Measurement of concentrated
chloride brines by water liquidvapour equilibration
Geoff Koehler
Environment Canada, University of Saskatchewan
Saskatoon, SK
Brines
Analytical difficulties with
composition methods
Must somehow remove
dissolved salts
Distillation – can result in backreaction to form HCl NaOH
Autosamplers
Increased memory effects
Slow equilibration times for CO2
Stable isotopic measurment of liquid
water
Composition methods
Activity Methods
•TCEA(H,O)
•Zn reduction (H)
•U reduction (H)
•Mn reduction (H)
•Cr reduction(H)
•Laser liquid injectors(H,O)
•AquaSIRA(H,O)
•BrF5(O)
•Guadadine hydrochloride(O)
•LiAl(OH)4 (H)
•H2 equilibration
•CO2 equilibration
=?
Only for pure water
Isotopic Salt Effect
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Henry Taube
•born Neudorf (pop 281), Saskatchewan
•B.Sc. 1935, M.Sc. 1937 - UofS.
•Berkeley, DOD, Chicago, Stanford
•Nobel prize for Chemistry, 1983 for his work
on the mechanisms of electron transfer
reactions, especially in metal complexes
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•Taube was studying coordination chemistry of
hydration spheres
•Discovered the isotope salt effect by equilibrating
CO2 with various ionic solutions
•Found that the 18O/16O of CO2 equilibrated with
solutions is different than that of pure water, and
that the difference varies linearly with
concentration.
•Varies with cation, but not anion
•Structure makers, breakers
Frank and Wen, 1957
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G = R(soln)/R(H2O)
1000lnG ~ d(soln) – d(H2O)
Few data for hydrogen salt effect
Involved in studies that require measuring porewater
in drill cores – is there a salt effect?
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OA-ICOS or WS-CRDS technology
Can measure D, 18O activity in solutions
using H20(l) – H2O(v) equilibration
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General method
Similar to measurement of liquid waters
24-48 hour equilibration times – w(l) – w(v) exchange is rapid
Normalization by determination of known reference waters
4-5 minute integration times
Precisions of about 2 per mil H, 0.3 per mil O
Problems
decreased vapour pressure from decreased water activity with Brines
Reference waters are pure water
Water activity is the effective mole fraction of
Water activity is defined as:
aw = λwxw = p/p0
Laser spectrometers measure directly water activity
Often have a significant isotope ratio dependence on vapour pressure in the laser cavity
Must be corrected for
With very concentrated solutions (ie 5m MgCl2), can be 10 per mil more enriched
Can measure non-linearity with a variable reference source (WVISS) or with
NaCl solutions
Measured G values
Oxygen salt effect
Hydrogen salt effect
Implication #1 – Measurement of Brines
Conclusions
Isotopic measurement of brines probematic at best
Equilibration methods yeild activity ratios
Reduction or distillation yeild concentration ratios
For years, dD and d18O measuments done by H20CO2 for oxygen and hot reduction for hydrogen
Plausible there might be differences between watervapour equilibration and CO2-water equilibration or H2water equilibration.
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