after dilution with water. Alkalization of these solutions changed the colour to green or blue, but also to yellow or brown, respectively. The reaction of phenols withp-nitrosophenol was also studied. Except for 4-iodophenol and hydroquinone, phenols substituted in thep-position did not react with either nitrite orp-nitrosophenol. Negative reactions were obtained with 2- and 3-nitrophenols, 2- and 3-hydroxybenzoic acids and 2,6-dichlorophenol, although these phenols have a freep-position. The sensitivity is much poorer than that of other reactions for detection of phenols, e.g. the Emerson or Gibbs reactions. The Liebermann test is often recommended as a generic test for phenols, but its importance, selectivity and sensitivity seem to have been considerably overestimated. Application of the reaction for spectrophotometric determination of phenols is impracticable, because of the very poor reproducibility, the small yield of reaction product and the inconvenience of working with concentrated sulphuric acid media.p-Nitrosophenol was found to be the operative species and the main product was phenolindophenol, which is present in its diprotonated form H₂In²⁺ in the medium of concentrated sulphuric acid. Further forms of the dye can exist, depending on the pH or Ho of the medium. The orange spiro(7-hydroxyacridine-9,1′-cyclohexa-2′,5′-dien)-2,4′-dione was found to be the main byproduct of the reaction. The formation of further by-products was observed, of which 1,4-benzoquinone and 4-nitrophenol were identified.