By A. WURTZ.
This paper is a report drawn up by Wurtz on a memoir of Rosenstiehl on the coloring matters of the madder root. From this root five separate coloring substances can be extracted: alizarin, purpurin, madder-orange, pseudopurpurin, and purpuroxanthin. The researches of Graebe and Liebermann have fixed the constitution of the two former, whilst Rosenstiehl has studied more especially the three latter substances. He has shown that purpuroxanthin is isomeric with alizarin, and can be converted into purpurin by fusion with potash; and inversely purpurin can be reconverted into purpuroxanthin by the action of reducing agents, but if the action be prolonged, hydropurpuroxanthin is formed. Rosenstiehl has also devised a new method of formation of purpurin. On heating the madder root with sulphurous acid, Kopp obtained a product known as commercial purpurin; as this substance is useless for dyeing purposes, it has been customary to heat it to 180° with glycerol to convert it into solid purpurin. This rationale, of the process Rosenstiehl has explained; the "commercial purpurpin" contains pseudopurpurin, which possesses no tinctorial properties, but is easily decomposed into carbonic anhydride and purpurin; this latter dyes a brilliant madder-red. Rosenstiehl has succeeded in separating the pseudopurpurin, and shows that it is a monocarboxyl-derivative of purpurin, thus: purpurin, C14H5O2(OH)3, pseudopurpurin, C14H4O2(OH)3COOH.
This fact has thrown an unexpected light on the practical industry of the madder; for it has long been observed that madder of Avignon gave a more solid dye-stuff than the madder of Alsace.. It is now shown that this fact is due to the greater quantity of lime in the Avignon soil, which serves to eliminate the pseudopurpurin as a lime compound, and prevents it being fixed to the tissue of the root.
It has also been customary in Alsace to add small quantities of chalk to the dye-baths; this also prevents the fixation of the pseudopurpurin which passes into the residues, where it may be decomposed by sulphuric acid and converted into useful purpurin. Rosenstiehl has also studied madder-orange, identical with the munjistin of Stenhouse, and has shown that it is a monocarboxyl-derivative of purpuroxanthin, bearing to it the same relation that pseudopurpurin does to purpurin. These researches also show that the madder-root contains besides alizarin existing as such, three glucosides, viz., one which gives pseudopurpurin or purpurinearboxylic acid, a second which gives alizarin carboxylic acid, and a third which gives munjistin or xanthopurpurincarboxylic acid. The memoir presented to the Academy contains a full account of the various substances obtained from madder, and their physical properties as absorption-spectra.—Jour. Chem. Soc. 1883, p. 598; Compt rend., 96, 465-471.