© Sauvé 2002

Sébastien SauvéSébastien SauvéDepartment of ChemistryDepartment of ChemistryUniversité de MontréalUniversité de Montréal

sebastien.sauve@umontreal.casebastien.sauve@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