Dear Alice, you may use Triethanolamine (TEA; http://staff.ustc.edu.cn/~liuyz/methods/buffer.htm). I am a soil scientist and used TEA sveral times to assess amount of cations sorbed on soil particles at pH 7 or 8.2 - buffered by TEA - i.e. Potential Cation Exchnange Capacity. However, you cannot avoid completely that EC is affected by using this organic buffer, thus, you need to include control experiments to adjust results on buffer impact. Nils

You are lucky you chose a pH range near 7 because conductivity is fundamentally related to the number of ions present.  Indeed in the absence of a small amount of a salt, measuring pH near 7 can be inaccurate.   pH instruments need a specific amount of ions to measure a current.   100% resistance equal no current and therefore no pH.

at pH 7   10^-7 moles of  H+ and 10^-7 moles  OH-  so  very little current.  At pH 6, H+ is present at 10^-6 level so ten times the number of ions.  

So if one added 10^-7 molar  Na+Cl-,  the current at pH 7 would double, but the same amount of Na+Cl- at pH 6 would be a minor component in the current.

Thus if one wants to run constant current experiments, one needs a salt concentration at least an order of magnitude higher than the maximum H+ or OH- concentration.

P.S.   CO2 itself is a buffer forming carbonate in di-ionized water which will acidify the water slightly depending upon its concentration.

I suggest not to buffer the water you use for preparation of your solution, but first to dissolve your salts and then to bring the solution to the desired pH. For example, if you dissolve KCl and your solution is still too acidic, add KOH, or too alkaline, add HCl.

I concur.   Peptides for example are actually all buffers.