Junction potentials elimination

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-... 

Although in certain cells the liquid junction can be eliminated by appropriate choice of electrolyte solution, this is not always possible. However, the liquid junction potential can be minimised by the use of a salt bridge (a saturated solution of KCl of about 4-2m), and the liquid junction potential is then only 1-2 mV this elimination of the liquid junction potential is indicated... 

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 ... 

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. 

Changes in the reference electrode junction potential result from differences in the composition of die sample and standard solutions (e.g., upon switching from whole blood samples to aqueous calibrants). One approach to alleviate this problem is to use an intermediate salt bridge, with a solution (in the bridge) of ions of nearly equal mobility (e.g., concentrated KC1). Standard solutions with an electrolyte composition similar to that of the sample are also desirable. These precautions, however, will not eliminate the problem completely. Other approaches to address this and other changes in the cell constant have been reviewed (13). 

If both liquid junction potentials are eliminated and supporting electrolytes MX are not adsorbed at the free surfaces of solvents, the compensating voltage is... 

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... 

When eliminating the liquid junction potential by one of the methods described in Section 2.5.3, we obtain a concentration cell without transport. The value of its EMF is given simply by the difference between the two electrode potentials. More exactly than by the described elimination of the liquid junction potential, a concentration cell without transport can be obtained by using amalgam electrodes or electrodes of the second kind. 

It would appear from Eq. (3.2.8) that the pH, i.e. the activity of a single type of ion, can be measured exactly. This is not, in reality, true even if the liquid junction potential is eliminated the value of Eref must be known. This value is always determined by assuming that the activity coefficients depend only on the overall ionic strength and not on the ionic species. Thus the mean activities and mean activity coefficients of the electrolyte must be employed. The use of this assumption in the determination of the value of Eref will, of course, also affect the pH value found from Eq. (3.2.8). Thus, the potentiometric determination of the pH is more difficult than would appear at first glance and will be considered in the special Section 3.3.2. 

Interesting results have been obtained from polarographic studies in various donor solvents. Measurements have been made of various metal perchlorates in solutions of donor solvents containing tetraalkylammonium perchlorate as supporting electrolyte against an aqueous saturated calomel electrode 113. In order to eliminate differences in liquid-liquid junction potentials bisbiphenylchromium (I) has been used as a reference ion 114 118). 

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. 

Various attempts have been made to circumvent these problems and to eliminate junction potentials, including (1) extrapolation procedures designed to eliminate the difference between the compositions of the two solutions in the appropriate limit, (2) separation of the two solutions by means of a doublejunction salt bridge, (3) the use of double cells with dilute alkali metal amalgam connectors, and (4) the use of glass or other types of ion-specific electrodes as bridging reference electrodes. 

In the cell diagrams the boundary between the electrode and the electrolyte, where the potential difference arises, is marked by a single vertical line (e. g. Zn Zn++). The line dividing two electrolytes (e. g. Zn+ f Cu++) expresses the existence of the liquid junction potential added to the electrode potentials. When this liquid junction potential is eliminated (e. g. by a salt bridge, see below), so that it can be disregarded, a double line is written instead of a single one (Zn++ Cu++). 

A single vertical bar ( ) should be used to represent a phase boundary, a dashed vertical bar ( ) to represent a junction between miscible liquids, and double, dashed vertical bars ( ) to represent a liquid junction, in which the liquid junction potential has been assumed to be eliminated (see also -> concentration cells). 

Both the - standard hydrogen electrode (SHE), which is the primary standard in electrochemistry [iv,v] and the relative hydrogen electrode (RHE) are widely used in aqueous acidic solutions. In RHE the nature and concentration of acid is the same in the reference and the main compartments. In general, it is advantageous to use the same solution in both compartments to decrease the - junction potential. By the help of RHE the -> activity effect can also be eliminated when the -> pH dependence of a — redox reaction is to be determined, since the H+ ion activity influences both the redox reactions under study and the redox reaction occurring in the reference system (1/2H2 -> H+ + e-) in the same way. 

Elimination of Liquid Junction Potentials.—Electromotive force measurements are frequently used to determine thermodynamic quantities of various kinds in this connection the tendency in recent years has been to employ, as far as possible, cells without transference, so as to avoid liquid junctions, or, in certain ca.ses, cells in w hich a junction is formed between two solutions of the same electrolyte. As explained above, the potential of the latter type of junction is, within reasonable limits, independent of the method of forming the boundary. 

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,... 

The theoretical basis of the use of a bridge containing a concentrated salt solution to eliminate liquid junction potentials is that the ions of this salt are present in large excess at the junction, and they consequently carry almost the whole of the current across the boundary. The conditions will be somewhat similar to those existing when the electrolyte is the same on both sides of the junction. When the two ions have approximately equal conductances, i.e., when their transference numbers are both about 0.5 in the given solution, the liquid junction potential will then be small [cf. equation (36a)]. The equivalent conductances at infinite dilution of the potassium and chloride ions are 73.5 and 76.3 ohins cm. at 25, and those of the ammonium and nitrate ions are 73.4 and 71.4 ohms cm. respectively the approximate equality of the values for the cation and anion in each case accounts for the efficacy of potassium chloride and of ammonium nitrate in reducing liquid junction potentials. 

A procedure for the elimination of liquid junction potentials, suggested by Nemst (1897), is the addition of an indifferent electrol3rte at the same concentration to both sides of the cell. If the concentration of this added substance is greater than that of any other electrolyte, the former will carry almost the whole of the current across the junction between the two solutions. Since its concentration is the same on both sides of the boundary, the liquid junction potential will be very small. This method of eliminating the potential between two solutions fell into disrepute when it was realized that the excess of the indifferent electrolyte has a marked effect on the activities of the substances involved in the cell reaction. It has been revived, however, in recent years in a modified form a series of cells are set up, each containing the indifferent electrolyte at a different concentration, and the resulting e.m.p. s are extrapolated to zero concentration of the added substance. 

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