Solubility from, 180 standard potentials

Although the standard potentials are the fundamental values for all thermodynamic calculations, in practice, one has more frequently to deal with the so-called formal potentials. The formal potentials are conditional constants, very similar to the conditional stability constants of complexes and conditional solubility products of sparingly soluble salts (see [2c]). The term conditional indicates that these constants relate to specific conditions, which deviate from the usual standard conditions. Formal potentials deviate from standard potentials for two reasons, i.e., because of nonunity activity coefficients and because of chemical side reactions . The latter should better be termed side equilibria however, this term is not in common use. Let us consider the redox system iron(II/ni) in water ... 

In Figure 2 the solubility and speciation of plutonium have been calculated, using stability data for the hydroxy and carbonate complexes in Table III and standard potentials from Table IV, for the waters indicted in Figure 2. Here, the various carbonate concentrations would correspond to an open system in equilibrium with air (b) and closed systems with a total carbonate concentration of 30 mg/liter (c,e) and 485 mg/liter (d,f), respectively. The two redox potentials would roughly correspond to water in equilibrium wit air (a-d cf 50) and systems buffered by an Fe(III)(s)/Fe(II)(s)-equilibrium (e,f), respectively. Thus, the natural span of carbonate concentrations and redox conditions is illustrated. 

It can be seen from this equation that the solubility product of silver chloride can be calculated from the known standard potentials of the silver... 

Solubility products can be derived indirectly from standard electrode potentials and other thermochemical data, and directly from tabulated standard Gibbs energies of formation, AfG°, of the ions in aqueous solution [12]. Thus, the use of... 

Ca salts differ from one another in terms of the anions and molecules that they are associated with. CCM s characteristic aqueous solubility is directly related to the citrate and malate anions. The in vitro solubility of any Ca salf is essenfially constant under standard conditions (e.g., at neutral pH in water). Once a Ca source is consumed, it encoimters a host of variable environmental factors, such as pH changes, interactions with other food components, and the hormonal milieu, that alter its solubility and/or potential for absorption. It is the net impact of these internal factors, together with the combination of introduced variables, such as the food matrix in which Ca is incorporated and the timing of Ca intake, that contribute to determining just how beneficial to health a Ca source will be. 

This herb has been part of folk medicine since pre-Christian times (247). It has been primarily used as a sedative and for the treatment of epilepsy. Consistent with this use, this herb reportedly can increase synaptic concentrations of GABA (248). GABA has also been isolated from Valeria and extracts of Valeria have been reported to bind to GABA receptors in rat brain. Although Valeria has been reported to be active in rodent models of depression, there have been no efficacy trials in humans. The potential adverse effects of Valeria include the sensation of strangeness ( 247) and several cases of liver damage (e.g., central lobular necrosis) (249). Mutagenicity in bacteria has been reported and attributed to unstable, water-insoluble valepotriates ( 238). As a result of these reports, many, but not all, commercial preparations of Valeria use water-soluble extracts standardized for their content of valeric acid. 

When a metal forms a soluble, highly dissociated chloride, e.g., zinc, the standard potential is best obtained from measurements on cells without liquid junction, viz.,... 

The release of uranium and thorium from minerals into natural waters will depend upon the formation of stable soluble complexes. In aqueous media only Th is known but uranium may exist in one of several oxidation states. The standard potential for the oxidation of U in water according to equation (2) has been re-evaluated as E° - 0.273 0.005 V and a potential diagram for uranium in water at pH 8 is given in Scheme 3. This indicates that will reduce water, while U is unstable with respect to disproportionation to U and U Since the Earth s atmosphere prior to about 2 x 10 y ago was anoxic, and mildly reducing, U " would remain the preferred oxidation state in natural waters at this time. A consequence of this was that uranium and thorium would have exhibited similar chemistry in natural waters, and have been subject to broadly similar redistribution processes early in the Earth s history. Both U " and Th are readily hydrolyzed in aqueous solutions of low acidity. A semiquantitative summary of the equilibrium constants for the hydrolysis of actinide ions in dilute solutions of zero ionic strength has been... 

Another important use of standard potentials is for the determination of sdubility products, for these are essentially equilibrium constants ( 39j). If M Al, is a sparingly solvble salt, a knowledge of the standard potentials of the electrodes M, M, A, (s), A - and M, M + permits the solubility product to be evaluated. A simple example is provided by silver chloride Ifor which the standard (oxidation) potential of the Ag, AgCl( ), Cl electrode is known to be — 0.2224 volt at 25 C. The activity of the chloride ion in the standard electrode is unity, and hence the silver ion activity must be equal to the solubility product of silver chloride. The value of Oa may be derived from equation (45.13), utilizing the standard potential of silver thus Eu i — 0.22 volt, E for silver is — 0.799, and z is 1, so that at 25 C,... 

Since at a given temperature S is a constant the first two terms on the right-hand side of this equation may be combined, yielding equation (5). This explains the positive (+) sign before the term 22771 In (C1-). Incidentally equation (9) illustrates a method for obtaining solubilities from emf data, or for finding standard potentials from solubility data, both of which are discussed elsewhere in this book. 

The standard potential for silver can however, be readily computed from that of the silver-silver chloride electrode and the solubility product, s, of silver chloride in terms of the activities of the ions for which Brown and Machines 28 have recently obtained (1.309 X 10 5 mol per liter)2 at 25°. The silver-silver chloride electrode is essentially a silver, Ag Ag+, electrode at which the silver ion activity is controlled by the solubility product... 

Information deduced from reduction potentials, and from lattice energies and solubilities, indicates that H+ and d-block ions have more negative absolute standard Gibbs energies of solvation in NH3 than in H2O for alkali metal ions, values of are about the same in the two... 

In aprotic solvents such as PC, DMSO, sulpholane, and DMF, silver chloride appears to be fairly soluble and forms complex ions such as AgClj whereas thallous chloride does not (see below). From the E°(I) values in water and in the aprotic solvent, the free energy of transfer (eqn. 2.6.35) gives the value for the transfer of 1 mole of LiCl. Once AG (LiCl) is known, the value for any cell involving the transfer of LiCl, such as cell (I), can be evaluated from eqn. 2.6.35 since E%T) is known when the solvent is water. The standard potential for cell (XI) (i.e. for M = Ag) may then be found from... 

Treatment of the data in this manner naturally prevents the evaluation of activity coejfficients, and the standard potentials calculated from solubility data should be verified by precise e.m.f. measurements. 

Information deduced from reduction potentials, and from lattice energies and solubilities, indicates that and d-block ions have more negative absolute standard... 

On the basis of this assumption, a scale for standard Gibbs energies of transfer of individual ions from one solvent to another can be obtained using standard Gibbs energy of transfer of a salt which can be calculated from partition coefficients, solubility and voltammetric measurements (9, 10, 33). Similarly, the standard potential difference for individual ions between phase w and o can be evaluated from equation (17.3.2.8). For example, the standard Gibbs energies of transfer of TPAs" and TPB between water and... 

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