Cam Sep Lect 1 Chromatografia Bronislaw K. Glod

Changes in the Concentration of the Background Electrolyte
Indirect detection is based on measurement of changes of the concentration of the background
(probe) electrolyte, the electric charge of which should be the same as that of the solute. This means that for
acid analysis the background electrolyte should also be acid – a dilute solution of a strong, completely
dissociated acid is usually used. Optimum conditions for detection are not always optimum for
separation, however. Changes in the concentration of the background electrolyte can be calculated from a
knowledge of the dissociation constant and the mass-conservation equation. For dilute solutes we can assume
that the concentration of hydrogen ions is constant throughout the column, i.e. [H + ] = const. Under these
conditions, outside the region of the solute (chromatographic peak) the electrical neutrality condition can be
expressed in the form:
[H + ] = [B – ] (1)
Injection of the test acidic solute, HR, onto the chromatographic column reduces the concentration
of the dissociated form of background electrolyte. In accordance with the electrical neutrality condition at the
peak maximum in the mobile phase:
[H + ] = [B – ] max + [R – ] (2)
where B – and R – denote the dissociated forms of the background electrolyte and solute, respectively.
Eqs (1) and (2) can be easily transformed to:
[R – ] = [B – ] – [B – ] max (3)