Cyclic Voltammetry of Copper (II) Ions at Various Ph Values. Mononuclear Copper...

Jamonline / 2(4); 2012 / 312–320 E. J. Ukpong & I. A. Udoetok

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Research Article

Journal of Atoms and Molecules An International Online Journal

ISSN – 2277 – 1247

CYCLIC VOLTAMMETRY OF COPPER (II) IONS AT VARIOUS PH

VALUES.

MONONUCLEAR COPPER (II)-DIETHYLENETRIAMINE COMPLEXES

E. J. Ukpong* and I. A. Udoetok

Department of Chemistry, Faculty of Science, Akwa Ibom State University, Ikot Akpaden, Nigeria

Received on: 09-08-2012 Revised on: 19-08-2012 Accepted on: 29-08-2012

Abstract:

The electrochemical behaviour of copper(II) diethylenetriamine complex species 1:1, 1:2, 1:10,

1:100 molar ratios at a various pH values was studied in aqueous solution of 1×10-3

M

Cu(ClO4)2.6H2O containing 0.2M NaClO4 supporting electrolyte by means of cyclic voltammetry.

The cyclic parameters were calculated. The first couple Cu2+/+

showed a diffusion controlled, quasi-

reversible, one electron charge transfer at glassy carbon electrode (GCE). It was observed that the

cathodic peak potential at a given [Cu (II)]: [dien] molar ratio and at a given pH value shifted

cathodically with increasing ligand concentration. The effects of scan rate, pH and ligand

concentration were examined. A linear relationship was obtained between the cathodic peak

current( Ipc1) and square root of scan rate in the range of 10 to 250mV/s with a correlation co-

efficient of 0.9971

.Key Words: Cyclic Voltammetry, Copper (II) complexes, Diethlenetriamine, Aqueous solution

Introduction:

Diethylenetriamine (DIEN) has been

of a very important practical application in the

removal of traces of transition metal ions in

waste waters [1]

, determination of cation

exchange capacities of clays [2,3]

.

* Corresponding author

E. J. Ukpong,

Email: [email protected]

Phone: +2348167731079



Diethylenetriamine is often used in the coordination chemistry and favors metallic

complexes formation in aqueous solution [4,5]

.

Copper (II) complexes have possible

medicinal use in the treatment of many

diseases including cancer [6]

. Copper (II)

complexes are commonly air and moisture

stable, having informative and easy to obtain

UV-vis and EPR spectroscopic signatures [7]

.

The stereochemical flexibility of copper (II)

complexes also means that they adopt a wider

range of coordination geometries shown for

other transition metal ions [8]

. Dien is highly

caustic, soluble in ethylalcohol, in water and

has almost the same physicochemical

properties like ammonia. Due to its high

causticity, its electrochemical redox reaction

is not too much investigated [4]

. Cyclic

voltammetry, offers several distinct

advantages over some methods of analysis

because of ease and rapidity with samples

preparation and analysis, ability to work

outside pH range of glass electrodes and to

use non-protic media [9]

.

In this work, the redox behaviour of copper-

diethylenetriamine complexes has been

studied under the conditions of pH and ligand

concentration and the electrode process

determined.

EXPERIMENTAL

Material

The cyclic voltammetric data were

obtained with a BAS Model CV-IB (Indiana,

USA) cyclic voltammograph instrument

having an electrochemical cell with a three

electrode system. The working electrode was

a glassy carbon electrode (GCE). Platinum

wire was used as an auxiliary electrode, while

a saturated calomel electrode (SCE) as

reference electrode with E0=0.242V vs NHE.

The voltammograms were recorded on an X–

Y recorder. Diethylenetriamine was obtained

from Aldrich (USA). The Copper

perchlorate and sodium perchlorate were of

analytical reagent grade.

Method

All the experiments were done in an

inert atmosphere achieved by purging the cell

solutions with nitrogen gas for about 20

minutes and maintained over the cell solution

during the recording of the voltammgrams.

The nitrogen gas was purified by bubbling

through alkaline vanadous sulphate solution

and passing through a calcium sulphate drying

tube before bubbling through the cell

solution(s). The experiments were carried out

at 25oC in aqueous solution 0.2M NaClO4

supporting electrolyte. Fresh solutions of

copper (II) dien complexes were prepared by

mixing the appropriate quantities of metal ion

and ligand solutions. The pH of the working

solutions were obtain with a digital pH meter

model MD21 after calibration using pH 4.00,

7.00 and 10.00 commercial standard buffers.

The pH of the solutions was adjusted by



controlled addition of sodium hydroxide or

hydrochloric solutions as required.

RESULTS AND DISCUSSION

1:1 Cu (II)-dien system

At pH 7.10, the cyclic voltammogram

(CV) scanned at 10mV/s cathodically from

+0.40 to -0.75V shows a redox couple c1/a1

corresponding to Cu2+/+

electron-transfer

reaction and three additional oxidation peaks

a2, a1, and a1

,, (Figure 1). However, at scan

rate ≥ 25 mV/s only the first couple c1/a1

remains with E0’

= -505mV. At pH ≥ 9.20 two

small irreversible reduction peaks c1’’ and

c1’’’ appear at -360 and -410 and -460mV at

pH 11.05 respectively with a formal electrode

potential (E0’

) = -520 and -562mV at pH 9.20

and 11.05 respectively. This suggests that the

electron-transfer at c1 is followed by chemical

reaction (EC mechanism) [10].

On extending the negative potential

limit to -1.15V, the reduction peak c2

attributable to Cu+/0

electron transfer reaction

is observed in the forward scan followed by a

complicated oxidation process a1,a2,a1’, a2

’’

(Figure 2) in the reverse scan. At the second

cycle a new reduction peak c1’ appear at -

150mV (pH 7.10), -130mV (pH 9.20), -

330mV (pH 11.05) and -330mV (pH

12.0).This cathodic peak c1’ may be assigned

[11] to electro-reduction of electrochemically

formed Cu(II) species to Cu(o). The

magnitudes of Ipc1 and Ipa1 as well as the

peak current ratio Ipa1/Ipc1, remain almost

similar over the pH range 7.10-11.05 and the

potentials become more negative with rise in

pH] [12]

. However, Ipa1/Ipc1, decrease at pH

12.0.

1:2 Cu (II) dien system.

The CV features at pH 7.10 scanned

cathodically from +0.40 to -0.60V at 10 and

25mV/s are similar to that in 1:1 at 10mV/s

(Figure 1). The anodic peaks a2 and a1’,

however disappear at V≥ 50mV/s, an

indication that the electron-transfer is

followed by chemical reaction [10,11]

. The CV

features at pH 9.20 show a very small

irreversible reduction peak c’’ and a well

define couple c1/a1. Further, it is noted that the

peak currents are smaller in 1:2 ratio at a

given pH (Table 1). This may be due to the

formation of elongated tetragonal-octahedral

complex [13,14]

, [Cu (dien)2] (ClO4)2 in 1:2

ratio as compared to square planer complex

[15], [Cu (dien)(H2O)(ClO4)2] in 1:1 ratio at pH

7.10.

1:10 and 1: 100 Cu (II) dien system

The CV features at pH 7.10 and 9.20

show a single couple (c1/a1) as scanned from

+0.20V to -0.80V at V = 25mV/s. However,

in 1:100 ratio at pH 9.20, the cathodic peak

c1is broad (Figure 3) which may be due to the

presence of two very close reduction

processes [16]

. At pH 11.05, the CV of 1:10

Cu(II) –dien system shows two irreversible



reduction peaks c1’’ and c1

’’’ and one quasi-

reversible couple c1/a1 in the forward scan

followed by a relatively large stripping

oxidation peak a2 at -0.30V at 25mV/s (Figure

4) which can be attributed [17]

to the electro-

oxidation of Cu(o) to Cu(1) species. The

deposition of Cu(o) to the surface of the

working electrode results due to chemical

decomposition of Cu(I)-dien-hydroxo

complex species [18]

. The peak current of a2

decrease markedly with increasing scan rate

indicating an EC mechanism [10]

. At pH 11.05,

the CV for 1:100 ratio shows similar features

with that of 1: 10 at the same pH, except that

in addition to the c1/a1 couple, only one

irreversible reduction peak c1’’appears.

However, three complicated oxidation peaks

a1 a2’ and a2 appear in the reverse cycle at V =

25mV/s (Table 1)

Effect Of pH:

The influence of solution pH on the

reduction of copper(II)dien complex has been

investigated at GCE. It shows that the value

of Epc1 at a given [Cu(II)]:[dien] molar ratio

shifts cathodically with increasing pH

value(Table1), indicating that protons take

part in these electrode reactions[19].

Effect of Scan Rate:

The effect of scan rates on the

electrochemical response of 1:1, 1:2, 1:10,

and 1:100 Cu(II)dien system at pH7.10, 9.20,

11.05 has been studied between the range 10

to 250 mV/s. It was found that the cathodic

peak current increases linearly with square

root of scan rate for all molar ratios and all pH

values. The best fit was obtained for the plot

of square root of scan vs the cathodic peak for

1:10 molar ratio at pH 11.05 with a

correlation coefficient of 0.9971, slope of

2.670 and intercept of+01245 (Figure 5). This

indicates that the electrode process is not fully

diffusion controlled [10,20,21]

and also the

electron-transfer is followed by a preceding

chemical reaction[10]

Effect of Ligand Concentration:

The effect of variation of

concentration of dien at each pH was studied.

A typical plot of cathodic current vs square

root of dien concentration at pH 7.10 ( Figure

6) shows that cathodic peak current decreases

linearly probably due to the fact that the Cu-

dien complex formed is much larger size and

hence there is the low value of diffusion

current with increased ligand concentration[

22]. However, the value of Epc1 at a given [

Cu(II)]:[dien] molar ratio at a given pH value

shifts more negatively, indicating that the

reduction becomes more difficult with

increasing ligand concentration and pH [12]

. It

may therefore be concluded that higher order

complex species are formed at higher pH

values and increased ligand concentration[23]

.

The cyclic voltammetric results for 1:1, 1:2,

1:10 and 1:100 Cu(II) dien systems show the



following features for c1/a1 couple (i) Ipa/Ipc,

is less than unity indicating that Cu(I) dien

complex is unstable, the electrode process is

not chemically reversible. (ii) ΔEp (Epa1–

Epc1) increases with increasing scan rate,

demonstrating a quasi-reversible electrode

process.

CONCLUSIONS

The electrochemical behaviour of

copper(II)-dien complex species was

investigated by cyclic voltammetry. The

electrode process for c1/a1 couple involves a

one electron charge transfer . The

voltammetric results show that a higher

order complex species are formed at higher

pH values and increased ligand concentration.

However, it would be interesting to

characterize the exact nature of these

Cu(II)/Cu(I) species with the help of

coulometric and spectroscopic techniques.

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FIQURE LEGEND.

Figure 1: CV of 1:1 Cu(II)-dien system at 10mV/s and pH 7.10

Fiqure 2: CV of 1:1 Cu(II)-dien system at 25mV/s and pH 7.10.

Fiqure 3: CV of 1:100 Cu(II)-Dien system at 100 mV/s and pH 9.10.



Figure 4: CV of 1:10 Cu(II)-dien system at 10mV/s and pH 11.05



Table 1: Cyclic Voltammetric Parameters for different Cu(II)-dien systems. at

25mV/s, (T= 25oc).

[Cu(II)]:[dien] pH Epc1 Epc2 Epa1 Epa2 Epa1’ Epa1

’’ ΔEp Ipa1/Ipc1

(pc1) (Ipc2) (Ipa1) (Ipa2’) (Ipa1

’) (Ipa1

”)

7.10 -535 -875 -475 -250 +10 +245

(13.0) (10.0) (8.0) (13.0) (N/M) (13.0) 60 0.61

1:1 9.20 -570 -980 -470 -270 +100 +210

(13.0) (9.0) (8.0) (10.0) (31.5) (N/M) 100 0.61

11.05 -600 -1070 -525 -350 -50

(13.5) (9.5) (8.0) (17.5) (N/M) N/A 75 0.59

12.0 -625 -1110 -550 -450 -110

(16.0) (7.0) (6.0) (N/M) (1.5) N/A 75 0.43

7.10 -515 -450 -180 +25

(9.5) (6.5) (0.70) (1.6) N/A 65 0.68

1:2 9.20 -565 -475

(8.5) (6.5) N/A N/A N/A 90 0.76

7.10 -536 -455

(8.5) (6.5) N/A N/A N/A 80 0.76

1:10 9.20 -600 -530

(7.0) N/A N/A N/A 70 0.82

11.05 -650 -1070 -580 -300

(12.5) (8.0) (8.0) (4.5) N/A N/A 70 0.64

7.10 -565 -465

(7.0) (5.2) N/A N/A N/A 100 0.74

1:100 9.20 -655 -545

(7.0) (5.0) N/A N/A N/A 110 0.71

11.05 -685 -605 -330

(11.0) (7.5) (3.5) N/A N/A 80 0.68

N/A, not appear; N/M, not measurable; all potentials measure in mV from base line;

all currents measured in µA.

Cyclic Voltammetry of Copper (II) Ions at Various Ph Values. Mononuclear Copper...

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