OK Stable pure water will be difficult to maintain but what ionic level will be?

Ah, now we are getting somewhere! All you want is a constant conductivity. You need to think about just how constant you want it (defluff!). Reasonably pure water might have a conductivity of about (say) 1-10 Mohm cm or 0.1-1 microS/cm. If you are happy with fluff at a level of 0.1%, you need to add enough electrolyte to make a solution having 1-10 milliS/cm. You can look up tables of equivalent conductance to work out the required concentration for a given electrolyte. Since I presume you are not going to electrolyse this solution (which would change the conductivity), NaCl is OK. KOH is NOT; it would absorb CO2 from the air and the conductance would drift.

I see that I misspelled "quartz" in my previous posting. Very embarrassing, so sorry.

Hi Dieter

My latest work involves establishing my experimental parameter space. Thus I need to remove the atmosphere and then re-introduce it one gas/substance at a time. So flush out my chamber with argon and run my tests contaminated with "pure water" (the combs with several with AC and several with DC), then repeat with CO2 then N2 then O2 etc... For the Dc I will use a galvanostat/potentiostat and for the AC I will use an impedance spectrometer. So you can see why I need the pure water as I am trying to simulate the effect each of the atmospheric condition has on water. Water being the most likely vehicle of contamination for electrical good (not forgetting beer :-]).

On another thread there was a discussion on overpotential(s). If I have two copper electrode and I need to find the overvoltage then won't they cancel each other out or do they add up so Cu2+(aq) + 2e- -> Cu(s) = 0.34 so is it

Cheers

Wayne

I don't expect O2 or N2 to affect conductance, but CO2, yes.

They are two different processes occuring at a given applied cell voltage, the anodic reaction being dissolution of Cu, with its own overpotential. They will be additive though.