Analytical Sciences

Precise Chelatometric Titrations of Zinc, Cadmium, and Lead with Molecular Spectroscopy

Toshihiro SUZUKI, Diwakar TIWARI, and Akiharu HIOKI

National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 3-9, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan

Spectral data during a chelatometric titration were utilized for determining an equivalence point of the titration. Three-dimensional titration data were analyzed by fitting to a theoretical equation that was derived from equilibrium equations among a metal ion, an indicator and EDTA. The equation with optimized parameters successfully explained the three-dimensional titration data. This method was applied to EDTA titration systems of zinc, cadmium and lead with xylenol orange (XO) as an indicator. These systems were analyzed while taking the formation of 1:1 and 2:1 metal-indicator complexes into account. By this method, an accurate equivalence point was determined as well as stability constants and molar absorption spectra of metal-indicator complexes in each titration system. Moreover, the comparability among the concentrations of the metal standard solutions prepared from pure metals (Zn, Pb, and Cd) was also confirmed by the proposed evaluation method.

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