p-Aminobenzoate ion determination in pharmaceutical formulations by using a potentiometric sensor immobilized in a graphite matrix

Talanta 63 (2004) 833–838

p-Aminobenzoate ion determination in pharmaceutical formulations by using a potentiometric sensor immobilized in a graphite matrix A.O. Santini, E.S. Silva, Jr., H.R. Pezza, A.L.M. Nasser, C.B. Melios, L. Pezza∗ Instituto de Qu´ımica—UNESP, P.O. Box 355, CEP 14801-970, Araraquara, SP, Brazil Received 23 July 2003; accepted 9 December 2003 Available online 13 February 2004

Abstract The characteristics, performance, and application of an electrode, namely, Pt|Hg|Hg2 (PABzt)2 | graphite, where PABzt stands for paminobenzoate ion, are described. This electrode responds to PABzt with sensivity of (58.1 ± 1.0) mV per decade over the range 1.0 × 10−4 to 1.0 × 10−1 mol l−1 at pH 6.5–8.0 and a detection limit of 3.2 × 10−5 mol l−1 . The electrode shows easy construction, fast response time (within 10–30 s), low-cost, and excellent response stability (lifetime greater than 6 months, in continuous use). The proposed sensor displayed good selectivity for p-aminobenzoate in the presence of several substances, especially, concerning carboxylate and inorganic anions. It was used to determine p-aminobenzoate in pharmaceutical formulations by means of the standard additions method. The results obtained by using this electrode compared very favorably with those given by an HPLC procedure. © 2004 Elsevier B.V. All rights reserved. Keywords: p-Aminobenzoate-sensitive electrode; Potentiometry; Pharmaceutical formulations

1. Introduction p-Aminobenzoic acid (PABA) is widely included as a member of the vitamin B complex in nutritional supplements. It is also an essential metabolite in the synthesis of folic acid by certain microorganisms. In clinical use, PABA is used for determining pancreatic function after oral administration of N-benzoyl-l-tyrosyil-paminobenzoic acid [1]. PABA has also been prescribed for various dermatological ailments including Peyronie’s disease, scleroderma, dermatomyositis and as a sunscreen agent in cosmetics [2]. At present, there are numerous products available in several dosage forms containing PABA or the alkali salts of PABA (sodium or potassium salts), which range in dosage level from 50 to 1000 mg. Several analytical methods have been reported for the determination of PABA in cosmetics products, human urine, biological fluids and in pharmaceutical preparations. These methods include liquid chromatography (LC) [3], high-performance liquid chromatography (HPLC) [4–7], miscellar electrokinetic capillary chromatography (MECC) [8], gas chromatography (CG) [9], room temperature phos∗

Corresponding author. Tel.: +55-16222-7932; fax: +55-16222-7932. E-mail address: [email protected] (L. Pezza).

0039-9140/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2003.12.039

phorimetry (RTP) [10–12], Raman spectrometry (RS) [13], and kinetic determination [14]. Some of these methods display good detection limits and accuracy. However, most of them are not simple enough for routine analyses, requiring expensive and/or sophisticated instrumentation. Potentiometric methods with ion-selective electrodes (ISE’s) can provide valuable and straightforward means of assaying PABA in complex mixtures, as they make possible the direct determination of ions in solution with high selectivity. Most ISE’s are low-cost, their use and maintenance being very simple; assay procedures involving such electrodes are generally simple and fast. These features, coupled with the reliability of the analytical information, make ISE’s very attractive for the assay of pharmaceutical products. To the best of our knowledge, there is a single report on the use of ion-selective potentiometric sensor for the determination of p-aminobenzoate [15]. However, this electrode has not been applied to pharmaceutical formulations and not even to synthetic mixtures or simulated dosage forms, thus, precluding the assessment of its usefulness in real analysis. Previous work from this laboratory dealt with the development of mercury(I)-carboxylate electrodes and their application to solution equilibria [16–20], food analysis [21], and pharmaceutical analysis [22,23] involving carboxylate bearing compounds.

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In this work, the preparation of an electrode, namely Pt|Hg|Hg2 (PABzt)2 | graphite, where PABzt stands for p-aminobenzoate ion, is described. The investigation of the experimental variables that contribute to the electrode response led to the development of a simple, selective, and reliable method for p-aminobenzoate determination. Studies on the determination of PABA in pharmaceutical formulations, particularly tablets dosage formulations and injectable ampoules were carried out to illustrate the feasibility of the proposed method. Furthermore, as both the electrode and the standard potentiometric equipment are low-cost, the developed procedure also allows small laboratories with limited resources to run p-aminobenzoate analyses for the aforementioned samples.

2. Experimental 2.1. Reagents High purity deionized water (resistivity 18.2 M cm) obtained by using a Milli-Q Plus system (Millipore Corp., Bedford, MA, USA) was used throughout. All reagents employed were of analytical grade and obtained from E. Merck (Darmstadt, Germany) except PABA and PABA sodium salt, which were supplied by Fluka (St. Louis, USA). Standardizations of carbonate-free sodium hydroxide, nitric acid, and sodium nitrate solutions were performed as described elsewhere [16,20]. Metallic mercury was purified according to a previously reported procedure [16]. The sodium p-aminobenzoate stock solution was analyzed by evaporating and drying to constant weight at 120 ◦ C. Mercury(I) p-aminobenzoate was prepared by mixing, in aqueous solution, the corresponding nitrate with an excess of sodium p-aminobenzoate. The resulting precipitate was filtered through a sintered glass funnel, washed with deionized water until nitrate free, and then dried in a desiccator, over calcium chloride under reduced pressure, at room temperature, to constant mass. A white powder was obtained as the final product. 2.2. Electrode preparation and conditioning The mercury(I) p-aminobenzoate indicator electrode was prepared as follows: mercury(I) p-aminobenzoate (1.4 g) and metallic mercury (ca. 0.2 g) were mixed in an agate mortar and the material was crushed until a homogeneous solid was obtained. Pure powdered graphite (0.7 g) was then added and the crushing process was continued until perfect homogenization was attained. Part of the resulting solid was transferred to a press mold, being compressed at 9 t for about 5 min. The black pellet (1.5 mm thick, 12 mm o.d., and 0.6 g mass) was fixed at one end of a glass tube (12 mm o.d., 80 mm long) with a silicone-rubber glue (“Rhodiastic,” Rhˆone-Poulenc, France) and allowed to dry for 48 h. Sufficient metallic mercury (ca. 0.6 g) was then introduced into

Fig. 1. Mercury(I) p-aminobenzoate electrode: (A) conductor cable, (B) banana plug, (C) metallic mercury, (D) Pt wire, (E) silicone glue, and (F) sensor pellet (graphite|Hg2 (PABzt)2 |Hg).

the tube to produce a small pool on the inner pellet surface; electric contact was established through a platinum wire plunged into the mercury pool and a subsequent conductor cable. The resulting electrode is diagrammed in Fig. 1, showing that it is sealed. This feature, coupled with the small amount of metallic mercury placed inside the electrode (ca. 0.6 g), stresses that the considered sensor does not offer significant risk to the operator’s health and can, thus, be recognized as safe. When not in use, the electrode’ s pellet was kept immersed in a small volume of 0.100 mol l−1 sodium p-aminobenzoate solution whose ionic strength (µ) was adjusted to 0.500 mol l−1 with a sodium nitrate solution. Before carrying out each experiment, the external surface of the aforementioned pellet was polished with an alumina paper (polishing strip 30144-001, Orion Instruments Inc., Cambridge, MA, USA), washed with deionized water and dried with absorbent paper. 2.3. Instruments The electromotive force (emf) values were read to the nearest 0.1 mV with a Metrohm model 692 pH|ion meter

A.O. Santini et al. / Talanta 63 (2004) 833–838

(Metrohm Ltd., Herisau, Switzerland). The reference electrode was a Metrohm Ag|AgCl double junction, model 6.0726.100. The pH of aqueous solutions was adjusted and monitored with the aid of a Metrohm pH electrode, model 6.0234.100. A thermostated titration cell (25.0 ± 0.1 ◦ C) was employed. Volume measurements (±0.001 ml) were performed with a Metrohm model 665 automatic burette. The HPLC system consisted of a Shimadzu model SPD-10A liquid chromatograph (Shimadzu Seisakusho, Kyoto, Japan), equipped with a LC-10 AS pump (Shimadzu), variable UV-Vis detector (model SR-10A, Shimadzu) set at 300 nm, gradient control (Waters, model 680; Waters Chromatography Div., Milford, MA, USA) and a “Rheodyne” 20 ␮l injector (Rheodyne, Inc., Berkeley, CA, USA). A stainless steel “Supelcosil LC-18” analytical column (150 mm × 4.6 mm i.d., Supelco, Bellefonte, PA, USA) with 5 ␮m particle size packing material was used. Before injection the samples were filtered through a Millex unit (Millex-HV, 0.45 ␮m, Millipore). Chromatograms were recorded and peak heights measured with an integrator (Waters, mod. 746 recording integrator). All experiments were performed in a thermostated room, maintained at 25 ± 1 ◦ C. 2.4. Potentiometric cell

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2.5.1. Liquid samples Liquid samples were appropriately diluted with deionized water to obtain a PABzt concentration level within the linear range of the electrode’s calibration curve. The ionic strength was adjusted to 0.500 mol l−1 with NaNO3 and the pH to 7.0 ± 0.1 with 10−2 mol l−1 HNO3 or 10−2 mol l−1 NaOH. Finally, each sample was analyzed with the p-aminobenzoate-sensitive electrode. 2.5.2. Solid samples Fifteen tablets of each sample were weighed to calculate the average tablet weight. They were finely powdered and homogenized. A quantity of the resulting powder equivalent to about 80 mg of p-aminobenzoate was accurately weighed and placed in a glass vessel; 70 ml of water was added and magnetically stirred for 20 min. The resulting mixture was filtered and its ionic strength was adjusted to 0.500 mol l−1 with NaNO3 and the pH to 7.0 ± 0.1 with 10−2 mol l−1 NaOH or 10−2 mol l−1 HNO3 before volume completion. For the tablet dosage forms containing ascorbic acid, oxygen was bubbled into the sample for 20 min (rate: 200 cm3 min−1 ). Transfer the resulting solution quantitatively to a 100 ml volumetric flask using deionized water (pH 7.0 ± 0.1) for rinsing and volume completion. An aliquot of 25 ml is employed for analysis with the p-aminobenzoate-sensitive electrode.

The following cell was used, (−) Ag|AgCl

[NaCl](aq) = 0.010 mol l−1 [NaNO3 ](aq) = 0.490 mol l−1

[NaNO3 ](aq) = 0.500 mol l−1

where PABzt stands for p-aminobenzoate ion and x was in the range 10−1 –10−5 mol l−1 . The ionic strength of the cell compartments was kept constant at 0.500 mol l−1 . No flow of chloride ions from the reference electrode into the test solution could be detected during the measurements. The performance of the mercury(I) p-aminobenzoate electrode was assessed by measuring the emf of the aforementioned cell for 10−1 –10−5 mol l−1 sodium p-aminobenzoate solutions. These solutions were freshly prepared by serial dilution of a 0.100 mol l−1 stock standard solution with deionized water, at constant pH (7.0 ± 0.1). The emf readings were obtained for solutions under stirring and recorded when they became stable. The data were plotted as observed emf versus benzoate ion concentration. 2.5. Determination of p-aminobenzoate ion in pharmaceutical formulations The analyzed products were purchased locally or directly from the manufacturers and all were tested prior to the listed expiration date. Eight pharmaceutical formulations containing PABA or PABzt and other components were analyzed with the p-aminobenzoate-sensitive electrode.

[NaPABzt](aq) = x mol l−1 [NaNO3 ](aq) (0.500 = x) mol l−1

Graphite|Hg2 (PABzt)2 | Hg|Pt(+)

3. Results and discussion 3.1. Electrode response Experiments carried out as described in Section 2.4. led to the following linear relationship between the measured emf (E in mV) and p-aminobenzoate ion concentration; E = E0 + S p[PABzt] where E0 is the formal cell potential and S represents the Nernst coefficient (59.16 mV per decade, at 25 ◦ C, for monovalent ions). Potentiometric parameters and other features associated with the mercury(I) p-aminobenzoate electrode are given in Table 1. The above calibration equation and the slope value (Table 1) shows that the electrode provides a near-Nernstian response to the p-aminobenzoate ion. The sensor response displayed good stability and reproducibility over the tests; the last mentioned feature is illustrated by the standard deviation (S.D.) values shown in Table 1.

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Table 1 Potentiometric response characteristics of the mercury(I) p-aminobenzoate electrodea Slope (mV per decade)b

Intercept, E0 (mV)b

Linear range (mol l−1 )

Detection limit (mol l−1 )

58.1 ± 1.0

43.1 ± 2.0

10−4 –10−1

3.2 × 10−5

T = 25.0 ± 0.10◦ C; pH 7.0 ± 0.1; µ = 0.500 mol l−1 (NaNO3 ). Average of 20 determinations over a period of 6 months. Number of data points: 21–24. Mean linear correlation coefficient: 0.992 ± 0.003. a

b

3.2. Response time and lifetime of the electrode The response time of the electrode was tested by measuring the time required to achieve a steady state potential (within ±0.5 mV min−1 ), for 10−1 –10−4 mol l−1 sodium p-aminobenzoate solutions at pH 7.0 [24]. The electrode yielded steady potentials within 10–15 s at high concentrations (≥5 × 10−3 mol l−1 ) and about 30 s at concentrations near the detection limit. The useful lifetime of the electrode for the evaluated concentration range is at least 6 months, in continuous use. 3.3. pH effect The influence of pH on the electrode response was tested over the pH range 6.5–9.0 for 1.00 × 10−1 , 1.00 × 10−2 , and 1.00 × 10−3 mol l−1 p-aminobenzoate ion concentrations. The resulting solutions’ pH(s) were adjusted with diluted HNO3 or NaOH solutions. For pH > 8.0, the hydroxide ion interferes with the electrode’s response. The emf values are independent of pH in the range 6.5–8.0; this can be taken as the working pH range of the electrode. 3.4. Electrode selectivity The most important characteristic of any ion sensitive sensor is its response to the primary ion in the presence of other ions present in solution, which is expressed in terms of the potentiometric selectivity coefficient. The potentiometric selectivity coefficients for the mercury(I) p-aminobenzoate electrode (KPABzt,M ) were determined, for a number of anions (M), by the matched potential method (MPM) [25–27]. In this method, the selectivity coefficient is defined by the ratio of the activity of the primary ion relative to an interfering ion, when they generate identical potentials in the same reference solution. In the MPM method, both monovalent and divalent ions are treated in the same manner and the valence of the ions does not influence the selectivity coefficient. Furthermore, the MPM can be used with no regard to the electrode slopes being Nernstian or even linear [28]. Mainly for these reasons, it has increased in popularity in the last few years [29]. The determined KBzt,M values are presented in Table 2. The results comprised in the aforementioned Table 2 shows that the selectivity of the mercury(I) p-aminobenzoate

Table 2 Selectivity coefficients (KPABzt ,M ) for various anionsa Anion

KPABzt,M

Formate Acetate Propionate Citrate Lactate Dehydroascorbate Pantothenate Salicylate Phthalate Glutamate Chloride Sulphate Perchlorate Nitrate

1.2 1.4 1.8 3.1 2.3 No 8.3 8.7 5.8 9.9 2.8 1.1 No No

a

× 10−5 × 10−3 × 10−3 × 10−3 × 10−3 interference × 10−4 × 10−3 × 10−3 × 10−4 × 10−6 interference interference

T = 25.0 ± 0.1◦ C; pH 7.0 ± 0.1.

electrode towards all tested organic acid anions is good. No interference was noted for most of the common components found along PABA or p-aminobenzoate in pharmaceutical formulations such as croscarmellose sodium, vegetable stearate, magnesium stearate, cellulose, l-tyrosine, l-arginine, ␤-alanine, vitamin B1, vitamin B2, vitamin B6, inositol, biotin, methyl-, ethyl-, and n-propyl-p-hydroxybenzoate. The mentioned esters of p-hydroxybenzoic acid are also extensively used as preservatives in pharmaceutical formulations. Sulphate has a very low selectivity coefficient (Table 2); no interference at all is caused by nitrate or perchlorate and they can therefore be used as background electrolytes or ionic strength adjusters for p-aminobenzoate solutions before performing potentiometric measurements. Chloride ion interferes as shown in Table 2. However, the influence due to this ion can be eliminated by a preliminary chloroform extraction procedure. In the samples analyzed in this work (tablets and injectable ampoules), chloride ion is seldom found and hence the proposed electrode can generally be used for direct determination of p-aminobenzoate in these pharmaceutical formulations without previous extraction procedures. Ascorbic acid converts mercury(I) to elemental mercury at the electrode’s surface and seriously affects its response. Previous oxidation of this species, as described in the analytical procedure (Section 2.5.2) completely eliminates its interference. Moreover, the oxidation product, i.e., dehydroascorbate, does not interfere (Table 2). 3.5. Analytical application A standard additions method [30,31] was employed for potentiometric PABA estimation in some pharmaceutical formulations by using the presently proposed p-aminobenzoate-sensitive electrode. Because no standard methods are available for determination of PABA, an HPLC method proposed by Carlson and Thompson [4] for PABA (as well as for its sodium and potassium salts) determination in pharmaceuticals was used as a comparative method.

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Table 3 PABA determination in pharmaceutical formulations Formulationa

Label to content

Proposed method Found (mg

Tabletsb 1 2 3 4 5 6

500b 500b 100b 100b 100b 200b

510 487 95 106 96 195

Ampoulesc 7 8

200c 100c

196 ± 5e 98 ± 2e

± ± ± ± ± ±

Comparative method [4]

unit−1 )

R.S.D.f

15d 10d 2d 3d 3d 4d

(%) (n = 5)

Found (mg unit−1 ) ± ± ± ± ± ±

10d 12d 3d 3d 2d 5d

2.9 2.0 2.1 2.8 3.1 2.0

506 497 98 101 98 197

2.6 2.0

194 ± 4e 99 ± 3e

R.S.D.f (%) (n = 5) 1.9 2.5 3.0 2.9 2.0 2.5 2.1 3.0

These contain many or all of the following substances/materials: vitamins B1 , B2 , B6 , B12 , C, pantothenic acid, niacinamide, d-biotin, l-yrosine, inositol, meso-inosite, vegetable oil, cellulose, microcrystaline cellulose, silica, vegetable stearate, vegetable stearine, potato starch, gelatin, magnesium stearate, copper stearate, silica, croscarmellose sodium, and dicalcium phosphate. Ampoules contain bi-destilled water. b Declared concentration of p-aminobenzoic acid (PABA) in mg unit−1 . c Declared concentration of sodium p-aminobenzoate in mg ampoule−1 . d Values found are the average of five independent analyses (n = 5) ± the corresponding S.D. Expressed as p-aminobenzoic acid (PABA). e Values found are expressed as sodium p-aminobenzoate. f Relative S.D. a

Table 4 Recovery data for PABA spiked in pharmaceutical formulations Formulation

Concentration added (mg l−1 )

Total concentration (mg l−1 )

Concentration found (mg l−1 )

3

50 100 200

150 200 300

143.9 197.2 297.1

95.9 ± 1.4 98.6 ± 0.9 99.0 ± 0.9

5

50 100 200

150 200 300

148.2 201.6 301.8

98.8 ± 1.2 100.8 ± 1.0 100.6 ± 0.9

8

50 100 200

150 200 300

145.8 198.4 303.9

97.2 ± 1.3 99.2 ± 1.1 101.3 ± 0.9

a

Recovery (%)a

Average ± S.D. of three determinations.

The results, along with those obtained by applying the mentioned HPLC method to the same samples, are given in Table 3. Comparison of the results presented in Table 3 shows very good agreement with the label values and also with the HPLC method thereby reflecting the utility of the proposed electrode. In order to further validate the method, a recovery study was carried out. In this study 50, 100, and 200 mg l−1 of p-aminobenzoate reference solutions were added in three representative pharmaceuticals (samples 3, 5, and 8) from those listed in Table 3.The results presented in Table 4 shows that the recoveries were found to be close to 100%; the S.D. were within 0.9–1.4. These results point out the accuracy and precision of the proposed method and the absence of significant matrix effects on the potenciometric measurements. The times required for performing analyses by HPLC [4] and potentiometry were 30 min and 15–30 min per sample, respectively.

Concerning analyses’ costs the HPLC, LC, CG, MECC, RTP, and RS techniques are more expensive than the electrode method, if expenses regarding reagents, solvents and initial investment on good quality standard equipment associated with each of the named techniques are considered.

4. Conclusions The proposed electrode exhibits long lifetime, good stability, sensitivity, precision, and selectivity. It is low-cost, easy to prepare and to use. Its usefulness for p-aminobenzoate determination in real samples, particularly for some commercial pharmaceutical formulations was demonstrated suggesting its use as a reliable and advantageous alternative to the comparative method [4] as well as to most other previously reported methods in the routine control of p-aminobenzoate concentration in these samples. The electrode developed in this laboratory is superior (especially concerning lifetime

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and simplicity) as compared with the p-aminobenzoate electrode described in the literature [15]. Acknowledgements We would like to thank FAPESP, CNPq, and CAPES Foundations (Brazil), for financial support.

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