Measurement of concentrated chloride brines by water liquid-vapour equilibration

1. Measurement of concentrated chloride brines by water liquid-vapour equilibration Geoff Koehler Environment Canada, University of Saskatchewan Saskatoon, SK

2. 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

3. 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

4. Isotopic Salt Effect

5. 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

6. 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

7. 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?

8. OA-ICOS or WS-CRDS technology Can measure D, 18O activity in solutions using H20(l) – H2O(v) equilibration

9. 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

10. 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

11. Measured G values

12. Oxygen salt effect

13. Hydrogen salt effect

14. Implication #1 – Measurement of Brines

15. 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.