Solubilised vat dyes:-
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Solubilised vat dyes
These are sodium salts of sulphuric esters of leuco vat dyes. In the cas of indigo itself, leuco-indigo is first prepared by bubbling carbon dioxide into a solution of vatted indigo (with alkali and hydrosulphite), and drying it in the absence of air. this is then added to pyridine-chlorosulphonic acid complex in a carbon dioxide atmosphere. The pyridine salt thus formed is converted into the sodium salt by the addition of caustic soda. In the case of chlorinated indanthrone, the solubilised vat dye is made from 2 chloro-3-amino anthraquinone whihc is acetylated treated with pyridine and chlorosulphonic acid. The vat dye and iron dust are added simultaneously. After the reaction is completed (it may take several hours), the reaction is completed (it may take several hours), the reaction mixture is blown by nitrogen into soda ash solution and pyridine distilled off with steam, The contents are filtered to remove iron and iron oxide, the solubilised vat dye salted out from the filtrate after concentration under vacuum if the dye is highly soluble in water. It is filtered, mixed with soda ash as a stabiliser and homogenised.
In the case of indigo, the dye is first reduced and solubilised with sodium hydrosulphite (Na2-S2O4) and sodium hydroxide, carbon dioxide is bubbled when the leuco dye or the vat acid is formed. This is dried in the absence of air. The vat acid is added to a complex formed between chlorosulphonic acid and pyridine in an atmosphere of carbon dioxide. The resulting pyridinium salt is converted into the sodium salt by the addition of sodium hydroxide.
For most of the other vat dyes, pyridine is first treated with chlrosulphonic acid. The vat dye in the power form and iron dust are added to this mixture.
