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Bull. Chem. Soc. Ethiop. 2003, 17(1), 95-106. ISSN 1011-3924
Printed in Ethiopia 2003 Chemical Society of Ethiopia
__________
*Corresponding author. E-mail: hm.alemu@nul.ls
ELECTROCHEMICAL OXIDATION OF NICLOSAMIDE AT A GLASSY CARBON
ELECTRODE AND ITS DETERMINATION BY VOLTAMMETRY
Hailemichael Alemu
*
, Ntai M. Khoabane and Potlaki F. Tseki
Department of Chemistry, National University of Lesotho, P.O. Roma 180, Roma, Lesotho,
Southern Africa
(Received August 8, 2002; revised February 22, 2003)
ABSTRACT. Cyclic voltammetry, square-wave voltammetry and controlled potential electrolysis
have been used to study the electrochemical oxidation behaviour of niclosamide at a glassy carbon
electrode. The number of electrons transferred, the wave characteristics, the diffusion coefficient
and reversibility of the reactions have been investigated. Following optimisation of voltammetric
parameters, pH, and reproducibility, a linear calibration curve over the range 1 x 10
-6
--- 1 x 10
-4
mol dm
-3
niclosamide was achieved. The detection limit was found to be 8 x 10
-7
mol dm
-3
. For eight
successive determinations of 1 x 10
-5
mol dm
-3
niclosamide, a relative standard deviation of 3.6%
was obtained. This voltammetric method was applied for the determination of niclosamide in
tablets.
KEY WORDS: Niclosamide, Electrochemical oxidation, Cyclic voltammetry, Square wave
voltammetry, Glassy carbon electrode, Determination of niclosamide
INTRODUCTION
Niclosamide (2',5-dichloro-4'-nitrosalicylanilide, NA, Figure 1) is a relatively selective, non-
cumulative chlorinated aromatic amide pesticide. It is principally used against aquatic snails but
also as an anti-parasitic drug that is effective against all the species of tapeworm infections [1-
3]. Its mode of action against tapeworm species is to uncouple oxidative phosphorylation and
blocking the glucose uptake and inhibits respiration in cestodes [4, 5].
NA is toxic to aquatic
vertebrates (e.g. fish and amphibians) and crustaceans, but has very low toxicity to mammals. It
is non-persistent in the aquatic environment, has a slight effect on aquatic plants and
zooplankton, but is not generally phytotoxic at field concentration. Formulated as the
ethanolamine salt, or piperazine salt, or niclosamide monohydrate, it is one of the most effective
and widely used molluscicides for the control of snail vectors of schistosomiasis, a parasitic
disease affecting over 200 million people in more than seventy countries [6].
Figure 1. Structure of niclosamide (2',5-dichloro-4'-nitrosalicylanilide).
C
N
O
H
NO
2
Cl
OH
Cl
Hailemichael Alemu et al.
Bull. Chem. Soc. Ethiop. 2003, 17(1)
96
Several methods have been reported for the determination of NA. These include
spectrophotomteric techniques on derivatives or complexes of niclosamide [7-14], high
performance liquid chromatography (HPLC) [15-17], gas-liquid chromatography [18], and
polarography [19]. Most of these techniques involve complex formation or derivatization of NA
that affects the sensitivity and selectivity of NA determination. The polarographic method is
based on the reduction of NA and has very narrow linear range. HPLC or GC methods have
been officially recognised [20, 21].
Very recently, we reported the electrochemical reduction of NA at a glassy carbon electrode
using cyclic voltammetry and its direct determination by square-wave voltammetry [22]. Very
sensitive and selective procedure was developed and it was demonstrated that using the
developed method NA can be determined over the range 5 x 10
-8
– 1 x 10
-6
mol dm
-3
.
The aim of the present study was to examine the electro-oxidation behaviour of this
substance at solid electrodes that has not been reported elsewhere and to develop analytical
procedure for its determination.
Hence, in the present study the electrochemical oxidation behaviour of NA at a glassy
carbon electrode is described for the first time and based on the oxidation wave a voltammetric
procedure has been developed. This method has a wider linear dynamic range (1 x 10
-6
– 1 x 10
-4
mol dm
-3
). The method was applied successfully to the determination of NA in pharmaceutical
tablets.
EXPERIMENTAL
Apparatus
A BAS 100B electrochemical analyser (Bioanalytical Systems) was used for cyclic and square-
wave voltammetry, with a three-electrode system consisting of a glassy carbon disk working
electrode (BAS MF-2012), an Ag/AgCl (3 M NaCl) reference electrode (BAS MF-2052), and a
platinum wire auxiliary electrode (BAS MW-1032). Before each experiment the glassy carbon
electrode was polished manually with alumina (φ: 0.01 µm) on a micro-cloth pad and rinsed
with distilled and de-ionized water.
The active surface area of the working electrode was determined experimentally using 0.05
mol dm
-3
K
3
[Fe(CN)
6
] in 0.1 mol dm
-3
KCl and cyclic voltammetry at different scan rates and by
the Randels-Sevcik equation. Using the diffusion coefficient of hexacyanoferrat, 8.96 x 10
-6
cm
2
s
-1
[23], the active surface area was determined 0.06 cm
2
. This electrode was used for all
voltammetric measurements except for controlled potential electrolysis.
The pH of the buffer solution was measured with Hanna instruments digital pH meter with a
glass combination electrode and with accuracy of ± 0.05 pH. All potentials are reported with
respect to Ag/AgCl (3 mol dm
-3
NaCl) reference electrode.
Reagents
Niclosamide was obtained from Sigma (USA), 2-chloro-4-nitroaniline from Aldrich (USA), 5-
chloro-2-hydroxybenzoic acid from Merck-Schuchardt (Germany), methanol from Merck (South
Africa), ethanol and ammonium chloride from Saarchem (South Africa), ammonia solution and
sodium perchlorate from Riedel-de Haen (Germany), and sodium hydroxide from ACE (South
Africa) and were used as received. Distilled, de-ionised water was used throughout.
Ammonia-ammonium chloride buffers in the pH range 8–11 were prepared from 0.1 mol
dm
-3
ammonia solution and 0.1 mol dm
-3
ammonium chloride in water. The pH of the solutions
was adjusted by adding acetic acid or 1 mol dm
-3
sodium hydroxide. Stock solutions of
niclosamide, 2 x 10
-3
mol dm
-3
were prepared daily in pure methanol and kept in the dark. The
Electrochemical oxidation of niclosamide at a glassy carbon electrode
Bull. Chem. Soc. Ethiop. 2003, 17(1)
97
working solutions for the voltammetric investigations were prepared by dilution of the stock
methanolic solution with aqueous buffer solutions. All stock solutions were protected from light
and were used within several hours to avoid decomposition.
Procedure
Cyclic voltammetric measurements were run from an initial potential of -0.2 to a switching
potential of 1.2 V at a glassy carbon electrode with a scan rate of 100 mV s
-1
. The scan rate was
varied from 0.005 to 0.2 V s
-1
to study the dependence of the peak current and the peak potential
on the scan rate. Square wave voltammetric measurements were run from 0.15 to 1.2 V using
the Osteryoung square wave voltammetric mode and the net current responses were recorded.
The step was 10 mV, the square wave amplitude was 35 mV, and the square wave frequency
was 45 Hz. All measurements were carried out at ambient laboratory temperature (22 ± 2
o
C)
and without purging the solution with inert gas.
Controlled potential electrolysis of NA was performed at another glassy carbon electrode of
large surface area (0.79 cm
2
) in 0.1 mol dm
-3
NH
3
-NH
4
Cl aqueous solution for three NA
concentrations (c = 1 x 10
-4
, 2 x 10
-4
, 8 x 10
-4
mol dm
-3
). Solutions were stirred during
electrolysis using a magnetic stirring bar. The electrolysis was terminated when the electrolytic
current decreased to the residual current value measured in the supporting electrolyte prior to
addition of the analyte.
Analysis of tablets
Five tablets of niclosamide (EPHARM), each weighing 650 mg and containing 500 mg
niclosamide were ground to a powder and thoroughly mixed. From the ground tablets 42.52 mg
were taken and dissolved in 100 cm
3
methanol in order to obtain 1 x 10
-3
mol dm
-3
NA. This was
diluted to 1 x 10
-4
mol dm
-3
with the supporting electrolyte solution. An aliquot of this solution
(0.025 cm
3
) was spiked into the electrochemical cell that contained 20 cm
3
of 0.1 M NH
3
-NH
4
Cl
buffer (pH 8.5) and the voltammogram was recorded following the above outlined voltammetric
procedure and optimised parameters for square wave voltammetry. The standard addition
method was applied, adding successive aliquots of 0.025 cm
3
of 1 x 10
-4
mol dm
-3
NA standard
solution to the electrochemical cell. Square wave voltammograms were recorded by scanning
anodically from 0.15 to 1.2 V. The net peak current of the oxidation wave at 0.67 V was
measured. The calibration graph was then constructed by plotting the net peak current against
NA concentration.
RESULTS AND DISCUSION
Cyclic voltammetry oxidation behaviour of NA
Electrochemical oxidation of NA can take place via the phenol group at the side chain of the
molecule. In the present study carbon paste, glassy carbon, gold and platinum electrodes were
tested in the oxidation of NA. No response was obtained with carbon paste, gold and platinum
electrodes in the potential range investigated.
The cyclic voltammogram for the oxidation of 4 x 10
-4
mol dm
-3
NA at a glassy carbon
electrode in NH
3
-NH
4
Cl (pH 8.5) is shown in Figure 2. During the first cycle an irreversible
oxidation peak appeared at 0.724 V on the anodic scan. On the reverse scan no corresponding
reduction peak was observed. The oxidation of NA was investigated at different pH buffer
solutions. In the pH range 8–11 only a single irreversible oxidation peak was exhibited. In this
pH range the peak potential showed a shift of about 30 mV per pH that decreased with increase
Hailemichael Alemu et al.
Bull. Chem. Soc. Ethiop. 2003, 17(1)
98
in pH indicating the involvement of H
+
ion in the electron transfer process. When the pH was
decreased below 7 the oxidation peak disappeared completely.
Figure 2. Cyclic voltammograms of aqueous 0.1 mol dm
-3
NH
3
-NH
4
OH, pH 8.5 (A) and 4 x 10
-4
mol dm
-3
NA (B).
Organic compounds whose oxidation potentials are pH dependent undergo deprotonation
reaction during oxidation. Below pH 7 it is apparent that the NH group of NA molecule is
protonated to a great extent and hence presumably the oxidation of NA is precluded. The
absence of the oxidation peak at low pH further suggests the involvement of the NH group in
the charge transfer as well as deprotonation steps of the process.
The oxidation of NA at glassy carbon electrode gave rise to chemically irreversible process
over the scan rate range of 5 mV s
-1
to 5 V s
-1
. Figure 3 shows the cyclic voltammograms of 8 x
10
-4
mol dm
-3
NA solution at different scan rates. The peak potential for the process becomes
more positive as the scan rate increases while the peak currents are proportional to the square
root of the scan rate, for the scan rate up to 200 mV s
-1
, as expected when the mass transport
process is diffusion controlled [24, 25]. At scan rates greater than 200 mV s
-1
, the NA oxidation
process loses the characteristic diffusion controlled peak shape and becomes broad and
sigmoidal shaped implying that surface based process becomes dominant at high scan rates [26].
The effect of the potential scan rate ν on the peak current for different concentrations of NA was
studied. The peak current is proportional to the square root of scan rate ν
1/2
for all
concentrations of NA studied as predicted for a diffusion controlled regime. The linearity of the
plots is described by the following equations:
i
p
/µA = -0.052/µA + 6.304ν
1/2
; r
2
= 0.998; for c = 1 x 10
-4
mol dm
-3
(1)
i
p
/µA = 0.044/µA + 10.542ν
1/2
; r
2
= 0.997; for c = 2 x 10
-4
mol dm
-3
(2)
i
p
/µA = -0.294/µA + 44.170ν
1/2
; r
2
= 0.999; for c = 8 x 10
-4
mol dm
-3
(3)
-300 0 300 600 900 1200
0
4
8
12
A
B
Current (
µ
A)
Potential (mV)
Electrochemical oxidation of niclosamide at a glassy carbon electrode
Bull. Chem. Soc. Ethiop. 2003, 17(1)
99
Figure 3. Cyclic voltammograms of 8 x 10
-4
mol dm
-3
NA at different scan rates; (1) 5; (2) 10;
(3) 20; (4) 40; (5) 60; (6) 80; (7) 100; (8) 120; (9) 140; (10) 160 mV s
-1
.
Figure 4. Dependence of the peak potential on the logarithm of the scan rate, (1) 2 x 10
-4
; (2) 8 x
10
-4
mol dm
-3
NA.
200 400 600 800 1000 1200
0
5
10
15
20
10
9
8
7
6
5
4
3
2
1
Potential (mV)
Current (
µ
A)
-5 -4 -3 -2
0.6
0.7
0.8
2
1
Peak potential (V)
Ln
ν
Hailemichael Alemu et al.
Bull. Chem. Soc. Ethiop. 2003, 17(1)
100
Figure 4 shows the dependence of the peak potential of NA on the logarithm of the potential
scan rate for two different concentrations of NA. The peak potential is directly proportional to
the logarithm of the scan rate and the linear plots are expressed as follows:
E
p
/V = 0.704 + 0.0158ln
ν
; r
2
= 0.999; for c = 1 x 10
-4
mol dm
-3
(4)
E
p
/V = 0.855 + 0.0165ln
ν
; r
2
= 0.996; for c = 1 x 10
-8
mol dm
-3
(5)
Constant potential electrolysis of NA was carried out at 0.800 V for three concentrations of
NA, (c = 1 x 10
-4
, 2 x 10
-4
, 8 x 10
-4
mol dm
-3
) to determine the number of electrons transferred in
the process. From the electrolysis results, the average number of electrons n transferred per
molecule was found to be 1.9
±
0.2.
For a totally irreversible oxidation reaction the peak current at 25
o
C is given by:
i = (2.99 x 10
5
)n[(1-
α
)n
α
]
1/2
Ac
b
D
1/2
ν
1/2
(6)
where A in cm
2
, D in cm
2
s
-1
, c
b
in mol cm
-3
,
ν
in V s
-1
and n
α
is the number of electrons
transferred up to, and including the rate determining step [24-27]. The peak potential is related
to the scan rate
ν
with the following relation:
E
p
= K + [RT/2(1-
α
) n
α
F] ln
ν
(7)
where K = E
o
+ [RT/(1-
α
) n
α
F][0.78 + (1/2)ln [(1-
α
) n
α
F D/k
o2
RT]
Using equation (7) and t = 25
o
C, the value of (1-
α
)n
α
was determined from the slope of E
p
vs ln
ν
(equations (4) and (5)) as 0.78 and 0.82, respectively. The electron transfer coefficient
α
for the oxidation of NA was determined (
α
= 0.67) from the Tafel slope of a linear scan
voltammogram at low scan rate (5 mV s
-1
) [28]. Thus the n
α
value was estimated to be 2. The
(1-
α
)n
α
values were then inserted into equation (6) and the diffusion coefficient was determined
for 1 x 10
-4
and 8 x 10
-4
mol dm
-3
NA to be 3.04 x 10
-6
and 3.76 x 10
-6
cm
2
s
-1
, respectively,
giving an average diffusion coefficient of 3.40 x 10
-6
cm
2
s
-1
. This value is reasonably in good
agreement when compared to the diffusion coefficient of salicylate (D = 9.6 x 10
-6
cm
2
s
-1
) that
has similar structure but much lesser molecular weight [23].
Scheme 1
O
Cl
H
NH
C
O
Cl NO
2
- e
-
Cl
O
N
C
OCl
NO
2
- e
-
-H
+
cyclize
- H
+
O
.
Cl
NH
2
C
O
Cl NO
2
+
O
.
Cl
NH
C
O
Cl NO
2
+
.
Electrochemical oxidation of niclosamide at a glassy carbon electrode
Bull. Chem. Soc. Ethiop. 2003, 17(1)
101
The electrochemical oxidation of a series of Schiff bases that have similar structures like NA
was studied [29, 30].
The mechanism was investigated and found to proceed by oxidation of the
protonated substrate to give a radical cation. The radical cation is deprotonated and further
oxidised to form a di-radical cation which after losing another proton cyclizes to give product.
On the basis of these literature findings and taking into account the results of pH effect,
logarithmic analysis, cyclic voltammetry and controlled potential electrolysis, an oxidation
pathway for NA at glassy carbon electrode is hereby proposed (Scheme 1).
Square wave voltammetry
For the determination of NA both Osteryoung square wave and differential pulse techniques
were tested. It was found out that square wave voltammetry (SWV) was superior in terms of
peak intensity and resolution than the differential pulse voltammetry. Hence, SWV was utilised
throughout this study. In order to establish the optimum conditions for the determination of NA
by means of SWV, the effects of various instrumental variables were studied. The SWV
parameters are interrelated and have a combined influence on the peak current [31].
For the optimisation of instrumental conditions, the square wave frequency (f), the potential
step (
∆
E
s
) and the pulse amplitude (
∆
E) were examined, varying one of them and maintaining
constant the others. The variable ranges were: 2–10 mV for the potential step; 5–70 Hz for the
frequency and 12–40 mV for the pulse amplitude. The net peak current increased by increasing
all of these instrumental parameters. At higher potential step values the peak width increased
and at higher frequency values the background current and the peak potential increased. Finally
the conditions selected were:
∆
E
s
= 10 mV, f = 45 Hz and
∆
E = 35 mV.
The influence of the initial sweep potential on
∆
I
p
was examined in the potential range -200
to 250 mV. The analytical signal size is influenced by the initial sweep potential and the net
peak current was found to increase rapidly with increasing the potential up to 100 mV and then
remained constant (Figure 5). Therefore, initial sweep potential of 150 mV was chosen for all
subsequent measurements.
Effect of buffer solutions
Different types of buffer solutions were tested for their suitability in the determination of NA:
phosphate buffer, KH
2
PO
4
-Na
2
HPO
4
; acetate buffer, CH
3
COOH-CH
3
COONa; borate buffer,
H
3
BO
3
-NaBO
2
; and ammonia buffer, NH
3
-NH
4
Cl. There was no voltammetric signal detected in
acidic buffer system. The most suitable buffer system was found to be 0.1 mol dm
-3
aqueous
NH
3
-NH
4
Cl, since the voltammogram of niclosamide was well defined with higher sensitivity.
Effect of pH
The influence of pH on the net peak current of niclosamide was investigated over the range of
pH 8–11 (Figure 6). Low and constant current signals are obtained between pH 10 and 11.
Between pH 8 and 9.5 the response increased with decreasing pH. The largest peak signal was
obtained at pH 8.5. Thus pH 8.5 was selected for the analysis.
Hailemichael Alemu et al.
Bull. Chem. Soc. Ethiop. 2003, 17(1)
102
Figure 5. The effect of the initial sweep potential on the net peak current for 1 x 10
-4
mol dm
-3
NA.
Figure 6. The influence of pH on the net peak current for 1 x 10
-4
mol dm
-3
(A) and 2 x 10
-4
mol
dm
-3
NA (B).
-200 -100 0 100 200 300
4
5
∆
I
p
(
µ
A)
Initial sweep potential (mV)
8 9 10 11
4
6
8
10
B
A
∆
I
p
(
µ
A)
pH
Electrochemical oxidation of niclosamide at a glassy carbon electrode
Bull. Chem. Soc. Ethiop. 2003, 17(1)
103
Linear range and detection limit
Under the optimum conditions, using the square-wave mode the peak current was linearly
dependent on NA concentration. Selected square wave voltammograms at different
concentrations of NA are shown in Figure 7A. The dependence of the net peak current as a
function of the concentration of NA is also shown in Figure 7B. Each data point in Figure 7B is
the mean value of the net peak currents for three SWV runs. The net peak current increased with
increasing concentration of NA. The response was found to be linear in the concentration range
1.00 x 10
-6
– 1.00 x 10
-4
mol dm
-3
NA and the correlation coefficient was r
2
= 0.999. At higher
concentrations (
≥
2 x 10
-4
M) deviation from linearity occurred due to saturation of the electrode
surface. The detection limit (three times signal-to-noise ratio) was found to be 8 x 10
-7
mol dm
-3
NA. For eight successive determinations of 1 x 10
-5
mol dm
-3
NA, a relative standard deviation
(RSD) of 3.6% was obtained. Consistent electrode surface cleaning after each experimental run
enhanced the reproducibility of the results. Thus, uniform electrode surface cleaning is
recommended after each measurement.
Figure 7. Osteryoung square wave voltammograms of NA (A). (a) 0; (b) 1 x 10
-6
; (c) 2 x 10
-6
;
(d) 4 x 10
-6
; (e) 6 x 10
-6
; (f) 8 x 10
-6
; (g) 1 x 10
-5
; (h) 2 x 10
-5
; (i) 4 x 10
-5
. The net peak
currents of niclosamide as a function of concentration of NA in the range 1 x 10
-6
– 1 x
10
-4
mol dm
-3
(B).
Hailemichael Alemu et al.
Bull. Chem. Soc. Ethiop. 2003, 17(1)
104
Interferences and selectivity
The effect of the concomitants associated with NA in its pure form and its formulations were
tested using the developed method. This method does not suffer any interference from
commonly associated sweetening and flavouring agents used in the preparation of tablets, such
as sucrose, lactose, dextrose, starch, talc, stearic acid and sodium alginate with respect to known
amount of NA. The mean recovery was 98.83%.
The selectivity of the method was tested by examining 2-chloro-4-nitroaniline and 5-
chlorosalicylic acid. These compounds are disintegration products of NA that are routinely
tested in the drug manufacturing of NA [2, 4]. 5-Chlorosalicylic acid and 2-chloro-4-nitroaniline
gave irreversible oxidation peaks at more positive potentials than NA. The presence of five fold
molar excess of 5-chlorosalicylic acid and 2-chloro-4-nitroaniline in NA, respectively, did not
affect the peak current of NA. These observations indicate that the method is very selective for
NA.
Figure 8. Osteryoung square wave voltammograms of NA tablets at a glassy carbon electrode in
0.1 mol dm
-3
NH
3
-NH
4
OH buffer (pH 8.5); (a) 1 x 10
-5
and (b) 4 x 10
-5
mol dm
-3
NA.
Other experimental conditions as in Figure 7.
Analytical application
Figure 8 shows the square wave voltammograms of NA tablet for two concentrations (c = 1 x
10
-5
and 4 x 10
-5
mol dm
-3
) prepared without any purification. The proposed method was applied
Electrochemical oxidation of niclosamide at a glassy carbon electrode
Bull. Chem. Soc. Ethiop. 2003, 17(1)
105
to the determination of niclosamide in tablets by using the standard additions method. The
procedure used for the determination of NA as described earlier gave a mean value of 495.2 mg
of NA per tablet. This is in very good agreement with the declared value of 500 mg.
CONCLUSION
This study presents the electrochemical oxidation behaviour and analytical determination of NA.
The cyclic voltammetric response obtained is chemically irreversible over the range of scan rates
employed, and is consistent with electron transfer being followed by fast chemical process.
Successful application of SWV for the determination of NA in pharmaceutical formulation is
demonstrated. The method is relatively cheap and rapid in comparison with other methods and
avoids time-consuming extraction steps to remove the excipients from tablets. Combining the
present method with the recently reported voltammetric method [22] gives a very wide linear
range for the determination of NA.
ACKNOWLEDGEMENTS
The authors are grateful for the procurement of instruments by the Ministry of Education for the
National University of Lesotho, Chemistry Department. The Department of Chemistry is
acknowledged for both the material and financial support.
REFERENCES
1. Jones, W.E. Am. J. Trop. Med. Hyg. 1979, 28, 300.
2. Remington's Pharmaceutical Sciences, Mack Publishing Co.: Easton, Pennsylvania; 1980, p
1182.
3. Data Sheet on Pesticides No 63, World Health Organization, Food and Agriculture
Organization; WHO/VBC/DS/88.63, 1988.
4. Korlkovas, A. Essentials of Medicinal Chemistry, John Wiley: New York; 1988; p 617.
5. Reynolds, E.F. Martindale The Extra Pharmacopoei, The Pharmaceutical Press: London;
1982; p 100.
6. Churchill, F.C.; Ku, D.N. J. Chromatogr. 1980, 189, 375.
7. Bergisadi, N.; Sarigul, D. Acta Pharm. Turc. 1986, 28, 51.
8. Zarapkar, S.S.; Mehron, S.S; Rele, S.R. Indian Drugs 1989, 26, 360.
9. Onur, F.; Tekin, N. Anal. Lett. 1994, 27, 229.
10. Zarapkar, S.S.; Deshpande, P.M. Indian J. Pharm. Sci.1989, 51, 136.
11. Sastry, C.P.; Amna, M.; Rao, A.R. Talanta 1988, 35, 23.
12. Sastry, C.P.; Amna, M; Rao, A.R. Indian Drugs 1988, 25, 348.
13. Fattah, S.A. Spectrosc. Lett. 1997, 30, 795.
14. Daabees, H.G. Anal. Lett. 2000, 33, 639.
15. Derek, C.G.; Norbert, P. Int. J. Environ. Anal. Chem. 1980, 8, 1.
16. Dawson, V.K. Can. J. Fish. Aquat. Sci. 1982, 39, 778.
17. Schreier, M.T.; Dawson, K.V.; Boogaard, A.M. J. Agric. Food Chem. 2000, 48, 2212.
18. Johnson, S.J.; Geoffrey, B.P. Pestic. Sci. 1979, 10, 531.
19. Mishra, A.K.; Gode, K.D. Indian Drugs 1985, 22, 317.
20. Muir, D.C.G.; Grift, N.P. Int. J. Environ. Anal. Chem. 1980, 8, 1.
Hailemichael Alemu et al.
Bull. Chem. Soc. Ethiop. 2003, 17(1)
106
21. Luhning, C.W.; Harmann, P.D.; Sills, J.B.; Dawson, V.K.; Allen, J. J. Ass. Off. Analyt.
Chem. 1979, 62, 1141.
22. Alemu, H.; Wagana, P.; Tseki, P. Analyst, 2002, 127, 129.
23. Lide, D.R. CRC Handbook of Chemistry and Physics, CRC Press: Boca Raton; 1997.
24. Southampton Electrochemistry Group Instrumental Methods in Electrochemistry, Ellis
Horwood; Chichester; 1985.
25. Brown, E.R.; Sandifer, J.R. Physical Methods of Chemistry, Volume II, Electrochemical
Methods, Rossiter, W.; Hamilton, J.F. (Eds.); Wiley-Interscience: New York; 1986.
26. Coomber, D.C.; Tucker, D.J.; Bond, A.M. Electroanalysis, 1998, 10, 163.
27. Golabi, S.M.; Zare, H.R.; Hamzehloo, M. Microchem. J. 2001, 69, 111.
28. Bard, A.J.; Faulkner, L.R. Electrochemical Methods Fundamentals and Applications, John
Wiley: New York; 1980.
29. Lacan, M.; Rogic, V.; Tabakovic, I.; Galijas, D.; Solomun, T. Electrochim. Acta 1983, 28,
199.
30. Pletcher, D. Specialist Periodical Reports, Electrochemistry, 1985, Royal Society of
Chemistry, Burlington House: London; pp 208-209.
31. Christie, J.H.; Turner, J.A.; Osteryoung, R.A. Anal. Chem. 1977, 49,1899.
