A General Test for Carbohydrates.

Abstract

A number of tests for carbohydrates depend upon interaction in the chromogenic system, phenol-aldehyde-acid. The Molisch, the Seliwanoff, the Bial, and the Tollen tests belong to this reaction system. The Molisch reagent employes the phenol, α-naphthol, the Seliwanoff reagent resorcin, the Bial reagent orcinol, and the Tollen reagent phloroglucin. The aldehyde in the reaction system is either the carbohydrate itself or the furfural obtained by the decomposition of carbohydrate by means of more or less concentrated acid. Either the reagent or the final reacting mixture contains hydrochloric or sulphuric acid. The aldehyde and the phenol react to form a compound of chromogenic power. The Seliwanoff, the Bial and the Tollen reagent serve to detect certain types of carbohydrates, while the Molisch reagent is universal in its ability to detect carbohydrates of any number of carbon atoms, aldehydic or ketonic in nature, with or without an open carbonyl group, free or in combination with other compounds.

We have developed a test for carbohydrates, which like the Molisch is of general application, by introducing thymol as the phenol in the system, phenol-aldehyde-acid. We have found that a 5% thymol in 95% alcohol serves as a useful, sensitive and practical reagent for carbohydrates. The thymol reagent has the advantage over the Molisch reagent, since the former is a colorless solution and does not deteriorate on long standing. We have kept the alcoholic thymol solutions in ordinary bottles exposed to window light for over a year without the development of any color.

To make the test we proceed as follows:∗ 3 or 4 drops of the 5% thymol solution are added to the test solution. A volume of concentrated sulphuric acid is added equal to that of the sugar-containing solution.