Sébastien Sauvé Department of Chemistry Université de Montréal sebastien.sauve@umontreal

1. Metal speciation using ion-selective electrodes Sébastien SauvéDepartment of ChemistryUniversité de Montréalsebastien.sauve@umontreal.ca

2. Ion selective electrodes • Prejudiced against • Often, presumed unreliable • Very easy to use • Give a simple, direct measurement of free ionic activity • Commercial combined electrodes can be used with as little as ~5 mL of solution sample • Cheap

3. Avdeef et al. 1983

4. Prejudice • Too often, confusion over the speciation vs. concentration comparisons, i.e., not accounting for complexation • The « limit of detection » in dilute salts given around 10-7 M is close the background concentration expected in clean solutions (resulting in a standard addition type of plateau)

5. Cupric Ion-Selective Electrodes • Linear, Nernstian response down to pCu2+ of: • 7 in dilute copper salts solutions (60 µg·L-1) • 19 using solutions copper-buffered with ligands of known stability constants (10-19 M or 60 ag·L-1) • Simple equipment • Extensive literature

6. Cupric Ion-Selective Electrodes • Interferences • Ionic strength variations • Need a relatively uniform IS • Aluminum • Mercury • Chloride • Electrode surface is sensitive

7. Cupric Electrode Calibration • Suggested Cu-IDA calibration solutions have: • 1·10-3M IDA • 1·10-4M Cu(NO3)2 • 6·10-3M NaOH • 2.5·10-3M KHphthalate • 1·10-2M CaCl2 (media) • pH adjusted with HNO3 • Use IDA stability constants reported in the literature, interpolated to 0.02 ionic strength

8. Calibration Simultaneously determine the pH for calculations of pCu2+

9. Calibration …

10. Electrode Calibration • I considered the electrode to be equilibrated when the potential stays within the same 0.3 mV range for 3 min • (Very slow equilibration time — about two hours in the lowest activity samples) • Calibration and samples are analyzed in order of increasing activities, otherwise a much longer equilibration time is neccessary (especially when there is a large decrease in activity between two samples)

11. Calibration Curve

12. Cu2+ by potentiometry

13. Procedures • Soil preparation • Soil is air-dried and ground to 2 mm • Shake 5.00 g of soil in 10.00 mL of 0.01 M CaCl2 for 20 min • Centrifuge 10 min at 10000 g • Determination of pCu2+ • Electrode potential measured in 20-mL polystyrene cups shaken by hand (or with stirrer, but systematically…)

14. Ionic Strength • Statistically significant but negligible ionic strength effect where EP is in mV and IS is the ionic strength • The IS in the soil extracts is 0.02±0.01 so, one SD = 0.314 mV (~0.01 pCu2+)

15. Aluminum Interference

16. Chloride Interference • Cu(II) is reduced at the electrode surface to Cu(I), which is stabilized by chloride complexation • The electrode the respond to a combination of Cu(II) and Cu(I), which also changes the Nernstian slope from 59 to 29 mV/decade • Critical Cl concentration around 10-1.4 M (Westall et al. 1979), which prevents the use of the Cu ISE in seawater (~0.5 M Cl)

17. Other ISEs

18. Other ISE’s • Cadmium and Lead • They are somewhat selective but could still possibly be used to measure Cu2+… • Might be prone to interferences from natural organic matter and/or oxides • Will be useful in synthetic solutions of known composition

19. Large selection • NH3, NH4+, Br+, Cd2+, Ca2+, CO2, Cl-, Cl2, Cu2+, CN-, F-, I-, Pb2+, NO3-, NO2-, NOx, O2, ClO4-, K+, Redox, Ag+/S2-, Na+, SCN- • Analytical confidence needs confirmation, but many environmental applications could be better exploited