| abstract |
A process for the preparation of di-sodium 5,5'-indigosulfonate, the starting material being indigo, which process comprises the following steps: i) the indigo is subjected to a sulfonation treatment leading to a mixture which contains 5,5'-indigosulphonic acid, this process being characterized in that: ii) a reduction treatment is applied to the mixture obtained in step i), and optionally a purification step, so as to obtain a composition comprising leuco-5,5'-indigosulfonic acid, iii) leuco-5,5'-indigosulfonic acid is isolated from the composition resulting from step ii), iv) leuco-5,5 acid '-indigosulfonique from step iii) is oxidized to di-sodium 5,5'-indigosulfonate. |