Re: what happens in a copper/zinc voltaic pile with salt water electrolyte

Hi Paddy--
Excellent question!  Your question is in an area where many get confused, 
and I'm glad to have this opportunity to clear things up for you and 
perhaps others.

You are entirely correct in your observations but fortunately there is a 
simple answer to the problem.  To get battery operation, you **must** 
have a source of Hydrogen ions (=protons) or electrons in your 
electrolyte.  A sulfuric acid solution does this very nicely, and you can 
get appreciable current out of your cell.  The voltage will be the 
difference in reduction potentials between the anode and the cathode.

With aqueous NaCl as the electrolyte, there is (to first order) no real 
source of protons or electrons present.  I know that you're measuring a 
voltage (we'll get to that in a second) but if you do the test, you'll 
find that the NaCl cell can supply virtually zero current (microamperes 
at best).  What current you do get is due to some residual 
protons/electrons arising from the residual protons/electrons in the 
water.  Remember that pure water has a pH of 7, which corresponds to a 
hydrogen ion concentration of 1E-7 moles/liter;  that low concentration 
allows for what little current you do observe.  

A sulfuric acid solution has a pH of about one, which corresponds to a 
Hydrogen ion concentration of 0.1 moles/liter--six orders of magnitude 
higher!  That's why you can get appreciable current from an H2SO4 
electrolyte wet cell.

With an aqueous NaCl electrolyte, as discussed above, there are virtually 
no protons or electrons present; the solution is not so much an 
electrolyte as a simple wet conductor of electricity.  As you correctly 
hypothesize, the voltage that you're measuring with an NaCl solution is 
just the difference in redox potentials between the anode and cathode 
metals;  it's exactly the same voltage as you would get if you welded the 
two metals together and measured the voltage between the two terminals.

Hope this helps!!!