Electromotive force measurement procedure

Three methods have been generally employed for the experimental determination of transference numbers the first, based on the procedure originally proposed by Hittorf (1853), involves measurement of changes of concentration in the vicinity of the electrodes in the second, known as the moving boundary method, the rate of motion of the boundary between two solutions under the influence of current is studied (cf. p. 116) the third method, which will be considered in Chap. VI, is based on electromotive force measurements of suitable cells. 

Tables 3 6 give a survey of literature data for the vapor-liquid equilibrium of aqueous solutions of a single polyelectrolyte with various counterions. Abbreviations (shown in Table 2) are used to characterize the polyelectrolyte and the experimental procedures (MO membrane osmometry DMO differential membrane osmometry VO vapor pressure osmometry ISO isopiestic experiments EMF electromotive force measurements including also measurements with ion-selective electrodes as well as titration FPD freezing point depression GDM gel deswelling investigations). Table 3 gives a survey for aqueous solutions of poly(styrene sulfonic acid).

Similar approaches are used for most steady-state measurement techniques developed for mixed ionic-electronic conductors (see -> conductors and -> conducting solids). These include the measurements of concentration-cell - electromotive force, experiments with ion- or electron-blocking electrodes, determination of - electrolytic permeability, and various combined techniques [ii-vii]. In all cases, the results may be affected by electrode polarization this influence should be avoided optimizing experimental procedures and/or taken into account via appropriate modeling. See also -> Wagner equation, -> Hebb-Wagner method, and -> ambipolar conductivity. 

Measurements of emf (electromotive force) are to be made with this cell under reversible conditions at a number of concentrations c of HCl. From these measurements relative values of activity coefficients at different concentrations can be derived. To obtain the activity coefficients on such a scale that the activity coefficient is unity for the reference state of zero concentration, an extrapolation procedure based on the Debye-Huckel limiting law is used. By this means, the standard electrode emf of the silver-silver chloride electrode is determined, and activity coefficients are determined for all concentrations studied. 

Values of Activity Coefficients.—Without entering into details, it is evident from the foregoing discussion that activities and activity coefficients are related to chemical potentials or free energies several methods, both direct and indirect, are available for determining the requisite differences of free energy so that activities, relative to the specified standard states, can be evaluated. In the study of the activity coefficients of electrolytes the procedures generally employed are based on measurements of either vapor pressure, freezing point, solubility or electromotive force. The results obtained by the various methods arc... 

For metals of high melting point, Holborn and Day recommend the following procedure A wire of the metal about I cm. in length is placed at the junction of a thermocouple, and the electromotive force is measured as soon as the wire melts. In certain cases precautions must be taken to prevent chemical action with the atmospheric air. The table on page 44 gives the melting point h> and the latent heat of fusion w of the more important elements. 

Many workers have been engaged in the measurement and calculation of thermodynamic data relating to solvates [Ha 58a, Ho 72]. Nowadays, the heat of solvation is mainly determined by means of calorimetric measurements, but procedures based on the measurement of the electromotive force, solubility studies and calculations with the modified Born equation [Co 54, Cr 68] are also used. Special mention should be made of the heats of solvation obtained via the mass spectrometric study of interactions between the ion and solvent molecule in the gas phase [Ke 68, 79]. 

The different procedures based on the measurement of electromotive force that may be applied in many varied ways in the equilibrium chemistry of aqueous solutions have a much more restricted use in non-aqueous solutions. It is difficult to construct measuring cells that have small and readily reproducible diffusion potentials or that are without a liquid junction. The glass electrode, perhaps the most extensively used type in aqueous solutions, does not function at all in some non-aqueous solutions, and with very low accuracy in others. Evaluation of the data obtained with its use is hampered by the limits of the acidity scales employed in such systems. The most promising type of ionselective membrane electrodes, the liquid ionic exchange membrane electrode, virtually cannot be employed in non-aqueous solutions. 

In this section, a procedure to carry out measurements of the electromotive force (emf) is described. This emf is known as the standard potential (E°) already introduced in Table 2.2. The standard hydrogen electrode (SHE) is used as the reference electrode in conducting these measurements. In general, a reference electrode one selects to measure E° of a metal has to be reversible since classical... 

---