Sulphonic \Sul*phon"ic\, a. (Chem.)
   Pertaining to, or derived from, a sulphone; -- used
   specifically to designate any one of a series of acids
   (regarded as acid ethereal salts of sulphurous acid) obtained
   by the oxidation of the mercaptans, or by treating sulphuric
   acid with certain aromatic bases (as benzene); as, phenyl
   sulphonic acid, C6H5.SO2.OH, a stable colorless crystalline
   substance.
   [1913 Webster]

   Sulphonic group (Chem.), the hypothetical radical,
      SO2.OH, the characteristic residue of sulphonic acids.
      [1913 Webster]