Effect of Complexation on Solubility

The solubility of a precipitate can be improved by adding a ligand capable of forming a soluble complex with one of the precipitate s ions. For example, the solubility of Agl increases in the presence of NH3 due to the formation of the soluble Ag(NH3)2° complex. As a final illustration of the systematic approach to solving equilibrium problems, let us find the solubility of Agl in 0.10 M NH3. 

We begin by writing the equilibria that we need to consider Agl(s) Ag+(aq) + J-(aq) 

Counting unknowns, we find that there are seven—[Ag+], [I ], [Ag(NH3)2 ], [NH3], [NH4+], [OH ], and [H3O+]. Four of the equations needed to solve this problem are given by the equilibrium constant expressions 

Three additional equations are needed. The first of these equations is a mass balance for NH3. 

Note that in writing this mass balance equation, the concentration of Ag(NH3)2i must be multiplied by 2 since two moles of NH3 occurs per mole of Ag(NH3)2i . The second additional equation is a mass balance on iodide and silver. Since Agl is the only source of N and Ag+, every iodide in solution must have an associated silver ion thus 

When any of the constituent ions of a solid participate in complex formation following dissolution, there will be an increase in the solubility of the solid. Consider the dissolution of cadmium hydroxide, Cd(OH)2(sj, in water, the pH of which is controlled at 9.0 using strong acid or base. Cadmium is of particular importance because of the adverse health effects that it can cause if it is consumed in sufficient quantities. The following reactions are important. 

This value can be compared to a solubility of lO - or 2.24 x 10 , the concentration of Cd2 in equilibrium with Cd(OH)z(s), that would result if there were no complexes. Thus for cadmium hydroxide the formation of hydroxocadmium(II) complexes increases the solubility by approximately 14 percent. As the pH is increased, the various complex forms becom more dominant at lower pH values they are not present in significant concentrations. 

Let us now assume that in addition to the hydroxide ion, there is sufficient Cl in solution so that the free 01 concentration is 10 3 M. To calculate CT.cd crnd Ct.ci th following reactions are important as well as those for the hydroxo complexes. 

Thus the presence of 10 M Cl has increased the total dissolved cadmium concentration even further. Although the increase is approxi mately 2 percent in this example because of the 01 concentration selected, quite significant increases in solubility are observed in solutions such as seawater, where the Cl- concentration is approximately 20 g/liter or 0.56 M. In seawater of pH 8 the solubility of cadmium hydroxide including all hydroxo and chlorocomplexes) is approximately 10+°-39 compared with lO- - M if no Cd(II) complexes were formed. This is an increase of 110 times in cadmium solubility. In seawater the major dissolved species is CdCla . The solubilization of metals such as cadmium and mercury by the formation of soluble chloro complexes has significance in relation to marine waste disposal. The discharge of freshwater streams containing these metals in suspension to a saline environment could well result in increased dissolved metal levels because of the formation of dissolved chloride complexes. 

The hydroxo complexes, or hydrolysis products, of the trivalent metal ions and many other divalent metal ions have a dramatic effect on the solubility of these ions. Consider the following equilibria that relate to the behavior of Fe3+ in pure water  

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The objective of this chapter is to present sufficient background information to enable the student to understoiid the principles of complex formation and stability. Methods for calculating the concentrations of complexes in fairly simple systems are presented the approaches to calculating species distribution in more complicated systems are discussed together with the results of such computations. A discussion of complex formation in several dilute aqueous systems is presented to illustrate important points. The effect of complexation on solubility is the major topic in Chapter 6. 

Note Additional problems on the effect of complexation on solubility appear at the end of Chapter 6. 

These studies were initiated by Higuchi, who studied the effect of complexation on the solubility of numerous drug compounds. The results obtained for more than 500 systems were cited in the primary review of this work [50]. 

SCF carbon dioxide is a lipophilic solvent since the solubility parameter and the dielectric constant are small compared with a number of polar hydrocarbon solvents. Co-solvents(also called entrainers, moditiers, moderators) such as ethanol have been added to fluids such as carbon dioxide to raise the solvent strength while maintaining it s adjustability. Most liquid cosolvents have solubility parameters which are larger than that of carbon dioxide, so that they may be used to increase yields, or to decrease pressure and solvent requirements. A summary of the large increases in solubility that may be obtained with a simple cosolvent is given at the top of Table I. Cosolvents, unlike carbon dioxide, can form electron donor-acceptor complexes (for example hydrogen bonds) with certain polar solutes to influence solubilities and selectivities beyond what would be expected based on volatilities alone. Several thermodynamic models have been developed to correlate and in some cases predict effects of cosolvent on solubilities( ,2). They are used extensively in SCF research and development... 

Now the effect of complexation on Cd activity is apparent. The values of the equilibrium constants, K, 2 3, and K, along with the activity of Cr in solution, determine the fraction of the total soluble Cd that is in the free Cd (uncomplexed) form. High cr activity would result in a small fraction of the )luble Cd being in the free cation form. For example, if the Cl" concentration were 0.01 Af, then equation 1.58 could be solved to show that 50 percent of the total soluble Cd is in the free Cd form, while the rest is almost completely in the form of CdCr. 

The effect of temperature on solubility is more complex and involves both a consideration of the solute vapor pressure as well as the density of the SCF. The solubility isotherms shown in Figure 1.2-9 are typical of most solid-SCF systems in that they intersect within a narrow range of pressure. For any two isotherms, the point of intersection, or crossover pressure, represents a change in the temperature dependence of solubility. 

Detailed studies of the effect of pH on solubility of amino acids have been canied out by Hitchcock (103) for tyrosine and by Sano (180) for cystine. The simple relations given by them may readily be generalised so as to apply to a complex molecule such as a protmn with a very large number of ionising groups. 

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

Adding ammonia decreases the concentration of Ag+ as the Ag(NH3)2 complex forms. In turn, decreasing the concentration of Ag+ increases the solubility of AgCl as reaction 6.27 reestablishes its equilibrium position. Adding together reactions 6.27 and 6.28 clarifies the effect of ammonia on the solubility of AgCl, by showing that ammonia is a reactant. 

The effect of temperature on the kinetics of the direct radiation method is quite complex. Increase in temperature increases the monomer diffusion rate but also increases transfer and termination reaction rates of the growing chains and reduces the importance of the gel effect. Solubility and radical mobility may also change as the temperature is varied [88,89]. 

The effect of complex formation on the solubility of a solid can be observed in the home. Silver dinnerware eventually becomes discolored by an unsightly black tarnish of Ag2 S, formed from the reaction of the silver surface with small amounts of H2 S present in the atmosphere. Silver sulfide is highly insoluble in water. Commercial silver polishes contain ligands that form strong soluble complexes with Ag ions. If a tarnished serving pan is rubbed with a polish, the black tarnish dissolves, returning the silver to its brilliant shine. 

In 1988 Bast and Haenen [201] reported that both LA and DHLA did not affect iron-stimulated microsomal lipid peroxidation. However, Scholich et al. [202] found that DHLA inhibited NADPH-stimulated microsomal lipid peroxidation in the presence of iron-ADP complex. Inhibitory effect was observed only in the presence of a-tocopherol, suggesting that some interaction takes place between these two antioxidants. Stimulatory and inhibitory effects of DHLA have also been shown in other transition metal-stimulated lipid peroxidation systems [203,204]. Later on, the ability of DHLA (but not LA) to react with water-soluble and lipid-soluble peroxyl radicals has been proven [205], But it is possible that the double (stimulatory and inhibitory) effect of DHLA on lipid peroxidation originates from subsequent reactions of the DHLA free radical, capable of participating in new initiating processes. 

The effect of temperature on the solubility of 5b was investigated in a series of experiments at the same CO2 density (p = 0.75 gcm ). The temperature that generally may affect the solubility of volatile compounds in compressed fluids has only a minor impact on the solubility of the relatively low volatile complex 5b in the investigated range (Fig. 12). At temperatures between 313 and 333 K, approximately the same quantities of 5b are extracted. 

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