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Home » Chemistry Homework Help » Physical Chemistry » Junction Potentials
Junction Potentials
If the electrode solutions are different, the end of the cell includes a junction potential.

The treatment of emf’s so far has ignored the problem that arises if one seeks to couple two electrodes which operate in different solutions. If, for instance, one studies the cell consisting of a Zn|Zn2+ electrode and a Cu | Cu2+ electrode, one must separate the two solutions, perhaps solutions containing ZnSo4 and CuSO4, so that they cannot mix with each other. Since the direct contact between solutions of different concentrations is not a balanced state, as required for reversible processes, the system is not directly susceptible to thermodynamic analysis.

The dilution of HCl was studied very comprehensibly.  Assume that two HCl solutions of different concentrations can be brought together and prevented from mixing. The flowing of two streams of solution together sometimes accomplishes this. One can set up the cell:

Pt | H2 | HCl (c1) |H2|Pt

The emf of the cell can be related to the overall reaction that occur when 1 mol of current flows. The reactions which occur at the electrodes and those which occur at the liquid junction can be written separately. The electrodes reactions are:



The emf of this cell, which has Ø˚ = 0, can be written,

Ø˚ = - 0.05915/1 log [(aH+) 1 9aC1-) 1]t-/ [(aH+)2 (aC1-)2]t- = - 0.05915t- log [a±]21/[a±]22

The emf of the cell, unlike that without a liquid junction, depends on the transference numbers. Such cells are frequently described as concentration cells and transference.

If the activities of HCl at the two concentrations are known, the measured emf allows the determination of the transference number. The method is satisfactory and the results compare well with those obtained by the Hittorf method. The difficulties with the method arise through the experimental problem of obtaining a liquid junction that prevents mixing of two solutions. Reproducible and meaningful emf’s are always easily obtained. The assumption has been made furthermore, that the transference numbers are independent of concentration in the concentration range of c1 to c2

Example: the emf of the following concentration cell with transference was studied.

Ag | AgCl | NaCl (aq, c1) | NaCl (aq, c2) | AgCl | Ag

The temperature was kept at 25˚C, and for each of the cells studied the value of c1 was 0.0498 mol L-1. The activity coefficient γ ± of NaCl in this solution is 0.0823.

Here are some of the results obtained in a study by J. Janz and A. R. Gordon, J, Am. Chem. Soc., 65:218 (1943).



Show that this emf’s can be accounted for by a transference number t+ of 0.39 over this concentration range.

Solution: the electrode reactions are:



The cell also involves junction reactions:

Junction reactions can also be described, since t+ = 1 – t-, as

t+NaCl (aq, c1) t+ (NaCl aq, c2)

Cl- (aq, c1) Cl- (aq, c2)

The net cell reaction, the sum of the electrode and the junction processes, is:

Cell reaction: t+ NaCl (aq, c1) t+ NaCl (aq, c2)

The cell emf, with δ˚ = 0, is

δ = -0.05915 log [(a ±)22]t+/[a ±)21]t+



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