For the cell $\mathrm{Zn}\left|{\mathrm{Zn}}^{2+}\right|\left|{\mathrm{Cu}}^{2+}\right|\mathrm{Cu}$, if the concentration of Zn²⁺ and Cu²⁺ ions is doubled, the EMF of the cell:

cell reaction : $\mathrm{Zn} + {\mathrm{Cu}}^{+2}\to {\mathrm{Zn}}^{+2}+\mathrm{Cu}$

${\mathrm{E}}_{\mathrm{cell}}={\mathrm{E}}_{\mathrm{cell}}^0-\frac{0.0591}{2}\log \frac{\left[{\mathrm{Zn}}^{+2}\right]}{\left[{\mathrm{Cu}}^{+2}\right]}$

when concentrations are doubled then log 1=0

${\mathrm{E}}_{\mathrm{cell}}={\mathrm{E}}_{\mathrm{cell}}^0$