Concise Physical Chemistry

c14 JWBS043-Rogers September 13, 2010 11:27 Printer Name: Yet to Come

226 ELECTROCHEMICAL CELLS

the electrode compartments. There will be a measurable potential that depends only

on the difference in concentrations of the aqueous solutions. Such cells are called

concentration cells.

An example would be any metal;metal ion pair with ion concentrations, say

Ag;Ag+(aq) of 0.10 molar in one half-cell and 0.010 m in the other:

Ag(s);Ag+(0.010 m)||Ag+(0.10 m); Ag(s)

The cell reaction is the half-cell potential of the cathode minus that of the anode (so

as to give a positive whole-cell potential):

Ag+(0.10 m)→Ag+(0.010 m)

that is, the direction of spontaneous change for any solution is to become more dilute.

(Solutions don’t spontaneously “concentrate themselves.”)Qis 0.010/0.10=0.10

and the Nernst equation is

E= 0 − 0 .0257 ln 0. 10 = 0 .0592 V= 59 .2mV

If the ion concentration in one of the half-cells is known and the other is unknown,

the concentration of the unknown half-cell can be determined from the whole-cell

potential. This is the principle behind the most widely used application of electro-

chemistry, the pH meter, in which a potential is measured that is linearly related

through the Nernst equation to the logarithm of the hydrogen ion concentration in a

half-cell. The negative logarithm (log base 10) of the hydrogen ion concentration is,

as we learn in elementary chemistry, the pH. pH meters consist of an H+-sensitive

(glass) electrode and a constant voltage (calomel) electrode with suitable electronics

to measure and display or record their algebraic sum. pH meters are calibrated to

display the hydrogen ion concentration directly in pH units, most frequently as a

digital readout.

14.8 FINDINGE◦

A silver electrode immersed in a solution of HCl soon becomes covered by a coating

of AgCl(s). This half-cell, connected to a standard hydrogen electrode gives the cell

Pt(s); H 2 (g); HCl(aq); AgCl(s); Ag(s)

The cell has no need for a salt bridge. One electrode is sensitive to H+and the other

is sensitive to Cl−which is precisely equal in concentration. The cell reaction is

AgCl(s)+^12 H 2 (g)←→H+(aq)+Cl−(aq)+Ag(s)