Oxidation—continued with sodium dichromate

An autoclave (Note 1) is charged with 200 g. (1.28 moles) of 2,3-dimethylnaphthalene (Note 2), 940 g. (3.14 moles, 23% excess) of sodium dichromate dihydrate, and 1.8 1. of water. The autoclave is closed, heated to 250°, and shaken continuously at this temperature for 18 hours. The autoclave is cooled with continued agitation (Note 3), the pressure is released, and the autoclave is opened. The contents are transferred to a large vessel (Note 4). To effect complete transfer, the autoclave is rinsed with several 500-ml. portions of hot water. Green hydrated chromium oxide in the reaction mixture is separated on a large Buchner funnel and washed with warm water until the filtrate is colorless. The combined filtrates (7-8 1.) are acidified with 1.3 1. of 6N hydrochloric acid. The acidified mixture is allowed to... 

Various methods have been used for the reoxidation of vat leuco dyeings atmospheric skying, hypochlorite, chlorite and acidified dichromate are now rarely employed. Atmospheric oxidation can be difficult to control and thus uneven with some dyes it is also too slow, particularly for continuous methods. Sodium hypochlorite is used only for those few black dyes that tend to become dark green when oxidised with peroxide obviously hypochlorite should be avoided with the many chlorine-sensitive dyes. Similarly sodium chlorite, acidified to below pH 5 with acetic acid, can only be used with certain dyes, although with these it certainly gives rapid oxidation. Dye selectivity is also a drawback with... 

This reaction can be carried out both in concentrated aqueous solution and, preferably continuously, in the molten state. In the latter case, sodium dichromate is mixed with sulfuric acid in a twin screw and the mixture fed into an externally heated rotary tube furnace. The water first evaporates, then the. sodium hydrogen sulfate melts (at I70°C) and finally the chromium(VI) oxide is formed (I98°C). Temperature control is critical, since chromium(Vl) oxide decomposes at temperatures slightly above this temperature. The reacted mixture then separates in a settling tank. Chromium(Vl) oxide is taken off from below and liquid sodium hydrogen sulfate is taken off from above, both being converted into solid material by cooling rollers. 

Aniline black (Cl 50440/Pigment Black 1), an indazine derivative, was first developed as a dye by Lightfoot aroimd 1860-63. The process involved using aniline, aniline hydrochloride and sodium chlorate in the presence of an oxidation catalyst the compound was then developed at 60-100 C and oxidised further with sodium chromate. However, Perkin had already synthesised a compound that he called aniline black around 1856 oxidising aniline containing toluidine with potassium dichromate, aniline violet was then separated from the resulting mixture (the aniline black). Aniline black continues to be used in a variety of media where carbon-based blacks are inappropriate (Herbst and Hunger, 1997). 

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