Successive determination of thorium and the rare earths with triethylenetetraminehexaacetic acid

Triethylenetetraminehexaacetic acid (TTHA) is proposed for the successive determination of thorium and rare earths in mixtures by titrating first for thorium at pH 2, then adding an excess of TTHA to complex the rare earth ions completely, and titrating the excess of TTHA with standard zinc solution. Xylenol Orange and 3',3'-bis {N,N-bis (carboxymethy 1)amino]methyl} thymolsulphonephthalein (TMS) have been tested as indicators for the titration.