Cent. Eur. J. Chem. • 11(11) • 2013 • 1830-1836
DOI: 10.2478/s11532-013-0312-6
Central European Journal of Chemistry
Flow-injection spectrophotometric determination
of dipyrone in pharmaceutical formulations using
a solid-phase reactor with copper(II) phosphate
Research Article
Viviane G. Bonifácio1, Orlando Fatibello Filho2,
Luiz H. Marcolino-Júnior1*
Departamento de Química, Universidade Federal do Paraná,
CEP 81531-990, Curitiba - PR, Brazil
1
Departamento de Química, Universidade Federal de São Carlos,
CEP 13565-905, São Carlos - SP, Brazil
2
Received 8 May 2013; Accepted 8 July 2013
Abstract: In this work, a flow-injection spectrophotometric method for dipyrone determination in pharmaceutical formulations was developed.
Dipyrone sample solutions were injected into a carrier stream of deionized water and the reaction was carried out in a solid-phase
reactor (12 cm, 2.0 mm i.d.) packed with Cu3(PO4)2(s) entrapped in a matrix of polyester resin. The Cu(II) ions were released from the
solid phase reactor by the formation of Cu(II)-(dipyrone)n complex. When the complex is released, it reacts with 0.02% m/v alizarin red
S in deionized water to produce a Cu(VABO3)3 complex whose absorbance was monitored at 540 nm. The calibration graph was linear
over the range 5.0×10-5 - 4.0×10-4 mol L-1 with a detection limit of 2.0×10-5 mol L-1 and relative standard deviation for 10 successive
determinations of 1.5% (2.0×10-4 mol L-1 dipyrone solution). The calculated sample throughput was 60 h-1. The column was stable
for at least 8 h of continuous use (500 injections) at 25oC. Pharmaceutical formulations were analyzed and the results from an official
procedure measurement were compared with those from the proposed FIA method in order to validate the latter method.
Keywords: Dipyrone • Flow injection analysis • Copper(II) phosphate • Solid-phase reactor
© Versita Sp. z o.o.
1. Introduction
The dipyrone (sodium salt of the 1-phenyl-2,3-dimethyl4-methyl aminomethane sulfonate-5-pyrazolone) is a
soluble white crystalline powder that presents analgesic
and antipyretic activity available in oral, rectal and
injectable forms [1]. It is widely used in several countries.
Its therapeutic relevance and importance of the side
effects have prompted the development of several
methods for its determination both in pharmaceutical
preparations and biological samples. This drug can also
cause occasional or rare reactions such as: transitory
renal disturbances and inflammation of the renal tissue,
mainly in patients with history of renal disease or in
cases of overdose [2].
The methods commonly used for dipyrone (Fig. 1)
determination in several pharmaceutical formulations are
based on its reaction with iodide [3,4]. For example, the
Brazilian Pharmacopoeia recommends the iodometric
titration of dipyrone [4] but this procedure is very slow and
laborious, thus less applicable to large-scale analysis.
In order to improve performance and robustness,
several methods are also reported for dipyrone
determination
such
as
spectrometric
[5,6],
electrochemical [7,8] and chromatographic [9,10]
methods. Moreover, new trends employing flow injection
procedures have been developed for the determination
of dipyrone in pharmaceuticals and biological samples,
with different detection systems: amperometric
[11-13], spectrophotometric [14-16] potentiometric [17]
and chemiluminescent [18-20].
Solid-phase reactor coupled in a flow injection
system is an interesting strategy due to the advantages
offered over the dissolved reagents, namely, the use of
* E-mail: [email protected]
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V. G. Bonifácio, O. Fatibello Filho, L. H. Marcolino-Júnior
Figure 1. Molecular structure of dipyrone.
reagents that are not available in soluble form, increased
sensitivity, and increased injection rate. Furthermore,
the use of solid-phase reactors can avoid the timeconsuming steps of reagent solution preparation and
contact with the sample. The immobilization procedure
is fairly expeditious, simple and non-specific [21].
The combination of flow-injection analysis with solidphase reactors has been used extensively in several
pharmaceutical analyses and for a variety of purposes,
including
preconcentration,
sample
conversion,
immobilized enzymes and generation of unstable
reagents [22-26].
The aim of this work is to propose and confirm
the use of a solid-phase reactor with Cu3(PO4)2 for
dipyrone determination (see Scheme 1). The method
is based on the chelating reaction between dipyrone
and immobilized copper(II) ions (step I). The released
Cu(II) is complexed by alizarin red S and monitored
spectrophotometrically at 540 nm (step II).
2. Experimental procedure
2.1. Apparatus
A twelve-channel Ismatec (Zurich, Switzerland) Model
7618-50 peristaltic pump supplied with Tygon pump
tubing was used for the propulsion of the fluids. The
manifold was constructed with polyethylene tubing
(0.8 mm i.d.). All reference or sample solutions were
injected manually into the carrier stream using an injectorcommutator [27]. Spectrophotometric measurements for
the proposed method were performed with a Femto (São
Paulo, Brazil) Model 435 spectrophotometer equipped
with a glass flow cell (optical path 1.0 cm) connected
to a Cole Parmer (Chicago, IL) model 1202-0000 twochannel strip-chart recorder. A Hewlett Packard (Boise,
ID, USA) model 8452AUV-visible spectrophotometer
was used in the preliminary experiments.
2.2. Reagents and chemicals
All reagents were of analytical-reagent grade and all
solutions were prepared with water from a Millipore
(Bedford, MA) Milli-Q system (Model UV Plus Ultra-
Low Organics water). A 10 mmol L-1 dipyrone standard
solution was prepared by dissolving the appropriate
amount of dipyrone (Boheringer Ingelheim) in 50 mL of
deionised water. Working standard solutions within the
5.0×10-5 mol L-1 and 4.0×10-4 mol L-1 dipyrone ranges
were daily prepared by serial dilutions of appropriate
volumes of the standard stock solution with deionised
water.
The borate buffer solution (pH 9.0) was prepared
by mixing appropriate volumes of 0.2 mol L-1 sodium
borate (Merck) and 0.2 mol L-1 boric acid (Merck), and
diluting to 500 ml with water. A 0.020% (m/v) alizarin red
S solution was prepared by dissolving 200 mg of this
reagent from Aldrich (Milwaukee, WI) in 1 L of borate
buffer solution (pH 9.0).
The immobilization of Cu3(PO4)2 was made using
a commercial polyester resin solution (Resapol T-208,
Resana, SP, Brazil) and methyl ethyl ketone used as
catalyst (Ibere, Ramires & Cia, Taboão da Serra, SP,
Brazil).
Pharmaceutical dosage forms of dipyrone analysed
were those commercially available in Brazil: Novalgina
capsules 500 mg (Hoechst Marion Roussel Ltda.),
Magnopyrol capsules 500 mg (FARMASA S.A.),
Magnopyrol solution 500 mg mL-1 (FARMASA S.A.),
Anador capsules 500 mg (Boheringer Ingelheim do
Brasil), Anador solution 500 mg mL-1 (Boheringer
Ingelheim do Brasil), Conmel solution 500 mg mL-1
(Sanofi Winthrop Farmacêutica Ltda). For analysis
of capsules of 500 mg, 20 capsules of the drug were
used. The material from each capsule was removed and
accurately weighed, allowing the mean weight of the
contents of a single capsule to be estimated. Thereafter,
the material was homogenised and an appropriate
amount was sampled, dissolved in deionised water
and filtered. For analyses of sample solutions of
500 mg mL-1, five samples were used and the solution
contents were mixed for representative purposes.
Thereafter, the required volume was sampled and
diluted with deionised water.
2.3. Preparation and immobilization of Cu3(PO4)2
Preparation and immobilization of Cu3(PO4)2 was similar
to that previously reported [28]: Cu3(PO4)2 was prepared
by reacting CuCO3.Cu(OH)2 (Vetec) with concentrated
phosphoric acid (Merck).
2.4. Immobilization method
Ten grams of polyester resin solution were transferred to
a polyethylene flask; then 10 g of Cu3(PO4)2 were added
and after manual homogenization, 0.5 mL of the catalyst
(methylethyl ketone) were added and stirred until there
was an increase of viscosity. After 3-4 h, a rigid solid
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Flow-injection spectrophotometric determination of dipyrone
in pharmaceutical formulations using a solid-phase
reactor with copper(II) phosphate
Figure 2.
Schematic diagram of the flow system used for evaluation of the FIA manifold employed for dipyrone determination. C – Carrier solution
(deionized water with a flow rate of 1.7 mL min-1); S - sample or reference solutions (sample volume of 100 mL); SPR – Solid-phase
reactor (12 cm × 2.0 mm i.d.); R – Reagent solution (alizarin red S 0.020% (m/v) with a flow rate of 0.5 mL min-1); x – confluence point;
TCR – tubular coiled reactor (75 cm); D – detector (l = 540 nm); W - waste.
is obtained, which was broken with a hammer and a
domestic liquefier was used to obtain small particles.
The particle size was selected by sieving on known
mesh sieves. With this amount, it is possible to prepare
more than 50 reactors, once its solid-phase reactor was
prepared by packing ca. 350 mg of polyester particles
(100±350 mm) containing the immobilized copper(II)
phosphate into 13 cm long polyethylene tubes (2.0 mm
i.d.) by means of suction with a syringe and fixed in the
column with small pieces of glass wool.
2.5. Flow injection procedure
A schematic diagram of the flow manifold is shown in
Fig. 2. As suggested in the literature [29], dipyrone can
form a complex with Cu(II). However there are no data
in the literature about the structure of this complex.
The Cu(II) ions can be spectrophotometrically detected
with alizarin red S through the ternary complex whose
structure is 1:3 (Cu:VABO3) [30]. When a solution of
dipyrone is injected, Cu(II) ions are released from the
solid-phase reactor due the formation of Cu(II)-dipyrone
complex.
Thus, when alizarin red S (pH 9.0) is mixed into the
carrier solution containing Cu(II)-dipyrone complex, it
replaces the aspartame ligand leading to the formation
of a colored complex (Cu(II)- alizarin) of which the
absorbance measured at 540 nm is proportional to the
dipyrone concentration.
3. Results and discussion
3.1. Chemical parameters
In order to obtain either a long lifetime of the solid
phase reactor or a high sensitivity of the proposed
methodology, several solutions were investigated as
carriers: deionized water, acetate buffer solutions
0.05 mol L-1 (pH 4.8 and 5.6), sodium borate
1.0×10-4 mol L-1 and borate buffer solutions 0.1 mol L-1
(pH 8.0 and 9.0).
The best results were obtained with deionized water
as the carrier solution because, for acid solutions, Cu(II)
ions were released from the solid reactor even without
the adding of dipyrone solution. For basic solutions,
the absorbance signal decreased significantly due the
Cu(OH)2 formation in equilibrium with the Cu3(PO)4 in
the reactor making it difficult for Cu(II)-dipyrone complex
formation.
Pyrogallol vilolet and alizarin red S were evaluated
as colorimetric reagents for determination of Cu(II). The
best results were obtained with alizarin red S that was
chosen for further studies. The absorption spectrum of
the Cu(II)-alizarin complex shows that the maximum
absorbance difference between the alizarin solution
and Cu(II)-alizarin complex is found at 540 nm. This
wavelength was selected for further studies.
3.2. Solid-phase reactor parameters
The performance of solid-phase reactors can be
affected by several parameters. In order to obtain the
best results for the proposed procedure, some factors
were evaluated such as: column length, column internal
diameter, Cu(II)/polyester ratio, and particle size. Before
the first injection, all tested solid-phase reactors were
conditioned by passing the carrier solution for 10 min
for the best compaction of the particles in the column.
Reproducible absorbance signals were obtained after
five injections at least. The preparation of the solidphase reactors was evaluated for three different ratios
of Cu3(PO4)2/polyester resin: 12.5%, 25% and 50%
(m/m). Reactors prepared with 50% (m/m) showed the
highest sensitivity and was chosen for further studies.
Two particle size ranges (100-350 and 350-500 mm)
selected by sieving the triturated material were tested
with the compromise of minimize difficulties in regard
to the operating conditions (operation at low pressure).
The best results in terms of sensitivity and
reproducibility were obtained with particle size of 100350 mm. Solid-phase reactors with different internal
diameters (1.0, 1.5 and 2.0 mm) were prepared in order to
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V. G. Bonifácio, O. Fatibello Filho, L. H. Marcolino-Júnior
Table 1.
Optimization of chemical and flow injection parameters.
Parameter
Studied range
Selected value
12.5% - 25% - 50% (m/m)
50% (m/m) Cu3(PO4)2
100 - 500 mm
100 – 350 mm
4 - 20 cm
12 cm
Sample loop length
50 - 500 mL
100 mL
Tubular coiled reactor
50 – 200 cm
Reactor composition
Particle size
Reactor length
75 cm
Carrier flow rate
0.5 – 3.5 mL min
-1
1.7 mL mim-1
Reagent flow rate
0.5 – 3.5 mL min-1
0.5 mL min-1
0.005% - 0.020% (m/v)
0.020% (m/v)
Reagent concentration
Figure 3.
Effect of the reactor length on the absorbance intensity
for 100 mL of 1.0×10 -3 mol L-1 dipyrone solution.
optimize that parameter. All columns were prepared with
the same length (8.0 cm) and particle size (100-350 mm).
Best results were performed with solid-phase reactor of
2.0 mm of internal diameter due the highest absorbance
signal and easily preparation of these columns.
Fig. 3 shows the influence of the column length on the
absorbance signal for a studied range of 4.0 to 20 cm
at a carrier flow rate of 3.0 mL min-1. The absorbance
signals increased gradually with the increase of the
column length. Nevertheless, columns larger than
12.0 cm allowed the increase in the hydrodynamic
pressure, reducing the effective flow rate and,
consequently, the analytical frequency. Additionally,
the long-time stability of the column was investigated
measuring the absorbance of dipyrone solutions
containing 5.0×10-5 to 4.0×10-4 mol L-1. Plot calibration
curves were done daily indicating a good reproducibility
with a variation in the slope around 5-6% after at least
500 injections.
3.3. Flow injection parameters
To determine the optimal conditions for the flow injection
system performance, parameters such as sample loop
volume and carrier flow rate were initially investigated.
Table 1 presents the optimization of chemical flow
injection parameters studied in this work.
The effect of varying sample loop volume from 10
to 100 cm (50-500 mL) on the analytical signal was
evaluated by injection of the same concentration of
dipyrone passed through the column with immobilized
copper phosphate. The height peak was found to
increase with the sample volume between 50 and
100 mL, staying practically constant for higher volumes.
This behavior can indicate that the quantity of dipyrone in
100 mL, for these conditions, to be the maximum
necessary for the complex formation. Therefore, a
sample volume of 100 mL was selected for showing
better engagement between sensibility and analytical
frequency.
Using a univariate approach optimized the
carrier flow rate. The effect of the carrier flow rate
from 0.5 to 3.5 mL min-1 over the analytical signal
was studied and the optimal flow rate found was
1.7 mL min-1 whereas for flow rates higher than
1.7 mL min-1, the absorbance signals decreased due
the lower contact between the sample zone and the
immobilized Cu3(PO4)2. The effect of reagent flow rate
on the sensitivity was studied using a 0.020% (m/v)
alizarin red S solution in borate buffer solution with a pH
9.0 and varying the flow rate from 0.5 to 3.5 mL min-1.
With the decrease of the flow rate, the analytical signals
increased gradually due the low dilution of the reagent
solution. Therefore, a flow rate of 0.5 mL min-1 was
selected as the best compromise between sensitivity
and sample throughput. By using these flow rates, the
sample throughput was 60 measurements per hour.
The influence of the length of the tubular coiled
reactor was studied in the range from 50 cm to
200 cm and was observed that the absorbance
decreased continuously with an increase in the reactor
length. This fact can be explained by the higher effect of
the sample zone dispersion in contrast with an efficient
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Flow-injection spectrophotometric determination of dipyrone
in pharmaceutical formulations using a solid-phase
reactor with copper(II) phosphate
Table 2. Results of addition-recovery experiments using FIA spectrophotometric procedure for three different standard concentrations of dipyrone.
Dipyrone* / x 10-4 mol L-1
Sample
A
B
C
Added
Found
Recovery / %
2.00
2.01
100
4.00
3.98
99.5
6.00
6.00
100
2.00
2.00
100
4.00
4.17
104
6.00
6.25
104
2.00
1.98
98.9
4.00
3.87
96.7
6.00
5.87
97.8
*n=5
mixture between the reagent and the sample zone that
could allow an efficient reaction and, consequently, the
increase in the absorbance signal. Due the rapid reaction
for the complex formation, the dispersion predominates
and the absorbance decreases. For this, a tubular coiled
reactor with 75 cm was chosen.
Table 2 presents results of recovery experiments
for three real samples spiked with different standard
dipyrone concentrations. The FI procedure showed a
good accuracy with recoveries varying from 96.7% to
104.0%.
3.4. Effect of the alizarin red S concentration
The proposed method was used for the dipyrone
determination in pharmaceutical formulations. Dipyrone
reference solutions and samples solutions were injected
in the flow system showed in Fig. 2. The transient signals
yielded are shown in Fig. 4 and the obtained results are
presented in Table 3.
The analytical curve was linear in a range of
5.0×10-5 to 4.0×10-4 mol L-1. {A = 0.003 + 502.8C} where A
is the absorbance and C is the dipyrone concentration in
mol L-1. The detection limit was 2.0×10-5 mol L-1 (three
times the signal blank/slope), the relative standard
deviation of 10 successive dipyrone determinations
was 1.5%, and the analytical frequency was
60 determinations per hour.
No significant variations on the analytical curve
slope (5% to 6%) were observed for a 6-7 h work
period, indicating that the proposed method could be
adopted for dipyrone determination in pharmaceutical
formulations with sensitive and reproducible results.
The concentration of alizarin red S solution was
evaluated in the range of 0.005 to a maximum
concentration of 0.020% (m/v) limited by the solubility
of this reagent in borate buffer solution (pH 9.0). For
injections of a 1.0×10-4 mol L-1 dipyrone solution, the
best results (highest absorbance signals) were obtained
for a concentration of 0.020% (m/v) that was chosen for
further studies.
3.5. Interferences and recovery studies
The possible interferences on the determination
of dipyrone caused by common excipients used in
pharmaceutical preparations were investigated by
comparison of the response produced by a standard
solution containing 2.0×10-4 mol L-1 dipyrone with that
produced by a similar dipyrone solution with additions of
2.0×10-4 mol L-1 and 2.0×10-3 mol L-1 (sodium chloride,
sodium citrate, saccharin, sucrose, lactose and caffeine)
of each one of these potential interferents. There
were no significant interferences for the evaluated
substances with exception of sodium citrate that cause
a positive interference even in a 10-fold high dipyrone
concentration. This interference applies the fact that
citrate anions are excellent at complexing with Cu(II)
ions and releases this cation more effectively than
dipyrone. Samples that contained these substances
could not to be analyzed for this procedure.
3.6. Analytical curve and sample determination
4. Conclusions
The proposed method showed a simple and sensitive
way for dipyrone determination using a Cu(II) phosphate
reactor. Results are in agreement with those obtained
by the Brazilian Pharmacopoeia procedure. In addition,
no pre-treatment of the samples was required. The
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V. G. Bonifácio, O. Fatibello Filho, L. H. Marcolino-Júnior
Table 3.
Mean results obtained for the determination of dipyrone in pharmaceutica l formulations by FIA spectrophotometric procedure in
comparison with the official method.
Samples
Label Value
Reference*
FIA*
Re1 / %
A1
539
541 ± 3
544 ± 3
+ 0.6
+ 0.9
B1
699
721 ± 4
734 ± 4
+ 1.8
+ 5.0
2
C
500
507 ± 3
511 ± 2
+ 0.8
+ 2.2
D1
860
845 ± 3
812 ± 3
- 3.9
- 5.6
2
E
500
492 ± 3
494 ± 2
+ 0.4
- 1.2
F2
500
483 ± 3
490 ± 2
+ 1.4
- 2.0
Re2 / %
average of five determinations ± SD
Re1 = relative error = FIA method versus reference method.
Re2 = relative error = FIA method versus label value.
1 - mg dipyrone per g tablet
2 - mg dipyrone per milliliter
*
Figure 4.
Typical transient signals for reference and sample solutions. From left to right, shown is a triplicate of reference dipyrone solutions in
the concentration range of 5.0×10-5 to 4.0×10-4 mol L-1, eight pharmaceutical sample solutions (A, B, C, D, F, G and H) and triplicate of
reference dipyrone solutions again. Experimental conditions reported on Fig. 1.
reproducibility and stability of the solid-phase reactor
make it an attractive alternative for the determination
of dipyrone in a flow system, and this method may
be suitable for routine determination of dipyrone in
pharmaceutical formulations.
Acknowledgements
The authors gratefully acknowledge financial support
from these Brazilian foundations: CNPq, CAPES, and
FAPESP.
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reactor with copper(II) phosphate
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