Activity of electrolyte

Freezing point methods are often applied to the measurement of activities of electrolytes in dilute aqueous solution because the freezing point lowering, 6= T — T, can be determined with high accuracy, and the solute does not dissolve in the solid to any appreciable extent. Equations can be derivedgg relating a to 9 instead of T and T. The detailed expressions can be found in the literature.16... 

On the basis of this definition, one can determine, for instance, the activity of electrolytic solutions in terms of the real hydrogen ion activity. Rybkin et al. found that the Ax effect may be stabilized by adding surface-active substances in small quantities to the solution. Ac-... 

Analogonsiy, soinbiiity measurements yieid the mean activity of electrolyte (s = w, o), and the corresponding standard Gibbs energy AG i of solution (in the molar scale). 

Roughly half of the data on the activities of electrolytes in aqueous solutions and most of the data for nonelectrolytes, have been obtained by isopiestic technique. It has two main disadvantages. A great deal of skill and time is needed to obtain reliable data in this way. It is impractical to measure vapor pressures of solutions much below one molal by the isopiestic technique because of the length of time required to reach equilibrium. This is generally sufficient to permit the calculation of activity coefficients of nonelectrolytes, but the calculation for electrolytes requires data at lower concentrations, which must be obtained by other means. 

Although potential measurements are used primarily to determine activities of electrolytes, such measurements can also be used to obtain information on activities of nonelectrolytes. In particular, the activities of components of alloys, which are solid solutions, can be calculated from the potentials of cells such as the following for lead amalgam ... 

All methods used in the study of nonelectrolytes also can be applied in principle to the determination of activities of electrolyte solutes. However, in practice, several methods are difficult to adapt to electrolytes because it is impractical to obtain data for solutions sufficiently dilute to allow the necessary extrapolation to infinite dilution. For example, some data are available for the vapor pressures of the hydrogen halides in their aqueous solutions, but these measurements by themselves do not permit us to determine the activity of the solute because significant data cannot be obtained at concentrations below 4 moM. 

A great deal of information on activities of electrolytes also has been obtained by the isopiestic method, in which a comparison is made of the concentrations of two solutions with equal solvent vapor pressure. The principles of this method were discussed in Section 17.5. 

A useful concept that is used when the activities of electrolytes are calculated is that of the ionic strength of the solution. This is defined (on the molar scale) as ... 

Let us now imagine plotting the activity of electrolyte as a function of molality, elevated to stoichiometric factor v ml), as shown in figure 8.8. Graphically in this sort of plot, Henry s law constant represents the slope of equation 8.23... 

The activity of electrolyte may be related to the individual ionic activities of constituting ions and a ... 

This solvent vapor pressure method for measuring the activity of electrolytes has the advantage that the actual experiments one has to do are simple. The method can be applied to any concentration (e.g., a 15 M solution ). The difficulty comes at low concentrations when the difference of the vapor pressure between the solution and that of the solvent becomes limitingly small. A huge amount of data (see Table 3.5) have been determined by this method, particularly in the 1950s by a long-term Australian-New Zealand collaboration between professors Stokes and Robinson. 

Activities of Electrolytes.—When the solute is an electrolyte, the standard states for the ions are chosen, in the manner previously indicated, as a hypothetical ideal solution of unit activity in this solution the thermodynamic properties of the solute, e.g., the partial molal heat content, heat capacity, volume, etc., will be those of a real solution at infinite dilution, i.e., when it behaves ideally. With this definition of the standard state the activity of an ion becomes equal to its concentration at infinite dilution. 

The soluble anion is usually not explicitly considered in cation exchange equations, because the activities of electrolytes can be expressed in terms of the activities of their constituent ions, that is,... 

The measurement of cell potentials is the most powerful method of obtaining values of activities of electrolytes. Experimentally it is, in many cases at least, much easier to handle than measurements of colligative properties. It has the additional advantage that it can be used over a wide range of temperatures. Although cell potentials can be measured in nonaqueous solvents, the electrode equilibria often are not as easily established so that the experimental difficulties are much greater. 

Petry OA. Activity of electrolytically deposited platinum and mfhenium on the electrooxidation of methanol. Dokl Akad Nauk SSSR 1965 160 871. ... 

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