Complex Formation Between Zinc(II) and Alkyl- N -iminodiacetic Acids in Aqueous Solution and Solid State
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Complex Formation Between Zinc(II) and Alkyl‑N‑iminodiacetic Acids in Aqueous Solution and Solid State Leif Häggman1,2 · Cecilia Lindblad1 · Anders Cassel3 · Ingmar Persson1 Received: 23 April 2020 / Accepted: 1 August 2020 / Published online: 24 October 2020 © The Author(s) 2020
Abstract Removal of metal compounds from wastewater using processes where metals can be removed and valuable chemicals recycled is of significant industrial importance. Chelating surfactants are an interesting group of chemicals to be used in such applications. Carboxylated polyamines are a promising group to be used in such processes. To apply carboxylated polyamines as chelating surfactants, detailed knowledge of the solution chemistry, including complex formation, kinetics and structures of pure fundamental systems, is required. In this study zinc(II) alkyl-N-iminodiacetate systems with varying length of the alkyl chain have been studied. Acidic and stability constants have been studied by potentiometry, and the structures of both solids and aqueous solutions have been determined by EXAFS. Zinc(II) forms two strong complexes with alkyl-N-iminodiacetates in aqueous solution. In an attempt to determine the acidic constants of these complexes, the deprotonation of the nitrogen atom in the complex bound ligands, it was observed that this reaction is very slow and no accurate values could be obtained. The bis(alkyl-N-iminodiacetato)zincate(II) complexes take, however, up two protons in the pH region 3–7, which means that this complex is approximately singly protonated in the pH region 3–7 and doubly protonated at pH 12. However, most attention should be paid to the titration functions in the pH region between 3 and 7 where the imine nitrogen atoms in the bound ligands are deprotonated. The buffering region is undefined and smeared out, most probably because of slow kinetics resulting in a poorly defined composition of the solution. Despite waiting for up to 30 min. for each pH reading and the pH values seemed stable, the pH titration curves show clearly that thermodynamic equilibrium was not reached (Fig. 1). Below pH = 3 and above pH = 9, equilibria were reached rapidly, except at the 1:1 stoichiometry, where solid zinc hydroxide was formed. To check the validity of the discussion above, a 1:5 stoichiometry sample of zinc(II) and hexyl-N-iminodiacetic acid was titrated, and
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Journal of Solution Chemistry (2020) 49:1279–1289
the interpretation was similar. The bis(methyl-N-iminodiacetato)zincate(II) complex has two acidic constants in the region 10−7–10−3 but they have not been possible to determine them accurately due to the unstable pH and zinc potential readings in this pH region.
3.2 EXAFS A number of zinc(II) alkyl-N-iminodiacetate complexes in both aqueous solution and the solid state have been studied by means of EXAFS. The samples differ in stoichiometric composition, pH and the alkyl chain length of the ligand. The EXAFS functions of all samples show two major back-scattering contributions to the EXAFS function, obs
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