Question

The junction potentials are dominated by the mobility of H⁺ and OH⁻. The  ---Select--- high low mobility of H⁺ makes the right side of 0.1 M HCl | 0.1 M KCl become  ---Select--- positive negative . The  ---Select--- high low mobility of OH⁻ makes the right side of 0.1 M NaOH | 0.1 M KCl become  ---Select--- positive negative .

Why is the junction potential for 0.1 M NaOH | 0.1 M KCl so much more negative than 0.1 M NaOH | KCl (saturated)?

The junction potential for 0.1 M NaOH | KCl (saturated) is  ---Select--- large small in magnitude because diffusion is dominated by the  ---Select--- high low concentrations of K⁺ and Cl⁻, which have  ---Select--- nearly equal vastly different mobility.

Answer 1

Junction potential arises because different ions diffuse at different rates across a liquid junction, leading to a separation of charge. The resulting electric field slows the fast-moving ions and accelerates the slow-moving ions until a steady-state junction potnetial is reached. This limits accuracy of potentiometric measurement because we do not know what part of a measured cell voltage is due to the process of interest and what is due to the junction potnetial.

Therefore,The junction potentials are dominated by the mobility of H⁺ and OH⁻ : H⁺ has a greater mobility than K⁺. The HCl side of HCl I KCl junction will be negative because H⁺ diffuses into the KCl region faster than K⁺ diffuses into the HCl region. K⁺ has greater mobility than Na+, so this junction has opposite sign. The HCl I KCl voltage is larger because the difference in mobility between H⁺ and K⁺ is greater than the difference in mobility between K⁺ and Na⁺ ----- Positive negative

The junction potential for 0.1 M NaOH | 0.1 M KCl so much more negative than 0.1 M NaOH | KCl (saturated)

nearly equal vastly different mobility : K⁺ has greater mobility than Na+, so this junction has opposite sign. The HCl I KCl voltage is larger because the difference in mobility between H⁺ and K⁺ is greater than the difference in mobility between K⁺ and Na⁺