Question

How does ammonium hydroxide react to distinguish ${\mathrm{FeCl}}_2$ solution from ${\mathrm{FeCl}}_3$ solution?

Answer 1

Part 1:- Reaction of Ammonium hydroxide and ${\mathrm{FeCl}}_2$

1. $\underset{\mathrm{Iron}\left(\mathrm{II}\right)\mathrm{chloride}}{{\mathrm{FeCl}}_2\left(\mathrm{aq}\right)}+\underset{\mathrm{Ammonium}\mathrm{hydroxide}}{2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)}\to \underset{\mathrm{Ammonium}\mathrm{sulphate}}{2{\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right)}+\underset{\mathrm{Iron}\left(\mathrm{II}\right)\hspace{0.17em}\mathrm{hydroxide}}{\mathrm{Fe}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)(\downarrow )}$
2. When Ammonium hydroxide is added to Ferrous sulphate a “dirty green precipitate” is formed.
3. This ppt slowly changes to reddish-brown after some time and is insoluble in excess Ammonium hydroxide.

Part 2:-Reaction of Ammonium hydroxide and ${\mathrm{FeCl}}_3$

1. $\underset{\mathrm{Iron}\left(\mathrm{III}\right)\mathrm{chloride}}{{\mathrm{FeCl}}_3\left(\mathrm{aq}\right)}+\underset{\mathrm{Ammonium}\mathrm{hydroxide}}{3{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)}\to \underset{\mathrm{Ammonium}\mathrm{sulphate}}{3{\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right)}+\underset{\mathrm{Iron}\left(\mathrm{III}\right)\hspace{0.17em}\mathrm{hydroxide}}{\mathrm{Fe}{\left(\mathrm{OH}\right)}_3\left(\mathrm{s}\right)(\downarrow )}$
2. When Ammonium hydroxide is introduced to an aqueous solution of Iron III chloride, it produces a "reddish-brown precipitate" of Iron (III) hydroxide as well as Ammonium chloride.
3. The crucial thing to remember is that the 'reddish-brown precipitate' of Iron (III) hydroxide is insoluble in excess Ammonium hydroxide.

Therefore, When ${\mathrm{NH}}_4\mathrm{OH}$ is added to Iron (II) chloride we observe that a dirty green ppt is formed slowly turns reddish-brown after some time which is insoluble in excess Ammonium hydroxide, whereas on the other hand when ${\mathrm{NH}}_4\mathrm{OH}$ is added to Iron (III) chloride we observe that a "reddish-brown precipitate" of Ferric hydroxide is formed which is insoluble in excess Ammonium hydroxide.