Stability constants cations

Chaput, Jeminet and Juillard measured the association constants of several simple polyethylene glycols with Na", K", Cs", and Tl". Phase transfer catalytic processes and most biological processes are more likely to involve the first two cations rather than the latter two, so we will confine the discussion to these. Stability constants for the dimethyl ethers of tetra-, penta-, hexa-, and heptaethylene glycols were determined poten-tiometrically in anhydrous methanol solution and are shown in Table 7.1. In the third column of the table, the ratio of binding constants (Ks/K s) is calculated. Note that Simon and his coworkers have referred to this ratio as the selectivity constant. ... 

By considering the stability constant and the lipophilicity of host molecules, Fyles et al. synthesized a series of carboxylic ionophores having a crown ether moiety and energetically developed the active transport of alkali metal cations 27-32). Ionophores 19-21 possess appropriate stability constants for K+ and show effective K+-selective transports (Fig. 5). Although all of the corresponding [15]crown-5 derivatives (22-24) selectively transport Na+, their transport rates are rather slow compared with... 

Table 8. Stability constants and competitive transport ability of ionophores (50-57) for potassium and sodium cations...

EDTA is a very unselective reagent because it complexes with numerous doubly, triply and quadruply charged cations. When a solution containing two cations which complex with EDTA is titrated without the addition of a complex-forming indicator, and if a titration error of 0.1 per cent is permissible, then the ratio of the stability constants of the EDTA complexes of the two metals M and N must be such that KM/KN 106 if N is not to interfere with the titration of M. Strictly, of course, the constants KM and KN considered in the above expression should be the apparent stability constants of the complexes. If complex-forming indicators are used, then for a similar titration error KM/KN z 108. 

In this part we shall use the nomenclature of Si lien and Martel 1 (76) to define stability constants, medium and experimental techniques. We shall only deal with overall stability constants of the cation M with the ligand L according to reaction (25)... 

Stability constants of the complexes formed between Pu(III), Pu(IV), and HSOi, were determined in 1 M acid media by measuring the decrease in extraction (either into TTA-toluene or ion-exchange resin) with increasing concentration of HSO4 in the aqueous solution. Because of very different degrees of extraction for Pu(III) and Pu(IV) and the imposed requirement of 1 M acidity, Pu(IV)-HSOit complexation was studied by TTA extraction, while the Pu(III)-HSOit system was studied by cation exchange. 

One of several studies of this system shows remarkable agreement with the present results. Fardy and Pearson (3) investigated this system by cation exchange at 2 M acidity and ionic strength and reported uncorrected stability constants 8i = 278 (+ 8) and B2 = 6.8 (+0.2) x 103 (K2 24). Assuming the difference in the medium (2 M HClOi, vs. 1 M NaClO, 1.0 M HClOi ) has a minimal effect on the activities of the various complexes,... 

Cation-exchange techniques, stability constant determination of Pu sulfate... 

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

Some of these divalent cations form part of the Irving-Williams series Mn, Fe, Co, Ni, Cu and Zn. Irving Williams (1953) examined the stability constants of complexes of a number of divalent ions and found that the order... 

This is in descending order of the stability constants of the cations. 

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... 

The sample solution contains a fixed concentration of supporting electrolyte E" L and a varying concentration of primary salt M X . The ionophore I is confined in the membrane. Only the primary cation can be complexed with the ionophore I (given stoichiometry 1 1 stability constant The complex MI and the anionic site are the lipophilic species that are present only in the membrane phase. In this system, the electroneutrality condition at the membrane bulk leads to... 

When a membrane based on a derivative of azobis(benzo-15-crown-5) in contact with a solution of a primary cation is exposed to visible light, we assume that the iono-phore within the membrane phase is exclusively in the trans isomer and forms a 1 1 ionophore (I)-cation (M+) complex with a stability constant, trans. According to Eq. (10), the corresponding charge density at the membrane side of the interface, o is > can be expressed as... 

Under UV light irradiation, cis-trans photoisomerization of the ionophore in the membrane occurs. We assume that the cis and trans isomers are both present in the membrane and the cis isomer forms a 1 1 (ionophore-cation) complex with a stability constant, Am.cb- In this case, the surface charge density, and the phase boundary... 

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

The functional characteristic of these compounds that is of interest from the viewpoint of textile processing is their capability to accommodate alkaline-earth and alkali metal cations, as well as a variety of other species, within their cavities. Stability constants (Equation 10.3) are again used, both as a measure of ligand strength and as a hierarchical indicator of... 

Calorimetric studies indicate that the enthalpies of complexation tend to show related trends to the observed stability constants and display selectivity peaks, although there is not necessarily a coincidence between the two sets of peaks. Complexation is characterized by the entropy becoming progressively less positive (less favourable) as the cation size decreases. This is illustrated in Figure 6.5 for the complexation of 2.2.1 with the alkali metals. 

The dissociation rates for a number of alkali metal cryptates have been obtained in methanol and the values combined with measured stability constants to yield the corresponding formation rates. The latter increase monotonically with increasing cation size (with cryptand selectivity for these ions being reflected entirely in the dissociation rates - see later) (Cox, Schneider Stroka, 1978). 

In water, the relatively low stability of the alkali metal and alkaline earth cryptates (except those for which there is a near-optimal fit of the cation in the intramolecular cavity) has resulted in difficulties in undertaking a wide-ranging kinetic study in this solvent. However, in non-aqueous media, the stability constants are larger and most of the studies have been performed in such media. 

Lamb, J. D., Izatt, R. M. Christensen, J. J. (1981). Stability Constants of Cation-Macrocycle Complexes and Their Effect on Facilitated-Transport Rates, Ch 2 in Progress in Macrocyclic Chemistry, Volume 2, ed. R. M. Izatt J. J. Christensen. Wiley-Interscience, New York. 

The complexing behavior of Ca2+ is put into context in Table VI (49,208,211,236-246), which provides a comparison of stability constants (logioifi, on the molar scale, in aqueous media at 298 K (a few at 293 K), generally at 7 0.1M) for a selection of complexes of Ca2+ with those for a range of other metal cations. Ionic strength effects are often significant, especially for ionic... 

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). 

Table XIX contains stability constants for complexes of Ca2+ and of several other M2+ ions with a selection of phosphonate and nucleotide ligands (681,687-695). There is considerably more published information, especially on ATP (and, to a lesser extent, ADP and AMP) complexes at various pHs, ionic strengths, and temperatures (229,696,697), and on phosphonates (688) and bisphosphonates (688,698). The metal-ion binding properties of cytidine have been considered in detail in relation to stability constant determinations for its Ca2+ complex and complexes of seven other M2+ cations (232), and for ternary M21 -cytidine-amino acid and -oxalate complexes (699). Stability constant data for Ca2+ complexes of the nucleosides cytidine and uridine, the nucleoside bases adenine, cytosine, uracil, and thymine, and the 5 -monophosphates of adenosine, cytidine, thymidine, and uridine, have been listed along with values for analogous complexes of a wide range of other metal ions (700). Unfortunately comparisons are sometimes precluded by significant differences in experimental conditions.

Stability Constants (LogU)K, at 298 K and I = 0.1M) for Phosphate, Phospho-nate, Nucleoside, and Nucleotide Complexes of Selected M2+ Cations... 

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