Contributions to the basic problems of complexometry--XXIII: determination of thorium, scandium and some lanthanides in the presence of each other

A new method is described for the determination of thorium, scandium, and the sum of light lanthanides, in the presence of each other. The method is based on the titration of thorium plus scandium with DCTA (1,2-diaminocyclohexanetetra-acetic acid) at pH 2.5-3.5, with Xylenol Orange as indicator. After addition of TTHA (triethylenetetraminehexa-acetic acid) the thorium-DCTA complex is converted into the thorium-TTHA complex according to the equation Th-DCTA + TTHA = TH-TTHA + DCTA. Simultaneously, free lanthanum, cerium, praseodymium and neodymium react with TTHA. The excess of TTHA and the liberated DCTA are then determined by titrations with zinc and with lanthanum solutions according to the equations DCTA + TTHA + 3Zn = Zn-DCTA + Zn(2)-TTHA and DCTA + TTHA + 2La = La-DCTA + La-TTHA. From the results, the amounts of thorium and scandium, and the sum of the lanthanide concentrations, can easily be calculated. Further possibilities, e.g., determination of heavier lanthanides, are briefly discussed.