how we can obtain the reduction potential of any single electrode???????

The electrode potential cannot be determined in isolation, but in a reaction with some other electrode.The electrode potential depends upon the concentrations of the substances, the temperature, and the pressure in the case of a gas electrode.In practice, the first of these hurdles is overcome by measuring the potentials with respect to a standard hydrogen electrode. It is the nature of electric potential that thezero of potentialis arbitrary; it is the difference in potential which has practical consequence. Tabulating all electrode potentials with respect to the same standard electrode provides a practical working framework for a wide range of calculations and predictions. The standard hydrogen electrode is assigned a potential of zero volts.
The second hurdle is overcome by choosing standard thermodynamic conditions for the measurement of the potentials. The standard electrode potentials are customarily determined at solute concentrations of 1 Molar, gas pressures of 1 atmosphere, and a standard temperature which is usually 25°C. The standard cell potential is denoted by a degree sign as a superscript.

E°Cell

Measured against standard hydroden electrode.
Concentration 1 Molar
Pressure 1 atmosphere
Temperature 25°C
The example below shows some of the extreme values for standard cell potentials.

Cathode (Reduction)
Half-Reaction Standard Potential
E°(volts)
Li+(aq) + e--> Li(s)
-3.04
K+(aq) + e--> K(s)
-2.92
Ca2+(aq) + 2e--> Ca(s)
-2.76
Na+(aq) + e--> Na(s)
-2.71
Zn2+(aq) + 2e--> Zn(s)
-0.76
Cu2+(aq) + 2e--> Cu(s)
0.34
O3(g) + 2H+(aq) + 2e--> O2(g) + H2O(l)
2.07
F2(g) + 2e--> 2F-(aq)
2.87