Question

A sodium hydroxide solution can be used to distinguish between iron (II)sulphate (or chloride) and iron (III) sulphate (or chloride) solutions. Give the colour of the precipitate formed with each solution?

Answer 1

i. When Iron (II)sulphate react with sodium hydroxide

• A dirty green precipitate is formed.
• This precipitate indicates the presence of ferric hydroxide.
• ${\mathrm{FeSO}}_4+2\mathrm{NaOH}\to \underset{\mathrm{dirty}\mathrm{gren}\mathrm{orecipitate}}{\mathrm{Fe}{\left(\mathrm{OH}\right)}_2}+{\mathrm{Na}}_2{\mathrm{SO}}_4$
• It is not soluble in excess reagent.
• It turns to reddish brown precipitate slowly due to the oxidation and form Fe(OH)₃
• $4\mathrm{Fe}{\left(\mathrm{OH}\right)}_2+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to \underset{\mathrm{reddish}\mathrm{brown}\mathrm{precipitate}}{4\mathrm{Fe}{\left(\mathrm{OH}\right)}_3}$

ii. When iron (III) chloride react with sodium hydroxide

• A reddish brown precipitate of Fe(OH)₃ is formed.
• ${\mathrm{FeCl}}_3+3\mathrm{NaOH}\to \underset{\mathrm{reddish}\mathrm{brown}\mathrm{precipitate}}{\mathrm{Fe}{\left(\mathrm{OH}\right)}_3}+3\mathrm{NaCl}$
• When excess reagent is added, no reaction takes place.