World Champion in der Elektroanalytik - Metrohm AG

Literaturreferate
• Square-wave voltammetric method for determination of molinate
concentration in a biological process using a hanging mercury
drop electrode
M. Fátima Barroso, O.C. Nunes, M. Carmo Vaz and C. Delerue-Matos
A square-wave voltammetric (SWV) method using a hanging mercury
drop electrode (HMDE) has been developed for determination of the herbicide
molinate in a biodegradation process. The method is based on
controlled adsorptive accumulation of molinate for 10 s at a potential
of –0.8 V versus AgCl/Ag. An anodic peak, due to oxidation of the adsorbed
pesticide, was observed in the cyclic voltammogram at approx.
–0.320 V versus AgCl/Ag; a very small cathodic peak was also detected.
The SWV calibration plot was established to be linear in the range
5.0×10 –6 to 9.0×10 –6 mol L –1 ; this corresponded to a detection limit of
3.5×10 –8 mol L –1 . This electroanalytical method was used to monitor
the decrease of molinate concentration in river waters along a biodegradation
process using a bacterial mixed culture. The results achieved
with this voltammetric method were compared with those obtained by
use of a chromatographic method (HPLC-UV) and no significant statistical
differences were observed.
Anal. Bioanal. Chem. 381 (2005) 879–883
• Signal enhancement in adsorptive stripping voltammetry of Pt by
forced convection during the measurement step
Günther Weber and Jürgen Messerschmidt
Adsorptive stripping voltammetry («formazone-method») is already
known as one of the most sensitive methods for platinum analysis with
a detection limit in the low picogram range. In this work, it is shown that
the detection limit can be lowered even more, namely by one order of
magnitude, to 0.2 pg (=1 fmol) Pt in 15 mL electrolyte, corresponding
to 68 fmol/L, by applying forced convection during the stripping step of
the voltammetric measurement. The sensitivity of the method (given in
nA/pg Pt) is enhanced by a factor of 3…5 (in differential pulse mode
and 15 mL vial), up to a factor of 30 (using square-wave mode and 3
mL vial). The maximum enhancement factor is limited by the maximum
stirrer speed that can be applied without negative effects on the hanging
mercury drop electrode.
To check for similar enhancement effects in other types of stripping methods,
the behavior of adsorptive stripping voltammetry for Pt is compared
to conventional anodic stripping voltammetry (ASV) of lead, and
to adsorptive stripping voltammetry of nickel and cobalt using their dimethylglyoxime
(DMG) complexes. No enhancement effect is observed
in ASV of lead upon stirring, and the nickel-DMG system exhibits only a
small enhancement factor of about 1.5. A reasonable explanation of the
higher signal enhancement in the catalytic Pt-formazone system is the
mass transport of reaction product, namely hydrogen, away from the
working electrode during the catalytic hydrogen evolution cycle.
Anal. Chim. Acta 545 (2005) 166–172
• Electrochemical determination of 2,4-dichlorophenoxyacetic acid
(2,4-D) at a mercury electrode
N. Maleki, A. Safavi and H.R. Shahbaazi
An indirect electrochemical determination of 2,4-dichlorophenoxyacetic
acid (2,4-D), has been presented. The method is based on the adsorption
and desorption of 2,4-D on a mercury electrode. The electrochemical
behavior of 2,4-D in aqueous solutions at different pH values and different
2,4-D concentrations was also studied. A simple and rapid method has
been developed for its extraction from water and soil. The subsequent
determination was carried out by a tensammetric method. Three calibration
curves could be obtained from different parts of the voltammogram.
Under the optimum conditions (pH = 2.3; E acc = −1100 mV; t acc = 60 s;
alternative current mode; n = 40 mV s −1 ; pulse height = 20 mV; modulation
frequency = 60 Hz; phase angle = 90 degrees) the limit of detection
was 50 μg L−1. The proposed method was applied to the determination
of 2,4-D in real samples such as soil and water.
Anal. Chim. Acta 530 (2005) 69–74
• Complexation of Ni, Cu, Zn and Cd by dissolved organic carbon
(DOC) in some metal-impacted freshwater lakes: a comparison of
approaches using electrochemical determination of free-metal-ion
and labile complexes and a computer speciation model, WHAM V
and VI
J.W. Guthrie, N.M. Hassan, M.S.A. Salam, I.I. Fasfous, C.A. Murimboh,
J. Murimboh, C.L. Chakrabarti and D.C. Grégoire
Complexation of Ni(II), Cu(II), Zn(II) and Cd(II) by dissolved organic
carbon (DOC) in some freshwater lakes in Rouyn-Noranda, Québec,
Canada, where they were impacted by effluents from a nearby copper
smelter, was measured by kinetic and equilibrium methods using
cathodic and anodic stripping voltammetry. The measured free-metalion
and labile metal-complex concentrations were compared with the
predictions made by a widely-used computer speciation model, the
Windermere Humic Aqueous Model (WHAM): WHAM V and its improved
version WHAM VI. If it is assumed that 65% of the DOC is «active»,
i.e. behaving as isolated humic substances such as fulvic acid,
both versions of WHAM are able to predict the labile and free-metalion
concentrations of Ni, Zn, and Cd reasonably well; however, both
underestimate the free-copper-ion concentration by one to two orders
of magnitude. WHAM VI is generally better than or equal to WHAM V
for successfully predicting most of the free-metal-ion concentrations.
The modeled competition by Al(III) and Fe(III) in the lake surface waters
showed that in most cases Cu(II) was most affected by this competition.
WHAM VI predicts a larger effect from the Al(III) and Fe(III)
competition than does WHAM V.
Anal. Chim. Acta 528 (2005) 205–218
• Determination of mobile form contents of Zn, Cd, Pb and Cu in soil
extracts by combined stripping voltammetry
T. Nedeltcheva, M. Atanassova, J. Dimitrov and L. Stanislavova
The amount of mobile forms of Zn, Pb, Cd and Cu in extracts obtained
by treating soil samples with ammonium nitrate were determined by an
appropriate combination of anodic and cathodic stripping voltammetry
with a hanging mercury drop electrode. Every analysis required three
mercury drops: zinc was determined on the first one, cadmium and lead
on the second and copper on the third. Zinc, lead and cadmium were
determined by conventional differential-pulse anodic stripping voltammetry.
For copper determination, adsorptive differential-pulse cathodic
stripping voltammetry with amalgamation using chloride ions as a complexing
agent was applied. The standard deviation of the results ranged
from 1 to 10% depending on the metal content in the sample. Voltammetric
results were in good agreement with the AAS analysis. No microwave
digestion of soil extracts was necessary.
Anal. Chim. Acta 528 (2005) 143–146
• A study of heavy metal complexation in grape juice
Íñigo Salinas, Irene Esparza, Sara Gómez, Carolina Santamaría, José M.
Fernández
Differential pulse anodic stripping voltammetry, DPASV, has been used
to monitor the initial stages of grape juice fermentation, focusing on Zn
interactions with naturally occurring ligands. Langmuir and Scatchard
linearization methods have been employed. A 1 : 1 ratio has been found
by either method; from Langmuir data analysis only one ligand population
was found, while the Scatchard approach gave rise to the detection
of two ligand types. Both data analysis procedures led to the same total
ligand concentration. When catechin was used as model ligand, a 1 : 1
ratio was found for Zn and also for Cu.
Electroanalysis 17/5–6 (2005) 469–475
28 Metrohm Information Heft 1/2006