- 6/5/2025
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LearningTranscript
00:01Hello everyone, welcome to this exciting lecture series on electrochemistry.
00:11In this video, we will study in detail about the function of salt bridge, its construction and its application.
00:20But before going into the study of salt bridge, let us go through the electrochemical cell.
00:26Basically, a salt bridge is used in electrochemical cells, so we have to study that what is an electrochemical cell.
00:36If you want to study about electrochemical cells, you can check out this video.
00:42Let us study about this electrochemical cell.
00:47First of all, we see that the cell has 4 main parts.
00:51There are two reservoirs where chemical reactions occur. These are the two reservoirs. One is this one and the other one is this one.
01:00A wire for the electron transfer between the reservoirs. Here we can see that there is an electro voltmeter.
01:07And these are the wires which allow for the electrons to pass from the anode to cathode.
01:13There is also a salt bridge that allows ionic species to pass. So fourth object is the ionic species.
01:22Okay. So these are the species are the main parts of the of an electrochemical cell.
01:29Each reservoir contains regions and an electrically conducting solid that act as a terminal and current collector.
01:37So this is a terminal and current collector which is usually called or known as the electrode and it is dipped in a region or an electrolyte.
01:50So both of these electrodes are dipped in some sort of solution.
01:55This solid is called an electrode as we already know that and may also chemically participate as a reactant or a catalyst.
02:02Okay. So this electrode can be inert or it can be an active electrode.
02:08If it is an active electrode, it can take part in the chemical reaction.
02:13But if the electrode is an inert electrode, it cannot take part in the chemical reaction.
02:21The combination of the regions and the current collector is called a half cell.
02:26So this electrode and the electrolytic solution, both of these is called a half cell.
02:34So basically electrochemical cells have two half cells and they both combine to make a fully functional electrochemical cell.
02:44The wire connecting the two half cells is the external circuit which often includes devices to measure the cell's electrochemical properties.
02:52So this was the just simple depiction of the electrochemical cell.
02:57So now we will study in detail about the salt bridge in the coming slides.
03:05So first of all, let us study about the electron transfer in separate locations.
03:11Let us consider a simple, this simple cell which don't have a salt bridge.
03:16We see that this one is the first half cell and this one is the second half cell.
03:22These two half cells are connected via a wire which provide electrical circuit.
03:28So we see that electron loss and gain can occur in different physical locations.
03:35This means that oxidation and reduction don't need to happen in the same beaker or container.
03:40As long as there is a way for the electrons or ions to move, a redox reaction can still proceed.
03:49In this example setup, silver metal ion is in contact with the silver ions, we can see from here, and it is also in contact with the nitrate ions.
03:59It is connected via a copper wire to a reservoir of sodium nitrate solution, which is the NaNO3 solution.
04:08We see that these are the two ions Na plus ion and Na3 minus ions.
04:13So we have two separate chemical environments connected via a conductor.
04:18But we see that despite this connection, no visible reaction occurs.
04:24The silver metal remains unchanged and the copper wire does not cause the water to turn blue.
04:32So nothing seems to be reacting in this setup.
04:37We see that this outcome is expected.
04:43For a redox reaction to actually proceed, the reactants must physically contact each other.
04:48Or there must be a complete circuit that allows both electrons and the ions movement.
04:56Without that ionic path like a salt bridge, the reaction cannot move forward even if the wire is in the place.
05:04So we need a salt bridge for the movement of ions.
05:07Next, we will look into the limitation of electron flow without an ion balance.
05:18We see that silver ions can be reduced in one location while the copper atoms are oxidized in another if a metal wire is allowed to pass the electron flow.
05:31This setup allows the reaction to begin and electrons move from copper wire to silver.
05:39We see that in this setup, the reaction starts but stops quickly.
05:44Even though electron flow is possible initially, something prevents it from continuing for so long.
05:49The reason is a charge imbalance that built up between the two solutions.
05:59On the silver side, as silver ions get electron and get reduced, more nitrate ions remain in the solution which causes the solution to becoming more negative.
06:13So this half cell becomes more and more negative.
06:16On the other side or the copper side, cupric ions which are the CO2 plus ions, they form from the copper oxidizes from the selector.
06:29They oxidize and they are converted into the CO2 plus ions.
06:33But there are no contour ions entering to balance the charge so that the side becomes positively charged.
06:40So we see that these sides have become positively charged and this side have become negatively charged.
06:50This actually leads to polarization in the metal wire.
06:56A positive charge bolts up at the copper end and a negative charge bolts at the silver end.
07:00These charges act like a built-in voltage opposing the flow of more electrons.
07:10Ok.
07:12So the positive charge bolts at the copper end and the negative charge bolts at the silver end.
07:16As a result, but we see that eventually the opposing charges prevent further electron flow.
07:28The system resists more current because of the imbalance of the charge accumulation.
07:33As a result, the reaction stops once the charge imbalance is large enough.
07:39Even though the redox pair is capable of reacting, without ion movement to maintain charge neutrality, electron flow cannot continue in this system.
07:48So, there is a need for an ion movement to maintain the electroneutrality.
08:01The anions which are the nitrate ions needed for the electroneutrality in the copper solution are actually present in the silver ion solution.
08:08But the problem is they cannot reach the copper side.
08:11The reaction stops because these anions cannot move between the reservoirs.
08:18Even though electron flow is allowed through the wire, there is no pathway for the ion migration, so the charge imbalance halts the redox process.
08:27The solution is that we need to modify the setup to allow ion movement between these two reservoirs.
08:34That is the only way to maintain charge balance while the electrons flow.
08:38One way is to allow nitrate ions to move from the silver to the copper wire, which helps balancing the positive charge building up where copper is oxidized.
08:52Another way is to allow copper ions to move in the opposite direction towards the silver side, which would help offset the growing negative charge there.
09:01Or, more effectively, we can allow both movements by introducing a salt bridge, typically a tube filled with neutral electrolytes like sodium nitrate solution.
09:14This creates a complete circuit for both electrons and the ions.
09:17And the ions.
09:22But, now we see the effect of adding a salt bridge.
09:28Here, we can see a different type of salt bridge.
09:31Or we can see just like a direct connection between these two electrolytes.
09:35We see that the nitrate ions can now move between the solutions, which help maintain electroneutrality on both sides.
09:45This solves the problem of charge buildup that previously stopped the reactions.
09:50Silver ions can also move and may eventually reach the copper reservoir, which adds complexity to the overall reaction process.
09:59Okay.
10:00So, the result is that, with the charge balance maintained, electron flow through the wire and oxidation and reduction occur at the separated location.
10:11Exactly what that we want in a working electrochemical cell.
10:16But, over time, silver ions diffuse and react directly with the copper metal.
10:23This unintended reaction means that two solutions begin to mix.
10:27When that happens, the experiment becomes uncontrolled and poorly reproducible.
10:34So, while the salt bridge allows the reactions to work, long-term mixing can still interfere with the clean results.
10:42So, now we will see the original or another depiction of the salt bridge.
10:47In this slide, let us now understand the purpose and function of a salt bridge, which is a crucial component in any electrochemical cell.
10:57We see that, a salt bridge allows ion exchange between these two reservoirs without transferring reactive species.
11:07So, we want only wanted ions to migrate, but do not want unwanted ions to migrate through the solutions.
11:14This means that, ions can move to balance charges, but they do not interfere with the redox chemistry.
11:21The main requirement of the salt bridge is to allow ion migration to maintain electroneutrality.
11:29Without this, reaction would stop due to charge built-up.
11:37But, at the same time, the salt bridge must prevent reactive ions, those involved in the electrode reactions, from migrating.
11:43This ensures the redox reactions occur only at the electrodes, not by mixing the solution directly.
11:55So, now, as we have understood the purpose and function of the salt bridge, we will now look into the actual picture of a salt bridge.
12:02So, this is a proper setup, which have the salt bridge between these two electrolytes.
12:10We will now study the design and composition of a salt bridge.
12:13In terms of design, a salt solution connects the two reservoirs in the electrochemical cell, acting as a medium for the ion exchange.
12:27The salt used must have ions that are not ex-reoxidized or reduced, so they do not interfere with the redox reactions happening in the electrolytes.
12:37Common choices include alkali metal salts, like potassium or sodium, with nitrate, perchlorate or halide anions.
12:47They are chemically stable and won't participate in unmounted side reactions.
12:52The ends of this salt bridge are plugged with porous material.
12:58This setup allows ion diffusion while inhibiting the bulk flows of the solution between the two reservoirs.
13:07So, it only allows the flow of the charges between these two reservoirs.
13:13It allows diffusion and inhibits the bulk flow of the solution.
13:18Also, the gel-filled bridges.
13:22These are often made with ajar or similar substances, which greatly reduce bulk motion.
13:28This provides better stability and control during the experiment, which makes the results more reliable and reproducible.
13:38Now, we will study about the function of salt bridge in completing the circuit.
13:49As we have studied earlier is that, the salt bridge enables movement of inner ions to maintain electroneutrality, which is crucial for the electrochemical reactions to continue.
14:01It also helps in the composition of the electrical circuit, making it possible for the electrons to continuously flow through the wire from one electrode to the other.
14:13So, the electron flow travels through the external circuit from the anode to the cathode.
14:27But for every electron that moves, a corresponding ion must move in the solution to prevent the charge built up.
14:33So, this ionic compensation happens via the salt bridge, which moves ions in the opposite direction of the electron flow to maintain the charge balance.
14:46If electrons are flowing from right to left, so the ions will move from left to right to maintain the electroneutrality.
14:54Anions move opposite to the direction of the electron flow, while the cations move in the same direction.
15:05This coordinated motion keeps the system electrically neutral and allows the redox reaction to proceed smoothly.
15:12So, anion will move opposite to the electron flow and the cations move in the same direction as the electron.
15:26Let us now look into the types of the salt bridges.
15:31First, the glass tube salt bridge.
15:33It is a U-shaped glass tube which is filled with an inert electrolyte like potassium chloride, sodium chloride or potassium nitrate.
15:44This electrolyte is often gelled with a jar-a-jar to help prevent mixing of the solution on either side.
15:51The conductivity of a glass tube salt bridge mainly depends on the concentration of the electrolyte inside it.
15:59The second type is the filter paper salt bridge.
16:04This uses porous paper soaked in an electrolytic solution.
16:10Its conductivity depends on several factors like electrolytic concentration, the porosity of the paper and how smooth or rough the paper surface is.
16:21Smooth absorbent filter paper is more conductive than the rough paper because it increases the ion movement.
16:30So, as we have understood the function and the basic structure of the salt bridge, let us now summarize our lecture for the salt bridge construction and its function.
16:46So, we see that a salt bridge is a low resistance device that establishes electrical connection between two electrolytes in an electrochemical cell.
16:58It overcomes the direct liquid-liquid junction which would otherwise cause unwanted mixing of the solution.
17:06Typically, the salt bridge consists of a glass tube which is U-shaped which is filled with potassium chloride and a jar-a-jar paste which sets into a gel.
17:19Alternatively, it can be made from the porous paper soaked in an electrolytic solution.
17:24The choice of the electrolyte in the gel depends on the nature of the electrolyte used in the cell, but these electrolytes must be chemically inert.
17:35Commonly used salt include potassium chloride, ammonium nitrate, potassium nitrate and potassium sulfate and some of the other ones, for example sodium chloride.
17:45Let us now summarize the importance of salt bridge from a little different angle.
17:56First we see that a liquid-liquid junction is a thermodynamically unstable state.
18:02When the two solutions or the two different electrolyte solutions come into contact with each other, they tend to intermix due to diffusion.
18:09This creates instability in the system because the mixing is uncontrolled and can affect the accuracy of the electrochemical measurements.
18:19You can study about the liquid-liquid junction from a separate video and in that video I have explained in detail the chemistry behind the liquid-liquid junction.
18:29The unequaled rates of the migration of the cation and anion across the liquid-liquid junction give rise to a potential difference across the junction.
18:40Not all ions move at the same speed, for example smaller ions or those with higher mobility might diffuse faster.
18:48This imbalance in the ionic movement creates a small voltage which we call a potential difference.
18:53This potential difference across the liquid-liquid junction is called liquid-liquid junction potential.
19:02Liquid junction potential is a non-equilibrium voltage that can distort the actual cell potential readings.
19:10It arises simply due to contact between two different solutions.
19:13So, a salt bridge eliminates direct contact between the two solutions and thus can minimize the liquid junction potential.
19:23By separating the solutions and providing a pathway for the ion movement through a neutral inert medium like KCL in the ajar,
19:32the salt bridge establishes the cell, it stabilizes the cell and ensures more accurate voltage reading in the cell.
19:41Now, at the end, let us now discuss some practical limitations of the salt bridge in the electrochemical cells.
19:52To maintain charge balance during the electron flow, the salt bridge must transfer an equivalent number of ions from one reservoir to another reservoir.
20:02However, ion transfer occurs only through the diffusion, which is inherently a slow process over the microscopic length of the salt bridge.
20:14Because of the slow motion and the slow diffusion, the rate of ion movement is limited, which in turn restricts the rate of electron flow and therefore limits the maximum current the cell can provide.
20:26Okay.
20:27As diffusion lags, a charge imbalance builds up at the ends of the salt bridge.
20:37So, at the both ends, a charge imbalance builds, which creates a potential gradient that opposes further the current flow through the cell.
20:45Additionally, so we see that due to the charge imbalance at the edges of the salt bridge, we can see a potential gradient which can further oppose the current movement from the external circuit or the overall capacity of the cell to produce electric current is reduced over time.
21:06Additionally, we see that in the gas phase, salt bridge stability can be limited due to mobility of protons, which may affect the bridge performance.
21:21So, that is all for today's lecture. Thank you very much.
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