Salt bridge

Introduction

The salt bridge is a very important component for maintaining electrical neutrality by allowing the flow of ions between the two half-cells. It was first discovered by the French chemist Nicolas-Gaspard de Damar in the early 19th century. The study of electrochemistry was sped up by the work of scientists like Alessandro Volta and Humphry Davy. These researchers were exploring the nature of electric currents and their chemical effects. Nicolas-Gaspard de Damar Although not as widely recognized as Volta or Davy, de Damar made significant contributions to the field. He introduced the salt bridge to address issues of ion flow and cell stability in electrochemical cells. In an electrochemical cell, the salt bridge serves to complete the circuit between the two half-cells. It prevents the solutions in the two half-cells from mixing directly while allowing the transfer of ions to maintain charge balance. This is crucial for maintaining the cell's functionality over time.

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The discovery of the salt bridge was very important for the maintenance of the electrochemical cell and the batteries. It is useful for maintaining the cell more consistently and for ensuring that the reaction in electrochemical reactions can continue without any disturbance in the charges as they do not get imbalanced. A salt bridge is a U-shaped tube containing a concentrated solution of an electrolyte like KCl, KNO₃, K₂SO₄, etc. or a solidified solution of such an electrolyte in agar-agar and gelatine An inert electrolyte is one whose ions do not take part in redox reaction and also does not react with electrolyte used.

A salt bridge's function is to allow ions to move from one solution to the other without mixing two solutions. Thus, whereas electron flow in the outer circuit in the wire the inner circuit is completed by the flow of ions from one Solution to another through the salt bridge. Moreover, it helps maintain the solution's electrical neutrality in the two half-cells.

Salt Bridge And Its Conditions

It maintains electrical neutrality in two compartments by allowing the movement of anions toward the anodic compartment and cations toward the cathodic compartment.

• It is a glass tube having KCl, KNO₃, and ammonium nitrate in a gelatin gel or agar-agar paste.
• The gelatin gel allows ionic movement through it but prevents any kind of mixing.
• In the case of KCl or ammonium nitrate, the ionic mobility of cation and anion are the same.

The function of a Salt Bridge

• A salt bridge acts as an electrical contact between the two half-cells.
• It prevents the mechanical flow of solution but it provides a free path for the migration of ions to maintain an electric current through the electrolyte solution. It prevents the accumulation of excess charges.
• A salt bridge helps maintain the charge balance in the two half-cells.
• A salt bridge minimizes/eliminates the liquid junction potential.
• To complete the electrical circuit by allowing the ion to flow from one solution to the other without mixing the other solution

Liquid Junction Potential: The unequal rates of migration of the cations and anions across a liquid-liquid junction give rise to a potential difference across the junction. This potential difference across the liquid-liquid junction is called liquid junction potential. In other words, Liquid junction potential is the potential difference that develops at the boundary where two different electrolyte solutions meet. It occurs due to the difference in the ionic mobilities and concentrations of the ions in the two solutions.

• When two electrolytes with different ionic concentrations or compositions come into contact, the ions from each solution will diffuse across the junction to reach equilibrium. This diffusion creates an imbalance in the ion distribution, resulting in a potential difference known as the liquid junction potential.
• The liquid junction potential can be measured using electrochemical cells. It is often determined experimentally by measuring the overall potential difference between two electrodes in different solutions and correcting for other known potentials.
• - In electrochemical measurements, such as in pH electrodes or other ion-selective electrodes, the liquid junction potential can introduce errors. Therefore, it's crucial to account for this potential to ensure accurate readings.

NOTE: If the salt bridge is removed the emf of the cell drops to zero.

• Sometimes, both the electrodes are dipped into the same electrolytic solution. in such a case, there is no requirement for the salt bridge.

Conditions for salt bridge

A salt bridge is crucial for maintaining electrical neutrality in electrochemical cells.

• Electrolyte Solution- It must contain a strong electrolyte, such as potassium chloride (KCl) or sodium sulfate (Na₂SO₄), which dissociates completely in solution.
• Electrolyte Concentration- The concentration should be high enough to provide a sufficient ionic flow but not so high that it causes precipitation.
• Inert Material- The salt bridge should be made from an inert material that does not react with the electrolytes in the cell.
• Conductivity- It should be designed to allow for the easy flow of ions between the two half-cells.
• Porosity- If it's a physical salt bridge, it should have a porous structure to allow ion movement while preventing the mixing of the two solutions.
• Stability- The bridge should be stable and not degrade or dissolve rapidly in the electrolyte solutions.

These conditions help ensure that the electrochemical cell operates efficiently by preventing charge buildup and maintaining electrical neutrality.

For a better understanding of the topic and to learn more about Salt Bridge with video lesson we provide the link to the

YouTube video:

Some Solved Example

EXAMPLE.1

1. A saturated solution of KNO₃ is used to make a 'salt bridge' because:

1)velocity of K⁺ is greater than that of NO^-₃

2)velocity of NO^-₃ is greater than of K⁺

3) (correct)velocity of both NO^-₃ and K⁺ are nearly the same

4)KNO₃ is highly soluble in water

Solution

The salt bridge possesses the electrolyte having nearly the same ionic mobilities as its cation and anion.

Hence, the answer is the option (3).

EXAMPLE.2

2. Which of the following statements is true about the salt bridge?

1)The ionic mobility of the ions in the salt bridge should be vastly differing in value.

2) (correct)The ions in the salt bridge must be inert to the ions of the cell.

3)The ions in the salt bridge must be reactive to the ions of the cell

4) A salt bridge provides an electric contact between the two solutions by allowing them to mix.

Solution

The salt bridge provides electric contact between the two solutions without allowing them to mix.

The ions in the salt bridge must be inert to the ions of the cell.

Therefore, the correct option is (2).

EXAMPLE.3

3. A current of 5 amp is passed for one hour through an aqueous solution of copper sulfate using copper electrodes. What is the change in the mass of the cathode?

1)3.17

2) (correct)5.92

3)4.92

4)5.72

Solution

The reaction occurring at the given cathode is

Cu²⁺+2e⁻→Cu (Cathode

Thus, 1 mole of Cu is deposited by passing 2 moles of electrons.

So, the charge required to deposit 1 mole of Cu=2×96500=193000C

Now,

The actual charge passed through the electrode 5×60×60=18000

So, moles of Cu deposited =18000/193000

Thus, the mass of copper deposited =18000/193000×63.5=5.92 g

Hence, the answer is (5.92).

EXAMPLE.4

4. If hydrogen electrodes dipped in two solutions of pH = 3 and pH = 6 are connected by a salt bridge, the EMF_cell (in V) is:

1)0.052

2)0.067

3)0.277

4) (correct)0.177

Solution

For the given concentration cell:

H₂|H⁻(c₁)‖H⁺(c₂)|H₂Ecell =−0.059(pHc−pHa)=−0.059(3−6)=0.1773 V

Cell will be feasible only when [c₂]c>[c₁]a or pHc<pHa

Hence, the answer is the option (4).

Summary

The salt bridge is very important for the electrochemical cell, and it is a crucial component of the galvanic cell. It is used to maintain the electrical neutrality of the cell or inside the cell. By allowing the flow of ions between two half cells. This prevents the solution from becoming saturated or becoming too positively and too negatively charged if this can happen the flow of ions inside the cell will stop and the cell will stop functioning. A salt bridge connects the two half-cells and completes the circuit which is very important for the flow of electrons. Generally, a salt bridge is filled with a gel or a porous medium that contains an electrolyte usually potassium chloride and sodium sulfate. This electrolytic solution allows the ions to migrate between the two half-cells. As in cation moves toward the cathode anion moves toward the anode to maintain electrical neutrality.

The salt bridge has various applications in galvanic cells and batteries as the galvanic cell salt bridge enables the redox reaction to occur by maintaining charge balance, allowing the cell to generate the electrical neutrality for the spontaneous chemical reaction. In the batteries, these porous components play a vital role in balancing the charge and enhancing battery performance. Salt bridge prevents the charge built up to stop the redox reaction and ensures that the operation or response is continuous and effective. Salt bridges are also but very rarely used in the electrochemical cell where they help maintain the proper ionic movement needed for electrolysis.