Galvanic Cell

[nautica]

I am attempting to put together a galvanic cell with a voltage of 2.68 V

Here is what I used

KCl salt brigde

Cu2+ (aq) + 2e- = Cu(s) Red Potent = +0.34
used 1 M CuSO4

Mg2+ (aq) + 2e- = Mg(s) Red Potent = -2.37
used .1 M MgSO4

Nrtz eq E = E - 0.0257/2 (ln Q)

It gave me a voltage of 1.6 NOT 2.68, which is what it calculated out to be.

So I tried Z(s) with the Cu and it worked

So I switched the MgSO4 for MgCl and got 1.6 again.

My guess is something is reacting with the Cu2+ that has a potential of -1.26
(I used my fingers instead of the KCL and the numbers were the same.

Any ideas of what could be happening???

Thanks
Nautica

[nautica]

Anythoughts on this??? Chem tch?

[nautica]

:frown: :frown:

[nautica]

back up, for one more shot at knowlege.

thanks
nautica

[chem_tr]

The more pure your materials, the better solutions you'll get. This is what I understood from your experiment. If the experiment guide says that you should use KCl bridge, then use it, don't substitute it for another thing. Both your metals and your metal ion solutions should be as much pure as possible, then I think you will get the desired voltage.

[nautica]

The salt bridge did not seem to be the problem. I replaced it with my fingers and the voltage was the same.

Also, I replaced my solid Mg with Zinc (s) and the voltage worked out perfectly. This led me to believe that my Cu was good, my salt bridge was good, but my either my Mg (s) or my Mg solution was wrong. So, I replaced my solid with another solid - same results. Then I replaced my Mg ions with another solution and the results were once again the same.

This led me to believe that my Mg (s) was only acting as a wire and there must be another reaction happening. Something that is more reactive with my Cu. But, I can not find what it would be. I searched the all reduction potentials that would give me this voltage, but I found none that could have been possible with what I had in solution.

Any thoughts.

Thanks
Nautica

[chem_tr]

Did you clean the surface of magnesium? Sometimes, oxides may prevent conductivity to some degree, whereas zinc may be of sufficient purity to give what you want.

I am not sure, but please use potassium sulfate instead of potassium chloride, as common ions should be better for a galvanic cell.

To sum up, use emery to clean both your copper and especially magnesium metals to get rid of any impurities. But you're saying that your copper sample is sufficiently pure (this is okay, since copper is a semi-inert metal); as Mg is a reactive metal, you'd better clean the surface and if possible, use Mg wire to speed up the transfer.

[nautica]

I used two types of Mg solid and attempted to clean. I will clean once again and switch to KSO4, then see what happens.

Thanks for you time. I will let you know what happens.
Nautica

[Astronuc]

So I tried Z(s) with the Cu and it worked.

I presume you mean Zn.

Also, I replaced my solid Mg with Zinc (s) and the voltage worked out perfectly. This led me to believe that my Cu was good, my salt bridge was good, but my either my Mg (s) or my Mg solution was wrong. So, I replaced my solid with another solid - same results. Then I replaced my Mg ions with another solution and the results were once again the same.

Actually it sounds like the Mg / Mg solution is working fine.

But the Cu or more likely the Cu solution is contaminated with Zn, and that is why if you try a cell with Zn // Cu, it would give the right potential.

Mg^{2+} + 2e^- \rightarrow Mg, -2.37 V

Zn^{2+} + 2e^- \rightarrow Zn, -0.76 V

Cu^{2+} + 2e^- \rightarrow Cu, +0.34 V

Mg is preferably oxidized to Zn, and Zn is prefereably oxidized to Cu, so one should expect:

Mg + Zn^{2+} \rightarrow Mg^{2+} + Zn, 1.61 V

Mg + Cu^{2+} \rightarrow Mg^{2+} + Cu, 2.71 V

I suspect that Zn is in the Cu solution, such that in the same cell you are getting

Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu, 1.1 V, but this is a short circuit in the same cell (subtract 1.1 V from 2.71 V), so you are meauring the difference between Mg and Zn reactions in your galvanic cell.

Take the Cu solution and see if you can do a qualitative analysis to find Zn. Is the CuSO4 solution clean? Did you make it yourself? Zn is often an impurity with Cu, especially in recycled materials.

Would you be able to do an assay of the CuSO4 - preferably using ICP and emission spectroscopy?

Alternatively, test the Mg and Cu cells (with respective solutions) against a standard electrode, either H or calomel, and see what potentials develop.

If my potentials are a little off, its because I am using an older reference.
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Traditionally, a copper alloy which contains zinc is a "brass"; a copper alloy which contains tin (not exceeding 11%) is a "bronze". Bronze composition may vary significantly however, and contemporary bronzes are typically copper alloys which may contain silicon (Si), manganese (Mn), aluminum (Al), zinc (Zn) and other elements, with or without tin (Sn).

So be careful. In critical matters, always do your own independent check.