what ion strength in pH calibration buffer?

William Tivol tivol at news.wadsworth.org
Tue Dec 24 13:36:41 EST 1996

Hans-Georg Klieber (klieber at mailer.uni-marburg.de) wrote:
: my current understanding of pH electrodes is that they
: measure hydrogen ion activity in the given solution.


: It should therefore not matter if the ion strength of the buffer
: solution that was used to calibrate the pH electrode 
: is different from the ion strength of the solution where the
: actual measurement is taken.

	Approximately correct, but there are small variations.

: The concentration/activity
: ratio for hydrogen (and all other ions) would of course be
: different in the two solutions - but the pH (=activity) reading
: should still be correct.

	pH electrodes measure a potential given by the Nernst equation
		E(pH) = Eo + ln(10)RTlog(H+)/nF
	The presence of other ions can modify the potential sensed by the
electrode, depending on the permeability of the electrode to the particular

: It has been suggested to me that this belief is not correct.
: Could some (expert)  please comment on this?

	I am not an expert, but the above are my 2 cents.
				Bill Tivol

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