Chelate processes

Experimentally deterrnined equiUbrium constants are usually calculated from concentrations rather than from the activities of the species involved. Thermodynamic constants, based on ion activities, require activity coefficients. Because of the inadequacy of present theory for either calculating or determining activity coefficients for the compHcated ionic stmctures involved, the relatively few known thermodynamic constants have usually been obtained by extrapolation of results to infinite dilution. The constants based on concentration have usually been deterrnined in dilute solution in the presence of excess inert ions to maintain constant ionic strength. Thus concentration constants are accurate only under conditions reasonably close to those used for their deterrnination. Beyond these conditions, concentration constants may be useful in estimating probable effects and relative behaviors, and chelation process designers need to make allowances for these differences in conditions. 

Kinetics and mechanism of metal chelation processes via solvent extraction techniques. H. Freiser, Acc. Chem. Res., 1984,17,126-131 (40). 

Metal alkoxide compounds, frequently represented as M(OR)x, where M is a metal and R is an alkyl group, are the most common precursors in sol-gel CSD processes and are also frequently used in chelate processes.3-12,30-34 Groups such as OR, which are bound to a metal center, are frequently referred to as ligands. Alkoxide compounds, including commonly used alkoxides such as... 

To address the hydrolysis sensitivity issue for materials with highly polar bonds, one of two strategies is typically employed (1) alcohol exchange or (2) chelation (reaction) of the starting alkoxide with a ligand that is less susceptible to attack by water. Chelation processes are discussed in detail in the next section. 

A second category among widely used solution synthesis approaches is referred to as the hybrid or chelate process.16,23-30,37 38 46-48 These processes... 

Because of these precursor modification reactions, the process chemistry of chelate processes is as complex, or more so, than that involved in sol-gel processes.78 However, it is typical for chelate processes that some control of process chemistry is sacrificed in return for more expedient solution preparation. For example, the hour-long (or longer) reflux processes that have been historically used in 2-methoxyethanol based sol-gel processing of ferroelectric films are not used. Rather, the entire solution preparation procedure is generally completed within one hour, with only the initial phase of the procedure being carried out under dry box and inert atmosphere conditions. Once the chelation reaction(s) has occurred, the hydrolysis sensitivity of the precursor solution is reduced to the point where the remaining process chemistry may be carried out under ambient conditions.46... 

A typical flowchart for a chelate process is shown in Fig. 2.6.46 In this process, titanium isoproproxide is first added to zirconium butoxide. Acetic acid is then added to the precursor mixture for suppression of hydrolysis. Finally, lead acetate is then added to the solution, followed by the addition of alcohol and water for control of solution viscosity, solution stability, and oligomer formation.46... 

As for sol-gel processes, the product of the reactions in chelate processes is small oligomeric species that generate either chemical or physical gelation during film preparation. Although the nature of the species has not been... 

We have to acknowledge that in most cases it is not really defined which ligand atoms are in fact involved in the complexation or chelation process. 

One strategy for avoiding the sludge problem is to use non-chelate process solutions wherever feasible. Non-chelate alkaline cleaners are available, as are non-chelate etchants... 

Switching an operation to use of non-chelate process baths is not without problems. One problem is the need for continuous filtration of those baths through 1 to 5 micron filters to control the buildup of solids. Typical filter system costs range from 400 to 1000 per tank (USEPA 1989). 

Spent Process Baths and Rinse Water Containing Metal Complexes Use of non-chelate process solutions SR DHS 1987 USEPA 1989 Couture 1984... 

The assessment of the chelating ability of macroligands and applicability of such systems requires the data on the quantitative evaluation of the chelation processes such a stability constants and formation function. Contemporary methods are largely based on the composition of the products formed. The major methods of analyzing the quantitative parameters of MX chelation use predominantly the same techniques that are applied to the description of complexing reactions with participation of monofunctional macroligands [5, 7b, 13a]. 

Two extreme cases should be considered in the analysis of the quantitative parameters of the chelation processes (1) the reaction occuring in the homogeneous... 

Sometimes the chelation process is characterized by the substitution constant defined as the equilibrium constant (lg Keq) for the following reaction... 

In the case of infinitely long chains (1 - oo) and small conversions ( - 0) the chelation process has no appreciable effect on the macromolecule shape and conformation, therefore AGmol - 0 and... 

Of some interest are chelation processes occuring along with the synthesis of macromolecular ligands. This can be exemplified by a mechano-chemical activation of polyethylene terephthalate and en polycondensation to give the following complexes [112] ... 

Assessment of a typical analytical reaction in solution requires consideration of the solvent participation. It is likely that both reactants and products will undergo significant solvation, and the solvation energy will materially affect AC and AH for the analytical reaction. Solvation energies may be high, typically in the range —400 to —4000 kj mol 1 for hydration. Solvent molecules participate in the formation. c. r.ecies in solution with the reactants and products of the reaction, tlv-i a.lec.r. g a5 also. Notably the interpretation of chelating processes depends upon solvent considerations (p. 52). 

After purchasing commercial metaUo-orgaific reagents or following the in-house synthesis of precursor compounds, one of three families of approaches is usually employed for solution synthesis. These include sol-gel processes, typically based on the use of metal alkoxides chelate processes, which utilize modified metal... 

Hybrid, chelate, or molecularly modified precursor routes also utilize alkoxide compounds for the B-site species. Unlike true sol-gel processes, these routes rely on molecular modification of alkoxide compounds through reactions with other reagents, namely chelating ligands, such as acetic acid, acetylacetone, or amine compounds. Despite this difference, chelate processes stiU share several common attributes with methoxyethanol-based sol-gel processes, most importantly, the formation of oligomeric species during solution synthesis. 

The most commonly used chelate processes are based on acetic acid, " and although several steps are typically required for solution synthesis, solution preparation generally requires less than 1 h. This compares to the 1 to 2 days... 

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