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THE REDUCTION OF QUINONE VAT DYES 361

pigmentation processes and the proportion of fine particles is higher than for vat
pigments in the traditional vatting process. Dye powders for pigmentation dyeing
have higher dispersant content. Pastes and liquid forms are useful for printing and
continuous dyeing. They will usually have less dispersant but contain appreciable
amounts of water. Some ranges of dyes are deliberate mixtures of vat and disperse
dyes for dyeing cotton/polyester blends.

17.3.2 Reducing vat dyes with hydros

Reduction of quinones is not easy and vatting requires a strong reducing agent.
The dihydrate of sodium dithionite, or hydrosulphite, Na2S2O4.2H2O, is used
almost exclusively and goes by the common name of ‘hydros’. Vatting is carried
out under strongly alkaline conditions for a number of reasons:
(1) the reduction of the quinone (QO2) with hydros to give the leuco dye

(QO22–), and the oxidation of the hydros by oxygen, consume alkali (Scheme
17.1);
(2) it is important that the insoluble vat acid does not precipitate;
(3) hydros is unstable in solution, particularly under acidic or neutral conditions
and at higher temperatures. Its decomposition forms thiosulphate and
bisulphate. This decomposition reaction consumes hydroxide ion and it is
faster as the solution becomes more acidic (Scheme 17.2).

QO2 + Na2S2O4 + 4NaOH QO22 + 2Na + 2Na2SO3 + 2H2O
Na2S2O4 + 2NaOH + O2 Na2SO3 + Na2SO4 + H2O

Scheme 17.1

2Na2S2O4 + 2NaOH Na2S2O3 + 2Na2SO3 + H2O
Scheme 17.2

The decomposition of hydros is exothermic. Decomposition of solid
Na2S2O4.2H2O can lead to spontaneous ignition. Cool, dry conditions are
therefore necessary for storage. Decomposition of hydros occurs rapidly in acidic
solution. Alkaline hydros solutions are more stable provided oxygen is absent.