A salt bridge (Ion bridge) is a device that connects oxidation and reduction half cells of a galvanic cell. The two half cells of a galvanic cell remain neutral as a result.
The galvanic cells are made up of two half cells, oxidation and reduction half cells. As the electrochemical reaction proceeds, charges start to build up at anodes and cathodes as a consequence of charge transfer.
|Electrochemical cells: Their types and functions|
|Daniel's (Cu-Zn) cell|
|Junction potential in electrochemistry|
Types of Salt bridges
Mainly, salt bridges exist in two forms.
1. Glass tube bridge
Glass tube bridges are U-shaped bridges commonly filled with potassium, ammonium, chloride, and nitrate ions combination. The filled-in materials are specifically chosen for being unreactive to the chemicals used in the cell. Moreover, migratory speed and relative molecular weight are also taken into consideration for making a suitable (glass tube) salt bridge.
The concentration of electrolyte in the salt bridge increases the conductance with it but up to a certain limit. After a specific limit, the increase in concentration renders a decrease in conductivity.
The width of the salt bridge is directly proportional to the conductance, so wider salt bridges are better conductors.
2. Filter paper bridge
A soaked porous filter paper may serve the purpose of a filter paper bridge as suggested by the name. They are the most commonly used salt bridges.
The conductivity of paper bridges depends upon the concentrations of electrolytes in cells and the absorbance capacity of the paper. Usually, more absorbance corresponds to better conductance.
Porous disks or partitions are generally used as salt bridges between two half cells preventing the two from intermixing.
Preparation of Salt bridges
To make a glass tube bridge, a glass tube is heated and bent on a burner to shape like U. Agar and bridge material such as sodium sulphate or potassium chloride is filled into it. The length of the glass tube bridge is adjustable according to the apparatus but its width is generally 0.86mm.
A filter paper bridge is easier to make as it needs porous paper or a string soaked into the electrolyte. This paper is then placed with each end in each half cell. A tissue paper, a cotton piece, a woolen cloth piece, a string, or a general filter paper may even serve the purpose.
Salt bridge in (Zn-Cu) zinc-copper galvanic (Daniel’s) cell
It is a typical galvanic cell that contains two metal pieces (zinc and copper) immersed in their corresponding salt solutions of considerable concentrations. Let us assume that we took zinc sulphate (ZnSO4) and copper sulphate (CuSO4) in two separate beakers, with concentrations of 1 molar.
Being soluble salts, [Zn+2][SO4-2] and [Cu+2][SO4-2] are present upon complete dissolution. When these solutions are subjected to connect to each other, they start oxidation and reduction processes based on electrode potential (electrochemical series).
Zn+2(aq) + 2e -> Zn(s) ( Eo = -0.763 V )
Cu+2(aq) + 2e -> Cu(s) ( Eo = +0.337 V )
The resultant voltage given out by zinc-copper cells is 1.10 V.
Metal atoms oxidize to form metal ions donating electrons to the circuit.
Having less electrode potential, zinc metal will oxidize to form zinc ions donating two electrons to the other half cell. This makes more and more [Zn+2] ions in the solution making it more and more positive with the reaction.
Metallic ions gain electrons coming from the anode end and plate out as metal atoms.
Having more electrode potential, copper ions will reduce to form copper metal accepting two electrons from the other half cell. This leaves [SO4-2] ions in the solution making it more and more negative as the reaction progresses.
This whole process does not require any source or support to build up a positive charge at the anode and a negative charge at the cathode.
The whole cell (Necessity of a salt bridge)
Over time, this accumulation of charged species on electrodes increases drastically depending on the electrode potentials. This built-up charged system of electrodes is not favorable for electrochemical cells to keep running, they eventually cease to play.
In order to keep electrochemical (galvanic) cells functional, or charge generating, both half cells need to be kept somewhat neutral. It means that the charge accumulation is totally undesirable here. Salt bridges come into play here.
Function of a salt bridge
The purpose of using salt bridges in electrochemical cells is to introduce spectator ions into the solutions. These spectator ions preserve the neutral nature of solutions enabling the electrochemical cell to keep running.
Significance of a Salt bridge
- Maintenance of electrical neutrality within a cell is the main significance of a salt bridge.
- It connects the two half cells as a path alternative to the main electrical pathway.
- It prevents liquid-liquid junctional potential to establish, thus keeping electrical neutrality.
- It prevents intermixing of two half-cell species even if it is just a disk or a separating plate.
- If it wasn’t for salt bridges, the two half cells would get charged and the charge flow would have stopped.
What happens if no salt bridge is used in a galvanic cell?
Salt bridges are required in a galvanic cell to keep the electrode potential established and to keep the electrical conductance, plating, etc going on. If there is no salt bridge, the cell will halt and the electrical process will cease gradually.
What does salt bridge mean?
In biochemistry, a salt bridge means the electrical link between acidic and basic groups, especially on the protein structures.
In electrochemistry, a salt bridge is a conducting electrolyte-filled tube or paper which connects two half cells of an electrochemical cell system.
What is a salt bridge?
A salt bridge is either a tube filled with electrolytes or just a piece of paper connecting two separate electrochemical systems in order to keep them neutral.
What is a salt bridge in proteins?
Bonds between oppositely charged residues very close to each other experiencing electrostatic attraction serve as salt bridges in proteins.
How does the salt bridge work?
A salt bridge works by keeping the two electrochemical systems separated from each other and by keeping them in electrical equilibrium with each other.
What is a salt bridge in DNA?
Molecular entities and hydrogen bonds between hydrogen and nitrogen, oxygen atoms form phosphate groups serve as salt bridges for histones of DNA.
What are the three functions of a salt bridge?
- Maintenance of junctional potential (Electrical neutrality).
- Maintenance of normal electron flow from the electrical pathway as a consequence of point no 1.
- Prevention of half cell mixtures from intermixing (in case of plates and sheets shaped salt bridges).
What would happen without a salt bridge?
Without a salt bridge, an electrochemical (galvanic) cell will halt as the electrical (electron) flow will stop. Moreover, a junction-potential will be established between two half cells, making them no longer able to transfer electrons to each other.
What will happen if the salt bridge is removed?
If a salt bridge is removed from an electrochemical cell, the electrochemical process will halt, or the voltage will drop to zero.
What is the importance of a salt bridge?
- Maintenance of electrical neutrality.
- Connection of the two half cells by an alternative path.
- Prevention of liquid-liquid junctional potential to establish.
- Prevention of mixing of two half-cell species.
- If there is no salt bridge, the two half cells would get charged and the charge flow would have stopped, making the potential come to zero.
What ions to use in a salt bridge?
Potassium, ammonium, chloride, nitrate, sulphate, or sodium ions may be used as salt bridge medium.
What are “salt bridges” in the context of tertiary structure?
The interaction of amino acids with oppositely charged species provided that the atoms lie at hydrogen bonding distance serves as a salt bridge in tertiary structures.
What moves in a salt bridge?
Negatively charged species especially chloride ion moves in a salt bridge particularly.
What exactly flows through porous disks or salt bridges?
Positive and negatively charged species move along a salt bridge in order to neutralize any junctional potential formed.
Why is sodium chloride not a good salt bridge?
The ions of salt bridges are supposed to have similar migration speeds, molecular weights, ionic sizes, and much more. In case of NaCl, individual atoms are not having these similarities which makes sodium chloride a bad salt bridge.
Why is KCl used for salt bridges?
The approximately same ionic sizes, similar molecular weights, almost the same number of electrons, and considerably matching migration speeds make KCl a good salt bridge material.
How does the salt bridge in Daniel’s cell work?
A salt bridge in Daniel’s cell keeps the electrochemical equilibrium established by migrating ions to nullify or neutralize the developing junction potential. It keeps Daniel’s cell working and lasts longer.
How does a salt bridge maintain the liquid junction potential?
A salt bridge maintains a liquid-liquid junction potential by neutralizing any developing charge on half cells. It does so by moving ions to the corresponding cells so that system remains at electrical equilibrium.
Does a salt bridge increase or decrease conductivity?
Generally, an increase in conductivity is seen by using a salt bridge. It is because;
- A salt bridge offers less resistance than a gel.
- Electrical equilibrium established by salt bridge leads to more conductance to fulfill an electrochemical process.
Why is a salt bridge not used in an electrolytic cell?
There is no need for a salt bridge in electrolytic cells because it has one type of ionic solution only which contains both anode and cathode. There is no need to separate different types of electrolytes here because there is only one type present.
Can any type of material be used in a salt bridge?
A glass tube filled with an electrolytic solution and agar, a string, a soaked filter paper, tissue paper, and many other types of materials can be used to make salt bridges.
Is HCl used to make a salt bridge?
Any ionic substance can work as a salt bridge material. HCl is commonly used for hydrogen ions because it is easier and faster to move.
Do electrons actually flow in a salt bridge?
Electrons never move through salt bridges. It is the electrical pathway usually containing a voltmeter through which electrons pass. Salt bridges just pass ionic species and that’s it.
Why is saturated KNO3 used in the salt bridge?
The mobility or velocities of (K+) and (NO3–) are nearly the same, along with their electron numbers and molecular weights. This makes potassium nitrate (KNO3) a good salt bridge material. It saturated solution is used to make salt bridges because concentration increases the conductivity of a salt bridge.