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We know that pure water does not conduct electricity, but salt water is a decent conductor. This is commonly explained by saying that “the ions carry the current through the solution”, which is an incomplete explanation, because it does not indicate what happens when all of the ions have migrated to the electrodes.

More complete explanations of conduction through a salty solution (like this one or this one) rationalize the conduction of electricity in terms of a reduction reaction that takes place at the anode and an oxidation reaction that takes place at the cathode. In the case of salt water, chlorine gas ($\ce{Cl2}$) is formed at the anode and hydrogen gas ($\ce{H2}$) is formed at the cathode.

This explanation seems reasonable, but it implies that the conduction of electricity through a solution is fundamentally different than the conduction of electricity through a wire. A copper wire is (usually) unchanged, even after a large amount of electricity passes through it. In contrast, when electricity passes through salt water, two chemical reactions occur (one at each electrode), which fundamentally change the composition material.

This implies that it is not possible for an aqueous solution to conduct electricity forever. Since we are driving a chemical reaction, we are either consuming our salt (forming $\ce{Cl2}$ gas in the case of an NaCl solution or plating it onto the electrodes in other cases), or we are consuming the water by forming $\ce{H2}$ or $\ce{O2}$ gas.

This is surprising to me. So, I’m asking if my thinking is correct: is it possible for an aqueous salt-containing solution to conduct electricity forever, or it will it always eventually consume the reactants and stop conducting electricity as I have surmised?

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  • $\begingroup$ No, unless you add more ions, or reverse the reaction for some time(rechargeable batteries). $\endgroup$ – user80551 Dec 30 '13 at 18:17
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    $\begingroup$ "Conduct" is a dangerously ambiguous term here. How much of your argument applies if the current is AC not DC? $\endgroup$ – matt_black Dec 30 '13 at 21:38
  • $\begingroup$ Good question Matt! I would expect that the usual 60 Hz AC current effectively acts like a DC current on the relevant timescales for the chemistry that takes place, but I am not sure about this. $\endgroup$ – DanHickstein Jan 2 '14 at 16:30
  • $\begingroup$ Pure water does conduct electricity due to presence of equilibrium hydrogen and hydroxide ions. It is a very (very) small current though. $\endgroup$ – khaverim Aug 15 '15 at 15:17
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Since nobody has posted a counter example, I will go ahead and say that my claim is true for direct current (which is what I originally had in mind). The current drives a chemical reaction and, at some point, the reactants will run out and the solution will no longer have enough ions to effectively conduct electricity.

Hopefully someone will post a counter-example and make things more interesting :)

In the case of alternating current, it seems possible to plate material onto one electrode and remove it from the other. When the current switches direction, this process would reverse and the electrodes would remain the same mass. In theory, this could go forever, but in reality I wonder if the electrodes would deteriorate and break due to the many cycles of deposition and erosion.

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It can be possible, but it depends on the experimental conditions/set up. I think the following setup could work “forever”:

Two electrodes made of $\ce{Pt}$ in solution of $\ce{CuSO4}$ and $\ce{H2SO4}$ provided with AC source, the bath must be sealed – to avoid evaporation, applying reasonable voltage.

The copper is plating onto one electrode and depleting it to the other electrode.

The reduction: $\ce{Cu^2+ + 2e- -> Cu}$

The oxidation: $\ce{Cu $-$ 2e- -> Cu^2+}$

The electrodes made form $\ce{Pt}$ will avoid change shape of the electrodes during the endless process. The additional $\ce{H2SO4}$ makes the electrolyte more conductive.

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  • $\begingroup$ So, what reactions are taking place at the electrodes? And how do they reverse to return ions to the solution? $\endgroup$ – DanHickstein Jan 3 '14 at 14:59
  • $\begingroup$ CuSO$_4$ $\Leftrightarrow$ Cu$^+$$^2$ + SO$^-$$^2$$_4$ on both electrodes $\endgroup$ – Jaroslav Kotowski Jan 3 '14 at 15:03
  • $\begingroup$ That reaction doesn't consume or produce electrons, it just splits CuSO4. So, I don't understand how electrons would be drawn from or returned to an electrode. In the case of salt water, we are taking two Cl- and producing Cl2 gas and 2 electrons at one electrode and taking two electrons from the other electrode to turn 2H+ into H2 gas. $\endgroup$ – DanHickstein Jan 3 '14 at 16:08
  • $\begingroup$ Since the soulutin contains CuSO$_4$, one reaction will be reduction: Cu$^2$$^+$ + 2e$^-$ $\Rightarrow$ $Cu$ and simultaneously (since the soulutin contains Cu$^2$$^+$) oxidation: $Cu$ - 2e$^-$ $\Rightarrow$ Cu$^2$$^+$ $\endgroup$ – Jaroslav Kotowski Jan 3 '14 at 16:37
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    $\begingroup$ So, the reaction is basically plating copper onto one electrode and depleting it from another electrode. For DC, one electrode would be consumed and the reaction would stop. But, for AC, the electrodes would alternate depositing and removing materials and end up staying the same size. $\endgroup$ – DanHickstein Jan 3 '14 at 16:41
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The plain answer to your question is "no" because the electric current going through the solution would break the $\ce{H2O}$ bonds and form $\ce{H2}$ and $\ce{O2}$ gas. Eventually you run out of water due to this process. And don't forget, in a $\ce{H2O + NaCl}$ solution, chlorine gas is produced which would eventually affect ion concentrations. I believe that Wikipedia has a very detailed description of the electrolysis of water.

And by the way a really simple answer to your question would be to carry out the experiment yourself and see whether it conducts electricity forever. Take a glass of water add sodium chloride, take a 9 volt battery connect two wires one at the anode and the other at the cathode and take two pencils sharpened on both ends (graphite is a good conductor of electricity and plus you don't want the wires to be"eaten") connect each of the wires to one end of each of the pencils. Then put the end of the pencils (the ones without the wires ) into the glass and see it happen for yourself.

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  • $\begingroup$ I think that Jeroslav demonstrated that the answer is actually "yes" for AC, at least in theory. Nobody here has proposed an example of a solution that can conduct forever for DC. But, I am hesitant to say that it is not possible just because we cannot think of an example. Your proposed experiment will produce gas and demonstrate that a particular solution cannot conduct forever, but it does not prove this generally. $\endgroup$ – DanHickstein Aug 19 '15 at 16:49
  • $\begingroup$ Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. I have updated your post with chemistry markup. If you want to know more, please have a look here and here. Please do not use markup in the title field, see here for details. $\endgroup$ – Martin - マーチン Sep 3 '15 at 5:06

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