Oxidation of ferrous ions means: $\mathrm{Fe}^{2+} \rightarrow \mathrm{Fe}^{3+} + e^{-};$ $E^\circ_{\mathrm{Fe}^{3+}/\mathrm{Fe}^{2+}} = 0.77\ \text{V}$ Only those substances can oxidise Fe$^{2+}$ to Fe which are stronger oxidising$^{3+}$agents and have positive reduction potentials greater than 0.77 V sothat EMF of the cell reaction is positive. This is for elements lying belowFe$^{3+}$/Fe$^{2+}$ in the electrochemical series, for example, Br$_{2}$, Cl$_{2}$ and F$_{2}$.