In the process of the wool fabric setting with monoethanolamine sulfite, the dye on the fabric is often reduced to fade.
In order to investigate the fading mechanism, the spectral changes of dyes by adding the reducing agents have been examined. In the result of the reductive reaction, the absorption spectra of the dyes shifted to the short wave length side, making an isobestic point and then decreased. Thus it was found that the reduction of azo dyes has two step reactions. In the first step reaction azo group seems to change into hydrazo group, and in the second step hydrazo group into amino group or other compounds produced by the intramolecular rearrangement.
The equilibrium relation with the oxidation of hydrazo group lay in the first step reaction, and it obeys the equation of the secondary reaction between the dye and monoethanolamine sulfite. In the first step reaction the rate of converting azo group to hydrazo group agrees with Hamett's equation, forming a linear relationship between the rates and the substituent constants, and the rate is decreased by the presence of the electron releaseable substituent.