Sciencemadness Discussion Board

Why does my Mg-Cu galvanic cell produce the wrong voltage?

SplendidAcylation - 29-11-2023 at 02:35

Hi,

Despite my absence lately, I have been doing chemistry, although nothing really interesting enough to post, until now;

I seem to have encountered a mystery that I am as yet unable to solve satisfactorily!


Having spent a long time learning about the theory of galvanic cells and the Nernst equation, I decided to make a simple galvanic cell and adjust the electrolyte concentrations to see if the results aligned with the values given by the Nernst equation.

I do not have a lot of metals and salts of their corresponding cations, so I was limited in which metals to use, however I did have magnesium sulfate, copper sulfate, and magnesium ribbon and copper foil, so an Mg-Cu cell seemed like a fun idea.

Magnesium cations have a standard reduction potential of -2.37v, and copper is +0.337v, so such a cell, constructed under standard conditions, should generate 2.7v.

I prepared 10mL each of a 0.1M solution of magnesium sulfate and copper sulfate (since the concentrations are equal, the reaction quotient is still 1, so the potentials should not differ from the standard potentials, and the Nernst equation is not needed).

I sat the beakers side-by-side, placed the corresponding electrodes in (both the magnesium and copper were cleaned with sandpaper until they were shiny).

For the salt-bridge, I just placed a dry piece of filter paper between the two and allowed each solution to soak into the paper and meet in the middle.

The voltage I obtained from this cell was 1.7v, a whole volt lower than the calculated value!

I also observed that simply touching the electrodes with each hand was sufficient to cause a voltage drop through the path from one hand to the other, which indicated that my cell was only able to output a feeble current.
It therefore occurred to me that perhaps the cell's resistance was causing a voltage drop somewhere even with the small load presented by the voltmeter.

So I tried a higher impedance voltmeter, but I still obtained 1.7v.
My cell was able to light an LED, but very dimly.

Nonetheless, due to the unsatisfactory current output, I decided to upgrade my ion-bridge.

I made an agar ion-bridge, following this procedure:

http://www.sciencemadness.org/talk/viewthread.php?tid=2707#p...

I used potassium nitrate instead.

This worked amazingly well, and, when inserted into my galvanic cell, a much larger current was able to be supplied, my cell could now illuminate the LED quite brightly, and the resistance between my hands was not able to cause any change in the voltage... However the voltage was still 1.7v!

I proceeded to partially remove the electrodes from the solution, and I found that, even when the electrodes were almost completely removed from the solutions, the voltage did not drop at all, so this eliminated electrode surface area as a possible source of the issue.


It was necessary at this stage to try a different anode half-cell, however I did not have any zinc metal or zinc salts lying around, and I couldn't think of much else suitable.

Luckily I had some old dry-cell batteries, so I disassembled one of those, recovered some zinc, and proceeded to dissolve some in hydrochloric acid.
I then evaporated off the excess hydrochloric acid, yielding crude zinc chloride; I prepared a solution of approximately 0.1M concentration thereof, and used a piece of zinc from the battery as the electrode.

With this Zn-Cu cell, I obtained a voltage of 1.05v, satisfactorily close to the theoretical 1.1v!

So the problem appears to lie with magnesium in particular.

With some further research, I have found that at least four other people have made Mg-Cu galvanic cells and found the voltage to be 1.7v rather than 2.7v too, including our very own Sulaiman!:

A SMALL SCALE BATTERY USING SMALL-SCALE Mg / Cu GALVANIC CELL - Mohammad Ali Amayreh (Journal of Technology, Vol. 37, No. 2, pp. 103-109 (2022))

Why are potentials of cells with magnesium electrode always lower than expected? - Stack Exchange

Sulaiman's Mg-Cu cell


So clearly there is something very strange going on here!

You will notice that the measured value is exactly 1.0v lower than the theoretical voltage, so it is tempting to imagine that perhaps the IUPAC simply made a typo with the magnesium reduction potential, but of course this is surely nonsense, because we would expect the reduction potential for magnesium to be in the region of 2.37v; 1.37v would simply be too low for a metal of high reactivity...

I am at a loss to explain this, because, as far as I know, I have fairly accurately replicated the half-cell that would be used when obtaining these standard electrode potential values, so I don't know where the error is creeping in.

Obviously the standard hydrogen electrode would be used in the cathode half-reaction when measuring these values, but I don't see why the cathode half-cell would influence the anode half-cell in any particular way.

So, to summarize the things I have tried:

  1. Changing the ion-bridge
  2. Cleaning the electrodes
  3. Trying two different voltmeters
  4. Using a zinc half-cell to confirm that there is nothing wrong with my copper half-cell
  5. Varying the electrode surface area




I have seen some mention of overpotentials as a potential issue (pun not intended!), and, although I don't yet fully understand the concept, I think this only applies when we try to draw some significant current from the cell, and shouldn't influence the no-load voltage.



Anyway, there is not much more to say as yet, thanks for reading all this, and I look forward to hearing any suggestions as to what is going on here!

^_^

clearly_not_atara - 29-11-2023 at 07:33

I believe you are observing the poisoning of the cell by the electrolyte — water! Normally water will only sustain a voltage window of around 1.3 volts.

But the reduction potential for the rxn
2 H2O + 2 e- <> H2 + 2 OH-
will be dependent on the concentration of hydroxide and in fact lower in strongly alkaline solution. Hence, the window gets wider due to Mg raising the pH to about 9.5.

SplendidAcylation - 30-11-2023 at 02:24

Quote: Originally posted by clearly_not_atara  
I believe you are observing the poisoning of the cell by the electrolyte — water! Normally water will only sustain a voltage window of around 1.3 volts.

But the reduction potential for the rxn
2 H2O + 2 e- <> H2 + 2 OH-
will be dependent on the concentration of hydroxide and in fact lower in strongly alkaline solution. Hence, the window gets wider due to Mg raising the pH to about 9.5.




Thanks for the reply :)

I hadn't even thought of this!


So how do they obtain these standard reduction potentials, if not using water as the solvent?


I tried making an Mg-Zn cell, and I obtained a voltage of only 0.83v.
If water can sustain a voltage window of 1.3v, and the theoretical cell potential for a Mg-Zn cell is around 1.61v, shouldn't I obtain a voltage much higher than 0.83v for this cell?

semiconductive - 30-11-2023 at 09:14

The table you are looking at is for 'standard' conditions.
The concentration partially determines when water will break down.

It may be possible to sustain 2.7V in water. It just can't be under standard conditions.
I'm not sure of what conditions are necessary for Mg & Cu.

I have observed a secondary cell, charging in ultra concentrated acid, surviving 2.8 volts without hydrolysis. I don't understand chemistry, well enough, to explain why.
I have never observed Mg withstanding 2.7V, but I haven't experimented with it enough to know.


Maurice VD 37 - 26-12-2023 at 09:38

This problem has puzzled me for years. How was the standard potential for Mg measured ?

It seems to be to same problem for Ca, K, Li and Na. Their IUPAC standard potentials are respectively : -2.76 V, -2.92 V, -3.04 V, -2.71 V. How were they determined ? It is impossible to consider an electrode made of potassium or sodium metal in water. They would immediately disappear by reaction with water. Does anybody know how the standard potentials fo Ca, K, Na, Li and Mg have been determined ?

[Edited on 26-12-2023 by Maurice VD 37]

RU_KLO - 23-1-2024 at 09:22

short Answer: buy a electrochemical measurement system

"The electrochemical characterization was performed on an electrochemical measurement system (CHI660E, China) using a standard three-electrode system comprising the graphite rod as the counter electrode, the saturated calomel electrode as the reference electrode...."

"Electrochemical behavior of Mg electrode in sodium salt electrolyte system"
https://www.frontiersin.org/articles/10.3389/fchem.2022.9924...).
"Electrochemical behavior of Mg electrode in sodium salt electrolyte system"


RU_KLO - 23-1-2024 at 09:29

he standard electrode potential of Mg2+ | Mg can be measured with respect to the standard hydrogen electrode, represented by
Pt(s), H2(g) (1 atm) | H+(aq)(1 M).

A cell, consisting of Mg | MgSO4 (aq 1 M) as the anode and the standard hydrogen electrode as the cathode, is set up.
Then, the emf of the cell is measured and this measured emf is the standard electrode potential of the magnesium electrode.

Now, do you have a standard hydrogen electrode?
maybe this can help you, but you need platinum.

https://www.youtube.com/watch?v=g6YEu_S3d0U

also found:
"Standard electrode potentials and temperature coefficient in water..."
https://srd.nist.gov/JPCRD/jpcrd355.pdf

maybe also gold or graphite can be used, but results maybe are different.
From wiki:

The sensing electrode acts as a platform for electron transfer to or from the reference half cell; it is typically made of platinum, although gold and graphite can be used as well. The reference half cell consists of a redox standard of known potential. The standard hydrogen electrode (SHE) is the reference from which all standard redox potentials are determined, and has been assigned an arbitrary half cell potential of 0.0 V. However, it is fragile and impractical for routine laboratory use. Therefore, other more stable reference electrodes such as silver chloride and saturated calomel (SCE) are commonly used because of their more reliable performance.

https://en.wikipedia.org/wiki/Reduction_potential

and

Three Electrode System
The three electrode system is made up of the working electrode, reference electrode, and the auxiliary electrode. The three electrode system is important in voltammetry. All three of these electrodes serve a unique roll in the three electrode system. A reference electrode refers to an electrode that has an established electrode potential. In an electrochemical cell, the reference electrode can be used as a half cell. When the reference electrode acts as a half cell, the other half cell's electrode potential can be discovered. An auxiliary electrode is an electrode makes sure that current does not pass through the reference cell. It makes sure the current is equal to that of the working electrode's current. The working electrode is the electrode that transports electrons to and from the substances that are present. Some examples of reference cells include:

Calomel electrode: This reference electrode consists of a mercury and mercury-chloride molecules. This electrode can be relatively easier to make and maintain compared to the SHE. It is composed of a solid paste of Hg2Cl2 and liquid elemental mercury attached to a rod that is immersed in a saturated KCl solution. It is necessary to have the solution saturated because this allows for the activity to be fixed by the potassium chloride and the voltage to be lower and closer to the SHE. This saturated solution allows for the exchange of chlorine ions to take place. All this is usually placed inside a tube that has a porous salt bridge to allow the electrons to flow back through and complete the circuit.

12Hg2Cl2(s)+e−⇌Hg(l)+Cl−(aq)

Silver-Silver Chloride electrode: An electrode of this sort precipitates a salt in the solution that participates in the electrode reaction. This electrode consists, of solid silver and its precipitated salt AgCl. This a widely used reference electrode because it is inexpensive and not as toxic as the Calomel electrode that contains mercury. A Silver-Silver Chloride electrode is made by taking a wire of solid silver and coding it in AgCl. Then it is placed in a tube of KCl and AgCl solution. This allows ions to be formed (and the opposite) as electrons flow in and out of the electrode system.

AgCl(s)+e−⇌Ag+(aq)+Cl−(aq)


https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Electrodes/Standard_Hydrogen _Electrode#:~:text=A%20Standard%20Hydrogen%20Electrode%20(SHE,different%20electrodes%20or%20different%20concentrations.


[Edited on 23-1-2024 by RU_KLO]

Sachingare - 25-1-2024 at 07:50

This sounds like one of those age old textbook Experiments which have been copy pasted everywhere for decades without anyone ever actually trying it for real

Well, generally the potential is very much dependant on the pH (acidic or basic) and it might not be possible in water at all as the people above have stated. There's also a huge influence of complexing agents on the potential, as is the case with the hydration enthalpy.

Have you done the lemon battery experiment with copper and magnesium? I'm curious what voltage that would give you. Probably not the one you're looking for, though....

Afaik, the potential for alkaline metals can be measured in a water free molten salt electrolyte.
So you could try to figure out a low temperature salt melt or look for a diy ionic liquid recipe and see if that's gives you a different result.
Or you could try solder flux paste as electrolyte substitute (could work or be complete bogus)


I'm a bit out of chemistry practice, so take everything with a grain of salt

Sachingare - 29-1-2024 at 07:40

Here's a open access paper on the whole topic, you might find it useful

Attachment: schmidt-rohr-2018-how-batteries-store-and-release-energy-explaining-basic-electrochemistry.pdf (2.4MB)
This file has been downloaded 188 times

Johanson - 29-1-2024 at 19:49

Out of curiosity I just set up simple Mg-Cu and Zn-Cu cells using parts laying around my workbench, and I'm getting the same open circuit voltages you are, about 1.7V and 1V respectively. It doesn't matter how much electrode surface area is submerged, it doesn't seem to change the OC volts. From what I can read, we aren't really getting a clean oxidation of Mg to Mg+2 at the anode, due to complex chemistry going on at that electrode surface, so the optimum reduction potential of -2.4V isn't realized. Apparently since Mg is so reactive the water itself attacks the anode creating a pretty complex, multi-layer, film right at the surface. There are various suggestions for overcoming this, but none are very fun or easy. Your comment about achieving only "feeble" current flow is understandable; commercially produced batteries have very large surface area, and the active material is pressed under about a bajillion psi. That's the only way they get a full amp out of a small cell. If you get even 20 or 30 mA for a few minutes out of a garage cell, that's fairly typical, so don't let that discourage you.