Copper Wire as a Salt Bridge: The Mystery of its Function in Voltaic Cells

[Wellsi]

in a recent chemistry experiment at school, two voltaic (galvanic) cells were set up with the same solutions in either beaker (one being Copper Sulfate, and the other was another metal solution) anyway, one had a potassium nitrate salt bridge, the other a copper wire salt bridge...
sooooooooooo how come the one with the copper wire as the salt bridge still worked and produced a current when the ions cannot flow in between the solutions?
the teacher was unable to answer this and apparently the school has emailed a university to ask and even they could not provide an answer does anyone have any idea?

 

[Gokul43201]

Can you please describe the situation completely?

1. What are the cells made of (electrodes, electrolytes, bridges)?
2. What were the observations (cell EMF, lifetime, other)?

 

[Wellsi]

Ok here you go -
Cells - CuSO4 and ZnSO4 (both cells)
Electrodes - Cu plate and a Zn plate (both cells)
Bridges - KNO3 soaked paper for the first cell
- Length of copper wire for second cell dipped into either solution

EMF - voltage was less than 1V for both cells (there was about 75ml of 1M solution in both beakers
lifetime - both cells were running for about 5 mintues - there was still a current in both when we stopped them

 

[Gokul43201]

Interesting! How were the electrolytes separated? Were they in different beakers?

 

[Wellsi]

yes different beakers about 5cm apart

 

[Wellsi]

does anyone have an answer or is this something that is going to have to go on without anyone knowing?

 

[Gokul43201]

Do you know what the current draw, or series effective resistance was?

If the average current was only of order a milliamp, then you've only had to make a micromole of ions (small compared to 75 millimoles). This might easily be some non-steady state behaviour that you could expect to see for several tens/hundreds of minutes before saturation became noticeable.

Here's a guess:

You might have what is nothing but a pair of galvanic cells connected in series:
Cell1 = Zn(plate) | Zn(2+) || H(+) | H2, Cu(wire)
Cell2 = Cu(wire) | Cu(2+) || Cu(2+) | Cu(plate)

Clearly, cell2 has a std EMF of 0V, but cell1 has a standard EMF of 0.76V. The series voltage is the sum of the two real EMFs (the second one being 0V so long as the copper in the electrode and wire had similar purity) and can be calculated from the Nernst equation (I get something like 0.96V).

 

[Wellsi]

the two galvanic cells were separated completely... what you said there is stuff we haven't convered yet so makes no sense can you reword that or make it "year 12" style for me? sorry

 

[GCT]

One of my guesses on what's happening in this situation pertains to the formation of some type of a junction potential...try the experiment again with the implementation of an adequate stirring mechanism.

Try measuring the mass of the "Copper salt bridge" before and after the experiment, be sure to have it dried adequately for the post-experimental weighing.

Also, weigh the Copper and Zinc electrode before and after the experiment.

Why did you use the Copper wire as a salt bridge in the first place?

 

[Wellsi]

we used the copper wire because it was an experiment from a textbook. I think it was designed to show us that the transfer of ions needs to happen in order for the cell to function properly and complete the circuit... apparently not?

 

[GCT]

Was the peculiar result observed for your experiment only, or was this the case with the other students as well?

 

[Wellsi]

it was only done once for the whole class... i hope you don't mind i gave ur explanation above to my chemistry teacher to see what she made of it so ill ask her tomorrow...
we could repeat the experiment again and i can get you some more results if you like? i would certainly be interested

 

[GCT]

...try the experiment again, and measure and record the EMFs at 30 second intervals, for a total time of 5 minutes. Afterwards to mock trial where you don't record anything, after 3 minutes take the "Copper salt bridge" out and assess the trend in EMF.

Measure the masses of the Copper and Zinc electrodes as well as the Copper wire that was used as a salt bridge before and after the experiment, wash each of them with distilled water afterwards and either air dry or oven dry the three wires.

Another thing that you can do is to setup a Galvanic cell with a Copper electrode wire at one end and the Copper wire you used as a salt bridge at the other, assess the initial trend in EMF. Place the Copper electrode into the Copper Sulfate solution which was employed in the first experiment and use a fresh 75 mL 1 M CuSO4 solution to immerse the "Copper salt bridge."

Make sure that the "Copper salt bridge" is not connected to anything else then the two beakers.

 

[Wellsi]

alright, ill see my chemistry teacher today and hopefully do the prac again and send u my results

 

[Wellsi]

ok, we are doing chemistry next period and my teacher wants to try the experiment again so ill post results for you guys :)
my teacher is quite intruiged with it too, and thanks for the responses they have helped immensely

 

[CronoSaturn]

Hi, I'm also in year 12 and did the same experiment and when we hooked the cell up to the voltmeter we had a voltage of 0.76, the experiment was conducted fastidiously and according to my teacher the exact same thing has been happening for years now and he's never ( been bothered or ) been able to find an answer. I'll see if we can try out the copper cell which was suggested as being the component producing charge. Thankyou for your help!

 

[DrDu]

I think Gokul43201 explanation is most plausible: hydronium ions get reduced to hydrogen at the copper wire in the Zn half element and copper ions get oxidized to elementary copper on the in the Cu half element.
The over-voltage (don't know whether this is the correct translation of german "Ueberspannung") is lower on Cu than at Zn for hydrogen formation. So the reaction would be a combination of a galvanic element and an electrolysis.

 

[SpectraCat]

I agree with Dr. Du, but I would suggest trying to test it. If it really is acting as a pair of cells in series, then the potential should depend on the material used for the bridge. If you use a non-redox active conductor, such as carbon foil (or cloth or paper), that should shut down the 2nd cell, so there is no voltage. You could even use two solid carbon electrodes connected by a wire, provided that the wire doesn't come into contact with the solutions.

It would also be interesting to observe the effects of using wires made of other metals.

 

[DrDu]

In fact, if Gokuls idea were true, the copper cell should not modify the voltage (but the current due to it's resistance). So you could try whether hanging a copper wire into the Zn half cell is sufficient to produce a current.

 

[SpectraCat]

In fact, if Gokuls idea were true, the copper cell should not modify the voltage (but the current due to it's resistance). So you could try whether hanging a copper wire into the Zn half cell is sufficient to produce a current.

No, that should not work .. Zinc lies above Copper in the activity series. However, hanging a zinc wire into the copper half cell should result in a spontaneous reaction. I know for sure that reaction goes in acidic solution, but it should go in neutral solution as well based on the free energy change.

[EDIT] Just to be completely clear ... if you acidify the solution in the zinc half-cell, then you will see a reaction of the zinc metal anyway .. zinc dissolves via redox in acidic solution, with the evolution of H2. However I don't think the process would go in neutral solution simply from adding copper metal, unless there is something special about the reduction of H+ on copper that I am not considering.

 

[DrDu]

But the copper wire is already in the Zn solution anyway. I just meant to leave away the beaker with the Cu solution and measure the current between the Zn electrode and the copper "brigde" of course, you have to make sure that the copper wire doesn t touch the Zn electrode.
Btw. pure Zn doesn t dissolve also in somewhat acidic solution. It is traces of Cu and the like in Zn which have a lower overvoltage than Zn which are responsible for the hydrogen production.

 

[SpectraCat]

But the copper wire is already in the Zn solution anyway. I just meant to leave away the beaker with the Cu solution and measure the current between the Zn electrode and the copper "brigde" of course, you have to make sure that the copper wire doesn t touch the Zn electrode.

What would be the source of such a current?

Btw. pure Zn doesn t dissolve also in somewhat acidic solution. It is traces of Cu and the like in Zn which have a lower overvoltage than Zn which are responsible for the hydrogen production.

Are you quite sure? Because I have done experiments where gram quantities of zinc were dissolved in excess HCl, with vigorous evolution of gas. I had assumed it was H2 based on the activity series. What is the alternative explanation?

 

[Borek]

Are you quite sure? Because I have done experiments where gram quantities of zinc were dissolved in excess HCl, with vigorous evolution of gas. I had assumed it was H2 based on the activity series. What is the alternative explanation?

It is all a matter of purity from what I know. Zn usually contains some small amounts of impurities that speed up the Zn/H⁺. If it is pure enough, dissolution is slow - but then it is enough to add some small amount of copper solution to get copper reduced on the zinc surface and speed up the reaction.

 

[Vanadium]

I doubt a high school class is using Analytic Grade reagents. The zinc is likely technical grade or even 95%. Likewise the copper bridge may be home wiring, etc. (<99% pure)

I agree that it would be enlightening to measure each half-cell, relative to the copper bridge (ideally with the other beaker entirely removed, to show if the copper wire is acting as a pure bridge or is a reactant in a galvanic battery.

 

[Ecclesiastic]

Is it possible the Copper Wire has some form of PVC shielding causing the bridge?