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Redox Titration

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Redox Titration

You have probably heard of titrations before. Chemists use titrations to figure out the exact amount of base needed to neutralize an acid. However, titrations can also be useful in determining the amount of oxidizing agents required to react with a reducing agent. These titrations are called redox titrations. Interested in learning about redox titrations? Keep reading!

  • First, we will talk about redox reactions and oxidation states.
  • Then, we will explain what redox titrations are and look at a redox titration curve.
  • After, we will follow a redox titration experiment and look at redox titration calculations.
  • Lastly, we will go over some examples involving redox titrations.

Oxidation-Reduction (redox) reactions

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

During a redox reaction, electrons are exchanged by the reactants, changing the oxidation states of atoms from reactants to products.

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.

Oxidation numbers (or oxidation states) tell chemists the charge on an atom and how many electrons it has gained or donated during a reaction.

Determining Oxidation States
When atoms are in their elemental form, they have an oxidation number of zero. For example, the oxidation number of neon, Ne, is zero. The oxidation state of the fluorine molecule, F2 , is also zero.
For a monatomic ion, the oxidation number is the same as its ionic charge. For example, K+ has an oxidation number of +1, while Ca2+ has an oxidation number of +2.
In ionic compounds, the alkali metals (Group 1A) will always have a +1 oxidation number. Group 2A will have an oxidation number of +2. Aluminum, in group 3A, will always have an oxidation number of +3 in ionic compounds.
Atoms in molecular compounds:
  • Oxygen has an oxidation number of -2, except in peroxide (O22-).
  • Hydrogen has an oxidation number of +1 when bonded with any nonmetals, and -1 when bonded to metals.
  • Fluorine has an oxidation number of -1
  • Group 7A (or halogens) have an oxidation number of -1, except with bonded to oxygen.
The oxidation numbers sum to zero for a neutral compound, and sum to the overall charge for a polyatomic ion.

Let's look at an example!

Determine the oxidation number of sulfur in: SCl2 and SO42-.

In SCl2, we expect chlorine to have an oxidation number of -1 since Cl is a halogen, and the sum of the oxidation number needs to be equal to zero. So, by solving for x, we can determine the oxidation number for sulfur.

$$ x + 2(-1) = 0 $$

$$ x = -2 $$

So, the oxidation state of sulfur is -2.

Now, notice that SO42- is a polyatomic ion with an overall charge of -2. This means that the sum of the oxidation numbers will be equal to -2. From the table above, we know that oxygen will have an oxidation number of -2. So, if we let x be the oxidation number for sulfur, we get:

$$ x + 4(-2) = -2 $$

$$ x = +6 $$

In this case, the oxidation state of sulfur is equal to +6.

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.

Redox Titration Reaction between Mg and HCl oxidation states StudySmarterReaction 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.

To learn more about redox reactions, spectator ions and net ionic equations, check out "Net Ionic Equations" and "Redox Reactions".

Redox Titration Explanation

First, let's define titration.

A titration is a technique used to determine the concentration of an unknown solution by slowly adding a solution of known concentration to another solution of unknown concentration.

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 Cr2O72- 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.

You can learn more about titrations by checking out "Titration"!

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.

A redox titration curve follows the change in potential (E) against the volume of the titrant added. The titrant is the substance of known concentration, whereas the analyte is the substance of unknown concentration.

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 (Ared), whereas the titrant is an oxidizing agent (Tox).

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

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 Erxn 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% H2O2. 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.

Let's say that you performed an experiment, similar to the one above, and you recorded the following the data:

The initial volume of buret

50.00 mL

Final volume of Buret

64.00 mL

Volume of H2O2 solution being titrated (density = 1.00 g/mL)

1.00 mL

Molarity of KMnO4 solution

0.025 M

Use the collected data to calculate the percentage (%) of H2O2 in the store-bought hydrogen peroxide solution.

Step 1: Calculate the moles of KMnO4.

$$ \frac{0.025 \text{ mol } KMnO_{4}}{1000\text{ mL solution}} \times 14.00\text{ mL solution} = 0.00035 \text{ mol }KMnO_{4} $$

Step 2: Use the moles of KMnO4 to find the moles of H2O2.

$$ 0.00035 \text{ mol }KMnO_{4} \times \frac{5 \text{ mol }H_{2}O_{2}}{2 \text{ mol }KMnO_{4} } = 0.000875 \text{ mol }H_{2}O_{2} $$

Step 3: Calculate the mass of H2O2.

$$ 0.000875 \text{ mol }H_{2}O_{2} \times \frac{34.02\text{ } g\text{ } H_{2}O_{2}}{1\text{ } mol\text{ } H_{2}O_{2}} = 0.0298 \text{ } g\text{ }H_{2}O_{2} $$

Step 4: Find the % H2O2 .

$$ \frac{0.0298 \text{ } g\text{ }H_{2}O_{2} }{1.00\text{ } mL}\times 100 = 2.97\text{%} $$

Notice that the store-bought bottle was said to contain 3% H2O2. However, the experimental data tells us that the % H2O2 is actually 2.97%.

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!

In a titration of a 25.0 mL solution of Fe2+ with MnO4-, 16.7 mL of the 0.0152 M MnO4- was used. Find the concentration of Fe2+.

To answer this question, there are some steps we need to follow:

Step 1. Write the balanced net ionic equation for the titration reaction. If you are unsure about this, check out the explanation on "Net Ionic Equations".

$$ 5Fe^{2+} + MnO_{4}^{-} + 8H^{+} \longrightarrow 5Fe^{3+} + Mn^{2+} + 4H_{2}O $$

Step 2. Calculate the number of moles of MnO4- reacted.

$$ 0.0167\text{ L } \text{ } \times 0.0152\frac{mol}{L} = 2.54\times 10^{-4}\text{ } \text{moles of }MnO_{4}^{-} $$

Step 3. Calculate the concentration of Fe2+ in the sample.

$$ 2.54\times 10^{-4}\text{ } \text{moles of }MnO_{4}^{-}\text{ }\times \frac{5\text{ mol }Fe^{2+}}{1 \text{ mol }MnO_{4}^{-}} =1.27\times 10^{-3}\text{ moles of }Fe^{2+} $$

$$ \left[ Fe^{2+}\right]=\frac{1.27\times 10^{-3}\text{ mol }Fe^{2+}}{0.0250\text{ L}} = \textbf{0.0508 M}\text{ } Fe^{2+} $$

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!

Redox Titration - Key takeaways

  • During a redox reaction, electrons are exchanged by the reactants, changing the oxidation states of atoms from reactants to products.
  • In redox titrations, an oxidizing agent is titrated with a reducing agent (or vice versa).
  • A redox titration curve follows the change in potential (E) against the volume of the titrant added. The titrant is the substance of known concentration, whereas the analyte is the substance of unknown concentration.

References

  1. AP ® Chemistry COURSE AND EXAM DESCRIPTION Effective Fall 2020. (n.d.). https://apcentral.collegeboard.org/pdf/ap-chemistry-course-and-exam-description.pdf ‌
  2. Dingle, A., & Research And Education Association. (2020). AP chemistry crash course. Research & Education Association. ‌
  3. Moore, J. T., & Langley, R. (2021). McGraw Hill : AP chemistry, 2022. Mcgraw-Hill Education. ‌
  4. Theodore Lawrence Brown, Eugene, H., Bursten, B. E., Murphy, C. J., Woodward, P. M., Stoltzfus, M. W., & Lufaso, M. W. (2018). Chemistry : the central science (14th ed.). Pearson. ‌
  5. AP Chemistry L4-4 Redox Titration Lab. (n.d.). Studylib.net. Retrieved July 9, 2022, from https://studylib.net/doc/8818137/ap-chemistry-l4-4-redox-titration-lab-determination-of-the ‌
  6. REDOX TITRATIONS. (n.d.). https://teachntest.files.wordpress.com/2017/11/redox-titrations.pdf ‌
  7. Redox Titration Curves. (n.d.). BrainKart. https://www.brainkart.com/article/Redox-Titration-Curves_29670/ ‌
  8. Redox Titrations. (n.d.). https://www.calstatela.edu/sites/default/files/dept/chem/05fall/201/lecture10-redox-titrations.pdf ‌

Frequently Asked Questions about Redox Titration

A redox titration is a titration in which an oxidizing agent is titrated with a reducing agent (or vice versa). For example, using potassium dichromate to titrate a solution of iron(II) chloride.

The main principle of a redox titration is to determine the concentration of an unknown solution by slowing adding a solution of known concentration to another solution of unknown concentration. In redox titrations, an oxidizing agent is titrated with a reducing agent (or vice versa).

To solve problems about determining the concentration of a substance by redox titration, we can follow some steps.

  1. Write the balanced net ionic equation for the titration reaction.
  2. Calculate the number of moles of MnO4- reacted.
  3. Calculate the concentration of Fe2+ in the sample using stoichiometry. 

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.

Types of redox titrations include permanganometry, cerimetry, dichrometry, bromatometry, iodimetry and iodometry. 

Final Redox Titration Quiz

Question

During a redox reaction, ______ are exchanged by the reactants, changing the oxidation states of atoms from reactants to products.

Show answer

Answer

electrons

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Question

If a substance accepts an electron, we say that it has been______.

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Answer

reduced

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Question

If substance loses an electron, it has been _______.

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Answer

oxidized

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Question

During a redox reaction, electrons get transferred from the species that is ______ to the species that is _______.

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Answer

oxidized;reduced

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Question

The species that reduces another is called a ______ agent.

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Answer

reducing

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Question

A ______ agent get oxidized and loses electrons.

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Answer

reducing

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Question

The ______ agent oxidizes another species, and therefore gains an electron and gets reduced. 

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Answer

oxidizing

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Question

True or false: Oxidation numbers (or oxidation states) tells chemists the charge on an atom and how many electrons it has gained or donated during a reaction. 

Show answer

Answer

True

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Question

When atoms are in their elemental form, they have an oxidation number of ____.

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Answer

0

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Question

_______ is a redox titration that uses potassium permanganate as an oxidizing agent.

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Answer

Permanganometry 

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Question

Iodometric titrations use iodide ion as a weak ______ agent. 

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Answer

reducing

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Question

A redox titration curve follows the change in potential (E) against the _____ of  the titrant added. 

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Answer

volume

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Question

The _____ is the substance of known concentration.

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Answer

titrant

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Question

The ______ is the substance of unknown concentration.


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Answer

analyte

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Question

True or false: In redox titrations, an oxidizing agent is titrated with a reducing agent (or vice versa).

Show answer

Answer

true

Show question

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