Question

$KCl$ is used in salt bridge because:
A. It forms a good jelly with agar –agar
B. It is strong electrolyte
C. It is a good conductor of electricity
D. The transference number of ${K^ + }$ and $C{l^ - }$ ions are almost equal

Answer 1

To solve this question, first we need to understand the concept of a salt bridge. A salt bridge is responsible for providing an electrical contact between the two solutions and therefore completes an electrical circuit. Also, it prevents the mixing of two electrode solutions. Thus, a salt bridge maintains the electrical neutrality in both the solution by the flow of ions.

Complete step by step answer:

As we know that salt bridge is used in a galvanic cell. So, the reaction occurring in a galvanic cell is:
At Anode we have: $Zn$
At Cathode we have: $Cu$
We can write the chemical reaction taking place at anode as,
$Zn \to 2{e^ - } + Z{n^{2 + }}$
Similarly, we can write the chemical reaction taking place at cathode as,
$C{u^{2 + }} + 2{e^ - } \to Cu$
Therefore, the overall reaction becomes: $Zn + C{u^{2 + }} \to Z{n^{2 + }} + Cu$.Here, we know that $Z{n^{2 + }}$ ions are formed by losing their two electrons whereas $C{u^{2 + }}$ ions are formed by gaining two electrons.
We also know that anode is always dipped in $ZnS{O_4}$ and cathode is dipped in $CuS{O_4}$.
In the case of $KCl$, there are ${K^ + }$ and $C{l^ - }$ ions.
When $KCl$ is used in a salt bridge then the reaction becomes:
$2{K^ + } + S{O_4}^{2 - } \to {K_2}S{O_4}$
$Z{n^{2 + }} + 2C{l^ - } \to ZnC{l_2}$
Thus, it neutralizes the reaction. Due to which ${K^ + }$ move towards $S{O_4}^{2 - }$ and $C{l^ - }$ ions move towards $Z{n^{2 + }}.$
Therefore, we can use only those electrolytes in the salt bridge whose ions have the same transference number as agar-agar gel i.e.$KCl$ (option D).

Hence, the correct answer is option D.

Note:
We must know that a salt bridge bond in proteins is formed between oppositely charged residues that are sufficiently close to each other in an electrostatic attraction. Thus, they contribute to the protein structure and to the specificity of its formation.