emf of the cell $\mathrm{Zn}\left|{\mathrm{Zn}}^{2+}\left({\mathrm{C}}_1\right)\parallel {\mathrm{Cu}}^{2+}\left({\mathrm{C}}_2\right)\right|\mathrm{Cu}$ is maximum when

Anode       $Zn\left(s\right)\to Z{n}^{2+}\left(aq\right)+2e$

Cathod     ${\mathrm{Cu}}_{\left(aq\right)}^{2+}+2e\to C{u}_{\left(S\right)}$

               $\overline{Z{n}_{\left(s\right)}+C{u}_{\left(aq\right)}^{2+}\to Z{n}_{\left(aq\right)}^{2+}+C{u}_{\left(s\right)}}$

$Q_C=\frac{\left[{\mathrm{Zn}}^{2+}\right]}{\left[C{u}^{2+}\right]}$

$E_{\text{cell }}=E_{\text{Cell }}^O-\frac{2.303RT}{2F}\log \frac{\left[{\mathrm{Zn}}^{2+}\right]}{\left[{\mathrm{Cu}}^{2+}\right]}$

$=E_{\text{Cell }}^O-\frac{2.303RT}{2F}\log \frac{\left[C_1\right]}{\left[C_2\right]}$

$E_{max}$will be achieved when

$\left[{\mathrm{Cu}}^{2+}\right]>\left[{\mathrm{Zn}}^{2+}\right];C_2>C_1$