Page 155 - Instant notes

Electrochemistry and ion concentration 141

that exclusively take up protons from solution, changing the cation distribution in the
membrane. Potential differences at the two membrane-solution boundaries are set up as a
result of this process. The size of these voltages depends on the pH in solution, as more
+
H will produce more proton uptake and a higher voltage. A constant pH on the left-hand
side of the membrane ensures a constant voltage here, but changes in pH in the test
solution cause a corresponding voltage change. The cell Nernst equation is:

For any glass electrode, the validity of this expression is determined and E′, the constant
in the Nernst equation, is found by calibration, which involves measuring E cell for two
(or more) buffer solutions of known pH (see Topic C4). Once calibrated an E cell
measurement can be used to determine the pH of any unknown solution.