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Frequency (cm- 1 )
Frequency (cm- 1 )
Figure 4. FTIR spectra of dyes on KBr powder (a) synthetic 6,6'-dibromoindigotin and (b) synthetic indigotin
behavior. Indigoid materials dissolve in alkaline sodium hydrosulfite solu tion as the almost colorless leuco base and are reprecipitated when such so lutions are exposed to air (oxygen). The chemistry of the leuco base reac tions is:
Aht
J0t>w
Na 2 S 2 0 4
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OH The reactions are characteristic of the vat dye family of which indigotin and 6,6'-dibromoindigotin were the only known members in antiquity. The halogenated indigoids, such as 6,6'-dibromoindigotin, dehalogenate to indigotin when their leuco base so lutions are exposed to UV radiation (daylight and fluorescent light). In the case of the above compound, this dehalogenation is accompanied by a col or change in the oxidized form from purple to blue. A sherd ~ 6 mm in diameter with purple deposit was placed on filter pa per on a hot plate and observed under low-power magnification. It was heat ed to 60-80 °C, and alkaline hydrosulfite solution at about the same tem perature was dropped onto the purple surface at a rate that produced some overflow. When a change in the color intensity was observed, the filter pa per directly under the sherd was ex amined and found to have a small pur ple spot. As the filter paper remained exposed to daylight and fluorescent light on the laboratory bench, the spot spread slightly and turned blue. Leuco base of 6,6'-dibromoindigotin, which had not yet oxidized back to the dye, had undergone photodebromination, which is characteristic of halogenated indigoids. The chemical behavior of the deposit confirms the conclusions drawn from the IR spectra that the
