Picrate ion determination using a potentiometric sensor immobilized in a graphite matrix

Sensors and Actuators B 107 (2005) 296–302

Picrate ion determination using a potentiometric sensor immobilized in a graphite matrix M. Moghimi∗ , M. Arvand, R. Javandel, M.A. Zanjanchi Department of Chemistry, Faculty of Science, Guilan University, P.O. Box: 1119, Rasht, Iran Received 29 May 2004; received in revised form 16 October 2004; accepted 22 October 2004 Available online 13 December 2004

Abstract The characteristics, performance, and application of an electrode, namely, Cu|Hg|Hg2 (Pic)2 |graphite, where Pic stands for picrate ion, are described. This electrode responds to picrate with a sensitivity of 56.8 ± 0.5 mV/decade over the range 2.5 × 10−5 to 1.0 × 10−1 M at pH 5.7–8.0 with a detection limit of 1.3 × 10−5 M and an ionic strength of 0.5 M, adjusted with NaClO4 . The inverse solubility product (Ks−1 ) for picrate precipitates with a range of cations was also estimated. The electrode shows easy construction, fast response time (about 25 s), low cost, and excellent response stability (lifetime > 6 months, in continuous use). The proposed sensor shows high selectivity towards picrate ion over many hydrophilic and lipophilic anions and exhibits a non-Hofmeister selectivity sequence, which is an improvement over most of the methods reported so far. The electrode was successfully applied to the potentiometric determination of picrate ions and indirect determination of some pharmaceuticals such as quinidine through precipitation reactions with picrates. The results obtained by using this electrode compared very favorably with those given by the official standard methods. © 2004 Elsevier B.V. All rights reserved. Keywords: Picrate sensor; Second kind electrode; Graphite matrix; Potentiometry

1. Introduction Nitroaromatic compounds (NACs) are used as basic chemicals for paints, agrochemicals, plastics and pharmaceuticals. Picric acid (2,4,6-trinitrophenol), one of the NACs, is an important organic acid and can be used as antiseptic, in electric batteries, leather industry, inks, manufacture of colored glass, textile mordants, and in the synthesis of chloropicrin, or nitrotrichloromethane, CCl3 NO2 , a powerful insecticide [1,2]. On the other hand, picric acid can react quantitatively with some drugs such as cinchonine and quinidine to form insoluble precipitates, therefore attention has been paid to its direct determination in the areas of biological chemical analysis and indirect determination of these drugs. The most frequently used methods for the determination of picric acid are liquid chromatography–mass spectrometry [3], spectrophotometry [4–9], classical chemical methods ∗

Corresponding author. Tel.: +98 131 3221827; fax: +98 131 3220066. E-mail address: [email protected] (M. Moghimi).

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[10], fluorometry [11–14], polarography [15], gas chromatography [16], ion-selective electrodes [17–28] and optical sensors [29–31]. These methods demand a cumbersome pretreatment of the sample solution, require expensive instrumentation or easily cause interference problems. For example, in liquid chromatography–mass spectrometry and gas chromatography it is necessary to extract picric acid by a volatile organic solvent and then concentrate it in order to be used. According to the above-mentioned reasons, it requires a fast, simple, low-cost, selective and reliable method that could be used routinely for picric acid determination. Most of the previously reported picrate electrodes are based on basic dyes [25–27] or anion-exchangers using quarternary ammonium salts [19–24] and suffer from interferences from lipophilic anions, such as ClO4 − [19,23,24] and aromatic compounds [22,26]. In general, membranes doped with anion-exchangers exhibit selectivity that depends only on the free energy of hydration of the anion species [32], which corresponds to the classical Hofmeister sequence [R− (aromatic anions) > ClO4 − > IO4 − > SCN− > I− > NO3 − >

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Br− > NO2 − > Cl− > HCO3 − = HPO4 2− ], where the more lipophilic anions are preferred [33]. In contrast, nonHofmeister behavior can occur in some cases such as metalloporphyrin-based membrane systems [34] where the selectivity is attributed to the coordination interaction between the ligand anions at the axial position of the carrier molecule and the metal center [35]. To date, there is no sensor-based ion association complex as sensing element responsive to picrate ion. In this paper we report on another system of non-Hofmeister behavior. The sensor shows high selectivity that deviates from the Hofmeister pattern, where lipophilic species, such as aromatic anions, 2-nitrophenol, 2,4-dinitrophenol and ClO4 − do not interfere. The proposed sensor was also used for estimation of inverse solubility product (Ks−1 ) of picrate precipitates with some cations. We have prepared an electrode, namely, Cu|Hg|Hg2 (Pic)2 | graphite, where Pic stands for picrate ion. The investigation of the experimental variables that contribute to the electrode response led to the development of a simple, selective and reliable method for picrate determination. Studies on pharmaceuticals and picrate precipitates were carried out to illustrate the feasibility and reliability of the proposed method. Furthermore, as both the electrode and the standard potentiometric equipment are low of cost and field applicable, the developed procedure also allows small laboratories with limited resources to run picrate analysis for the above-mentioned samples and also using the proposed method in the field analysis.

2. Experimental 2.1. Reagents High purity deionized water was used throughout the experiments (resistivity 18.2 M cm) which was obtained by using a Milli-Q Plus system (Millipore Corporation, Bedford, MA, USA). All solutions were prepared with analytical reagent grade chemicals, obtained from Merck or Fluka. Metallic mercury was purified according to a previously reported procedure [36]. The sodium picrate stock solution was standardized potentiometrically with standard perchloric acid solution. Mercury(I) picrate was prepared by mixing, in aqueous solution, the corresponding nitrate with an excess of sodium picrate. The resulting precipitate was filtered through a sintered glass funnel, washed with deionized water until became perfectly nitrate free, and then dried in a desiccator, over calcium chloride under reduced pressure, at room temperature, to constant mass. A yellow powder was obtained as the final product. 2.2. Electrode preparation and conditioning The picrate indicator electrode was prepared as follows: mercury(I) picrate (600 mg) and metallic mercury (ca. 0.2 g)

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were mixed in an agate mortar and the material was crushed until a homogeneous solid was obtained. Pure powdered graphite (0.17 g) was then added and the crushing process was continued until perfect homogenization was attained. Part of the resulting solid was transferred to press mold, being compressed at 12 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., 100 mm long) with a silicone–rubber glue and allowed to dry for 24 h. Sufficient metallic mercury (ca. 0.5 g) was then introduced into the tube to produce a small pool on the inner pellet surface, electric contact was established through a copper wire plunged into the mercury pool with a subsequent conductor cable. When not in use, the electrode’s pellet was kept immersed in a small volume of 1.0 × 10−1 M sodium picrate solution whose ionic strength (I) was adjusted to 0.5 M with a sodium perchlorate solution. Before carrying out each experiment, the external surface of the above-mentioned 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 (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 was employed. The official spectrophotometric method [4] was carried out by a UV-2100 Shimadzu spectrophotometer. All experiments were performed in a thermostated situation, maintained at 25 ± 1 ◦ C (except for study of temperature effect). 2.4. Potentiometric cell The following cell was used: Ag/AgCl/KCl (sat d)|NaClO4 (0.5 M)|NaPic(x M), NaClO4 (0.5 − x M)|graphite, Hg2 (Pic)2 , Hg|Cu where Pic stands for picrate ion and x was in the range 1.0 × 10−6 to 5.0 × 10−1 M. The ionic strength of the cell compartments was kept constant at 0.5 M. 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) picrate electrode was assessed by measuring the emf of the above-mentioned cell for 1.0 × 10−6 to 5.0 × 10−1 M sodium picrate solutions. These solutions were freshly prepared by serial dilution of a 0.1 M 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.

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The data were plotted as observed emf versus picrate ion concentration. 2.5. Determination of Ks−1 for some picrate salts by potentiometry To determine Ks−1 (inverse solubility product) [37] using the proposed picrate selective electrode, a 5–8 point calibration for picrate was first performed using the spiking method. Known volumes of 1.0 × 10−2 to 1.0 × 10−1 M related cation solution were then added to the same solution and the steadystate positive change in electrode potential was noted. Ks−1 was calculated from the total picrate concentration, the free picrate concentration determined from the electrode readings and the amount of cation added. Generally, for the cation Mn+ n+

Ks = [M

]free [Pic]nfree

(1)

n[Mn+ ]free = n[Mn+ ]total + [Pic]free − [Pic]total

(2)

Fig. 1. Potentiometric response of the picrate solid-state sensor (pH 7.0, T = 25 ◦ C).

3. Results and discussion

2.6. Indirect determination of quinidine

3.1. Electrode response

Picrate can react quantitatively with some pharmaceuticals to form insoluble precipitates, and the sensor can be exploited for the indirect determination of these substances. Quinidine was taken as an example for such an analysis. The procedure was carried out as follows: an accurately weighed portion of finely powdered sample obtained from three tablets, equivalent to about 200 mg of quinidine sulphate was transferred to a 100 ml flask and dissolved in 25 ml of Britton–Robinson buffer solution (pH 7.0). To this solution, an excess of sodium picrate solution was added with stirring. The solution with the precipitate was diluted to an appropriate volume. An aliquot of filtered or decanted filtrate was taken for the determination of the unreacted picrate by using the proposed sensor:

Experiments carried out as described in Section 2.4 led to the following linear relationship between the measured emf and picrate ion concentration:

[picrate]consumed = [picrate]initial − [picrate]free

(4)

[quinidine]initial = [picrate]consumed

(5)

E = E◦ − m log[Pic]

(6)

where E◦ is the formal cell potential and m represents the Nernst coefficient (59.16 mV/decade, at 25 ◦ C, for monovalent ions). Potentiometric parameters and other features associated with the mercury(I) picrate electrode are given in Table 1. The above calibration equation and the slope value (Table 1) show that the electrode made from 550 mg (electrode no. 4) provides a near-Nernstian response to the picrate ion (Fig. 1). The sensor response displayed good stability and reproducibility over the tests; the last mentioned feature is illustrated by the standard deviation values shown in Table 1. The effect of thickness of the sensing disk of the electrode was also verified (Table 1). As can be seen, the electrode

Table 1 Potentiometric characteristics of the picrate selective electrode No.

Weight (mg)

Slope (mV/decade)

Dynamic range (M)

Detection limit (M)

Regression coefficient

1 2 3 4 5 6 7 8 9 10 11

400 450 500 550 600 650 700 750 800 850 900

– 51.7 ± 0.7 55.4 ± 0.7 56.8 ± 0.5 55.5 ± 0.5 52.9 ± 0.7 48.5 ± 0.7 47.6 ± 0.8 43.2 ± 0.9 43.3 ± 1.2 42.3 ± 1.5

– 2.8 × 10−5 2.5 × 10−5 2.5 × 10−5 2.6 × 10−5 2.9 × 10−5 4.5 × 10−5 7.5 × 10−5 6.5 × 10−5 3.5 × 10−5 5.5 × 10−5

– 2.0 × 10−5 2.0 × 10−5 1.3 × 10−5 2.1 × 10−5 1.9 × 10−5 2.1 × 10−5 5.2 × 10−5 4.0 × 10−5 5.0 × 10−5 5.0 × 10−5

– 0.9916 0.9918 0.9962 0.9924 0.9926 0.9837 0.9898 0.9857 0.9980 0.9960

to 1.0 × 10−1 to 1.1 × 10−1 to 1.0 × 10−1 to 1.2 × 10−1 to 1.0 × 10−1 to 8.0 × 10−2 to 4.0 × 10−2 to 1.1 × 10−1 to 1.0 × 10−2 to 2.3 × 10−2

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disk, which weighs less than 450 mg, cannot be used because they crack in early steps of handling. The disks with higher weights reduce the slope and dynamic range of electrode, which is due to the increase of electrode matrix. The results in Table 1 also show that the graphite behaves as an inert substance and conductor in the immobilization process of the sensor of the electrode. 3.2. Response time, lifetime and reversibility of the electrode The dynamic 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 2.5 × 10−5 to 3.1 × 10−1 M sodium picrate solutions at pH 7.0. The measurement sequence was from the lower (2.5 × 10−5 M) to the higher (3.1 × 10−1 M) concentrations. The actual potential versus time trace is shown in Fig. 2. As can be seen, the electrode yielded steady potentials within 20–25 s at high concentrations (>1.0 × 10−3 M) and about 30 s at concentrations near the detection limit. To evaluate the reversibility of the electrode, a similar procedure in the opposite direction was adopted. The measurements were performed in the sequence of high-to-low (from 1.0 × 10−2 to 1.0 × 10−3 M) sample concentrations and the results are shown in Fig. 3. Fig. 3 shows that the potentiometric response of the electrode is reversible, although the times needed to reach equilibrium values were longer than that of low-to-high sample concentrations [38]. The useful lifetime of the electrode for the evaluated concentration range is at least 6 months, in continuous use. During this period, slope of the electrode was not changed dramatically (58.7 ± 0.7) and the electrode keeps its performance in the estimated concentration range, and reproducible results obtained, so that, the relative standard deviation of response slope was less than 1.5% (for 10 determinations).

Fig. 2. Dynamic response time of the proposed electrode towards picrate ions (pH 7.0).

Fig. 3. Dynamic response characteristics of the picrate electrode for several high-to-low sample cycles (pH 7.0).

3.3. Effect of pH The influence of the pH on the electrode response was tested over the pH range 0.0–12.0 for 1.0 × 10−2 and 1.0 × 10−3 M picrate ion concentrations. The resulting solutions’ pH(s) were adjusted with diluted HClO4 or NaOH solutions. For pH values below 2, significant fractions of picrate ion (pKa = 0.38) changes to the corresponding protonated form which is not detected by the electrode. For pH > 8.0, the hydroxide ion interferes with the electrode’s response (Fig. 4). The emf values are independent of pH in the range 6–8, this can be taken as the working pH range of the electrode. 3.4. Selectivity coefficients The most important characteristic of any ion-sensitive sensor is its response to the primary ion in the presence of

Fig. 4. Effect of pH of the test solution on the potential reading: (䊉) 1.0 × 10−2 M picrate solution, ( ) 1.0 × 10−3 M picrate solution at 25 ◦ C.

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Table 2 Selectivity coefficients of the picrate selective electrode obtained by matched potential method pot

pot

Xn−

KPic,X

Xn−

KPic,X

Phenolate Salicylate Oxalate Phthalate Citrate Benzoate Formate Acetate Chloride

7.0 × 10−3 6.5 × 10−2 7.3 × 10−3 4.9 × 10−3 6.3 × 10−3 2.9 × 10−2 1.0 × 10−3 3 × 10−3 1.05

2,4-Dinitrophenol Dehydroascorbate 0-Methylphenol 2-Nitrophenol Sulfate Perchlorate Nitrate Sulphite Ascorbic acid

4.3 × 10−4 7.3 × 10−4 6.8 × 10−4 2.3 × 10−4 No interference No interference No interference – –

Initial concentration of picrate ion was 1.0 × 10−3 M

other ions present in solution, which is expressed in terms of the potentiometric selectivity coefficient. The potentiometric selectivity coefficients for the mercury(I) picrate electrode (KPic,M ) were determined, for a number of anions (X− ), by the matched potential method (MPM) [39–43]. 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 [38]. Mainly for these reasons, its popularity has increased in the last few years [44]. The determined KPic,M values are presented in Table 2. The results given in the above-mentioned table reveal that the sensor showed high selectivity towards picrate ion over structurally similar anions, such as phenolate, 2-nitrophenol, 2,4-dinitrophenol and many lipophilic aromatic anions, salicylate, phthalate, citrate, benzoate. From the selectivity coefficient values of oxalate and acetate, it can be seen that the picrate electrode is about 130 and 330 times more responsive to picrate than to oxalate and acetate, respectively. As previ-

ously pointed out, these substances relatively interfere with picric acid analysis [29,30]. For salicylate, the associated selectivity coefficient (Table 2) shows that the picrate electrode can tolerate about 15 times higher concentration of salicylate than picrate without suffering interference. No interference at all is caused by nitrate, sulphate or perchlorate and they can, therefore, be used as background electrolytes or ionic strength adjusters for picrate solutions before performing potentiometric measurements. Chloride ion interferes seriously as shown in Table 2. This interference is due to formation of corresponding mercury(I) salt on the electrode surface. Strong reducing agents, such as sulfite, ascorbic acid and reducing sugars, convert mercury(I) to elemental mercury at the electrode’s surface and seriously affect its response. Previous oxidation of these species, coupled with the subsequent chloroform extraction of picric acid from the aqueous samples, completely eliminates their interference. Moreover, the oxidation products, i.e., sulfate and dehydroascorbate, do not interfere (see Table 2). Therefore when using this electrode in solution containing reducing agents, it is necessary to oxidize these agents using a suitable oxidant, and then carries the picrate ion determination. The results indicate that the anion selectivity of the sensor did not follow the classical Hofmeister selectivity sequence, which depends solely on the lipophilicity [33]. When the selectivity of the sensor differs from the Hofmeister selectivity sequence, it is generally not governed by simple anion lipophilicity, but by specific chemical interactions between the sensing elements and anions [45–48]. Herein, the unique interaction between ion association complex and the picrate anion makes the sensor highly selective for picrate anion over other anions. 3.5. Determination of picrate in non-aqueous solvents The proposed electrode was applied for picrate ions determination in non-aqueous media. As is shown in Table 3, the

Table 3 Application of the proposed picrate selective electrode in non-aqueous solutions Solvent type

Percent

Slope (mV/decade)

Usable concentration range (M)

Detection limit (M)

1.1 × 10−1

Ethanol

1 5 8 10 15 20

59.0 58.1 57.7 54.6 47.7 48.3

2.7 × 10−5 2.5 × 10−5 3.9 × 10−5 5.5 × 10−5 4.8 × 10−5 4.4 × 10−5

to to 1.0 × 10−1 to 5.0 × 10−2 to 1.0 × 10−1 to 5.0 × 10−2 to 8.0 × 10−2

2.5 × 10−5 2.4 × 10−5 2.9 × 10−5 4.0 × 10−5 4.0 × 10−5 4.3 × 10−5

Methanol

1 5 8 10 15 20

57.1 57.2 56.7 57.0 58 51.5

3.8 × 10−5 3.7 × 10−5 2.9 × 10−5 3.0 × 10−4 4.8 × 10−5 4.5 × 10−5

to 9.8 × 10−2 to 1.0 × 10−1 to 1.0 × 10−1 to 8.8 × 10−2 to 6.9 × 10−2 to 7.0 × 10−2

3.0 × 10−5 3.5 × 10−5 2.8 × 10−5 2.5 × 10−5 4.5 × 10−5 3.7 × 10−5

Acetone

1 5 8 10

56.8 58.2 60.1 48.5

7.8 × 10−5 4.2 × 10−5 2.9 × 10−5 7.3 × 10−5

to 1.0 × 10−1 to 1.0 × 10−1 to 2.1 × 10−1 to 5.6 × 10−2

5.0 × 10−5 4.0 × 10−5 2.9 × 10−5 7.0 × 10−5

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electrode performance is acceptable with respect to slope, usable concentration range, and detection limit up to 10% ethanol, 15% methanol, and 8% acetone. Due to the changes in the activity coefficient of picrate ion in the higher percentages of organic solvents, sensitivity of the electrode reduces. 3.6. Analytical application The reliability of the developed picrate sensor for the quantification of picrate ion was assessed by determining 0.045–2.15 mg ml−1 standard picrate solutions using the calibration graph and standard addition methods. The obtained results show average recovery of 99.2 ± 0.1% (n = 5). The electrode has been advantageously employed to the indirect determination of quinidine in pharmaceutical preparation. Quinidine present in drug was determined as described in the Section 2.6. For a sample containing 197.4 mg of quinidine, the mean result of three determinations was 195.8 mg with an R.S.D. of 2.23%. The result was in agreement with that obtained by a spectrophotometric method, which gave a mean of 198.6 mg. As the other application the inverse solubility product (Ks−1 ) for picrate precipitates with a range of cations was also estimated: Ba2+ , 1.04 × 10−8 ; Pb2+ , 1.82 × 10−9 ; Zn2+ , 1.84 × 10−7 ; Cu2+ , 1.15 × 10−7 ; Al3+ , 1.88 × 10−9 ; NH4 + , 7.74 × 10−5 .

4. Conclusion 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 picrate determination in real samples was demonstrated suggesting its use as a reliable and advantageous alternative to the official as well as to most other previously reported methods in the routine control of picrate concentration in these samples. The electrode stability in the solutions containing higher percentages of organic solvents reveals that this electrode could be well used in non-aqueous media. The most important characteristics of the electrode developed in this laboratory are superior (especially concerning lifetime, tolerance to aromatic anions, simplicity and linear range) as compared with other potentiometric sensors previously suggested based on charge transfer complexes and PVC-based membranes.

Acknowledgment The authors acknowledge the financial support of the Guilan University.

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Biographies M. Moghimi received his Ph.D. degree (1975) in physical chemistry from the London University, England. The overall objective of his research is to investigate and apply new techniques for modification of electrode surface, design and construction of sensors and optodes and chemometrics. M. Arvand received his bachelor degree in chemistry from Guilan University (1996), M.Sc. degree in applied chemistry from Tehran University (1998) and Ph.D. degree in analytical chemistry from the Tarbiat Modarres University (2003). He joined the faculty of Guilan University in 2003.

His field of interest is development of new chemical sensors for cations and anions species and application of molecular sieves such as clay and zeolites in electrochemistry. R. Javandel is a M.Sc. student of the Chemistry Department of the Guilan University. M.A. Zanjanchi received his Ph.D. degree at the chemistry department of University of Manchester (1980). His research interests are on study of material properties and investigation for the preparation and characterization of new zeolite molecular sieves.