Overview of Salt Bridge

The conversion of chemical energy to electrical energy and vice versa is called electrochemistry. It is primarily concerned with electricity production during spontaneous reactions and the use of electrical energy for non-spontaneous chemical transformations. The reactions carried out utilising electrochemical cells are more energy-efficient and less polluting.

One such device used to form an electrochemical cell is a salt bridge. The article explains the salt bridge definition, classification, significance, and device examples.

Salt Bridge Definition

According to the salt bridge definition, it is a laboratory device that connects the reduction and oxidation of half-cell components of a voltaic cell (galvanic cell). A voltaic cell is a form of an electrochemical cell. A salt bridge is also called an ion bridge. 

The function of the salt bridge is to keep the internal circuit’s electrical neutrality. If there were no salt bridges, the solution present in the one-half cell would build a negative charge with the advancement of the reaction. On the contrary, the solution in the other half cell would accumulate a positive charge as the reaction progressed. Therefore, it would block further reaction and, thus, power production. The functions of a salt bridge will be discussed in detail in the latter part of the article. 

Classification of Salt Bridge

Primarily, a salt bridge is divided into two types, namely, 

• Filter Paper Bridge

The filter paper salt bridges are most regularly used. As the name suggests, an absorbent filter paper soaked in electrolyte solution can be used as a filter paper salt bridge. The conductivity of filter paper bridges is influenced by the paper’s absorbance capacity, the texture of the filter paper,  and the electrolyte concentrations in cells. The filter paper’s absorbance and smoother texture usually equate to higher conductivity. Porous discs or partitions are often applied as salt bridges connecting two half cells to prevent intermixing.

• Glass Tube Bridge

Glass tube salt bridges are U-shaped structures. Usually, this salt bridge contains a mixture of chloride,  potassium, nitrate, and ammonium ions. The materials utilised to fill the gaps were chosen for their inertness to the chemical substances used in the cell. For creating a proper (glass tube) salt bridge, relative molecular weight and migratory speed are also considered. For gelification, agar is used in a glass tube salt bridge. 

The conductivity of the glass tube salt bridge rises with the concentration of the electrolyte in it, but only to a certain extent. The rise in concentration causes a reduction in conductivity after a certain point.

Since the conductivity of a salt bridge is directly related to its width, broader salt bridges are generally better conductors.

Example of a Salt Bridge

Let’s look at an example of a salt bridge in a zinc copper galvanic cell. This cell is also called Daniel’s cell. 

In a standard galvanic cell, zinc and copper metal pieces are submerged in salt solutions with high concentrations of their respective salts. Assume we put 1 molar amount of copper sulphate (CuSO₄) and zinc sulphate (ZnSO₄) in 2 different beakers. When these two salts are thoroughly dissolute, we are left with 

[Cu+2 ] [SO4-2 ] and [Zn+2 ] [SO4-2]

When these two solutions are connected, oxidation and reduction reactions are initiated depending on electrode potential. The resultant voltage measured in the zinc-copper cells is 1.10 V. 

The copper ions at the cathode side reduce to form copper metals gaining electrons coming from the anode side. It leaves [SO4-2] in the solution, which gets more negative as the reaction progresses. 

The zinc metal at the anode side oxidises to generate zinc ions by giving out two electrons to the cathode side. This occurs due to the less electrode potential of zinc. Consequently, more and more zinc ions are produced, making the solution more and more positively charged as the reaction continues. 

Based on their electrode potential, the charged species accumulate on the electrodes as time passes. This overcharged system of electrodes makes it difficult for the electrochemical cells to continue operating, and they eventually shut down. Therefore salt bridges are used in the electrochemical cells. As per the salt bridge definition, the device links both the half cell components of the cell, thereby maintaining the internal circuit’s electrical neutrality. 

Salt Bridge Significance

• The fundamental objective of a salt bridge is to maintain electrical neutrality inside an electrochemical cell.
• A salt bridge keeps electrical neutrality by preventing the formation of liquid-liquid junctional potential.
• Even if it’s merely a disc or a dividing plate, it prohibits intermixing of the 2 half-cell species

• A salt bridge links the two half cells as an alternative to the primary electrical pathway.

• The 2 half cells would have been charged, stopping the charge flow entirely if it weren’t for salt bridges.

Conclusion

Electrochemical cells are usually made up of two halves. The half-cells separate the oxidation and reduction halves of the reaction, allowing current to flow through an external wire. The circuit is completed when a salt bridge is added, allowing current to flow. The anions in the salt bridge flow toward the anode, while the cations flow towards the cathode. According to the salt bridge definition, it maintains a neutral charge in each cell and prevents the cell from quickly reaching equilibrium.