Understanding Standard Hydrogen Electrode and Electrode Potentials

The Standard Hydrogen Electrode (SHE) serves as a fundamental reference point in electrochemistry, assigned a potential of 0.000 V. This standard is realized through a specific setup involving a platinum electrode immersed in an acidic solution of hydrochloric acid, where the concentration of hydrogen ions is unity. At 25°C (298 K), the conditions reflect a molality of approximately 1.2, enabling precise measurements of electrode potentials.

In practice, the SHE setup involves "platinizing" the platinum electrode with finely divided platinum powder, which provides a large effective surface area for hydrogen absorption. Pure hydrogen gas is introduced at atmospheric pressure, allowing it to interact intimately with the acid and the platinum. This configuration is crucial for studying the reduction and oxidation reactions that dictate the behavior of various electrodes.

The direction of spontaneous reactions at an electrode determines the sign of its potential. For example, when copper ions are reduced to copper metal, the reaction produces a positive potential, while the reverse reaction of hydrogen ions forming hydrogen gas results in a negative potential. This relationship is essential for understanding the electrochemical series and predicting reaction feasibility.

In practical applications, while the SHE is accurate, it can be cumbersome to use. Therefore, alternative reference scales, such as the Saturated Calomel Electrode (SCE) and Silver/Silver Chloride (Ag/AgCl), have been developed. It's important to note that these sub-standard scales have shifted zero points compared to the SHE, which affects the interpretation of measured potentials.

The potential at any electrode is influenced by its inherent properties as well as the activities of the species involved in the reaction. Therefore, when comparing electrode potentials, it is essential to ensure they are measured on the same scale and under identical conditions. This standardization allows for more accurate assessments across different electrochemical systems.

Lastly, the temperature dependence of standard electrode potentials is linked to the Gibbs free energy changes associated with the reactions. Understanding these relationships is vital for applications in corrosion science, where electrodes like the silver/silver chloride cell are utilized to measure solubility products of sparingly soluble salts, further expanding the practical utility of electrochemical principles.