Frans_M_Everaerts_Isotachophoresis_378342.pdf

92 CHOICE OF ELECTROLYTE SYSTEMS
In the pH* measurements carried out in the work described in this chapter, the same
procedure was used as described by De Ligny et al. Standard buffer solutions and pH*
values used were as determined De Ligny et al.
Determination of pK values in methanolic solutions
The determinations of pK values [13] can be carried out in several ways, the most
important of which are the conductivity, electrometric, spectrometric and colorimetric
methods. In this section, the electrometric method is discussed,
Rorabacher et al. [14] gave some definitions relating to the pK. The activity
equilibrium constant is defined as
The equilibrium constant based on concentrations is
K,* = [HI [A1 /[HA1
and the mixed-mode equilibrium constant is defined as
K$ = a$ [A] /[HA] or K z = K:rfi
Hence
In the electrometric method, the concept of the half neutralization point (HNP) is
used. The HNF’ is the point in the acid-base titration at which half of the amount of
acid (or base) present has been neutralized. According to eqn. 5.1 1 , this means that the
pK; is determined. The thermodynamic equilibrium constant can be calculated from
the mixed-mode constant by correcting for the activity coefficients (eqn. 5.20).
(5.17)
(5.18)
(5.19)
Ex penmen tal values
In order to choose an optimum electrolyte system in isotachophoresis, the pK values
of the buffers and the pK values of the sample ionic species must be known. Some
pKk values for anionic species and bases have been determined in 95% methanol by
the electrometric method and the results are given in Table 5.6.
5.3. CHOICE OF THE BUFFERING COUNTER IONIC SPECIES
(5.20)
With regard to the choice of the buffering counter ionic species, three important
points can be distinguished. Firstly, the buffering counter ions act as a counter ion by
which the principle of electroneutrality is obeyed. At the same time, the counter ionic
species can be used to form complexes with the sample ionic species in order to affect
the effective mobility of these ionic species in a favourable manner, particularly if the
ionic species to be separated have similar mobilities. Even buffering counter ionic species
with which particular sample ionic species do not migrate or are precipitated can be
chosen; by this they do not disturb the separation procedure. Deman [ 11 used this