Dear Zhang,

In case of the  high pH and high salinity waters the most of the CO2 will be
liquid phase. I hope you inject same amount of CO2 in all the types of samples.
Try to measure the amount of CO2 after extraction in different types of water
samples. You will find the extracted amount of CO2 will be very less in case of
high pH and high salinity waters. Even you can try to do some theoretical
calculations for the carbonic speciation in such systems.

Thus in my openion you need extra precautions to analyse highly pure or highly
saline water samples.

I hope it answers your quest.

Best regards

Mahendra


Zhang, Dachun escribis:

> Dear Mahendra:
>
> We routinely acidify water samples if their pH are 7 or higher to facilitate
> the CO2-water equilibration. High pH water will give erratic numbers. We
> also distill water with high salinity to get better results. This seems to
> be contrary to your experience. Please advise.
>
> Dachun Zhang
> Global Geochemistry Corp.
>
> -----Original Message-----
> From: Mahendra P. Verma <[log in to unmask]>
> To: [log in to unmask] <[log in to unmask]>
> Date: Friday, April 09, 1999 10:00 AM
> Subject: Re: slow equilibration
>
> >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?
> >>
> >> ********************************
> >> Steve Nelson
> >> Dept. of Geology
> >> S389 ESC
> >> Brigham Young University
> >> Provo. Utah  84602
> >>
> >> voice:  1-801-378-8688
> >> lab:    1-801-378-5124
> >> FAX:    1-801-378-8143
> >>
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> >> *********************************
> >