Equilibrium constant chelates

Chelation is an equilibrium system involving the chelant, the metal, and the chelate. Equilibrium constants of chelation are usually orders of magnitude greater than are those involving the complexation of metal atoms by molecules having only one donor atom. 

The product is equal to the equilibrium constant X for the reaction shown in equation 30. It is generally considered that a salt is soluble if > 1. Thus sequestration or solubilization of moderate amounts of metal ion usually becomes practical as X. approaches or exceeds one. For smaller values of X the cost of the requited amount of chelating agent may be prohibitive. However, the dilution effect may allow economical sequestration, or solubilization of small amounts of deposits, at X values considerably less than one. In practical appHcations, calculations based on concentration equihbrium constants can be used as a guide for experimental studies that are usually necessary to determine the actual behavior of particular systems. 

Whilst this will be satisfactory when dealing with kinetic data in which reactions involving the solvent will not explicitly appear in the rate equations, it is not appropriate when we consider equilibrium constants. As an exercise, consider the formation of [Ni(en)3] from aqueous solutions of nickel(ii) chloride and en (en = H2NCH2CH2NH2) write the equations with the inclusion and the omission of the water molecules. Can you recognize the driving force for the formation of the chelate in each case ... 

The experimental results imply that the main reaction (eq. 1) is an equilibrium reaction and first order in nitrogen monoxide and iron chelate. The equilibrium constants at various temperatures were determined by modeling the experimental NO absorption profile using the penetration theory for mass transfer. Parameter estimation using well established numerical methods (Newton-Raphson) allowed detrxmination of the equilibrium constant (Fig. 1) as well as the ratio of the diffusion coefficients of Fe"(EDTA) andNO[3]. 

Published equilibrium constants for monocarboxylato complexes are summarized in Table VII. All that can be deduced with certainty from these data is that the anions derived from monocarboxylic acids form rather weak complexes with beryllium. In all probability they act as monodentate ligands. The possibility of bidentate chelation using both carboxylate oxygen atoms can be ruled out on the grounds... 

Stary, J. Freiser, H., Compilers, Part IV "Chelating Extractants" in "Equilibrium Constants of Liquid-Liquid Distribution Reactions" IUPAC Chemical Data Series No. 

The enthalpy value of Eq. (3.23) is very small as might be expected if two Cd-N bonds in Cd(NH3) 2 are replaced by two Cd-N bonds in Cd(en). The favorable equilibrium constants for reactions [Eqs. (3.22) and (3.23)] are due to the positive entropy change. Note that in reaction, Eq. (3.23), two reactant molecules form three product molecules so chelation increases the net disorder (i.e., increase the degrees of freedom) of the system, which contributes a positive AS° change. In reaction Eq. (3.23), the AH is more negative but, again, it is the large, positive entropy that causes the chelation to be so favored. 

Selected entries from Methods in Enzymology [vol, page(s)] Buffer capacity, 63, 4 choice, 63, 19, 20, 285 metal ion chelation effects, 63, 225, 226, 287, 298, 299 dielectric constant effect on pK, 63, 226 dilution, 63, 20 equilibrium constant effects, 63, 18 heavy water, 63, 226, 227 ionic strength effects, 63, 226,... 

In addition, the reduction of a-11 and (3-11 was s tudied in dimethyl sulfoxide as solvent, in the presence of pyridiniump-toluensulfonate. This medium makes mutarota-tion slower than the redox process.73 The two anomeric forms could reduce Cr(VI) and Cr(V) by formation of a Cr(VI) and Cr(V) ester intermediate. The equilibrium constant for this step and the rate of the redox step were different for each anomer for a-11, the equilibrium constant for ester formation is higher than for (3-11, but the redox process within this complex is faster for the latter anomer. These differences can be explained by the better chelating capacity of the 1,2-cri-diol moiety of a-11. Room-temperature CW-EPR spectra of these mixtures revealed for the a anomer several five-coordinated Cr(V)-bischelates (giso= 1.9820 [crilso 15.9xl(r4cm 1 (=47.7MHz)],... 

Extraction of transition metals from low grade ores. Factor (c) (formation of a chargeless chelate complex) can be illustrated by considering the formation of complexes between 8-hydroxyquinoline (HQ) and a mixture of metal ions, say, M2+ = Fe2+, Co2+, Ni2+, and Cu2+. This is in fact the order of increasing stability constants of the complexes MQ2 (equilibrium constants /J2 for Eq. 17.14) log 2 = 15.0, 17.2, 18.7, and 23.4, respectively, in dilute solution at 20 °C. This commonly encountered sequence for complex formation by the divalent Fe, Co, Ni and Cu ions is known as the Irving-Williams order (cf. the susceptibility of Ni2+ and Cu2+ to complexing by NTA3-, noted in Sections 14.4 and 16.5). 

The best place to start is with a measure of the ability to chelate individual metal ions. This is expressed by the stability constants, that is, equilibrium constants for the reaction of chelant with metal, as shown in Equation 10.1 ... 

The stability constant is normally shown as the logarithmic value, logK, and so the larger the value of logic, the further to the right the equilibrium is and the stronger the chelate is and the less free metal ion is in solution. These are equilibrium constants and so give no information on the kinetics of the reaction [31]. 

The equilibrium between Mg2+ and the edta anion L4 can be compared to that between NH3 and H+, because the corresponding equilibrium constants are very dose in magnitude. In the latter case, it is possible to titrate NH3 with a solution of a strong acid in order to determine quantitatively its total concentration. It is therefore quite evident that, based on the values of the equilibrium constants of Scheme 3, the quantitative determination of the Mg2+ using edta should be possible. Because the other cations form more stable complexes than Mg2+, the complexometric titration should be of wide application. Some caution is necessary concerning the pH value at which the determination is done, because the ligand can be protonated, with consequent decrease of its chelating power. However, in the case of copper(II), its edta complex is already completely formed at pH 3 and therefore a titration is possible under these conditions. 

Ethyl Acetate. The esterification of ethanol by acetic acid was studied in detail over a century ago (357), and considerable literature exists on determinations of the equilibrium constant for the reaction. The usual catalyst for the production of ethyl acetate [141-78-6] is sulfuric acid, but other catalysts have been used, including cation-exchange resins (358), a-fluoronitrites (359), titanium chelates (360), and quinones and their pardy reduced products. 

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