Transcript Speciation and Complexation of Cd in Soil Solutions

Metal speciation using
ion-selective electrodes
Sébastien Sauvé
Department of Chemistry
Université de Montréal
[email protected]
© 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 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 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
© 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(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
Calibration
Simultaneously
determine the pH for calculations of pCu2+
IDA-Cu Calibration Buffer
-3
Free Cu2+ log(x) M
-5
-7
-9
-11
-13
-15
2.00
3.00
4.00
5.00
6.00
pH
© Sauvé 2002
7.00
8.00
9.00
10.00
Calibration
pH
10.00
9.95
9.90
9.85
9.80
9.75
9.70
9.65
9.60
…
© Sauvé 2002
pCu2+
13.53
13.52
13.51
13.49
13.48
13.46
13.44
13.42
13.39
pH
8.00
7.95
7.90
7.85
7.80
7.75
7.70
7.65
7.60
pCu2+
11.43
11.34
11.26
11.17
11.08
11.00
10.91
10.82
10.74
pH
6.00
5.95
5.90
5.85
5.80
5.75
5.70
5.65
5.60
pCu2+
8.51
8.46
8.40
8.35
8.30
8.24
8.19
8.14
8.09
pH
4.00
3.95
3.90
3.85
3.80
3.75
3.70
3.65
3.60
pCu2+
6.45
6.40
6.35
6.29
6.24
6.18
6.13
6.08
6.02
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)
© Sauvé 2002
Calibration Curve
Calculated pCu2+
2
4
pCu = -0.03027 EP + 10.23
corr. R 2 = 0.997***
6
8
10
12
14
-100
-50
0
50
100
150
Electrode Potential (mV)
200
250
Cu2+ by potentiometry
Free Cu (pCu2+)
2.0
4.0
6.0
8.0
10.0
y = -0.0299x + 10.107
12.0
R2 = 0.9948
14.0
300
200
100
0
Electrode potential (mV)
© Sauvé 2002
-100
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…)
© Sauvé 2002
Ionic Strength

Statistically significant but
negligible ionic strength effect
EP  32.07 pCu2  32.45 IS  340.4
(corr .R 2  0.991* * * )
where EP is in mV and IS is the ionic strength

© Sauvé 2002
The IS in the soil extracts is 0.02±0.01 so,
one SD = 0.314 mV (~0.01 pCu2+)
Aluminum Interference
3
pCu2+
4
5
pCu2+
0.1 mM Al
No Al
6
7
125
150
175
200
Electrode Potential (mV)
225
250
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)
© Sauvé 2002
Other ISEs
Electrode Potential (mV)
Pb-ISE Calibration
-100
-150
-200
-250
-300
-350
y = -24.571x - 87.952
2
R = 0.9431
-400
-450
13 12 11 10 9
8
7
pPb2+
© Sauvé 2002
6
5
4
3
2
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
© Sauvé 2002
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
© Sauvé 2002