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Cyclic voltammetric studies of thymoquinone with Iron (III).

Introduction

Thymoquinone (2-methyl-5-isopropyl-1,4-benzoquinone) (TQ) is the major component of the essential oil of Nigella sativa, but it is also present in the fixed oil of the seed (Ali and Blunden, 2003) and it is an active principle responsible for many of the seed's beneficial effects (Mehta et al., 2009; Xin et al., 2008). Among various bioactivities, examined for TQ, one of the most important is its antioxidant activity (Badary et al., 2003; Mansour et al., 2002). The compound has been observed to decrease cellular oxidative stress (Mohamed et al., 2003) and has a potent chemo-preventive potential of inhibiting the process of carcinogenesis (Badary et al., 2007; Badary et al., 1999). In addition, several research studies have shown its other pharmacological activities such as anti-inflammatory (Syed, 2008; Gazzar et al., 2007), anti-tumor (Gali-Muhtasib et al., 2008; Shoieb et al., 2003), antidiabatic (Abdelmeguid et al., 2010; Fararh et al., 2005), antitussive (Hosseinzadeh et al., 2008), antimicrobial (Mouhajir et al., 1999), apoptosis induction (El-Mahdy et al., 2005) and neuro-protective (Al-Shabanah et al., 1998) activities.

Structurally, TQ (Fig. 1) belongs to 2,5-di-substituted benzoquinone class of compounds having methyl and isopropyl groups at carbon-2 and carbon-5, respectively (Padhye et al., 2008). TQ can be prepared by oxidation of thymol (Dockal et al., 1955). The crystal structure of TQ has been determined using high-resolution X-ray powder diffraction, which showed that TQ belongs to the triclinic system. Weak Vander Walls forces have been found in the molecules by thermal analysis (Pagola et al., 2004).

Iron is the second most abundant metal after aluminium and the fourth most abundant element in the earth's crust. The earth's core is believed to consist mainly of iron and nickel (Cotton and Wilkinson, 1988). Iron has a key role in our life also, as it is an essential component of several proteins and enzymes. Iron can serve different functions because of the fact that it can exist in two different ionic states, ferrous and ferric iron, e.g., it can serve as a cofactor to enzymes involved in oxidation-reduction reactions. Iron forms a part of the electron carriers that participate in the electron transport chain. In the final step, these carrier transport hydrogen and electrons form energy-yielding nutrients to oxygen, forming water, and in the process make ATP. Most of the body's iron is found in two proteins: hemoglobin in the red blood cells and myoglobin in the muscle cells. In both iron helps to accept, carry and then release oxygen. Iron is also required by enzymes, which are involved in the making of amino acids, collagen, hormones and neurotransmitters (Eleanor and Sharon, 2002).

Iron (III) forms complexes with a number of ligands such as; chloride, fluoride, cyanide, amines etc., but oxygen containing ligands have high affinity for it. Iron (III) also forms complexes with oxalate and phosphate ion, glycerol and sugars etc. Formation of oxo and/or hydroxo bridges is one of its characteristic features (House croft and Sharpe, 2005). Iron (III) in aqueous solution has tendency to hydrolyze and form complexes such as [[Fe[([H.sub.2]0).sub.6]].sup.3+](Cotton and Wilkinson, 1988). Regarding stability, iron (II) and iron (III) lie much closer together as compared to (II) and (III) oxidation states of other transition elements. This is the reason that ferrous ([Fe.sup.2+]) and ferric ([Fe.sup.3+]) ions are readily inter-convertible by the use of mild oxidizing or reducing agents. The standard electrode potential of [Fe.sup.2+]/[Fe.sup.3+] (E[degrees]=0.77) shows that iron (III) is a good oxidizing agent (House croft and Sharpe, 2005).

Since most of the biological activities of TQ are due to its antioxidant behaviour, its electrochemical study is very important. Although polarographic behaviour of TQ has been examined (Michelitsch and Rittmannsberger, 2003), which shows a single reversible peak, not much work is done on electrochemical study of complexes of TQ. Keeping this point in view electrochemical study of complex of TQ with iron has been performed.

Materials and Methods

Reagents and glassware. All reagents used were of analytical grade, purchased from Merck, and MP Biochemicals LLC. All glassware used were of standard quality, properly cleaned and rinsed with distilled-deionized water before use. For cyclic voltammetric studies ferric chloride (hexa hydrate), thymoquinone (TQ) and sodium chloride were used.

Instrumentation. Cyclic voltammeter. CHI-760 D Electrochemical work station, cyclic voltammeter was used for electrochemical studies. The instrument consists of a computer under Windows environment, a potentiostat and a cell assembly. The cell assembly consisted of a cell stand and a cyclic voltammetric (CV) glass cell. There were three electrodes, a glassy carbon electrode as working electrode, a saturated calomel electrode as reference electrode and a platinum wire electrode as an auxiliary or counter electrode. Repolishing and resurfacing of working electrode was done time to time, especially when supporting electrolyte or analyte was changed. Alumina polishing compound was used to polish working electrode. Then the electrode was thoroughly washed with distilled water in order to remove alumina compound from its surface. Finally, nitrogen purging was checked for the system, but its presence or absence was found to produce no change in voltammograms, so all experimental work was performed without nitrogen.

Sample preparation. Supporting electrolyte solution.

In a 500 mL volumetric flask 2.9220 g of NaCl was taken then dissolved in distilled deinoized water to prepare 0.1M solution of NaCl.

Analyte solutions. 0.005M solution of TQ and equimolar solution of ferric chloride were prepared as analyte by taking sodium chloride (0.1 M) as electrolyte solution.

Cyclic voltammetric studies. Fresh solutions of analyte and supporting electrolyte were prepared every time. The cell assembly was rinsed thrice with the analyte every time and the working electrode was repolished time to time throughout the experiment. First of all, the base-line of the supporting electrolyte was taken and then 15.0 mL of analyte was run to get overlay of NaCl (0.1 M), Fe(III) solution (5 x [10.sup.-4] M), TQ (5 x [10.sup.-4] M) and Fe(III)-TQ (5 x [10.sup.-4] M). The scan rate was 0.1 V and current sensitivity was 1 x [10.sup.-4] A/V. The potential range was set from -0.20 V to + 0.80 V and then reversed back to -0.20 V. The complexation of Fe(III) and TQ was studied in the light of effects of several parameters such as, effect of metal ligand ratio, effect of scan rate, effect of concentration and effect of repeated scan.

To check effect of metal-ligand ratio on complexation, complex solutions having metal ligand ratios 1:1 to 1:4 were prepared. To study the effect of concentration, complex solutions having concentrations 0.02 x [10.sup.-3], 0.1 x [10.sup.-3], 0.2 x [10.sup.-3], 0.4 x [10.sup.-3], 0.6 x [10.sup.-3], 0.8 x [10.sup.-3], 1.0 x [10.sup.-3] and 1.2 x [10.sup.-3] M were prepared. In order to examine effect of scan rate Fe(III)-TQ complex was analyzed at different scan rates ranging 0.05-200 V by keeping all parameters constant. Repeated scan of the complex of Fe(III) and TQ was also recorded up to 14 sweep segments. The metal-ligand ratio of the complex solution for the effect of concentration, scan rate and repeated scan was 1:3.

Results and Discussion

Cyclic voltammetric study of Fe(III)-thymoquinone complex revealed valuable information which is as follows:

Confirmation of complex formation. Complex formation was checked by comparing voltammograms of supporting electrolyte NaCl (0.1M), Fe(III) (5 x [10.sup.-4] M), thymoquinone (5 x [10.sup.-4] M) and Fe(III)-thymoquinone complex (1:1) (5 x [10.sup.-4] M) (Table 1, Fig. 2a-d). Analysis was performed at 0.1 V/sec at current sensitivity 1 x [10.sup.-4] A/V. Linearity in baseline confirms absence of impurity and cleanliness of working electrode (Fig. 2a). Fe(III)-thymoquinone complex showed anodic and cathodic peaks at 0.282 V and 0.066 V, respectively. Neither Fe(III) nor thymoquinone showed peaks within this potential range which clearly indicates that complex formation has occurred in aqueous medium (Table 1, Fig. 2a-d).

Effect of scan rate on voltammograms of Fe(III)-thymoquinone complex. Effect of scan rate from 0.05 V/s to 0.2 V/s was analyzed on Fe(III)-thymoquinone complex (1:3) (5 x [10.sup.-4] M) and different electrochemical parameters [E.sub.pa], [E.sub.pc], [I.sub.pa], [I.sub.pc] etc. were determined (Table 2, Fig. 3). Voltammograms fulfill the criteria of quasi-reversible behaviour (Table 3) because by increasing scan rates anodic peak potential shifts from 0.298 V to 0.326 V. Similarly cathodic peak shifted from 0.049 V to 0.018 V. Another quasi-reversible behaviour found was that [I.sub.pa]/[I.sub.pc] was not equal to 1. Furthermore. [E.sub.pa]-[E.sub.pc] was observed greater than 59/n mV and it increased with the increase in v. [I.sub.p] was also found to increase with [v.sup.1/2] (Fig. 4). Negative shift of Epc with increase of v further favours the quasi-reversible behaviour (Table 2 and 3). Plot of the peak potential against log of scan rates gave very good [R.sup.2] values (Fig.5). The values of [alpha] and [beta] were also determined using the relation 0.048/([E.sub.p]-[E.sub.p/2]). Values of [alpha] and [beta] at different scan rates were found 0.786 [+ or -] 0.01-0.923 [+ or -] 0.02 and 0.813 [+ or -] 0.01-1.021 [+ or -] 0.01, respectively.

Effect of concentration on voltammograms of Fe(III)-thymoquinone complex. Effect of concentration was judged by calibration curve method. To understand the effect of concentration for current study Randles-Sevcik equation is helpful as given below:

[I.sub.p] = 0.4463 [nFACo.sup.*] [(nFv[D.sup.0]/RT).sup.1/2]

Where:

[I.sub.p] = peak current (A)

v = scanrate (V/s)

n = Number of electron transfer

F = Faraday's constant

A = Area of electrode ([cm.sup.2])

[Co.sup.*] = Concentration of Fe(III)-thymoquinone complex (moles/[cm.sup.3])

[D.sup.0] = Diffusion coefficient of Fe(III)-thymoquinone complex ([cm.sup.2] [s.sup.-1])

T = 25 [+ or -] 2[degrees]C

R = Rate constant

More precisely this equation can be written as follows:

[I.sub.p] = (2.69 x [10.sup.5]) [D.sup.1/2][(n).sup.3/2]AC[(v).sup.1/2]

Effect of concentration (0.02 x [10.sup.-3] M to 1.2 x [10.sup.-3]M) followed Randles-Sevcik equation, because current was directly proportional to concentration (Fig. 6). Calibration curve along with least square fit line showed no major deviation from zero. So no adsorption occurred on electrode surface. These results indicate that calibration curve method can be used for quantification of Fe(III)-thymoquinone complex within a wide range i.e. (0.02 x [10.sup.-3] M to 1.2 x [10.sup.-3] M).

Effect of concentration on peak potential ([E.sub.pa]) gives a straight line similarly [E.sub.pc/2] against log of concentration also shows a straight line with good [R.sup.2] value (Fig. 7).

Effect of ratio. Here as well, cyclic voltammograms followed the criteria for quasi-reversible reactions because [I.sub.pa]/[I.sub.pc] was not equal to one and [E.sub.pa]-[E.sub.pc] showed values greater than 59/n mV and it increased with the increase in v. The values of [alpha] and [beta] were found in the range of 0.827 [+ or -] 0.01 to 0.923 [+ or -] 0.02 and 0.842 [+ or -] 0.01 to 0.979 [+ or -] 0.01, respectively (Table 4). Effect of metal ligand ratio on peak potential ([E.sub.pa] and [E.sub.pc]) gave a straight line, similarly the plot of [E.sub.p/2] against M: L ratio also gave a straight line with good [R.sup.2] value (Fig. 8).

It was found that by increasing the ratio [I.sub.pa] become approximately constant at 1:3 metal to ligand ratio which may be due to maximum complexation at this ratio (Table 4, Fig. 9).

Effect of repeated scan. Effect of repeated scan on Fe(III)-thymoquinone complex was analyzed up to 14 repeated scans in aqueous medium (NaCl) (Fig. 10). Results revealed that the shape of the cyclic voltam-mograms of complex remain unchanged in terms of peak potentials ([E.sub.pa] and [E.sub.pc]) but a little bit change in anodic and cathodic peak heights was noticed in these cycles, however, only up to 3rd to 4th cycle limiting values were achieved. The shape of single and multiple scan was similar which confirms neither adsorption nor deposition of complex on electrode surface.

Analysis of diffusion coefficient for Fe(III)-thymoquinone complex. Cyclic voltammetry is a cheap and easy technique for the determination of diffusion coefficient of different complexes and compounds (Anwer, 2006; Ali, 1995). So present study has used cyclic voltammetry for calculation of diffusion coefficient of Fe(III)-thymoquinone complex. For this purpose diffusion coefficient of the complex was determined using Randles-Sevcik (Greef et al., 1985) equation by varying scan rates, concentrations and metal-ligand ratios (Table 5a-c). Its value was found to be approximately same and no reasonable effect of varying scan rates, concentration or metal-ligand ratio was observed. Area of electrode (A) was 0.0706 [cm.sup.2] whereas number of electron transfer (n) was 1.

Analysis of E, a characteristic property. E (the potential corresponding to 85% of the peak current) is a characteristic property and is constant for a particular system. Effects of scan rate, concentration and ratio were observed on E and it was found to be constant at all scan rates, concentration and ratios (Table 6).

Cyclic voltammetric study of Fe(III)-thymoquinone complex was performed at glassy carbon elelctrode against Standard calomel electrode. Horizontal base line (NaCl) indicates the purity of system. The qualitative and quantitative analyses were performed for this complex. Qualitative analysis clearly showed the formation of complex on mixing of Fe(III) and thymoquinone. Quantitative analyses included determination of E, D, [alpha] and [beta]. Effects on complexation were observed at different parameters. Present research reveals that calibration curve method by cyclic voltammetry can be used for quantification of this complex. Effect of repeated scanning on cyclic voltammograms showed no pre or post peak indicating no adsorption of the complex on electrode surface. E being a characteristic property is constant for a particular system. In present study, effects of scan rate, concentration and ratio were observed on E and it was found to be constant in all above mentioned cases. Diffusion coefficient was calculated using Randles-Sevick equation. The values of transfer coefficients, [alpha] and [beta] were also determined by varying scan rates and metal ligand ratios.

Electrochemical study revealed quasi-reversible behaviour for Fe(III)-thymoquinone complex. It was supported by diagnostic criteria for a quasi-reversible reaction (Table 3).

References

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Farah Kishwar * and Qamar-ul-Haq

Department of Chemistry, Federal Urdu University of Arts, Science and Technology, Gulshan-e-Iqbal Campus, Karachi-75300, Pakistan

(received November 28, 2012; revised February 11, 2013; accepted March 6, 2013)

* Author for correspondence; E-mail: farahkishwar@yahoo.com

Table 1. Electrochemical parameters of cyclic voltammograms
of thymoquinone, Fe(III), and Fe(III)-thymoquinone complex

Component            [I.sub.pa]            [I.sub.pc]
/Complex                 (A)                   (A)

Thymoquinone     1.017 x [10.sup.-5]   2.717 x [10.sup.-5]
                    [+ or -] 0.01         [+ or -] 0.01
Fe(III)                  --                    --
Fe(III)-         5.875 x [10.sup.-6]   1.149 x [10.sup.-5]
  thymoquinone      [+ or -] 0.01         [+ or -] 0.01
  complex

Component          [E.sub.pa]        [E.sub.pc]
/Complex               (V)               (V)

Thymoquinone     -0.242 [+ or -]   -0.326 [+ or -]
                      0.01              0.01
Fe(III)                --                --
Fe(III)-         0.282 [+ or -]    0.066 [+ or -]
  thymoquinone        0.01              0.01
  complex

Table 2. The values of [E.sub.p], [E.sub.p-2],
[E.sub.p]-[E.sub.p-2], [E.sub.pa]-[E.sub.pc] and [I.sub.p] from
cyclic voltammograms of Fe(III)-thymoquinone complex with
different scan rates

Scan rate   [E.sub.pa]            [E.sub.pa/2]
(V/s)       (V)                   (V)

0.05        0.298 [+ or -] 0.01   0.25 [+ or -] 0.01
0.10        0.309 [+ or -] 0.01   0.26 [+ or -] 0.01
0.15        0.318 [+ or -] 0.01   0.266 [+ or -] 0.01
0.20        0.326 [+ or -] 0.01   0.269 [+ or -] 0.01

Scan rate   [E.sub.pa]-            [I.sub.pa]
(V/s)       [E.sub.pa/2] (V)       x [10.sup.-5]
                                   (A)

0.05        0.048 [+ or -] 0.011   0.798 [+ or -] 0.01
0.10        0.049 [+ or -] 0.011   1.035 [+ or -] 0.01
0.15        0.052 [+ or -] 0.011   1.743 [+ or -] 0.01
0.20        0.057 [+ or -] 0.011   2.541 [+ or -] 0.01

Scan rate   [I.sub.pa]/           [beta][n.sub.b]
(V/s)       [I.sub.pc]            =0.048/[E.sub.pa]
                                  -[E.sub.pa/2]

0.05        0.274 [+ or -] 0.01   1.00 [+ or -] 0.01
0.10        0.253 [+ or -] 0.01   1.021 [+ or -] 0.01
0.15        0.355 [+ or -] 0.01   0.923 [+ or -] 0.01
0.20        0.463 [+ or -] 0.01   0.842 [+ or -] 0.01

Scan rate   [E.sub.pc]            [E.sub.pc/2]
(V/s)       (V)                   (V)

0.05        0.049 [+ or -] 0.01   0.11 [+ or -] 0.01
0.10        0.034 [+ or -] 0.01   0.09 [+ or -] 0.01
0.15        0.024 [+ or -] 0.01   0.082 [+ or -] 0.01
0.20        0.018 [+ or -] 0.01   0.076 [+ or -] 0.01

Scan rate   [E.sub.pc]-             Epa-Epc
(V/s)       [E.sub.pc/2]            (V)
            (V)

0.05        -0.061 [+ or -] 0.011   0.249 [+ or -] 0.01
0.10        -0.056 [+ or -] 0.011   0.275 [+ or -] 0.01
0.15        -0.058 [+ or -] 0.011   0.294 [+ or -] 0.01
0.20        -0.058 [+ or -] 0.012   0.308 [+ or -] 0.01

Scan rate   [I.sub.pc]            [alpha][n.sub.a]
(V/s)       x [10.sup.-5]         =0.048/[E.sub.pc]
            (A)                   -[E.sub.pc/2]

0.05        2.917 [+ or -] 0.01   0.786 [+ or -] 0.01
0.10        4.084 [+ or -] 0.01   0.857 [+ or -] 0.01
0.15        4.912 [+ or -] 0.01   0.828 [+ or -] 0.01
0.20        5.488 [+ or -] 0.01   0.828 [+ or -] 0.01

Table 3. Comparison of diagnostic criteria for reversible,
irreversible and quasi-reversible systems at 25 [+ or -]
1[degrees]C and results obtained from Fe(III)-thymoquinone
complex

S.No.   Comparison of
        diagnostic criteria

Criteria for reversible system *

1       [absolute value of [I.sub.p]][alpha][v.sup.1/2]

2       [absolute value of [I.sub.pa]/[I.sub.pc]] = 1
3       [DELTA][E.sub.p] = [E.sub.pa]-[E.sub.pc] = 59/n mV

4       [E.sub.p] is independent of v
5       [absolute value of [E.sub.p]-[E.sub.p/2]] = 59/n mV

Criteria for irreversible system *

1       [absolute value of [I.sub.pc]] [alpha] [v.sup.1/2]
2       No reverse peak
3       Epc shifts -30/acnamV for each decade increase in v

4       [absolute value of [E.sub.p]-[E.sub.p/2]] = 48/
          [[alpha].sub.c][n.sub.[alpha]] mV

Criteria for Quasi-reversible system **

1       [absolute value of [I.sub.p]] is not proportional to
          [v.sup.1/2], but increases with increase in [v.sup.1/2]
2       [absolute value of [I.sub.pa]/[I.sub.pc]] = 1,
          provided [[alpha].sub.c] = [[alpha].sub.a] = 0.5
3       [E.sub.pa]-[E.sub.pc] > 59/n mV and increases with
          increase in v
4       [E.sub.pc] shifts negatively as v increases

S.No.   Results obtained from Fe(III)-thymoquinone complex

Criteria for reversible system *

1       [absolute value of [I.sub.p]] is not proportional
          to [v.sup.1/2]
2       [absolute value of [I.sub.pa]/[I.sub.pc]] [not equal to] 1
3       [DELTA][E.sub.p] = [E.sub.pa]-[E.sub.pc] > 59/n mV and
          increases as v increases
4       [E.sub.p] is dependent of v
5       [absolute value of [E.sub.p]-[E.sub.p/2]] [not equal to]
          59/n mV

Criteria for irreversible system *

1       [absolute value of [I.sub.pc]] [alpha] [v.sup.1/2]
2       Reverse peak is present
3       [E.sub.pc] shift [not equal to] -30/[[alpha].sub.c]
          [n.sub.[alpha]] mV for each decade increase in v
4       [absolute value of [E.sub.p]-[E.sub.p/2]] [not equal to]
          48/[[alpha].sub.c][n.sub.[alpha]] mV

Criteria for Quasi-reversible system **

1       [absolute value of [I.sub.p]] increases with increase
          in [v.sup.1/2]
2       [absolute value of [I.sub.pa]/[I.sub.pc]] [not equal to] 1

3       [E.sub.pa]-[E.sub.pc] > 59/n mV and increases with
          increase in v
4       As v increases [E.sub.pc] shifts negatively

References * = (Greef et al., 1985); References ** = (Bard
and Faulkner, 2004; Greef et al., 1985; Nicholson, 1965).

Table 4. The values of [E.sub.p], [E.sub.p-2], [E.sub.pa]-
[E.sub.pc], [I.sub.p] and [alpha][n.sub.a] and [beta][n.sub.b]
from cyclic voltammograms of Fe(III)-thymoquinone complex with
different metal-ligand ratios

Ratio   [E.sub.pa/2]          [E.sub.pa]-           [I.sub.pa]/
L/M     (V)                   [E.sub.pa/2] (V)      [I.sub.pc]

1       0.23 [+ or -] 0.03    0.052 [+ or -] 0.03   0.511
2       0.241 [+ or -] 0.01   0.057 [+ or -] 0.01   0.343
3       0.26 [+ or -] 0.01    0.049 [+ or -] 0.01   0.253
4       0.27 [+ or -] 0.02    0.051 [+ or -] 0.02   0.237

Ratio   [[beta].sub.nb]       [E.sub.pc/2]
L/M     = 0.048/              (V)
        [E.sub.pa]-
        [E.sub.pa/2]

1       0.923 [+ or -] 0.01   0.12 [+ or -] 0.01
2       0.842 [+ or -] 0.01   0.1 [+ or -] 0.01
3       0.979 [+ or -] 0.01   0.09 [+ or -] 0.01
4       0.941 [+ or -] 0.02   0.08 [+ or -] 0.02

Ratio   [E.sub.pc]-            [E.sub.pa]-
L/M     [E.sub.pc/2]           [E.sub.pc] (V)
        (V)

1       -0.054 [+ or -] 0.01   0.216 [+ or -] 0.01
2       -0.052 [+ or -] 0.02   0.25 [+ or -] 0.02
3       -0.056 [+ or -] 0.01   0.275 [+ or -] 0.01
4       -0.058 [+ or -] 0.02   0.299 [+ or -] 0.02

Ratio   [[alpha].sub.na]
L/M     =0.048/[E.sub.pa]
        -[E.sub.pa/2]

1       0.889 [+ or -] 0.01
2       0.923 [+ or -] 0.02
3       0.857 [+ or -] 0.01
4       0.827 [+ or -] 0.01

Table 5a. Diffusion coefficients ofthe Fe(III)-thymoquinone
complex at different scan rates [D.sup.1/2] = [I.sub.p]/
(2.69 x [10.sup.5])[(n).sup.3/2] AC [(v).sup.1/2]

v       [v.sup.1/2]   [I.sub.pa]            [D.sup.1/2]
(V/s)                 x [10.sup.-5]
                      (A)

0.05    0.224         0.798 [+ or -] 0.01   3.572 x [10.sup.-3]
0.10    0.316         1.035 [+ or -] 0.01   3.449 x [10.sup.-3]
0.15    0.387         1.743 [+ or -] 0.01   4.743 x [10.sup.-3]
0.20    0.447         2.541 [+ or -] 0.01   5.986 x [10.sup.-3]

v       [v.sup.1/2]   [I.sub.pc]            [D.sup.1/2]
(V/s)                 x [10.sup.-5]
                      (A)

0.05    0.224         2.917 [+ or -] 0.01   1.371 x [10.sup.-2]
0.10    0.316         4.084 [+ or -] 0.01   1.361 x [10.sup.-2]
0.15    0.387         4.912 [+ or -] 0.01   1.34 x [10.sup.-2]
0.20    0.447         5.488 [+ or -] 0.01   1.292 x [10.sup.-2]

v       D
(V/s)   ([cm.sup.2]
        [s.sup.-1])

0.05    1.408 x [10.sup.-5]
0.10    1.189 x [10.sup.-5]
0.15    2.25 x [10.sup.-5]
0.20    3.583 x [10.sup.-5]

v       D
(V/s)   ([cm.sup.2]
        [s.sup.-1])

0.05    1.880 x [10.sup.-4]
0.10    1.852 x [10.sup.-4]
0.15    1.796 x [10.sup.-4]
0.20    1.669 x [10.sup.-4]

Table 5b. Diffusion coefficients of Fe(III)-thymoquinone
complex at different concentrations

Conc.               [I.sub.pa]              [D.sup.1/2]
(M)                 x [10.sup.-5]
                    (A)

0.1 x [10.sup.-3]   0.4839 [+ or -] 0.01]   8.064 x [10.sup.-3]
0.2 x [10.sup.-3]   0.7272 [+ or -] 0.01]   6.06 x [10.sup.-3]
0.4 x [10.sup.-3]   0.9398 [+ or -] 0.01]   3.916 x [10.sup.-3]
0.6 x [10.sup.-3]   1.188 [+ or -] 0.01]    3.299 x [10.sup.-3]
0.8 x [10.sup.-3]   1.735 [+ or -] 0.01]    3.614 x [10.sup.-3]
1.0 x [10.sup.-3]   2.199 [+ or -] 0.01]    3.664 x [10.sup.-3]
1.2 x [10.sup.-3]   2.788 [+ or -] 0.02]    3.871 x [10.sup.-3]

Conc.               [I.sub.pc]              [D.sup.1/2]
(M)                 x [10.sup.-5]
                    (A)

0.1 x [10.sup.-3]   1.007 [+ or -] 0.01]    1.678 x [10.sup.-2]
0.2 x [10.sup.-3]   1.738 [+ or -] 0.01]    1.448 x [10.sup.-2]
0.4 x [10.sup.-3]   2.582 [+ or -] 0.01]    1.076 x [10.sup.-2]
0.6 x [10.sup.-3]   4.236 [+ or -] 0.01]    1.176 x [10.sup.-2]
0.8 x [10.sup.-3]   5.87 [+ or -] 0.01]     1.223 x [10.sup.-2]
1.0 x [10.sup.-3]   7.088 [+ or -] 0.02]    1.181 x [10.sup.-2]
1.2 x [10.sup.-3]   8.73 [+ or -] 0.01]     1.212 x [10.sup.-2]

Conc.               D
(M)                 ([cm.sup.2]
                    [s.sup.-1])

0.1 x [10.sup.-3]   6.503 x [10.sup.-5]
0.2 x [10.sup.-3]   3.672 x [10.sup.-5]
0.4 x [10.sup.-3]   1.533 x [10.sup.-5]
0.6 x [10.sup.-3]   1.088 x [10.sup.-5]
0.8 x [10.sup.-3]   1.306 x [10.sup.-5]
1.0 x [10.sup.-3]   1.342 x [10.sup.-5]
1.2 x [10.sup.-3]   1.498 x [10.sup.-5]

Conc.               D
(M)                 ([cm.sup.2]
                    [s.sup.-1])

0.1 x [10.sup.-3]   2.816 x [10.sup.-4]
0.2 x [10.sup.-3]   2.097 x [10.sup.-4]
0.4 x [10.sup.-3]   1.158 x [10.sup.-4]
0.6 x [10.sup.-3]   1.383 x [10.sup.-4]
0.8 x [10.sup.-3]   1.496 x [10.sup.-4]
1.0 x [10.sup.-3]   1.395 x [10.sup.-4]
1.2 x [10.sup.-3]   1.469 x [10.sup.-4]

Table 5c. Diffusion coefficients of Fe(III)-thymoquinone
complex at different metal-ligand ratios

L/M     [I.sub.pa] x          [D.sup.1/2]
Ratio   [10.sup.-5] (A)

1       0.588 [+ or -] 0.02   1.96 x [10.sup.-3]
2       0.919 [+ or -] 0.01   3.063 x [10.sup.-3]
3       1.035 [+ or -] 0.01   3.449 x [10.sup.-3]
4       1.187 [+ or -] 0.03   3.956 x [10.sup.-3]

L/M     [I.sub.pc] x          [D.sup.1/2]
Ratio   [10.sup.-5] (A)

1       1.149 [+ or -] 0.01   3.829 x [10.sup.-3]
2       2.683 [+ or -] 0.02   8.941 x [10.sup.-3]
3       4.084 [+ or -] 0.01   1.36 x [10.sup.-2]
4       5.002 [+ or -] 0.02   1.67 x [10.sup.-2]

L/M     D ([cm.sup.2]
Ratio   [s.sup.-1])

1       3.84 x [10.sup.-6]
2       9.38 x [10.sup.-6]
3       1.19 x [10.sup.-5]
4       1.565 x [10.sup.-5]

L/M     D ([cm.sup.2]
Ratio   [s.sup.-1])

1       1.466 x [10.sup.-5]
2       7.995 x [10.sup.-5]
3       1.852 x [10.sup.-4]
4       2.779 x [10.sup.-4]

Table 6. Halfwave potential ([E.sup.0] = [E.sub.1/2])
for Fe(III)-thymoquinone complex

Scan      [([E.sup.0]).sub.a]   Conc.           [([E.sup.0]).sub.a]
rates     (V)                   x [10.sup.-3]   (V)
(v) V/s                         M

0.05      0.274 [+ or -] 0.01   0.1             0.195 [+ or -] 0.01
0.10      0.286 [+ or -] 0.01   0.2             0.237 [+ or -] 0.01
0.15      0.292 [+ or -] 0.01   0.4             0.266 [+ or -] 0.01
0.20      0.297 [+ or -] 0.01   0.6             0.295 [+ or -] 0.01

Scan      Ratio   [([E.sup.0]).sub.a]
rates     LM      (V)
(v) V/s

0.05      1       0.256 [+ or -] 0.01
0.10      2       0.27 [+ or -] 0.01
0.15      3       0.285 [+ or -] 0.01
0.20      4       0.296 [+ or -] 0.02
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Author:Kishwar, Farah; Qamar-ul-Haq
Publication:Pakistan Journal of Scientific and Industrial Research Series A: Physical Sciences
Article Type:Report
Geographic Code:9PAKI
Date:May 1, 2013
Words:5759
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