Iodine is a trapping agent in Gram’s stain, an important method for classifying bacteria. The crystal or methyl violet is first applied, followed by the iodine in solution with potassium iodide. This forms a complex with the dye which resists removal by solvents.
Iodine in ethanol is also used as an oxidising agent to convert hematoxylin to hematein in Cole's hemalum formula.
The PAS reaction is an essential procedure in carbohydrate histochemistry. Periodic acid is used to oxidise 1-2-glycols to aldehydes. The aldehydes then condense with Schiff’s reagent forming a red product.