Disodium salt with iodine

FD C Red No. 3 is principally the monohydrate of 9-(o-carboxy-phenyl)-6-hydroxy-2,4,5,7-tetraiodo-3//-xanthen-3-one disodium salt, with smaller amounts of lower iodinated fluoresceins. 

The amount of unreacted iodine was determined by titration with sodium thiosulfate [Thiosulfuric acid (H2S203), disodium salt], and the amount of iodine initially present was determined by a separate blank titration.3... 

It has been reported that a tetrahydrofuran solution of the disodium salt of the tetramer of a-methyl styrene reacts with iodine to form coupled products with molecular weights of up to 3 x 103 331). Although it would be expected that iodine would favor the coupling reaction, the lack of specific product and yield information precludes further discussion of this result. 

Erythrosine occurs as a brown powder or granules. It is principally the disodium salt of the monohydrate of 9-(o-carboxy-phenyl)-6-hydroxy-2,4,5,7-tetraiodo-3//-xanthen-3-one. It dissolves in water to give a solution red at neutrality, with a yellow-brown precipitate in acid, and with a red precipitate in base. When dissolved in concentrated sulfuric acid, it yields a brown-yellow solution that evolves iodine and a precipitate of the free acid when heated. It is insoluble in ethanol. 

The authors of this work were able to carry out iodination only when they used iodine monochloride. Compound 233 is unstable when stored in air and loses iodine quite rapidly. We were unable to isolate reaction products in the case of sulfonation with sulpfuric acid. The most convenient sulfonating reagent was a solution of sulfur trioxide in dichlorethane. The sulfonic derivative 234 was very hygroscopic and was further converted into the disodium or bis(isobutylammonium) salt. 

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