Indigo carmine has two different CI numbers and CI names as it may be made either from synthetic indigo (CI 73015, acid blue 74) or from natural indigo (CI 75781, natural blue 2).
It was more popular in the past than at present. It has been used as a collagen stain in a Van Gieson type method giving blue collagen and is sometimes used in methods having red nuclear stains, with which it contrasts well.
It is an indicator at pH 11.5 - 14, changing from blue to yellow.
R. D. Lillie.
Conn's Biological Stains
Williams & Wilkins, Baltimore, MD., U.S.A.
Green, Floyd J, (1990)
The Sigma-Aldrich handbook of stains, dyes and indicators.
Aldrich Chemical Company, Milwaukee, WI, USA.
Gurr, Edward, (1971)
Synthetic dyes in biology, medicine and chemistry
Academic Press, London, UK