Measurement of concentrated chloride brines by water liquid-
vapour equilibration
Geoff KoehlerEnvironment Canada, University of SaskatchewanSaskatoon, SK
Brines
Analytical difficulties with composition methods
Must somehow remove dissolved salts
Distillation – can result in back-reaction to form HCl NaOH
Autosamplers
Increased memory effects
Slow equilibration times for CO2
Stable isotopic measurment of liquid water
Composition 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)
Activity Methods
• H2 equilibration• CO2 equilibration
=?
Only for pure water
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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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Can measure D, 18O activity in solutions using H20(l) – H2O(v) equilibration
OA-ICOS or WS-CRDS technology
Problems
decreased vapour pressure from decreased water activity with Brines Reference waters are pure water
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
aw = λwxw = p/p0
Water activity is the effective mole fraction of Water activity is defined as:
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
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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 H20-CO2 for oxygen and hot reduction for hydrogen
Plausible there might be differences between water-vapour equilibration and CO2-water equilibration or H2-water equilibration.
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