saccharin

A purple or violet coloration proves the presence of salicylic acid, which in turn indicates the presence of saccharin.

Salicylic acid, benzoic acid, and saccharin are used to preserve fruits.

Saccharin possesses some antiseptic properties, but its main use is as a sweetener.

If a bright cherry-red color forms where the two liquids meet, either benzoic acid or saccharin is present.

The saccharin will have been converted into salicylic acid, which may be identified by the usual test for that acid.