Weak electrolytes, activity coefficients

The impact of these liquid phase reactions on the phase equilibrium properties is thus an increased solubility of NH3, CO2, H2S and HCN compared with the one calculated using the ideal Henry s constants. The reason for the change in solubility is that only the compounds present as molecules have a vapour pressure, whereas the ionic species have not. The change thus depends on the pH of the mixture. The mathematical solution of the physical model is conveniently formulated as an equilibrium problem using coupled chemical reactions. For all practical applications the system is diluted and the liquid electrolyte solution is weak, so activity coefficients can be neglected. 

The secondary salt effect is important when the catalytically active ions are produced by the dissociation of a weak electrolyte. In solutions of weak acids and weak bases, added salts, even if they do not exert a common ion effect, can influence hydrogen and hydroxide ion concentrations through their influence on activity coefficients. 

The previous derivation was made under the implicit assumption that the activity coefficients of A and B are both equal to unity. This assumption matches the definition of E° as a standard potential. There are two cases of practical interest, where these conditions are not fulfilled. One is when the activity coefficients differ from unity but do not depend on the relative amounts of A and B in the film. This type of situation may arise when the interactions between the reactants are weak but the presence of the supporting electrolyte decreases the activity coefficients of A and/or B, yA and yB, to below 1 while they remain constant over the entire voltammo-gram. The only change required is thus to replace the standard potential by the formal potential ... 

In primary salt effect, addition of an electrolyte (salt) or variation of ionic strength affects the activity coefficients and hence the rate of reaction. However, in a reaction where H+ or OH ions produced from a weak acid or weak base act as catalyting agent, the addition of salt influences the concentration of H+ or OH ions. Since the rate of reaction depends upon the concentration of H+ or OH, it will be affected by the salt concentration. This phenomenon is known as secondary salt effect. 

About the same time Beutier and Renon (11) also proposed a similar model for the representation of the equilibria in aqueous solutions of weak electrolytes. The vapor was assumed to be an ideal gas and < >a was set equal to unity. Pitzer s method was used for the estimation of the activity coefficients, but, in contrast to Edwards et al. (j)), two ternary parameters in the activity coefficient expression were employed. These were obtained from data on the two-solute systems It was found that the equilibria in the systems NH3+ H2S+H20, NH3+C02+H20 and NH3+S02+H20 could be represented very well up to high concentrations of the ionic species. However, the model was unreliable at high concentrations of undissociated ammonia. Edwards et al. (1 2) have recently proposed a new expression for the representation of the activity coefficients in the NH3+H20 system, over the complete concentration range from pure water to pure NH3. it appears that this area will assume increasing importance and that one must be able to represent activity coefficients in the region of high concentrations of molecular species as well as in dilute solutions. Cruz and Renon (13) have proposed an expression which combines the equations for electrolytes with the non-random two-liquid (NRTL) model for non-electrolytes in order to represent the complete composition range. In a later publication, Cruz and Renon (J4J, this model was applied to the acetic acid-water system. 

Recently, there have been a number of significant developments in the modeling of electrolyte systems. Bromley (1), Meissner and Tester (2), Meissner and Kusik (2), Pitzer and co-workers (4, ,j5), and" Cruz and Renon (7j, presented models for calculating the mean ionic activity coefficients of many types of aqueous electrolytes. In addition, Edwards, et al. (8) proposed a thermodynamic framework to calculate equilibrium vapor-liquid compositions for aqueous solutions of one or more volatile weak electrolytes which involved activity coefficients of ionic species. Most recently, Beutier and Renon (9) and Edwards, et al.(10) used simplified forms of the Pitzer equation to represent ionic activity coefficients. 

From the foregoing discussion we conclude that some sophisticated tools are now available by which the activity coefficient in hydrometal— lurgical systems can be addressed. What is lacking is the actual application of these tools by the industry. The next step in establishing the accuracy of the available approaches lies in providing a broader data base for complex multicomponent systems which can be used for parameter refinement. TTte lack of data is most serious in the weak electrolyte area, but even familiar systems such as those encountered in sulfuric acid leaching need attention. 

As it is possible to measure (or closely approximate) the ionic concentrations of a weak electrolyte, it is convenient to define ionic activity coefficients for weak electrolytes in the same way, based on the actual ionic concentrations, or m. Thus,... 

Quirk JP, Posner AM (1975) Trace element adsorption by soil minerals. In Nicholas DJ, Egan AR (eds) Trace elements in soil plant animal system. Academic Press, New York, pp 95-107 Randall M, Failey CF (1927) The activity coefficient of the undissociated part of weak electrolytes. Chem Rev 4 117-128... 

Although these effects are often collectively referred to as salt effects, lUPAC regards that term as too restrictive. If the effect observed is due solely to the influence of ionic strength on the activity coefficients of reactants and transition states, then the effect is referred to as a primary kinetic electrolyte effect or a primary salt effect. If the observed effect arises from the influence of ionic strength on pre-equilibrium concentrations of ionic species prior to any rate-determining step, then the effect is termed a secondary kinetic electrolyte effect or a secondary salt effect. An example of such a phenomenon would be the influence of ionic strength on the dissociation of weak acids and bases. See Ionic Strength... 

Which electrolytes are called strong, and which are called weak In what form can the law of mass action be applied to strong electrolytes Define the activity, the activity coefficient, and the ionic strength of a solution. 

For weak 1 1 electrolytes, the dissociation constant (Kd) reflects the degree of ionization of the salt a. Ignoring the activity coefficient,... 

Activity coefficients of non-ionized molecules do not differ appreciably from unity. In dilute solutions of weak electrolytes the differences between activities and concentrations (calculated from the degree of dissociation) is negligible. 

From all that has been said about activity and activity coefficients, it is apparent that whenever precise results are to be expected, activities should be used when expressing equilibrium constants or other thermodynamic functions. In the present text however we shall be using simply concentrations. For the dilute solutions of strong and weak electrolytes that are mainly used in qualitative analysis, errors introduced into calculations are not considerable. 

The equilibrium constant given by Eq. (9) using a values obtained from Eq. (8) differs from K, the true equilibrium constant in terms of activities, owing to the omission of activity coefficients (y ) from the numerator of Eq. (9) and the approximations inherent in Eq. (8). At the very low ionic concentrations encountered in the dissociation of a weak electrolyte, a simple extrapolation procedure can be developed to obtain from the values of Since y is an excellent approximation, it follows that... 

Nonetheless, the activity coefficient is not determined by the dielectric constant alone. In this connection, it is interesting to note that acetic acid is much weaker in NMP than in water (13). When NMP is added to the aqueous solvent, the dissociation of the protonated form of tris-(hydroxymethyl )aminomethane is enhanced initially (12). In pure NMP, however, this acid is weaker than in water (14), despite the greatly increased dielectric constant (e = 176 at 25°C). These results point to the controlling influence of solute-solvent interactions on the behavior of these weak electrolytes. 

In this chapter some aspects of the present state of the concept of ion association in the theory of electrolyte solutions will be reviewed. For simplification our consideration will be restricted to a symmetrical electrolyte. It will be demonstrated that the concept of ion association is useful not only to describe such properties as osmotic and activity coefficients, electroconductivity and dielectric constant of nonaqueous electrolyte solutions, which traditionally are explained using the ion association ideas, but also for the treatment of electrolyte contributions to the intramolecular electron transfer in weakly polar solvents [21, 22] and for the interpretation of specific anomalous properties of electrical double layer in low temperature region [23, 24], The majority of these properties can be described within the McMillan-Mayer or ion approach when the solvent is considered as a dielectric continuum and only ions are treated explicitly. However, the description of dielectric properties also requires the solvent molecules being explicitly taken into account which can be done at the Born-Oppenheimer or ion-molecular approach. This approach also leads to the correct description of different solvation effects. We should also note that effects of ion association require a different treatment of the thermodynamic and electrical properties. For the thermodynamic properties such as the osmotic and activity coefficients or the adsorption coefficient of electrical double layer, the ion pairs give a direct contribution and these properties are described correctly in the framework of AMSA theory. Since the ion pairs have no free electric charges, they give polarization effects only for such electrical properties as electroconductivity, dielectric constant or capacitance of electrical double layer. Hence, to describe the electrical properties, it is more convenient to modify MSA-MAL approach by including the ion pairs as new polar entities. 

A kinetic electrolyte effect ascribable solely to the influence of the ionic strength on activity coefficients of ionic reactants and transition states is called a primary kinetic electrolyte effect. A kinetic electrolyte effect arising from the influence of the ionic strength of the solution upon the pre-equilibrium concentration of an ionic species that is involved in a subsequent rate-limiting step of a reaction is called a secondary kinetic electrolyte effect. A common case encountered in practice is the effect on the concentration of a hydrogen ion (acting as catalyst) produced from the ionization of a weak acid in a buffer solution. 

Define the mean ion activity coefficient of a salt and comment on its significance in a weak versus a strong electrolyte solution. 

Weak electrolyte model of RS has been employed to calculate activity coefficients and to use activity coefficients to detennine the activation barriers for conductivity. The agreement between the experimental and theoretical activation energies has been found to be satisfactory (Ravine and Souquet, 1977). 

The activity coefficients of unionised molecules do not differ considerably from imity and for weak electrolytes in which the ionic strength is small, the true or thermodynamic expression reduces to... 

The reason why Qstwald s dilution law, equation (17), Chapter 3, is moderately successful in accounting for the conductances of weak electrolytes is now evident. Arrhenius equation, a = A/Ao, yields degrees of dissociation which are too low. This error, from our present point of view, was more or less offset by the tacit assumption made by Arrhenius and Ostwald, that activity coefficients are unity, whereas, for dilute solutions at least, they are less than unity. 

Figure 7.2 summarizes the relative weathering rates of major minerals in igneous and metamorphic rocks. Actual weathering rates depend also on soil temperature and moisture, particle size, and planes of physical weakness (cleavage) in the crystal. The effect of moisture includes both the flow rate of soil solution past mineral surfaces and the composition of the solution. Solids dissolve more slowly if the solution already contains their constituent ions. High electrolyte concentrations, on the other hand, can maintain higher ion concentrations at equilibrium because of lower activity coefficients and because of complex-ion and ion-pair formation. 

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