“On a method of determining indol,” with demonstrations

The method described by the authors constitutes a rapid and accurate means of determining indol. It is based on the fact that indol, in slightly alkaline solution, readily condenses with naphthoquinon sodium mono-sulfonate, and forms a blue crystalline compound which is only very slightly soluble in water and is readily extracted by chloroform from a watery solution or suspension. The condensation compound results from the union of two molecules of indol with one of the naphthoquinon compound. The union does not occur as in the case of compounds with amins, with the elimination of the sulfonic acid group, but occurs between one of the carbonyl groups of the naphthoquinon compound and the imid group of the indol. The new compound is, therefore, a di-indyl naphtho-ketone mono-sulfonate. The solubility of this substance in chloroform is about one part in 4,000 of the solvent, and is sufficiently great to permit a rapid and thorough extraction of the substance. Chloroform containing the di-indyl compound has a red color, very like that of hemoglobin. Owing to this circumstance, the condensation compound in chloroform can be approximated colorimetrically in a convenient manner by comparing the tint of the solution with that of the orange-red glass scale of the Fleischl hemoglobinometer. When more accurate results are desired, the chloroform is evaporated and the residue of the di-indyl compound weighed.

It was found that the method here indicated serves for the recovery of a very large percentage of indol from peptone solutions or bouillon. From solutions containing a little protein, the indol may be recovered almost quantitatively. The presence of a large proportion of protein may cause the retention of considerable indol. The distillation should be carried on directly, without steam, from the acidified fluid. The presence of indol in a small fraction of distillate is best ascertained by boiling the acid solution with a few drops of a 2 per cent. alcoholic solution of di-methyl amido-benz-aldehyde.