Sébastien Sauvé Department of Chemistry Université de Montréal sebastien.sauve@umontreal
description
Transcript of Sébastien Sauvé Department of Chemistry Université de Montréal sebastien.sauve@umontreal
© Sauvé 2002
Sébastien SauvéSébastien SauvéDepartment of ChemistryDepartment of ChemistryUniversité de MontréalUniversité de Montréal
[email protected]@umontreal.ca
Metal speciation Metal speciation using ion-selective using ion-selective
electrodeselectrodes
© Sauvé 2002
Ion selective electrodesIon 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 Avdeef et alet al. 1983. 1983
© Sauvé 2002
PrejudicePrejudice
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)
© Sauvé 2002
Cupric Ion-Selective Cupric Ion-Selective ElectrodesElectrodes
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
© Sauvé 2002
Cupric Ion-Selective Cupric Ion-Selective ElectrodesElectrodes
Interferences
Ionic strength variations
• Need a relatively uniform IS
Aluminum
Mercury
Chloride
Electrode surface is sensitive
© Sauvé 2002
Cupric Electrode CalibrationCupric Electrode Calibration Suggested Cu-IDA calibration solutions
have:• 1·10-3 M IDA
• 1·10-4 M Cu(NO3)2
• 6·10-3 M NaOH
• 2.5·10-3 M KHphthalate
• 1·10-2 M CaCl2 (media)
• pH adjusted with HNO3
Use IDA stability constants reported in the literature, interpolated to 0.02 ionic strength
© Sauvé 2002
CalibrationCalibration
IDA-Cu Calibration Buffer
-15
-13
-11
-9
-7
-5
-3
2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
pH
Fre
e C
u2+
lo
g(x
) M
Simultaneously determine the pH for calculations of pCu2+
© Sauvé 2002
CalibrationCalibration
pH pCu2+ pH pCu2+ pH pCu2+ pH pCu2+
10.00 13.53 8.00 11.43 6.00 8.51 4.00 6.459.95 13.52 7.95 11.34 5.95 8.46 3.95 6.409.90 13.51 7.90 11.26 5.90 8.40 3.90 6.359.85 13.49 7.85 11.17 5.85 8.35 3.85 6.299.80 13.48 7.80 11.08 5.80 8.30 3.80 6.249.75 13.46 7.75 11.00 5.75 8.24 3.75 6.189.70 13.44 7.70 10.91 5.70 8.19 3.70 6.139.65 13.42 7.65 10.82 5.65 8.14 3.65 6.089.60 13.39 7.60 10.74 5.60 8.09 3.60 6.02
…
© Sauvé 2002
Electrode CalibrationElectrode 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 CurveCalibration Curve
pCu = -0.03027 EP + 10.23corr. R2 = 0.997***
2
4
6
8
10
12
14
-100 -50 0 50 100 150 200 250
Electrode Potential (mV)
Cal
cula
ted
pC
u2+
© Sauvé 2002
CuCu2+2+ by potentiometry by potentiometry
y = -0.0299x + 10.107R2 = 0.9948
2.0
4.0
6.0
8.0
10.0
12.0
14.0
-1000100200300
Electrode potential (mV)
Free
Cu
(pC
u2+)
© Sauvé 2002
ProceduresProcedures 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…)
© Sauvé 2002
Ionic StrengthIonic Strength Statistically significant but negligible ionic
strength effect
)991.0.(
4.34045.3207.32***2
2
Rcorr
ISpCuEP
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+)
Aluminum InterferenceAluminum Interference
3
4
5
6
7
125 150 175 200 225 250
Electrode Potential (mV)
pCu2
+
pCu2+
0.1 mM Al
No Al
© Sauvé 2002
Chloride InterferenceChloride 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)
© Sauvé 2002
Other ISEsOther ISEs
Pb-ISE Calibration
y = -24.571x - 87.952
R2 = 0.9431
-450
-400
-350
-300
-250
-200
-150
-100
2345678910111213
pPb2+
Electrode P
ote
ntial (m
V)
© Sauvé 2002
Other ISE’sOther 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
© Sauvé 2002
Large selection 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