Electrochemical studies of Schiff base compounds derived from antipyrine nucleus in ethanolic buffer solutions

Article abstract of DOI:10.1139/v00-108

The electrochemical behaviour of Schiff base compounds derived from an antipyrine nucleus was investigated in 30% (v/v) ethanolic buffer solutions (pH 3-11) using various electrochemical techniques at mercury electrode. The results showed that, the total

Full text of DOI:10.1139/v00-108

1170
Electrochemical studies of Schiff base compounds
derived from antipyrine nucleus in ethanolic
buffer solutions
Ibrahim S. El-Hallag, Gad B. El-Hefnawy, Youssef. I. Moharram,
and Enass. M. Ghoneim
Abstract: The electrochemical behaviour of Schiff base compounds derived from an antipyrine nucleus was investi-
gated in 30% (v/v) ethanolic buffer solutions (pH 3–11) using various electrochemical techniques at mercury electrode.
The results showed that, the total limiting current of each of the studied compounds corresponds to 2-electron transfer
process. The mechanistic pathway of the electrode reaction of the investigated compounds at mercury electrode, the
effect of the medium, and the evaluation of the electrode reaction parameters were illustrated and discussed.
Key words: Schiff base, antipyrine nucleus, electrode reaction, electrochemical parameters, cyclic voltammetry.
Résumé : Opérant dans des solutions tampons (pH allant de 3 à 11) contenant 30% (v/v) d’éthanol et faisant appel à
diverses techniques électrochimiques avec électrode de mercure, on a étudié le comportement électrochimique de diverses
bases de Schiff dérivées du noyau antipyrine. Les résultats montrent que, le courant limite total de chacun de composés
étudiés correspond à un processus de transfert d’électrons 2-. On illustre et on discute des voies mécanistiques de la
réaction à l’électrode, de l’effet du milieu et de l’évaluation des paramètres de la réaction à l’électrode.
Mots clés : base de Schiff, noyau antipyrine, réaction à l’électrode, paramètres électrochimiques, voltampérométrie
cyclique.
[Traduit par la Rédaction] El-Hallag et al. 1177
Introduction
nol to an ethanolic solution of 0.01 mol of salicyladehyde,
p-hydroxybenzaldehyde, 2,4-dihydroxybenzaldehyde, and
2-hydroxy-1-naphthaldehyde. Then, the mixture was heated
and refluxed for about 2 h. On cooling the mixture, the solid
compounds were separated and filtered off. The crude prod-
ucts were recrystallized several times from hot ethanol,
washed with diethyl ether, and finally dried in a vacuum
desiccator. The purity of the compounds was ascertained by
crystallization until a constancy of the mp was reached. The
structure of the prepared compounds were confirmed by ele-
mental and spectral analysis. The prepared compounds have
the general structure formula
The electroreduction of Schiff base compounds was the
subject of several interesting investigations (1–6). Generally,
the electroreduction of such compounds at mercury elec-
trode in acidic and alkaline solutions consume two electrons
per reactant molecule for saturation of a carbon–nitrogen
double bond or four electrons for its reductive splitting.
In this paper we report the electrochemical behaviour of
some Schiff base compounds derived from an antipyrine nu-
cleus in ethanolic buffer B.R. solutions. The electrochemical
parameters of the electrode process were evaluated using
dc–polarography, cyclic voltammetry and controlled poten-
tial coulometry. Verification of the electrochemical parame-
ters determined experimentally via digital simulation method
was also considered. The mechanistic pathway of the elec-
trode reaction was discussed.
Scheme 1.
Experimental
Chemicals and solutions
The compounds under consideration were prepared by adding
a solution of 4-aminoantipyrine (0.01 mol) dissolved in etha-
Received January 12, 2000. Published on the NRC Research
Press website on September 1, 2000.
I.S. El-Hallag1, G.B. El-Hefnawy, Y.I. Moharram, and
E.M. Ghoneim. Chemistry Department, Faculty of Science,
Tanta University, 31527, Tanta, Egypt.
1Author to whom correspondence may be addressed.
Can. J. Chem. 78: 1170–1177 (2000)
© 2000 NRC Canada

El-Hallag et al.
1171
Fig. 1. Dc–polarograms of III in buffer solutions of different pH
values, containing 30% (v/v) ethanol.
Table 1. Dc–polarographic data obtained for compounds under
investigation at 25°C.
–E1/2 (V)
(ii)b
(α , na = 1)
(i)a (ii)b
(i)a
Comp.
pH
I
4.0
6.0
9.3
4.0
6.0
8.6
3.8
5.9
9.6
3.9
6.6
9.1
0.85
0.98
—
1.05
1.18
—
1.02
1.16
—
0.89
1.02
1.25
—
0.42
0.31
—
0.29
0.35
—
0.26
0.37
—
0.25
0.39
0.35
—
1.25
1.35
—
1.34
1.48
—
1.35
1.60
—
1.23
1.38
0.40
0.52
—
0.20
0.49
—
0.40
0.51
—
0.29
0.31
II
III
IV
a(i) First wave.
b(ii) Second wave.
show any reduction wave except the hydrogen wave which
appears at a potential not more than –1.1 V. However, in so-
lution of 3 ≤ pH ≤ 4.0 the polarograms of the studied
compounds show a single reduction wave (A) Fig. 1. At pH > 4.0
a second ill-defined wave (B) appeared. This wave becomes
well-defined on increasing the pH up to 7.0, its height in-
crease at the expense of first wave (A) until the two waves
merge together and become one wave. This behaviour is
characteristic for compounds (I–III) while the polarograms
of compound (IV) exhibit two waves of almost equal height
in slightly acidic solutions and the height of the first wave
decreases slightly with increasing pH. It is important to note
that the total limiting current of the studied compounds cor-
responds to a two-electron transfer process as evidenced
from coulometry studies. Figure 1 gives an example of the
recorded dc–polarograms of compound (III).
The plots of il for compounds (I–III) as a function of pH
of the medium recall a dissociation curve of two steps denot-
ing the presence of an acid–base equilibrium. The height of
the reduction waves is proportional to the square root of
mercury height (h). The straight lines obtained on plotting
log il vs. log h have slope values amounting to 0.45–0.53 re-
vealing that the reduction process of the electrode reaction is
mainly diffusion controlled (7). Also, the reduction waves of
the Schiff base compounds under consideration were ana-
lyzed using the fundamental equation of irreversible
polarographic waves (8)
Stock solutions (2.5 × 10–3 M) of the prepared compounds
were obtained by dissolving the accurate mass of the solid
compound in appropriate volume of ethanol. Britton–Robin-
son buffer series (pH 2–12) containing 30% (v/v) ethanol
were prepared from analytical grade chemicals and used as
supporting electrolyte.
Instrumentation
The dc–polarograms were recorded using a Sargent Welch
polarograph model 4001. The capillary characteristics were
m = 1.03 mg s–1, t = 3.3 s drop–1 at mercury height (h) =
60 cm. The cyclic voltammograms were recorded using a
polarographic Analyzer model 264A-PAR (from EG & G) and
electrode assembly 303 A with a hanging mercury drop elec-
trode (HMDE) of area = 2.6 × 10–2 cm2 as a working electrode,
Ag–AgCl as a reference electrode, and a Pt-wire as a counter
electrode. The number of electrons consumed in the overall
electrode reaction was determined using coulometry system
model 380-PAR with a mercury pool cathode, saturated calo-
mel electrode as a reference, and a Pt-wire as counter elec-
trode. The electronic spectra were measured using Shimadzu
UV–vis spectrophotometer model 160 A. Digital simulation
of the extracted parameters for the cyclic voltammetric exper-
iments were run on an PC computer using Condesim software
packages (from EG & G) which allows accurate digital simu-
lation for a wide range of mechanisms.
0.0591
i
[1]
E = E1/ 2
−
log
α na
(id − i)
The low values (<0.5) of the cathodic symmetry coefficient
(α ) obtained from the slope S1 (S1 = 0.0591/α na) of the E
vs. log (i/id – i) plots indicate the irreversible nature of the
electrode process. The most probable α-values are obtained
for na = 1.0 (Table 1), where na is the number of electrons
participating in the rate-determining step.
On increasing the pH of the electrolysis solution, the half-
wave potentials (E1/2) of the polarographic wave get shifted
to more negative values (Table 1), denoting that the
Results and discussion
Dc–polarography
The dc–polarographic behaviour of 2.5 × 10–4 M of Schiff
base compounds (I–IV) was investigated in Britton–Robin-
son buffer solutions of pH 3–11 containing 30% (v/v) etha-
nol. It was found that in strong acidic solutions (pH < 3.0),
the polarograms of all compounds under investigation do not
© 2000 NRC Canada

1172
Can. J. Chem. Vol. 78, 2000
Table 2. Data of cyclic voltammetry for compounds under inves-
tigation at 25°C, v = 0.5 V/s.
RT
[2]
Ep = −1.14
α na F
Ep – Ep/2 (mV)
α (na = 1)
(i)a (ii)b
(kf,h
D1/ 2
)
(i)a
(ii)b
—
RT
RT
Comp.
pH
5.2
Mol wt.
307.34
+
ln
−
ln(α nav)
I
112
120
115
128
110
125
120
132
0.42
0.40
0.41
0.37
0.43
0.38
0.40
0.36
—
0.35
—
α na F
2α na F
10.5
6.3
7.5
6.6
9.6
135
—
II
323.34
307.34
357.39
On plotting Ep or Ep/2 for the investigated compounds vs. log
v, linear correlations are obtained and the cathodic symmetry
coefficient (α ) values at selected pH values were calculated
from the slopes of these plots. Values of the symmetry coef-
ficient (α ) were also determined from the difference of peak
and half-peak cathodic potentials by means of the eq. (10)
—
—
III
IV
115
130
126
138
0.41
0.36
0.38
0.34
6.6
10.5
a(i) First wave.
b(ii) Second wave.
[3]
Ep – Ep/2 = 1.857(RT/α naF)
and were found to be less than 0.5 when na = 1 (Table 2),
confirming the irreversible nature of the reduction process.
Good agreement is found between α and na values calculated
from the cyclic voltammetry data and their values obtained
from dc–polarographic measurements.
For the irreversible charge transfer processes (12), the
peak current (ipirr) can be expressed by the following equa-
tion:
hydrogen ions are consumed in the reduction process (9).
The E1/2–pH plots of the first reduction wave of the studied
compounds give a break at pH 4.5–5.5, whereas the E1/2–pH
plots of the second one exhibit a break at pH 8–9, indicating
the protonation of -CH=N- center and the ionization of OH
group of the reactants at those pH values. From the slope
values of logarithmic analysis (S1) and that of E1/2–pH plots
(S2) (S2 = (0.0591/α na)ZH+ ), the number of hydrogen ions
participating in the rate-determining step is found to equal
unity (Z+H = S2/S1).
[4]
ipirr = 2.99 × 105n(α na)1/2AD1/2Cv1/2
in which ipirr is the peak current in amp, and C is the con-
centration of the reactant species which equals 1.25 × 10–4
M
in the present study, and the other terms have their usual
meaning.
On plotting the ipirr vs. v1/2 at different pH values straight
lines with some slight deviation from the origin are obtained
revealing the diffusion character of the current (13). Table 2
revealed that the peak width (Ep – Ep/2) grows larger for the
higher molecular weight compounds as well as with increas-
ing pH of the medium, indicating increased irreversibility
nature of the electrode process as the molecular weight and
(or) the pH are increased (14). The Ep vs. pH plots of the
first peak of the investigated compounds give two segments
with slope of 60–95 mV/pH which is not much different
from the values obtained from dc–polarography experiments and
indicates that the species in solution undergoes a fast proton-
ation at the interface, prior to the reduction process (14).
Diffusion coefficient (D) of Schiff base compounds was
determined from the following equation (15, 16)
Controlled potential coulometry (CPC)
A coulometry measurement is performed by adding
10 mL of Britton–Robinson buffer (slightly acidic or alka-
line) to the coulometric cell. The specified potential for each
compound was applied to the cell and the electrolysis was
continued until the background current attained a constant
value. An amount of the reactant solution was then added
(1 mL of 10–3 M) and the electrolysis was allowed to pro-
ceed until completion. The accumulated charge (Q) was read
directly from the digital coulometer and the number of elec-
trons (n) consumed for complete reduction of the reactants
was calculated using the relation Q = nM/Fw), in which w is
the mass (in grams) of the reactant, M is the reactant molec-
ular weight and F is the Faraday’s constant. The results ob-
tained denoted that (n) equals two electrons per reactant
molecule in both acidic and alkaline media.
ipirr
[5]
I lim =
3.099(α nav)1/2
Cyclic voltammetry
Also, in the same manner, the ratio [(dI1/dt)f]irr/ipirr for irre-
versible charge transfer can be used for determining the dif-
fusion coefficient of the electroactive species via the
following relation (16, 17)
The voltammograms of the studied compounds in B.R.
buffer solution at different scan rates ranging (10–500 mV/s)
exhibit one or two well-defined cathodic peaks depending on
the pH of the medium and nature of the substituent. In the re-
verse scan (anodic direction) no oxidation peaks were ob-
served which confirms the irreversible nature of the reduction
wave. The extent of the cathodic shift of the peak potential
(Ep) as a function of the sweep rate, the difference between
the potentials at half-peak (Ep/2) and at the peak (Ep), confirm
the irreversible nature of the electrode process (10). So, the
cathodic peak potential (Ep) varies with the logarithm of the
potential sweep rate according to the eq. [2] (11)
[dIl /dt)f ]irr
[6]
= 3.73(α /na)1/2nv1/2
ipirr
where [(dI1/dt)f]irr = α n2F2SCD1/2v/3.367RT is the height of
the forward sweep of the deconvoluted current, and the other
terms have their usual meaning. Hence from relation [6], the
ratio [(dI1/dt)f]irr can be determined which gives another ac-
curate route for determination of the diffusion coefficient.
© 2000 NRC Canada

El-Hallag et al.
1173
Table 4. Values of kfh determined via ref. (11) and simulation
Table 3. Values of the diffusion coefficient (D) of the compounds
under consideration determined via different methods at 25°C.
method and its corresponding ψ of the investigated compounds
at 25°C.
D x1010 (m2/s)
kfh (m/s)
(i)a
2.9
2.5
3.1
2.3
3.3
3.1
2.9
2.3
(ii)b
2.8
2.2
2.7
2.2
3.2
2.9
2.7
2.2
(iii)c
2.7
2.3
2.9
2.5
3.4
3.3
3.1
2.4
Comp.
pH
5.2
10.5
6.3
7.5
6.6
9.6
6.6
10.5
ψ
Comp.
pH
Ref. (11)
Simulation method
I
I
5.2 8.2 × 10–6
10.5 5.6 × 10–6
6.3 7.1 × 10–6
7.5 7.9 × 10–7
6.6 8.5 × 10–6
9.6 9.1 × 10–7
6.6 6.1 × 10–5
10.5 6.3 × 10–7
9.5 × 10–6
6.2 × 10–6
6.8 × 10–6
8.5 × 10–7
9.3 × 10–6
9.4 × 10–7
6.3 × 10–5
6.1 × 10–7
4.04 × 10–3
1.28 × 10–3
1.81 × 10–3
4.04 × 10–4
4.00 × 10–4
9.00 × 10–4
1.28 × 10–3
2.86 × 10–4
II
II
III
IV
III
IV
a(i) Value of D determined from CV via eq. [4].
b(ii) Value of D determined from Ilim via eq. [5].
c(iii) Value of D determined from [(dI1/dt)f]irr via eq. [6].
Table 5. Values of peak characteristics, kc, ks, D, α, and E° determined via theoretical work at sweep rate 0.5 V/s.
Comp.
pH
5.2
10.5
6.3
7.5
6.6
9.6
6.6
10.5
ip (uA)
Ep – Ep/2 (mV)
kc (s–1)
ks × 106 (m/s)
D × 1010 (m2/s)
α
–E° (V)
I
3.33
5.52
5.31
4.62
1.70
5.35
4.75
2.91
111
121
117
130
109
126
121
133
7
8
5
7
6
8
9
10
8.7
6.5
6.1
8.0
9.7
0.9
6.7
0.62
3.1
1.9
2.9
2.3
3.1
2.6
2.8
1.9
0.40
0.35
0.42
0.34
0.41
0.33
0.36
0.32
0.92
1.30
1.13
1.18
1.08
1.24
1.14
1.26
II
III
IV
Values of diffusion coefficient (D) determined via different
methods (Table 3), indicate a good agreement between the
values of D estimated via these methods. As a result of the
disappearing of the anodic peak coupled with the cathodic
one of the investigated compounds the heterogeneous rate
constant (kfh) of the irreversible electrode reaction can be
calculated from cyclic voltammetric data using eq. [2] (11).
The plot of Ep vs. log v gives the value of the heterogeneous
rate constant (kfh). Also, from the values of Ep – Ep/2 vs. the
dimensionless parameter ψ, the heterogeneous rate constant
is estimated. The values of kfh is calculated by performing
several simulated cyclic voltammograms with various values
of kfh. The true value of kfh is established when the values of
ip, Ep, and Ep – Ep/2 of the simulated cyclic voltammograms
agree well with those values of the experimental one. Values
of kfh determined from these methods were reported in Ta-
ble 4 confirming the agreement and the accuracy of the het-
erogeneous rate constant estimated via different methods.
comparison between the peak characteristics (ip, Ep, Ep –
Ep/2), of the experimental and theoretical cyclic voltammo-
grams (18, 19). The generated cyclic voltammograms have
been carried out using CONDESIM software package (from
EG & G).
In this work, the nature of the electrode reaction was de-
termined by running digital simulation for the following
types of electrochemical reactions: (i) EirrCirr; (ii) ErevCirr;
(iii) ErevCrev; (iv) EqCirr; and (v) Eirr. The electrochemical pa-
rameters used are: (i) diffusion coefficient of the
electroactive species (D); (ii) reduction potential (E°);
(iii) concentration of the electroactive species (C); (iv) heter-
ogeneous rate constant (kfh); (v) scan rate (v); (vi) tempera-
ture (T); (vii) forward homogeneous chemical rate constant
(kf), (viii) backward homogenous chemical rate constant (kb)
(kc = kf + kb); and (ix) symmetry coefficient (α).
Figure 2 shows the comparison between the experimental
and generated cyclic voltammograms of EirrCirr model with
its convolution and deconvolution voltammetry of the first
peak of compound I at pH 6.2 and scan rate 0.2 V/s. The
value of reduction potential (E°) and the values of kc for
chemical steps which following the slow charge transfer
were determined using theoretical treatment and their values
at the selected pH values are given in Table 5. The values of
the measured peak characteristics and the electrochemical
parameters used for generating the voltammograms are listed
in Table 5 and confirm the accuracy of calculated parameters
from experimental cyclic voltammograms. The slight devia-
tion between the shape of experimental and simulated cyclic
Verification of the calculated electrochemical
parameters via digital simulation
Digital simulation was used for testing the determined ac-
curate values of the cyclic voltammetry parameters as well
as obtaining the true information of the type of electrochem-
ical process of the electrode mechanism. In general the test-
ing of parameters is made by matching the simulated
voltammogram on the experimental data, using the average
experimentally determined parameters. Also, the accuracy
of the electrochemical parameters can be tested via the
© 2000 NRC Canada

1174
Can. J. Chem. Vol. 78, 2000
Fig. 2. Matching between experimental (….) and generating (—) voltammograms of I (A), its convolution (B) and deconvolution
voltammetry of experimental voltammogram (C) at T = 298 K, sweep rate 0.2 V/s, pH 6.2.
Fig. 3. Generating voltammograms of the models ErevCrev (A) and ErevCirr (B) with their convolution voltammetry at sweep rate
0.5 V/s, T = 298 K.
Spectrophotometric determination of the dissociation
constants
voltammograms (Fig. 2) is attributed to the weak adsorption
of the reactant at electrode surface (20, 21). Figure 3 gives
an example response of the simulated cyclic voltammogram
along with its convolution voltammetry for the systems
The spectra of compounds (I, III, and IV) in universal
buffer solutions of varying pH values containing 30% (v/v)
ethanol are studied within the wavelength range 250–
600 nm.
E
revCrev and ErevCirr at v = 0.5 V/s which indicate a discrep-
ancy between captured CV with these models confirming the
validity of the EirrCirr mechanism.
The changes in the absorption spectra on varying the pH
of the buffer solution have been used successfully for the de-
© 2000 NRC Canada

El-Hallag et al.
1175
Table 6. pKa values of the compounds (I, III,
and IV) determined from electronic absorption
spectra methods.
termination of the dissociation constant of azomethine group
and OH group for the compounds under investigation apply-
ing the following methods:
(i) The half-height method (22) for which the pK is equal
to the pH at half height of the absorbance–pH curve, and can
be calculated from the following equation
Comp.
pK1
pK2
I
III
IV
5.2
5.5
4.5
8.70
9.00
8.00
[7]
pK = pH at A1/2 = (Amax – Amin)/2;
(ii) limiting absorbency method (23) using the relation
to zero. The values of pK obtained from the above three
methods are collected in Table 6.
A
[8]
pH = pK + log
Amax − A
Electrode reaction pathway
The compounds under consideration are characterized by
the presence of azomethine group C=N+H. The number of
electrons involved in the reduction is found to be two in
acidic as well as in alkaline media as confirmed from a con-
trolled potential coulometry experiments in acidic and alka-
line media.
Accordingly, a plot of the log (A/Amax – A) vs. pH pro-
duces a straight line with intercept at log (A/Amax – A) = 0
equal to pK;
(iii) The modified limiting absorbency method (24, 25) in
which the pKa is calculated via the following relation
IR (KBr optics) studies of the electrolyzed products gave
a broad band at ~3310 cm–1 (stretching) which represent the
presence of a secondary amine group in the products. Also,
it was found that the mp of all the electrolyzed products in
the range 338–341°C. All this experimental evidence indi-
cates that the electrode process is irreversible and consumed
two-electron reduction transferring the initial electroactive
species to an inactive secondary amine (CH2-NH-).
The E1/2–pH plots of the first reduction wave of these
compounds give a break at pH 4.5–5.5, whereas the E1/2–pH
plots of the second one exhibit a break at pH 8–9, indicating
the protonation of the azomethine group and dissociation of
the OH group of the reactants at these pH values.
On the other hand, calculation of the pK values of these
compounds using spectrophotometric studies indicated the
presence of two pK values (Table 6). The equilibria in solu-
tion can be represented as follows.
A − Amin
[9]
pH = pKa + log γ + log
Amax − A
where the limiting absorbency Amin and Amax corresponding
to the absorbency of the neutral and ionized species liable to
exist in the solution of the measuring wavelength and γ is
the activity coefficient of the ions which are present in equi-
librium. Since in very dilute solution, log γ = 0.0, then
eq. [9] takes the following form
A − Amin
[10] pH = pKa + log
Amax − A
The pK value can be evaluated by plotting log [(A –
min)/(Amax – A)] against pH. Thus, the pK value is that cor-
responding to the value at log [(A – Amin)/(Amax – A)] equals
A
Scheme 2.
© 2000 NRC Canada

1176
Can. J. Chem. Vol. 78, 2000
Based on the above results the mechanism of the electrode reaction of the investigated Schiff base compounds can be repre-
sented as follows: (a) In acidic and slightly alkaline solutions (pH < 8).
Scheme 3.
The electrode reaction mechanism follows the sequence H+, e, e, H+. (b) In strong alkaline solutions (pH > 8).
Scheme 4.
© 2000 NRC Canada

El-Hallag et al.
1177
12. P. Delahay. J. Am. Chem. Soc. 75, 1190 (1953).
The electrode reaction mechanism follows the sequence e,
H+, e, H+.
13. A.J. Bard and L.R. Faulkner. In Electrochemical methods fun-
damentals and applications. John Wiley, New York. 1982.
p. 218.
14. I.S. El-Hallag, A.M. Hassnien, and M. Gaber. Egypt. J. Chem.
40, 39 (1997).
15. A. Blagg, S.W. Carr, G.R. Cooper, I.D. Dobson, J.B. Gill, D.C.
Goodal, B.L. Shaw, N. Taylor, and T. Boddington. J. Chem.
Soc. Dalton Trans. 1213 (1985).
16. I.S. El-Hallag, M.M. Ghoneim, and E. Hammam. Anal. Chim.
Acta. 414, 173 (2000).
Acknowledgement
The authors are grateful to Prof. M. Gaber for preparing
the investigated compounds.
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