# Redox titrations

### Redox titrations

#### Lessons

In this lesson, we will learn:

• To recall the practical use of titration experiments
• How titration applies to redox reactions.
• How to calculate chemical quantities required in redox reactions.

Notes:

• We learned the basics of a titration with its use in acid-base chemistry in Acid-base-titration.
Just like acid-base titrations are used to find the concentration of acids and bases, a redox titration can be done to find the unknown concentration of a chemical in a redox process. The working out and calculations are detailed in Acid-base-titration and is summarized in the image below. Chemical A and chemical B in a redox titration would simply be the two chemicals in the redox (the reducing and oxidizing agent):

• Redox titrations will involve a reducing and oxidizing agent reacting together, but indicator is normally not used like it is in acid-base titrations. This means that one of the reactants used has to be one with a color difference between its reduced and oxidized form. There are two good options:
• Potassium permanganate (KMnO4) is an oxidizing agent that is purple in solution, but turns colorless when reduced to Mn2+ ions.
• Potassium iodide (KI) in solution gives I- ions that get oxidized (lots of chemicals can be used for this part) into brown-colored I2 in solution. Then in a redox titration, I2 can be reduced back to colorless I- ions. Starch can be added (it acts like an indicator for I2) to this, which is blue-black when I2 is present, the color fading when I2 becomes I- again.

• WORKED EXAMPLE:
A solution containing Co2+ ions of unknown concentration is made. 25mL of this Co2+ solution was measured and was titrated by 0.2M MnO4- solution until equivalence point was reached. 19.40 mL of the MnO4- solution was required.
The first thing that needs doing is the finding out of the two half-reactions:
• Manganese in MnO4- will be reduced to Mn2+ ions as shown in the half-equation:

MnO4- + 8H+ + 5e-$\enspace$$\enspace$Mn2+ + 4H2O

• Co2+ ions can be oxidized to Co3+ according to the half-equation:

Co2+$\enspace$$\enspace$Co3+ + e-

The method for working out half-equations in redox was covered in Half equations.

Next, the combining of the two half-reactions will give us the overall equation

1 x [ MnO4- + 8H+ + 5e-$\enspace$$\enspace$Mn2+ + 4H2O ]

5 x [ Co2+$\enspace$$\enspace$Co3+ + e- ]

These balance for electrons and give the overall equation:

MnO4- + 8H+ + 5Co2+$\enspace$$\enspace$Mn2+ + 4H2O + 5Co3+

This reaction has the cobalt solution as the unknown, so MnO4- with known concentration is being added by burette. MnO4- is purple and as it is added to the cobalt solution, the purple color will disappear as Co2+ reacts it away. When equivalence point is reached the purple color will no longer be removed as there will be no more Co2+ to remove the MnO4- and the purple color that it causes. Therefore the equivalence point is shown by the appearance of the purple color of the MnO4- that’s now in excess.

The number of moles of MnO4- can be calculated using the information in the question:

Mol MnO4- = 19.40 mL * $\frac{1\; L}{1000 \; m L} \;* \; \frac{0.2\; mol \; MnO_{4}^{-}}{1 \; L}$ = 3.88 * 10-3 mol MnO4-

Looking at the equation, we can see a 1:5 MnO4- to Co2+ ratio. The equivalence point will have five times as many moles of cobalt as manganese, then.

Mol Co2+ = 3.88 * 10-3 MnO4- $\,$* $\frac{5\; mol \; Co^{2+}}{1 \; mol \; MnO_{4}^{-}}$ = 0.0194 mol Co2+

With the moles of Co2+ ions now found in 25 mL volume of the sample used, we can calculate the concentration.

[Co2+] = $\frac{0.0194\; mol \; Co^{2+}}{0.025 \; L }$ = 0.776 M Co2+
• Introduction
Using titration for redox reactions.
a)
Recap of titration.

b)
Titration for redox reactions.

c)
Redox titration: Worked example

• 1.
Find the full equation and concentration of substances in a redox titration.
A solution containing Co2+ ions of unknown concentration is made. 25mL of this Co2+ solution was measured and was titrated by 0.25M MnO4- solution until equivalence point was reached. An average titre of 16.20 mL MnO4- solution was required. The reaction produces Mn2+ and Co3+ ions.

Write the full redox equation for this reaction and find the concentration of the aqueous Co2+ solution.

• 2.
Find the full equation and concentration of substances in a redox titration.
A solution containing I- ions of unknown concentration is made. 25mL of this solution is measured precisely and is titrated by 0.18M MnO4- solution until the equivalence point is reached. This is repeated, to find an average titre of 19.55 mL MnO4- solution being needed to completely react the I- ions.

Write the full redox equation for this reaction and find the concentration of the aqueous I- solution.