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I am having problems understanding why we do need salt added to the water to make this shown galvanic cell work.

I understand that Zinc wants to dissolve into the electrolyte, resulting in $\ce{Zn^2+}$. Since there is $\ce{Cl^-}$ present, it will bond with the Zinc.

At the cathode, the $\ce{H_2O}$ will be electrolysed, leaving $\ce{OH^-}$ behind. I assume the $\ce{OH^-}$ will bond with $\ce{Na^+}$.

I assume this is the reason, why we need the electrolyte: If there would be no electrolyte, $\ce{Zn^2+}$ would remain close to the anode, while $\ce{OH^-}$ would remain close to the cathode, resulting in an electric field that counteracts against the redox-reaction. I think, adding NaCl results in negative Cl-ions moving towards the anode and positive Na-ions moving towards the cathode, resulting in a closed electric circuit, keeping the whole thing running. If we wouldn't add salt, the electric current should disappear very quickly.

Please let me know if my assumptions are correct. I wouldn't mind if you would explain the whole thing as easy and detailed as possible, since I have obviously not studied chemistry.

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Little update: I have just built that galvanic cell and let it sit there for 15min. Voltage was around 0.7V. Current was in the mA-range, drastically increasing when adding table salt.

As you can see, the Zinc-plate got some whitish residue on it. I am unable to scratch it off. Any idea what that is? Could be areas where Zinc-ions left the metal?

enter image description here

The copper plate also shows some change below the waterline, but I have no possible explanation why it did change.

enter image description here

I was also able to see that the current through the wire was decreasing steadily. After adding salt and stirring the water, it showed ~30mA. 15min later it showed ~3mA. When I was stirring the water again, the current was increasing again to ~30mA and was decreasing again thereafter.

I assume after some time cations accumulate around the anode and anions accumulate around the cathode, inhibiting further flow of electric charge.

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    $\begingroup$ Any process leading to cumulation of electric charge quickly forms strong coulombic forces stopping such a process. $\endgroup$
    – Poutnik
    Nov 15, 2021 at 11:11
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    $\begingroup$ The cations do not bond with the anions in solution and this is basically just a variant of the lemon ‘battery’. See here for another variant: chemistry.stackexchange.com/a/116181/79678. $\endgroup$
    – Ed V
    Nov 15, 2021 at 12:17
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    $\begingroup$ Imagine you managed to take from such an electrolyteless cell the current 250 mA for 1 minute. It would mean there is the negative charge 15 C near the anode and the same but positive charge near the cathode. In air, it would be enough for 1 lightning. Something tells me the voltage of such a cell would drop faster than a falcon at attack. $\endgroup$
    – Poutnik
    Nov 15, 2021 at 15:03
  • $\begingroup$ Without the electrolyte, the cell would be a poor reference cell, capable of providing only tiny current and power. Even trying to measure the open circuit voltage, with an ordinary digital multimeter (DMM), would yield poor results because of the $\pu{10 M\Omega}$ input impedance of the DMM. $\endgroup$
    – Ed V
    Nov 15, 2021 at 15:13
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    $\begingroup$ I ran the experiment a few minutes ago. My photos and results are at this temporary link. So using distilled water gave poor results, as expected: declining voltage and 11 micro amperes of short circuit current. Using saltwater gave about 0.8 V, jumpy, and low mA short circuit current. Better, but still pathetic, as expected for such a simple cell with no salt bridge, etc. $\endgroup$
    – Ed V
    Nov 15, 2021 at 18:02

2 Answers 2

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If you recall, the earliest Voltaic cell also consisted of copper and zinc with a piece of paper soaked in salt. For a cell, you do need an electrolyte to maintain charge balance and reduce the resistance of the solution. Pure water has a resistivity of 18,000,000 $\Omega$.cm (~ million ohms!). This is too high and current will be wasted or may not be generated at all. Consider a similar case, remove the two electrodes from the solutions and hold them in air. Does your meter still register a current? Hopefully not, because air has very high resistance too.

Whenever direct current passes through a solution, there must be a chemical decomposition (by definition of an electrolytic cell). Consequently, in your salt water battery, in a closed circuit, zinc should lose electrons and become zinc ions. Zinc ion needs a counter balancing charge which is kindly provided by the salt.

Simultaneously, there is no free cation in water which can be reduced at the cathode, so water has to sacrifice itself to the electrons. As a result of water reduction hydrogen is produced, along with hydroxyl ions. Now hydroxyl ions cannot stay alone, you need a counter ion, $\ce{Na+}$, to form $\ce{NaOH}$.

You can use a pH indicator to see how pH is locally changing near the electrode. There are beautiful demos with a nail rusting in salt water. Prof. Ed did experiments to demonstrate this pH idea using Zn and Cu sitting in salt water. It is clear that Zn side is almost neutral and Cu side has become alkaline (Photo courtesy Ed V).

Zinc

Copper

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    $\begingroup$ Updated showing the pH test strip on the copper cathode is deep blue: chemistry.meta.stackexchange.com/a/4757/79678. Took way under a minute to start showing blue. The photo was after about a minute, with short circuit condition. Maximum current was about 3 mA, but it dropped a great deal. Too bad this “salt water battery” gets its power, feeble as it is, from the zinc oxidation and not the sea water. And the deficiency of no salt bridge, so the salt water electrolyte has to be replaced continually. $\endgroup$
    – Ed V
    Nov 15, 2021 at 19:13
  • $\begingroup$ Dear M. Farooq, thank you, this explanation helped me a lot. So Cl-ions balance the charge of the Zn-ions close to the anode (but do not bond to ZnCl_2). And Na-ions will bond with OH-ions close to the cathode. Makes sense. It also makes sense that the current increases when I stirr the water after some time, since new Cl-ions can get close to the anode and new Na-ions can get close to the cathode. Now I have just one last question: Why does stirring the water close to the zinc-anode increases the current temporarily, while stirring the water close to the copper-cathode does not increase it? $\endgroup$ Nov 15, 2021 at 19:31
  • $\begingroup$ Sodium ions do not bond with hydroxide ions in aqueous solution! Please stop saying such things! All the ions are mobile in solution. Their local concentrations in this cell are not the same as their average concentrations. Stirring just averages the concentrations for a short time. You could always add a stirring mechanism. $\endgroup$
    – Ed V
    Nov 15, 2021 at 19:41
  • $\begingroup$ @Rainer_Zoufal, Yes, remember these are ions, all you need is a charge balance. For each Zn ion, you need two chloride ions. I have not investigated the salt water battery formally but oxygen is known to cause a current enhancement, and so does nitrogen. While you are stirring you might be increasing the amount of dissolved air. Continue stirring, do you see a steady increase in current? I do not know the theoretical reasons. $\endgroup$
    – AChem
    Nov 15, 2021 at 19:43
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    $\begingroup$ @Rainer_Zoufal Glad to be of a little help and it was fun to do the experiment! If either you or M. Farooq would have any use of the photos I posted, please download them: they are in a temporary location and I cannot leave them longer than a day or two. You can do what you want with them: I am not posting an answer and have already upvoted you and M. Farooq. $\endgroup$
    – Ed V
    Nov 15, 2021 at 22:28
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Pure Water, $\ce{H_2O}$, is a bad conductor of electricity. By adding $\ce{NaCl}$ to it, it becomes a good conductor of electricity and allows the electricity to pass through it. Water has only few ions in it that can move. $\ce{NaCl}$, when dissolved in water greatly increases the number of free-flowing ions in it, making salt water a good conductor of electricity.

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    $\begingroup$ Try to type $\ce{H2O}$ to get $\ce{H2O}$ // For better site experience, you can find useful these links: notation , expressions. ( Not to be applied to titles ) and upright vs italic $\endgroup$
    – Poutnik
    Nov 15, 2021 at 15:08
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    $\begingroup$ Please note that grave accents in Markdown are reserved for source_code. Chemical name isn't capitalized unless the first word in a sentence. Instead of operating in terms of good-bad scenarios, just use numbers (i.e. cited physical quantities for conductivities). Finally, "NaCl makes water gain charge" doesn't seem to make much sense. $\endgroup$
    – andselisk
    Nov 15, 2021 at 16:57
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    $\begingroup$ Sodium chloride does not make water gain charge: this is nonsense. Water is a poor electrolyte because it has few charge carriers, i.e., ions, that can move, i.e., constitute a current flow, in response to an applied electrical potential. Dissolving salt in the water greatly increases the concentration of mobile charge carriers, so the salt water is a good electrolyte. $\endgroup$
    – Ed V
    Nov 15, 2021 at 20:01

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