E.m.f. measurements

Acid K, X From conductance measurements" 10s From e.m.f. measurements... 

An element of uncertainty is introduced into the e.m.f. measurement by the liquid junction potential which is established at the interface between the two solutions, one pertaining to the reference electrode and the other to the indicator electrode. This liquid junction potential can be largely eliminated, however, if one solution contains a high concentration of potassium chloride or of ammonium nitrate, electrolytes in which the ionic conductivities of the cation and the anion have very similar values. 

Binary Systems.—Halides. The heats of formation of Crp2 and CrFj have been evaluated as —655 and —958 kJ mol respectively, from e.m.f. measurements. ... 

Figure 9 Steady-state rate and e.m.f measurements as a function of oxygen in feed during CO oxidation at 500°C and 6%C0 over a La( ) catalyst (from 35). Closed (open) symbols signify the measurements at decreasing (increasing) percentages of oxygen.

This may be further emphasized in that dffwatar for the acetic acid dimerization is very small [5, 6]. This conclusion has also been established for a number of other monocarboxylic acids in water from conductance and e.m.f. measurements (Table 1, [7]). 

We now look at some examples of redox reactions involving simple cations in aqueous solution. Electrochemical terminology will often be encountered, since e.m.f. measurements on electrochemical cells are important sources of thermodynamic data in this area. For example, the reduction potential ° for the half-reaction ... 

MoCr)4C3 x has been shown by X-ray powder diffractometry to be isomorphous with -Hf4N3.64 The standard free energy of formation of UWC2 has been determined by e.m.f. measurements for the temperature range 981—1094K.384 The crystal structure of PuWC, 15 has been reported.385... 

The heat of formation in solution of the addition compound of picric acid and naphthalene determined by Bronsted [131], using e.m.f. measurements, was found to be 2.15 kcal/mole. Similar figures were obtained by cryometric measure... 

Table 8.2. Free energy values calculated from e.m.f. measurements for the system Ca0-V205...

Table 8.3 Free energy values calculated from the e.m.f. measurement of the cell Involving CaCr04...

The majority of e.m.f. measurements are made at the present time by means of special potentiometers, operating on the Poggendorff principle, which are purchased from scientific instrument makers. They generally consist of a number of resistance coils with a movable contact, together with a slide wire for fine adjustment. A standard cell is used for calibration purposes, and the e.m.f. of the cell being measured can then be read off directly with an accuracy of 0.1 millivolt, or better. 

Although the so-called saturatedWeston cell, containing a saturated solution of cadmium sulfate, is the ultimate standard for E.M.F. measurement, a secondary standard for general laboratory use has been recommended this is the unsaturated Weston cell, which has an even smaller temperature coefficient than the saturated cell. The form of unsaturated cell generally employed contains a solution which has been saturated at 4 c., so that it is unsaturated at room temperatures its temperature coefficient is so small as to be negligible for all ordinary purposes and its e.m.p. may be taken as 1.0186 volt. ... 

Some of the results obtained in the calculation of heat content changes from E.M.F. measurements arc recorded in Table XLII the values de-... 

TABLE XLII. HEAT CHANGES FROM E.M.F. MEASUREMENTS... 

Determination of Transference Numbers.—Since activity coefficients can be derived from e.m.f. measurements if transference numbers are known, it is apparent that the procedure could be reversed so as to make it possible to calculate transference numbers from e.m.f. data. The method employed is based on measurements of cells containing the same electrolyte, with and without transference. The e.m.f. of a concentration cell without transference E) is given by equation (11), and if the intermediate electrodes are removed so as to form a concentration cell with transference, the e.m.f., represented by Et, is now determined by equation (25), provided the transference numbers may be taken as constant within the range of concentrations in the cells. It follows, therefore, on dividing equation (25) by (11), that... 

The method for determining transference numbers from e.m.f. measurements was first suggested by Helmholtz in 1878. 

This result is of particular interest because it can be used to determine the activities of metals in amalgams or other alloys by e.m.f. measurements such determinations have been carried out in a number of cases. If the amalgams are sufficiently dilute, the ratio of the activities may be taken as equal to that of their mole fractions, i.e., Xi/x2, or even to that of their concentrations C1/C2 in the latter case equation (47) takes the approximate form... 

The Oxygen Electrode.—The standard potential of the oxygen electrode cannot be determined directly from e.m.f. measurements on account of the irreversible behavior of this electrode (cf. p. 353) it is possible, however, to derive the value in an indirect manner. The problem is to determine the e.m.f. of the cell... 

An entirely different method of arriving at this standard free energy change is based partly on e.m.f. measurements, and partly on equilibrium data. From the dissociation pressure of mercuric oxide at various temperatures it is possible to obtain the standard free energy of the reaction... 

Standard Electrode Potentials.—By the use of methods, such as those described above, involving either e.m.f. measurements or free energy and related calculations, the standard potentials of a number of electrodes have been determined some of the results for a temperature of 25° are recorded in Table XLIX. It should be noted that the signs of the... 

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