Solubilised vat dyes

The vat dyes section of the Colour Index incorporates a subgroup called solubilised vat dyes. These are sodium salts of sulphuric acid esters of the parent leuco vat dyes, such as Cl Solubilised Vat Blue 6 (1.50). In contrast to the leuco compounds, the vat leuco esters dissolve readily in water at neutral pH. They have relatively low substantivity for cellulose and thus have been used mainly in continuous dyeing and printing. In the presence of an oxidant in mineral acid solution (sodium nitrite and sulphuric acid, for example) the leuco ester is rapidly decomposed and the insoluble vat dye regenerated. Thus application of a vat leuco ester represents a simpler (but more costly and less versatile) alternative to conventional dyeing methods via the alkaline leuco compound. 

Solubilised vat dyes (Figure 7.7) are a group of water-soluble, prereduced forms of vat dyes, stabilised as the sodium salts of sulfate esters, which are prepared via the leuco form by chlorosulfonation. These dyes may be applied directly to the cellulosic fibres and subsequently generate the vat dye within the fibres by acid hydrolysis and oxidation with hydrogen peroxide. 

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