Dear Steve,

We had the same experience. An tentative explanation for it could be find in
the chemistry of carbonic system. When the water sample is ultra-pure, the pH
decreases drastically. It could be in the range 3-5 or lower. Under this
situation the most of the CO2 will be in gas phase. In my openion the CO2
(inclduing all the carbonic species) in water gets first in equilibrium with
the water sample and then there is an exchange of CO2 between water and vapor
phase. Therefore if there is less CO2 in water, it takes longer to reach the
whole system in equilibrium.

To avoid this problem, one can add some salts (dry) like NaCl, Na2SO4, etc. to
the water samples. We haven't work on this problem in detail. If you do some
experiments with adding salts, I appreciate your considerations to inform us
your results.

Regards

Mahendra


Steve Nelson escribis:

> I have a quick question for you folks who analyze a lot of waters. Have any
> of you experienced slow equilibration kinetics between water and CO2 when
> using ultra-pure water?
>
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