The junction potentials are dominated by the mobility of H + and OH . The ---Sel

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.

Explanation / Answer

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+

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