A salt bridge is a type of laboratory device used in an electrochemical cell to connect its reduction and oxidation half cells wherein a weak electrolyte is used. In other words, it is a junction that connects the anodic and cathodic compartments present in a cell or electrolytic solution. Besides, we can give the salt bridge definition in many different ways.
Usually, the salt bridge consists of a strong electrolyte that is further made up of ions. The salt bridge examples are KCl, AgNO3, and more. Generally, Salt bridges are used in a galvanic cell like a voltaic cell or Daniel cell.
If there is no salt bridge, the solution in the one-half cell will accumulate the negative charge. The solution that belongs to the other half cell would accumulate a positive charge as the reaction proceeded, quickly preventing further reaction, thereby the production of electricity takes place.
Salt Bridge Function
A salt bridge’s major function is to help maintain the electrical neutrality within the internal circuit. Also, it helps to prevent the cell from taking its reaction to equilibrium. If salt bridges are not used or absent, then the reaction will continue, and the solution in one-half electrodes will find a negative charge. Likewise, the electrodes would accumulate a positive charge in the other half. Further, this will result in the stoppage of the reaction, and no electricity will be produced.
Therefore, basically, a salt bridge helps to prevent the accumulation of both positive and negative charges around the respective electrodes and allows a smooth reaction further to take place. The salt bridge also helps in the continual electrons flow. However, the salt bridge’s objective is not to transfer the electrons from the electrolyte, but rather to maintain the balance of the charge because the electrons move from one to the other half of the cell.
Some important points of Salt Bridge can be listed as,
Salt bridge prevents the mechanical flow or diffusion of a solution from one-half cell to another
It also minimizes or prevents the liquid-liquid junction potential. (Potential arises between the two solutions when they are in contact with each other)
The bridge acts as an electrical contact between the two half cells
Types of Salt Bridges
Mainly, there are two types of salt bridges that are used in electrochemical cells. They are,
Glass Tube Bridge
Filter Paper Bridge
Glass Tube Bridge
Generally, these are the U - Shaped Tubes, filled with electrolytes. Generally, Potassium Chloride (KCL), Sodium Chloride (NaCl), and Potassium Nitrate (KNO3) are used as an electrolyte. The electrolyte is required to be relatively unreactive with other chemicals in the cell and has cations and anions with the same migratory speed (comparable molecular weight and ion charge).
Often, the electrolytes are held as a gel, like Agar-Agar. The salt solution’s concentration and the diameter of the glass tube play an essential role in conductivity. Lowering the diameter of the tube and the concentration decreases the conductivity.
Filter Paper Bridge
They are other most commonly used bridges, consisting of a porous material or filter paper soaked in electrolytes. Here, commonly, potassium chloride (KCL) or sodium chloride (NaCl) is used as an electrolyte. Electrolytic concentration, roughness, and porosity of filter paper affect the conductivity. A filter paper with a smooth absorbent is used for higher conductivity, and they yield higher conductivity than the rough paper with a lower absorbent.
As declared above, the primary function of a salt bridge is to maintain the electrical neutrality between two beakers. To perform this, the used salt must be inert. The ions are required to move to and forth between the two half cells. Unlike other salts, potassium nitrate (KNO3) and potassium chloride (KCl) are better inert salts. Whereas, inert salt is used to prevent the reactions that occur between the salt and solution. Inert salt potassium chloride (KCl) is widely used as salt since potassium and chloride ions have a very similar diffusion coefficient and reduce the junction potential. But it is unwise to use potassium chloride as an electrolyte when the electrode used is silver or lead because they form a precipitate.
Working of Salt Bridge
Let us see the working of the salt bridge.
The oxidations that occur in an anode generate positive ions and electrons. Now the electrons flow via the wire in a beaker, leaving the unbalanced positive charge. The negatively charged (NO3–) ion moves towards the positively charged beaker (anodic half cell) in order to maintain the electrical neutrality.
The same situation develops in the cathode cell but in reverse order, and here, the Cu2+ ions are consumed. So, the K+ ions are migrated into this half cell from the salt bridge to maintain electrical neutrality. Hence, the electrical neutrality of a solution is maintained by using the salt bridge.