Simultaneous Speciation, Structure, and Equilibrium Constant Determination in the Ni-EDTA-CN Ternary System via High-Resolution Laboratory X-ray Absorption Fine Structure Spectroscopy and Theoretical Calculations.

Even quite simple chemical systems can involve many components and chemical states, and sometimes it can be very difficult to differentiate them by their hardly separable physical-chemical properties. The Ni-EDTA-CN (EDTA = ethylenediaminetetraacetic acid) ternary system is a good example for this problem where, in spite of its fairly simple components and numerous investigations, several molecular combinations can exist, all of them not having been identified unambiguously yet. In order to achieve a detailed understanding of the reaction steps and chemical equilibria, methods are required in which the structural transitions in the different reaction steps can be followed via element-selective complex spectral feature sets. With the help of our recently developed von Hámos type high-resolution laboratory X-ray absorption spectrometer, both the structural variations and stability constants of the forming complexes were determined from the same measurement series, proving that X-ray absorption spectroscopy can be considered as a multifaced, table-top tool in coordination chemistry. Furthermore, with the help of theoretical calculations, independent structural evidence was also given for the formation of the [NiEDTA(CN)] mixed complex.