Ionic medium activity scale

The infinite dilution activity scale is useful for ionic equilibria in fresh waters, but for equilibria in sea water one gains precision by applying an ionic medium activity scale. Measuring pH in sea water gives less information than total alkalinity and total carbonate. Calculations on redox equilibria are simplified by introducing the master variable pE -----log e. ... 

The "ionic medium activity scale, on the other hand, is so defined that the activity coefficient, yA = A / [A] approaches unity as the solution approaches the pure solvent (in this case the ionic medium)—Le., when the concentrations go toward zero for all other species than water and the medium ions. 

Activity coefficients defined within the infinite dilution activity scale cannot be formulated theoretically for the ionic medium of seawater. Since the oceans contain an ionic medium of practically constant composition, the ionic medium activity scale might be used advantageously in studying acid-base and other equilibria in seawater (see also Appendix 6.2 in Chapter 6). 

Equilibrium constants are either or are defined in terms of the constant ionic medium activity scale. 

Both activity scales are thermodynamically equally well defined. In constant ionic medium, activity (= concentration) can frequently be determined by means of emf methods. 

All equilibrium constants in the present discussion are based on the concentration (not activity) scale. This is a perfectly acceptable thermodynamic scale, provided the ionic strength of the solvent medium is kept fked at a reference level (therefore, sufficiently higher than the concentration of the species assayed). This is known as the constant ionic medium thermodynamic state. Most modern results are determined at 25 °C in a 0.15 M KCl solution. If the ionic strength is changed, the ionization constant may be affected. For example, at 25 °C and 0.0 M ionic strength, the pXj of acetic acid is 4.76, but at ionic strength 0.15 M, the value is 4.55 [24]. 

Experience shows that the activity coefficients on this scale stay near unity (usually within experimental error) as long as the concentrations of the reactants are kept low, say less than 10% of the concentrations of the medium ions. The activity ( concentration) of several ions, notably H+, can be determined conveniently and accurately by means of e.m.f. methods, either with or without a liquid junction. In the latter case the liquid junction potential is small (mainly a function of [H+] ) and easily corrected for (3). The equilibrium constant for any reaction, on the ionic medium scale, may then be defined as the limiting value for the concentration quotient ... 

The Ionic Medium Scale This convention can be applied to solutions that contain a swamping concentration of inert electrolyte in order to maintiiin a constant ionic medium. The activity coefficient, f = A /[A], beconries unity as the solution approaches the pure ionic medium, that is, when all concentrations other than the medium ions approach zero ... 

Figure 3.2. Activity coefficients depend on the selection of the reference state and standard state, (a) On the infinite dilution scale the reference state is an infinitely dilute aqueous solution the standard state is a hypothetical solution of concentration unity and with properties of an infinitely dilute solution. For example, the activity coeffic ent of in a HCl solution, / hcm varies with [HCl] in accordance with a Debye-Hiickel equation (dashed line, left ordinate) (see Table 3.3) only at very great dilutions does /become unity. On the ionic medium scale, for example, in I M KCl, the reference...

State is the ionic medium (i.e., infinitely diluted with respect to HCl only). In such a medium /hci (solid line, right ordinate) is very nearly constant, that is,/Hci = 1- Both activity coefficients are thermodynamically equally meaningful. (Adapted from P. Schindler.) (b) A comparison of activity coefficients (infinite dilution scale) of electrolytes and nonelectrolytes as a function of concentration (mole fraction of solute) m = moles of solute per kg of solvent (molality) = number of moles of ions formed from 1 mol of electrolyte 1 kg solvent contains 55.5 mol of water. (From Robinson and Stokes, 1959. Reproduced with permission from Butterworths, Inc., London.)... 

As equation 23 illustrates, a change in the activity scale convention merely changes kf. In an ideal constant ionic medium, equation 23 becomes... 

In dealing with redox equilibria, we are also confronted with the problem of evaluating activity corrections or maintaining the activities under consideration as constants. The Nernst equation rigorously applies only if the activities and actual species taking part in the reaction are inserted in the equation. The activity scales discussed before, the infinite dilution scale and the ionic medium scale, may be used. The standard potential or standard pe on the infinite dilution scale is related to the equilibrium constant for / = 0 of the reduction reaction... 

These authors used activities instead of concentrations, but at constant ionic strength (constant ionic medium scale), the rate law can be written in terms of concentrations. 

Acidity scales are used commonly to assess the chemical and biological state of seawater. The international operational scale of pH fulfills the primary, requirement of repro ducibility and leads to useful equilbrium data. Nevertheless, these pH numbers do not have a well defined meaning in respect to all marine processes. Seawater of 35%o salinity behaves as a constant ionic medium, effectively stabilizing both the activity coefficients and the liquid junction potential. It may be possible, therefore, to determine hydrogen ion concentrations in seawater experimentally. One method is based on cells without a liquid junction and is used to establish standard values of hydrogen ion concentration (expressed as moles of H /kg of seawater) for reference buffer solutions. 

In the step from the raw experimental data to data that can be treated by conventional computer programs, transformations are often necessary. For titrations, the raw data are EMF values as a function of titrant added (volume changes may be more or less important). These would typically be transformed to log[H ] (or pH), total acid concentration, and dilution factors (e.g., for FITEQL). With LAKE [32], raw data can be treated directly. An input file or the code itself must, in such a case, provide all the information necessary for the calculation of equilibria (e.g., LAKE must perform the transformations to obtain concentrations from the raw data). For the majority of eodes, however, pH or log[H ] as a function of titrant added is required. EMF values must be transformed to pH or log[H" ] using the apphed calibration procedures. Here, the calibration and, sometimes, assumptions in the data treatment may be important. Calibration can be performed on the eoneentration or on the activity scale. The concentration scale has the advantage that, a priori, no assumptions about activity coefficients are necessary the disadvantage is that such an approach is limited to one ionic medium (the constant ionic medium approach), although in work on suspensions, the variation of the ionic medium certainly is of importance. The activity scale requires assumptions concerning the treatment of activity coefficients it must be realized that sueh assumptions must lead to self-consistency between the finally presented experimental data and the model calculations. 

It is obvious that the expression enclosed in the brackets by the author of the present book is nothing but the primary medium effect of O2- expressed via the difference in the values of the equilibrium constants of equation (1.3.6) for the media compared the molten equimolar KCl-NaCl mixture, which was chosen as a reference melt, and for which pKHa/H20 was found to be 14 at 700 °C, and the melt studied. As to the physical sense of the common acidity function Cl, this is equal to the pO of the solution in the molten equimolar KCl-NaCl mixture, whose acidic properties (oxide ion activity) are similar to those of the solution studied. Moreover, from equation (1.3.7) it follows that solutions in different melts possess the same acidic properties (f ) if they are in equilibrium with the atmosphere containing HC1 and H20 and Phc/Ph2o — constant. This explanation confirms that the f function is similar to the Hammett function. Therefore, Cl values measured for standard solutions of strong bases in molten salts allow the prediction of the equilibrium constants on the background of other ionic solvents from the known shift of the acidity scales or the f value for the standard solution of a strong Lux base in the solvent in question. According to the assumption made in Refs. [169, 170] this value may be obtained if we know the equilibrium constant of the acid-base reaction (1.3.6) in the solvent studied. 

To be applied industrially, performance must be superior to that of the existing catalytic systems (activity, regioselectivity and recyclability). The use ofionic liquid biphasic technology for nickel-catalyzed olefin dimerization proved to be successful and this system has been developed and is now proposed for commercialization. However, much effort remains if the concept is to be extended to non-chloroaluminate ionic liquids. In particular, the true potential ofionic liquids (and mixtures containing ionic liquids) could be achievable if an even more substantial body of thermophysical and thermodynamic properties were amassed in order that the best medium for a given reaction could be chosen. As far as industrial applications are concerned, the easy scale-up of two-phase catalysis can be illustrated by the first 0X0 commercial unit with an initial capacity of 100 000 tons extrapolated by a factor of 1 24 000 (batch-wise laboratory development to production reactor) after a development period of 2 years [4]. 

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