The Collaborative International Dictionary of English v.0.48:

Euthiochroic \Eu`thi*o*chro"ic\, a. [Gr. e'y^ well + ? sulphur +
   ? color.] (Chem.)
   Pertaining to, or denoting, an acid so called.
   [1913 Webster]

   Euthiochroic acid (Chem.), a complex derivative of
      hydroquinone and sulphonic (thionic) acid. -- so called
      because it contains sulphur, and forms brilliantly colored
      (yellow) salts.
      [1913 Webster]