a
Oxidised form \(+ ne^- \rightarrow\) Reduced Form

The substance which has lower reduction potential are stronger reducing agent while the substances which have higher reduction potential are a stronger oxidising agent.

\(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}+\mathrm{e}^{-} \rightarrow\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4} ; \mathrm{E}^{\circ}=0.35 \mathrm{V}\)

\(\mathrm{Fe}^{3+}+\mathrm{e}^{-} \rightarrow \mathrm{Fe}^{2+} ; \quad \mathrm{E}^{\circ}=0.77 \mathrm{V}\)

The reduction potential of \(\mathrm{F} \mathrm{e}^{3+/} \mathrm{Fe}^{2+}\) is higher, hence, \(\mathrm{Fe}^{3+}\) is a strongest oxidising agent.