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Potentiometric titration File

الكلية كلية العلوم للبنات     القسم قسم الكيمياء     المرحلة 3
أستاذ المادة محمد هاشم مطلوب الغرابي       26/12/2017 15:28:58

POTENTIOMETRIC TITRATIONS
INTRODUCTION
General Principle
A typical cell for potentiometric analysis consists of a reference electrode, an indicator electrode
and a salt bridge. This cell can be represented as
Reference electrode | salt bridge | analyte solution| indicator electrode
Eref Ej Eind
A reference electrode, Eref, is a half-cell having a known potential that remains constant at
constant temperature and independent of the composition of the analyte solution. The reference
electrode is always treated as the left-hand electrode in potentiometric measurements. Calomel
electrodes and silver/silver chloride electrodes are types of reference electrodes.
An indicator electrode has a potential that varies with variations in the concentration of an
analyte. Most indicator electrodes used in potentiometry are selective in their responses.
Metallic indicator electrode and membrane electrodes are types of indicator electrodes.
The third component of a potentiometric cell is a salt bridge that prevents the components of
the analyte solution from mixing with those reference electrode. A potential develops across the
liquid junctions at each end of the salt bridge. The junctions potential across the salt bridge, Ej,
is small enough to be neglected.
The potential of the cell is given by the equation;
Ecell= Eind – Eref + Ej
The Glass Electrode for pH Measurements
Typical cell for measuring pH consist of a glass indicator electrode and a saturated calomel
reference electrode immersed in the solution whose pH is unknown. The indicator electrode
consists of a thin, pH sensitive glass membrane sealed onto one end of a heavy-walled glass or
plastic tube. A small volume of hydrochloric acid saturated with silver chloride is contained in the
tube. A silver wire in this solution forms a silver/silver chloride inner-reference electrode, which
is connected to one of the terminals of the potential-measuring device, pH-meter. The calomel
electrode is connected to the other terminal.
The electric potential created between the glass electrode, and the inner-reference electrode is
a function of the pH value (activity of hydronium ion, aH3O+ ) of the measured solution. So once

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