pH problem

Mon Oct 5 08:05:13 EST 1992

> I am having problems adjusting the pH of various 1 M solutions (citrate,
> succinate etc.). My pH electrode, if left in these for a while (few minutes)
> seems to drift, in the sense that when I rinse it with distilled water, and put
> it back into my reference buffer, I get a lower pH value than I know it should
> be. This obviously doesn't bode well for accurate pH's in the above mentioned
> solutions! What exactly is the influence of ionic strength on pH? I also don't
> always get corresponding pH values with pH-paper. Can anyone tell me what is
> happening to the electrode?
> Regards,
> Pierre Janssens
> Zoology Dept.
> University of Cape Town

The influence of ionic strength on pH is fairly straight
forward.  pH is defined as -log activity hydrogen activity.
The relationahip between hydrogen ion activity and hydrogen
ion concentration can be calculated from the Debye Huckel equation,
although the approximation implicit in the equation begins to fall
apart at high ionic strengths, so be sure to use the extended form.

Drifting--If your solutions are not buffered and made from distilled
water, you could be picking up CO2.  Or else perhaps you have
a bad glass electrode.  Why it should not return to the
proper pH using a reference buffer suggests that.  Also check
the reference electrode.  Perhaps the porous plug is clogged.

Hope this helps

Jim Cassatt

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