CO2 measurement in the presence non-carbonate buffers
To: Aquatic-Plants at actwin_com
Subject: CO2 measurement in the presence non-carbonate buffers
From: psears at emr_ca (Paul Sears)
Date: Thu, 23 Nov 1995 10:25:11 -0500 (EST)
In-Reply-To: <199511212039.PAA08521 at looney_actwin.com> from "Aquatic-Plants-Owner at actwin_com" at Nov 21, 95 03:39:02 pm
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.