Page 376 - Basic Principles of Textile Coloration

THE REDUCTION OF QUINONE VAT DYES 365

Obviously, such a large equilibrium constant for the reduction means that hydros
solution is a sufficiently powerful reducing agent to completely reduce the vat dye
pigment to the leuco form. Similar considerations apply to the oxidation of the
leuco derivative. Any oxidising agent with a redox potential below about –0.60 V
will give quantitative oxidation back to the quinone form of the dye.

QO2(ox) + 2e– QO22– (red) Eo = –0.80 V (7)
O2 + H2O + 2e– HO2– + HO– Eo = –0.10 V

Thus, combination of the second with the reverse of the first equation gives

QO22– + O2 + H2O QO2 + HO2– + HO– (8)

Eo = –0.10 – (–0.80) = + 0.70 V

For oxidation of the leuco derivative with oxygen, E° = +0.70 V and the
equilibrium constant is enormous.

Standard redox potentials are thermodynamic values related to the position of
equilibrium and give no information about reaction rates. Hydros is a very strong
reducing agent, more effective at high pH and at higher concentration. Its
potential is unstable but negative enough that it will reduce almost all vat dyes,
but not necessarily rapidly. Indigo dyes, for example, have less negative standard
potentials than anthraquinone vat dyes but reduce more slowly. Attempts to
control the amount of hydros needed in vatting and during dyeing, based on redox
potential measurements of the dyebath solution, have not been particularly
successful. It is difficult to interpret the recorded potentials in a way that allows
calculation of the amount of excess hydros and to keep the required platinum
electrodes in good condition.

The reducing action of hydros is complex. The dithionite ion is a good reducing
agent but much of its reducing power depends upon formation of the dithionite
radical formed by breaking the weak bond between the two sulphur atoms in
dithionite (Scheme 17.3). The kinetics of the reduction process depend on the
temperature and concentrations of chemicals in solution, the rate increasing with
increase in the temperature and the concentration of hydros. The rate of
reduction of a vat pigment is, however, complicated by the heterogeneous nature
of the system and depends upon the size distribution and habit of the crystals in
dispersion. The crystalline form affects the rate of vatting more than the particle