Frans_M_Everaerts_Isotachophoresis_378342.pdf

CHOICE OF THE SOLVENT
pH and pKvalues can be determined in methanolic solutions is dealt with in the next
section.
5.2.1.2. Determination of pK values in methanolic solutions*
Determination of pff in methanolic solutions
The operational definition of the pH determined electrometrically in water is based
on E measurements of cells of the type
Aqueous solution
?t:ti: 1 I of standard; pH,
KC1, reference electrode
Aqueous solution [ KCI, reference electrode
In general, the indicator electrodes are glass electrodes and the reference electrodes are
calomel electrodes. Es and Ex can be expressed as (25°C)
E s = E ref -Eo md -0.05916l0ga~,~+l$~ (5.9)
Ex = Emf- Eqmd - 0.05916 log^^,^ +E;:,
Combination of eqns. 5.9 and 5.10 gives
Ex-Es -E),x-Ej,s
- log aH,x = - log aH,s + ____
0.0591 6 0.0591 6
The operational definition of the pH is
EX -4
pHx = pHs + ___
0.0591 6
Comparison of eqns. 5.11 and 5.12 shows that
PH, = -logaH,x
if pH, = -log I Z ~ and , ~
89
(5.10)
(5.1 1)
(5.12)
(5.13)
Ej,x = Ej,s (5.14)
In general, this is not exactly true. Bates [5] of the National Bureau of Standards
determined the pH, values of some standard buffer solutions for which
PHs = -log aH,s
If the solution x has about the same ionic strength in the solvent used for the standxd
solution, Ej,S can be considered to be equal to Ej,x and then pH, can be interpreted as
-log aH,x'
For pH measurements in methanolic solvents, the same procedure can be followed.
Because of the different liquid junction potentials for aqueous buffer solutions and
*For other solvents or combinations of soIvents, a simiIar procedure can be followed.,.