By Sunil Bhardwaj

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Suppose we construct concentration cell in such a way that two solutions of the same electrolyte having different concentration are not separated by salt bridge but are in contact through a porous partition i.e. diffusible membrane. Then higher concentration solution diffuse into dilute solution and will continue till the equilibrium is established. The ions of the electrolyte diffuse at different rates. It is found that rate of diffusion of each ion is proportional to its velocity. But velocity or mobility is related to transport number of ions because, $$\frac { { t }_{ + } }{ { t }_{ - } } = \frac { { u }_{ + } }{ { u }_{ - } }$$ The transport number of $${ H }^{ + }$$ ions $$({ t }_{ + })$$ is 0.83 and $${ Cl }^{ - } \ ({ t }_{ - })$$ is 0.17 in HCl.

As the $${ t }_{ + } > { t }_{ - }$$ it means velocity of $${ H }^{ + }$$ is greater than $${ Cl }^{ - }.$$ Therefore in the same time interval more of $${ H }^{ + }$$ ions will move from higher concentration to lower concentration resulting the formation of electrical double layer across the membrane. This electrical double charge layer will develop a potential which is known as liquid-liquid junction potential and is represented as Ej and is in volts. Liquid junction potential is also known as Diffusion potential .

The magnitude of liquid junction potential depends on the difference between the speed of ions. i.e. transport number of ions. Please note that the liquid junction potential can be eliminated by using a salt-bridge.

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