Water complexation stability constants

Figure 4. Copper complexation by a pond fulvic acid at pH 8 as a function of the logarithm of [Cu2+]. On the x-axis, complex stability constants and kinetic formation rate constants are given by assuming that the Eigen-Wilkens mechanism is valid at all [M]b/[L]t. The shaded zone represents the range of concentrations that are most often found in natural waters. The + represent experimental data for the complexation of Cu by a soil-derived fulvic acid at various metakligand ratios. An average line, based on equations (26) and (30) is employed to fit the experimental data. Data are from Shuman et al. [2,184]...

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... 

For a ligand concentration at the nanomolar level, as frequently detected in oceanic waters, the stability constants which can be explored range between about 10 and lO" AT . In the case of stability constants higher than lO" M only the ligand concentration can be evaluated by direct titration (end-point detection), the constant remaining undefined (but >10" M ). In the case of K< Qp Af no complexation can be observed at all (when reasonable quantities of titrant are added) and neither Cl nor K are obtainable. 

Recent applications have shown the potential of flow titration as a modem tool in analytical chemistry. As the required amount of titrand is associated with the analytical signal, important parameters, e.g., oxidis-ability in wastewaters [339], bromine number in foodstuffs [340], bitterness of beers and similar [341], total acidity in wines and vinegars [342] and total alkalinity in natural waters [343], are efficiently determined. In addition, the total concentration of several analytes belonging to the same family, e.g., amines [344], can be determined. The entire titration curve is generally available, allowing the determination of weak acids, complex stability constants and acid dissociation constants [345]. The determination of humidity by the Karl Fischer method [346] is another important application of flow titrations. For single analyte determinations, the analytical characteristics inherent to titrimetric procedures, such as enhanced accuracy and precision, should be emphasised. 

These data reveal some important aspects of the inclusion phenomenon displayed by the anion hosts 24 and 25 Many small and heavily hydrated anions can be bound in water with stability constants exceeding those with simple cyclodextrinsby factors of 20-150. The specificity of binding, however, is rather poor. The stability pattern does not follow the hydration enthalpies of the anions as is most obviously realized in the halide series. Though bromide ion possesses an intermediate enthalpy of hydration it forms the most stable inclusion complex with 24. 

The development of routine and easy handling procedures for continuous and real-time speciation of trace metals in waters has led, in the last years, to the development of microsensors coupled to voltammetric techniques. Microelectrodes offer several advantages for speciation measurements in real-world samples, including their application in low ionic strength media (e.g., freshwaters), reproducibility, and sensitivity. Some Cd speciation studies carried out in river waters, heavily loaded with suspended material, using microelectrodes demonstrated that most of Cd was associated with colloidal material. In addition, this technique also enables the determination of the corresponding complexation stability constants for Cd and protons. 

Salts of diazonium ions with certain arenesulfonate ions also have a relatively high stability in the solid state. They are also used for inhibiting the decomposition of diazonium ions in solution. The most recent experimental data (Roller and Zollinger, 1970 Kampar et al., 1977) point to the formation of molecular complexes of the diazonium ions with the arenesulfonates rather than to diazosulfonates (ArN2 —0S02Ar ) as previously thought. For a diazonium ion in acetic acid/water (4 1) solutions of naphthalene derivatives, the complex equilibrium constants are found to increase in the order naphthalene < 1-methylnaphthalene < naphthalene-1-sulfonic acid < 1-naphthylmethanesulfonic acid. The sequence reflects the combined effects of the electron donor properties of these compounds and the Coulomb attraction between the diazonium cation and the sulfonate anions (where present). Arenediazonium salt solutions are also stabilized by crown ethers (see Sec. 11.2). 

Considering the anion concentration ranges in natural waters (Table II) and the magnitude of the corresponding plutonium stability constants (Table III), the chemistry of plutonium, as well as that of uranium and neptunium, is almost entirely dominated by hydroxide and carbonate complexation, considering inorganic complexes only (41, 48, 49). ... 

Fluoride ion selective spectrometry was used to determine the stability constants for zinc fluoride complexes in water at 25 °C, giving values j3i[ZnF(aq)]+ = 3.5 0.1 and /l2[ZnF2(-aq)] = 3.8 0.5.643 These results demonstrate that the complexation of fluoride is very weak and in aqueous chemistry no species beyond ZnF+ is of much importance. Organotitanium fluorides have been used as matrices for trapping molecular ZnF2 and MeZnF. 4... 

Bioavailable trace elements in soil correlate with plant uptake and concentrations in plants. Extractants for bioavailable trace elements include chelating agents, diluted inorganic acid, neutral salt solutions, and water (Table 7.2). The most popular extractant for bioavailable trace elements in arid and semi-arid soils is DTPA-TEA (triethanolamine), which was developed by Lindsay and Norvell (1969, 1978) to extract available Cu, Zn, Fe and Mn from neutral and calcareous soils. Use of this chelating agent, DTPA, is based on the fact that it has the most favorable combination of stability constants for simultaneous complexation of Cu, Fe, Mn and Zn... 

Mapsi et al. [16] reported the use of a potentiometric method for the determination of the stability constants of miconazole complexes with iron(II), iron(III), cobalt(II), nickel(II), copper(II), and zinc(II) ions. The interaction of miconazole with the ions was determined potentiometrically in methanol-water (90 10) at an ionic force of 0.16 and at 20 °C. The coordination number of iron, cobalt, and nickel was 6 copper and zinc show a coordination number of 4. The values of the respected log jSn of these complexes were calculated by an improved Scatchard (1949) method and they are in agreement with the Irving-Williams (1953) series of Fe2+ < Co2+ < Ni2 < Cu2+ < Zn2+. 

We shall ignore for the moment the fact that the solvent plays a role and will represent the formation of the successive complexes as shown in Eqs. (19.17) to (19.19). However, we should not lose sight of the fact that in aqueous solutions, the total coordination number of the metal is m, and if x sites are bonded to water molecules and y sites are where ligands are attached, then x + y = m. Because the constants Ku I<2,..., Km represent the formation of complexes, they are called formation constants. The larger the value of a formation constant, the more stable the complex. Consequendy, these constants are usually called stability constants. 

Another factor that affects trends in the stability constants of complexes formed by a series of metal ions is the crystal field stabilization energy. As was shown in Chapter 17, the aqua complexes for +2 ions of first-row transition metals reflect this effect by giving higher heats of hydration than would be expected on the basis of sizes and charges of the ions. Crystal field stabilization, as discussed in Section 17.4, would also lead to increased stability for complexes containing ligands other than water. It is a pervasive factor in the stability of many types of complexes. Because ligands that form tt bonds... 

Figure 6.3. Stability constants for the 1 1 potassium complexes of some polyether ligands at 25 °C. The values for (223) and (169) were determined in methanol those for 2.2.2, (276) and (277) in methanol/water (95 5).

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