Preparation of Dyes and Indicators

Dyes are also classified on the basis of their application. The water-soluble dyes which are the salts of sulfonic acid or phenolic compounds are named acid dyes] those which are the salts of amino compounds are called basic. If the dyeing is accomplished without use of mordants the dyes are called direct. Dyes which require the use of metallic oxides, tannin, and other substances to give fast shades are called mordant dyes. The water-insoluble dyes are known as vat dyes. The insoluble colored substance is reduced in a fermentation vat or by hydrosulfite to a soluble form which is applied to the fiber then oxidized by air to the insoluble color. Finally ingrain dyes are produced by performing one or more of the chemical reactions used for the preparation of the dye directly on the fiber. 

The simplest way of dyeing a fiber is by a direct dye. The dye is dissolved in water so that its concentration is about 0.02 to 0.1 per cent. The amount of dye depends on the weight of the cloth. For light shades the amount of dye is 0.05-0.3 per cent, and for deep shades 4-10 per cent, of the weight of the cloth. A small amount of sulfated alcohol soap is added to reduce the surface tension of the solution and thereby aid in the penetration of the dye. Inorganic salts are added to the dye bath, such as sodium chloride for cotton dyes and sodium sulfate for wool dyes, in amounts varying from 5 to 10 per cent. The fiber is steeped in the dye bath and heated at 80-100° until the proper shade has been 

In the preparation of dyes it is often necessary to heat the reaction mixture 3-10 hours at a particular temperature, which may vary from 110° to 180°. The usual procedure of using oil baths is highly unsatisfactory. Whether the mixture to be heated is 5 or 100 g, an oil bath cannot be heated by a flame and adjusted to keep a constant temperature even within a range of 10°. The pressure of the gas in the main varies from hour to hour a small flame is easily extinguished by drafts these conditions, and the time required to watch the process, are difficulties which call for some other arrangement. The semimicro constant-temperature bath which is described has been used with satisfactory results for a number of years. 

The extra large opening is fitted with a cork having a diameter of 45 mm or more at the top. A hole is bored through this large cork to fit an eight-inch tube which protrudes about 50 mm into the interior of the bath. In this manner the lower end of the tube 

The nitro and nitroso derivatives of a few aromatic hydroxy compounds are used as yellow-green dyes. The nitro compounds are prepared by direct action of nitric acid in presence of sulfuric acid, or by first sulfonating and then nitrating the sulfonic acid. In the preparation of naphthol yellow, a-naphthol is first sulfo-nated to the 2,4,7-trisulfonic acid, which on nitration undergoes replacement of the sulfonic acid groups at positions 2 and 4 by nitro groups. 

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