In the titration of Mohr salt solution with ${\mathrm{KMnO}}_4$ solution, dilute ${\mathrm{H}}_2{\mathrm{SO}}_4$ is used to provide acidic medium. The titration gives unsatisfactory result when we use $\mathrm{HCl}$ in place of ${\mathrm{H}}_2{\mathrm{SO}}_4$. This is because.