Frans_M_Everaerts_Isotachophoresis_378342.pdf

EFFECTIVE IONIC MOBILITY 33
comparison with the duration of the experiment. As the equilibrium adjustments are
very slow, the ionic species seems to consist of two components (an example is the
esterification of oxalic acid in methanol, see section 16.2, Fig.16.1) and sometimes
disturbances can be expected (see Chapter 9).
For the equilibrium states, we shall distinguish two types of interactions, viz.,
protolysis and complex formation.
3.4.1.1. Protolysis
Here a proton takes part in the dissociation reaction, as shown in the dissociation of
acetic acid (see eqn. 3.19). The degree of dissociation depends on the pH and the
equilibrium constant.
The relationship between pH and pKa and the degree of dissociation is given by the
Henderson-Hasselbalch equation:
(3.24)
(positive for anionic species and negative for cationic species). Also, for ionic species
with more than one pK value, thls equation can be used if the differences between the
pK values are not too small. In Fig.3.3, a nomogram is given by which the degree of
dissociation can be obtained for given pH and pK values.
The relationship between the degree of dissociation and the pH for some anionic
species is shown in Fig.3.4, from which it can be concluded that changes in pH are
important between k 2 pH units from the pK value. Between these values, the degree
of dissociation changes from about 1% to 99%, and hence the effective mobility changes
from 1% to 99% of the absolute ionic mobility, neglecting other influences on the
absolute ionic mobility.
In Fig.3.5, some relationships between effective mobility and pH are shown for
anionic species, cationic species and amphiprotic substances. Of course, the mobilities
depend on the absolute ionic mobility chosen. Fig.3.5 shows clearly that differences
in pH have a great effect on the effective mobility near the pK values.
3.4.1.2. Complex formation
Now a particle different from a proton takes part in the dissociation reaction, e.g.
Pb(CH3C00)2 =+ Pb(CHjCOO)++ CH3COO- (3.25)
The degree of complex formation depends mainly on the partial concentrations and the
complex constant. Corrections can be made for this effect in a manner similar to that
described above. Often, both types affect the effective mobility, e.g., for Al*:
Al(H20)F* Al(OH)(H,O)T+ H+
AI(CH3COO)3 =+ Al(CH3COO)T + CH3COO-
Further dissociations are possible.
(3.26)
(3.27)