Yellow natural dyeings, testing

Testing of Yellow Natural Dyeings. When a yellow dyeing is boiled in glacial acetic acid (cf. solvent stripping tests), berberine (C.I. Natural Yellow 18), the only natural basic yellow, is dissolved. This dye can be identified by TLC comparison on silica gel with the solvent butanol-glacial acetic acid-water (5 1 2). 

The 4,7-dinitro-4,5,6,7-tetrahydro[l,2,5]oxadiazolo[3,4-6]pyrazine (67) has been tested as an explosive <85USP4539405>. The o-quinonoid nature of the [l,2,5]thiadiazolo[3,4-d]pyridazines results in their 4,7-diaryl derivatives being used as fluorescent yellow-red dyes for organic polymers... 

Testing of Black Natural Dyeings. Black dyeings are first boiled in tin(II) chloride solution (15). During this treatment, dyeings with iron tannate (C.I. Natural Brown 6) become almost colorless, whereas dyeings with tanners sumach (Rhus coriaria L., Herba, C.I. Natural Brown 6) turn yellow-orange because of their quercetin content. 

Testing of Violet Natural Dyeings. Violet dyeings are first boiled in 102 sulfuric acid. If the dye dissolves with a yellow color, a madder dyeing on iron mordant is indicated. If the sulfuric acid turns orange and a blue remains on the fiber, the dyeing is a mixture of cochineal and indigo, which can be identified separately by the methods described earlier. 

To test for indigo, a sample was warmed in a standard reducing solution (1 g of NaOH, 1 g of sodium dithionite, and 100 mL of water) until fibers and solution were yellow. A few drops of ethyl acetate was then added, and the tube was shaken gently. If indigo was present, the ethyl acetate layer became blue as any indigo present was reoxidized. Comprehensive outlines of spot tests for other natural dyes can be found in References 1 and 2. 

The flavone morin (700) (Natural Yellow C.I. 75660) dyes wool yellow, the actual shade being determined by the mordant which is used. It has also been mentioned as a spot test reagent for salts of aluminum, beryllium, zinc and other metals (B-54MI22401). 

It has been found that many of the reactive dyes have not the highest degree of fastness to natural weathering. When exposed over prolonged periods to the atmosphere they fade more rapidly than normal laboratory fastness tests would predict. Procion Yellow H-A, Procion Yellow M-6G, Procion Yellow M-R, and Procion Yellow H-SG are all deemed to be of borderline fastness to weathering. They are, nevertheless, in many cases superior to vat dyes because they do not induce accelerated oxidation of the cellulose on exposure to light. The deeper the shade the less apparent is the alteration due to weathering. 

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