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Date: | Tue, 11 Nov 1997 12:16:14 -0700 |
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>> Hi, Everybody: When I use hydrochloric gas to acidify high pH water
>>samples for oxygen isotope ratio analysis using CO2-H2O equilibration
>>method, I found the Delta O-18 of the water shifts to lighter values as
>>the pH goes lower. Any explanation? Dachun Zhang
Dear Dachun,
Maybe the O isotope fractionation between H3O+ and H2O is responsible?
Looking in Friedman and O'Neil (1977), Table 4, #8, the O isotope
fractionation between H3O+ and H2O is given as +22.8 per mil [actually the
reaction is given as H2O+1 - H2O(l), but I assume this should be H3O+. The
original reference, Thornton (1962), is given]. If you are acidifying to
sufficiently low pH, the concentration of H3O+ may be high enough to affect
your measured delta18O values. Moreover, it will shift them to lighter
values (as you observe), as H3O+ is isotopically heavy vs. H2O(l). I hope
this helps.
Cheers,
Simon
*****
Simon R. Poulson
Dept. of Geology & Geophysics
University of Wyoming
Laramie, WY 82071-3006
USA
(307) 766-2885
Fax: (307) 766-6679
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*****
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