JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech I Year ENGINEERING CHEMISTRY Previous Question Paper (July - 2021 - R13)

A Calomel electrode is a reference electrode consisting of mercury in contact with a paste of mercurous chloride (calomel) and a solution of potassium chloride (KCl). It is represented as Hg | Hg₂Cl₂ | KCl.

Construction: It consists of a glass tube containing a layer of mercury at the bottom, covered by a paste of Hg and Hg₂Cl₂. The remaining part of the tube is filled with a KCl solution of known concentration (usually saturated). A platinum wire is sealed into the glass tube to provide electrical contact with the mercury.

Working: The electrode reaction is reversible and is given by:

The potential of the electrode depends on the concentration of the KCl solution. Because the concentration of Cl⁻ ions is fixed by the saturated KCl solution, the electrode maintains a constant potential, making it a reliable reference electrode for measuring the potentials of other half-cells.

Cathodic Protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. This is achieved by supplying electrons to the metal structure, thereby preventing the oxidation (corrosion) of the metal.

The Sacrificial Anode Method involves connecting the metal structure to be protected (e.g., iron pipe or tank) to a more active, electropositive metal (the sacrificial anode, such as magnesium, zinc, or aluminum). The more active metal acts as the anode and undergoes oxidation, effectively 'sacrificing' itself to protect the structural metal.

Key points of the sacrificial anode method:

• The sacrificial anode corrodes preferentially, while the protected metal remains intact as the cathode.
• The anode must be replaced periodically as it is consumed over time.
• This method is commonly used for protecting underground pipelines, ship hulls, and water storage tanks.
• It requires no external power source, making it ideal for remote locations.