#azo_dye  any dye containing one or more azo groups
  supertype:  dye__dyestuff  a usually soluble substance for staining or coloring e.g. fabrics or hair
  subtype:  congo_red  a red-brown azo dye especially as a chemical pH indicator (congo red is red in basic and blue in acidic solutions)
  subtype:  methyl_orange  an azo dye used as an acid-base indicator; used for titrations involving weak bases

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