Page 153 - Instant notes

Electrochemistry and ion concentration 139

Measurement of activity coefficients
+
−
For a cell such as M(s)|M (aq, c)||Cl (aq, c)|AgCl(s)|Ag (s), the cell reaction is
−
+
AgCl (s)+M (s)→Ag (s)+M (aq)+Cl (aq) and the Nernst equation for the cell is:

From the relationship of activity to concentration (see Topic E2):

−3
where is the standard concentration of 1 mol dm . Measurements of E cell as the
concentration c is varied therefore allow measurements of γ ± (see Topic E2) if is
known or calculated. can be determined from tabulated data (see Topic E4) or by
determining as c→0 since γ ±→1 (see Topic E2).

Electrolyte concentration cells

An electrolyte concentration cell involves connecting two identical half-cells via a salt
+
+
bridge, for example M(s)|M (aq, a=x)||M (aq, a=y)|M(s). The only difference in the two
+
half-cells is the difference in ion activity, x and y. The cell reaction is M (aq,
+
a=y)→M (aq, a=x) and the Nernst equation is:

as is the same for both half-cells. This reaction also occurs when there is a membrane
+
+
between the two M solutions across which only M can transfer. An example is the
junction between the inside and outside of a neuron cell membrane, where an imbalance
+
in K activity is developed, producing the potential difference E cell that drives the nerve
+
impulse. The potential is positive in the solution of high M concentration and negative in
+
the low M solution and is established to increase the rate of transfer from high
+
concentration to low and equalize the M activity.
pH dependence of cell voltage
Cells that incorporate the hydrogen gas electrode have a cell potential which
−
+
isdependent on pH. For example Pt|H 2(g, p=1 atm)|H (aq)||Cl (aq)|AgCl(s) |Ag(s) has a
−
+
cell reaction AgCl (s)+½H 2(s)→Ag (s)+H (aq)+Cl (aq) and the cell Nernst equation is: