Standard Hydrogen Electrode Definition in Physical Chemistry

Standard Hydrogen Electrode

Definition and meaning of Standard Hydrogen Electrode in chemistry.

The Standard Hydrogen Electrode (SHE) is a setup used as a baseline to measure other electrical potentials in chemistry. This electrode uses a piece of platinum metal in contact with hydrogen gas and an acid solution. The potential of this reaction is defined as exactly 0.00 volts at all temperatures.

In more detail

In electrochemistry, we cannot measure the electrical potential of a single half-cell by itself. We must connect two half-cells to make a complete circuit. Therefore, chemists chose one specific half-cell to act as a universal reference point.

The SHE serves this role by bubbling hydrogen gas at one atmosphere of pressure over a platinum surface. The platinum is placed in a solution that has a hydrogen ion concentration of one molar. Platinum does not react, but it lets electrons transfer between the gas and the solution.

Many students think that the hydrogen electrode has a natural potential of zero volts. In reality, scientists simply agreed to define its potential as zero. This acts like setting sea level as zero for measuring mountain heights.

The SHE is rarely used in daily school labs because hydrogen gas is highly explosive. The setup is also very bulky and hard to keep at exact conditions. Instead, chemists use other stable reference electrodes that they calibrate against the SHE.

Key facts

Field

Physical Chemistry

Half-reaction

2H+ + 2e- → H₂

Standard Potential

0.00 V

Standard Conditions

1 atm H₂, 1 M H+, 25°C

Role in Chemistry

Universal reference electrode

Electrode Material

Platinum coated with platinum black

Example

We want to find if copper metal reacts with dilute nitric acid to make copper ions. We compare the copper half-cell potential of 0.34 volts with the nitric acid reduction potential of about 0.96 volts. This reaction is written as NO₃- + 4H+ + 3e- → NO + 2H₂O. Nitric acid has a much higher potential than the copper half-reaction. This difference makes the reaction spontaneous, so the copper metal will dissolve in the acid.

Frequently asked questions

Why is the Standard Hydrogen Electrode defined as zero volts?

Scientists needed a common starting point to measure and compare other electrodes, much like setting sea level to zero meters.

Why does the electrode require a platinum wire instead of a cheaper metal?

Platinum is highly unreactive, so it will not dissolve or corrode in the acidic solution during the chemical measurements.

Can the SHE be used to measure basic or alkaline solutions?

No, the SHE requires a hydrogen ion concentration of exactly one molar, which only exists in strongly acidic solutions.

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