CO2 measurement in the presence non-carbonate buffers

	The CO2 measurement kits of which I am aware work by titrating
the CO2 (H2CO3, in effect) against a strong base, using phenolphthalein
as an indicator.  The end point of this titration will be at about pH 9.
	This means that it will measure H2CO3 changing to HCO3-, but
also any acid with a pKa much below 9, i.e., just about any acid.
	Buffers are mixtures of weak acids with their salts (both
components are necessary), so a solution that is buffered with a
non-carbonate buffer will contain weak acid other than the H2CO3 you
are trying to measure.  That acid will be measured by the titration,
and will give a false high reading for the CO2.

Paul Sears       Ottawa, Canada.