Abstract
The calcium(II), iron(III) and chromium(III) alkyl-N-iminodiacetate systems have been studied in aqueous solution with respect to stability, acid–base properties and structure. The calcium(II) ion forms only one weak complex with methyl-N-iminodiacetic acid in water, K1 = 12.9 (2) mol–1⋅dm3, while iron(III) and chromium(III) form very stable complexes with alkyl-N-iminodiacetic acids. The calcium(II)–methyl-N-iminodiacetate complex is octahedral in the solid state with most probably water in the remaining positions giving a mean Ca–O bond distance of ca. 2.36 Å. The iron(III) alkyl-N-iminodiacetate complexes have low solubility due to a strong tendency to form polymeric structures. Depending on pH in the solution at their preparation, the degree of hydrolysis in the resulting compound(s) may differ. In the solid state, the polymeric iron(III) alkyl-N-iminodiacetate compounds seem to have the mean composition Fe2O(Cx-IDA)5; the mean Fe–O bond distances to the oxo group and the alkyl-N-iminodiacetate ligands are 1.92 and 2.02 Å, respectively. In these complexes the nitrogen atoms are bound at much longer bond distances, 0.1–0.2 Å, than the carboxylate oxygens. This distribution with short strong Fe–O bonds and much longer and weaker Fe–N bonds is also found in most other structurally characterized iron(III) carboxylated amine/polyamine complexes. The chromium(III) alkyl-N-iminodiacetate complexes are octahedral in both solution and solid state, and the low solubility of the solid compounds indicates a polymeric structure with the ligands bridging chromium(III) ions. Also, chromium(III) binds oxygen atoms in carboxylated amines at significantly shorter distance than the nitrogen stoms. The chromium(III) alkyl-N-iminodiacetate complexes display such slow kinetics at titration with strong base that the back-titration with strong acid shows completely different acid–base properties, thus the acid–base reactions are irreversible.
Highlights
Interest is growing for chelating surfactants in technical applications such as separation of metal ions and complexes from, e.g., industrial wastewater and polluted soils
The stability constants of the calcium(II) formate and acetate systems, log10 K1 = 1.43 [28, 29], and 1.24 [29], 1.12 [30, 31] and 1.2 [32], respectively, at zero ionic strength giving, after extrapolation, log10 K1 ≈ 1.0 and 0.7 at 0.1 mol⋅dm–3 ionic strength, while the sta(bIi l=it 0y.1comnostla⋅dnmts–o3 fNtahCel)ca[3lc3i]uamn(dII2).3o0xa(lIa =te 0s.1ysmteoml⋅damre–3siKgNniOfic3)an[3tl4y].laTrhgeers,talobgil1i0tyKc1o =ns 2t.a5n4t of the calcium(II)–methyl-N-iminodiacetate complex measured in this study has the same order of magnitude as for the monodentate formate and acetate ligands, while the bidentate oxalate ion forms significantly stronger complexes
If it is assumed that the nitrogen atom is involved in the complex formation it will cause that larger stability constants are obtained for the same experimental pH data than they are in reality
Summary
Interest is growing for chelating surfactants in technical applications such as separation of metal ions and complexes from, e.g., industrial wastewater and polluted soils. Many chelating surfactants on the market are technical products containing several similar compounds, but this means that detailed physico-chemical studies cannot be performed accurately. The most common type of chelating surfactants, alkyl-polyamine carboxylates, so called APACs, consists of a hydrocarbon chain and a hydrophilic chelating part [1]. In this project we have chosen to study the simplest chelating surfactants of this kind, alkyl-N-iminodiacetic acids (IUPAC name alkyl-N-iminodiethanoic acids), as they can be prepared with high purity. The acidic constants of the iminodiacetic and alkyl-N-iminodiacetic acids show that they are present as zwitter ions in aqueous solution as the imino nitrogen is a much stronger base than the carboxylate groups, see supplementary Table S1. The acidic constants show that the acetic acid groups are significantly stronger acids in iminodiacetic and alkyl-
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