pH Induced Difference Spectrophotometric Assay of Paracetamol and Mefenamic Acid

FABADJ. Pharm. Sci., 25, 91-95, 2000

RESEARCH ARTICLES /BİLİMSEL ARAŞTIRMALAR

pH Induced Difference Spectrophotometric Assay of Paracetamol and Mefenamic Acid

in Tablets

Cem YÜCESOY*^0, Keziban TÜLEK*

pH lnduced Difference Spectrop/wtometric Assay of Paracetamol and Mefenamic AcUl in Tablets

Swnmary : Binary combimtions ef pamce!f1mfll (P) atUi mefenamic acid (M) wm deıemıined by pH indl.!ced difference spectrophotonıeıry. For !his pwpose ıwo sets ef solııtinns ef tlıe compoıınds in 0.02 N melhanDlic NaOH atUi 0.02 N melhanDlic HCT were pnqxıred Acidic solııtinns were ıısed as b/ank in tlıe measıımnenıs. Difference absorlxw:es ef sıandard atUi sompie

Ja/ı.aions >Wre read at272.5 nmfor P amf 3620 nmfor M Ihe recoveries ef

P amf M from sıandard mixıures were 98.3 % for P atUi 98.9 % for M re- spectively. Assay results ef tlıe cmımen:ialfomıulaıion ""'~ in good agree- menJ witlı tlıe staJed conJen!s ef tlıe dosage uniJs atUi wilfı tlıe resulıs ef Vie- ronit's metlınd Ihe reprodı.u::ibilİ1y ef tlıe as&ı)! resulıs as reloJive standard devia!ion is0.79%forPami1.19%forM, respectively.

Keywords : Paracetamol, mefenamic acid, silnultaneous determination, pH induced difference spec- trophotometry

Received Revised Accepted

04 03.2000 26.04.2000 26.04.2000

INTRODUCTION

Paracetamol (P) is commonly used as an analgesic and antipyretic agent1. On the other hand mefenamic acid (M) is prescribed as an antiinflammatory agent1.

The combination of P and M is useful in the treat- ment of pains connected with rheumatoid arthritis.

Paracetamol has been determined in pharmaceuticals and biological samples by spectrophotometry2-4, flu- orometry5, GC4, HPLC4,6, capillary electrophoresis7 and NIRS while mefenamic acid has been determined

pH Değişimine Dayanan Fark Spektrofotoınetrisi ile Tabletlerde Parasetamol ve Mefenamik Asit Tayini Özet : Parasetamol (P) ve mefenamik asitin (M) ikili ka-

rışımları pH değişimine dayanan fark spektrofotometrisiyle tayin edildiler. Bu amaçla bileşiklerin 0.02 N metanollü NaOH ve 0.02 N metanollü HCl de obnak üzere 2 seri çö- zeltisi hazırlandı. Asidik çözeltiler ölçünılerde kör olarak

kullanıldılar. Standart ve nunıunelerin fark absorbansları P için 272.5 nm de, M için 362.0 nm de okundu. P ve M için standart karışunlardan geri kazanını sırasıyla % 98.3 ve o/o 98.9 bulundu. Ticari formülasyonun analiz sonuçları etikette belirtilen miktarla ve Vierordt nıetodııyla bulunan so- nuçlarla uyumlu bulundu. Analiz sonuçlarının tekrar

edilebilirliği -bağıl standart sapma olarak- P için o/o O. 79, M için% 1.19 dur.

Anahtar kelimeler : Parasetanıol, nıefenanıik asit, bir arada tayin, fark spektrofotometrisi

by titrimetry2, spectrophotometry9, fluorometrylD, HPLCll,12, GC-MS13 and capillary electrophoresis14.

On the other hand, there are only three methods re- ported for simultaneous determination of P and M in binary mixtures. These were based on spectro- photometry15, first derivative UV spectrophotometry16 and HPLC17. This paper describes a very simple and accurate method for simultaneous determination of P and M by difference spectrophotometry18. This tech- nique was used to eliminate spectral interferences caused by co-drugsı⁹,ıo, matrix ingredients21 and degradation products22.

* Ankara Üniversitesi Eczacılık Fakültesi, Analitik Kimya Anabiliın Dalı, 06100 Tandoğan-Ankara

°

Yücesoy, Tülek

EXPERJMENT AL Apparatus

A Shimadzu UV-160 recording spectrophotometer ( spectral bandwidth ⁼ 2 nm) was used far ali UV measurements. Spectra between ^225-400 nm were recorded and plotted using a Lexmark ¹⁰²⁰ Printer.

Scan speed was 8 nm / s. Ordina le settings in dif- ference spectra were ^+0.500 and -0.250.

Chemicals and dosage form

Paracetamol was kindly donated by Roche Pharm.Co. (Turkey) and mefenamic acid by Nobel Pharm. Co. (Turkey). Lanagesic® tablets were kind- ly supplied by Tata Pharma, Bombay; India.

The declared content per tablet was : Paracetamol (500 mg) + mefenamic acid (250 mg). Ali other chemicals were analytical reagent grade.

Solutions and plotting of calibration graphs

Stock solutions of paracetamol ^{(!: 1000} µg.mJ-1) and mefenamic acid (il : 500 µg.mı-1) were prepared in methanol. Alkaline working solu ti ons of P ^(1-14

µg.ınJ-1) and of M (1-8 µg.mJ-1) were prepared by ap- propriate dilution of 1 and il with 0.02 N methanolic NaOH, separately.

Acidic working solutions were prepared analogously using 0.02 N methanolic HCI. Difference absorbance values (M) were obtained by reading the ab- sorbances between equimolar so]utions prepared in 0.02 N methanolic NaOH relative to solutions pre- pared in 0.02 N methano!ic HCI. ^ılA of working so]u- tions of P and M were measured at 272.5 nm far P and 362.0 nm far M. Calibration equations of ^P and M were calculated usir,g M values and cor- responding concentrations of working solutions.

Sample preparation and prpcedure

Twenty tablets were weighed and powdered. Tab- let powder equivalent to ⁵⁰ mg of paracetamol was accurately weighed and placed in a ⁵⁰ mi volumetric flask. The flask was filled up with methanol and stirred for ²⁰ min by a magnetic stirrer. The resulting suspension was filtered through a Whatrnan No. ⁴² filler paper and 5 mi a!iquots of the filtrate were diluted to ¹⁰⁰ mi with 0.02 N methanolic NaOH (III A) and 0.02 N methanolic HCI (III B), separately.

W orking solutions of the ^sanıp le were prepared by diluting 5 mi aliquots of ^fil A and ^fil B to 25 mi with 0.02 N methanolic NaOH (SA) and ^0.02 N methanolic HCI (SB), separately. Difference ab- sorbance values (M) were obtained by reading the absorbance of SA relative to SB at ^272.5 nm for P and 362.0 nm far M. Concentrations of P and ^M were found using the corresponding calibration equations.

RESULTS AND DISCUSS!ON

Mefenamic acid

Paracetamol and mefenamic acid have different spec- tral characteristics in acidic and alkaline media, which are attributed to the effects of the auxochromic groups !hey possess.

However in either media, their spectra overlap between 225-400 nm so !hat !hey cannot be determined simultane- ously by direct UV measurements (Fig. ¹ A and B).

o •

c

•

1.l5.0

Figure 1.

A

400,0

Wavelength (nm)

B

25M

'°'

Wavclcngth ^(nnı)

Zero-order spectra of paracetamol, 5 µg.rnl-1 ( ... ) and mefenamic acid, 5 µg.m!-1 ( ----) in (A) 0.02 N methanolic HCl and (B) 0.02 N methanolic N aOH.

FABAD J. Pharm. Sci., 25, 91-95, 2000

The difference spectrophotometric method proposed to take advantage of the spectral shifts induced by pH changes to overcome this problem. in this meth- od, difference absorbances between equimolar solu- tions prepared in 0.02 N methanolic NaOH relative to solutions prepared in 0.02 N methanolic HCI were measured.

Difference spectra of mefenamic acid have a zero- crossing point at 272.5 nm, which means that differ- ence absorbance (ılA) of it at this wavelength is zero (Fig.2). Consequently, ılA values of the binary mix- ture at this wavelength result only from paracetamol and can be used far the determination of this drug.

Since paracetamol has na ılA values above 330 nm, mefenamic acid can be determined using ılA values of the binary mixtures at 362.0 nm, where difference spectra of mefenarnic acid are ata minimum.

2<5.0 2Sll.O E.0 _fül.0

Wavelength (nm)

Figure 2. Difference spectra of paracetamol, 5 µg.mJ-1 ( ... ) and mefenamic acid, 5 µg.mJ-1 (---) in 0.02 N methanolic NaOH relative to equimolar solu- tions in 0.02 N methano!ic HCL

Calibration equations evaluated by plotting ılA val- ues of P and M separately against concentration are in Table 1. They obey Beer's law in the concentra- tion range of 1-14 µg.mJ-1 far P and 1- 8 µg.mJ-1 far M, respectively.

The m/m ratio of P ta M in the pharmaceutical preparation is 2:1 (500mg+250mg).

Recovery studies were perfarmed with synthetic

mi'<tures containing different concentrations of P and M ta determine the ratios at which one sub- stance can be accurately measured in the presence of the other. The recoveries of both drugs far mix- tures investigated were ranged between 97.6 % and 100.2 % (Table 2). They indicate !hat each com- ponent in binary mixtures of 2 : 1 composition can be reliably determined by the proposed method.

Assay results of the combined formulation of P and M were presented in Table 3. The concentra- tions faund were in good agreement with the stated contents of the dosage units and the pre- cisions were betler than 0.79 % (RSD) far P and 1.19

% (RSD) far M, respectively. The results were com- pared with the results obtained by Vierordt's meth- od, which was described by Das at aJ15_ However the method was applied ta alkaline solutions using ab- sorbance-values at 263.8 nm (Amax of P) and 289.9 nm (A.max of M). According ta statistical <lata, the Table l. Calibration data of P and M by difference

spectrophotometry (Li)

Paracetrunol MefenamicAcid

Metlıod (A. as nrn) Ll (2725 )* Li(362.0)

Slope(a) 5.2•10-2 -1.6°10-2

Intercept (b) 9.5•10-3 -l.1 •10-3 Correlationcoeff. (r) 0.9999 0.9994 Conc.Range (µg•mI-1) 1-14 1-8

Datapoints 7 7

* Li (272.5), Li (362.0) : Difference absorbances meas- ured at 272.5 nm and 362.0 nm.

difference between the results and precisions are not significant, showing the applicability of the proposed method to tablets.

The disadvantage of Vierordt's method is that it re- quires the solution of simultaneous equations, which may lead to calculation errors. HPLC method de- scribed formerly by Rau at eJ1⁷ suffers from high as- say-costs and the accuracy of this method ranging be- tween 98.4 % and 100.4 % is na better than the pre- sented method. Moreover the reproducibility of P and M by this method were 2.79 % (RSD) and 2.17 % (RSD), respectively, which were less than the pre- sented method. Derivative spectrophotometric meth-

Yücesoy, Tülek

Table 2. % Recovery of paracetamol and mefenamic acid from synthetic mixtures

Added(mg)

PARACETAMOL MEFENAMICACID

500 500*

500 500 450 550 600 Mean

Standard deviation ( SD )

% Relative standard deviation (RSD) 200 250*

300 350 250 250 250

'The m/m ratio of P and M in Lanagesic tablets

%Recovery

PARACETAMOL MEFENAMIC ACID

99.6 99.4

98.4 98.2

97.9 97.9

98.0 98.7

98.4 98.2

97.6 99.6

98.5 100.2

98.3 98.9

0.64 0.86

0.65 0.87

Table 3. Assay results far paracetamol and mefenamic acid in Lanagesic tablets

PARACETAMOL MEFENAMIC ACID

Content (mg) ^{il (} 272.5) VM* Content (mg) il ( 362.0) VM*

500 498.5 489.5 250 246.5 245.0

500 498.5 498.5 250 251.3 248.8

500 494.5 493.0 250 251.8 246.8

500 489.5 494.0 250 245.8 246.8

500 492.5 487.5 250 246.3 243.8

Mean 494.7 492.5 Mean 248.3 246.2

SD 3.90 4.26 SD 2.95 2.01

% RSD 0.79 0.86 % RSD 1.19 0.82

t = 2.31 ( for p=0.05 ) tcalc = 0.85 t = 2.31 ( for p=0.05 ) tcalc

=

F = 6.39 ( for p=O. 05 ) Fcaıc

=

=

• VM: Vierordt's method. The calculations are based on measurements made at 263.8 nm (Amax of P) and 289.5 nm (Amax of M) in alkaline media.

od described by Tora! et al¹⁶ was applied only to synthetic mixtures and need to be validated. This technique requires spectrophotometers with <lata processing units, which are abundant in this par- ticular case.

During the study, determination of M by direct UV measurements at 351.4 nm {absorption maxirna of M in acidic media) and 335.8 nm (absorption maxirna of M in alkaline media), where P has no absorbance, was also attempted. Since the results were > 2% high- er than the results of both difference spectro- photometry and Vierordt' s method, this possibility was excluded.

As conclusion, the difference spectrophotometric method is siınpler and faster compared with the methods described formerly for the siınultaneous de-

terınination of P and M. !ts precision and accuracy are in acceptable ranges far use in content uniforınity

tests of combined formulations.

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FABADJ. Pharm. Sci., 25, 91-95, 2000

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