Analysis of mixtures of hafnium and zirconium by the methylthymol blue-hydrogen peroxide method

The reaction of hydrogen peroxide with the zirconium(IV) and hafnium(IV) Methylthymol Blue complexes (MeMTB) has been investigated. The conditional stability constants of the Zr(IV) and Hf(IV) complexes with hydrogen peroxide [K'(Me(H(2)O(2)))] were determined spectrophotometrically. The K'(Me(H(2)O(2))) values found, which depend on the acidity, are 3.91 x 10(2) 3.24 x 10(2), 2.63 x 10(2) at [HCl] = 0.2, 0.3, 1.0M respectively for Me = Zr(IV) and 0.828, 0.523, 0.319 for Me = Hf(IV). The ratios of the conditional stability constants, K'(Me(H(2)O(2)))/ K'(MeMTB), are: 5.52 x 10(-4), 5.79 x 10(-4), 8.23 x 10(-4) for Me = Zr(IV) and 2.08 x 10(-6), 2.74 x 10(-6), 1.48 x 10 (-5) for Me = Hf(IV) at the three acidities. The maximum of the ratio of the relative conditional stability constants is obtained in 0.2M hydrochloric acid. The conditions which should be complied with for the determination of hafnium in the presence of zirconium are discussed. The results were compared with those obtained by the Xylenol Orange-hydrochloric acid method. They are superior for samples containing less than 20 mole% of hafnium in admixture with zirconium.