>>    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
[log in to unmask]
*****