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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>> *********************************
>