I don't understand how a salt bridge work exactly: what happens inside it and how it affect the cell's lifetime.

I have already read:

Salt Bridges/Porous Disks

Salinic bridges in voltaic cells

Difficulties understanding how salt bridge works

What I understand:

The salt bridge's goal is to maintain electro-neutrality in each of the half-cells so that the voltaic cell keeps working and it helps avoiding the mixing of the half-cells' ions in solutions. When the salt bridge is being used in the cell, its ions (anions and cations which constitute the salt bridge's electrolyte) are moving towards the half-cell that attracts them.

What I don't understand:

  1. Do the salt bridge's ions leave the salt bridge to go all the way within the hall-cells' solutions or do they stay at the border ? Is it the case in all bridges ?
  2. Do the half-cells' ions move inside the salt bridge or do they stay in the salt bridge at the border solution/salt-bridge ? Is it the case in all bridges ? And is it in the same proportions as the salt bridge's ions ?
  3. How do the ions move exactly in the salt-bridge ? It seems like the propagation in, let's say agar-agar, is much slower than the displacement of ions in the liquid solutions in the half-cells, is it not a problem for the whole cell ? Is it like a longitudinal wave, ions pushing others ? If so, are they two speeds of propagation (the wave speed and the ions speed) ?
  4. If ions are moving inside it, we can consider that there is a current in the salt bridge, correct ?
  5. Are they families of salt bridges depending on how they work ?
  6. Finally, can the salt bridge be exhausted before the half-cells (whether the solutions or the electrodes) ? It seems the salt bridge is quite small in basic cells examples compared to the sheer volumes of the solutions and sizes of the electrode, thus containing much fewer ions.

3 Answers 3

  1. Ions leave the salt bridge and move further, without accumulating at the bridge end. For all salt bridges where there is no other reactions involved, so all simple salts in solution or molten salts.
  2. Ions move all the way through. There is some concentration difference in salt bridge and main half cell volume. It is true for simple salt solutions and molten salts. Same ions move through half cell and salt bridge.
  3. The speed of a wave is on the order of 100 000 km/s. The speed of ions moving is on the order of 0.001 mm/s. You can estimate the ion speed from calculating amount of ions participating in a reaction per second and dimensions of the bridge and ion density in the solution or molten salt.
  4. Yes. But it is important to remember that there are two cancelling each other currents in the salt bridge. Thus magnetic field is close to zero.
  5. Most important is a number of ion types. If there are just two types, such as in solutions and molten salts, then this intuition applies. If more ion types are present, and especially if ions can convert between types, things will get more complex. Another option to consider is a fuel cells where only one type of ions passes through, and electrons use outside route to complete the loop. So, cases with one negative and one positive ion types are considered here. All other collections of ions will behave very differently.
  6. No. Ions are constantly being replenished by the bigger volume of the both half cells.

Electroneutrality strictly speaking is not conserved when the bigger volume than an atom is considered and precise measurments are available. There is a gradient of potential due to different concentration of ions. Concentration of ions is the reason for the effect you are interested in. Important parts to consider are electrodes, where ions react, thus dissapearing from what we calculate, and narrow areas, where ions' speed increase, such as in salt bridge.

  • $\begingroup$ Are you sure about #2 and #6 ? Salt bridges are prepared with $\ce{KNO3}$ or $\ce{KCl}$ or some inert salt like that, in a way that both cation and anion has similar mobility through the agar-agar. If the ions from solution could get into the salt bridge, there would be no need for all of that. $\endgroup$
    – S R Maiti
    Apr 19, 2021 at 9:18
  • $\begingroup$ @ShoubhikRMaiti I am sure. How else would conductivity work? Gel is beneficial because ions also move a lot of other material around, as they are pulled through the liquid. Without gel ions would grab a lot of neutral solution nearby. Ratio of ion movement and neutral matter movement can be as high as x1000, due to friction. $\endgroup$ Apr 19, 2021 at 9:26
  • $\begingroup$ @SurprisedSeagull, thank you for your excellent answer, it explains almost everything. However I have two small issues before validating the answer: (a) if the ions from one half-cell go all the way in the salt bridge, they may eventually end up in the other half-cell (11,5 days for a one-meter bridge at 1μm/s): is it not what we are trying to avoid with a salt bridge: the half-cells' ions producing unwanted reactions together ? (b) Also, could you edit your answer (I cannot yet) to use the Greek letter μ instead of u for the speed so it does not confuse people reading it later ? Thanks. $\endgroup$ Apr 20, 2021 at 7:28
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    $\begingroup$ Changed to mm, that I hope everyone knows. We would want to avoid side reactions, but we cant. We just accept that some contamination will inevitably occur. If you want to reduce contamination, you use inert electrodes. $\endgroup$ Apr 20, 2021 at 7:35

Here are the answers :

  1. The ions form the salt bridge must get out of this bridge. But they usually stays at the border, because they push other ions already present in the solution to compensate for the new charge having appeared on the electrode, like a locomotive or an engine pushes its trucks on the other end of the train. It is the same in all bridges.
  2. The same movement appears in the salt bridge. The "train" of similar ions are pushed as a whole through the bridge.
  3. There is a current of ions through the bridge, exactly like the current of electrons in the outer circuit.
  4. The bridge cannot be exhausted, because it gets the same number of ions from one electrode as what it looses on the other electrode.
  5. The bridge must be as small as possible, if the cell has to deliver a large current.
  • $\begingroup$ Thank you for your answer. I will however probably validate @Surprised Seagull's answer since it is a more exhaustive. $\endgroup$ Apr 20, 2021 at 7:31
  • $\begingroup$ If we have $\ce{ Zn/Cu}$ cell with $\ce{ ZnSO4/CuSO4}$ electrolyte and $\ce{ KNO3}$ salt bridge and we run the cell . 1- Do $\ce{ K^+}$ travel from the salt bridge to the solution around the cathode to neutralize $\ce{SO4^{2-}}$ 2- Do $\ce{ Zn^{2+}}$ travel from the solution around the anode to the solution around the cathode through the salt bridge 3- Do $\ce{ SO4^{2-}}$ enter the bridge salt? 4-Do $\ce{NO3-}$ migrate from salt bridge to the solution around the anode to neutralize $\ce{Zn^{2+}}$ $\endgroup$ Apr 21, 2021 at 18:15
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    $\begingroup$ In the $\ce{Zn/Cu}$ cell (made with sulfates of these metals), the cathode loses positive ions ($\ce{Cu^{2+}}$. As the number of negative ions are not modified, something must be made to compensate this excess of negative ions. First these ions $\ce{SO4^{2-}}$ may pass into the bridge was they are attracted by the excess of positive charges on the other side of the bridge. Second, some positive ions ($\ce{K+}$ from the bridge) may move into the cathode zone. Both phenomena are occurring simultaneously. It is difficult to state which is the most important. it is a question of ionic mobility. $\endgroup$
    – Maurice
    Apr 21, 2021 at 19:57
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    $\begingroup$ On the other hand, $\ce{Zn^{2+}}$ ions do enter the bridge, pusj^hing in front of them $\ce{K+}$ ions. So it is highly improbable that the $\ce{Zn^{2+}}$ ions can cross the whole bridge, at least in a reasonable amount of time. Then, as I told you before, $\ce{SO4^{2+}}$ do also enter the bridge. And finally, nitrate ions form the bridge do get out of the bridge to compensate for the excess amount of $\ce{Zn^{2+}}$ ions appearing in the anode region. $\endgroup$
    – Maurice
    Apr 21, 2021 at 20:06
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    $\begingroup$ Not exactly. The $\ce{SO4^{2-}}$ ions releasing the cathode zone and will enter the bridge. But here, they first push the nitrate ions which are already here. On the other side of the bridge, one or two nitrate ions will enter the anode zone. Later one, another sulfate ion make the same and push the precedent sulfate ion plus the other nitrate ions. After a very long time, all nitrate ions have been pushed into the anode region. And then, but only then, the sulfate ions will be able to enter the anode region. $\endgroup$
    – Maurice
    Apr 22, 2021 at 20:04

This is just about question number 6. The answer by Maurice does not address it, and the answer by Surprised Seagull is a bit short.

[OP] 6. Finally, can the salt bridge be exhausted before the half-cells (whether the solutions or the electrodes)?

Most salt bridges start out with a higher concentration of ions than that of the electrolytes in the half cells. This ensures efficient charge transfer, and minimizes electrolyte traveling through the salt bridge. Even if you run the cell for a long time, the concentration of ions will never go to zero; if the concentration of ions in the salt bridge is lower than in the half cells, diffusion will transport ions into the salt bridge. You could even start with a salt bridge that contains no ions, and you would still be able to detect a current.

[OP] It seems the salt bridge is quite small in basic cells examples compared to the sheer volumes of the solutions and sizes of the electrode, thus containing much fewer ions.

I don't think salt bridges are used in applications where you want a lot of charge transport (i.e. such as a battery). They are used to study electrode potentials, with very low currents sufficient for measuring a voltage.

Why is a large concentration of inert ions good to have in a salt bridge?

If the ions in the salt bridge are effective in transporting charge, you won't have a large electric potential at the salt bridge that would speed up the mixing of the two electrolyte solutions. Eventually, they will mix anyway (through diffusion). Once electrolytes start mixing, the redox reaction can happen directly (not sending electrons through the wire), so that will lower the voltage measured between electrodes. Having a high ion concentration in the salt bridge and having a gel-based salt bridge (to minimize convection mixing) slows down the process of redox-active ions making their way through the salt bridge.

  • $\begingroup$ "you won't have a large electric potential at the salt bridge that would speed up the mixing of the two electrolyte solutions" meaning that ;you will have a small electric potential at the salt bridge that would slow the speed of the mixing of the two electrolyte solutions" $\endgroup$ Apr 20, 2021 at 19:04
  • $\begingroup$ Explain" convection mixing" $\endgroup$ Apr 20, 2021 at 19:05
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    $\begingroup$ Convection is when an entire part of the solution moves, like it does in a stirred solution. Even without stirring, there might be temperature fluctuations resulting in density fluctuations that cause bulk flow that is faster than diffusion. The agar prevents this from happening. $\endgroup$
    – Karsten
    Apr 20, 2021 at 20:18
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    $\begingroup$ If a copper ion is supposed to be reduced at the copper electrode but travels to the zinc electrode via the salt bridge instead, it could react directly without electrons going through the wire. $\endgroup$
    – Karsten
    Apr 20, 2021 at 20:22
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    $\begingroup$ @AdnanAL-Amleh Yes, you are trying to minimize the gradient of net charge between the electrodes by making ion transport fast. If you remove the salt bridge alltogether, the redox reactions stops very quickly. $\endgroup$
    – Karsten
    Apr 20, 2021 at 21:44

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