Alcian blue is the most specific stain to detect acid mucous substances. This anion dye forms an electrovalent bond with acid group. Alcian blue can be used to separate and identify different mucous substances at various pHs. At pH 1, sulfonic group (OSO3H) is stained; and at pH 2.5, carboxyl group is well stained. The capsule of Cryptococcus neoformans consists of mucous substance and can be stained blue with Alcian blue. Other mildews and their capsules cannot be stained.