A reddish brown sol (containing ${\mathrm{Fe}}^{3+}$) is obtained by :

the addition of $\mathrm{NaOH}$ to ${\mathrm{FeCl}}_3$ solution dropwise

 the addition of small amount of ${\mathrm{FeCl}}_3$ solution to freshly prepared $\mathrm{Fe}(\mathrm{OH}{)}_3$ precipitate

the addition of ${\mathrm{NH}}_4$ OH to ${\mathrm{FeCl}}_3$ solution dropwise

the addition of $\mathrm{Fe}(\mathrm{OH}{)}_3$ to freshly prepared ${\mathrm{FeCl}}_3$ solution