Iron(II) ions catalyse the oxidation of iodide ions by peroxodisulphate ions. The steps are: $\text{S}_2\text{O}_8^{2-} + 2\text{Fe}^{2+} \rightarrow 2\text{SO}_4^{2-} + 2\text{Fe}^{3+}$, then $2\text{Fe}^{3+} + 2\text{I}^- \rightarrow 2\text{Fe}^{2+} + \text{I}_2$. Which line names the catalyst and the intermediate?