Simultaneous equilibria with complex ions

The introduction of conditional stability constants permits us, from a theoretical standpoint, to treat a complexation equilibrium to which are superimposed other equilibria of any nature as if it were alone, at least in some conditions. Conditional stability constants were introduced by G. Schwarzenbach 1957 to take into account acido-basic phenomena occurring simultaneously with complexation phenomena. The first case for which conditional stability constants were inuoduced concerned the titration of some metallic ions with EDTA tetraanion. The scope of their application scope was extended to other parasitic phenomena by A. Ringbom 1959. 

Two simultaneous equilibria are involved the solubility equilibrium (horizontal equation), and the dissociation of the complex ion (vertical equation). The Ag+ is shared in common with both equilibria. 

The Lewis acid catalyst, such as trimcthylsilyl triflate, reacts simultaneously with the peracylated sugar bearing a 2-acyloxy group to generate in situ the electrophilic 1,2-acyloxoni-um ion D and with the silylated purine A to form in situ the trimcthylsilyl tr-complex at Nl, the center of highest electron density, B. The a-complex B can also exist in equilibrium with the Nl trimethylsilyl derivative C. Either of the silylated intermediates reacts with the acyloxonium cation to result in a 1-, 3-, 7-, and 9-ribosylation. Under optimum reaction conditions, the 1-, 3-, and 7-isomers rearrange to the thermodynamically most favored N9 isomer... 

The probability that a reaction should really take place in this way is, however, extremely small. Not only does the equation demand the simultaneous collision of three molecules, but of molecules with multiple positive charges which would exert strong mutual repulsions. The observed relations between the rate and the concentrations of the various ions present are consistent with the idea that the principal reacting species are actually the ferric ion and the complex ion SnClf", which participates in the equilibrium... 

The apparently twice-Nemstian responses of ISEs based on acidic ionophores (Figure 7.8A) are the first examples of apparently non-Nemstian responses that could be explained on the basis of equilibrium phase boundary potentials (44). In this system, the acidic ionophores such as lasalocid (Sr -1) in their deprotonated form, L , can bind both to dications, as primary ions and to H+ ions as secondary ions. With the assumption of 1 1 complexes for both ions and equilibrium exchange of the ions at the membrane/ sample interface, the potentiometric response was demonstrated to depend simultaneously on the two ions over several orders of magnitude of the sample primary-ion activity in pH-bn fered solntions. In the twice-Nemstian response region, the ionophores in the complexed forms with H and are present simultaneously, whereas free ionophores are depleted (Figure 7.8B). The free concentration in the membrane phase is inversely proportional to the sample activity (Figure 7.8C), which results in a... 

The reversible reaction between a solid solute and its ions in aqueous solution is never the sole process occurring. At the very least, the self-ionization of water also occurs, although we can generally ignore it. Other equilibrium processes that may occur include reactions between solute ions and other solution species. Two possibilities are acid-base reactions (Section 18-7) and complex-ion formation (Section 18-8). Calculations based on the Ksp expression may be in error if we fail to take into account other equilibrium processes that occur simultaneously with solution equilibrium. We have encountered the dissolution of Pbl2(s) several times already in this chapter. However, the dissolution process is, in fact, a lot more complex than we ve shown. As suggested below, there are many competing processes ... 

For the precise examination of the complexation equilibria in linear carboxylate polyion systems, the Kj values of the Ag -CmDx and the Ca -CmDx binding equilibria have been examined at various a values [42,43]. In this case, the Kj, values resolved in the presence of trace-level concentrations of metal ion have been substituted for the intrinsic constant, Km values. Concurrent measurements of p[H] and p[M], at equilibrium, of the M /(Na, H )CmDx/Na (excess) system enabled the simultaneous analyses of the acid-dissociation and the metal complexation equilibria. The log K(P, and the log K g values determined on the metal association studies are plotted versus a in Fig. 29. The increase in the log KjS value with a is more pronounced with the higher valent metal ion. It should be pointed out as well that log K, , is greatly influenced by the added salt concentration in the Ca ion-binding system. 

Some believe that there are two main template effects kinetic and thermodynamic [8]. The latter is responsible for an increase in the yield of the complex with ligands formed in situ in the presence of metal ions, which bind products that result from ordinary reactions and to withdraw them from the reaction medium. These procedures are not true template reactions since they do not satisfy the above-mentioned conditions, and the metal ion causes equilibrium shift only. It is impossible to distinguish between kinetic and thermodynamic contributions to the template effect, since the coordination to the metal ion simultaneously causes both steric... 

A similar model for the case of thallium cuprate is complicated by the large number of parameters that affect the state of the system, and also by the necessity to introduce two experimentally unknown quantities simultaneously into the model. However, taking into account the character of the pH dependences of all the processes that occur with the participation of thallium and copper, one can suppose that the shape of the thallium cuprate stability region would be close to that for the mixed oxide. That is, its width would increase with increasing concentration of copper ions (hydroxo complexes) within the crystallization zone. Although the crystallization process took place under a fortiori non equilibrium conditions, the results of the preparative analysis agree qualitatively with the concept of stability of the products. The interval over which formation of thallium cuprate on copper substrates occurs and the formation of the mixed oxide on platinum and carbon substrates can be observed is wider for higher values of the pH [352-354]. 

So far, only the reactions with the solvent assumed to be water have been considered. However, in practical situations other components are usually simultaneously present in the solutions, including impurities, reaction products, and cosolvents. We thus need a simple method to present the equilibrium of interest. One approach to resolve this problem is the use of conditional constants originally introduced by Schwarzenbach as apparent constants and developed further by Ringbom and Kolthoff. For a simple reaction between an L(igand) and a M(etal) ion, the ML complex is formed ... 

This section deals with the equilibria involving metal ions, their complexing ligands, and the resulting coordination complexes (see Chapter 9). Some of these complexes are so stable that salts can be prepared from them that show no appreciable amounts of the separate constituents. An example is the ferricyanide ion, Fe(CN)6", solutions of which are quite different in properties from those of Fe or of CN . More commonly, the complex is not so stable and in solution is partly dissociated into its components. In such a case there is an equilibrium constant which regulates the allowable simultaneous values of the concentrations of the various species. An example is FeBr, which may be formed or dissociated easily, depending on slight modifications of experimental conditions. 

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