Physical Chemistry Third Edition

8.4 The Determination of Activities and Activity Coefficients of Electrolytes 373

Unknown

or reference

solution

Pt Pt

KCl solution

Calomel

electrode

Porous plug

H 2

Figure 8.10 A Cell for the Measurement of pH.

electrode side of the liquid junction is much more concentrated than the solution on the

other side, the magnitude of the liquid junction potential should not change appreciably

when a different solution is placed in the cell.

We do not try to use Eq. (8.4-5) directly since the liquid junction potential is pre-

sumably not known. Instead, we first put into the cell a reference solution (solution I)

that has a known value of its pH. We measure the voltage of the cell at a known hydro-

gen pressure, and then remove this solution and put a solution whose pH we wish to

determine (solution II) into the cell, and measure the voltage with the same hydrogen

pressure as before. We assume that the liquid junction potential has the same value in

both cases and that the activity of the chloride ion is the same in both cases, so that

E(II)−E(I)−

RT

F

ln

[

a(H+,II)

]

+

RT

F

ln

[

a(H+,I)

]

(8.4-6)

whereE(I)andE(II)are the two voltages measured. Equation (8.4-6) is the same as

pH(II)−pH(I)

E

RTln(10)

[

E(II)−E(I)

]

(8.4-7)

We can regard Eq. (8.4-7) as an operational definition of pH. The pH values obtained

are probably accurate to two digits after the decimal point.

Exercise 8.11

Show that Eq. (8.4-7) is correct.

Ag

AgCl

Thin glass
membrane

Buffered

solution

Unknown

or reference

solution

Figure 8.11 A Glass Electrode (Sche-
matic).

Use of the hydrogen electrode can pose a fire hazard. Theglass electrodehas come

into common use to replace the hydrogen electrode in pH meters. This electrode is

schematically depicted in Figure 8.11. It consists of a silver–silver chloride electrode

inside a thin-walled glass bulb filled with a buffered solution of nearly constant pH. To

measure the pH of an unknown solution, a glass electrode and a calomel electrode are

placed in the solution. The special glass of which the bulb is made allows hydrogen