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.