Controlling pH of solution without changing its conductivity

[Urmi Roy]

Hi,
I need to conduct experiments at fixed pH. The complication is that the solution I'm using will have NaCl in it. So if I add HCl or NaOH to make the solution more acidic or basic, the conductivity of the solution (as measured by a conductivity meter) will change due to the added Na+ or Cl-.
So I'm wondering if there's a way to control pH without changing the ion concentrations. Would using a pH buffer be a good solution to this?

Thanks,
Urmi

 

[Borek]

Changing pH means changing the concentrations, there is no simple workaround.

You may be able to keep the conductivity constant by adding not only a buffer but also another inert salt and varying its amount. Tricky, and it still doesn't have to work the way you want it to, as you will be changing ionic strength of the solution.

But if you are planning on experiments at constant pH using a buffer, conductivity due to the buffer presence will be constant throughout the experiments. As long as it is not orders of magnitude higher than the signal you are looking for, you should be able to just subtract the background.

 

[Urmi Roy]

Changing pH means changing the concentrations, there is no simple workaround.

You may be able to keep the conductivity constant by adding not only a buffer but also another inert salt and varying its amount. Tricky, and it still doesn't have to work the way you want it to, as you will be changing ionic strength of the solution.

But if you are planning on experiments at constant pH using a buffer, conductivity due to the buffer presence will be constant throughout the experiments. As long as it is not orders of magnitude higher than the signal you are looking for, you should be able to just subtract the background.

Thanks for the reply.I find that the commercially available buffer solutions have multiple constituents so calculating it's contribution to the conductivity (in order to subsequently be able to subtract it out) is not trivial.
So now I'm thinking of just adding NaOH or HCl, which dissociate fully. Using a pH meter I could test for the excess number of OH-/H+ ions.Then using a conductivity meter I could test for the total contribution of NaCl and NaOH (or NaCl and HCl). From the link below [1] (table 9.4), it seems that the correction factor from conductivity to concentration for HCl and NaOH are linearly dependent on temperature. These correction factors would allow me to calculate the contribution of the base/acid to the conductivity and so I could calculate concentration of NaCl.

Please let me know if I'm thinking of this in the wrong way.

[1] http://msdssearch.dow.com/Published...seps/pdfs/noreg/609-02127.pdf&fromPage=GetDoc

 

[epenguin]

Hard to answer without knowing exactly what you are trying to do. But in most cases e.g. if this is biophysical, wild and uncontrolled variations of pH are likely to affect your system much more than small variations of conductivity! In many cases a constant buffer concentration 10 or 20% of your total salt concentration would satisfy your needs.