KEY2CHEM

Redox Titrations

A redox titration allows for the quantitation of amounts of oxidized and reduced species in a redox reaction. At the equivalence point in a redox titration, the moles of oxidized species = moles of reduced species. The permanganate ion ($$\require{mhchem}\ce{MnO4-}$$) is commonly used in redox titrations because its deep purple color fades as it is reduced to $$\require{mhchem}\ce{Mn^2+}$$.

Example 1.

An iodometric titration is a reliable redox titration in which iodine ($$\require{mhchem}\ce{I2}$$) reacts with thiosulfate ion ($$\require{mhchem}\ce{S2O3^{2-}}$$) in acidic solution to generate $$\require{mhchem}\ce{I-}$$ and $$\require{mhchem}\ce{S4O6^{2-}}$$. Write a balanced equation for this redox titration.

A. $$\require{mhchem}\ce{I2(aq) + 2 S2O3^{2-} -> 2 I^{-}(aq) + S4O6^{2-}(aq)}$$

B. $$\require{mhchem}\ce{I2(aq) + S2O3^{2-} -> I^{-}(aq) + S4O6^{2-}(aq)}$$

C. $$\require{mhchem}\ce{I2(aq) + 2 S2O3^{2-} + H+(aq)-> 2 I^{-}(aq) + S4O6^{2-}(aq)}$$

Solution

A. $$\require{mhchem}\ce{I2(aq) + 2 S2O3^{2-} -> 2 I^{-}(aq) + S4O6^{2-}(aq)}$$

The atoms in this redox reaction can be balanced using simple coefficients in front of the species, followed by balancing the number of electrons. Adding $$\require{mhchem}\ce{H2O}$$ or $$\require{mhchem}\ce{H+}$$ is not required to balance.

Example 2.

A breathalyzer is a device used to measure blood alcohol content, based on exhaled alcohol. If the redox reaction occurring in a breathalyzer is

$$\require{mhchem}\ce{3 C2H5OH + 2 Cr2O7^{2-} + 8H+-> 2 Cr^{3+} + 3 CH3COOH +11 H2O}$$

which species is reduced?

A. $$\require{mhchem}\ce{C2H5OH}$$

B. $$\require{mhchem}\ce{Cr2O7^{2-}}$$

C. $$\require{mhchem}\ce{H+}$$

Solution

B. $$\require{mhchem}\ce{Cr2O7^{2-}}$$

Dichromate ion ($$\require{mhchem}\ce{Cr2O7^{2-}}$$) is reduced to $$\require{mhchem}\ce{Cr^{3+}}$$ and ethanol ($$\require{mhchem}\ce{C2H5OH}$$) is oxidized to acetic acid ($$\require{mhchem}\ce{CH3COOH}$$).

Example 3.

Which is true at the equivalence point in a redox titration?

A. There is a color change.

B. The moles of oxidized species equal the moles of reduced species.

C. There must be an acid and a base present.

Solution

B. The moles of oxidized species equal the moles of reduced species.

At the equivalence point in a redox titration, the moles of oxidized species equal the moles of reduced species.