Stability constants estimator

The Stability Constant Estimator by Hay et al.10 for 1 1 complexes in water applies empirical correlations 1-3, equations by Hancock8 and by Brown-Sylva-Ellis59 as well as correlations with some physical properties of metals. This program includes 88 ligands and 74 metal ions. The correlations were trained on 1,431 stability constants. 

Hay, B.P. Castleton, K.J. Rustad, J.R. Stability Constant Estimator User s Guide PNNL-11434. Pacific Northwest National Laboratory Springfield, 2001 pp. 1—47. 

The introduction to the symposium details the current constraints on research in general and chemical modeling in particular. The 37 technical papers presented fall naturally into the areas of 1) redox equilibria, 2) organic ligand characterization and stability constant estimation, 3) adsorption processes, 4) evaluation and estimation of thermodynamic and kinetic data, 5) interfacing chemical and biological models, and 6) comprehensive chemical models and their applications. 

The stability of halide complexes is known to increase in the sequence Cl-— Br — I- [81, 365-369]. This follows from the fact that the corresponding stability constants, estimated on the basis of molten alkali-metal nitrates, increase in the sequence given. However, alkali-metal nitrate melts are referred to media possessing the lowest ability of metal-cation solvation. Moreover, the molalities of the metal cations studied in the above-said works do not exceed 0.02 mol kg-1 and the corresponding molalities of halide ions are in the order of 0.3 mol kg-1. It is obvious enough that there is no appreciable association of halide ions in complexes with alkali-metal halides in such diluted solutions. 

This is a study of the complexation of zirconium by fluoride employing the potenti-ometric technique, using a Fe VFe electrode. Experiments were conducted at (25.0 0.2)"C and in 0.5 M NH4CIO4. Results were analysed using least squares minimisation. The stability constants estimated are given in the table, according to the reaction... 

Thus, a can be calculated (it is sometimes negligible), and the rate constants are evaluated graphically or by least-squares analysis the estimates of k and k must be consistent with the known stability constant. 

In view of the magnitude of crystal-field effects it is not surprising that the spectra of actinide ions are sensitive to the latter s environment and, in contrast to the lanthanides, may change drastically from one compound to another. Unfortunately, because of the complexity of the spectra and the low symmetry of many of the complexes, spectra are not easily used as a means of deducing stereochemistry except when used as fingerprints for comparison with spectra of previously characterized compounds. However, the dependence on ligand concentration of the positions and intensities, especially of the charge-transfer bands, can profitably be used to estimate stability constants. 

Exercise 6.2. Chemical substitution experiments have indicated that the presence of the negatively charged Asp 52 changes the pKa of Glu 35 by 1.1 units. Using the distances between Asp 52 and Glu 35 and between Asp 52 and Cj (which are 6.2 and 3.8 A, respectively) and a uniform dielectric constant, estimate the stabilization of C/ by Asp 52. 

If the photoequilibrium concentrations of the cis and trans isomers of the photoswitchable ionophore in the membrane bulk and their complexation stability constants for primary cations are known, the photoinduced change in the concentration of the complex cation in the membrane bulk can be estimated. If the same amount of change is assumed to occur for the concentration of the complex cation at the very surface of the membrane, the photoinduced change in the phase boundary potential may be correlated quantitatively to the amount of the primary cation permeated to or released from the membrane side of the interface under otherwise identical conditions. In such a manner, this type of photoswitchable ionophore may serve as a molecular probe to quantitatively correlate between the photoinduced changes in the phase boundary potential and the number of the primary cations permselectively extracted into the membrane side of the interface. Highly lipophilic derivatives of azobis(benzo-15-crown-5), 1 and 2, as well as reference compound 3 were used for this purpose (see Fig. 9 for the structures) [43]. Compared to azobenzene-modified crown ethers reported earlier [39 2], more distinct structural difference between the cis... 

M.M. Shultz, O.K. Stefanova, S.S. Mokrov, and K.N. Mikhelson, Potentiometric estimation of the stability constants of ion-ionophore complexes in ion-selective membranes by the sandwich membrane method theory, advantages, and limitations. Anal. Chem. 74, 510-517 (2002). 

Batch adsorption experiments by Yee and Fein (2002) using aqueous Cd, B. subtilis, and quartz as a function of pH showed that the thermodynamic stability constants, determined from binary systems, could successfully describe the distribution of Cd between the aqueous phase and the bacterial and mineral surfaces. The constants could also be used to estimate the distribution of mass in systems, and construct a surface complexation model. 

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

The temperature dependence of stability constants has been documented for a few calcium complexes, giving estimates for... 

Stability constants for a series of M2+ cations, including Ca2+, with A23187 and with ionomycin have been determined in methanol-water mixtures (Taylor, R. W. Pfeiffer, D. R. Chapman, C. J. Craig, M. E. Thomas, T. P. Pure Appl. Chem. 1993, 65, 579-584) the estimation of stability constants for complexes of the anti-fungal antibiotic pradimicin (BMY-28864) with alkaline earth cations is complicated by the tendency of this antibiotic to aggregate (Hu, M. Ishizuka, Y. Igarashi, Y. Oki, T. Nakanishi, H. Spectrochim. Acta A 1999, 56, 181-191). 

Values from Refs. (272,646) are derived from measurements of the solubility of calcium sulfate in solutions of the respective sugars (at 298K, 0.2stability constants from solubility measurements has been reviewed - see Ref. (225). 

Playle, R.C., D.G. Dixon, and K. Bumison. 1993b. Copper and cadmium binding to fish gills estimates of metal-gill stability constants and modelling of metal accumulation. Canad. Jour. Fish Aquat. Sci. 50 2678-2687. 

Galceran, J., Cecilia, J., Salvador, J., Monne, J., Torrent, M., Companys, E., Puy, J., Garces, J. L. and Mas, F. (1999). Voltammetric currents for any ligand-to-metal concentration ratio in fully labile metal-macromolecular complexation. Easy computations, analytical properties of the currents and a graphical method to estimate the stability constant, J. Electroanal. Chem., 472, 42-52. 

Under these conditions, the formation rate constant, k, can be estimated from the product of the outer sphere stability constant, Kos, and the water loss rate constant, h2o, (equation (28) Table 2). The outer sphere stability constant can be estimated from the free energy of electrostatic interaction between M(H20)q+ and L and the ionic strength of the medium [5,164,172,173]. Consequently, Kos does not depend on the chemical nature of the ligand. A similar mechanism will also apply to a coordination complex with polydentate ligands, if the rate-limiting step is the formation of the first metal-ligand bond [5]. Values for the dissociation rate constants, k, are usually estimated from the thermodynamic equilibrium constant, using calculated values of kf ... 

Acetate complexes with Pu(III),Pu(IV) and Pu(VI). In the case of Pu(III) and Pu(IV) five acetate molecules interact with the cations. In the case of trivalent plutonium the stability constant of Pu(acetate) + has been estimated to be 5 x 1016(i<5) while that of tetravalent plutonium, Pu(acetate)j has been estimated to be between 8.06 x 1022 and 3,98 x 1022 (18). Evidence indicates that Pu(VI) forms mono, di-, tri- and tetra-acetate complexes (19). 

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