Hanna Instruments HI 904 - Manual (Page 155)

7
TITRATION THEORY
Figure 1.
2 TYPES OF TITRATIONS
2.1 Titrations According to The Measurement Method
2.1.1 Amperometric Titrations
An amperometric titration is performed by placing two electrodes (often a metal electrode
and a reference electrode) into the sample solution and holding the potential of the metal
electrode at a selected voltage. The current that flows, due to the oxidation or reduction of a
reactant or product, is plotted vs. volume of titrant to provide the titration curve and locate
the equivalence point. Changes in the current are due to changes in the concentration of a
particular species (being oxidized or reduced at the electrode).
Generally the reaction between the analyte and titrant forms a new species. Depending on
the titration, the reactants are electroactive and the products are not, or vice-versa.
Amperometric titration curves look like two straight lines intersecting at the equivalence
point, this is due to the change in the electroactivity of the solution.
Many metal ions can be amperometrically titrated using a precipitation, complexation or
redox reaction. Some metal ions and species that can be determined in this manner include
silver, barium, halides, potassium, magnesium, palladium, molybdate, sulfate, tungstate,
zinc, bismuth, cadmium, fluoride, indium, thallium, iodine, and gold.
Figure 1 shows four amperometric titrations and their endpoints. In graph “A” the analyte is
electroactive and gives current but the reacted species does not. In “B” the reactant is not
active but the titrant is. In “C” both the analyte and titrant are active and both give current
flow. Graph “D” shows the same situation as “B”; however, the current has an opposite sign
(the titrant is reduced).
2.1.2 Potentiometric Titrations
Potentiometric titrations are done by measuring the voltage across the solution using an
electrode system. An electrode system consists of an indicator electrode and a reference
electrode. As titrant is added the variations in the potential of the indicator electrode, with
respect to the reference electrode, are monitored to show the progress of the titration.