Question

How will you distinguish between ammonium hydroxide and sodium hydroxide using copper sulfate solution?

Answer 1

1. Reaction between NH₄OH and CuSO₄

• When an aqueous solution of ammonium hydroxide $\left({\mathrm{NH}}_4\mathrm{OH}\right)$ is added to a copper sulfate solution $\left({\mathrm{CuSO}}_4\right)$, a pale blue precipitate of Copper hydroxide $\left(\mathrm{Cu}{\left(\mathrm{OH}\right)}_2\right)$ is obtained.
• The chemical reaction is depicted below.

${\mathrm{CuSO}}_4\left(\mathrm{aq}\right)+2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\to \mathrm{Cu}{\left(\mathrm{OH}\right)}_2\downarrow +{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)$

• This precipitate is soluble in excess ammonium hydroxide solution and produces an intense deep blue solution of tetraamine copper (II) sulfate $\left(\mathrm{Cu}{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\right)$.

$\mathrm{Cu}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)+{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\to \mathrm{Cu}{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

2. Reaction between NaOH and CuSO₄

• When an aqueous solution of sodium hydroxide is added to a copper sulfate solution, a pale blue precipitate of Copper hydroxide $\left(\mathrm{Cu}{\left(\mathrm{OH}\right)}_2\right)$ is obtained.

${\mathrm{CuSO}}_4\left(\mathrm{aq}\right)+2\mathrm{NaOH}\left(\mathrm{aq}\right)\to \mathrm{Cu}{\left(\mathrm{OH}\right)}_2\downarrow +{\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)$

• This precipitate is insoluble in excess sodium hydroxide solution.

Thus, ammonium hydroxide and sodium hydroxide can be distinguished using copper (II) sulfate.