C - laboratoire PROTEE

protee.univ.tln.fr

C - laboratoire PROTEE

Application of Solid

Microelectrodes for in situ

Voltammetric Measurements

Ivanka Pižeta

Center for Marine and Environmental Research, Ruđer Bošković Institute,

P.O. Box 180, 10002 Zagreb, Croatia

in cooperation with

Gabriel Billon, Jean-Claude Fischer and Michel Wartel

Université des Sciences et Technologies de Lille 1, Laboratoire de Chimie

Analytique et Marine, Bât C8, 59655 Villeneuve d'Ascq Cedex, France – UMR

CNRS 8013


Ex situ voltammetric determination of heavy metals


In situ


Solid microelectrodes

General features

Cathodic measurements

Anodic measurements


General features

Dimensions: disks or strips of few microns to few tens

of microns in diameter

Material: Au, Ag, Ir with a layer of mercury

Ratio signal/ noise – convenient

Concentration gradient does not depend on stiring

iR drop negligible

Steady state condition:


Small A/V conditions: where the electrode in

experiments lasting few seconds to a few

minutes does not transform the bulk system.

Large A/V conditions: where the electrode is

intended to transform the bulk system.


Cottrell equation for planar diffusion:

i( t)

= id

( t)

=

nFAD

1/

2

π t

1/

2

0

1/

2

Concentration profile in time and space after

the start of a Cottrell experiment:

C

0

( x,

t)

C

⎡ x

erf ⎢

⎣2(

D0t)

C

*

= 0

1 / 2

One definition of diffusion layer

*

0




Semi-infinite spherical diffusion:

i d

* ⎡ 1 1 ⎤

( t)

= nFAD0C0

⎢ + 1/

2 ⎥

⎣(

πD0t)

r0


nFAD0C

( spherical)

= i ( linear)

+

r

id d

here the first term (linear) dominates at short times, when the

diffusion layer is thin compared to r 0 , and the second

dominates at long times, when the diffusion layer grows much

larger than r 0 .

For planar electrode:

For spherical case:

0

*

0

lim i =

t→∞

limi

d

t→∞

d

=

0

nFAD

r

0

0

C

*

0


At microelectrodes such steady state is readily realized, where

diffusion field need only grow to a thickness of 100 µm (or less).

So steady-state current i ss is:

nFAD

*

0 0

*

iss = iss = 4πnFD0C 0r0

r

more generally

0

C

or

i ss =

where m0 is a mass-transfer coefficient and depends on

geometry of the electrode.

In our case we used the formula:

*

nFAm0C

0

i =

αnFD

where α = 4 for a disk geometry, so we calculated (veryfied)

the radius of the electrode from the following cyclic

voltammogram:

lim

0

C

*

0

r

0


i / A

1.E-09

0.E+00

-1.E-09

-2.E-09

-3.E-09

-4.E-09

-5.E-09

-6.E-09

-7.E-09

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6

E / V

Cyclic voltammogram in 6 x 10 -3 mol L -1 K 3 Fe(CN) 6 : start potential =

0.6 V, vertex potential = –0.4 V, step incr. = 2 mV, scan rate = 5 mV/s

with a Ir microelectrode.

For i lim = 6.36 nA, D 0 = 7.84 x 10 -6 cm 2 /s, C 0 = 6 x 10 -3 mol L -1 , F =

96500, n = 1, calculated radius is 3.5 µm.


-7 0.050x10

-7 0.025x10

0

-7

-0.025x10

-7

-0.050x10

-7

-0.075x10

-7

-0.100x10

-7

-0.125x10

-7

-0.150x10

3 different Ir electrodes

characterisation

-7

-0.175x10

-0.400 -0.200 0 0.200 0.400 0.600

Cyclic voltammograms in 6 x 10 -3 mol L -1 K 3 Fe(CN) 6 : start

potential = 0.6 V, vertex potential = –0.4 V, step incr. = 2 mV,

scan rate = 5 mV/s with three different Ir microelectrodes


The drawing of the Au and Ag microelectrode:

2 mm

contact

19 mm

5 mm

100 mm

Adapter for the Metrohm VA663 (Stopfen aud PVC 6.2709.050 – modified accordingly):

17 mm

9 mm

5.1 mm

10 mm 7.5 mm

16.1 mm

10.5 mm

36.6 mm

4.1 mm

14.6 mm

14.8 mm

4 mm

1 mm


Image of a silver microelectorde

of 30 µm in diameter – three

enlargements

Images thanks to Michel GRIMALDI


Preparation of the electrode

Polishing with sand paper, finishing with 0.3 µm Al 2O 3 wet

powder

Amalgamation: in 0.05 M [Hg(NO 3) 2 + HNO 3] (deareated)

at –0.4 V (vs. Ag/AgCl) until cca 0.3 mC charge

Stabilisation of mercury by applying –9V between working

electrode and a platinum one for 120 s in 1 M NaOH

Rinsing for few seconds in 0.01 M HClO 4 (to get rid of sodium

species)

Stabilisation by cycling the potential in the range of interest (in

our case from –0.1 V to –1.8 V 10 scans, scane rate 1 V/s)


15-25 cm

10-20 cm

RE

CE

WE


General features

Cathodic measurements

Anodic measurements


Own measurements

Instrument: µAutolab, Eco Chemie

working electrode: Ag, 30 µm diameter

counterelectrode Pt, referent electrode Ag/AgCl

Method: square wave voltammety (SWV)

parameters:

frequency 50 Hz

amplitude 25 mV

potential and acc. time –0.1 V, 20s

start potential –0.1 V

final potential –1.8 V

“stand by” potential –1 V

Place of measurement:

cell (models and calibration)

sediment in a box


E p / V

i p /nA

14

12

10

8

6

4

2

0

-1.38

-1.40

-1.42

a)

b)

0 50 100 150 200 250

f / Hz

i / nA

20

15

10

5

0

-1.0 -1.5

E / V vs. Ag/AgCl

Fig. 1 Dependence of peak

height of reduction of

Fe(II) on SW frequency.

Solution: seawater, salinity

= 31 ‰, pH = 5.47,

[Fe(II)] = 1 · 10 -3 mol L -1 .

a) peak heights, b) peak

positions. Measuring

conditions: SWV, step

increment 2 mV, amplitude

25 mV, conditioning

potential –1 V,

conditioning time 120 s,

deposition potential –0.1

V, deposition time 3 s,

initial potential –0.1 V,

final potential –1.8 V, cell

on after measurement,

standby potential –0.8 V.

Inset: original signals of

the reduction of Fe(II).


E p / V

i p /nA

60

50

40

30

20

10

0

-1.49

-1.50

-1.51

a)

(1)

after measurement

in the sediment

b)

(2)

(2)

(1)

0 500 1000 1500

i / nA

60

50

40

30

20

10

[Mn 2+ ] / µmol L -1

0

-1.0 -1.5

E / V vs. Ag/AgCl

Fig. 2 a) Calibration lines constructed

from measurement of reduction

current peak heights of Mn(II) in a

filtered seawater of pH 7.8 – 8.2: (1)

in the concentration range from 23.8

to 1550 µmol L -1 , salinity = 31 ‰

and (2) in the range from 50 to 405

µmol L -1 of salinity 15.5 ‰,

respectively; - measurements in

the cell for the case (2) after

measurement in the sediment.

Sensitivity for (1) is (0.0331 ±

0.0003) nA / (µmol L -1 ) , r = 0.9994,

n = 67 points, sensitivity for (2) is

(0.048 ± 0.001) nA / (µmol L -1 ) , r =

0.9996, n = 14 points. b)

corresponding peak potentials of the

reduction current. Electrode: Ag, 30

µm diameter. Frequency was 50 Hz,

other measuring conditions as in

Figure 1. Inset: some of the original

signals of the reduction of Mn(II),

used for constructing of the line (1).


E p / V

i p /nA

60

50

40

30

20

10

0

-1.30

-1.35

-1.40

-1.45

a)

b)

(2)

(1)

(2)

after amalgamation

0 1000 2000 3000 4000

i / nA

60

40

20

[Fe 2+ ] / µmol L -1

(1)

0

-1.0 -1.5

E / V vs. Ag/AgCl

Fig. 3 a) (1) Calibration line

constructed from measurement

of height of reduction peak of

Fe(II) in a filtered seawater of

pH 5.5 – 3.2 in the range from

50 to 4450 µmol L -1 , salinity =

31 ‰, measured by the silver

electrode, 30 µm diameter,

sensitivity (0.0130 ± 0.0003) nA

/ (µmol L -1 ), r = 0.9976, n = 51

points; (2) Peak heights of

reduction of Fe(II) in a filtered

seawater of pH 5.6 – 3.4 in the

range from 100 to 2400 µmol L -

1 , salinity 31 ‰, Au electrode,

40 µm diameter; b)

corresponding peak potentials of

the reduction peak currents in a).

Measuring conditions: same as

in Figure 2. Inset: some of the

original signals of the reduction

of Fe(II), used for constructing

of the line (1).


i / nA

6

4

2

0

0.5 1.0 1.5

-E / V vs. Ag/AgCl

Square wave voltammogram of sea water

(s = 3.1 %o, pH = 7.8) before purging


i / nA

6

4

2

0

0.5 1.0 1.5

-E / V vs. Ag/AgCl

Square wave voltammogram of sea water

(s = 3.1 %o, pH = 7.8) after 60 s purging


i / nA

6

4

2

0

0.5 1.0 1.5

-E / V vs. Ag/AgCl

Square wave voltammogram of sea water

(s = 3.1 %o, pH = 7.8) with addition of 45 x 10 -6 M Mn 2+


i / nA

6

4

2

0

0.5 1.0 1.5

-E / V vs. Ag/AgCl

Square wave voltammogram of sea water

(s = 3.1 %o, pH = 7.8) with addition of 45 x 10 -6 M Mn 2+

and 225 x 10 -6 M Mn 2+


i / nA

6

4

2

0

i / nA

7

6

5

4

3

2

1

after sediment

0

0 50 100 150 200 250

[Mn 2+ ] / µM

0.5 1.0 1.5

-E / V vs. Ag/AgCl

Voltammograms of reduction of Mn 2+ in sea water (s = 3.1

%o, pH = 7.8). Additions: 45, 90, 135 i 225 x 10 -6 M Mn 2+ .

Sensitivity of the electrode: 0.023 ± 0.001 nA/µM


i / nA

1 nA

i / nA

-0.5

5

4

3

2

1

0

5

4

3

2

1

-0.5 -1.0

E / V vs. Ag/AgCl

-1.0

E / V vs. Ag/AgCl

-1.5

-1.5

Fig. 5 SW

voltammograms of

seawater, salinity 31 ‰,

pH = 6.9. 1 – after

addition of 122 µmol L -1

Fe(II); 2 – after further

addition of 2.65 µmol L -

1 of S(-II); 3 – 5 after

further addition of 91

µmol L -1 of Mn(II).

Deposition time at –0.1

V for curves 1- 3 was 2

s, while for curves 4 and

5 it was 7 and 22 s,

respectively. Other

measuring conditions

same as in Figure 2.

Inset: original signals

before smoothing and

shifting.


i / nA

40

30

20

10

0

0.5 1.0 1.5

-E / V vs. Ag/AgCl

Voltamogramm in the sediment (s ~ 10 %o, pH ~ 7.8,

pe ~ -150 – -200 mV). Sensitivity of the electrode for

Mn 2+ : 0.023 ± 0.001 nA/µM


i / nA

40

30

20

10

0

0.5 1.0 1.5

-E / V vs. Ag/AgCl

Voltamogramm in the sediment (s ~ 10 %o, pH ~ 7.8,

pe ~ -150 – -200 mV). Sensitivity of the electrode for

Mn 2+ : 0.023 ± 0.001 nA/µM


i / nA

40

30

20

10

0

0.5 1.0 1.5

-E / V vs. Ag/AgCl

Voltamogramm in the sediment (s ~ 10 %o, pH ~ 7.8,

pe ~ -150 – -200 mV). Sensitivity of the electrode for

Mn 2+ : 0.023 ± 0.001 nA/µM


i / nA

40

30

20

10

0

0.5 1.0 1.5

-E / V vs. Ag/AgCl

Voltamogramm in the sediment (s ~ 10 %o, pH ~ 7.8,

pe ~ -150 – -200 mV). Sensitivity of the electrode for

Mn 2+ : 0.023 ± 0.001 nA/µM


i / nA

40

30

20

10

0

0.5 1.0 1.5

-E / V vs. Ag/AgCl

Voltamogramm in the sediment (s ~ 10 %o, pH ~ 7.8,

pe ~ -150 – -200 mV). Sensitivity of the electrode for

Mn 2+ : 0.023 ± 0.001 nA/µM


i / nA

40

30

20

10

0

O 2

S 2- FeS

0.5 1.0 1.5

-E / V vs. Ag/AgCl

Fe 2+

Mn 2+

Voltamogramm in the sediment (s ~ 10 %o, pH ~ 7.8,

pe ~ -150 – -200 mV). Sensitivity of the electrode for

Mn 2+ : 0.023 ± 0.001 nA/µM


i / nA

i / nA

40

30

20

10

20 nA

I

0

-0.5

I

II

-0.5 -1.0

E / V vs. Ag/AgCl

II III

5

4

3

2

1

-1.0

III

IV

E / V vs. Ag/AgCl

V

-1.5

IV

V

-1.5

Fig. 6 SW voltammograms of

a boxcore of salinity 10 ‰, pH

= 7.0, E ~ -200 mV. Curves 1

– 5: repetitions at one depth in

intervals of 60 s. The

sensitivity of the electrode for

Mn(II) was (0.023 ± 0.001)

nA/(µmol L -1 ) , for salinity of

3.1 ‰; dep. pot. –0.1 V, dep.

time = 20 s. Other measuring

conditions: same as in Figure

2. Inset: original signals before

smoothing and shifting.


profondeur / mm

0

-10

-20

-30

-40

-50

-60

-70

ip / nA

Sediment La Madelon (baie d'Authie)

0 5 10 15 20 25

S-

Mn2+

Peak a -0.8 V

Peak a -1 V

profondeur / mm

0

-10

-20

-30

-40

-50

-60

-70

-Ep / V

Sediment La Madelon (baie d'Authie)

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

S-

Mn2+

Peak a -0.8 V

A depth profile taken in the boxcore sediment with a Ag 100 µm electrode

Peak a -1 V


Summation for cathodic measurements:

sensitivity [ nA/µmol L -1 ] M.D.L. [µmol L -1 ]

Mn(II) 0.05 3.5

Fe(II) 0.01 28

I(-I) 0.16 0.3

S(-II) 1.10 0.05


General features

Cathodic measurements

Anodic measurements


i / A

1.5n

1.3n

1.0n

0.8n

0.5n

0.3n

0

-0.3n

C:\Ivanka\Experiments\Juin\270603\ag270603.iew

Ir cca 10 µm

l'eau de mer pH 3.5

Additions: 2.5, 5, 7.5, 10, 12.5, 17.5 and 22.5 ppb Pb2+

5 6

-0.5n

-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3

1

E / V

SWV peaks of standard additions of Pb 2+ in seawater,

measured by 10µm iridium electrode. Dep. t. = 300 s

3

7

4

2


ip / nA

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

y = 0.0418x - 0.0471

R 2 = 0.9986

0 5 10 15 20 25

[Pb2+] / ppb

Calibration line constructed from the voltammograms.

Sensitivity of the electrode:

(0.0418 ± 0.0009) nA/ppb; MDL = 0.8 ppb

(0.0087 ± 0.0002) nA/nM; MDL = 3.76 nM


ip / A

Area

3.5E-08

3.0E-08

2.5E-08

2.0E-08

1.5E-08

1.0E-08

5.0E-09

0.0E+00

-1.3 -1.25 -1.2 -1.15 -1.1 -1.05 -1 -0.95 -0.9

3.0E-09

2.5E-09

2.0E-09

1.5E-09

1.0E-09

5.0E-10

0.0E+00

Edep / V

-1.3 -1.25 -1.2 -1.15 -1.1 -1.05 -1 -0.95 -0.9

Edep / V

Ep / V

Width1/2 / V

-0.40

-0.45

-0.50

-0.55

-0.60

-0.65

-0.70

-1.3 -1.25 -1.2 -1.15 -1.1 -1.05 -1 -0.95 -0.9

0.12

0.11

0.10

0.09

0.08

0.07

0.06

0.05

Edep / V

0.04

-1.3 -1.25 -1.2 -1.15 -1.1 -1.05 -1 -0.95 -0.9

Edep / V

Pseudopolarogram - Interstitial water, pH 4.07-4.90, Ir electrode, φ = 70 µm


ip / A

1.4E-07

1.2E-07

1.0E-07

8.0E-08

6.0E-08

4.0E-08

2.0E-08

0.0E+00

-1.4 -1.2 -1 -0.8 -0.6 -0.4

Ep /

-0.40

-0.41

-0.42

-0.43

-0.44

-0.45

f101-164

f165-228

f229-292

f293-356

Edep / V

f101-164

f165-228

f229-292

f293-356

-1.4 -1.2 -1 -0.8 -0.6 -0.4

Edep / V

Pseudopolarograms –

four repetitions

Interstitial water, pH 2,

Added Pb

Ir electrode, φ = 70 µm


ip / n

-Ep / V

40

35

30

25

20

15

10

5

0

0.43

0.42

0.41

0.2 0.4 0.6 0.8 1 1.2 1.4

0.4

0.39

0.38

0.37

0.36

-Edep / V

0.2 0.4 0.6 0.8 1 1.2 1.4

-Edep / V

Pseudo 1

Pseudo 6

Pseudo 1

Pseudo 6

Interstitial water,

filtered, pH 2,

Added Pb

Ir electrode, φ = 70

µm


ip / A

/ V

Ep

1.4E-07

1.2E-07

1.0E-07

8.0E-08

6.0E-08

4.0E-08

2.0E-08

0.0E+00

-1.4 -0.9 -0.4

-0.30

-0.35

-0.40

-0.45

-0.50

-0.55

-0.60

-0.65

Edep / V

-0.70

-1.4 -0.9 -0.4

Edep / V

13.06.-1

13.06.-2

13.06.-3

19.06.

20.06.-1

20.06.-2

21.06.-1

21.06.-6

24.06.-1

24.06.-rev

25.06.-1

25.06.-2

25.06.-3

25.06.-4

20.06.-1n

20.06.-2n

20.06.-3n

13.06.-1

13.06.-2

13.06.-3

19.06.

20.06.-1

20.06.-2

21.06.-1

21.06.-6

24.06.-1

24.06.-rev

25.06.-1

25.06.-2

25.06.-3

25.06.-4

20.06.-1n

20.06.-2n

20.06.-3n

20.06.-4n

“All” Pseudopolarograms

and “all” repetitions taken

in:

-Sediment

-Interstitial water

-Filtered interstitial water

(Withour and with addition

of Pb 2+ )

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