Frans_M_Everaerts_Isotachophoresis_378342.pdf

CHOICE OF THE SOLVENT 91
TABLE 5.4
LIQUID JUNCTION POTENTIALS BETWEEN STANDARD SOLUTIONS IN AQUEOUS
MIXTURES 11 ]
Methanol and a saturated solution of potassium chloride in water for oxalate and succinate buffers.
Reproduced by permission of Dr. C.L. de Ligny.
Oxalate buffer Succinate buffer
Methanol
(%I
0
39.13
70
84.2
84.21
94.2
100
100
0.0046
0.0091
0.01 14
-0.009
-0.0082
-0.0435
-0.1338
-0.1347
Methanol E;
(%)
0 0.0041
43.31 0.0083
64.2 0.0132
84.1 -0.0091
84.2 -0.0086
94.19 -0.0485
100 -0.1329
De Ligny et al. determined the liquid junction potential between the buffer solutions in
methanol and a saturated solution of potassium chloride in water.
The liquid junction potentials between standard solutions in methanol-water mixtures
and a saturated solution of potassium chloride in water are given in Table 5.4. When the
pH of a solution of methanol-water mixtures is measured by means of a pH meter, standardised
by a solution of potassium chloride in water, the error due to the liquid junction
potential can be calculated by means of the equation
E:* (methanol-water) - E:,...-&-
(5.16)
For higher percentages of methanol, this dpH* value can be very high. In order to
calibrate the pH meter for measurements in methanolic solutions, a standard buffer
solution in water can also be used [lo, 111 and then the correct pH* can be calculated
from the observed pH by subtracting a correction factor (6) [12]. The 6 values are given
in Table 5.5.
The liquid junction potentials at the standard solution (alcohol-water mixtures)/
saturated aqueous potassium chloride boundaries are independent of the nature of the
buffering solution.
TABLE 5.5
6 CORRECTION TERMS FOR SOME METHAhOL-WATER MIXTURES
Methanol
(%I
6 (pH* units)
0 -
43.3 0.11
64 0.22
94.29 -0.86