Frans_M_Everaerts_Isotachophoresis_378342.pdf

DISTURBANCES DUE TO CO, 263
Electropherograms are given for these systems in Fig.9.6. Li' at these pH values has
normal step heights,but Tris+ at pH 3.5 shows a retardation and at pH 3.25 a large and a
lower step height are present between the Li' and Tris' zones (zone electrophoresis).
Note that the traces at pH 4.25,4.0 and 3.75 are nearly identical, but at pH 3.5 and 3.25
the step heights of Li' are about the same whereas those of Tris' decrease owing to the
presence of H+. Tris'already shows no real zero points at pH 3.75 in the system potassium
formate-formic acid, and indeed at this pH the isotachopherogram (see Fig.9.7) shows
a large and a low step height between Li' and Tris'. Similar results can be obtained at
high pH in anionic separations.
9.3. DISTURBANCES DUE TO THE PRESENCE OF CARBON DIOXIDE*
In section 9.2, some disturbances due to the presence of H+ and OH-, migrating through
all zones according to the moving-boundary principle, have been described. A flow of
HCO; ions can also cause such a disturbance, because of the presence of carbon dioxide
in air and solvents (even if all carbon dioxide is removed from the solution, it can still
diffuse through the capillary walls into the solution). Carbon dioxide can react in an
aqueous solution as follows:
COZ + HzO =+ HzCO3
HzCO3 + H,O =+ H30+ + HCO;
K, = 2.6 - 10-~
Kz = 1.72 *
(9.1)
(9.2)
Fig.9.7. Electropherogram of Li' between Tris' (terminator) and K'. The leading electrolyte was
prepared by adjusting KOH (0.01 N) to pH 3.75 by addition of formic acid. T = Increasing temperature.
A thermometric detector was used.
*See also Chapter 13.