Frans_M_Everaerts_Isotachophoresis_378342.pdf

90 CHOICE OF ELECTROLYTE SYSTEMS
unknown methanolic solutcons, one must look for standard buffer solutions in the same
kind of methanolic solution as that used for the unknown solution. Using this standard
solution, the two liquid junction potentials will cancel each other again and the measured
pH can be interpreted in terms of hydrogen ion activity.
De Ligny et ul. [6,7] determined the pH (- log c,yk) for some standard solutions in
methanolic solvents according to the method of the National Bureau of Standards for
aqueous solutions.
In the determination of the pH* of standard solutions ( the asterisk here and on other
symbols indicates that the quantities refer to the solutions considered and not to aqueous
solutions) for methanolic solvents, corrections have to be made for the slight association
of ions to give ion pairs. Fuoss and Onsager [8,9] developed a method for the calculation
of the closest approach b and the dissociation constant K of an incompletely dissociated
electrolyte in water, but because for methanolic solvents no accurate values of the
conductivity of electrolytes were available, De Ligny et al. did not correct for the ion
pair association.
For the estimation of log y*, De Lgny et el. used the Gronwdl-LaMer-Sandved
equation:
(5.15)
The pH* values for some buffers as determined by De Ligny et al. are given in Table 5.3.
In the experiments, the reference electrode (calomel) was placed in a potassium chloride
solution of the solvent that was used to prepare the buffers. Using the values mentioned,
TABLE 5.3
pH* VALUES FOR THE OXALATE AND SUCCINATE BUFFER IN METHANOLIC SOLUTIONS
AS DETERMINED BY DE LIGNY [ 11 ]
Reproduced by permission of Dr. C.L. de Ligny.
0.01 MH,Ox + 0.01 MNH,HOx 0.01 MH, Succ + 0.01 MLiHSucc
Methanol PH* Methanol PH*
(%I (%I
0 2.15
10 2.19
20 2.25
30 2.30
40 2.38
50 2.47
60 2.58
70 2.76
80 3.13
90 3.73
100 5.19
0
10
20
30
40
50
60
70
80
90
100
4.12
4.30
4.48
4.67
4.87
5.07
5.30
5.57
6.01
6.73
8.75