2. ENVIRONMENTAL ChEMISTRy & TEChNOLOGy 2.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Environmental Chemistry & Technology
P58 SIMuLTANEOuS DETERMINATION OF
MERCuRy, LEAD AND CADMIuM IN
AQuEOuS SAMPLES uSING PECTIC ACID-
MODIFIED CARbON PASTE ELECTRODE
JOSE H. SAnTOS and SEAn GERARD WARD
Department of Chemistry, University of Brunei Darussalam,
Tungku Link, Gadong BE1410, Brunei Darussalam,
joey@fos.ubd.edu.bn
Introduction
Anodic stripping voltammetry (ASV) and its variants are
the most sensitive electroanalytical techniques employed for
trace heavy metal analysis. Mercury electrodes have been widely
used for ASV due to their ease of use and compatibility
with a number of heavy metal species. Recently, however,
there are major concerns regarding the use of mercury in the
laboratory because of hazards it poses to humans and other
living organisms. Prompted by environmental issues, some
of the applications of modified carbon paste electrodes (CPE)
are aimed at the development of mercury-free electrodes for
ASV 1 . In this paper, we investigated the use of pectic acid as
a modifier for CPE and its utilization for the analysis of some
representative heavy metal species.
Pectic acid, also known as polygalacturonic acid, is a
natural polymer found in citrus rinds. It consists of chains of
300 to 1000 units of galacturonic acid monomer joined with
α 1 → 4 linkages. When incorporated in a CPE, the exposed
carboxylic acid groups are responsible for the accumulation
of heavy metal ions on the electrode surface presumably through
ion-exchange or complex-formation processes.
The aim of this study is to fabricate pectic acid-modified
CPE and examine various experimental conditions that affect
the analytical signal when used as a working electrode for the
ASV of mercury, lead and cadmium in aqueous samples.
Experimental
Pectic acid isolated from orange peel was obtained from
Fluka and used without further purification. Mineral oil,
graphite powder were purchased from Sigma-Aldrich while
1,000-ppm standard solutions of Hg (II), Pb (II) and Cd (II)
were from Sharlau (Spain). All other chemicals used were at
least AR grade.
All electrochemical experiments were carried out using
a BAS 100B Electrochemical System (BioAnalytical System)
in the Osteryoung square wave stripping voltammetry mode
(OSWSV) or cyclic voltammetry (CV) utilizing Ag/AgCl
reference electrode and a Pt wire counter electrode.
Modified carbon paste was prepared by thoroughly
mixing 4 : 1 (w/w) ratio of graphite powder to powdered
pectic acid and enough mineral oil to form a paste typical
for conventional CPEs. A portion of prepared paste was then
tightly packed on the cavity (2-mm dia.) of previously cut
200-ml pipette tip where a copper rod was inserted on the
other end to establish electrical contact.
s448
The modified CPE was first immersed in a sample solution
containing the heavy metal ion being analyzed. After a
predetermined period of time, referred to as accumulation
time, the electrode was removed from the sample and rinsed
thoroughly with water. The electrode was then transferred
into a voltammetric cell containing deoxygenated 0.1M
HCl for ASV. The accumulated metals were first reduced by
applying a sufficiently negative potential of –800 mV for
60 s, then re-oxidized while anodically scanning the potential.
The peak-type I-E plots resulting from the anodic scan
were recorded and evaluated.
Results and Discussion
P r e l i m i n a r y S t u d i e s
Cyclic voltammetric experiments revealed that pectic
acid-modified CPE possesses a useful potential window
ranging from –900 mV to 1200 mV relative to the Ag/AgCl
reference electrode when the supporting electrolyte used was
deoxygenated 0.1M HCl. Preliminary investigations also
showed that Cd, Pb and Hg undergoes redox transformations
at about –690, –470 and +100 mV, respectively, using the
above CPE and electrolyte combination. In principle, these
metal species may be simultaneously detected and conveniently
analyzed with well resolved analytical peaks using the
modified CPE.
I n d i v i d u a l A n a l y s i s
Results of ASV experiments using laboratory prepared
solutions containing a single metal species are consistent
with the literature 2,3 . For all the three heavy metal species,
as the concentration increases or the accumulation time is
prolonged, the peak current also increases in a linear fashion
until such an instance where current signal plateaus and further
increase in concentration or accumulation time does
not anymore amplify the peak height. This is due to the fact
that the higher the concentration or the longer the electrode
is immersed into the solution containing the analyte; more
metal ions are able to accumulate on the surface, which consequently
provides a higher current. Saturation point is attained
when the active sites for metal accumulation are maximized
resulting to levelling of current response. Using an
accumulation time of 2 min and other parameters described in
the experimental section, the sensitivities and detection limits
are reported in Table I. Sensitivities were calculated from the
slopes of individual calibration curves within the linear dynamic
range while detection limits (LOD) were estimated based
on three times the standard deviation of the blank. Further
Table I
Analytical data for pectic acid-modified CPE
Metal
Sensitivity LOD LOL
[μA ppm –1 ] [ppm] [ppm]
cadmium 20 0.25 20
lead 50 0.15 50
mercury 10 0.40 50