Acta Chim. Slov. 2006, 53, 357–362 357 Scientific Paper Spectrophotometric Determination of Salicylamide and Paracetamol in Biological Samples and Pharmaceutical Formulations by a Differential Kinetic Method Abbas Afkhami1, Nahid Sarlak1 and Ali Reza Zarei2 1 Department of Chemistry, Faculty of Sciences, Bu-Ali Sina University Hamadan, Iran 2 Faculty of Materials & Chemical Engineering, Malek-Ashtar University of Technology, Tehran, Ira n Received 18-04-2005 Abstract A rapid, simple and sensitive spectrophotometric method is presented for the determinations of salicylamide and paracetamol in mixture. The method is based on their complexation and oxidation reactions by Fe3+ ion. Two sets of conditions were established such that in one set of conditions only salicylamide reacts with Fe3+ ion, but in the other set of conditions both of the salicylamide and paracetamol are oxidized by Fe3+ ion in the presence of 1,10-phenantroline. In both sets the reactions can be monitored spectrophotometrically by measuring the absorbance of the produced copmplexes at 510 nm. The data were evaluated by proportional equations. The method allowed the determination of salicylamide and paracetamol at concentrations between 2.0 – 300 µg mL–1 and 0.50 - 10 µg mL–1 with relative standard deviations of 2.58% and 3.47%, respectively. The method was applied to the determination of salicylamide and paracetamol in human serum and pharmaceutical formulations. Key words: salicylamide, paracetamol 1. Introduction Differences in kinetic behavior have been used extensively for the determination of two or more components in mixtures. Many differential kinetic methods have been proposed for the analysis of mixtures of closely related species without prior separation.1-5 Proportional-equation method is a mathematical method of wide use in differential kinetic methods for the resolution of closely related species. This method depends on changing the ratio of two rate constants by varying the reaction medium or the conditions.5 Salicylamide (o-hydroxy benzamide, SAL) and paracetamol (N-acetyl-p-aminophenol, PRC, also known as acetaminophen) are important and extensively used antipyretic-analgesic drugs. They are frequently prescribed in mixture with each other or with other related drugs. Therefore their determinations in mixtures are required. Several methods have been reported for determination of salicylamide and paracetamol in mixtures. These include HPLC,6 spectrofluorimetric7 electrochemical8 and spectrophotometric9 methods. Recently we reported a spectrophotometric method for the determination of salicylamide and paracetamol in mixtures using partial least squares regression (PLS) and H-Point standard addition method (HPSAM).10 In this paper we describe a simple, precise and accurate method for rapid spectrophotometric determination of salicylamide and paracetamol in mixture. The method is based on their complexation and oxidation reactions with Fe3+ ion at two different sets of conditions. Under both sets of conditions the reactions were monitored spectrophotometrically by measuring the increase in absorbance at 510 nm. 2. Results and discussion 2.1. Preliminary Investigations In pH 3.5 acetate buffer solution and at 25 °C, salicylamide reacts with Fe3+ ion and forms a colored complex (Eq. 1). This reaction was performed through the possession of a free phenolic group adjacent to the COR group (R=NH2). The spectrum of complex is shown in Fig. 1. Under this set of conditions paracetamol does not react with Fe3+ ion even up to 500 µg mL–1. Salicylamide + Fe -> colored complex (1) But in pH 4.5 acetate buffer solution and at 60 °C salicylamide and paracetamol are oxidized by Fe3+ ion and Fe2+ ion is produced (Eq. 2). The produced Fe2+ ion reacts with 1,10-phenantroline and forms a colored complex which is called ferroin (Eq. 3).11 The spectrum of produced ferroin is shown in Fig. 1. Afkhami and Sarlak Determination of Salicylamide and Paracetamol 358 Acta Chim. Slov. 2006, 53, 357–362 Paracetamol Salicylamide + Fej ^ 2+ Oxidized ->¦ Fe + /r,\ products (/) Fe + 3 Phen -> Fe(Phen)3 + (Ferroin) (3) 400 450 500 550 600 650 700 Wavelength / nm Figure 1. Absorption spectra of (a) complex of salicylamide with Fe3+ ion and the ferroin produced from oxidation reaction of (b) salicylamide (c) paracetamol and (d) their mixture by Fe3+ ion. Conditions for (a) : salicylamide, 40 µg mL–1;pH, 3.5; Fe3+, 2.58×10-3 M; T= 25 oC; for (b): salicylamide, 20 µg mL-1; paracetamol, 5.0 µg mL–1; pH, 4.5; 1,10-phenanthroline, 1.8×10-3 M; Fe3+, 6.45×10-4 M; T=60 oC. The reactions could be monitored spectrophoto-metrically by measuring the absorbance of the solution at 510 nm with time. Based on the above results, salicylamide and paracetamol could be determined in the presence of each other by choosing suitable conditions. Two simultaneous equation were solved to give the salicylamide and paracetamol concentration. 2.2. Effect of Variables The effect of reaction variables was studied separately for salicylamide and paracetamol and their optimum values for procedures were selected. Two sets of conditions must be fulfilled. Under the first set of conditions (Procedure 1) only salicylamide reacted and under the second set of conditions (Procedure 2) both the salicylamide and paracetamol reacted. The effect of pH on the reaction of salicylamide and paracetamol with Fe3+ was studied in the range of 2-7. The results are shown in Fig. 2. As Fig. 2 shows under the first set of conditions only salicylamide forms complex with Fe3+ ion and paracetamol did not react. Even when its concentration was in 10-fold excess over salicylamide. The absorbance for salicylamide reaction increased with increasing pH up to 3.5 and decreased at higher pH values. Therefore a pH of 3.5 was chosen for the first set. Fig. 2 shows the effect of pH on both the oxidation reactions of salicylamide and paracetamol with Fe3+ ion under the second set of conditions. As Fig. 2 shows the absorbance for both 12345678 pH Figure 2. Effect of pH on the complex formation reaction of (a) paracetamol and (b) salicylamide with Fe3+; and oxidation of (c) salicylamide and (d) paracetamol. Conditions for (a) and (b): salicylamide, 30 µg mL–1; paracetamol, 300 µg mL–1; Fe3+, 2.58×10-3 M; T= 25 oC; for (c) and (d): salicylamide, 20 µg mL-1, paracetamol, 5.0 µg mL–1; 1,10-phenanthroline, 1.8×10-3 M; Fe3+, 6.45×10-4 M; T=60 oC. the reactions increased by increasing pH up to 4.5 and decreased at higher pH values. Therefore a pH 4.5 was chosen for second set. The effect of Fe3+ ion concentration on the reaction of salicylamide and paracetamol was studied separately. The results are shown in Fig. 3. As the results showed under the first set of conditions even at high concentrations of Fe3+ only salicylamide reacted with Fe3+ ion and paracetamol did not react even when its concentration was 10-fold excess over salicylamide. The effect of Fe3+ ion concentration on the complex formation reactions of salicylamide in the range of 6.45×10-5– 5.81×10-3 M was studied. 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 d c f MIA -A---- i—A----- b -----A---- — A a -At--------------------- —i-------------------A 012345 Concentration of Fe3+ / mM Figure 3. Effect of Fe3+ ion concentration on the complex formation reaction of (a) paracetamol and (b) salicylamide with Fe3+, and oxidation reaction of (c) salicylamide and (d) paracetamol. Conditions for (a) and (b): salicylamide, 30 µ g mL–1; paracetamol, 300 µg mL–1; pH, 3.5; T= 25 oC; for (c) and (d): salicylamide, 20 µg mL-1; paracetamol, 5.0 µg mL–1; 1,10-phenanthroline, 1.8×10-3 M; pH, 4.5; T=60 oC. 1.4 1.2 0.8 2.5 0.6 2 0.4 1.5 0.2 0 Afkhami and Sarlak Determination of Salicylamide and Paracetamol Acta Chim. Slov. 2006, 53, 357–362 359 The absorbance for salicylamide reaction increased with increasing Fe3+ ion concentration up to 1.90×10-3 M and remained constant at higher concentrations. Therefore a concentration of 2.58×10-3 M Fe3+ ion was chosen for the first set of conditions. Fig. 3 also shows the effect of Fe3+ ion concentration on the oxidation reactions of salicylamide and paracetamol with Fe3+ ion under the second set of conditions in the range of 6.45×10-5– 5.81×10-3 M. As Fig. 3 shows the absorbance for paracetamol and salicylamide reactions increased by increasing Fe3+ ion concentration up to 3.87 ×10-4 and 6.45×10-4 M, respectively, and remained constant at higher concentrations. Therefore, a concentration of 6.45×10-4 M Fe3+ ion was used for second set of conditions. The effect of 1, 10- phenanthroline concentration on the absorbance of the solution under the second set of conditions was studied in the range of 6.06×10-5 – 2.424×10-3 M. The results are shown in Fig. 4. As Fig. 4 shows the absorbance for both the reactions increased by increasing 1,10-phenanthroline concentration up to 1.30×10-3 M and remained nearly constant at higher concentrations. Therefore, a concentration of 1.8×10-3 M 1,10- phenanthroline was selected as optimum. at higher temperatures. Therefore, a temperature of 60 oC was chosen for second set of conditions. A- ^^* ^* 1 1.5 Concentration of 1,10-phenanthroline /mM Figure 4. Effect of 1,10-phenanthroline concentration on the absorbance of the ferroin produced from the oxidation of (a) paracetamol and (b) salicylamide with Fe3+. Conditions: salicylamide, 20 µg mL-1; paracetamol, 5.0 µg mL –1; pH, 4.5; Fe3+, 6.45×10-4 M; T= 60 oC. The effect of temperature on the reaction of salicylamide and paracetamol were studied in the range of 15-80 oC. As Fig. 5 shows that the absorbance remained constant with changing temperature. This meant that temperature had no effect on the complexation reaction of salicylamide with Fe3+ and paracetamol did not react with Fe3+ in the investigated temperature range. Therefore, a temperature of 25 oC was chosen for the first set of conditions. Fig. 5 also shows the effect of temperature on the oxidation reactions of salicylamide and paracetamol with Fe3+ ion under the second set of conditions. As Fig. 5 shows by increasing temperature the absorbance for both the reactions increased rapidly up to 60 oC and slowly 1.4 d 1.2 0.8 c 0.6 0.4 b 0.2 --------A-------- i-------------------------*-------------------------i— -----*--- ---*— a -*— —*— 10 20 30 40 50 60 70 80 90 Temperature / oC Figure 5. Effect of temperature on the complex formation reaction of (a) paracetamol and (b) salicylamide with Fe3+, and oxidation reaction of (c) paracetamol and (d) salicylamide with Fe3+. Conditions for (a) and (b):salicylamide, 30 µg mL-1; paracetamol, 300 µg mL –1; pH, 3.5; Fe3+, 2.58×10-3 M; for (c) and (d): salicylamide, 20 µg mL-1; paracetamol, 5.0 µg mL –1; pH, 4.5; Fe3+; 6.45×10-4 M. The effect of paracetamol concentration on the reaction of 30 µg mL-1 salicylamide under the first set of conditions was studied in the range of 0.0 – 1000 µg mL-1. The results showed that paracetamol had no effect on the reaction of 30 µg mL-1 salicylamide with Fe3+ up to 700 µg mL-1 and interfered slightly at higher concentrations. 2.3. Analytical Parameters Under the optimum conditions described above, calibration graphs for salicylamide and paracetamol were constructed by plotting absorbance values as a function of the analyte concentration. The calibration graphs for the individual determinations were linear in the range of 2.0-300 µg mL-1 for salicylamide under the first set of conditions and 1.0-40 µg mL -1 for salicylamide and 0.50- 10 µg mL –1 for paracetamol under the second set of conditions. Paracetamol had no contribution to the absorbance under the first set of conditions. The results are summarized in Table 1. Table 1. Characteristics of the calibration graphs for the determination of salicylamide and paracetamol Analyte Procedure Range Slope (µg mL-1) (mL µg1) Intercept R2 Salicyl-amide Paracetamol 1 2.00 - 300 0.0091 0.0455 0.9988 2 1.0 - 40 0.019 0.2443 0.9984 2 0.50 - 10 0.25 0.0864 09974 0.5 2.5 Afkhami and Sarlak Determination of Salicylamide and Paracetamol 360 Acta Chim. Slov. 2006, 53, 357–362 The absence of synergistic effects ensured that the absorbance values obtained at 20 min after initiation of the reaction for a mixture of the two analytes were the sum of the absorbance values obtained for each individual analyte. Therefore the following equations can be used for the determination of salicylamide and paracetamol in mixture. A1= 0.0455 + 0.0091C paracetamol (4) A2=0.33 + 0.25C paracetamol + 0.019 C salicylamide (5) where A1 and A2 are the absorbance for the first and second set of conditions, respectively, and the concentrations are in µg mL-1. To evaluate the precision and the detection limit, a series of independent standard samples was used. The relative standard deviation for five replicate measurement of salicylamide and paracetamol was 3.47% and 2.58% in a 30 : 1 mixture salicylamide = 30 µg mL-1 and paracetamol = 1.0 µg mL-1). The limit of detection which can be calculated on the basis of YLOD = YB + 3SB, in which YLOD, YB and SB are signal of limit of detection, signal of blank and standard deviation of blank,12 respectively was 0.134 µg mL-1 for salicylamide under the first set of conditions and 0.27 and 0.19 µg mL-1 for salicylamide and paracetamol respectively, under the second set of conditions. 2.4. Complexation Reaction of Salicylamide The stochiometry and formation constant of complex of salicylamide with Fe3+ ion were determined spectrophotometrically. The stochiometry was determined using the Job’s method and the formation constant was determined using the variation of the absorbance of the complex with the metal to ligand mole ratio and non-linear least-squares curve fitting program KINFIT.13 The results show that the stochiometry of complex of salicylamide with Fe3+ ion was 1:1 and complex formation constant was log Kf = 5.23 ± 0.09. 2.5. Determination of Salicylamide and Paracetamol in Synthetic Mixtures Various mixtures of standards of salicylamide and paracetamol solutions were prepared and tested according to the recommended procedure. The test was carried out covering concentrations within the dynamic ranges of the species, using different concentration ratio of these ions. The results are given in Table 2. As Table 2 shows the relative error of the measurements were ? 6.0%, which confirms the good accuracy of the proposed method. Table 2. Analyses of mixtures of salicylamide and paracetamol by the proposed method Saicylamide: Salicylamide Relative Paracetamol Relative paracetamol (|ig mL ) (|ig mL ) (%) (%) Taken Found Taken Found 30:1 30 30.2 +0.70 1.0 0.948 -5.2 80:1 40 41.2 +3.0 0.50 0.48 -4..0 4:1 20 19.3 -3.5 5.0 4.9 -2.0 1:1 5.0 4.89 -2.1 5.0 5.3 +6.0 2:5 2.0 2.1 +5.0 5.0 4.7 -6.0 5:2 10 10.4 +4.0 4.0 4.2 +4.2 2.6. Application To evaluate the analytical applicability of the proposed method, it was applied to the determination of salicylamide and paracetamol in pharmaceutical preparations and in human serum. The results are given in Table 3 and Table 4. The recoveries are close to 100% which indicates that there is no serious interference in the determination of salicylamide and paracetamol in such samples. Table 3. Determination of paracetamol and salicylamide in pharmaceutical preparations. Samplea Nominal value (mg) Foundb (mg) Salicylamide Paracetamol Salicylamide Paracetamol Yendol granular packet 500 200 495.2 208.6 (Faes) Rinomicine pellets 50 50 48.5 52.2 (Fardi) Coricidin Capsules (Schering 190 192.9 plough) a Composition: Yendol: Salicylamide, 500 mg; paracetamol, 200 mg; Chlorpheniramine maleate, 3 mg; Caffeine, 30mg; Saccharin, 10mg; Saccharose, 6.5 g; Rinomicine: Paracetamol, 50 mg; Chlorpheniramine maleate, 4.0 mg; Salicylamide, 50 mg; Lactose, 10 mg; Saccharose, 225 mg. ; coricidin : Salicylamide . 190 mg; Chlorphenir amine maleate , 4 mg; caffeine , 30mg; as corbic acid, 50mg. b mean+ S.D. (n=3) Table 4. Determination of paracetamol and salicylamide in human serum. Spiked / µg mL-1 Found / µg mL-1 Recovery (%) Salicylamide Paracetamol Salicylamide Paracetamol Salicylamide Paracetamol 30.0 1.00 20.0 5.00 5.00 10.0 10.0 4.00 25.0 3.00 31.0 0.970 19.4 5.23 4.81 10.16 10.3 3.88 24.8 3.11 103 97.0 97.0 104.2 96.2 101.6 103 97.0 99.2 104 error error Afkhami and Sarlak Determination of Salicylamide and Paracetamol Acta Chim. Slov. 2006, 53, 357–362 361 3. Conclusion Determination of salicylamide and paracetamol in mixture by proposed method is based on their complexation and oxidation reactions by Fe3+ ion. Two sets of conditions were established that in one set of conditions only salicylamide forms complex with Fe3+ ion, but in the other set of conditions both the analytes are oxidized by Fe3+ ion in the presence of 1,10-phenanthroline as indicator. In both sets the reactions can be monitored spectrophotometrically by measuring the increase in absorbance at 510 nm. The proposed method offers good selectivity, accuracy and precision that can be applied for a wide range of paracetamol and salicylamide in synthetic and real samples. 4. Experimental 4.1. Apparatus A Pharmacia model LKB3 UV-visible Ultraspect (III) single beam Spectrophotometer that connected to a Pentium II computer with 1- cm quartz cells was used for absorbance measurements. All spectral measurements were performed using the blank solution as a reference. Measurements of pH were made with a Jenway C15 pH- meter using a combined glass electrode. 4.2. Reagents Triply distilled water and analytical-reagent grade chemicals were used. A 1000 µg mL-1 standard solution of salicylamide (Aldrich) was prepared by dissolving 0.1000 g salicylamide in 5% (v/v) ethanol and diluting to the mark in a 100-mL volumetric flask, working solutions were prepared by diluting the standard solution in water; this solution was stable at least for two weeks. A 1000 µg mL-1 standard solution of paracetamol (Merck) was prepared by dissolving 0.1000 g paracetamol in water and diluting to the mark in a 100-mL volumetric flask, working solutions were prepared by diluting the standard solution in water. A 0.02580 M Fe3+ ion solution was prepared by dissolving 0.69789 g FeCl3.6H2O (Merck) in water and diluting to the mark in a 100-mL volumetric flask. A 1.212 ×10-2 M 1,10-phenanthroline solution was prepared by dissolving 0.2400 g 1,10-phenanthroline (Merck) in ethanol and diluting to the mark in a 100-mL volumetric flask. Acetate buffer solutions of pH 3.5 and 4.5 were prepared and pH checked by pH meter. 4.3. Determination of Salicylamide and Paracetamol in Mixture Two runs are needed for each sample. 4.3.1. Procedure 1 An aliquot solution containing 2-300 µg of salicylamide and 0.5-20 µg of paracetamol was transferred into a 10 mL volumetric flask containing 1 mL of 2.58×10-2 M Fe3+ solution and 1 mL of pH 3.5 acetate buffer. The solution was diluted to the mark with triply distilled water. A portion of the solution was transferred into a glass cell to measure the increase in absorbance at 510 nm against a blank solution that was prepared in the same method except that distilled water was used instead of analytes. The dependence of the absorbance (A1) on the concentration of salicylamide was found to conform the following equation: A1 = a1+ b1 Csalicylamide (6) 4.3.2. Procedure 2 An aliquot solution containing 1-40 µg of salicylamide and 0.50-10 µg of paracetamol was transferred into a 10 mL volumetric flask containing 1.5 mL of 1.212 × 10-2 M 1,10-phenantroline and 1 mL of pH 4.5 acetate buffer solution. The solution was diluted to ca. 9 mL with triply distilled water and was placed in a water bath at 60 °C for 5 min. Then 1.0 mL of 6.45×10-3 M Fe3+ ion solution was added. The stop watch was started and a portion of the solution was transferred into a glass cell to measure the increase in the absorbance at 510 nm against a blank solution that was prepared in the same method except that distilled water was used instead of analytes at 20 min after initiation of the reactions. The dependence of the absorbance on the concentration of salicylamide and paracetamol was found to conform the following equations: A2 = a2+ b2 Csalicylamide + b2' Cparacetamol (7) 4.3.3. Preparation of Human Serum Human serum was separated from blood by centrifugation at 3000 rpm for 10 min.14 A 0.5 mL of the supernatant was transferred into a 10 mm × 75 mm glass tube containing 1.0 mL of ethyl acetate and a few crystals of NaCl and mixed vigorously for 30 s using a vortex mixer. Then exactly 0.5 mL of the upper organic layer was pipetted into a 10 mL volumetric flask. The determinations were carried out according to procedures 1 and 2. 5. References and Notes 1. A. Afkhami, A. R. Zarei, Talanta, 2001, 53, 815–821. 2. A. Afkhami, A. R. Zarei, Talanta, 2003, 60, 63–71. 3. A. Afkhami, M. Bahram, Anal. Chim. Acta, 2004, 526, 211–218. 4. A. Afkhami, T. Madrakian, M. Bahram, J. Hazard. Mater. B, 2005, 123, 250–255. 5. D. Perez-Bendito, M. Silva, Kinetic Methods in analytical Chemistry, Ellis Horwood, Chichester, 1988. Afkhami and Sarlak Determination of Salicylamide and Paracetamol 362 Acta Chim. Slov. 2006, 53, 357–362 6. M. E. EI-Kommos, K. M. Emara, Talanta, 1989, 36, 678–679. 7. K.W. Jun Street, G. H. Schenk, J. Pharm. Sci., 1981, 70, 641–645. 8. M. I.Walash, A. M. El-Brashy, M. A.Sultan, Mikrochim. Acta, 1994, 113, 113–124. 9. A.Ruiz Medina,; M. L. Fernández de Córdova, A. Molina Díaz, Anal. Chim. Acta, 1999, 394, 149–158. 10. A. Afkhami, N.Sarlak, Acta Chim. Slov., 2005, 25, 98–103. 11. Z. Marczenko, Separation and Spectrophotometric Determination of Elements, Ellis Horwood Limited, 1980, pp. 330–333. 12. V. Thomsen, D. Schatzlein, D. Mercuro, Spectroscopy, 2003, 18, 112–114. 13. V. A. Nicely, J. L. Dye, J. Chem. Edu., 1971, 48, 443–446. 14. M. A.Koupparis, P. I. Anagnostopoulou, J. Pharm. Biomed. Anal., 1988, 6, 35–46. Povzetek Predstavljena je hitra, enostavna in občutljiva spektrofotometrična metoda za določevanje salicilamida in paracetamola v zmesi. Metoda temelji na njuni oksidaciji z ionom Fe3+ in tvorbi kompleksov železa s salicilaminom in 1,10-fenantrolinom. Izbrani so bili taki reakcijski pogoji, da v prvem primeru reagira z Fe3+ samo salicilamin, v drugem pa Fe3+ oksidira obe spojini v prisotnosti 1,10-fenantrolina. V obeh primerih merimo koncentracijo nastalih kompleksov spektrofotometrično pri 510 nm. Metoda omogoča določevanje salicilamida in paracetamola v koncentracijskih območjih 2,0 do 300 µg mL–1 in 0,50 do 10 µg mL–1, z relativnimi standardnimi odmiki meritev 2,58% oziroma 3,47%. Metoda je bila uporabljena za določevanje salicilamida in paracetamola v krvni plazmi in farmacevtskih pripravkih. Afkhami and Sarlak Determination of Salicylamide and Paracetamol