Water blue

Water blue, also known as aniline blue, Acid blue 22, Soluble Blue 3M, Marine Blue V, or C.I. 42755, is a chemical compound used as a stain in histology. Water blue stains collagen blue in tissue sections. It is soluble in water and slightly soluble in ethanol.

Water blue's molecular formula is C32H25N3O9S3Na2.

Water blue is also available in mixture with methyl blue, under name Aniline Blue WS, Aniline blue, China blue, or Soluble blue.

It can be used in the Mallory's connective tissue stain and Gomori's one-step trichrome stain. It is used in differential staining.