Cells with eliminated liquid junction potentials

2 Cells with eliminated liquid junction potentials 

Consider now the same half-cells as used in the previous section but joined 

since individual ion activity coefficients are inaccessible to measurement, the cell e.m.f. must be related to determinable mean ion activities. By definition, 

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Electrode Potential (E) the difference in electrical potential between an electrode and the electrolyte with which it is in contact. It is best given with reference to the standard hydrogen electrode (S.H.E.), when it is equal in magnitude to the e.m.f. of a cell consisting of the electrode and the S.H.E. (with any liquid-junction potential eliminated). When in such a cell the electrode is the cathode, its electrode potential is positive when the electrode is the anode, its electrode potential is negative. When the species undergoing the reaction are in their standard states, E =, the stan-... 

An electrode potential varies with the concentration of the ions in the solution. Hence two electrodes of the same metal, but immersed in solutions containing different concentrations of its ions, may form a cell. Such a cell is termed a concentration cell. The e.m.f. of the cell will be the algebraic difference of the two potentials, if a salt bridge be inserted to eliminate the liquid-liquid junction potential. It may be calculated as follows. At 25 °C ... 

The experimental apparatus consists essentially of a narrow vertical glass tube down the inner surface of which one liquid is made to flow, the other liquid emerges from a fine glass tip in the form of a narrow jet down the axis of the tube. The two solutions are connected with calomel electrodes employing potassium chloride or nitrate as junction liquids. The E.M.F. of the cell is measured by means of a sensitive quadrant electrometer. The greatest source of error in the method is the elimination of or the calculation of the exact values of the liquid-liquid junction potentials in the system. For electrolytes which are not very capillary active, the possible error may amount to as much as fifty per cent, of the observed E.M.F. 

Cells with Liquid Junctions and Elimination of Junction Potentials. When electrochemical cells are employed to obtain thermodynamic data, high accuracy ( 0.05 mV) requires the use of cells that are free from liquid junction (in the sense that the construction of the cell does not involve bringing into contact two or more distinctly different electrolyte solutions). Otherwise, the previously discussed uncertainties in the calculation of liquid-junction potentials will limit the accuracy of the data. 

In many instances, however, it has not yet been found possible to avoid a junction involving different electrolytes. If it is required to know the e.m.f. of the cell exclusive of the liquid junction potential, two alternatives are available either the junction may be set up in a reproducible manner and its potential calculated, approximately, by one of the methods already described, or an attempt may be made to eliminate entirely, or at least to minimize, the liquid junction potential. In order to achieve the latter objective, it is the general practice to place a salt bridge, consisting usually of a saturated solution of potassium chloride, between the two solutions that w ould normally constitute the junction (Fig. 70). An indication of the efficacy of potassium chloride in reducing the magnitude of the liquid junction potential is provided by thf. data in Table XLVII 3 the values iucorded are the e.m.f.of the cell, with free diffusion junctions,... 

It is important to emphasize here the difference between cells without transfer and the cell with transfer. A cell with transfer has two additional potential differences between the salt bridge and the electrolytes at each end of the bridge. These potentials can be minimized and almost eliminated in a number of ways. The additional potentials are referred to as the diffusion or liquid junction potentials, which will be discussed in Chapter 3. 

In the Daniell cell, although both electrodes are reversible, diffusion of ions will occur at the liquid junction and, as a result, the e.m.f. will change continuously with time. The liquid junction, and the corresponding liquid junction potential (q.v.), can be almost eliminated by interposing a salt bridge between the two electrolyte solutions. 

The same reference electrode can be used to characterize electrodes in contact with different electrolytes therefore, the cell used to determine the electrode potential often includes a liquid junction (electrolyte-electrolyte interface). In this case the electrode potential is understood as being the corrected OCV value, which is the value for this cell after elimination of the hquid-junction potential. For instance, for the zinc electrode (2. b), the expression for the electrode potential can be written as... 

Practically all liquid cells with reversible interfacial equilibria examined can be considered as liquid galvanic cells of the Nernst, Haber, or intermediate type [3]. Usually, a dashed vertical bar ( ) is used to represent the junction between liquids. A double dashed vertical bar ( ) represents a liquid junction in which the diffusion potential has been assumed to be eliminated. 

When measuring an unknown electrode potential the half cell under examination M M+ is combined with a reference electrode (e. g. a calomel one) in the manner illustrated in Fig. 13. In this figure A is the calomel electrode, B the element to be measured and C the salt bridge, which forms the electric connection between A and B and eliminates at the same time the liquid junction... 

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