Conventions single-ion

Note that in all ion interaction approaches, the equation for mean activity coefficients can be split up to give equations for conventional single ion activity coefficients in mixtures, e.g., Eq. (6.1). The latter are strictly valid only when used in combinations that yield electroneutrality. Thus, while estimating medium effects on standard potentials, a combination of redox equilibria with H " + e 5112(g) is necessary (see Example 3). 

OCp]3 can be calculated directly if the concentrations of all ligands and of all competing cations are known (21). Where this information is not available, ISE s can in principle enable conventional single-ion activites to be measured directly. The limited sensitivity of present-day ISE s precludes their use in natural waters,/ although they Ccin be. used in experimental systems involving elevated concentrations of trace metals. Provided that the salinity remains constant, a cell without liquid junction, composed of perfectly selective chloride and lead ISE s could be used. The difference between the emfs measured in the sample (E ) and in a standard solution with the same tanperature and major ion composition (Eg) would be given by... 

Pitzer and co-workers have developed an ion interaction model and published a series of papers (Pitzer, 1973a-b, 1974a-b, 1975, 1977, 1995, 2000 Pabalan Pitzer, 1987) which gave a set of expressions for osmotic coefficients of the solution and mean activity coefficient of electrolytes in the solution. Expressions of the chemical equilibrium model for conventional single ion activity coefficients derived are more convenient to use in solubility calculations (Harvie Weare, 1980 Harvie et al.l984 Felmy Weare, 1986 Donad Kean, 1985). 

Fig. 11.17. Simulated mass chromatograms resulting from precursor ion and constant neutral loss tandem mass spectra (middle and bottom traces), illustrating the selectivity that those MS/MS scan modes can bring to chromatographic analyses. The top trace in the figure represents a total ion chromatogram obtained using a conventional single stage of mass analysis.

AH% would be the heat of reaction of (41). Note that AHX is dependent on n and when n is small it differs from the conventional heat of vaporization. Neglecting surface effects, the whole single-ion solvation processes can be constructed from reactions (40 c) and (41)... 

However, there are cases, especially for the configurations with dN and fN shells as well as for some excited configurations of multiply charged ions whose energy levels lie very close or even overlap, in which the conventional single-configuration approach is completely unfit (see also Chapter 31). 

Although for solutions at infinite dilution the absolute enthalpy of hydration of a single ion is clearly defined, there is no purely thermodynamic way to separate the sum of the hydration energies of a cation and an anion into constituent parts. Hence it has been common practice to use conventional standard enthalpies of hydration related in the case of a cation Mz+ and an anion Xz to the absolute enthalpies of hydration by... 

Probably the most satisfactory approach which has been used to estimate absolute enthalpies of hydration of single ions is that of Halliwell and Nyburg (39). The method makes use of a simple model of aqueous solutions, and is based on the significance of differences between thermodynamic functions for ions of opposite charge rather than attempting to split the enthalpy of hydration of pairs of ions. Values of the differences between conventional enthalpies of hydration are plotted against (R -fa) - , where R is the effective radius of the ion and a is the effective radius of the water molecule. This plot yields the absolute enthalpy of hydration of the proton. 

Even with the definition of the Reference State, chemical thermodynamics alone cannot provide a unique methodology for the measurement of single-ion activity coefficients. An infinitude of possibilities exists, each of that calls upon its own extra thermodynamic set of conventions according to criteria of experimental convenience and intended application. However, chemical thermodynamics does provide general constraints that limit any set of arbitrary conventions defining single-ion activities. 

Only mean activity coefficients can be experimentally determined for salts, not activity coefficients for single ions. The Maclnnes Convention is one method for obtaining single ion activity coefficients and states that because of the similar size and mobility of the potassium and chloride ions ... 

The mean ion activity coefficient values can be obtained from experiments where the effect of electrolyte concentration on the A sp value for a salt is determined. The mean values are then compared with those for KCl under the same solution conditions. The single-ion activity coefficient for Ca " " can then be computed if an assumption is made about the individual values for and Cl. These ions have the same magnitude of charge and similar electronic configuration, ionic radii, and ionic mobilities. On the basis of these properties, the Macinnes convention (1919) states that... 

As already mentioned, it is not possible to measure the thermodynamic properties of single ions. However, it would be highly desirable to set up such a compilation, so that an experimentalist does not have to measure the literally thousands of cationic-anionic combinations whose properties are of interest. Since the thermodynamic characteristics if such pairings are algebraically additive one may set up the following convention ... 

Electroneutrality is required in all solutions thus, it is not possible to measure the properties of a single ion without influence from an ion of opposite charge. By convention, the standard enthalpy of formation and the standard entropy of the... 

Apparent molar volumes of ions are often considered additive. The a and b parameters from Equation (22) cannot be measured for single ions, but the parameter values for salts can be split appropriately between the anions and cations of the salt if the parameter values for one ion are fixed by convention. Unfortunately, several conventions are in use. As data based on different conventions are incompatible, it is important to check some reference values when using this type of data. 

MD studies on the solvation of sodium chloride in [C4mim][NTf2] were performed by Sieffert and Wipff [110]. Dissociation of a single [Na][Cl] ion pair in ILs and in conventional solvents, [Na][Cl] condensation in the IL, and solvation of [Na] [Cl] microcrystals were investigated. Characteristics of the solid/IL interface was analyzed. The single ion pair preferred to be associated rather than dissociated. In a concentrated [Na][Cl] solution the ions spontaneously associated. Simulations of Na13Cl 4 and Na14Cl 3 microcrystals in the IL showed that both remain associated [110]. The crystal surface was solvated by the less polar IL components rather than by the polar ones. In the first IL layer the ions were ordered rather parallel to the surface, whereas in the second layer they were more perpendicular. The solvation of the crystal seemed to be rather apolar due to the mismatch between the IL and the crystal ions [110]. 

In any work applying single ion activities, the convention used should be clearly stated and carefully analyzed. 

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