© Sauvé 2002 Sébastien Sauvé Department of Chemistry Université de Montréal Metal speciation using ion-selective electrodes

© Sauvé 2002 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

© Sauvé 2002 Avdeef et al. 1983

© Sauvé 2002 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 M is close the background concentration expected in clean solutions (resulting in a standard addition type of plateau)

© Sauvé 2002 Cupric Ion-Selective Electrodes Linear, Nernstian response down to pCu 2+ of:  7 in dilute copper salts solutions (60 µg·L -1 )  19 using solutions copper-buffered with ligands of known stability constants ( M or 60 ag·L -1 ) Simple equipment Extensive literature

© Sauvé 2002 Cupric Ion-Selective Electrodes Interferences  Ionic strength variations Need a relatively uniform IS  Aluminum  Mercury  Chloride Electrode surface is sensitive

© Sauvé 2002 Cupric Electrode Calibration Suggested Cu-IDA calibration solutions have: 1·10 -3 M IDA 1·10 -4 M Cu(NO 3 ) 2 6·10 -3 M NaOH 2.5·10 -3 M KHphthalate 1·10 -2 M CaCl 2 (media) pH adjusted with HNO 3  Use IDA stability constants reported in the literature, interpolated to 0.02 ionic strength

© Sauvé 2002Calibration Simultaneously determine the pH for calculations of pCu 2+

© Sauvé 2002 Calibration …

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)

Calibration Curve

© Sauvé 2002 Cu 2+ by potentiometry

© Sauvé 2002Procedures Soil preparation  Soil is air-dried and ground to 2 mm  Shake 5.00 g of soil in mL of 0.01 M CaCl 2 for 20 min  Centrifuge 10 min at g Determination of pCu 2+  Electrode potential measured in 20-mL polystyrene cups shaken by hand (or with stirrer, but systematically…)

© Sauvé 2002 Ionic Strength Statistically significant but negligible ionic strength effect where EP is in mV and IS is the ionic strength l The IS in the soil extracts is 0.02±0.01 so, one SD = mV (~0.01 pCu 2+ )

Aluminum Interference

© Sauvé 2002 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 M (Westall et al. 1979), which prevents the use of the Cu ISE in seawater (~0.5 M Cl)

© Sauvé 2002 Other ISEs

© Sauvé 2002 Other ISE’s Cadmium and Lead  They are somewhat selective but could still possibly be used to measure Cu 2+ …  Might be prone to interferences from natural organic matter and/or oxides  Will be useful in synthetic solutions of known composition

© Sauvé 2002 Large selection NH 3, NH 4 +, Br +, Cd 2+, Ca 2+, CO 2, Cl -, Cl 2, Cu 2+, CN -, F -, I -, Pb 2+, NO 3 -, NO 2 -, NO x, O 2, ClO 4 -, K +, Redox, Ag + /S 2 -, Na +, SCN - Analytical confidence needs confirmation, but many environmental applications could be better exploited