Food Chemistry 172 (2015) 794–801

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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem

Analytical Methods

A novel citrate selective electrode based on surfactant modified nano-clinoptilolite Mahdieh Hasheminejad a, Alireza Nezamzadeh-Ejhieh a,b,⇑ a b

Department of Chemistry, Shahreza Branch, Islamic Azad University, P.O. Box 311-86145, Shahreza, Isfahan, Iran Razi Chemistry Research Center (RCRC), Shahreza Branch, Islamic Azad University, Isfahan, Iran

a r t i c l e

i n f o

Article history: Received 1 November 2013 Received in revised form 19 June 2014 Accepted 10 September 2014 Available online 17 September 2014 Keywords: Citrate selective electrode Surfactant modified zeolite Nano clinoptilolite particles

a b s t r a c t A citrate-selective sensor was prepared by modification of a PVC membrane with modified nanoclinoptilolite particles by hexadecyltrimethyl ammonium surfactant (SMZ). A Nernstian slope of 29.9 ± 0.2 mV per decade of citrate concentration was obtained over the concentration range of 5.0  105–5.0  102 mol L1 of citrate. The electrode showed a fast response time (610 s) and a detection limit of 1.3  10–5 mol L1 of citrate. The linear range and detection limit were respectively changed to 1.0  104–5.0  102 mol L1 and 1.0  104 mol L1 of citrate when the micronized clinoptilolite particles were used. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Citric acid (CA) is a predominant organic acid of citrus fruits and is a common parameter used in evaluating the quality of agricultural products and food control points in the food process (Saidani Tounsi & Aidi, 2011). Because of wide use of the citrus-based beverages in the world, the determination of citrate is very important regarding quality control of the citrus products. The determination of citrate has been commonly performed by GC, HPLC or enzymatic method (Mello & Kubota, 2002). Citric acid is an antibacterial and also pH controlling agent (Juliane Guerreiro-Tanomaru, Renata, & Morgental, 2011), and hence, it has been extensively used in food industries. It is also utilized within the pharmaceutical field because of its anticoagulant and antacid properties, and for preventing kidney calculi as well (Ribeiro, Matos, Goreti, & Sales, 2002). In the recent decades, a variety of classical and instrumental methods have been used for direct or indirect determination of citrate. These methods include amperometry (Balasoiu & Stefan-van Staden, 2011), spectrophotometry (Issa, El-Hawary, Youssef, & Senosy, 2010), chemiluminescence (Zhang, Nie, & Jiuru, 2009), flame atomic emission spectrometry (Yebra & Bollaín, 2010) and potentiometry (Broncová, Shishkanova, Krondak, Volf, & Král, 2008). Except potentiometry, in the most of the mentioned methods there are serious interferences in the determination of citrate due to presence of ⇑ Corresponding author at: Department of Chemistry, Shahreza Branch, Islamic Azad University, P.O. Box 311-86145, Shahreza, Isfahan, Iran. Tel.: +98 31 53292515; fax: +98 31 53291018. E-mail address: [email protected] (A. Nezamzadeh-Ejhieh). http://dx.doi.org/10.1016/j.foodchem.2014.09.057 0308-8146/Ó 2014 Elsevier Ltd. All rights reserved.

ascorbic acid, malic acid, oxalic acid, lactic acid, tartaric acid, EDTA, glucose, fructose and sucrose in the media (Araújo, Melo, & Coelho, 2011). In recent years, ion selective electrodes (ISE) have been used for the determination of various organic and inorganic ions due to their advantages including: low cost, ease of preparation, simple instrument, small response time, wide linear range and good selectivity and sensitivity (Cheung, Marriott, & Small, 2007). Thus, we prepared a novel citrate selective electrode by the modification of a PVC membrane using nano-particles of natural clinoptilolite and hexadecyltrimethyl ammonium surfactant. Zeolites involving three-dimensional networks of SiO4 and AlO4 tetrahedra, linked by the sharing of all oxygen atoms, have net negative charge due to the partial substitution of Si4+ by Al3+ which is compensated by alkali and alkaline earth cations. The natural zeolite clinoptilolite is one of the world’s most abundantly occurring and most abundantly used zeolitic minerals (Charkhi, Kazemian, & Kazemeini, 2010; Nezamzadeh-Ejhieh & Amiri, 2013). The cation-exchange properties of zeolites can be exploited to modify their surface. The quaternary amine hexadecyltrimethyl ammonium (HDTMA) is a long-chain cationic surfactant that possesses a permanent positive charge. When brought into contact with clinoptilolite, HDTMA cations can exchange with the inorganic cations on the external surfaces of the zeolite crystals and forms a surfactant bilayer with anionic exchange property. Furthermore, the surfactant bilayer is a solvent-like medium into which organic compounds tend to dissolve. The resultant surfactant-modified zeolite (SMZ) is capable of simultaneous sorption of anions, cations, and non-polar organic molecules from water (Wang & Peng, 2010). On the basis our search, HDTMA onto the different zeolitic and clay beds has shown different behaviour towards different

M. Hasheminejad, A. Nezamzadeh-Ejhieh / Food Chemistry 172 (2015) 794–801

inorganic and organic species (Dong, Wu, Chen, & Lin, 2010; Schick, Caullet, Paillaud, Patarin, & Mangold-Callarec, 2010; Slade & Gates, 2004; Zhoua, Xi, Hea, & Frost, 2008). Thus, we believe that support in different SMZs has a major role on their adsorption behaviour and we used different modified electrodes based on different SMZs for potentiometric determination of different anions. Our previous works confirmed our speculation, so HDTMA onto the different zeolites showed Nernstian behaviour towards different anions (Nezamzadeh-Ejhieh & Esmaeilian, 2012; Nezamzadeh-Ejhieh & Masoudipour, 2010; Nezamzadeh-Ejhieh & Mirzaeian, 2011; Nezamzadeh-Ejhieh & Nematollahi, 2011). In this work, modification of a nano-sized clinoptilolite by HDTMA showed good Nernstian behaviour towards citrate when it was used as a modifier in a PVC membrane electrode. 2. Experimental 2.1. Reagents and chemicals Tri-sodium citrate pentahydrate, hydrochloric acid (37%), sodium hydroxide (98%), ethanol (98%), high relative molecular weight polyvinyl chloride (PVC) and other analytical grade chemicals were obtained from Merck. Reagent grade dioctylphthalate (DOP), tetrahydrofuran (THF) and hexadecyltrimethyl ammonium bromide (HDTMA-Br) were purchased from Aldrich or Fluka and used as received. Natural clinoptilolite, belongs to Semnan region in the north-east of Iran, was purchased from Afrand Tuska Company (Isfahan–Iran). The pH of solutions was appropriately adjusted with sodium hydroxide or hydrochloric acid solution. Doubly distilled-deionized water was used throughout the experiments. 2.2. Apparatus The potentiometric measurements were performed with a potentiometer model 691 pH/Ion Meter (Metrohm) at 25.0 ± 1 °C. The proposed citrate-selective PVC electrode and a standard Ag/AgCl electrode, shielded by an intermediate salt-bridge compartment containing the background electrolyte in order to prevent any transfer of ions into the measuring solution, were respectively used as the indicator electrode and external reference electrode. All potentials were recorded versus this reference electrode. The pH measurements were performed using a corning model 125 pH-meter equipped with a combined electrode. Infrared spectra were recorded on a FT-IR Spectrum 65 instrument using Nujol mulls. Nano-clinoptilolite particles were prepared by using a planetary ball mill (PM100; Retsch Corporation). The X-ray diffraction patterns of samples were recorded using a Bruker diffractometer (D8 Advance) with Ni-filtered copper radiation (Ka = 1.5406 Å) and 2h range of 5°–80°. The particles sizes of the NCP and SMZ were recorded using Transmission Electron Microscope (TEM) S-3500N with Absorbed Electron Detector S-6542 (Hitachi Science System Ltd.). The surface morphology of the samples was studied using a Philips XL30 scanning electron microscope (SEM). Bismuth subcitrate and Clomiphene Citrate tablets were prepared from Amin Pharmaceutical Company (Isfahan, Iran). Soft drink was prepared from Alis Company (Iran).

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study dry milling was applied and the milling parameters such as: the dry milling speed and time and also the ball to powder ratio were optimized with respect to size reduction and crystallinity retention. The milling speed, milling duration and balls to powder ratio were 600 rpm, 6 h and 4.0, respectively. In order to remove water soluble and also magnetic impurities, the obtained nanoparticles were refluxed at 70 °C for 8 h in distilled water while the suspension was stirred magnetically. This repeated four times to reach complete purification. In order to obtain a powder with fixed water content, after centrifuging, washing and drying, the purified powder was stored in desiccators over saturated sodium chloride solution for 2 weeks. For preparation of SMZ, nano-particles of zeolite (1 g) were mixed with 100 mL of 100 mmol L1 HDTMA solution and stirred for 24 h on a magnetic stirrer. The mixture was then centrifuged at 6000 rpm for 20 min and the resulting SMZ was dried in air. Same method was done for 50 and 200 mmol L1 HDTMA solutions. 2.4. Preparation of real samples solutions To prepare the solutions of pharmaceutical Bismuth Subcitrate (BS) and Clomiphen Citrate (CC) tablets, 3 tablets of BS (each tablet = 0.4212 g) were crushed, powdered and dissolved in sufficient water, filtered (to remove any un-dissolved materials) and the filtrate was diluted to 100 mL. The same procedure was done for 3 CC tablet (each tablet = 0.2008 g). The soft drink sample was also diluted 100 times with water. To remove the matrix interferences in the determination of citrate in the prepared solutions, standard addition method was applied in both proposed potentiometric and standard spectrophotometric methods. 2.5. Preparation of the modified electrode For the preparation of the membrane, 0.10 g of the SMZ, 0.30 g PVC and 0.60 g DOP in 2 mL THF were mixed and homogenized for 1 h on a magnetic stirrer. A Pyrex tube (1 mm o.d.) was dipped into the mixture for approximately 10 s, so that a membrane with approximately 0.5 mm thickness was formed. The tube was then withdrawn from the mixture and maintained at room temperature about 2 h. Semi-transparent PVC membrane was obtained with an average thickness of about 0.5 mm. A 1.0  102 mol L1 citrate ion solution was used as internal reference solution for the electrode. The filled electrode was conditioned by soaking into a 1.0  102 mol L1 of citrate solution. An Ag/AgCl electrode was used as an internal reference electrode. 2.6. Potentiometric system and EMF measurements All EMF measurements were carried out with the following cell assembly: Ag/AgCl | internal citrate solution (1.0  102 M) | membrane electrode | test solution | Ag/AgCl The potential reading of each solution was recorded when it became stable, and then plotted as a logarithmic function of citrate anion concentration. The activities of anions, based on the activitycoefficient data, were calculated from the modified form of Debye– Huckel equation (Tao et al., 2010).

2.3. Preparation of the nano-particles of zeolite and the SMZ 3. Results and discussion Natural clinoptilolite zeolite was mechanically pretreated, by crushing in an agate mortar and sieving in analytical sieves for the separation of 12.0 can be related to increasing the concentration of the triple charge citrate species, C6H5O3 7 , and also interference from OH (Nezamzadeh-Ejhieh & Masoudipour, 2010), while the drift in the electrode response at strong acidic conditions is due to the interferences from Cl and mono valent citrate anions. The calibration slopes as a function of solution pH were also obtained and the Nernstian slopes of 29.2 ± 0.6, 29.4 ± 0.4 and 29.8 ± 0.3 mV per decade (n = 5) were respectively obtained for pHs: 4, 6 and 7. Statistical comparing of the results confirms that random errors affect the response of the electrodes in this pH range. On the other hand, divalent citrate anions as the predominant citrate species at pH range between 4 and 7 respond to the active centres in the membrane, while out of this range mono and three valent citrate species and other interfering species such as hydroxyl and chloride anions affect the electrode response. In the strong acidic pHs (pH 6 2) citric acid is predominant form of citrate species present in the solution and hence the chloride anions present in the solution can affect the electrode response not neutral citric acid molecules. Based on the above discussion, pH 6 was selected as the optimum pH in the next studies. According to the results, the slopes of 57.8 ± 1.7, 56.9 ± 1.9 and 18.7 ± 0.9 mV per decade (n = 5) were respectively obtained at pHs 2, 3 and 10. Comparing these values with 59.2 (for mono valent citrate) and 19.7 mV per decade (for three valent citrate)

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3.2.5. Selectivity coefficients When an ISE respond to the primary ion in the presence of other ions in solution, the response of the sensor will expressed in terms of the potentiometric selectivity coefficient which counts the main characteristics of any ISE. The experimental selectivity coefficients depend on the activity and the method of their determination. pot Hence, the potentiometric selectivity coefficients (kCitrate;A ), describing the preference of the membrane for an interfering ion A relative to citrate, were studied by the separate solutions method (SSM), and also (FIM) method (Umezawa, muhlmann, Umezawa, & tohda, 2000) using the following equations.

40 0 -40

80 40 0 -40 -80 1

2

3

4

5

-log C

-80 0

2

4

6

8

10

-log C

b

80 40

SMZ-MCP SMZ-NCP PVC-NCP PVC-Surf

0 -40 -80 1

2

3

4

5

-log (C)

c

pot

pot

a

80

log kCitrate;A

80

¼ ½nFðEj  Ei Þ=ð2:303RTÞ þ log ðai =an=m Þ j FIM :

3.2.6. Response characteristics of the electrode Using the optimized composition and conditions described above, the EMF response of the electrode to varying concentrations of citrate indicates a linear range from 5.0  105 to 5.0  102 mol L1 (r = 0.9997) (Fig. 4a). The slope of calibration curve was 29.9 ± 0.2 mV per decade of citrate concentration. The lower and

½n=m

kCitrate;A ¼ ai =ðaj

Þ

ð3Þ ð4Þ

E(mV)

SSM :

Confirms that the proposed electrode has good selectivity towards divalent citrate compared to common mineral anions (see Complementary informations).

E(mV)

3.2.4. Effect of temperature Another key operating factor which significantly affects the electrode response is temperature. To investigate the thermal stability of the electrode, calibration curves (Ecell versus log Ccitrate) were constructed at different temperatures covering the range from 25 to 60 °C. The experimental and Nernstian slopes were statistically compared and the results confirmed that random errors affect the electrode response in the range of 25–40 °C and hence only in this region electrode shows good Nernstian behaviour (see Complementary informations). At higher temperatures (above 40 °C) due to presence of systematic errors (such as probably leaching of SMZ into test solution) affect the electrode response and deviates the experimental slopes from the corresponding Nernstian value.

  2 Citrate  CN > I > OAC > Cl > C2O2 4 > CO3 > ClO4 > NO3

E(mV)

3.2.3. Response time The response time of the sensor (SMZ-NCP), as a main factor for every selective electrode, was studied when the citrate concentration was rapidly increased from 1.0  105 to 1.0  101 mol L1 (Fig. 3c). From the results, response time of the proposed electrode was about 10–20 s. To study the effect of the size of clinoptilolite particles on the response time of the electrode, same experiments were done using the surfactant modified micronized clinoptilolite (SMZ-MCP) which of results are also presented in Fig. 3c. As shown, the required time to reach a constant response is longer for SMZ-MCP membrane. This is due to the higher homogeneity and also effective surface area of the SMZ-NCP due to smaller particle size of nano-particles. The steady state response time of the SMZ-NCP was also checked and the results are shown by inset of Fig. 3c. As shown, potential was fixed during 180 s with tolerance of ±1 mV, and afterward slowly deviates.

for calculating k-values using Eq. (4). The following selectivity trend:

E (mV)

confirms that the proposed electrode showed Nernstian behaviour to mono and three valent citrate species. Hence the electrode can be used at pH 2–3 and also pH 10 for the determination of citrate.

60

pot

where (kCitrate;A ) is the potentiometric selectivity coefficient, Ei the measured cell potential at primary ion activity ai citrate, Ej the measured cell potential at interference ion activity aj, n and m are the charges of the primary and interference ion, respectively. R, T and F have their common meaning. In FIM method, a constant concentration of the interfering anion (0.2 mol L1) was used while the concentration of citrate was changed from 1  105 to 1.0  102 mol L1. In the SSM method the cell potential of each of two separate solutions, one containing the ion ‘i’ at the activity ‘ai’, the other containing the ion ‘j’ at the same activity aj = ai = 0.01 M was measured and used

40 0

2

4

6

8

10

12

14

V(mL) Fig. 4. (a) Calibration curve for citrate using SMZ-NCP-PVC electrode, 10% SMZ (prepared in 100 mmol L1 HDTMA solution), pH = 6, (b) calibration curve for citrate using different electrodes, 10% SMZ (prepared in 100 mmol L1 HDTMA solution) and 10% other used modifiers, pH = 6 and (c) potentiometric titration of 10 mL 0.001 mol L1 citrate anion with 0.001 mol L1 CuSO4 using SMZ-NCP electrode, 10% SMZ (prepared in 100 mmol L1 HDTMA solution), pH = 6.

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upper detection limits were 1.3  105 and 0.22 mol L1 of citrate, respectively. The potentiometric behaviour of the SMZ-MCP-PVC electrode and the membrane electrodes constructed using unmodified nano zeolite alone and also surfactant alone was studied which of results are presented in Fig. 4b. According to the results, presence of raw zeolite alone and also surfactant alone did not cause a suitable response for the corresponding electrodes towards citrate species. In the case of surfactant modified micronized zeolite (SMZ-MCP), with respect to SMZ-NCP, in addition to decreasing the linear range (from the range of 5.0  105–5.0  102 mol L1 of citrate to the range of 1.0  104–5.0  102 mol L1 of citrate) the sensitivity of the electrode was also decreased from 3.7 to 2.9 lV per mg of citrate. Using SMZ-MCP modified electrode, detection limit was also increased from 1.3  105 mol L1 of citrate (in the case of SMZ-NCP) to 1.0  104 mol L1 of citrate. The average experimental slope and also regression of the calibration curve in case of SMZ-MCP electrode (S = 28.8 ± 2.5 mV per decade of citrate concentration, r = 0.9944 ± 0.0531 for n = 15) was poor with respect to SMZ-NCP electrode. These confirm that with reducing the size of clinoptilolite particles, the effective surface area was increased which increased both loaded surfactant molecules onto zeolite surface and homogeneity of the prepared membrane. Hence higher active centres of SMZ-NCP are available to respond to citrate anions. Long term stability of the electrode was also investigated by taking the response of the optimized electrode in a period of 70 days. The electrode showed best responses during 60 and 54 days for SMZ-NCP and SMZ-MCP electrodes, respectively. The repeatability of both SMZ-MCP and SMZ-NCP membrane electrodes was also studied by 15 replicate measurements and the standard deviations of slopes were determined. The relative standard deviations of 2.5% and 1.1% were respectively obtained for SMZ-MCP and SMZ-NCP membrane electrodes, indicating high repeatability of both electrodes. In this study the SMZ-NCP has also a higher repeatability. The reproducibility of both SMZ-MCP and SMZ-NCP membrane electrodes was also investigated by carrying out the measurements on four independent electrodes with the same composition. These measurements were also performed in 5 replicates. The results were compared by g-test (Anderson, 1987). The calculated ‘g’ values were 0.35 and 0.56 for SMZ-NCP and SMZ-MCP, respectively, which are smaller than the critical ‘g’ value at 95% confidence interval, g0,05,5,4 = 0.6287 (4 class, 5 replicate in each class). This confirms that there is no significant difference between the behaviours of the four independent electrodes. Also, lower RSD values confirmed that the SMZ-NCP-PVC electrode has also a higher reproducibility with respect to SMZ-MCP-PVC electrode. Some characteristics of the proposed electrode were compared with previous citrate ISEs based on different ionophores (Araújo et al., 2011; Broncová, Shishkanova, Volf, & Král, 2004; Broncová et al., 2008; Kim, 2006; Ribeiro et al., 2002). Based on the results (see Complementary informations), response time, slope, pH range and linear range for the proposed SMZ-PVC electrode are comparable and in some cases superior with respect to the other electrodes. 3.2.7. Practical application The proposed SMZ-NCP-PVC electrode was also used as indicator electrode in the titration of a 10 mL 0.001 mol L1 of citrate solution with 0.001 mol L1 of cupper(II) aqueous solution. Changes in cell potential due to changing the citrate concentration during the titration reaction were recorded which of results are presented in Fig. 4c. Potentiometric titration was conducted manually. The differentiation end point curve was used to obtain more accurate and precise end point. The molar ratio of copper(II) to

HC6H5O2 in the titration reaction is 1:1 (Naumenko, Buneeva, 7 Korneeva, Selemenev, & Nemtsev, 2004) and thus the end point should appear at 10 mL. The new citrate-selective electrode was applied for the determination of citrate in drug and tap water samples with satisfactory results. The analysis was performed by using the standard addition technique. The citrate content of the used samples was also determined by UV–vis spectrophotometry as a standard method (Alexis & Holden, 1996; Harikrishna, Nagaralli, & Seetharamappa, 2008; Nora, 2007). For example, the citrate values of 60.4 ± 2.5 and 62.6 ± 1.9 were respectively obtained for Bismuth subcitrate sample using the ISE and spectrophotometric methods. The obtained results which are summarized in Complementary informations confirm the applicability of the proposed electrode for the determination of citrate in real samples.

4. Conclusion The results of this work confirm that SMZ retains its anionic adsorption behaviour when entered in a PVC matrix. Although, both modified PVC electrodes with SMZ-MCP and SMZ-NCP showed good potentiometric responses towards citrate anion, but presence of nano-particles of clinoptilolite showed wider linear range, better sensitivity, smaller response time and finally better repeatability and reproducibility. On the other hand, nano-dimensions of zeolite increase the surface area and hence more active centres in the membrane. The results showed that the electrode behaviour is extremely pH dependent, so the electrode respond to divalent citrate anion in the pH range of 4–7, to mono valent citrate anion in pH range of 2–3 and to three valent citrate anion in pH 10.

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