Instant Notes: Analytical Chemistry

platinum. Consider a cell set up with a zinc electrode dipping into a zinc sulfate

solution, and connected through a porous disc to a copper sulfate solution in

contact with a copper electrode, as shown in Figure 1.

The cell is conventionally written as:

ZnÔZn^2 +SO 42 - ÔÔCu^2 +SO 42 - ÔCu

The cell reaction will result in simultaneous reduction or addition of electrons to

a species in solution at the one electrode and oxidation, or removal of electrons

at the other.

Cu^2 ++2e-=Cu reduction (note: ‘reduction on the right’)

Zn -2e-=Zn^2 + oxidation (note: ‘oxidation on the left’)

Adding these gives the cell reaction (excluding the sulfate anions).

Cu^2 ++Zn =Cu +Zn^2 +

If the cell reaction is allowed to take place, by connecting the copper and zinc

electrodes electrically, then it acts as a galvanic cell or battery, and does work. If

the potential difference is measured with no current flow, it is defined as the

electromotive force, oremf, E, and this is equivalent to reversible conditions. If

an external voltage is applied to the cell it can cause electrolysis, driving the cell

reaction.

In order to prevent the zinc sulfate and the copper sulfate mixing, a porous

disc, or a conducting salt bridge of potassium sulfate in a tube, or a wick can be

used to connect the solutions. This helps to reduce the liquid junction

potential, which occurs when two solutions of unequal concentration, or

containing dissimilar ions are placed in contact. Because of the different rates of

diffusion of the ions, a liquid junction (or diffusion) potential, EL, is set up and

this affects the total cell emf. For example, between a solution of 0.1 M HCl and

a solution of 0.01 M HCl, ELis about 38 mV.

By using a third solution, which must not react, and in which the cation

and anion have similar mobilities and diffusion characteristics, to connect the

two electrode solutions the liquid junction potential may be minimized.

Concentrated potassium chloride, about 4 M, is often used, since K+and Cl-ions

have almost the same mobility (see Topic C10). Potassium nitrate or ammonium

nitrate is also suitable if chloride ions react. Use of a salt bridge reduces the

effective ELto about 1 mV.

The laws of thermodynamics, which are described in detail in textbooks of

physical chemistry, provide the connection between the chemical reaction and

the electrochemical quantities that are measured in analysis.

62 Section C – Analytical reactions in solution

Zinc electrode Copper electrode

Copper sulfate solution

Porous plate

Zinc sulfate solution

Meter

Fig. 1. Daniell cell.