Redox titration is an extremely valuable and widely used titrimetric analysis method in the chemistry laboratory. Before understanding in detail what redox titration is, how it is performed, and why it is needed, you need to have a thorough knowledge of a redox reaction. In this article, we will explain to you all about redox reactions and redox titration. So, continue reading!
What is redox titration-Definition
A redox titration, also known as an oxidation-reduction titration, is a titrimetric analysis involving a redox reaction between the titrant and the titrand. The redox indicator changes color as a result of the oxidation-reduction reaction in the titration flask. This color change marks the end point of the titration experiment. The volume of titrant used is noted, which helps determine the unknown analyte concentration.
What are redox reactions
A redox reaction is defined as a chemical reaction taking place between an oxidizing agent and a reducing agent. It is based on the complete transference of electrons between two chemical entities. In a redox reaction, the reducing agent gets oxidized by losing one or more electrons. The electrons lost by the reducing agent are transferred to the oxidizing agent, which consequently gets reduced by gaining these electrons.
Example:
The chemical reaction shown above is a redox reaction between zinc metal and an aqueous solution of copper sulfate.
The zinc metal is oxidized, while copper ions are reduced. The oxidation-reduction taking place simultaneously is called a redox reaction.
What is oxidation
Oxidation is:
- The loss of electrons from a chemical substance.
- The increase in the oxidation state of the substance.
- Gain of oxygen or loss of hydrogen.
In a redox reaction, the substance oxidizing another substance while itself getting reduced is known as an oxidizing agent or oxidant.
In the redox reaction example given above, zinc undergoes oxidation. Its oxidation state increases from 0 to +2. A +2 increase in the oxidation state represents a loss of 2 electrons.
In this example, zinc is the reducing agent. The 2 electrons lost are transferred to the oxidizing agent, which is then reduced.
What is reduction
Reduction is:
- The gain of electrons by a chemical substance.
- The decrease in the oxidation state of the substance.
- Loss of oxygen or gain of hydrogen.
In a redox reaction, the substance reducing another substance while itself getting oxidized is known as a reducing agent or reductant.
In our example, zinc reduces copper sulfate, so zinc is the reducing agent, while copper sulphate is the oxidizing agent.
The copper ions present in the reaction mixture initially have a +2 oxidation state. But, as it gains the 2 electrons lost by Zn, copper ions get reduced to copper metal.
Now let’s see how a redox reaction can be used for titrimetric analysis in redox titrations.
How to perform redox titrations-Procedure
The oxidizing or reducing agent of unknown concentration is called the titrand. It is taken in the titration flask while performing the redox titration.
A standard solution of known concentration is prepared and taken as the titrant in the burette.
The following step-by-step guide will lead you through the process of determining an unknown analyte concentration, such as the concentration of ferrous (Fe2+) ions, by titrating it against a standard solution of potassium permanganate (KMnO4).
Step I: Preparation of standard solution.
A known concentration of potassium permanganate (KMnO4) solution is prepared by completely dissolving KMnO4 crystals in distilled water.
To prepare 0.1 M KMnO4 solution in 100 mL distilled water, 1.58 grams of KMnO4 solid is accurately weighed using an analytical mass balance.
It is then dissolved in distilled water, and the solution is raised up to the 100 mL mark in a volumetric flask. The exact concentration of this solution can be determined by standardizing it against oxalic acid, a primary standard, prior to using it in the redox titration.
The 50 mL burette is then filled with this purple-colored solution. The initial burette reading is recorded by reading the upper meniscus of the colored liquid.
Step II: A specific volume of analyte solution is pipetted out in the titration flask.
A specific volume, such as 10 mL of the analyte, i.e., FeSO4 solution, is measured and added into the titration flask using a pipette.
A small volume of H2SO4 solution is also added to the titration mixture. An acidic medium fulfils the stoichiometric requirements of the redox reaction between Fe2+and Mn7+ ions.
It appears as a blue-green solution due to the presence of Fe2+ ions. Note that we are not adding any external indicator in this case, unlike acid-base titrations. This is because the oxidizing and/or reducing agents act as self-indicators in a redox titration.
The transition metal ions, such as Mn7+ ions in KMnO4 and Fe2+ ions in FeSO4 solution, exhibit specific colors. As the redox reaction takes place and the oxidation state of the transition metal ions in the aqueous solution changes, the color of the solution also changes. This color change is used to detect the endpoint of the redox titration.
Let us see how it’s done in the next step.
Step III: The titrant is dropwise added into the titration flask containing titrand.
As KMnO4 solution is added dropwise into the titration flask, some streaks of purple color appear in the titration mixture. But these purple streaks immediately disappear to give a colorless solution.
However, the endpoint is marked as the analyte solution turns faint pink, and this color persists for more than 30 seconds. This is the point where all Fe2+ ions present in the reaction mixture are oxidized to Fe3+ while Mn7+ ions from the titrant are reduced to Mn2+. Excess Mn7+ ions in the titration flask give the solution a permanent pink/purple color.
The final burette reading is recorded at this point. The difference between the initial and final burette reading is known as the titre volume.
The titration experiment is repeated thrice to obtain an average value for the volume of 0.1 M KMnO4 solution used against an acidified solution of Fe2+ ions.
This titre volume is then used to determine the concentration of Fe2+ ions in the analyte mixture as per the calculations shown below.
Calculations
In the above redox reaction, KMnO4 is the oxidizing agent, while FeSO4 is the reducing agent. An oxidation-reduction reaction occurs in the titration flask as KMnO4 is added from the burette. The loss of 5 electrons from a manganate (Mn7+) ion reduces it to manganese (Mn2+).
These 5 electrons are gained by ferrous (Fe2+) ions, which are consequently oxidized to ferric (Fe3+).
The net ionic equation for the above redox reaction is:
2 moles of MnO4– completely react with 10 moles of Fe2+ in the above reaction.
The redox titration curve for this example is given below.
Redox titration curve
Additionally, you can follow this article to learn more about a redox titration curve following some extensive calculations.
Types of redox titrations
1. Permanganometry
Permanganometry is the redox titration in which potassium permanganate (KMnO4) is used as the titrant, as we used in the example above. Potassium permanganate is a strong oxidizing agent. It acts as a self-indicator by changing color from purple to colorless in its oxidized and reduced forms.
2. Dichromate titrations
Dichromate titration is a type of redox titration in which a dichromate salt such as potassium dichromate (K2Cr2O7) is used as the titrant. The dichromate ion change color from orange to green in its oxidized (Cr6+ in Cr2O72-) and reduced (Cr3+) forms.
3. Bromometry
Bromometry is the redox titration in which bromine (Br2) is used as the titrant. Bromine changes color from red brown (Br2) to colorless (Br–) as a redox reaction takes place.
4. Cerimetry
It is a redox titration in which ammonium ceric sulfate (NH4)4Ce(SO4)4 is used as the titrant. It is an oxidizing agent that changes color from orange red to pale blue as it is reduced in a redox reaction.
5. Iodometry
Iodometry is the redox titration performed to determine the iodine (I2) concentration in a solution. Starch is used as an indicator. Iodine-starch complex gives a blue-black color to the solution. The color becomes light brown as I2 is reduced to iodide (I–) ions.
6. Iodimetry
It is a type of redox titration performed to determine the unknown concentration of an analyte that reacts with iodine. Here iodine is used as a titrant.
Learn more about iodometry and iodimetry here.
What are the uses of a redox titration
Redox titrations are used for:
- Determining the unknown concentration of elements such as iron in pharmaceutical formulations.
- Wastewater treatment by analyzing by-products of organic waste oxidation.
- Determining the unknown concentration of sugar, salt or specific vitamins in food products.
- Determining unknown analyte concentration in dentistry, metallurgy and other related industries.
Revise the concepts learned in this article by watching this video.
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References
1. B.Kiruthiga. 2012. Redox titration: Oxidation-Reduction titration.
2. C.Harris, Daniel. 2010. Quantitative Chemical Analysis (W.H Freeman and Company).
