Thermo Products REDOX/ORP 9678BNWP - Manual (Page 12)

0 Redox/ORP Electrode User Guide
Oxidation-Reduction Titrations
Oxidation-reduction or redox titrations provide a simple,
reliable method for determining many substances in solution.
A redox titration consists of adding to an unknown sample
small increments of a titrant that converts the unknown to
a different oxidation state. After each addition of titrant, the
platinum redox electrode develops a potential proportional to
the logarithm of the ratio of the activities of the two oxidation
states. At the inflection point, or endpoint, the titrant has
completely oxidized or reduced the unknown, causing a sharp
change in the logarithm of the ratio of the activities of the two
oxidation states. A corresponding sharp change in the potential
is developed by the platinum electrode. Often several oxidizing
or reducing species can be precisely determined in the same
solution by a single titration with several inflection points.
Table 2 lists suitable titrants for different unknowns, the
equation for the titration reaction, interferences, and a reference
where information about preparing the sample and running the
titration can be found. For similar information about unknowns
not listed in the table, consult the standard texts listed as
references.
The following is a general procedure for performing a redox
titration once the sample is prepared.
1. Fill a 10 mL buret with a standard titrant solution whose
normality is 5-10 times that of the sample.
2. Connect the electrode to the meter and set the meter to
the mV mode.
3. Pipet 50 mL of sample into a 150 mL beaker. Stir the
solution thoroughly throughout the titration.
4. At first, add titrant in 0.5 to 1 mL increments, recording the
potential after each addition. Near the endpoint, when large
potential changes appear, add increments of 0.1 to 0.2 mL.
Continue the titration 3 to 4 mL past the endpoint.
5. Plot the electrode potential versus volume of added titrant
on linear graph paper and draw a smooth curve through the
points. The endpoint is the point of inflection (the point of
greatest slope).