Weld(CHAPTER V. The Flavone Group.)(Osa artikkelista)

The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

Weld is the dried herbaceous plant known as Reseda luteola formerly cultivated to a considerable extent in France, Germany, and Austria. Its cultivation in this country has nearly ceased, because not only is the quantity of colouring matter it contains very small, but the carriage of the plant, owing to its bulky nature, is expensive. A special interest, however, attaches to weld, for it is said to be the oldest European dyestuff known, and was used by the Gauls and other nations dwelling north of the Alps in the time of Julius Caesar.

The plant attains a height of about 3 feet, is pale brown in colour, and is sold in sheaves like straw. The colouring matter is disseminated throughout the entire plant, but the greater quantity occurs in the upper extremity and the seeds.

Luteolin, the main colouring matter of weld, was examined by Chevreul (J. Chim. Med, 6, 157; Annalen, 82, 53), who obtained it in a crude condition; its isolation in a state of chemical purity was first achieved by Moldenhauer (Annalen, 100, 180), who assigned to it the formula C₂₀H₁₄O₈. It was subsequently investigated by Schützenberger and Paraf (Bull. Soc. Chim., 1861, (i.), 18), who proposed the formula C₁₂H₈O₅ and purified it in a somewhat novel manner which is worthy of mention. Weld was exhausted with alcohol, the extract evaporated, and treated with water, which threw down a dirty greenish precipitate. This was collected, introduced with a little water into a sealed tube and heated to 250°. On cooling the sides of the tube were found to be coated with golden-yellow needles of luteolin, and the impurities had collected at the bottom of the tube to form a resinous cake.

Hlasiwetz suggested that luteolin had the formula C₁₅H₁₀O₆ and was isomeric with the paradiscetin, which he obtained during the fusion of quercetin with alkali (Annalen, 112, 107).

For the preparation of luteolin in quantity, Perkin (Chem. Soc. Trans., 1896, 69, 206, 799) employs weld extract.

300 gms. of the extract dissolved in 3 litres of water is treated with 100 c.c. of hydrochloric acid (33 per cent.), and the mixture is digested at the boiling temperature for some hours. A quantity of a black resinous substance separates, which is collected while hot, and the filtrate, which contains the colouring matter, is allowed to stand for twelve hours. A brown precipitate of impure luteolin is slowly deposited, and is collected, washed, and dissolved in a little hot alcohol. On pouring this solution into ether, the main bulk of the impurity is precipitated, and the ethereal liquid on evaporation yields a yellow residue, which is crystallised from dilute alcohol. The product in addition to luteolin contains apigenin (Chem. Soc. Trans., 1900, 77, 1315), and the latter can only be removed with certainty by the following method: -

The mixture dissolved in boiling glacial acetic acid is treated with a few drops of strong hydrochloric acid; this causes the almost immediate separation of luteolin as hydrochloride, whereas the apigenin remains in solution. The hydrochloride is collected, decomposed by water, and the luteolin crystallised from dilute alcohol.

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It has already been stated that weld contains a second colouring matter, Apigenin (v. Parsley).

Dyeing Properties of Weld.

The importance of weld as a dyestuff in silk and wool dyeing has greatly diminished in consequence of its low colouring power compared with quercitron bark, flavin, and old fustic. This in one respect is unfortunate, because, of all the natural yellow colouring matters, it yields the purest and fastest shades. In conjunction with aluminium and tin mordants it gives very bright pure lemon-yellow colours, and these do not change to an olive or reddish tint as in the case with other vegetable yellows. With chromium and iron mordants weld gives yellowish and greenish olives respectively. For yellow, wool and silk are mordanted with alum and tartar in the usual manner and dyed subsequently in a decoction of weld with the addition of chalk to the dye-bath. Weld alumina yellow is to some extent still employed in this country for certain army cloths and braid. For silk dyeing, weld extract is manufactured in small quantity, and is used for the production of yellow and olive colours.