Instant Notes: Analytical Chemistry

Section C – Analytical reactions in solution

C3 POTENTIOMETRY

Cells When a cell is set up, but not connected to any outside circuit, no reaction
should take place. If the cell is now ‘shorted out’ by connecting the electrodes on
right and left, electrons will flow and the cell reaction will occur. This reaction
changes the concentrations of the original solutions. Therefore, to measure the
original sample concentrations within the cell, a device must be used that does
not allow current, and hence electrons, to flow. While older systems used a
potentiometer, where the potential difference was balanced by adjusting an elec-
trical circuit so that an external source gave exactly the same potential difference
detected by the null-point of a galvonometer, modern potentiometry uses
digital voltmeters (DVM), where the current used to take the measurement is
extremely small. A suitable experimental arrangement is shown in Figure 1.
If a check is needed on the correctness of the measured value for an experi-
mental cell, a standard cell, such as the Weston cadmium cell, may be used as a
calibration, since the value of its emf is accurately known over a range of
temperatures. The electrode potential is defined using the standard hydrogen
electrode as reference, as described in Topic C2.

There are many types of indicator electrode used in analyses to construct electro-

chemical cells. They may be classified as shown in Table 1.

When two electrodes are combined in a cell, the measured emf may be sepa-

rated into ‘half-cell emfs’ that relate to the individual electrodes. For a Daniell

cell discussed in Topic C2:

Indicator
electrodes

Key Notes

In order to make measurements of electrode potentials, or to study the

changes that take place in a solution reaction, an appropriate

electrochemical cell must be set up.

The indicator electrode makes electrical contact with the solution and acts

as a sensor, which responds to the activity of particular ions in solution

and acquires a potential dependent on the concentration of those ions.

The ideal electrode should respond to a single ion, but this is not often

the case. The effectiveness of any indicator electrode is determined by its

selectivity.

The direct measurement of concentrations is possible using electrodes of

high selectivity and reproducibility. The measurement may also be used

to follow titrations.

Related topics pH and its control (C4) Titrimetry II: complexation,

Titrimetry I: acid-base titrations precipitation and redox titrations

(C5) (C7)

Cells

Indicator electrodes

Selectivity

Direct potentiometry