Abstract
Trypan blue, Niagara blue 2B, Niagara blue 4B, Afridol blue, Evans blue, Niagara sky blue 6B, Niagara blue 5B and Chlorazol blue G cannot be distinguished by their colour or their behaviour on paper chromatography. They may, however, be identified by paper electro-phoresis at pH 4 of the products obtained by reduction of the azo linkages with sodium dithionite (Na2S2O4).