Standard Hydrogen Electrode

For the thermodynamic scale of redox potentials, the standard hydrogen electrode is the standard measurement of electrode potential. The standard hydrogen electrode, also known as a normal hydrogen electrode, is frequently abbreviated as SHE (NHE). 

A SHE and an NHE are not the same thing. The NHE measures a platinum electrode’s potential in a 1 N acid solution, whereas the SHE measures a platinum electrode’s potential in an ideal solution (current standard of zero potential at all temperatures).

The potential of a platinum electrode in the redox half-reaction:

 2 H+(aq) + 2 e-  →  H2(g) at 25 °C is used to determine the standard.

Platinum black is added to platinum wire or a platinum plate to create a hydrogen electrode.It is submerged in the test solution and given an electric charge,as well as platinum black and hydrogen gas. 

Among the several ways for measuring Ph,the hydrogen-electrode method is a standard. Only when the values obtained using other methods match those obtained using the hydrogen electrode method are they considered reliable. However because of the time and money required, as well as the discomfort of handling hydrogen gas and the significant influence of highly oxidising or reducing compounds in the test solution,this method is not suitable for everyday usage.

Construction:

A conventional hydrogen electrode is made up of five parts:

platinum electrode platinized

1 mol/dm3 acid solution with 1 mol/dm3 hydrogen ion (H+) activity

Bubbles of hydrogen gas

Reservoir to attach the second half-element of the galvanic cell. Hydroseal to prevent oxygen interference.

To prevent mixing, a salt bridge or a thin tube can be employed.

The platinized platinum electrode is where the redox process takes place. Hydrogen gas bubbles through the electrode when it is dipped into the acidic solution. The concentration of the reduced and oxidised forms is maintained, resulting in a pressure of 1 bar (100 kPa) for hydrogen gas. The formal concentration multiplied by the activity coefficient equals the hydrogen ion activity.

The choice of platinum for the hydrogen electrode is based on a number of factors:

Platinum’s inertness (it doesn’t corrode).

Platinum’s capacity to catalyse proton reduction reactions.

Great reproducibility of the potential (bias of less than 10V when two well-made hydrogen electrodes are compared) 

High intrinsic exchange current density for proton reduction on platinum

Increase total surface area,the surface of platinum is platinized (i.e., covered with a layer of fine powdered platinum,also known as platinum black). This enhances reaction kinetics and allows for the most current possible.

Use a surface material with a high hydrogen adsorption capacity at its contact. This also improves the kinetics of the process.

Other metals, such as palladium and hydrogen, can be used to make electrodes with comparable functions.

Working of a Standard Hydrogen Electrode: 

A standard hydrogen electrode can function as both an anode and a cathode.

When a conventional hydrogen electrode in a cell is oxidised,it becomes an anode. Hydrogen is converted into H+ ions, which are then released into the solution.

H2→2H+ +2e–

When a standard hydrogen electrode in a cell is reduced,it becomes a cathode.H+ ions in the solution are converted to hydrogen gas during this process.

2H+ +2e– →H2

In terms of H+ ions, the electrode is thus reversible.

At 298K, the electrode potential of the standard hydrogen electrode is zero.

Electrode Potential Measurement:

To determine an electrode’s potential, a cell is set up with the electrode as one of the electrodes and the standard hydrogen electrode as the second electrode. The cell’s emf is then determined. The cell’s emf is equal to the sum of the cell’s oxidation and reduction potentials.

Because one of the electrodes in the conventional hydrogen electrode has a zero electrode potential, the cell’s emf will be precisely proportional to the electrode potential of the electrode under inquiry.The direction of the current flow reveals whether oxidation or reduction occurs on the hydrogen electrode under consideration. The electrode potential is referred to as oxidation potential or reduction potential in this case.

Standard Hydrogen Electrode Benefits:

1. It is applicable to a wide variety of pH values.

2. The potential of a conventional hydrogen electrode is zero.As a result,calculating the electrode potential of another electrode is simple.

Standard Hydrogen Electrode Disadvantages:

1. Platinum is costly, and platinized platinum is difficult to work with.

2. Maintaining the hydrogen pressure and acid concentration is difficult.

3. Pure hydrogen is difficult to obtain.

Conclusion: 

The Standard Hydrogen Electrode is the potential of a platinum electrode in an ideal gas solution. To elaborate, the theoretical notion solution includes the present requirement for zero potential at all temperatures. The process of donating or giving electrons from a substance to generate a new salt is known as oxidation.

The process of taking electrons from a chemical in order to generate a new salt is known as reduction.