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Aqueous NaCl and overvoltage

Question asked by Shannon Seebeck on Jul 17, 2013
Latest reply on Jul 22, 2013 by Joseph Castellano

In lab we constructed an electrolytic cell using a U tube, an adapter, aligator clips, and graphite electrodes (pencil lead).

we added 3g NaCl to 150mL of deionized water and used this to fill the U tube.  At the cathode there was immediate evolution of gas (H2), while nothing happened at the anode for some time...eventually larger bubbles that were much slower to form appeared on the electrode.  The lab instructor told us that the reaction at the anode was the oxidation of H20.


The lab manual mentioned that overvoltage is something to be considered in the electrolysis of H20.  And from the documents I found it seemed that Cl2(g) not O2(g) should have been forming at the anode.  I emailed my professor to ask about what I had found and she asserted that it was in fact O2(g). So now the professor, the lab instructor, and the tutor in the science center all claim it was the oxidation of H2O that formed O2(g).  But no one can tell me why they have ruled Cl2(g) out as a possible product of the reaction.  We only tested pH with litmus paper and while the cathode side showed plenty of activity clearly OH- ions were present the Anode solution did not have much of an effect on the litmus paper.  


So ....How quickly might NaOH build up on the cathode side?  Is a U tube effective in keeping the OH- Ions from migrating over to the anode side?  Is it possible that both gasses were being released?  Does Cl- have a significant overvoltage with graphite electrodes? Does its overvoltage increase with increasing current like H2O's does?  Does concentration have a significant effect?


Thank you in advance for any help offered!