ELECTROLYTES, EMF, AND CHEMICAL EQUILIBRIUM

8.115

pH x pH s

E x E s

2.3026 RT /F

where

R is the gas constant per mole,

T is the temperature on the absolute scale, and

F is the faraday.

The pH

x of the unknown medium is calculated

from that of an

accepted standard

(pH

s ) and the

measured difference in the emf (

E ) of the electrode

combination when the standard solution is

removed from the cell and replaced by the unknown. The double vertical line marks a liquid junction.

Electrodes as fabricated exhibit variations in the reproducibility of the reference electrode, in the

liquid-junction potential, and, with glass electrodes, in the asymmetry potential. These differences

are all eliminated in the standardizing procedure with standard reference pH buffers. (See R. G.

Bates,

Determination of pH, Theory and Practice,

Wiley, New York, 1964.)

Electrode reversible

Standard reference

Salt bridge

Reference

to hydrogen ions

buffer or unknown

(KCl. 3.5

M

electrode

solution

or saturated)

An electrometric pH-measurement system consists of (1) pH-responsive electrode, (2) reference

electrode, and (3) potential-measuring device— some form of high-impedance electronic voltmeter

for glass-electrode combinations and this or a potentiometer arrangement for other pH-responsive

electrodes. Electronic pH meters are simply voltmeters with scale divisions in pH units which are

equivalent to the values of 2.3026

RT /F (in mV) per

pH unit. Values of this function at several

temperatures are given in Table 8.22. There is no compensation incorporated in the meter for the

changes in pH of the test solution as a function of temperature. Reliability of an indicator– reference

electrode combination must be ascertained by standardization of the pH meter with one standard

buffer and checking the pH response by immersing the combination in a second and different ref-

erence buffer.

The temperature compensator on a pH meter varies the instrument definition of a pH unit from

54.20 mV at 0

C to perhaps 66.10 mV at 60

C. This permits one to measure the pH of the sample

TABLE 8.22 Values of 2.3026

RT /F at Several Temperatures

In millivolts.

t C

Value

t C

Value

t C

Value

t C

Value

0

54.197

25

59.157

50

64.118

80

70.070

5

55.189

30

60.149

55

65.110

85

71.062

10

56.181

35

61.141

60

66.102

90

72.054

15

57.173

38

61.737

65

67.094

95

73.046

18

57.767

40

62.133

70

68.086

100

74.038

20

58.165

45

63.126

75

69.078