Question

KCl Used for the construction of salt bridge because:
A. ${{\text{K}}^ + },\,{\text{C}}{{\text{l}}^ - }$ are isoelectronic
B. ${\text{KCl}}$ is an ionic substance
C. ${{\text{K}}^ + },\,{\text{C}}{{\text{l}}^ - }$ have unit charges
D. ${{\text{K}}^ + },\,{\text{C}}{{\text{l}}^ - }$ move with equal mobilities

Answer 1

Hint: Salt bridge is usually an inverted U-shaped tube filled with non- concentrated solution of inert electrolytes. Where an inert electrolyte is one whose ions are neither involved in any electrochemical change nor they react with electrolytes in the two half cells. Generally salts like ${\text{KCl}}\,{\text{,}}\,{\text{KN}}{{\text{O}}_3}\,,\,{\text{N}}{{\text{H}}_4}{\text{N}}{{\text{O}}_3}$ etc.. are used. The saturated solutions of the electrolyte are prepared in agar-agar jelly or gelatine.

Complete step by step answer:
A salt bridge is often employed to provide ionic contact between two half cells with different electrolytes and prevent the solution from mixing and causing unwanted reactions.
In a galvanic cell , if a salt bridge is not used then the two solutions should be either in contact with each other or they are separated by a porous membrane. In both cases positive and the negative ions of the electrolyte will migrate across the junction in opposite directions as there is an increase of positive and negative charges at anode and cathode respectively. Inside the cell the current is carried by the ions that are negative ions from cathode to anode and positive ions from anode to cathode. There is a difference in migration velocities of these ions, accumulation of charges across them occurs. This will give rise to the formation of an electrical double layer and develop a potential called liquid junction potential across the junction.in such situation the emf of the cell includes liquid junction potential and we get incorrect measurement.
If we use a salt bridge it will avoid the liquid junction potential and also provides a contact between the two solutions. It completes the electrical circuit and maintains the electrical neutrality of two half cells. Also it consists of saturated solutions of salts such as ${\text{KCl}}$ whose ions have almost similar migration velocities. The positive \[{{\text{K}}^ + }\] and negative ions ${\text{C}}{{\text{l}}^ - }$ of the salt in the salt bridge migrate with equal speed into cathode and anode respectively. Thereby avoid liquid junction potential.
From the above explanation it is clear that salt bridge is used to avoid the liquid junction potential. For that we need a saturated solution of electrolytes whose ions move at equal speed, that is salt have ions with equal mobilities.

So, the correct answer is Option B.

Note: Main functions of salt bridge include
Allows the passage of electric current internally.
Maintain electrical neutrality of the solution.
Prevents the intermixing of the solution.
It doesn’t take part in cell reaction.