S

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Sébastien SauvéDepartment of ChemistryUniversité
de Montréalsebastien.sauve_at_umontreal.ca
Metal speciation using ion-selective electrodes
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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

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Avdeef et al. 1983
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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)

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Cupric Ion-Selective Electrodes

• Linear, Nernstian response down to pCu2 of
• 7 in dilute copper salts solutions (60 µgL-1)
• 19 using solutions copper-buffered with ligands
  of known stability constants (10-19 M or 60
  agL-1)
• Simple equipment
• Extensive literature

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Cupric Ion-Selective Electrodes

• Interferences
• Ionic strength variations
• Need a relatively uniform IS
• Aluminum
• Mercury
• Chloride
• Electrode surface is sensitive

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Cupric Electrode Calibration

• Suggested Cu-IDA calibration solutions have
• 110-3 M IDA
• 110-4 M Cu(NO3)2
• 610-3 M NaOH
• 2.510-3 M KHphthalate
• 110-2 M CaCl2 (media)
• pH adjusted with HNO3
• Use IDA stability constants reported in the
  literature, interpolated to 0.02 ionic strength

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Calibration

• Simultaneously determine the pH for calculations
  of pCu2

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Calibration

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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)

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Calibration Curve
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Cu2 by potentiometry
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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)

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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 0.020.01 so, one
  SD 0.314 mV (0.01 pCu2)

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Aluminum Interference
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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)

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Other ISEs
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Other ISEs

• 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

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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