Redox titrations involve oxidative reduction reactions. This types of titrations are quite common in usage next to acid-base titrations.
As the name indicates, these redox titrations are used to analyze the reducing agents or oxidizing agent.
The reaction involves the transfer of electrons, hydrogen or oxygen.
The principle of redox titration:
The test substance if it is a reducing or oxidizing one, it is titrated against an oxidizing agent or reducing respectively to determine the concentration. The reaction of the experiment is denoted as
Oxidized form+ electrons ——————–> Reduced form.
In the reaction, there is the addition of hydrogen, removal of oxygen or addition of electrons to the reduced form. While, for the oxidized form, there is the removal of hydrogen, the addition of oxygen or removal of electrons.
The experimental technique.
This is simple titration and the sample under test is analyzed by titrant till there is an endpoint. However, there are other methods too.
Requirements for the experiment:
1. The test substance
2. A standard oxidizing or reducing agent
3. An indicator of endpoint.
The test substance is mostly dissolved in a suitable solvent. It is taken in a conical flask and titrated with a suitable oxidizing agent or reducing agent. The standard oxidizing agents are like cerric ammonium sulfate. Standard reducing agent includes titanous chloride.
And the indicators are quite different here unlike in acid-base titrations. The indicator system could be
b) External indicator
c) Internal indicator
d) Instrumental techniques
Self-indicator: Here the sample under test itself acts as an indicator. It shows a distinct color change at the end point. This is seen in the titration of potassium permanganate. Initially, it is a dark brownish color. And as the endpoint arises, the solution turns pink.
External indicator: Few small drops of the reaction solution is taken on to a tile. Then indicator is put on it to see changes in color. When there is change, it indicates the approach of an endpoint. This method is a bit outdated with the possibility of errors.
Internal indicator: Here a redox indicator is used. This indicator is present in the flask along with the titrated sample. When the reaction reaches the endpoint during the titration, the indicator changes its color. This is possible because the indicator has redox potential which lies in between that of titrate and titrant.
Example of this type of indicator is ferroin solution (1, 10 phenanthroline).
Instrumental technique: The endpoint is detected by use of a potentiometer. The indicator electrode is a platinum electrode and a reference electrode is a saturated calomel electrode.
Types of redox titrations:
1. Direct titration: These are suited for compounds with high chemical oxidative or reducing property. Here the given sample is titrated against a know concentration of titrant. The volume of titrant consumed until the endpoint help to determine the sample concentration.
2. Back titration: This method is suitable for a compound with less oxidative or reducing properties. They do not react with the titrant. Hence, a known excess volume of a specific concentration of reactant is added into the flask containing the sample. The excess unconsumed reactant is estimated with suitable titrant.
Example: Dye brilliant green is not easily reduced. Hence, an excess volume of 0.1N titanous chloride is added into the flask containing the dye. Then the unconsumed titanous chloride is back titrated with 0.1N ferric ammonium sulfate.