Introduction

The voltage or potential difference between a cell formed from a standard hydrogen electrode and another electrode with the required potential is electrode potential. 

It is the potential difference between a point on the electrode surface and a point in the electrolyte’s bulk due to charged particle transport and polar molecule adsorption. Except for noble gases, all substances tend to gain or lose electrons; hence, the scientific community introduces two essential principles to comprehend electrode potential: oxidation potential and reduction potential.

Oxidation Potential

• The potential difference between a half-cell containing an aqueous solution of the oxidised and reduced versions of the test substance and the standard hydrogen half-cell is used to measure the oxidation potential.
•  For example, for Fe2+ → Fe3+ + e−, Eθ = 0.77, for Mn2+ → Mn3+ + e−, Eθ = 1.51. 
• The reduced form of a couple (e.g. Fe2+) will lower the oxidised form of a couple with a higher oxidation potential (e.g. Mn3+) as the oxidation potential decreases.
• The oxidation potentials obtained under these controlled conditions are standard electrode potentials or sometimes standard reduction potentials. 

Reduction Potential

• A chemical species’ tendency to gain or lose electrons from an electrode, leading it to be reduced or oxidised, is measured by redox potential (also known as oxidation/reduction potential). Redox potential is measured in volts (V) or millivolts (mV).
• Each species has its redox potential; for example, the higher the affinity for electrons and the higher the reduction potential, the more positive it is (reduction potential is used due to general formalism in electrochemistry). 

Applications of Electrode Potential

Electrode potential can be used for a variety of purposes:

• Corrosion-related chemical or electrochemical reactions can be predicted.
• It is used to select chemicals and equipment for reaction control.
• As electrode potential in crevices and pits is examined for controlling reactions, it aids in researching crevice corrosion and pitting.
• The potential of an electrode can be used as a predictor of corrosion responses. The protective mechanism is effective when it is within the potential protective range.

Standard Electrode Potential

• According to electrochemistry, the standard electrode potential is the value of the standard emf (electromotive force) in which molecular hydrogen is oxidised to solvated protons at the left-hand electrode at standard pressure. 
• It is a measurement of the reducing power of an element or compound. An electrochemical cell, such as a galvanic cell, is often based on a redox reaction that can be divided into two halves: oxidation at the anode (electron loss) and reduction at the cathode (electron gain). The difference in electric potential between the two metal electrodes individual potentials in relation to the electrolyte generates electricity. 

Electrochemical Series

• The order in which elements are grouped in order of increasing electrode potential values is described by an electrochemical series. The series was made by comparing the potential of several electrodes to the potential of a standard hydrogen electrode (SHE). As is normal, the standard hydrogen electrode is always at zero voltage.
• For reduction half-reactions, electrodes with positive E° values act as cathodes versus SHE, while those with negative E° values act as anodes versus SHE. The electrochemical series is listed in the table below. A cell’s standard state potential is at standard state conditions, approximated at 1 mole per litre (1 M) concentrations and 1-atmosphere pressure at 25 degrees Celsius.

[Potassium]                                                        [ K ]                      (Most reactive metal)

Sodium                                                               Na 

Calcium                                                              Ca                                        

Magnesium                                                       Mg 

Aluminium                                                        Al

Zinc                                                                    Zn                                              ↓

Iron                                                                    Fe

Tin                                                                      Sn

Lead                                                                   Pb

[Hydrogen]                                                       [H ]    

Copper                                                              Cu

Mercury                                                             Hg

Silver                                                                  Ag

Gold                                                                   Au                             (Least reactive metal)

Applications of Electrochemical Series

The major applications of electrochemical series are: 

• Oxidising and reducing strengths.
• Displacement reactions.
• Metals’ liberation of hydrogen gas from acids: Prediction
• Predicting a redox reaction’s feasibility.
• The calculations of EMF of the cell.
• Metal reactivities are compared.

Conclusion

The potential difference between the electrodes, measured in volts (V), is determined by the compounds that make up the electrodes. The total potential of any electric cell is the sum of the potentials produced by the reactions at the two electrodes: EMF oxidation + EMF reduction = EMF cell. Electromotive force (EMF) is another term for electrical potential. The potential of an electrode is determined by the direction of electron flow—whether the electrode is serving as the cathode or anode in its cell. It is also contingent on the concentration of electrolytes and temperature vs pressure.