Activity weak electrolytes

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. 

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. 

Conclusions The correlation of vapor-liquid equilibria in aqueous solutions of weak electrolytes is important for the separation of undesirable components from gases and liquids. The major problem in such correlations is the estimation of the activity... 

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. 

In principle, this system of 20 equations can be solved provided the equilibrium constants, activities, Henry-constants and fugacities are available. While some results for most of these properties are available, there exists no approved method for calculating activities in concentrated aqueous solutions of weak electrolytes therefore, several approximations were developed. ... 

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. 

Such a chemical approach which links ionic conductivity with thermodynamic characteristics of the dissociating species was initially proposed by Ravaine and Souquet (1977). Since it simply extends to glasses the theory of electrolytic dissociation proposed a century ago by Arrhenius for liquid ionic solutions, this approach is currently called the weak electrolyte theory. The weak electrolyte approach allows, for a glass in which the ionic conductivity is mainly dominated by an MY salt, a simple relationship between the cationic conductivity a+, the electrical mobility u+ of the charge carrier, the dissociation constant and the thermodynamic activity of the salt with a partial molar free energy AG y with respect to an arbitrary reference state ... 

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... 

From Eqn. (14) it follows that with an exothermic reaction - and this is the case for most reactions in reactive absorption processes - decreases with increasing temperature. The electrolyte solution chemistry involves a variety of chemical reactions in the liquid phase, for example, complete dissociation of strong electrolytes, partial dissociation of weak electrolytes, reactions among ionic species, and complex ion formation. These reactions occur very rapidly, and hence, chemical equilibrium conditions are often assumed. Therefore, for electrolyte systems, chemical equilibrium calculations are of special importance. Concentration or activity-based reaction equilibrium constants as functions of temperature can be found in the literature [50]. 

For the case where diffusion of the corrosive ions is the rate controlling reaction, it has been found that P = po (1 + Ac/Aa) where p is the penetration that is proportional to the corrosion rate and p0 is the corrosion rate of the less noble uncoupled metal Ac and Aa are the areas of the more noble and active metal respectively (Uhlig and Revie, pp. 101-103).7 If a galvanic cell is not avoidable, a large anode and a limited size of cathode are recommended. Stagnant conditions and weak electrolytes may lead to pitting in spite of the large area of the exposed active metal. 

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. 

The arguments in favor of such a scheme are (juite strong but relatively complex and depend on a number of auxiliary data, kinetic, thermodynamic, and structural-chemical. The large activity effects on ions and Kion of weak electrolytes in the low-dielectric media have been quantitatively considered, and it appears that the active species in the system are not ions but rather ion pairs. This makes it likely that the same is true of some of the other work done in glacial acetic acid which has been probably incorrectly interpreted as ionic. 

When the source of the catalytically active hydrogen ion is a weak acid, one has to consider the weak electrolyte equilibrium involved and the change of the dissociation constant with electrolyte concentration, medium, and temperature. Br0nsted (7) termed this phenomenon secondary kinetic salt effect, but the writer would prefer to omit the word kinetic and substitute electrolyte for salt. The understanding of these... 

The Finkelstein reactions of alkyl bromides with chloride ion show the same differences in rate (Fig. 5), no matter whether the solvent is acetone or DMF or whether the nucleophile is introduced as the weak electrolyte, lithium chloride in acetone, or the strong electrolyte, NEt4Cl in DMF. The calculated differences in activation energy in acetone (de la Mare et al., 1955) correlate well with observed activation energies in DMF, observed differences in acetone show less satisfactory correlation, but the behaviour of the activation energy is quite similar in... 

Lactic acid, C2H4(OF[)COOH, is found in sour milk. It is also formed in muscles during intense physical activity and is responsible for the pain felt during strenuous exercise. It is a weak monoprotic acid and therefore a weak electrolyte. The freezing point of a 0.0100 m aqueous solution of lactic acid is —0.0206°C. Calculate (a) the / value and (b) the percent ionization in the solution. 

This clearly establishes a root of activity of the modifier oxide dependence of the conductivity. The finding was attributed to the dissociation equilibrium of alkali oxide in the silicate glass. The alkali oxide M O is assumed to dissociate as a weak electrolyte. 

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). 

Muller, G., Radke, C.J., and Prausnitz, J.M. (1985). Adsorption of weak electrolytes from dilute aqueous solution onto activated carbon. Part I. Single-solute systems. J. Colloid Interface Sci., 103, 466—83. 

This result shows that the chemical potential of the weak electrolyte system may be expressed in terms of the activities of the ions only, without explicitly including the activity of the undissociated molecule. Equation (3.6.38) is no different in form from those for a strong electrolyte (equations (3.6.1) and (3.6.2)). Of course, the activities of the ions are much less for the weak electrolyte than those for the strong electrolyte for a given molality. Thus, on the basis of the present analysis for a weak electrolyte... 

This result shows that the chemical potential of the electrolyte can be expressed in terms of the activities of the two ions without considering that of the ion pair. It was obtained earlier for weak electrolytes as equations (3.6.38) and (3.6.40). 

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