Redox Titration

Oxidation-Reduction (redox) reactions

Before diving into redox titrations, let's review what oxidation-reduction reactions are.

If a substance accepts an electron, we say that it is reduced, whereas if a substance loses an electron, it is oxidized. During a redox reaction, electrons get transferred from the species that is oxidized to the species that is reduced!

We can use one of the following mnemonics to remember this:

• OIL RIG: Oxidation Is Loss, Reduction Is Gain
• LEO the lion GERS: Losing Electron Oxidation, Gaining Electron Reduction

Now, let's define oxidizing and reducing agents. The species that reduces (adds electrons to) another is called a reducing agent. A reducing agent gets oxidized and loses electrons. On the other hand, the oxidizing agent oxidizes (takes electrons from) another species, and therefore gains an electron and gets reduced.

Oxidation States

We can also look at their oxidation numbers to determine which molecule gets oxidized or reduced in a redox reaction.

Let's look at an example!

Now, to put everything we learned so far together, let's look at the reaction between Magnesium and Hydrochloric acid and the oxidation states involved.

Reaction between Magnesium and HCl, StudySmarter Originals.

Notice that the oxidation number of Mg increased from 0 to +2 meaning that it lost electrons and was oxidized. H+ in the acid decreases from +1 to 0, suggesting that it gained electrons has been reduced.

• If the oxidation number increases, then we say that the species has been oxidized.
• If the oxidation number decreases, then we say that the species has been reduced.
• If the oxidation number stays the same (as in the case of Cl), we call them spectator ions. Spectator ions are usually taken off when writing net ionic equations.

Redox Titration Explanation

First, let's define titration.

In redox titrations, an oxidizing agent is titrated with a reducing agent (or vice versa). For example, we can use potassium dichromate to titrate a solution of iron(II) chloride. During this titration, the Cr₂O₇^2- solution is added to the Fe²⁺ solution. The dichromate ion gets reduced to Cr³⁺ as it oxidizes the Fe²⁺ to Fe³⁺.

There are different types of redox titrations, and these are commonly named after the reagent being used!

• Permanganometry is a redox titration that uses potassium permanganate as an oxidizing agent. Titrations with potassium permanganate do not need an indicator because KMnO₄ already acts as an indicator! In a titration of iron(III) with permanganate, chemists use phosphoric acid to mask the color of iron(III) so that it does not interfere with the color change at the end-point of the titration.
• Cerimetry is a type of redox titration that uses ammonium ceric sulphate as an oxidizing agent. Titrations are usually performed on perchloric acid or sulfuric acid.
• Dichrometry uses potassium dichromate (K₂Cr₂O₇) as an oxidizing agent. In this type of redox titration, a redox indicator (for example, barium diphenylamine sulphonate) is used to tell us the end-point of the titration.
• Bromatometry is a redox titration that uses potassium bromate as an oxidizing agent.
• Iodimetric titrations use iodine (I₂) as an oxidizing agent to titrate reducing agents. Iodimetric titrations are usually performed in neutral or slightly basic/acidic solutions.
• Iodometric titrations, on the other hand, uses iodide ion as a weak reducing agent.

Redox Titration Curve

Redox titrations tend to be a little more complex to evaluate than titration curves involving acid-base titrations. Chemists can monitor redox titrations by monitoring electrochemical potential.

As an example, we can use the titration of 50.0 mL of 0.100 M Fe²⁺ with 0.100 M Ce⁴⁺ in 1 M HClO₄. The reaction in this case is \( Fe^{2+} (aq) + Ce^{4+}(aq) \leftrightharpoons Ce^{3+}(aq) + Fe^{3+} (aq) \). Notice that this is an example of a titration where the analyte is a reducing agent (A_red), whereas the titrant is an oxidizing agent (T_ox).

$$ A_{red} + T_{ox} \rightleftharpoons T_{red} + A_{ox} $$

The electrochemical potential for the reaction is referred to as the difference between the reduction potentials for the oxidation and reduction half-reactions.

$$ E_{rxn} = E_{\frac{T_{ox}}{T_{red}}} - E_{\frac{A_{ox}}{A_{red}}} $$

When the reaction between the analyte and titrant reaches equilibrium, then we say that the E_rxn is zero and that \( E_{\frac{T_{ox}}{T_{red}}} = E_{\frac{A_{ox}}{A_{red}}} \) .

To be able to monitor the progress of the redox titrations, we can use the potential for either half-reaction. Now, look at the redox titration curve below.

What is exactly happening here? Let's take a look! Before the equivalence point is reached, the mixture consists of mostly the oxidized and reduced form of the analyte. The equivalence point is reached when 50 mL of Ce⁴⁺ has been added to the mixture. At the equivalence point, the moles of Fe²⁺ and Ce⁴⁺ are equal. After the equivalence point, significant amounts of the titrant's oxidized and reduced forms are present.

The image below shows an experimental setup for this redox titration. The Pt electrode responds to the relative concentration of Fe³⁺/Fe²⁺ and Ce⁴⁺/Ce³⁺, while the calomel electrode is used as reference.

Redox Titration Experiment

Curious about how redox titrations are performed in the lab? Let's look at an example involving the determination of the percentage of H₂O₂ in a store-bought bottle of 3% H₂O_2. We will use a standardized solution of KMnO₄ as the titrant.

1. Record the mass of a clean 125mL Erlenmeyer flask using an analytical balance.
2. Pipette 1.0 mL of the sample H₂O₂ solution into the flask and record the mass of the H₂O₂ solution added.
3. Add 10mL of 3M H₂SO₄ to the flask, and then add 50mL of distilled water.
4. Fill a buret with 50mL of your standard 0.025 M KMnO₄ solution. Record the initial volume of the buret.
5. Now, slide a piece of white paper under the flask containing the analyte, and titrate it with the KMnO₄ solution until the analyte solution changes color to a very pale pink/purple color permanently.
6. Record the final volume of the buret. This difference between the initial and final buret volume will tell us the volume and concentration of KMNO₄ needed to reach the end-point of the titration.
7. Then, we can calculate the actual percentage of H₂O₂ in the store-bought bottle using stoichiometry.

Redox Titration Calculations

We learned that the concentration of a substance can be determined by redox titration. So, let's solve a problem involving calculations!

Redox Titration Examples

Let's finish up by looking at some examples of redox titrations. We can use Iodimetric titration to find the mass of ascorbic acid in a vitamin C tablet, by titration with 0.005 M I₂ solution.

Chemists can also use another type of redox titration, iodometric titration, to find the percentage of copper in an unknown brass sample.

Now, I hope that you feel more confident in your understanding of redox-titrations!