Photometric complex-formation titration of submicromolar amounts of zirconium

Even in strongly acidic solution Zr(IV) forms polynuclear hydroxide complexes. In these compounds tetrameric units play a dominant role. When the Zr atoms in these units are interconnected through double OH-bridges (olated bonds) quantitative formation of Zr-EDTA is possible at room temperature. When the double OH-bridges are replaced by single oxygen-atom bridges (oxolated bonds) the units are less reactive. The oxolated tetramers are still capable of reacting quantitatively with EDTA if heated to about 100 degrees ; higher oxolated polymers are not. When these higher polymers are formed during the initial adjustment of conditions for a titration, large errors will be observed. By suitable treatment of the solutions oxolation can be restricted sufficiently to permit titrations with an error of less than 1%. Semi-Xylenol Orange has been found to be a suitable indicator for direct titrations; back-titrations are performed with Bi(3+), with PAR as indicator.