Oxidation-Reduction Titrations
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
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).
10 | Redox/ORP Electrode User Guide |