Blue natural dyeings, testing

Hou (1992) used a simple screening test to determine whether acid and direct dyes precipitate at calcium concentrations typical of hard waters of the SE Piedmont region of the U.S. Of the 52 dyes tested, only three direct dyes (Direct Black 19, Direct Black 22, and Direct Blue 75) and seven acid dyes (Acid Red 88, Acid Red 114, Acid Red 151, Acid Brown 14, Acid Black 24, Acid Orange 8, and Acid Blue 113) precipitated. Although the Ca salts of acid and direct dyes were thought to be the most likely metal salts to precipitate after dye discharge to natural waters, the precipitation is not likely to occur unless dye concentrations exceed 0.02 to 0.6 mg/L, a level far greater than reported concentrations of dyes in surface waters. 

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. 

These two types of emulsions are easily distinguished (1) An emulsion can readily be diluted with more of the outer phase, but not as easily with the inner phase. Consequently O/W emulsions disperse readily in water W/O ones do not, but they do disperse readily in oil. This method works best on dilute emulsions. (2) O/W emulsions have electrical conductivities similar to that of the water phase W/O emulsions do not conduct current significantly. (3) W/O emulsions will be colored by oil-soluble dyes, whereas O/W emulsions show the color faintly, if at all, but will be colored by water-soluble dyes. (4) If the two phases have different refractive indices, microscopic examination of the droplets will determine their nature. A droplet, on focusing upward, will appear brighter if its refractive index is greater than the continuous phase and darker if its refractive index is less than that of the continuous phase. This clearly identifies the substance in the droplet if one knows the relative refractive indices of the two phases. (5) In filter paper tests, a drop of an O/W emulsion produces an immediate wide, moist area a drop of a W/O emulsion does not. If the filter paper is first impregnated with 20% cobaltous chloride solution and dried before the test, the area around the drop immediately turns pink if the emulsion is O/W and remains blue (shows no color change) if it is W/O (Tronnier, 1960). 

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