Instant Notes: Analytical Chemistry

The cell reaction is the reduction of copper ions by hydrogen:

Cu^2 ++H 2 =Cu +2 H+

Writing the Nernst equation for this cell:

E =En-(RT/nF) ln [(a(Cu).a(H+)^2 )/(a(Cu^2 +).a(H 2 ))]

The superscript, n, means the standard value, when the activities are all 1. Note

that the quantity (RT/F) has the value 0.02569 V at 25∞C. Here, n =2, since two

electrons are transferred at each electrode. For a standard hydrogen electrode,

a(H+) =1 and a (H 2 ) =p(H 2 ) =1, so, we may write:

E =En-(RT/2F) ln [a (Cu)/(a (Cu^2 +)]

which is the equation for the copper electrode potential. It may also be written:

E =En+(RT/2F) ln [a (Cu^2 +)/(a (Cu)]

and for a pure copper electrode, a(Cu) =1, so at 25∞C:

E =En+0.01284 ln [a (Cu^2 +)]

This means that the electrode potential of a copper electrode depends logarith-

mically on the activity of the copper ions in solution. This is known as the

Nernst equation. Standard electrode potentials are listed in Table 1 of Topic C3.

Although the SHE is essential to define electrode potentials, it is not suitable

for many routine analytical uses, and the saturated calomel electrode (SCE)is

often used in practical cells. This is described in Topic C3.

The Nernst equation is very useful in analytical measurements, since it allows

the analyst to measure variations in concentration during reactions or experi-

ments. It is important to note that, strictly, electrochemical cells measure the

activity, although in many cases this may be related to the concentration.

Electrolysis If an electric current is passed through a cell consisting of two electrodes in
contact with an electrolyte solution, the current causes the ions to migrate
towards the electrode of opposite charge, where a reaction takes place. For
example, when acidified water is electrolyzed:

(i) the hydrogen ions are attracted to the negative electrode (cathode) and gain

electrons to form hydrogen gas:

2H++2e-=H 2

(ii) the hydroxyl ions are attracted to the positive electrode (anode) and lose

electrons to become water and oxygen gas:

2OH-- 2e-=H 2 O +^1 ⁄ 2 O 2

The total reaction is therefore:

2H 2 O =2H++2OH-=H 2 O +H 2 +^1 ⁄ 2 O 2

In order to cause 1 mole of reaction involving the transfer of nelectrons, then n

moles of electronic charge must be transferred or nF Coulombs cause 1 mole of

such a reaction, where F is the Faraday equal to 96458 C.

Electrolysis is the basis of the analytical techniques of polarography,voltam-

metry, amperometry and coulometry(see Topic C9). Electrolysis may also be

used for the deposition, production and purification of materials. For example,

64 Section C – Analytical reactions in solution