Activity coefficient without liquid junction

The potentiometric measurement of physicochemical quantities such as dissociation constants, activity coefficients and thus also pH is accompanied by a basic problem, leading to complications that can be solved only if certain assumptions are accepted. Potentiometric measurements in cells without liquid junctions lead to mean activity or mean activity coefficient values (of an electrolyte), rather than the individual ionic values. 

To illustrate a use of activity coefficients, consider the cell without liquid junction... 

The difference between cells with and without liquid junctions is one of degree rather than of kind. The cell used to establish the mean ionic activity coefficients of HC1... 

Propose cells, without liquid junction, which might be used for determining the activities (or activity coefficients) of (i) H Oi, (ii) KCl, (iii) NaOH, (iv) CdBr2, in solution. Why is it not posrable to determine the activity of a nitrate in this manner ... 

NB Very careful work using an electrochemical cell without liquid junction potentials. Activity coefficients were calculated for measurements made on carbonate-free solutions at the following temperatures ... 

The basis for accurate activity coefficients and standard potentials are cells without liquid junctions. Studies on cells (Xni) and... 

The standard acidity scale, although well defined theoretically, has the limitation in practice that only the mean activity coefficient, but not the single-ion activity coefficient, is thermodynamically assessible. The single-ion coefficient depends on the composition of the solution as well. One way to circumvent this problem would be to have a defined value of the activity coefficient for one selected ion. Given that, all other activity coefficients could be obtained firom the activity coefficients of the particular electrolytes and that special single-ion coefficient. The value of this selected coefficient could be used, then, as the base of the conventional acidity scale. This single-ion activity coefficient is derived for chloride by the Debye-Hiickel theory. This choice is made by convention, initially proposed for aqueous solutions it is accepted also for other amphiprotic, polar solvents. Note that the measurements of the proton activity are carried out in cells without liquid junction. 

Note that single ion activity coefficients cannot be rigorously assessed experimentally. Potentiometric measurements in cells without liquid junctions where a cation-selective indicator electrodes is measured against an anion-selective electrode gives information on the mean activity coefficient ... 

Experience shows that the activity coefficients on this scale stay near unity (usually within experimental error) as long as the concentrations of the reactants are kept low, say less than 10% of the concentrations of the medium ions. The activity ( concentration) of several ions, notably H+, can be determined conveniently and accurately by means of e.m.f. methods, either with or without a liquid junction. In the latter case the liquid junction potential is small (mainly a function of [H+] ) and easily corrected for (3). The equilibrium constant for any reaction, on the ionic medium scale, may then be defined as the limiting value for the concentration quotient ... 

Acidity scales are used commonly to assess the chemical and biological state of seawater. The international operational scale of pH fulfills the primary, requirement of repro ducibility and leads to useful equilbrium data. Nevertheless, these pH numbers do not have a well defined meaning in respect to all marine processes. Seawater of 35%o salinity behaves as a constant ionic medium, effectively stabilizing both the activity coefficients and the liquid junction potential. It may be possible, therefore, to determine hydrogen ion concentrations in seawater experimentally. One method is based on cells without a liquid junction and is used to establish standard values of hydrogen ion concentration (expressed as moles of H /kg of seawater) for reference buffer solutions. 

---