Cation-exchange method for the detection of pyrrole derivatives

Abstract

Recently, ion-exchange resins have been used for the detection of uranium [11, nitroso compounds [21, EDTA [3], silver [4], barbiturates [5], mercapto compounds [6], hydroxamic acids [7], hydrazines [8], and zinc [9] by the "Resin Spot Test Technique". Many methods for the detection of indole and pyrrole are based on the colour reaction with p-dimethylaminobenzaldehyde (pDAB). However, these procedures are often of limited selectivity and are lacking simplicity. We have therefore worked out a microqualitative detection combining the colour reaction with p-DAB with cation-exchange. Accordingly, pyrrole derivatives react analogously to pyrrole, whereas 50 gg of isatine, acetphenolisatine and strychnine do not react. We found that under our experimental conditions the quinoidal violet compound is formed on the surface of weakly acidic macroreticular resin. It is essential that the resin grains are first treated with the analyte solution and subsequently with p-DAB solution as otherwise no satisfactory results are obtained. Various types of resins have been tested and Amberlite IRC-50 (in H-form) has been found to give best results (i.e. the most favourable limit of identification). The coloured product is stable within the pH range of 1.4 to 5.3 and can still be clearly recognized even after 60 min. Limits of identification and other characteristics are shown in Table 1. The intensity of the colour depends on the amount of analyte and the reaction time. The selectivity of the proposed procedure was examined by using 106 typical organic compounds. Tests have shown that the following compounds do not cause interferences: alcohols, aldehydes, aliphatic amines, amides, amino acids, carboxylic acids, esters, heterocyclic bases, ketones, nitro compounds, phenols, sulphonic acids and thio compounds. Certain compounds can only be tolerated up to a certain amount (see Table 2).