Metal-organic complexes stability constants

Formally, complexation stability constants in water (log K) and extraction constants (logA j.J can be related via partitioning coefficients of the free ligand and its complex between the aqueous and organic phases.16 However, the latter are rarely available, and therefore the relationships between log/f and log Ktx are not widely used. Nevertheless, in many cases, the binding ability of ligands to metal in complexation and extraction processes follows the same trend. In this respect, information... 

Table 4 Concentration and stability constants for metal organic complexes in seawater...

TUrating Waters with Metal Ions The application of these techniques often involves titration of the natural water with metal ions, from which the conditional stability constant of metal-organic complexes and the ligand concentrations can be obtained (Coale and Bruland, 1988). These parameters are significant in evaluating the complexation of metals by natural organic ligands they are used to estimate by extrapolation the free metal-ion concentration at the ambient total concentrations when sensitivities are insufficient. The extrapolation provides only an upper limit. 

Rossotti et al [Fa 70] found that the ratio of stability constants obtained by equilibrium measurements in solvent mixtures similar to the above was independent of the composition of the solvent. McBryde et al showed that in most of the systems they examined, the ratio of the first two stepwise complex stability constants did not depend on the solvent. In the case of the stepwise acid dissociation constants this correlation was approximately true only for ethylenediamine for glycine the ratio of the protonation constants increase with increase in the concentration of the organic component. However, if the formation of the glycine complexes is defined as a reaction between the protonated ligand and the metal ion, accompanied by the liberation of a hydrogen ion, the ratio of the equilibrium constants is approximately independent of the composition of the solvent mixture. 

UF has also been applied to the determination of stability constants for trace metal-HS complexes, equilibrium constants for the distribution of organic compounds between water and synthetic micelles, the binding of metal ions and small compounds to proteins and other macromolecules in biochemistry, etc. This type of application is essential to know precisely to what degree the complexing agent, the complexed species, and the free ions and molecules can pass through the membrane. 

Due to the anionic nature of rhamnolipids, they are able to remove metals from soil and ions such as cadmium, copper, lanthanum, lead and zinc due to their complexation ability [57-59], More information is required to establish the nature of the biosurfactant-metal complexes. Stability constants were established by an ion exchange resin technique [60], Cations of lowest to highest affinity for rhamnolipid were K+ < Mg + < Mn + < Ni " " < Co " < Ca2+ < Hg2+ < Fe + < Zn2+ < Cd2+ < Pb2+ < Cu2+ < M +. These affinities were approximately the same or higher than those with the organic acids, acetic, citric, fulvic and oxalic acids. This indicated the potential of the rhamnolipid for metal remediation. Molar ratios of the rhamnolipid to metal for selected metals were 2.31 for copper, 2.37 for lead, 1.91 for cadmium, 1.58 for zinc and 0.93 for nickel. Common soil cations, magnesium and potassium, had low molar ratios, 0.84 and 0.57, respectively. 

L. G. SiLLfiN and A. E. Martell, Stability Constants of Metal-ion Complexes, The Chemical Society, London, Special Publications No. 17, 1964, 754 pp., and No. 25, 1971, 865 pp. Stability Constants of Metal-lon Complexes, Part A. Inorganic Ligands (E. Hcigfeldt, ed.), 1982, pp. 310, Part B. Organic Ligands (D. Perrin, ed.), 1979, pp. 1263. Pergamon Press, Oxford. A continually updated database is now provided by L. D. Pettit and K. J. Powell (eds.), IVPAC Stability Constants Database, lUPAC and Academic Software. 

The above account of selectivity of inorganic plus organic chemistry in synthesis is given rather extensively to stress three points. All the four (Mg, Fe, Co and Ni) porphyrin products came from one source, the synthesis of uroporphyrin. The basis of selection is very different from that in primitive centres which use thermodynamic stability constant selectivity based on different donor atoms for different metal ions. Here, all ion complexes have the same donor atoms, nitrogen, the most constrained being the coordination of Mg2+ by five nitrogens exactly as is seen for Fe in haemoglobin. Hence, there also has to be a new control feedback to ensure that the appropriate quantities of each metal cofactor is produced in a balanced way, that is synthesis from uroporphyrin has to be divided based upon... 

Furia, T. E. (1972). "Sequesterants in Foods," Chapter 6 in CRC Handbook of Food Additives, 2nd ed. CRC Press, Boca Raton, FL. Comprehensive tables of stability constants for many complexes of organic and biochemical ligands of metals. 

Martell, A. E. 1964. Section II Organic ligands in stability constant of metal-ion complexes. Special Publication 17. London The Chemical Society. 

---