Spectrophotometric determination of paracetamol with microwave assisted alkaline hydrolysis

Article abstract of DOI:10.1016/j.saa.2003.10.003

A novel and rapid spectrophotometric method for the determination of paracetamol is proposed in this paper. The proposed method is based on the microwave assisted alkaline hydrolysis of paracetamol to p-aminophenol that reacts with S^2- in the presence of Fe³⁺ as oxidant to produce a methylene blue-like dye having an absorptivity maximum at 540nm. The experiment showed that paracetamol could be hydrolysed quantitatively to p-aminophenol in only 1.5min under radiation power 640W using a microwave in NaOH medium. The system obeys Beer's law in the range of 0-3.0×10 ^-4moll^-1 paracetamol. The molar absorptivity and Sandell's sensitivity were found to be 3.2×10³lmol^-1cm ^-1 and 0.047μgcm^-2, respectively. The relative standard deviation (n=11) was 1.7% for 8.0×10^-5moll^-1 paracetamol. The method has been applied successfully to analysis of paracetamol in pharmaceutical preparation.

Full text of DOI:10.1016/j.saa.2003.10.003

Spectrochimica Acta Part A 60 (2004) 1861–1864
Spectrophotometric determination of paracetamol with microwave
assisted alkaline hydrolysis
Chunli Xu, Baoxin Li∗
Department of Chemistry, School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an 710062, PR China
Received 1 March 2003; received in revised form 6 October 2003; accepted 6 October 2003
Abstract
A novel and rapid spectrophotometric method for the determination of paracetamol is proposed in this paper. The proposed method is based
on the microwave assisted alkaline hydrolysis of paracetamol to p-aminophenol that reacts with S2− in the presence of Fe3+ as oxidant to
produce a methylene blue-like dye having an absorptivity maximum at 540 nm. The experiment showed that paracetamol could be hydrolysed
quantitatively to p-aminophenol in only 1.5 min under radiation power 640 W using a microwave in NaOH medium. The system obeys Beer’s
law in the range of 0–3.0×10−4 mol l−1 paracetamol. The molar absorptivity and Sandell’s sensitivity were found to be 3.2×103 l mol−1 cm−1
and 0.047 ␮g cm−2, respectively. The relative standard deviation (n = 11) was 1.7% for 8.0 × 10−5 mol l−1 paracetamol. The method has
been applied successfully to analysis of paracetamol in pharmaceutical preparation.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Spectrophotometry; Paracetamol; Microwave assisted alkaline hydrolysis
1. Introduction
the conventional hydrolysis step was major limitation to im-
proving the sample throughput of this frequently ordered
test because the hydrolysis step needed 30 min in boiling
water bath [15,17], which was the time-limiting step in the
analytic procedure.
Paracetamol (N-acetyl-4-amino-phenol) is a popular anal-
gesic and antipyretic medication that is readily absorbed
after administration and has few side effects and little tox-
icity when used in recommended dose. After ingestion of
an overdose quantity of paracetamol, the accumulation of
toxic metabolites may cause severe and sometimes fatal
hepatotoxicity and nephrotoxicity [1]. So, the accurate de-
termination of paracetamol in pharmaceutical preparations
and biological fluids has appeared especially attractive. For
its measurement, many methods have been developed, such
as fluorometry [2], chemiluminescence [3], electrochemical
method [4], nuclear magnetic resonance-mass spectrometry
[5], gas chromatography [6], liquid chromatography [7,8]
and capillary electrophoresis [9].
Microwave digestion is an effective alternative to con-
ventional procedures as it reduces digestion time and
problems associated to analyte losses and atmospheric
contamination [18]. Microwave oven power has also been
used to carry out the hydrolysis of organic compounds in
acid and alkaline media, reactions involving the hydrolysis
of specific bonds to yield compound suitable for further
derivatisation/determination are of especial interest [19].
Microwave technology has been used into the hydroly-
sis of paracetamol to p-aminophenol [13,14,16]. How-
ever, in these systems, the derivatisation of p-aminophenol
needed some special reagent, such as o-cresol [14,16] and
8-hydroxyquinoline [13], which was inconvenient for the
operation.
In this paper, a novel and simple spectrophotometric
method for the determination of paracetamol is proposed.
The proposed method is based on the microwave assisted
alkaline hydrolysis of paracetamol to p-aminophenol that re-
acts with S2− in the presence of Fe3+ as oxidant to produce
a methylene blue-like dye having an absorptivity maximum
As simple and routine analysis method, spectrophotomety
has also been successfully applied to determine paracetamol
[10–17]. These methods usually required paracetamol to be
hydrolyzed to p-aminophenol, which could form a colored
derivative. Although the analytical reaction was fairly fast,
∗
Corresponding author. Tel.: +86-29-5300986; fax: +86-29-5307774.
E-mail address: libx29@hotmail.com (B. Li).
1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2003.10.003

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C. Xu, B. Li / Spectrochimica Acta Part A 60 (2004) 1861–1864
at 540 nm [15]. In this system, paracetamol could be hy-
drolyzed quantitatively to p-aminophenol in only 1.5 min,
and the derivatisation of p-aminophenol could be accom-
plished with very simple reagent, S2− and Fe3+, quickly
at room temperature. The proposed method is rapid and
simple, and has been applied satisfactorily to the analysis
of paracetamol in pharmaceutical preparations.
3. Results and discussion
3.1. Hydrolysis of paracetamol
The effects of experimental parameters on the hydroly-
sis of paracetamol, such as HCl and NaOH concentration,
the radiation power supplied, reaction hydrolysis time were
studied using PTFE reactors.
2. Experimental
3.1.1. Effect of hydrolysis medium
The paracetamol can be hydrolysed to p-aminophenol
in HCl or NaOH medium. The effect of HCl or NaOH
medium on the hydrolysis of paracetamol was investigated
at a power level of 400 W and using irradiation time of
2 min. The result showed that hydrolysis yield of paraceta-
mol in NaOH was higher than in HCl for the same radi-
ation power and irradiation time, which is consistent with
reference [13]. So, NaOH was selected for the hydroly-
sis of paracetamol in subsequent experiments. The effect
of NaOH concentrations was also examined in the range
1.0–6.0 mol l−1. The result showed that the hydrolysis yield
increased with increasing NaOH concentrations. However,
above the concentration of 4 mol l−1, hydrolysis yield de-
clined. Therefore, a NaOH concentration of 4 mol l−1 was
selected.
2.1. Apparatus
A Hitachi model U-2000 UV-Vis double-beam spec-
trophotometer was used. A household microwave oven (LG
Electronics Tianjin Appliances Co. Ltd., Tianjin. China)
equipped with a magnetron of 2450 MHz with a nominal
maximum power of 800 W as marked was used.
2.2. Materials and reagents
All chemicals and solvents were of analytical grade. Dis-
tilled water was used throughout.
A stock solution of paracetamol (The Institute of Phar-
maceutical and Bio-material Authentication, China) 4.3 ×
10−3 mol l−1 was prepared by dissolving 325 mg of the solid
product in 500 ml distilled water using an ultrasonic water
bath to ensure a complete dissolution. This solution was kept
in a dark bottle, and was highly stable for long time. Work-
ing standard solutions were prepared daily from the stock
solution by diluting the stock solution.
3.1.2. Effect of microwave oven power and hydrolysis time
The effect of microwave oven power was studied in
the range 160–800 W. The hydrolysis yield increased
with increasing microwave oven power up to 640 W, and
then decreasing thereafter. Therefore power 640 W was
selected.
The effect of the hydrolysis time was studied. The hy-
drolysis yield increased with time increased in the range
0.5–1.5 min. However, above 1.5 min, the hydrolysis yield
decreased.
Na2S (3.2 × 10−3 mol l−1) was freshly prepared daily by
dissolving 384.5 mg in 100 ml distilled water. Fe(III) solu-
tion of 9.2 × 10−2 mol l−1 was prepared by dissolving am-
monium iron(III) sulfate (Xi’an Reagent Plant, Xi’an) in
500 ml 0.03 mol l−1 sulfuric acid.
Endophy tablets (Shenzhen Neptunus Pharmaceutical Co.
Ltd., China), Jin Gang Cold tablets (Fujian Desheng Phar-
maceutical Co. Ltd., China) and Puerxitong tablets (Shaanxi
Xi’an Pharmaceutical Plant, China) were purchased from
the local market.
The above result showed that the hydrolysis yield de-
creased with power more than 640 W or time more than
1.5 min. The probably reason is that the degradation of parac-
etamol or p-aminophenol (or both) occurred [14].
3.2. Optimization of the derivatisation reaction
2.3. Procedure
The proposed method is based on the microwave assisted
alkaline hydrolysis of paracetamol to p-aminophenol that
reacts with S2− in the presence of Fe3+ as oxidant to produce
a methylene blue-like dye as follows (Scheme 1).
The absorption spectra of the reaction product using
Fe(III) as oxidant was examined. The final product shows a
maximum absorbance at 540 nm [15].
To a PTFE reactor, a 5.0 ml volume of the sample solu-
tion and a 5.0 ml 8 mol l−1 solution of NaOH were added.
The reactor was placed inside the domestic microwave
oven irradiated at 640 W power for 1.5 min. The contents
of the reactor were transferred quantitatively to a 50 ml
volumetric flask, to which 13.0 ml of 6 mol l−1 HCl, 6 ml
of sodium sulfide solution and 6 ml Fe(III) solution was
sequentially added. The mixture was quickly diluted to a
final 50 ml volume with distilled water, shaken for about
0.5 min and allowed to stand for 10 min. The absorbance
was measured at 540 nm against the corresponding reagent
blank.
3.2.1. Order of reagent addition
The order of reagent addition was very important. The
result showed that HCl solution, sodium sulfide solution and
Fe(III) solution should be orderly added, and changing the
order produced low results.

C. Xu, B. Li / Spectrochimica Acta Part A 60 (2004) 1861–1864
1863
Scheme 1. Suggested mechanism for the system.
increasing the Fe(III) concentration up to 1.0×10−2 mol l−1
.
3.2.2. Effect of HCl concentration
Various concentrations of HCl, ranging from 0.8 to
2.0 mol l−1, were tried to acidify the microwave hydrolysis
product. The results indicate that 1.6 mol l−1 is necessary
for maximum absorbency.
Above it, the absorbance decreased. Therefore, the Fe(III)
concentration of 1.0 × 10−2 mol l−1 was chosen as
optimum.
3.2.5. Color stability
3.2.3. Effect of sodium sulfide concentration
Under the optimum reaction conditions described above,
the absorbance of final product gradually reached maximum
throughout the first 10 min and remained constant for more
than 3 h. Therefore, absorption measurements were carried
out 10 min after the mixture was diluted to a fixed volume
with distilled water.
The effect of the sodium sulfide concentration on ab-
sorbency was studied and presented in Fig. 1. The results
showed that the absorbency increased with increasing s−u1l-
fide concentration from 0.6 × 10−4 to 3.8 × 10−4 mol l
For sodium sulfide concentration higher 3.8×10−4 mol l−1
.
,
the absorbance remained almost constant. The sodium sul-
fide concentration of 3.8×10−4 mol l−1 was used in all sub-
sequent work.
3.3. Quantification
Under the proposed experimental conditions, the system
obeys Beer’s law in the range of 0–3.0 × 10−4 mol l−1
paracetamol. The molar absorptivity (ε) and Sandell’s sen-
sitivity (s) were found to be 3.2 × 103 l mol−1 cm−1 and
0.047 ␮g cm−2, respectively. The relative standard devi-
ation (n = 11) was 1.7% for 8.0 × 10−5 mol l−1 para-
cetamol.
3.2.4. Effect of Fe(III) concentration
The effect of Fe(III) concentration on absorbance was
investigated (Fig. 2). The absorbance increased with
1.4
1.2
1
0.8
0.6
0.4
0.2
3.4. Interference study
The influences of foreign species were investigated by an-
alyzing the interference of a series of typical active species
(acetylsalicylic acid, codeine, caffeine, chloropheniramine
and ascorbic acid) that are in currently present in pharma-
ceutical preparations of paracetamol to 8.0 × 10−5 mol l−1
paracetamol. The results indicted that they did not interfere
with the determination of paracetamol.
0
2
4
0
6
Sulfide Concentration (X 10 -4mol/l)
Fig. 1. Effect of S2− concentration on the absorbance: microwave ir-
radiation, 640 W and 1.5 min; paracetamol, 1.0 × 10−4 mol l−1; HCl,
1.6 mol l−1; NaOH, 4.0 mol l−1; Fe3+, 9.0 × 10−3 mol l−1
.
3.5. Sample analysis
0.6
0.5
0.4
0.3
0.2
Several commercial paracetamol tablets were grounded
to powder and a portion of the powder was weighted and
dissolved in water. After filtering, aliquots of the filtrate were
further diluted with water in order that the concentration of
paracetamol was in the working range.
Following the procedure detailed in Section 2, the pro-
posed method was applied to the determination of parac-
etamol concentration in pharmaceutical preparations. The
contents of paracetamol in three kinds of pharmaceuti-
cal preparations were determined. The results (showed in
Table 1) agree with the manufacturers’ stated contents of
paracetamol.
0.3
0.7
1.1
1.5
Fe(III) Concentration (X 10 -2 MOL/l)
Fig. 2. Effect of Fe3+ on the absorbance: microwave irradiation, 640 W
and 1.5 min; paracetamol, 1.0 × 10−4 mol l−1; HCl, 1.6 mol l−1; S2−
,
3.8 × 10−4 mol l−1; NaOH, 4.0 mol l−1
.

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C. Xu, B. Li / Spectrochimica Acta Part A 60 (2004) 1861–1864
Table 1
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[3] D. Easwaramoorthy, Y. Yu, H. Huang, Anal. Chim. Acta 439 (2001)
95.
[4] H. Tanaka, P.K. Dasgupta, J. Huang, Anal. Chem. 72 (2000)
4713.
[5] J.P. Shockcor, S.E. Unger, I.D. Wilson, P.J.D. Foxall, J.K. Nicholson,
J.C. Lindon, Anal. Chem. 68 (1996) 4431.
[6] S. Murray, A.R. Boobis, J. Chromatogr. 33 (1991) 355.
[7] A.G. Goicoechea, M.J.L. de Alda, J.L. Vila Jato, J. Liq. Chromatogr.
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[8] V. Bari, U.J. Dhorda, M. Sundaresan, Indian Drugs 35 (1998)
222.
Determination of paracetamol in pharmaceutical preparation using the
proposed method
Sample
Stated
content
(mg U−1
Detected
contenta
R.S.D. %
(n = 4)
Relative
error
)
(mg U−1
)
Endophy
Jin Gang Cold
Puerxitong
335
250
500
330
258
494
1.5
1.6
1.5
−5
8
−6
a
Average of four measurements.
4. Conclusions
[9] S. Heitmeier, G. Blaschke, J. Chromatogr. B 721 (1999) 93.
[10] J.T. Afshari, L. Tsan-Zon, Anal. Chim. Acta 443 (2001) 165.
´ıa, C. Sánchez-Pedreo, M.I. Albero, Talanta
The proposed method offers several advantages over pre-
viously reported procedure. The proposed method is rapid
and simple, and has been applied satisfactorily to the anal-
ysis of paracetamol in pharmaceutical preparations.
[11] V. Rodenas, M.S. Garc
52 (2000) 517.
[12] A. Ruiz-Medina, M.L. Fernández-de córdova, M.J. Ayora-Canˇada,
M.I. Pascual-Reguera, A. Molina-Diaz, Anal. Chim. Acta 404 (2000)
131.
[13] Z. Bouhsain, S. Garrigues, A. Morales-Rubio, M. de la Guardia,
Anal. Chim. Acta 330 (1996) 59.
[14] A. Criado, S. Cárdenas, M. Gallego, M. Valcárcel, Talanta 53 (2000)
417.
[15] F.A. Mohamed, M.A. AbdAllah, S.M. Shannat, Talanta 44 (1997)
61.
Acknowledgements
The Committee of the Education Foundation of Huo Ying-
dong (Grant No. 81013) supported this study.
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(2000) 1179.
[17] A. Dasgupta, G. Kinnaman, Clin. Chem. 39 (1993) 2349.
[18] J.L. Burguera, M. Burguera, Analyst 123 (1998) 561.
[19] K.D. Khalaf, A. Morales-Rubio, M. de la Guardia, Anal. Chim. Acta
284 (1993) 249.
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