In the electrochemical series, Cu is below hydrogen. Hence, it has positive value of standard reduction potential (EM2+∕M). The standard reduction potentials are Ni(Z=28)‌⇒−0.257V Cu(Z=29)‌⇒+0.337V Cr(Z=24)‌⇒−0.90V Ti(Z=21)‌⇒−1.63V Highest reduction potential is of Cu as, it has positive value of reduction potential, while Ni,Cr and Ti have negative reduction potential value.