Identification of anode and cathode in an electrochemical cell is made by the use of — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Understand the Electrical Polarity in an Electrochemical Cell In a galvanic (voltaic) electrochemical cell, the electrodes are characterized by their chemical reactions and corresponding electrical charges: * **Anode:** The electrode where **oxidation** (loss of electrons) occurs. It is the source of electrons for the external circuit and is designated as the **negative terminal ($-$)**. * **Cathode:** The electrode where **reduction** (gain of electrons) occurs. It accepts electrons from the external circuit and is designated as the **positive terminal ($+$)**. To identify which electrode is the anode and which is the cathode, we must determine the electrical polarity of the cell's terminals. Step 2 - Analyze how a Voltmeter Identifies Terminal Polarity A **voltmeter** is an instrument designed to measure the electrical potential difference between two points in a circuit. It has two input terminals with defined polarities: a positive terminal (typically color-coded red) and a negative terminal (typically color-coded black). * When connected to an operating electrochemical cell, a digital voltmeter will display a **positive voltage reading** if and only if its positive terminal is connected to the positive terminal of the cell (the **cathode**) and its negative terminal is connected to the negative terminal of the cell (the **anode**). * If the connections are reversed, the digital voltmeter will display a negative sign ($-$), indicating **reversed polarity**. * Because a voltmeter has extremely high internal resistance, it performs this measurement while drawing a **negligible amount of current**, thereby preserving the electrode potentials without polarizing the electrodes. Thus, a voltmeter directly identifies the anode and cathode by indicating terminal polarity. Step 3 - Evaluate and Differentiate the Other Options * **Option (A) is incorrect:** A **galvanometer** is designed to detect and indicate the presence, direction, and strength of small electric currents. While the direction of current deflection can theoretically be used to deduce polarity, a galvanometer has very low internal resistance and draws significant current. This current flow polarizes the electrodes, changing their concentrations and potentials, making it unsuitable for accurate thermodynamic electrode identification. * **Option (B) is incorrect:** A **salt bridge** is a physical component of the cell containing an inert electrolyte. Its role is to complete the internal circuit and maintain electrical neutrality by allowing ions to migrate between the half-cells. It is not a measuring instrument and cannot identify polarity. * **Option (C) is correct:** A **voltmeter** measures potential difference and directly indicates the polarity of the terminals under near-zero-current conditions, allowing for the direct identification of the cathode and anode. * **Option (D) is incorrect:** A **potentiometer** is used to measure the electromotive force (EMF) of a cell under zero-current conditions at the balance (null) point. While highly accurate, it is a complex, active balance system requiring a standard auxiliary cell, a sliding contact wire, and a galvanometer to find the null point. It is not used as a simple, direct diagnostic tool to identify anode/cathode polarities. $$\text{Correct Option: } \boxed{\text{C}}$$