Journal of Pharmaceutical and Biomedical Analysis
26 (2001) 873– 881
www.elsevier.com/locate/jpba
Electroanalysis of dapsone, an anti-leprotic drug
P. Manisankar *, A. Sarpudeen, S. Viswanathan
Department of Industrial Chemistry, Alagappa Uni6ersity, Karaikudi-630 003, India
Received 24 November 2000; received in revised form 4 April 2001; accepted 15 April 2001
Abstract
The electrochemical oxidation and adsorption of dapsone, an anti-leprotic drug were studied in aqueous alcohol
medium at a stationary glassy carbon electrode. Cyclic voltammetry studies showed one well-defined oxidation peak
in the potential range 1.2– 1.9 V at pH conditions 1.0, 4.0, 7.0, 9.2 and 13.0. The oxidation was irreversible and
exhibited diffusion controlled adsorption. Controlled potential coulometry revealed one electron oxidation of the
amino group in the molecule. A systematic study of the experimental parameters that affect the squarewave stripping
response was carried out and the optimized experimental conditions were arrived at. A calibration plot was derived
for the determination of the compound in solution. This method was used for the determination of dapsone in tablets
and urine. The limits of determination was 0.0036 and 3.56 mg/ml and the relative standard deviation (n= 10) was
4 ppt (0.4%) at a concentration level 0.100 mg/ml. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Dapsone; Cyclic voltammetry; Squarewave voltammetry; Tablets; Urine
1. Introduction
Dapsone,
4,4-diamino-biphenyl-sulphone
(DDS) is one of the most important and main drug
available for the treatment of leprosy [1]. It is also
used in the treatment of malarial diseases and as
an anti-inflammatory [2] in acute ileitis. Dapsone
with clofazimine prevents the inhibitory effect on
neutrophil motility [3]. With ethambutol and rifampin it is used for the treatment of human eye
disease [4]. The structural formula of dapsone is:
* Corresponding author. Fax: + 91-4565425-202.
E-mail address: pms11@rediffmail.com (P. Manisankar).
Perusal of literature reveals the availability of
methods like HPLC [5,6], LC [7], spectrophotometry and polarography for the determination of
DDS in tablets, blood and urine [1– 11]. The
cleavage of DDS using a mercury cathode and
tetraalkyl ammonium salts in both protic and
aprotic media has also been reported [12]. A
number of procedures were available for the determination of dapsone using HPLC technique.
HPLC, at its current stage of development, is
clearly not a method for analytical problems with
a high repetition rate because the receptive condition of the system requires 24– 36 h. On the other
hand, electroanalysis is a manageable method,
which is suitable for various problems [13]. Hence
the development of electrochemical determination
0731-7085/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 7 3 1 - 7 0 8 5 ( 0 1 ) 0 0 4 8 0 - 0
874
P. Manisankar et al. / J. Pharm. Biomed. Anal. 26 (2001) 873–881
assumes importance. No electroanalytical data
based on stripping voltammetry are available for
the determination of this compound. Stripping
voltammetry is a simple, highly sensitive and selective technique for the determination of electroactive compounds at ppm and ppb levels.
Determination of drugs in pharmaceutical formulations and urine samples using differential pulse
polarography was already reported [8]. Mercury is
unjustly marked as one of the most insidious
hazards within the laboratory. Solid electrode is
better than the liquid electrode. Hence we selected
glassy carbon as a working electrode. The present
paper reports the cyclic voltammetric behaviour
of DDS and square wave stripping voltammetric
procedures for the determination of this drug. The
analytical procedure proposed for the determination of DDS is easy to adopt and the lowest
concentration level possible is better than the
existing LC and polarographic methods. This
method can be adopted for the determination of
DDS in pharmaceutical formulations and urine
samples.
2. Experimental
2.1. Apparatus and reagents
The voltammetric experiments and stripping
analysis were performed using a Bioanalytical
Systems (BAS 100A) electrochemical analyser. A
glassy carbon working electrode (BAS Model MF
2012; geometric area=0.0774 cm2), a silver– silver
chloride reference electrode (BAS Model
MF2021), a platinum counter electrode and a
standard one-compartment three-electrode cell of
15 ml capacity were used in all experiments. The
compound DDS was prepared [14] by the action
of thionyl chloride on acetanilide followed by the
oxidation with CrO3 to its sulphone. Then it was
hydrolysed to 4,4%-diamino-biphenyl-sulphone
(DDS). DDS was recrystallized from ethanol to
white crystals (m.p. 178–179 °C). The stock solution was prepared in alcohol. The supporting
electrolytes solutions, 10% H2SO4 in 50% aqueous
alcohol (pH 1.0), B.R. buffer in 50% aqueous
alcohol (pH 4.0 and 9.2), 0.1 M KCl in 50%
aqueous alcohol (pH 7.0) and 0.1 M NaOH in
50% aqueous alcohol (pH 13.0) were prepared
and used for the voltammetric studies.
2.2. Procedure
Under the experimental conditions, a solution
of DDS was placed in the electrochemical cell and
purified nitrogen gas was passed for 20 min to
remove the dissolved oxygen under stirred conditions. The glassy carbon electrode was pretreated
in two ways to get reproducible results: (1) mechanical polishing over a velvet micro-cloth with
alumina suspension, and (2) electrochemical treatment by applying a potential of + 1.5 V for 4 s in
the same solution in which the measurements was
carried out. The electrode cleaning procedures
were carried out for each and every experiment
and this pretreatment requires only 5 min. The
suitability of glassy carbon electrode was checked
after every 100 stripping analysis by running
cyclic voltammogram of potassium ferricyanide
and measuring the potential difference between
reversible peaks. If the potential difference was
greater than 60 mV, then the electrode was boiled
in 10% potassium hydroxide solution for 10 min,
washed with water and the usual treatments mentioned above were done. The electrode was fairly
stable for long periods. The Ag/AgCl reference
electrode and the platinum counter electrode were
used. The analyte solution was prepared in the
ratio of 1:9 using the analyte and appropriate
amount of the buffer solution and the total volume was kept constant (10 ml). Measurements
were made using cyclic voltammetry, squarewave
voltammetry, chronocoulometry and controlled
potential coulometry for the reaction mechanism
study. Squarewave stripping voltammetry was employed for the analytical study at pH 1.0. Exactly
9 ml 10% sulphuric acid in 50% aqueous ethanol
and 1 ml of the ethanolic solution of DDS were
taken in a 15 ml undivided cell for normal studies.
There was no change observed in the stripping
response when the volume of the pH solution was
reduced to 8 ml and the substrate volume was
increased to 2 ml. The total analyte volume was
always kept at 10 ml. All the three electrodes were
inserted, deaerated and the experiment was car-
P. Manisankar et al. / J. Pharm. Biomed. Anal. 26 (2001) 873–881
875
Fig. 1. Cyclic voltammogram of 7.7 mM DDS at various scan rates in 0.1 M H2SO4 in 50% aqueous ethanol.
ried out depending upon the necessity of the
medium volume. Background values were
recorded and subtracted.
3. Results and discussion
3.1. Cyclic 6oltammetric beha6iour
Cyclic voltammograms of DDS at all the five
pH conditions exhibited one distinct and welldefined anodic peak in the potential range 1.2– 1.9
V at concentrations ranging from 3.2 to 29.4 mM
and sweep rate from 0.1 to 1.0 V/s (Fig. 1). Apart
from this, a cathodic peak with low peak current
is observed at pH 7.0, 9.2 and 13.0. However, it
does not satisfy the criteria for reversibility [15].
The cyclic voltammetric study was not considered
beyond the concentration 29.4 mM of DDS in
this medium due to the precipitation of substrate
in the solution.
As the sweep rate is increased from 0.10 to 1.00
V/s at a fixed concentration of DDS: (i) the peak
potential shifts anodically, (ii) the peak current
increases steadily, (iii) the peak current function,
ip/ACw 1/2 exhibits almost constancy. If the polishing was not done between two sweep rate changes
then a decrease in the peak current is observed.
This indicates the possibility of adsorption. A
straight line is obtained when ip is plotted against
w 1/2 (Fig. 2). This reveals that the anodic oxidation is diffusion controlled. A straight line (Ep =
0.088 log w+ 0.0017) is observed when Ep is
plotted against log w at a particular concentration
in pH 1.0 medium. From the slope of the straight
line (DE/Dlog w), the hna value is calculated by
using the formula, DE/Dlog w = −30/hna. The hna
value is found to be 0.34 and is taken for further
calculation for the number of electron transferred.
Fig. 2. Dependance of the peak current on the square root of
scan rate.
876
P. Manisankar et al. / J. Pharm. Biomed. Anal. 26 (2001) 873–881
Fig. 3. Dependence of the peak current on the various concentrations of DDS.
Similar correlations were made for other pH conditions. With the increase of concentration from
7.7 to 29.4 mM: (i) the peak potential shifts
anodically, (ii) the peak current increases, (iii) the
peak current function decreases steadily. As the
compound has the character of adsorption during
the cycle itself on electrode surface, the polishing
was given for every concentration changes. As a
result of linear variation between ip and concentration C, a straight line is obtained when ip is
plotted against C (Fig. 3). However, the peak
current function exhibits a decrease with increase
of concentration. This reveals the blocking effect
due to adsorption of the substrate on the electrode surface.
The peak current, ip and the peak current function, Ip values almost decrease gradually from pH
1.0 to 13.0 except in basic medium (pH 13.0)
where an increase in the peak current is observed.
The peak potential value decreases from pH 1.0 to
4.2 and then slight increase is observed at pH 7.0.
Then it is decreased up to alkaline pH conditions
(Fig. 4). This shows that the mechanism is different at acid, neutral and basic conditions. Since, no
cathodic peak is observed when the potential scan
direction is reversed the anodic oxidation is irreversible reaction. The fractional hna value confirms the irreversible oxidation of DDS. After the
first cycle, the peak current decreases tremendously and comes closer to the background current as the number of cyclic voltammetric cycles
increased from 2 to 10 for 20 mM and sweep
0.300 V/s (Fig. 5). This indicates and confirms the
adsorption behaviour of substrate.
3.2. Controlled potential coulometric beha6iour
By using controlled potential coulometry, the
number of electrons transferred, n values were
found out from the charge consumed by 1×10 − 9
M concentration of the compound DDS. The
charge consumed for every electrolysis was found
as 774×10 − 7 C in acid medium. The coulometric
n was calculated by using the equation, Q= nFN,
where Q is charge in coulombs, F is Faraday’s
constant and N is number of moles of the substrate. The n value is found to be one (rounded
Fig. 4. (a) Plot of peak versus pH; (b) plot of peak potentials versus pH.
P. Manisankar et al. / J. Pharm. Biomed. Anal. 26 (2001) 873–881
877
Fig. 6. Plot of log ip versus log Hz.
Fig. 5. Ten continuous cycles voltammagram of 7.7 mM DDS
in pH 1.0 at scan rate 0.3 V/s.
value) for anodic peak of the compound DDS in
all mediums (Table 1).
3.3. Chronocoulometric beha6iour
In chronocoulometry, a plot of charge versus
square root of time, Q versus t 1/2 transforms the
data into a linear relationship whose
slope was
1
given as DQ/Dt 1/2 =2 nFACD 1/2/y2. The number
of electrons transferred, n is one, taken from the
controlled potential coulometric studies. By using
forward slope and above equations, diffusion coefficient of DDS was calculated. It exhibits almost
constancy except in basic medium (Table 2).
3.4. Square wa6e 6oltammetric beha6iour
To confirm the results found in the cyclic
voltammetry studies, the same experiments were
carried out in pH 1.0 with the squarewave technique, maintaining the pulse amplitude at 25 mV
and modifying the frequency used. The logarithm
of the peak current was directly proportional to
the logarithm of the frequency (Fig. 6).
Table 1
Controlled potential coulometry
pH
Electrolytic potential (mV)
Current (A)
Charge (C)
Number of electrons transferred
1.0
4.0
7.0
9.2
13.0
1800
1725
1750
1600
1200
2023×10−8
1994×10−8
1983×10−8
2017×10−8
2239×10−8
774×10−7
781×10−7
789×10−7
799×10−7
895×10−7
0.80
0.81
0.82
0.83
0.93
Table 2
Chronocoulometry
pH
Concentration
(mol/cm3)
Forward slope
(mC/s)
Intercept
Correlation coefficient Diffusion coefficient (cm2/s)
1.0
4.0
7.0
9.2
13.0
3.2×10−6
9.1×10−6
3.2×10−6
9.1×10−6
9.1×10−6
8.5456
8.5290
2.6976
8.0956
15.1921
37.7749
−39.7049
4.9286
−80.8418
11.6859
0.9986
0.9975
0.9994
0.9999
0.995
1.238×10−5
1.230×10−5
1.001×10−5
1.113×10−5
3.918×10−5
878
P. Manisankar et al. / J. Pharm. Biomed. Anal. 26 (2001) 873–881
All the above facts reveal that the anodic oxidation of DDS at a glassy carbon electrode in acid
medium is an irreversible one-electron transfer,
the overall reaction is diffusion-controlled and the
compound is having adsorptive behaviour. At
acid medium, the NH2 group is protonated and
hence the loss of electron becomes difficult. This
is understood from higher Ep values observed in
acid medium. In alkaline medium the oxidation
becomes easier because of the easy availability of
electrons from the NH2 group and removal of H+
from NH2 as H2O. Thus hydroxide present in the
medium facilitates the oxidation. Hence lowest Ep
values are observed here. In neutral, slightly
acidic and basic medium, intermediate values are
possible because of the maximum possible oxidation of neutral substrate. The formed intermediates, radical cations or radical may undergo some
coupled chemical reactions. Further deelectronation is not realized because of the absence of
second anodic peak. On the basis above discussion the probable oxidation mechanism is depicted as follows:
To carry out the square wave stripping voltammetry the optimum conditions of various instrumental parameters which lead to well-defined peaks
with maximum peak current were fixed by varying
them over a range (Table 3). A lower concentration of 1.42 mM was chosen to establish optimum
conditions (deposition time 10 s, a.c. amplitude 50
mV, frequency 15 Hz, quiet time 5 s, step potential 15 mV and deposition potential 50 mV).
Under optimum conditions the square wave stripping voltammograms were recorded for various
concentrations varying from 0.0142 to 14.2 mM.
(0.0036– 3.56 mg/ml). As an illustration, the
squarewave stripping voltammogram recorded for
DDS under optimum conditions is presented in
Fig. 7. The peak currents obtained for various
concentrations were plotted against concentration, which give a straight line, ip(mA) =60.534 C
where ip is the peak current in mA and C is the
concentration in mg/ml with good correlation
(r 2 =0.9923).The relative standard deviation for
10 measurements (n =10) was 4 ppt (parts per
3.5. Squarewa6e stripping 6oltammetric beha6iour
Since highest peak current with no complication from other peaks was observed in pH 1.0, the
acid medium was chosen for stripping. The test
solution was purged with nitrogen for about 20
min and an accumulation potential of 0.0 V was
applied to the electrode. The stirring was stopped
after accumulation time of 15 s. After a 10 s rest
period, Osteryoung squarewave stripping pulse
sweep varying a.c. amplitude and frequency was
carried out.
The set conditions given in the instrument were
square wave amplitude= 50 mV, frequency= 15
Hz, quiet time=10 s, sampling point=256, step
potential =4 mV and deposit potential=0.0 V.
thousand; 0.4%) at a concentration level 0.100
mg/ml.
4. Determination of DDS in pharmaceutical
sample
One tablet of dapsone is dissolved in 100 ml of
50% aqueous alcohol and 1.5 ml of dapsone solution was pipetted out into a cell containing 5.5 ml
of buffer solution (pH 1.0). By using optimum
conditions, the squarewave stripping voltammogram was recorded (Fig. 7) and the peak current value was noted. The corresponding amount
of DDS present in 1 ml of the tablet solution was
P. Manisankar et al. / J. Pharm. Biomed. Anal. 26 (2001) 873–881
found out from the calibration equation. From
this the amount of DDS present in the whole of
the 100 ml of solution was calculated. The experimentally calculated value of DDS present in one
tablet was 109.1 mg. The theoretical calculated
value is 110.0 mg. Deviation in percentage is 0.82.
This experiment was repeated 5 times to get concordant value. Similarly by pipetting out 0.5, 1.0
and 2.0 ml of tablet solution and keeping the
buffer volume at 7.0 ml, the stripping voltammograms were recorded. By employing the same
879
procedure the amount of DDS present was calculated. The results are presented in Table 4. A
minimum of 3.6 ppm (0.0036 mg/ml) concentration of DDS was determined by this procedure.
5. Determination of DDS assay in urine
Measurement of DDS in an urine sample collected after 8 h from administration was demonstrated and 0.5 ml of the urine sample was mixed
Table 3
Square wave stripping voltammetry
Constant parameters
Varied parameter
C 1.42
ACA 50
FR 15
QT 10
SP 04
DP 0.0
C 1.42
DT 10
FR 15
QT 10
SP 04
DP 0.0
Deposition Time (DT, s)
C 1.42
DT 10
ACA 50
QT 10
SP 04
DP 0.0
C 1.42
DT 10
ACA 50
FR 15
SP 04
DP 0.0
C 1.42
DT 10
ACA 50
FR 15
QT 05, DP 0.0
C 1.42
DT 10
ACA 50
FR 15
QT 05, SP 15
Frequency (FR, Hz)
C= concentration (mM).
a.c. Amplitude (ACA, mV)
Quiet time (QT, s)
Step potential (SP, mV)
Deposition potential (DP, mV)
10
20
30
40
50
60
25
50
75
100
125
150
175
200
5
7
9
11
13
15
5
10
15
20
25
30
5
10
15
20
25
0
50
300
500
600
Ep (mV)
ip (mA)
1724
1624
1628
1620
1636
1632
1572
1568
1532
1504
1460
1452
1400
1344
1508
1512
1532
1508
1508
1516
1516
1524
1508
1544
1512
1544
1530
1560
1495
1500
1525
1465
1495
1525
1495
1525
36.23
29.73
26.53
31.01
27.90
28.54
29.98
44.43
34.73
28.12
36.93
40.53
38.42
33.70
16.01
19.73
17.23
25.89
27.84
28.21
30.74
30.62
28.88
28.82
29.37
26.35
31.53
34.00
60.17
54.22
30.04
63.61
116.05
44.28
42.63
32.17
P. Manisankar et al. / J. Pharm. Biomed. Anal. 26 (2001) 873–881
880
Fig. 7. Square wave voltammogram of DDS under optimum conditions for: (a) 11.8 mM; (b) tablet; (c) urine sample; (d) urine
sample before DDS dosage.
Table 4
Determination of DDS in dapsone tablet
Volume (ml)
Ep (mV)
ip (mA)
Experimental weight per tablet
(mg)
Theoretical weight per tablet
(mg)
Deviation (%)
0.5
1.0
1.5
2.0
1568
1555
1554
1574
35.40
66.61
96.84
127.39
109.0
109.1
109.1
109.1
110
110
110
110
0.91
0.82
0.82
0.82
with 6.5 ml of the supporting electrolyte solution
and the pH was brought to 1.0. The squarewave
stripping voltammogram was recorded under the
optimum experimental conditions. The squarewave voltammogram was presented in Fig. 7.
This experiment was repeated 5 times to get
concordant value. There is no appreciable interference due to the presence of a small amount
urine present in the electrolyte hence the same
calibration plot can be used. By employing
the calibration plot the amount of DDS present
in 1.0 ml of the urine sample was calculated.
Similar experiments were carried out with vary-
ing amount of urine sample, 1.0, 1.5 and 2.0 ml.
In all the cases the suitability of the procedure
was verified. The results are presented in Table
5.
There is no degradation of the analyte in solution during experiment. Only electrooxidation of
the amino group of DDS is taking place. The
other matters present in tablets and urine samples are not interfering with the study. This
method is a stable method. Repetition rate is
found to be high. Hence the proposed method
can be used as a stable method like spectrophotometric or chromotographic methods.
P. Manisankar et al. / J. Pharm. Biomed. Anal. 26 (2001) 873–881
881
Table 5
Determination of DDS in urine sample
Volume (ml)
Ep (mV)
ip (mA)
Concentration (mg/ml)
Experimental weight per ml of urine (mg)
0.5
1.0
1.5
2.0
1551
1556
1560
1564
9.26
23.51
27.64
37.01
0.1531
0.3067
0.4569
0.6117
0.3061
0.3067
0.3046
0.3059
6. Conclusion
References
DDS is anodically oxidized irreversibly on
glassy carbon electrode in the potential range of
0.10–0.18 V at all pH conditions. The oxidation is
diffusion-controlled adsorption and loss of one
electron is observed. Effect of pH leads to the
conclusion that pH 1.0 is suitable for analytical
studies. The adsorptive stripping voltammetric
studies are carried out by employing Osteryoung
squarewave stripping voltammetry. Optimum
conditions are arrived at. The concentration effect
was studied and a calibration plot was obtained.
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the unknown concentration of DDS present in the
pharmaceutical tablet and urine samples. Dapsone is determined over the range of 0.5–20.0
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limit of determination of DDS between 0.0036
and 3.56 mg/ml is obtained in this stripping
voltammetry method. This technique is simple
and easy to carry out. Once the instrument is set,
just by changing the analyte and polishing the
electrode, within a few minutes, the amount of
DDS can be determined. Hence stripping voltammetry is a better technique over chromatography
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