Question

What causes a junction potential? How does this potential limit the accuracy of potentiometric analysis? What does a selectivity coefficient tell us about an ion selective electrode?

Answer 1

Different ions diffuse at different rates across a liquid junction, leading to a separation of charge - the resulting electric fields retards fast moving ions and accelerates slow-moving ions until a steady-state potential is reached.

We don't know what part of the measured cell voltage is due to the process of interest and what is due because of the junction potential.

Selectivity coefficient tells us the relative response of an ion selective electrode to the ion of interest and an interfering ion.