PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 406

Example

An example is the standard hydrogen electrode, see11.8.6. Another example is a chlorine

electrode—a platinum electrode washed by chlorine of the partial pressurep. The electrode is

dipped into a solution of hydrogen chloride of the molalitym.

Reaction:

Cl 2 + 2 e−= 2 Cl−.

Nernst equation:

Φ = Φ◦−

RT

2 F

ln

γ^2 ±m^2 /(mst)^2

p φ/pst

,

whereφis the fugacity coefficient of gaseous chlorine.

11.8.10.5 Reduction-oxidation half-cell

This half-cell is comprised of an inert electrode dipped into an electrolyte containing a substance
in two different oxidation degrees, e.g. ions Fe2+and Fe3+.

Example

Another example is the so-called quinhydrone electrode. It is a platinum electrode dipped into

a saturated solution of quinhydrone, an equimolar mixture of quinon C 6 H 4 O 2 and hydroquinon

C 6 H 4 (OH) 2 containing hydrogen ions of the activity aH+. It is assumed that the activities of

quinon and hydroquinon are equal.

Reaction:

C 6 H 4 O 2 + 2 H++ 2 e−=C 6 H 4 (OH) 2.

Φ = Φ◦−

RT

F

ln

1

aH+

.

11.8.10.6 Ion-selective half-cell

This is a cell exchanging only ions of one type with another half-cell.