Solubility stability constants

The data refer to various temperatures between 18 and 25°C, and were compiled from values cited by Bjerrum, Schwarzenbach, and Sillen, Stability Constants of Metal Complexes, part II, Chemical Society, London, 1958, and values taken from publications of the lUPAC Solubility Data Project Solubility Data Series, International Union of Pure and Applied Chemistry, Pergamon Press, Oxford, 1979-1992 H. L. Clever, and F. J. Johnston, J. Phys. Chem. Ref Data, 9 751 (1980) Y. Marcus, Ibid. 9 1307 (1980) H. L. Clever, S. A. Johnson, and M. E. Derrick, Ibid. 14 631 (1985), and 21 941 (1992). 

The extent of hydrolysis of (MY)(n 4)+ depends upon the characteristics of the metal ion, and is largely controlled by the solubility product of the metallic hydroxide and, of course, the stability constant of the complex. Thus iron(III) is precipitated as hydroxide (Ksal = 1 x 10 36) in basic solution, but nickel(II), for which the relevant solubility product is 6.5 x 10 l8, remains complexed. Clearly the use of excess EDTA will tend to reduce the effect of hydrolysis in basic solutions. It follows that for each metal ion there exists an optimum pH which will give rise to a maximum value for the apparent stability constant. 

The study of carbonate complexes of Pu is complicated by various experimental difficulties. The low solubility of many carbonates (7), leaving a very dilute Pu concentration in solution, results in difficulties to the experiments with electrochemical or spectrophotometric methods. However, the radiometric method with solvent extraction or solubility measurement is easily applicable for the purpose. Unlike the solution with anions, like Cl, N03 etc., the concentration of which can be varied at a constant pH, the preparation of solutions with varying carbonate concentration accompanies indispensably the change of pH of the solution. As a result, the formation of carbonate complexes involves accordingly the hydrolysis reactions of Pu ions in solutions under investigation. It is therefore prerequisite to know the stability constants of Pu(IV) hydroxides prior to the study of its carbonate complexation. 

Table 6. Free calcium concentrations in equilibrium with common complexing agents. A low free calcium concentration implies effective complexation, whether the complex formed is soluble or insoluble. The data were derived from either stability constants (soluble complexes) or solubility products (insoluble complexes).

For hydrophobic, (virtually) nonionizable substances [i.e., those that show no ionic species of significance in the pH range 1 to 10 (e.g., diazepam)], solubility can usually be improved by addition of nonpolar solvents. Aside from solubility, stability is also affected by solvents in either a favorable or a nonfavorable direction [6], Theoretical equations for solubility in water [7] and in binary solvents [8] have been reported in literature, but in general the approach in preformulation is pseudoempirical. Most often the solubility changes as the concentration of nonpolar solvent C2, increases. For binary systems it may simply be a monotonely changing function [9], as shown in Fig. 2. The solubility is usually tied to the dielectric constant, and in a case such as that shown by the squares, the solubility is often log-linear when plotted as a function of inverse dielectric constant, E, that is,... 

Note The above potentials, E, are for pH 7 at equal concentrations of oxidised and reduced species. These equilibrium values are as important as stability constants and solubility products for an understanding of cellular chemical systems. These are free energy changes in volts, E, and where n3E is in kilocalories. The [Fe3+]/[Fe2+] is related to an equilibrium constant, K (see Section 4.17). 

In order to determine the stability constants for a series of complexes in solution, we must determine the concentrations of several species. Moreover, we must then solve a rather complex set of equations to evaluate the stability constants. There are several experimental techniques that are frequently employed for determining the concentrations of the complexes. For example, spectrophotometry, polarography, solubility measurements, or potentiometry may be used, but the choice of experimental method is based on the nature of the complexes being studied. Basically, however, we proceed as follows. A parameter is defined as the average number of bound ligands per metal ion, N, which is expressed as... 

Values from Refs. (272,646) are derived from measurements of the solubility of calcium sulfate in solutions of the respective sugars (at 298K, 0.2stability constants from solubility measurements has been reviewed - see Ref. (225). 

Stability constants (ethylendiamine, glycinate, oxalate), surface complex formation constants and solubility products (sulfides) of transition ions. The surface complex formation constant is for the binding of metal ions to hydrous ferric oxide =Fe-OH + Me2+ =FeOMe++ H+ K. ... 

Van den Berg, C. M. G., and J. R. Kramer (1979), "Conditional Stability Constants for Copper Ions with Ligands in Natural Waters", in E. Jenne, Ed., On Chemical Modeling Speciation, Sorption, Solubility and Kinetics in Aqueous Systems, ACS Symp. Series. 

Direct comparisons of the stability constants for formation of the dmpp complexes of molybdenum(VI) with those for uranium(VI) are not possible. The mono-ligand complexes have different stoichiometries, MoOaldmpp) versus U02(dmpp), while although log P2 is available for Mo02(dmpp)2 (175) the value of 40.2 refers to formation from Mo04 rather than from MoOg laq), and U02(dmpp)2 is too sparingly soluble for its formation constant to be determined (231). 

The most important applications of Cu ISEs are in the direct determination of Cu " in water [169, 372,410], complexometric titration of various metal ions using Cu " as an indicator [30, 143,269, 385] and complexometric titrations of Cu " [409]. This ISE has also been used in the determination of the equilibrium activity of Cu in various Cu complexes in order to determine the stability constants (see [46, 285, 317, 318,427, 445]), in the determination of the solubility of poorly soluble salts [122] and in the determination of the standard Gibbs transfer energies [58]. It can also be used in concentrated electrolytes [170]. 

The solubility properties of salen-type complexes can be tailored by employing substituents such as t-butyl and triphenylphosphonium-methyl (—CFl2PPh3+), with its associated chloride, in the aromatic rings.The stability constant for A,A -o-phenylenebis(salicylideneimine)-iron(III) in 80% (w/w) DMSO-water has been compared with stability constants for complexes of six metal ions with this ligand. 

Such equilibria and their stability constants are summarized in Table 9.1. The total concentration of dissolved iron (Fe-r) at any pH is given by the sum of the concentrations of the free metal iron and all the soluble hydrolysis species, i. e. 

Solubility diagrams have nearly always been calculated using solubility and stability constants. Experimental determination of the solubility of iron oxides as a function of pH has been concerned predominately with ferrihydrite. Lengweiler et al. 

Physical and Chemical Properties. Some of the physical and chemical properties (i.e., K°w K°<= and Henry s law constant) that are often used in the estimation of environmental fate of organic compounds are not useful or relevant for most inorganic compounds including thorium and its compounds. Relevant data concerning the physical and chemical properties, such as solubility, stability, and oxidation-reduction potential of thorium salts and complexes have been located in the existing literature. 

For complex formation between aldehydes and S(IV) to be important in the troposphere, the aldehydes not only must have high solubility but also be present in air at significant concentrations and form stable adducts with S(IV) at a sufficiently fast rate that it can occur during the lifetime of a typical cloud or fog event. Table 8.4 gives the rate constants /c,4 and kt5 for formation of the S(IV) complexes as well as the stability constants Ku and apparent stability constant K p, defined as... 

Solubility studies indicate the formation of strong Zr carbonate complexes (Pouchon et al. 2001). This is not a surprise when considering the behaviour of other tetra-valent metals. However, the available experimental data are not sufficient to derive the stoichiometry of the limiting carbonate complex, that is, to discern between a tetra- and a pentacarbonato complex. This prevents the derivation of any meaningful stability constant and no value can be recommended at present. 

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