Explain normal or standard hydrogen electrodes.

A Normal Hydrogen Electrode (NHE), also known as a Standard Hydrogen Electrode, is a fundamental reference electrode used in electrochemistry to measure and define the standard electrode potential of other half-cells in electrochemical cells. It serves as a reference point against which the electrode potentials of other half-cells can be compared.

Here's how a Normal Hydrogen Electrode works and its key components:

Hydrogen Gas (H2): The heart of the NHE is a glass tube or vessel containing pure hydrogen gas at a known pressure. The hydrogen gas is bubbled through a solution, typically an acidic electrolyte like hydrochloric acid (HCl) or sulfuric acid (H2SO4). This generates a continuous supply of hydrogen ions (H+) in the solution.

Platinum Electrode (Pt): A platinum electrode is used as a conductor to facilitate the electron transfer reactions occurring at the electrode-electrolyte interface. Platinum is chosen because it does not readily react with hydrogen or the electrolyte and provides a stable interface for electron exchange.

Hydrogen Ion Concentration (pH): The pH of the solution is maintained at a constant and well-defined value, usually close to 0 or 1, to ensure a known concentration of hydrogen ions (H+) in the solution. The pH is controlled by the choice of acidic electrolyte and is an essential aspect of the NHE's operation.

The half-reaction at the NHE can be represented as follows:

2H2(g) ⇌ 4H+(aq) + 4e-

In this equation, hydrogen gas (H2) dissociates into hydrogen ions (H+) and electrons (e-). The electrode potential of the NHE is defined as 0 volts at all conditions of temperature and pressure when the hydrogen gas is at 1 atmosphere (standard pressure) and the hydrogen ions are at a specific concentration, usually 1 molar (1 M), and the pH is maintained at a specific value.

The NHE is essential in electrochemical measurements because it provides a fixed reference point for the determination of electrode potentials (also known as redox potentials) of other half-cells in electrochemical cells. By comparing the voltage of other electrodes to that of the NHE, scientists and engineers can measure the relative electrochemical reactivity of different substances and reactions. The potential of the NHE is often used as the reference electrode in the construction of the electrochemical cell notation for chemical reactions.