Cyclic voltammetric study on the effect of the ...

J.Natn.Sci.Foundation Sri Lanka 2010 38 (2): 91-99

RESEARCH ARTICLE

Cyclic voltammetric study on the effect of the introduction of
secondary ligands on the redox behaviour of the copper-saccharin
complex

Monira Laiju, H.M. Naseem Akhtar, M.A. Mamun, M.d. Abdul Jabbar and M.Q. Ehsan*

Department of Chemistry, Faculty of Science, University of Dhaka, Dhaka-1000, Bangladesh.

Revised: 25 August 2009 ; Accepted: 16 October 2009

Abstract: The detailed redox behaviour of copper-saccharin Saccharin (Sac) is used as a pharmaceutical excipient
(Cu-sac) complex was examined using the cyclic voltammetric in the formulation of different medicinal products like
technique. It was found that the adsorption process suppresses syrups, suspensions and as a sweetening agent. It is a
the Faradaic process of the Cu-sac complex. The effect of the good ligand and coordinates through its N atom. Due
introduction of secondary ligands such as 1,10-phenanthroline mainly to the fact that it can act as biologically active
(phen), pyridine (py) and bypyridine (bp) on the redox behaviour substance and thus affect the living systems, the metal
of the Cu-sac complex in aqueous solution was studied where complexes of saccharin have drawn certain scientific
these ligands contribute on the charge-transfer kinetics of the attention during the last two decades. X-ray diffraction
complex. The heterogeneous charge transfer rate constants are and Fourier transform infrared (FT-IR) spectroscopic
found to follow the order Cu(NO3)2 > Cu-sac-bp > Cu-sac > studies on the mixed ligand complex of Cu(II)with
Cu-sac-phen > Cu-sac-py. saccharin and pyridine in the solid state have been
reported. It has been concluded that [Cu(II)(sac) (H O) ]
Keywords: Copper-saccharin complex, cyclic voltammetry,
ligands, redox behaviour. 224

INTRODUCTION complex shows an unusually high lability towards
water interchange with aromatic nitrogen bases such
Copper is essential for many diverse functions in the as pyridine2. Because of their potential pathological
biological system such as formation of melanin, electron- effects a study of the structural properties of metal(II)
transport, phospholipid synthesis, collagen synthesis saccharin compounds containing aromatic nitrogen bases
and integrity of myelin sheath. Three Cu-containing as secondary ligands has provoked a pronounced interest
proteins, such as, cerebrocuprein, erythrocuprein and recently. The spectroscopic properties of saccharin in
hepatocuprein occur in the brain, blood (RBC) and metal complexes with pyridine3–4, imidazol5–7, 2,2’-
liver respectively1. Many Copper(II)-complexes show bipyridine8 and 1,10-phenanthroline9 have been reported.
biological activity. For example, complexes of Cu(II) with No references on the electrochemical behaviour of the
bioactive carboxyamide ligands N′,N″-bis(3-carboxy-1- mixed ligand complexes of cysteine, Cu(II)–saccharinate
oxoprop-2-enyl)2-amino-N-arylbenzamidine, N′,N″-bis(3- complex and saccharin–cysteine interaction in aqueous
carboxy -1-oxopropanyl)2 amino-N-arylbenzamidine medium have been traced so far in the literature.
and N′,N″-bis(3-carboxy-1-oxophenelenyl)2-amino-N-
arylbenzamidine show bioactivity against the growth of Due to the diverse uses of the metal-ligand-
bacteria and pathogenic fungi, and the results indicate complexes, scientists have been trying to improve the
that the ligand and its metal complexes possess notable performance of metal-ligand complexes. One of the
antimicrobial properties2. ways to improve the performance of a complex either for
its electrochemical and/or-photochemical activities is to
introduce a second ligand into the complex. For example,
photoluminescence and redox properties of europium
complexes increase by introducing 1,10-phenanthroline,

* Corresponding author ([email protected], [email protected])

92 Monira Laiju et al.

into the complex10. The role of the second ligand is not polishing cloth. After polishing, the electrode was rinsed
only to saturate the co-ordination number of the central with a continuous flow of deionized water and wiped-
metal ion but also to improve the volatility and stability off with a clean tissue paper to remove the last drop of
of the complex11. A Study12 reported that carrier-transport water. The reference and the counter electrodes were also
characteristics and light-emitting properties can be thoroughly rinsed with deionized water prior to use.
improved by using bathophenanthroline as the second
ligand. This situation lead to the pesent study of the Preparation of the complexes: Copper-saccharin (Cu-
effect of secondary ligand in a complex. In this paper, the sac), copper-sac-phenanthroline (Cu-sac-phen), copper-
following is reported i) the redox behaviour of Cu-system saccharin-bipyridine (Cu-sac-bpy) and copper-saccharin-
in Cu-saccharin complex, ii) redox behaviour of Cu- pyridine (Cu-sac-py) complexes were synthesized and
saccharin complex in the presence of secondary ligands characterized following published procedures15,16.
such as, 1,10-phenanthroline, 2,2’-bipyridine and pyridine
at a glassy carbon electrode in aqueous KCl solution and Details of the cyclic voltammetric study of different
acetate buffer solutions of different pH. Among all the systems: The redox behaviour of 200 ppm solutions of
voltammetric techniques, cyclic voltammetry (CV) is copper complexes were studied in 0.1M KCl at room
extremely popular in electrochemical research, because temperature using cyclic voltammetric technique at a
it can provide useful information about redox reactions glassy carbon electrode. The effect of co-ordination
in an easily interpretable form 13. was described by comparing the cyclic voltammograms
(CVs) between un-coordinated copper and various
METHODS AND MATERIALS copper-ligand complexes. The effect of presence of the
secondary ligands, such as, 1,10-phenanthroline, 2.2’-
Reagents and solutions: All the reagents and solutions bipyridine and pyridine, respectively, were also studied.
were prepared using analar grade chemicals. The
ligands used in the synthesis were from BDH, England The redox properties of the corresponding complexes
and E-Merck, Germany. To study pH effect, solutions in aqueous KCl and acetate buffer were studied by
of the complexes were prepared using acetate buffer comparing with that of pure Cu(II) and the Cu-sac
solutions of pH 4.1, 4.5, 4.9, 5.2 and 5.4, respectively. complex.
The preparations of acetate buffer solutions of different
pH values were done using sodium acetate (MERCK, RESULTS AND DISCUSSION
Germany) and acetic acid (Sigma-Aldrich)' according to
the literature14. Redox behaviour of Cu(II) in Cu-sac complex

Equipment: The current-voltage measurements were The redox behaviour of Cu(II) in cu-sac complex was
performed with an Epsilon electrochemical workstation studied at a glassy carbon electrode within the potential
of Bioanalytical System Inc., (BAS), USA. The pH window of 1500 to –1000 mV at room temperature
meter (Orion, Thermo Electron Corporation) was used to by comparing the cyclic voltammograms of Cu(II) in
measure the pH of experimental solutions.Avoltammetric Cu(NO ) . CV of 200 ppm Cu and 200 ppm Cu-sac
cell (three-electrode cylindrical shape micro-cell system)
made of borosilicate glass was used in this work. Glassy 32
carbon electrode (GCE) with geometric area 0.05 cm2
was used as the working electrode, Ag/AgCl (saturated complex in 0.1 mol dm-3 KCl are shown in curve ‘(a)’
KCl) as the reference electrode and platinum wire as the and ‘(b)’ respectively in Figure1(A).
counter electrode. All the electrodes were procured from
BAS, USA. The mixing and purging of N2 gas (99.97% Compared to the pure metal salt [curve (a)] of
pure, procured from Bangladesh Oxygen Limited) were Cu(II), the peak positions and the peak currents in the
done by using a BAS C-3 Cell stand combined with a Cu-sac complex [curve (b)] are drastically changed.
Faraday Cage and a magnetic stirrer. All of the potentials The reversible peak pair of pure Cu(II) [curve (a)] in
reported in this paper are with respect to Ag/AgCl/KCl aqueous KCl solution observed at 0.480 V and 0.420 V
electrode. respectively, is shifted to more cathodic region in the case
of Cu-sac complex [curve (b)]. The two anodic peaks of
Preparation of the electrode surface: Prior to use, the Cu-sac complex were found at potentials 0.178 V and
glassy carbon electrode surface was polished with -0.007 V respectively, compared to those in case of
0.5 π fine alumina powder slurry in deionized water on a pure copper in aqueous solution. In the case of Cu-sac
complex, the anodic peaks become closer to each other.

June 2010 Journal of the National Science Foundation of Sri Lanka 38 (2)

Redox behaviour of the copper - saccharin complex 93

The electrode reactions for pure Cu(II) may be shown peak current is reduced at least two fold as compared to
according to the following scheme: the pure Cu(II) [Figure 1(B)]. This may be due to some
other process occurring simultaneously at the electrode
Cathodic Reaction Cu2+ + e Cu+ + e Cuo surface. Such behaviour has been ascribed to slower
charge propagation, probably due to the difference in
Anodic Reaction Cuo - e Cu+ - e Cu2+ solvation and or permeability17. The chemical reactions,
which occur in this process, may follow the electron
The redox behaviour of the Cu-sac complex in aqueous transfer process18. Therefore, the mass transfer process
KCl solution, CV of Cu-sac complex has been performed is limited due to the charge transfer kinetics19 indicating
at different scan rates and represented in Figure 1(B). that the peak current increases and the ratio of the peak
The current-potential data and calculated parameters for current (ipa/ipc) decreases with the increase of scan
the Cu-sac complex derived from Figure 1(B) are listed rate. The second cathodic process, where the peak has
in Table 1. It shows that the first cathodic peak becomes broadened, is probably influenced by the redox behaviour
sharper and the second one becomes broader with higher of the first redox process (quasi-reversible)20 occurring
scan rate. For the first cathodic peak, there is no change at the positive side in the CV. This also causes the
in peak position, and the second cathodic peak shifts redox process to become quasi-reversible as compared
towards negative potential. For the anodic peak, the first to the pure Cu(II) system, where the same peak-pair is
one is shifted towards more positive potential and the reversible20. Figure 2 shows that the peak currents (ip)
second peak shows no change. Interestingly, the second for both the cathodic and anodic peaks for the Cu-sac
cathodic peak at ca. – 0.45 V peak has broadened in the complex increase linearly with the square root of the
case of Cu-sac complex with increase in scan rate and the scan rates.

15 (a) (b) 16 iv
(A) (B)
iii
10 8 ii
i
5

Current (mA)
Current (mA)
0
0

-5

-10 1.0 0.5 0.0 -0.5 -1.0 -8 1.0 0.5 0.0 -0.5 -1.0

-15 Potential (mV) -16 Potential (E/V vs Ag/AgCl)
1.5
-20
1.5

Figure 1: (A) Combined cyclic voltammograms of (a) 200 ppm Cu(II)-nitrate and (b) 200 ppm Cu-sac-complex at
scan rate 100 mV s-1 and (B) Cyclic voltammograms of Cu-sac-complex (200 ppm) at different scan rates
i) 50, ii) 100, iii) 150, iv) 200 and v) 300 mVs-1 in 0.1moldm-3 KCl vs. Ag/AgCl

Table 1: Current-potential data, peak separation and peak current ratio of the voltammograms of 200 ppm copper- saccharin complex in 0.1M
KCl at different scan rates

(ν) Epa1 Epa2 Epc1 -Epc2 -ipa1 -ipa2 ipc1 ipc2 ΔEp1 ΔEp2 ipa1/ ipa2/
V/sec V V V V mA mA mA mA V V ipc1 ipc2

0.050 0.162 0.020 0.124 0.574 2.979 8.01 3.766 5.229 0.038 0.596 0.791 1.532
0.100 0.178 0.017 0.118 0.597 2.102 12.09 4.907 8.257 0.060 0.614 0.429 1.464
0.150 0.195 0.019 0.113 0.614 2.453 13.32 5.783 9.771 0.082 0.633 0.424 1.363
0.200 0.195 0.019 0.113 0.614 2.453 14.03 7.009 11.66 0.082 0.633 0.350 1.203
0.300 0.195 0.017 0.118 0.641 3.154 15.64 8.411 14.79 1.352 0.631 0.375 1.058

ν = scan rate, Epa1 = anodic peak potential for the 1st peak, Epa2 = anodic peak potential for the 2nd peak, Epc1 = cathodic peak potential for the
1st peak, Epc2 = cathodic peak potential for the 2nd peak, ipa1 = anodic peak current for the 1st peak, ipa2 = anodic peak current for the 2nd peak,
ipc1 = cathodic peak current for the 1st peak, ipc2 = cathodic peak current for the 2nd peak, DEp1 = peak separation for the 1st pair, DEp2 = peak
separation for the 2nd pair.

Journal of the National Science Foundation of Sri Lanka 38 (2) June 2010

94 Monira Laiju et al.

Though the peak currents vary linearly, the intercepts does not pass through the origin (figure not shown) as
it is expected from the Randless-Sevcik equation. This
for both the lines at the Y-axis indicate that the process strongly suggests that the electrogenerated cathodic
is adsorption controlled21. Figure 3 indicates that with species generated from the reduction of Cu-sac complex
are adsorbed on the electrode surface. Therefore, the Cu-
increasing scan rate peak potential separation (ΔE ) also sac system in aqueous KCl electrolytic system is limited
p by an adsorption controlled process21.

increases. This may be due to the slow electron transfer
kinetics or Ohomic Potential (iR) drop22.

Effect of pH: The CV of the Cu-sac complex was studied Cu(II)-saccharin-phenanthroline(Cu-sac-phen)
at different pH values (4.1, 4.5, 4.9, 5.2 and 5.4) using complex
the acetate buffer. In the acetate buffer, we observe
significant change in the CV of the Cu-sac complex. At A CV of the pure copper Cu(II) and Cu-sac-phen
pH 4.1, two significant cathodic peaks can be seen but complex in aqueous KCl solution at scan rate 100 mVs-1
the anodic peaks merge to give one peak. At higher pH is shown in Figure 4A. The presence of the secondary
(pH 5.4), the cathodic peaks become significantly sharp ligand, phenanthroline has brought about changes in the
and anodic peak splits. number of the peaks as well as the shapes of the peaks of
the Cu-sac complex.
Concentration effect: CVs of Cu-sac complex of
various concentrations (150, 200, and 300 ppm) were The Cu-sac-phen complex shows that there are two
studied, which shows that the cathodic peak current peaks in the cathodic scan [Figure 4(A), curve b]. The
increases linearly with increase in concentration as it first one is observed as a hump and the second one as a
is expected from the diffusion controlled process21. In broadened. Peak observed ca. – 0.500 V vs. Ag/AgCl.
this experiment, it is also observed that the linear line
0.09
10

8 (a) 0.08

Peak current/mA 6 0.07

4 DEp/mV

0.06

2

0.05

0 0.04

(b)

-2

-4 0.1 0.2 0.3 0.4 0.5 0.6 0.03
0.0 (Scan rate1/2, v1/2)/mVs-1 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

Scan rate, v (mV/s)

Figure 2: Variation of peak current with square root of Figure 3: Variation of peak potential separation against
scan rate for Cu-sac complex for (a) anodic and scan rate for Cu-sac complex (200 ppm solution)
(b) cathodic peak currents in 0.1M KCl

-5.9998 a -6.9998 (v)
(A) b (iv)
(B) (iii)
-5.9999 (ii)
-6.9999
Current (mA) -6.0000 Current (mA) (i)
-7.0000

-6.0001

-7.0001

-6.0002

-6.0003 -7.0002

1.0 0.5 0.0 -0.5 -1.0 1.0 0.5 0.0 -0.5
Potential ( E/V vs Ag/AgCl)
Potential (mV)

Figure 4: (A) Combined cyclic voltammograms of (a) 200 ppm Cu(II)-nitrate and (b) 200 ppm Cu-sac-phen complex at
scan rate 100 mV s-1 and (B) Cyclic voltammograms of Cu-sac-phen complex (200 ppm) at different scan rate:
i) 50, ii) 100, iii) 150 and iv) 200 mVs-1 in 0.1M KCl

June 2010 Journal of the National Science Foundation of Sri Lanka 38 (2)

Redox behaviour of the copper - saccharin complex 95

With the increase in scan rate, the first cathodic peak ligand bp. Figure 5(B) represents the CV of the Cu-sac-bp
becomes sharp and the second cathodic peak becomes complex at different scan rates, where it is observed that
an individual. Peak [(Figure 4(B)]. The redox behaviour the cathodic peaks shift to more negative potential and
of both the cathodic processes become prominent with the anodic peaks slightly towards positive potential by
increase in the scan rate. The peak potential separation increasing the scan rates. The peak currents also increase
(ΔEp) increases slowly with increase in scan rate due to with increase in scan rates but the cathodic peak becomes
the prominent charge transfer kinetic controlled process. broader. The respective cathodic to anodic peak potential
It is also attributed that the Faradaic process is limited separation (ΔEp) and peak current ratio also increases
in this case19. The plot of peak current (both the anodic with increasing scan rates. Such phenomena have taken
and cathodic peak current) vs. square root of the scan place in the case of diffusion process proceeding with a
rate are linear at the higher scan rate but deviates from secondary process such as adsorption or other chemical
linearity at the slower scan rate( figure not shown). process22. The current function (ip/v1/2) for the second
This fact may be explained that at a higher scan rate cathodic process has also been found to increase with
the process is diffusion controlled and at a slower scan increasing scan rates. A significant increase in the current
rate there is a contribution of both Nernstian and kinetic functions at faster scan rates is a strong indication of the
controlled processes at the electrode surface. It may also presence of weak adsorption23.
be deduced from the observation that the attachment of
the ligand 1,10-phenanthroline to Cu-sac complex makes Copper-saccharin-pyridine (Cu-sac-py) complex
the system (Cu-sac-phen) bulky, as a result the redox
behaviour becomes more prominent at the higher scan Comparative CVs of pure Cu(II) (curve a) and Cu-sac-
rate. py complex (curve b) are represented in Figure 6(A)
at 100 mV/s scan rate. The first cathodic peak shift
Cu(II)-saccharin-bipyridine (Cu-sac-bp) complex towards the anodic direction indicates that the second
ligand py catalyzed the first reduction process. However,
Comparative CVs between the Cu-sac-bp system the second cathodic peak has appeared as a broad peak
(curve b) and pure Cu(II) (curve a) at scan rate 100 mVs-1 indicating that the process is adsorption controlled.
are shown in Figure 5(A). The CV of Cu-sac-bp system Thus, the system shows almost the same type of redox
[curve b in Figure 5(A)] shows one cathodic and one behaviour of the Cu-sac complex with a difference in
anodic peak. The peak-pair shows one-step process and the magnitude of the peak potential due to the presence
exhibit reversible redox behaviour. Interestingly, the of the secondary ligand, pyridine. The similar nature of
first cathodic peak of pure Cu(II) completely vanishes CVs between Cu-sac and Cu-sac-py complexes suggests
and the second cathodic peak shifts towards the anodic that the secondary ligand pyridine has a very small effect
direction in the case of Cu-sac-bp complex. The shifting (relative to the other mixed ligand complexes of Cu-sac
of the potential of the cathodic peak towards the complex system) on their redox behaviour. The CVs of
positive direction suggests that the reduction process of the complex at different scan rates have been presented
Cu-sac-bp mixed ligand complex is catalytic-controlled20. in Figure 6(B), which show changes in peak current but
It is found that the redox behaviour is enhanced for the no change in peak position with increasing scan rates.
Cu-sac complex with the introduction of secondary

Table 2: Current-potential data, Tafel slope b, diffusion coefficient, D and the charge-
transfer rate constant, kf calculated from the voltammograms of 1.0 mM
metal salt and metal complexes in 0.1 M KCl at 100 mV s-1 and at ambient

temperature

Sample ID Epc ipc b D x1011 kf x 106
V µA cm-2 s-1 (cm s-1)
Cu(NO3)2 0.077
Cu-sac 0.174 13.980 0.041 0.280 3.15
Cu-sac-phen 0.134 5.170 0.038 0.020 1.17
Cu-sac-bp -0.029 3.590 0.2076 0.009 0.80
Cu-sac-py -0.177 6.870 0.0413 0.180 1.55
0.134 3.010 0.007 0.68

Epc= cathodic peak potential, ipc = cathodic peak current, b = Tafel slope = (2.303 RT /
α Na F), T = 298 K, D = diffusion coefficient, n = number of electron transferred = 2,
R = 8.314 J K mol , F = 96500 C, A = surface area of the electrode = 0.05 cm2,

-1 -1

kf = heterogeneous electron transfer rate constant

Journal of the National Science Foundation of Sri Lanka 38 (2) June 2010

96 Monira Laiju et al.

15 (b) 15 v

Current ( mA) (A) Current ( mA) (B) iv
iii
10 10 ii

5 (a) 5 i

0

-5
0

-10

-5
-15

-20 -10
1.5 1.0 0.5 0.0 -0.5 -1.0 1.5 1.0 0.5 0.0 -0.5 -1.0

Potential (mV) Potential (mV)

Figure 5: (A) Combined cyclic voltammograms of (a) 200 ppm Cu(II)-nitrate and (b) 200 ppm Cu-sac-bp complex at
scan rate 100 mV s-1 and (B) cyclic voltammograms of Cu-sac-bpy complex (200 ppm) at different scan rates
i) 50, ii) 100, iii) 150, iv) 200 and v) 300 mVs-1 in 0.1M KCl vs. Ag/AgCl

15 (a) (b) 15 v

(A) Current ( mA) (B) iv
iii
10 10 ii
i
5 5
Current ( mA)
0

-5 0

-10 -5 1.0 0.5 0.0 -0.5 -1.0
Potential (E/V vs Ag/AgCl)
-15 -10
1.5
-20
1.5 1.0 0.5 0.0 -0.5 -1.0

Potential (mV)

Figure 6(A): Combined CVs of (a) 200 ppm Cu(II)-nitrate and (b) 200 ppm Cu-sac-py complex at scan rate
100 mV s-1 and (B) Cu-sac-py complex (200 ppm) at different scan rates: i) 50, ii) 100,
iii) 150, iv) 200 and v) 300 mVs-1 in the supporting electrolyte (0.1M KCl) vs. Ag/AgCl

10 vv -5.99985 ..... pH=4.5
(B) ----------------pH = 5.2 -5.99990 - - - - pH=5.4
-5.99995
Current ( mA) 8 iviv Current ( mA) -6.00000
6 -6.00005
4 iiii -6.00010
2 iii -6.00015
0 ii -6.00020
-2
-4 ----------------pH = 5.4 1.5
-6
-8 -----pH=5.2
1.5 ---- pH = 4.9

----------------pH = 4.9 ----- pH=4.1
1.0 0.5 0.0
----------------pH = 4.5
----------------pH = 4.1 Potential ( E/V vs Ag/AgCl)

-0.5 -1.0

1.0 0.5 0.0 -0.5 -1.0

Potential (mV)

Figure 7: Cyclic voltammograms of Cu-sac-phen (200 ppm solution) Figure 8: Cyclic voltammograms of Cu-sac-bp (200 ppm solution)
complex in acetate buffer at different pH 4.1, 4.5, 4.9, 5.2 complex in acetate buffer at different pH
and 5.4

June 2010 Journal of the National Science Foundation of Sri Lanka 38 (2)

Redox behaviour of the copper - saccharin complex 97

20 20 20

pH=4.1 pH=4.5 pH=4.9

0 0 0

Current (PA)
Current (PA)
Current (PA)
-20 -20 -20

-40 -40 -40

-60 -60 -60

1.5 1.0 0.5 0.0 -0.5 -1.0 1.5 1.0 0.5 0.0 -0.5 -1.0 1.5 1.0 0.5 0.0 -0.5 -1.0

Potential (E/ VvsAg/AgCl) Potential (E/ VvsAg/AgCl) Potential (E/ VvsAg/AgCl)

20 20

pH=5.2 pH=5.4

0 0

Current (PA)
Current (PA)
-20 -20

-40 -40

-60 1.0 0.5 0.0 -0.5 -60 1.0 0.5 0.0 -0.5 -1.0
1.5 Potential (E/ VvsAg/AgCl) -1.0 1.5 Potential (E/ VvsAg/AgCl)

Figure 9: Cyclic voltammograms of Cu-sac-py (200 ppm solution) complex in acetate buffer of different
pH(4.1, 4.5, 4.9, 5.2 and 5.4)

Effect of pH interesting feature of the CV of Cu-sac-bp system is the
shifting of potential and decreasing the peak current
CVs of Cu(II)-sac-phen complex at different pH values magnitude. It has been noticed that by increasing pH
are represented in Figure 7. The CVs show that there are by a factor of 1 (from pH 4.1 to 5.2), the peak potential
small differences in the peak shapes. As it is expected, shifted about 0.060 V towards the cathodic position. This
the anodic peak potentials are shifted to a more anodic suggests that the catalytic activity of Cu-sac complex
position with increase in pH. With one-unit of pH increases by introducing the second ligand bp and only
deviation, the peak potential is shifted to about 60 mV, one-electron is involved in the redox process19,24.
suggesting the involvement of one-electron in the redox
reaction24. CVs for Cu-sac-py complex at scan rate 100 mVs-1
using the acetate buffer are presented in Figure 9. CVs
The CVs of Cu-sac-bp complex, at scan rate at 4.1 and 4.5 have two cathodic peaks and one anodic
100 mV s-1 in acetate buffer solution at different pH peak, whereas at pH 5.2 and 5.4 the trend is the opposite.
values are shown in Figure 8. At pH 4.9, there is only one pair of peaks, i.e. with
increase of pH, separation of cathodic peaks decreases
There are only small differences in the CVs consisting and ultimately fuse to one and the peak separation of
of mainly one pair of cathodic and anodic peaks. The anodic peak increases and ultimately split into two anodic
anodic peak becomes broader and the peak current peaks.
decreases with increase in pH values. A small peak at
-0.50 V is due the presence of small amount of oxygen in Concentration effect: The CV for all three mixed
the medium despite the removal of oxygen by nitrogen ligand complexes were also analyzed for different
purging. Sonicating the GCE surface for the removal concentrations of solutions (150 ppm, 200 ppm and 300
of oxygen was avoided, which sometimes disconnect ppm). In all cases the peak current increases due to the
the carbon disc and the copper rod of the electrode. An increased concentration of metal ion. Plot of current vs.

Journal of the National Science Foundation of Sri Lanka 38 (2) June 2010

98 Monira Laiju et al.

concentration for both cathodic and anodic peak current 7. Li J., Ke Y., Wang Q. & Wu X. (1997). Synthesis, crystal
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controlled18.
8. Hergold-Brundic A., Grupcˇe O. & Jovanovski G. (1991).
Charge transfer rate constant: The heterogeneous charge Structure of bis(2,2'-bipyridyl)(saccharinato-N)copper(II)
transfer rate constants for the copper salt and the copper saccharinate dehydrate Acta Crystallographica C 47:
complexes were calculated. To calculate the heterogeneous 2659-2660.
charge transfer rate constant, kf , the current-potential data
obtained from the cyclic voltammograms of metal salts 9. Li J., Chen H., Wu Q. & Wu X. (1993). Structure of a
and metal complexes at 100 mV s-1 and 200 mV s-1 were manganese (II) complex containing saccharin and 1,10-
used. The current-potential data, Tafel slope, diffusion phenanthroline:[Mn(Sacch)2(o-Phen)2(H2O)2]H2O.Crystal
coefficient and charge transfer rate constants at room Research and Technology 28(2): 181-186.
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(2001). Bright red electroluminescent devices using novel
The results in Table 2 illustrates that the kf valuves second-ligand-contained europium complexes as emitting
for all the copper complexes are lower than that of the layers. Journal of Material Chemistry 11: 790-793.
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be due to the formation of complexes of copper with the 11. Sano T., Fujita M., Fujii T., HamadaY., Shibata K. & Kuroki
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