Instant Notes: Analytical Chemistry

Section C – Analytical reactions in solution

C2 ELECTROCHEMICAL

REACTIONS

Important electrochemical reactions have already been noted in Topic C1, for

example:

● acid-basereactions, where the acid donates a proton to the base;

● precipitation reactions, where the reactants form an insoluble product;

● complexation reactions, where a ligand coordinates to an acceptor;

● oxidation-reduction reactions, where the oxidizing agent gains electrons

from the reducing species.

All of these reactions involve charged species and all may be studied by electro-

chemical methods and used for analysis.

In order to study an electrochemical reaction, the appropriate cell must be

constructed. It is impossible to measure the electrical properties using a single

contact. For example, connecting to one end of a resistor will not allow measure-

ment of its resistance. Connections must be made to two electrodes, and a cell must

be constructed. Electrical connection to the solution, whether to measure a cell emf

or to conduct electrolysis reactions, must be made through two electrodes.

Cells with two similar, inert electrodes placed in the same solution may be

used for measuring conductance, discussed in Topic C10. Cells where electro-

lytic reactions occur are used in voltammetry, which is discussed in Topic C9.

For potentiometric methods, discussed here and in other parts of this section,

two dissimilar electrodes are used to construct a cell whose emf depends on the

solutions and electrodes used.

Many different types of electrodes are available and the most important are

described in Topic C3. The simplest are made of a metal, such as zinc, copper, or

Electrochemical
cells

Electrochemical
reactions

Key Notes

Reactions involving charged species in solution are important in many

analyses. Their chemistry and the laws governing them must be known.

The construction of an electrochemical cell using two electrodes allows

the study of the cell reaction, changes in concentration, and electrolysis.

The potential difference between electrodes depends on the properties of

the electrodes and the concentrations in the solution.

If the cell reaction is driven by applying an external voltage supply, then

useful chemical reactions may occur.

Related topics Solution equilibria (C1) Other topics in Section C

(C3-C10)

Electrochemical

reactions

Electrochemical cells

Electrode potentials

Electrolysis