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Author Topic: A 10 euro (overrated) copper concentration cell experiment  (Read 4788 times)

Offline sorincosofret

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A 10 euro (overrated) copper concentration cell experiment


A copper concentration cell is device  that plates out copper from a concentrated Cu2+(aq) solution and dissolves copper from an electrode into a relatively dilute solution of copper ion. The purpose of this experiment is to measure the mass transfer in case of a concentration cell.

1 M copper sulfate solution,
0.01 M copper sulfate solution
two PVC containers (fig.1 in section), with copper electrodes, passing taps  and the possibility of connection through a  membrane filter or a thicker layer.

In the experiment the containers was 2 liter volume and the filter membrane used was a nylon 0,22 micrometer pore size. One container was filled with 1 M solution and the other with a 0.01M of CuSO4. Other barrier layer are foreseen to be used in the future (maybe someone suggests one as described in actual scientific texts able to permit a unidirectional circulation of a ionic species !!!). The taps of system were open and the potential was measured in order to check the actual prediction (Nernst equation) and the system was leaved for a week in order to equilibrate the concentrations in both compartments.
After this time both electrodes were weighed with a precision balance (analytical balance) and used again for another week, filling the containers with fresh solutions. At the end of second week again both electrodes were again weighed and the procedure of forming a new concentration cell was repeated. In few days the 3rd cycle will be finished, but the conclusions are clear at this stage.
During these 2 weeks of concentrations equilibration there was no transfer of copper electrodes mass from one to another. Both electrodes remain at the same weight as before experiment.  In the experiment were measured both electrodes in order to avoid some experimental errors. When for example copper is transported from solution (or from another electrode) to electrode, there is no a really metallic deposition and this deposit can remain in solution at electrode extraction. Therefore both electrode were weighed, in order to have a correlation between an eventually decrease of a electrode mass and increase of another.

How is the experimental reality fitted to the actual theory?

It is known from thermodynamic that the process
solute (concentrated) → solute (dilute)
is accompanied by a fall in free energy.
A concentration cell use this energy and some chemical processes take place according to:
cathode: Cu2+(1 M) + 2e– → Cu(s)
anode: Cu(s) → Cu2+(.01 M) + 2e–
The net reaction regard a transfer of copper mass from anode (diluted container) to cathode (concentrated container).
What is the theoretical mass of transported copper?
If anode generates a Cu cation, another sulfate anion must pass the membrane in order to have the neutrality of solution and simoultaneously a copper atom is deposited on the cathod. The final concentration of both solutions is 0.5M (the initial concentration of anode compartment is neglected).
 In order to have a 0.5 M sulphate concentration in anode compartment, the quantity of copper released in solution must be equal with 1 mole, which means 63.5 g.
On the other hand the mass of cathode should appear increased with 63.5 g.
Even in a case of a fluffy deposited copper, it is impossible to not observe this quantity at solution change.
The experimental reality is completely different. Both electrodes at concentration equilibration appears clean and without any visible modification; the mass of electrodes remain constant with a precision lower then 0.1 mg.   
The up presented calculus was made in considering the concentration cell working with a yield of 100 %. Of course there is a Fick diffusion between compartments which diminishes the copper transfer.
Even in case of a concentration cell working with a yield of 0.1 % (which is absurd because the cells are recognized to have a better yield then another thermodynamic processes), at a single concentration equilibration should appear a mass transfer of 0.1*63.5 =6,35 g.
In fact even in case of a yield of 0.001 and two cycle of concentration equilibration using the same electrodes, there should appear a mass transfer equal with: 0.001*65.3*2= 0.13 g, which is more then enough to be detected.
Other experiments will be posted soon focused on the concept of salt bridge. To date, quite all scientific texts avoid presenting a description of a concentration cell with a salt bridge and the transfer of ions in solution in this case.

The mass transfer and what’s happened at interface metal solution can be very easy verified with isotopes techniques.
Having a metal electrode in a solution that contains ions of that metal, a potential difference between the metal and the solution appears according to actual interpretation due to the following equilibrium :
M reversible arrow Mn+ + ne-

When the metal strip contain only one isotope and the solution of its salt contain another isotope after a period of time there will be a process of isotopic change between metal and solution. Consequently is very easy for example to use a radioisotope in the metal strip and non radioisotope for salt solution and after a time to measure the isotopic exchange (the lows of isotopic exchange are well known).
Again I put here only the idea, without any prediction or interpretation. Who is interested in the problem will find the whole material

« Last Edit: 08/08/2008 07:54:13 by sorincosofret »


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