sulfuric

Evaporate a portion of the extract to dryness and add sulfuric acid to the residue.

If sulfurous acid is present, it will be oxidized to sulfuric acid and precipitated as barium sulfate by the barium chlorid.

A white precipitate of barium sulfate will form if sulfuric acid is present.

Mix a bit of the powder with a small quantity of dry resorcin and add a few drops of concentrated sulfuric acid.

Heat at 200° C. till sulfuric acid fumes begin to come off and only a dry mass remains.