Peter A. Rock University o f California Davis, California 95616
The Design of Electrochemical Cells without Liquid Junction
It is well recognized that when electrochemical cells are employed to obtain thermodynamic data it is desirable in work aiming for high accuracy to use 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. This is because in many cases uncertainties surrounding the magnitude of the liquid junction potential constitute the limiting factor on the reliability of the thermodynamic analysis of data from such cells. It has been established experimentally (1-9) that only when certain rather restrictive conditions are met is it possible to make a reliable (*0.2 mV) estimate of the liquid junction potential. Briefly, the.restrictive conditions are: (1)-the junction muat be formed from the same charge type salts having an anion or cation in common, e.g., KCl(aq) IKBr(aq), or Na2S04(aq)IK2S04(aq); (2) the solvent must be the same, and the salt concentrations in the two solutions must be equal and 50.1 AI; and, (3) the transport numbers of the differing ions should not be very different in the two solutions. Because it is im~ossibleto meet these conditions for a wide variety of cells involving electrodes of interest, various methods have been devised in an attempt to circumvent the problem, the most commonly employed of which are: (1) extrapolation procedures designed to eliminate the difference between the compositions of the two solutions in the appropriate limit (10-15); and, (2) separation of the two solutions by means of a salt bridge. The first of these two alternatives is very timeconsuming experimentally and the data analysis is not without pitfalls owing to the lack of any generally satisfactory theoretical expression to guide the extrapolation, or certainty that the liquid junction potential will actually decrease perceptibly over the experimental range. Although salt bridges are very frequently employed, they are regarded by many electrochemists as makeshift devices surrounded by considerable experimental uncertainty (4). There is even some direct experimental evidence that salt bridges can actually increase the liquid junction potential in certain cells (16). The reason that these admittedly unsatisfactory procedures are still widely employed is that it is believed by many that with certain types of electrodes they are the best that can be done. That is, it is widely believed that there are certain types of electrodes for which cells without liquid junction cannot be devised. However, as we shall now proceed to demonstrate, this is not the case. The types of electrodes with which one encounters the problem of cells with liquid junction are: (I) Electrodes of the second kind
where the anion, X-, is unstable in acidic solution, and the salt, MX, is unstable in basic solution, e.g., Pb(s)l PbzFe(CN)e. 3H20(s)lFe/(CN)64-(aq), and, Hg(l)5Hg2Cr04(s)1 Cr042-(aq). Such electrodes cannot be investigated in cells of the type (Pt)H~(g)lHX(soln)IMX(s)lM(s)
(1)
or Hg(l)~HgO(s)~KOH(soln),KX(soln)~MX(s)~M(s) (2)
owing to the decomposition of H X in cell (I), and R/IX(s) in cell (2). Nor can such elect
