1. Speciation and Lability of Zn(II) in River Waters
Paper by R. Jensen, H. van Leeuwen, R. Cleven, and M. van Den Hoop
Presented by Slil Siripong
February 17, 2000

2. Overview Introduction Theory Voltammetry DPV-SV Experimental Results
Discussion
Conslusions

3. Introduction
Study the relationship between bioavailability of Zn(II) and its speciation.
Determine equilibrium speciation of Zn(II) and the labilities of its complex species in different European river water samples.
Consider different rate determining.
Techniques: DPV and SV to provide information on speciation and lability.

4. Theory
Sufficient excess of ligand so that the association reaction becomes quasi-monomolecular
Lability requires dynamic complexation equilibria and abundance of metal flux toward the interface
Introduce a basic lability parameter and diffusion coefficient ratio
Assume steady state
Limiting case: Davison criterion
ka of Zn is in the order of 108

5. Voltammetry
Class of electroanalytical measurements in which the controlled parameter, the potential of the indicator electrode, varies with time and in which the current flowing through the indicator electrode is the measured parameter

6. Electrodes Working electrode Static Mercury Drop Electrode (SMDE)
Reference electrode
Ag/AgCl/3 M KCl
Counter electrode
Glassy carbon

7. Differential Pulse Voltammetry

8. Differential Pulse Voltammetry
A 50-mV pulse is applied
Two current measurements are made: prior to the pulse and at the end of the pulse
Increase the sensitivity
Enhance faradaic current
Reduce nonfaradaic current
Effective time scale: pulse time

9. Differential Pulse Voltammetry
Excitation Signals for DPV

10. Stripping Voltammetry
Preconcentration step
Analyte first deposited on microelectrode
Analyte redissolved or stripped from microelectrode
Performed in the differential pulse mode (DPASV)
Increase the concentration of analyte on the surface of microelectrode
Lower the detection limit
Effective time scale: stirring rate

11. Stripping Voltammetry
Excitation Signals for SV

12. Stripping Voltammetry
Stripping Voltammogram

13. Experimental Results: Labilities
DPV peak current is linearly dependent to t -1/2
Systematic shift of DPV peak potential toward more negative values
Peak current is linearly dependent to the square root of the stirring rate

14. Experimental Results: Cottrell
Cottrell plot of DPV peak currents for different values of the pulse
duration t for the sample Eindergatsloot. The linear behavior illustrates
diffusion-controlled character of the current.

15. Experimental Results: Complexation Curves
Increase in complexation of Zn(II) with increasing ligand concentration
Use  and c*L to compute K
The influence of  on K is small
Not reaching the plateau of p

16. Experimental Results: Complexation Curves
Complexation curves for the samples Eysden (a) and
Hultabacken-1 (b)

17. Experimental Results: K
Sample
Log K
Free Zn(II)/
Total Zn(II) 
Hultabacken 1
6.5
0.12
0.10
Hultabacken 2
6.6
0.38
0.25
Larjean
6.4
0.28
0.14
Langemossen
0.39
0.18
Keisersveer
6.7
0.24
0.22
Eysden
7.0
Lillan
0.45
Table 2 Values found for the Zn(II) complex stability (log K), the Corresponding fraction of free Zn(II), and the Resulting  for different European River Waters

18. Discussion K is in the range of 106.4 – 107 M-1
Free Zn(II) is about 30% of total Zn(II)
 = [Zn(II)bound]/[total available ligand]
Concentrate the ligands
More accurate diffusion coefficients of Zn(II) complexes
No interference from competing cations

19. Conclusions
30% free Zn(II), 70% Zn(II) complexes labile on the voltammetric time scale
Flux of Zn(II) into an organism in the system with complexes compared with the system without complexes
The actual interfacial uptake process is rate-determining: 30%
The coupled diffusion of free and complexed Zn(II) is rate-determining: 50%