Frans_M_Everaerts_Isotachophoresis_378342.pdf

DISTURBANCES CAUSED BY H+ AND OH 25 I
T
1
5 4 3 2 1 5 4 3 2 1
Fig.9.3. Separation of a mixture of cations in an unbuffered electrolyte system. (a) With fresh termi-
nating electrolyte; (b) with old solution the pH of which is changed by the electrode reaction. 1 = H';
2 = Cst; 3 = Na+; 4 = Li+; 5 = Cd". T = increasing temperature; r = time. A thermometric detector was
used.
electrolyte has not been renewed for some time is shown. The terminator solution became
increasingly acidic and a flow of rmigrates through all zones towards the cathode
compartment.
From the phenomena described above, it can be concluded that it is not advisable to
work with unbuffered electrolyte systems, where regular renewal of the electrode compartments
is necessary. The use of terminator solutions at low pH is undesirable in cationic
separations.
A similar disturbance can be expected in anionic separations in unbuffered systems as
a terminator of high pH is used. A flow of OH- from the terminator zone penetrates all
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9.2.2. Disturbances from the leading zone in unbuffered systems
In the previous section, disturbances caused by the presence of H' and OH- from the
terminator zone and penetrating all preceding zones have been described. Sometimes
disturbances can also be caused by H' and OH- from the leading zone penetrating all
proceeding zones. Although it is sometimes difficult to recognize whether the disturbances
originate from the terminator or from the leading zone, these disturbances have different
characteristics. In order to recognize the difference, two detectors have to be used, and
from the two signals obtained it can be concluded from which side the disturbance is
coming.
In this section we discuss the disturbances from the leading zone. In an anionic separa-
tion, this disturbance is due to a flow of H', whereas in a cationic separation it is caused
by a flow of OH-. Some examples of the latter situation are considered below.
If cations are separated in an unbuffered system, a leading electrolyte of, e.g., hydro-
chloric acid could be used. In such a case, hydrogen will be evolved at the cathode. OH-,
possibly formed in the cathode compartment and migrating in the direction of the anode,
will meet the H' ions of the leading electrolyte and will be neutralized because the