Materials Science and Engineering C 36 (2014) 187–193
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Construction and performance characteristics of polymeric membrane electrode and coated graphite electrode for the selective determination of Fe3 + ion Koteswara Rao Bandi, Ashok K. Singh 1, Anjali Upadhyay Department of Chemistry, Indian Institute of Technology-Roorkee, Roorkee 247667, India
a r t i c l e
i n f o
Article history: Received 21 October 2013 Received in revised form 20 November 2013 Accepted 6 December 2013 Available online 14 December 2013 Keywords: Potentiometric electrodes Polymeric membrane electrode Coated graphite electrode Ion selective electrode
a b s t r a c t Novel Fe3+ ion-selective polymeric membrane electrodes (PMEs) were prepared using three different ionophores N-(4-(dimethylamino)benzylidene)thiazol-2-amine [L1], 5-((3-methylthiophene-2yl) methyleneamino)-1,3,4thiadiazole-2-thiol [L2] and N-((3-methylthiophene-2yl)methylene)thiazol-2-amine [L3] and their potentiometric characteristics were discussed. Effect of various plasticizers and anion excluders was also studied in detail and improved performance was observed. The best performance was obtained for the membrane electrode having a composition of L2:PVC:o-NPOE:NaTPB as 3:38.5:56:2.5 (w/w; mg). A coated graphite electrode (CGE) was also prepared with the same composition and compared. CGE is found to perform better as it shows a wider working concentration range of 8.3 ×10−8–1.0 × 10−1 mol L−1, a lower detection limit of 2.3 × 10−8 mol L−1, and a near Nernstian slope of 19.5 ± 0.4 mV decade−1 of activity with a response time of 10 s. The CGE shows a shelf life of 6 weeks and in view of high selectivity, it can be used to quantify Fe3+ ion in water, soil, vegetable and medicinal plants. It can also be used as an indicator electrode in potentiometric titration of EDTA with Fe3+ ion. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Iron is the fourth most abundant element of the Earth's crust and is widely and uniformly distributed. It generally presents in surface waters as salts containing Fe3+ ion. Most of those salts are insoluble and settled out, thus, the concentration of iron in well-aerated waters is seldom high. Siderite, hematite, limonite and magnetite are the most important commercial iron ores. It finds many applications both in support of animal life and in industrial processes. At trace concentration, iron promotes many important biological processes of human life [1]. However, at higher concentration, it is toxic and causes liver and kidney damages. Tissue damage has also been reported from prolonged consumption of acidic foodstuffs cooked in iron kitchenware. Thus it is important to monitor the concentration of iron in various environmental samples [2–6]. A number of instrumental techniques such as energy dispersive Xray fluorescence [7], neutron activation analysis [8], isotope dilution multiple collector inductively coupled plasma mass spectrometry (IDMC-ICP-MS) [9], flame atomic absorption spectrometry [10], cathodic stripping voltammetry [11] and spectrophotometry [12] are available for its estimation. These methods provide accurate determination of Fe3+ ion content but require large infrastructure backup and support of expertise. Therefore in the search of attaining such requirements, we found the simple potentiometry technique ion-selective electrodes.
1
E-mail address: [email protected] (A.K. Singh). Tel.: +91 9412978289.
0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.12.010
Ion selective electrodes can analyze a large number of samples practically in small time, in the presence of different interferents and can even be adapted to online monitoring. In view of significant advantages of ion selective electrodes (ISEs), a number of good ISE systems have become commercially available for the determination of various chemical species. A number of ISEs for the estimation of Fe3+ concentration have also been recently reported [13–23]. However they exhibit shortcomings in terms of (i) a narrow working concentration range, (ii) a nonNernstian potential response, (iii) a narrow pH range, and (iv) a high response time. These limitations have affected their widespread use in quantification of Fe3+ ion. Thus a more sensitive and selective electrode with good performance parameters for Fe3 + ion quantification is required and needs to be developed. To develop a good ISE, a selective ionophore is required that which has a property of showing strong affinity for a particular ionic species and poor affinity for others. When such a compound is present in the membrane separating two solutions of an electrolyte, the membrane first facilitates the transport of the ions for which the ionophore has high affinity, from high concentration to low concentration and hinders the passage of all other ions. Different groups of compounds such as porphyrins [13], calixarenes [14], Schiff bases [15–17], macrocycles [18] and different groups of compounds [19–23] have been used as ionophore in the construction of ion-selective electrodes. The difficulty in the development of ISE is that good ionophores are not available. However with the advancement of chemical research, novel compounds are being synthesized. Schiff base is another group of compounds which may show selective affinity. We have therefore chosen Schiff bases as ionophores to prepare ion selective electrode. During
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the past years, the lower detection limit and the selectivity behavior of polymeric membrane ion-selective electrodes (ISEs) have been dramatically improved. ISEs with solid internal contact, i.e., without an inner solution, have been reported to produce better values of lower detection limits, selectivity values and smaller size of the conventional electrodes with optimized inner solutions [24,25]. Therefore in the present study, three schiff bases N(4-(dimethylamino)benzylidene)thiazol-2-amine [L 1 ], 5-((3methylthiophene-2yl)methyleneamino)-1,3,4-thiadiazole-2-thiol [L2] and N-((3-methylthiophene-2yl)methylene)thiazol-2-amine [L3] have been synthesized (Fig. 1) for their possible use as ionophores. Preliminary complexion study of these Schiff bases has shown that they form stable complexes only with Fe3 + ion and weak complexes with other metals, thus indicating the strong affinity for Fe3+ ion and the poor affinity for other metals. Thus these Schiff bases are the ideal choice to be taken as ionophore for preparing a selective membrane to be used as an Fe3 + ion-selective electrode. Poly(vinylchloride) based membranes of L1, L2 and L3 were prepared and investigated as ISEs for Fe3+ ion. 2. Experimental 2.1. Reagent Reagent grade sodium tetraphenylborate (NaTPB), dibutylphthalate (DBP), tetrahydrofuran (THF) and high molecular weight poly(vinylchloride) were procured from E. Merck (Germany). Dioctylphthalate (DOP), acetophenone (AP), o-nitrophenyloctyl ether (o-NPOE), 3-methyl-2-thiophenecarboxaldehyde, 4-(dimethylamino) benzaldehyde, 5-amino-1,3,4-thiadiazole-2-thiol and thiazol-2-amine were obtained from Sigma Aldrich. Potassium tetrakis p-(chloro phenyl)borate (KTpClPB) was taken Fluka. 1-Chloronapthalene (1-CN) from LOBA Chemie. The nitrate and chloride salts of all the cations used were of analytical grade and used without any further purification. The solutions of metal salts were prepared in doubly distilled water and standardized whenever necessary. 2.2. Apparatus and instrumentation The potential measurements were carried out using a 5652 digital pH meter/millivoltmeter (ECIL, India) and a Cyberscan 510 bench pH/ion/mV meter (Eutech Instruments, Singapore). Metal concentrations were determined on a Perkin Elmer-3100 atomic absorption spectrophotometer. IR spectra were recorded with a Perkin Elmer FT-IR 1000 spectrophotometer as films between KBr. The UV–Vis spectra of the compounds were recorded on a Specord S600 PC single beam spectrophotometer using a 10 mm path length silica cell. 1H NMR spectra were recorded on a Bruker DRX 500 MHz spectrophotometer. The elemental analyses were performed with a Vario EL III instrument.
2.3. Synthesis of Schiff bases L1, L2 and L3 2.3.1. Synthesis of Schiff base N-(4-(dimethylamino)benzylidene)thiazol2-amine [L1] 0.14919 g (1 mmol) of 4-(dimethylamino)benzaldehyde was dissolved in 15 mL of methanol. To this 15 mL of thiazole-2-amine (0.1001 g, 1 mmol) was added by continuous stirring and the mixture was refluxed for 5–6 h at 60 °C. A yellow colored solution was formed. The product was purified by column chromatography (ethyl acetate/ hexane). A yellow colored solid was obtained. Yield: 44%. Anal. calc. for C12H13N3S: C, 62.31; H, 5.66; N, 18.17; and S, 13.86. Found: C. 63.11, H. 5.02, N. 17.52, and S. 14.35; UV, λmax (nm): 207.4, 256, and 406.7; IR (KBr, cm− 1): 1593; and 1H NMR (CDCl3, 500 MHz): δ 3.08 (s, 6H), 6.71 (d, 3H), 7.74 (d, 3H), and 9.73 (s, 1H). 2.3.2. Synthesis of Schiff base 5-((3-methylthiophene-2yl)methyleneamino)1,3,4-thiadiazole-2-thiol [L2] 0.1332 g (1 mmol) of 5-amino-1,3,4-thiadiazole-2-thiol was dissolved in 15 mL of methanol in a 100 mL round bottom flask, to this 3-methylthiophene-2-carbaldehyde (1 mmol) in 10 mL of methanol was added. The mixture was stirred for 2 h and it was refluxed for 4 h. A yellow colored solution was formed. The product was purified by column chromatography (ethyl acetate/hexane). A yellow colored powder was obtained. Yield: 67%. Anal. calc. for C8H7N3S3: C, 39.81; H, 2.92; N, 17.41; and S, 39.86. Found: C. 38.13, H. 3.47, N. 18.01, and S. 40.39; UV, λmax (nm): 208, 316.6, and 394.5; IR (KBr, cm−1) 3095 (_CH), 1560 (C_N), and 1279 (C\O); and 1 H NMR (CDCl 3 , 500 MHz): δ 8.69 (s, 1 H), 7.66 (d, 1H), 7.02 (d, 1H), 3.54 (s, 3H), and 2.34 (s, 1H). 2.3.3. Synthesis of Schiff base N-((3-methylthiophene-2yl)methylene) thiazol-2-amine [L3] 3-Methyl-2-thiophene carboxaldehyde (1 mmol) was dissolved in 15 mL of methanol. To this 15 mL of thiazole-2-amine (1 mmol) was added by continuous stirring and this solution was refluxed for 5–6 h at 60 °C. A yellow colored solution was formed. The product was purified by column chromatography (ethyl acetate/hexane). A yellow colored liquid was obtained. Yield: 54%. Anal. calc. for C9H8N2S2: C, 51.89; H, 3.87; N, 13.45; and S, 30.79. Found: C. 53.59, H. 4.02, N. 12.52, and S. 29.87; UV, λmax (nm): 207, 267.5, and 361.2; IR (KBr, cm−1): 1574 (\CH_N); and 1H NMR (CDCl3, 500 MHz): δ 2.51 (s, 3H), 6.95 (d, 1H), 7.19 (d, 1H), 7.51 (d, 1H), 7.62 (d, 1H), and 9.24 (S, 1H). 2.4. Electrode preparation It is well known that the performance of an ion selective electrode depends to a great extent on the composition of its membranes. Therefore in order to arrive at an optimum composition for the membranes of L1, L2 and L3, PVC based membranes of different compositions were
Fig. 1. Structure of ligands N-(4-(dimethylamino)benzylidene)thiazol-2-amine [L1], 5-((3-methylthiophene-2yl)methyleneamino)-1,3,4-thiadiazole-2-thiol [L2] and N-((3methylthiophene-2yl)methylene)thiazol-2-amine [L3].
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Fig. 2. SEM images of graphite rod (A) without coating and (B) with coating.
prepared. The ionophore (L1, L2 and L3), PVC, cation excluder (NaTPB/ KTpClPB) and different plasticizers viz. o-NPOE, DBP, 1-CN, DOP and AP were dissolved in different relative amounts in 2 mL of THF. All the ingredients dissolve readily and a slight viscous solution was prepared by continuous stirring. The solution was poured in to an acrylic ring placed on a clean and smooth glass plate. The solution was allowed to stand overnight which resulted in the slow evaporation of THF and resulted in the form of a transparent membrane of 0.1 mm in thickness and 1 cm in diameter. The membrane was removed carefully and attached to end of a Pyrex tube with araldite. In this way membranes of different compositions were prepared and studied. The membranes were equilibrated with 0.01 mol L−1 Fe(NO3)3 solutions for 2 days. To prepare the coated graphite electrodes, spectroscopic grade graphite rods 10 mm long and 3 mm in diameter were used. A shielded copper wire was glued to one end of the graphite rod, and the electrode was sealed into a PVC tube of about the same diameter with epoxy resin. The working surface of the exposed end of the graphite rod was polished with fine alumina slurry on a polishing cloth, sonicated in distilled water and dried in air. The polished graphite electrode was dipped into the membrane solution mentioned above, and the solvent was evaporated. A membrane was formed on the graphite surface, and the electrode was allowed to stabilize overnight. The coating of polymeric membrane electrode on the graphite rod surface was observed by SEM studies (Fig. 2) and it is found that the coating is homogenous throughout the graphite rod. 2.5. Conditioning of membranes and potential measurements
20 mL of 1.0 × 10− 4 mol L− 1 metal ion solution was titrated with a 1.0 × 10− 2 mol L− 1 ligand solution and the conductance of the mixture after each addition of ligand, was measured. The observed conductance was plotted against [L]/[Fe3 +] ratio and shown in Fig. 3 for the titration of Fe3+ ion. It is seen that the conductance decreases in the beginning on the addition of the ligand which shows that Fe3+ ions are getting complexed with the added ligand. After the completion of complexation reaction conductance becomes constant, the break in this plot corresponds to 1:2 (metal to ligand) stoichiometry of the complex. For other metal ions conductance decreases gradually and does not show the clear change. Non-constancy in the conductance shows that the formed complexes are weak. Thus conductometric study also shows that these Schiff bases have strong affinity for Fe3+ ion and poor affinity for other metals. Thus they can act as a good ionophore for Fe3+. The conductance studies of Schiff bases (L1, L2 and L3) against various metal ions revealed that the Schiff bases were showing high affinity for Fe3+ ion and poor affinity for other metal ions. 3.2. Determination of stability constants The stability constants of the resulting 1:2 metal–ionophore complexes were calculated according to the sandwich membrane method [27]. The formation constants of various ion–ionophore complexes were evaluated from the following relation: nR −n EM zI F βILn ¼ LT − T exp RT zI
ð1Þ
The electrode was equilibrated with a 0.01 mol L−1 Fe(NO3)3 solution. The optimized equilibration time was found to be at 2 days for CGE and PME. The following cell setup and the potential measurement were made on an Orion pH meter. The potential of the cell was determined as a function of Fe3+ ion concentration in the range 1.0 × 10−9 to 1.0 × 10−1 mol L−1. Hg=Hg2 Cl2 jKClðsatd:Þjjinternal solution ð0:01molL
−1
FeðNO3 Þ3 Þj
PVC membranejsample solutionjjHg–Hg2 Cl2 ; KClðsatd:ÞðPMEÞ CGEjtest solutionjjHg–Hg2 Cl2 jKCl ðsatd:Þ Activity coefficients were calculated according to the following Debye–Huckel equation [26]. 3. Results and discussion 3.1. Complexation study of Schiff bases L1, L2 and L3 In order to know the complexation property of the Schiff bases with metals, conductometric studies were carried out. In this study
Fig. 3. Variation in the conductance of the Fe3+ solution on the addition of ligands L1, L2 and L3.
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Table 1 Formation constants of metal complexes with ligands L1, L2 & L3. Metal ions
L2
L1 Fe3+ La3+ Cd2+ Ce3 Zn2+ Co2+ Cr3+ Ni2+ a
Metal ions
Formation constant (log βILn)a ± SD sandwich membrane method
5.02 2.84 2.63 2.67 2.51 2.44 2.32 2.21
± ± ± ± ± ± ± ±
0.03 0.04 0.03 0.02 0.02 0.05 0.07 0.06
6.23 3.12 3.05 2.76 2.63 2.56 2.44 2.38
Formation constant (log βILn)a ± SD sandwich membrane method
L3 ± ± ± ± ± ± ± ±
0.01 0.03 0.04 0.03 0.06 0.03 0.02 0.06
5.23 3.01 2.77 2.89 2.59 2.49 2.37 2.31
L1 ± ± ± ± ± ± ± ±
0.04 0.05 0.02 0.04 0.06 0.05 0.04 0.05
Cu2+ Ca2+ Ag+ Pb2+ Mn2+ Al3+ Fe2+
2.02 1.75 1.62 1.52 1.44 1.29 3.04
L2 ± ± ± ± ± ± ±
0.03 0.02 0.03 0.02 0.05 0.02 0.02
2.27 2.11 2.03 1.89 1.73 1.59 3.33
L3 ± ± ± ± ± ± ±
0.02 0.03 0.05 0.03 0.03 0.03 0.03
2.16 1.82 1.71 1.66 1.53 1.45 3.16
± ± ± ± ± ± ±
0.05 0.03 0.06 0.05 0.04 0.02 0.03
Mean value ± standard deviation (three measurements).
where LT is the total concentration of ionophore in the membrane segment, RT is the concentration of lipophilic ionic site additives, n is the ion–ionophore complex stoichiometry, EM is membrane potential, and R, T and F are constants having their usual meaning. zI is the charge on the ion I. The stability constants of different complexes calculated by the sandwich membrane method are given in Table 1. These values show that these ligands form most of the stable complex with Fe3 + ion having stability constants 5.02 ± 0.03, 6.23 ± 0.01 and 5.23 ± 0.04 for L1, L2 and L3 respectively. The stability constant values for other metal complexes are much smaller, indicating their weak stability. The stability constant values also indicate the strongest affinity of Schiff bases for Fe3 + and poor affinity for all other metals listed in Table 1. Therefore Schiff bases are the potential ionophores for preparing Fe3+ ion selective electrodes. 3.3. Potential response study and optimization of membrane composition It is well established that the performance of membrane electrode depends on its composition i.e., relative amounts of various ingredients (PVC, ionophore, plasticizer and ionic additives) present in the membrane phase. Therefore to arrive at an optimum composition of the membrane performing best, first of all potential response of the sensor with blank membranes having only ionophores L1, L2 and L3 was investigated. Potential response of the sensor was then plotted as a function of log a3+ Fe (Fig. 4). These plots were analyzed and it was evaluated that the membrane generates a linear potential response over a narrow range of working concentration range of 3.5 × 10− 5–1.0 × 10− 1,
7.0 × 10−5–1.0 × 10−1 and 1.2 × 10− 4–1.0 × 10−1 mol L− 1, a subNernstian slope of 13.0 ± 0.4, 14.2 ± 0.6 and 16.8 ± 0.6 mV decade−1 of activity and a high detection limit of 1.7 × 10−5, 5.2 × 10−5 and 7.5 × 10−5 mol L−1 for L1, L2 and L3 respectively. In view of a limited concentration range, a sub-Nernstian slope and a high detection limit it is required to improve the performance of the membrane sensor. One way of doing it is to prepare membranes by adding plasticizers. Thus the membranes containing different plasticizers were prepared and their potential response was investigated. Potential response plot clearly indicates that the addition of plasticizer considerably improves performance characteristics. A perusal of potential response plots of all plasticized membranes indicated that the addition of plasticizer to the membrane phase widens the working concentration range in all cases, and increases the slope and lower the detection limit. Of all the 5 plasticizers (DBP, DOP, AP, 1-CN, o-NPOE) studied, the addition of o-NPOE produces the best effects as the membrane sensor produces the widest working concentration range of 1.9 × 10− 6–1.0 × 10− 1, 6.7 × 10− 7 –1.0 × 10 − 1 and 8.1 × 10 − 7 –1.0 × 10 − 1 mol L − 1, a Nernstian slope of 19.9 ± 0.7, 19.6 ± 0.5 and 19.8 ± 0.8 mV decade − 1 of the Fe3 + activity and a lower detection limit of 3.7 × 10− 7, 1.4 × 10− 7, and 2.8 × 10− 7 mol L− 1 for sensors of L1, L 2 and L 3 respectively. Not only this, the effect of other ingredients and the amount of ionophore in the membrane phase were also studied. After carefully evaluating the performance of different membranes it was concluded that the sensor having a membrane with the composition of L2:o-NPOE:NaTPB:PVC as 3:56:2.5:38.5 (w/w; mg) gives the best performance characteristics. 3.4. Dynamic response time behavior of the proposed electrode It is well known that the dynamic response time of an ionselective electrode is one of its most important characteristics. To measure the dynamic response time of the electrode the concentration of the test solution was changed in steps from 1.0 × 10 − 6 mol L− 1 to 1.0 × 10− 1 mol L− 1. The average time required for the electrode to reach a potential response within ± 1.0 mV of the final equilibrium value after successive immersion in Table 2 Selectivity coefficients of various interfering ions for Fe(III) ion-selective electrodes. Metal ions
Selectivity coefficient (KPot A,B) PME
Fig. 4. Calibration plot for Fe3+ selective polymeric membrane electrodes based on L1, L2 and L3.
Co2+ Ni2+ Ce3+ Pb2+ Ca2+ Al3+ Cd2+ Cu2+
4.89 3.89 7.07 1.20 2.18 7.76 9.77 3.16
Metal ions
CGE × × × × × × × ×
10−4 10−4 10−4 10−4 10−4 10−5 10−4 10−4
1.54 7.94 1.73 3.71 5.62 1.86 2.23 6.45
Selectivity coefficient (KPot A,B) PME
× × × × × × × ×
10−4 10−5 10−4 10−5 10−5 10−5 10−4 10−5
Zn2+ K+ Cr3+ Na+ La3+ Ag+ Mn2+ Fe2+
6.16 1.05 4.37 1.48 1.15 1.82 8.70 2.18
CGE × × × × × × × ×
10−4 10−4 10−4 10−4 10−3 10−4 10−5 10−3
1.65 3.16 1.12 4.46 2.45 5.13 2.04 5.27
× × × × × × × ×
10−4 10−5 10−4 10−5 10−4 10−5 10−5 10−4
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Fig. 5. Calibration characteristics of the electrodes (PME and CGE) based on L2.
a series of Fe3 + ion solutions, each increasing in concentration by a factor of 10, was 13 s for proposed PME and 10 s for CGE. The potentials generated by the electrodes remained stable for about ~ 5 min after which a slow divergence was recorded. 3.5. Effect of soaking time and lifetime It is well established that the loss of membrane components due to chemical process at the membrane sample interface is the main cause for the limited lifetime of neutral carrier based ion selective electrodes [28]. To determine the shelf life of the electrodes, their performance was monitored daily over a period of 12 weeks, their potential response plots were drawn on a daily basis and their performance characteristics measured. No significant change was noticed for 5 weeks for PME and 6 weeks for CGE. After this period, performance of the electrodes started deteriorating with the slope becoming smaller and the detection limit becoming higher. However it is important to mention that the electrodes were kept equilibrated with a 1.0 × 10−2 mol L−1 Fe3+ solution when not in use. 3.6. Effect of pH on electrode performance The pH dependence of the best polymeric membrane electrode and coated graphite electrodes based on ionophore L2 was examined at a 1.0 × 10−3 mol L−1 concentration of Fe3+ ions. The pH of the solution was varied by the small addition of a 0.1 mol L−1 solution of either HCl or NaOH. The potential remains constant over the pH range for proposed PME 2.0–5.0 and 1.5–6.5 for CGE. Therefore, the same was taken as the working pH range of the electrodes. The significant change in potential response observed at a lower pH may be due to the
191
Fig. 6. Potentiometric titration curve of 25 mL of 1.0 × 10−4 mol L−1 solution of Fe3+ ion with 1.0 × 10−3 mol L−1 EDTA at pH 6.0 using PME and CGE as indicator electrode.
interference of hydrogen ions. On the other hand, the observed potential drift at higher pH values could be due to the hydrolysis of Fe3+. 3.7. Effect of interfering ions on electrode performance The most important feature of an ISE is its selectivity which is measured in terms of selectivity coefficient. The potentiometric selectivity coefficient values for these electrodes were determined by the IUPAC recommended fixed interference method (FIM) [29]. For this purpose, a fixed concentration of interfering ion (aB = 1.0 × 10−2 mol L− 1) was added to the primary Fe3+ ion solutions ranging from 1.0 × 10−9 to 1.0 × 10−2 mol L−1 and the potentials were measured. The potential values obtained were plotted versus the activity of the Fe3+ ion. The linear portions of the potential response curve were extrapolated and the value of a3+ Fe was obtained from the intersection point. Potentiometric selectivity coefficients were then calculated using the expression: Pot
KFe3þ ;B ¼
aFe3þ : ðaB ÞZA =ZB
ð2Þ
The potentiometric selectivity coefficient values given in Table 2 indicate that the electrodes are highly selective to Fe3+ over a number of monovalent (Ag+, Na+, K+), divalent (Pb2+, Co2+, Ni2+, Zn2+, Cu2+, Ca2+) and trivalent (Ce3+, Al3+, La3+, Cr3+) ions. 3.8. Comparative performance characteristics of polymeric membrane electrode (PME) and coated graphite electrode (CGE) The investigations on PVC based membranes of three ligands, viz. L1, L2 and L3 have shown that they act as Fe3+ ion-selective electrodes. However, of the three ligands, the electrode with the membrane composition
Table 3 Response characteristics of Fe(III) ion selective PME and CGE. Properties
Optimized membrane composition Soaking time Working concentration range (mol L−1) Detection limit (mol L−1) Slope (mV decade−1 of activity) Response time (s) Life span pH range
Electrode response PME
CGE
L2(3):NaTPB(2.5):o-NPOE(56):PVC(38.5) 2 days in 0.01 mol L−1 Fe(NO3)3 6.7 × 10−7–1.0 × 10−1 1.4 × 10−7 19.6 ± 0.5 13 5 weeks 2.0–5.0
S2(3):NaTPB(2.5):o-NPOE(56):PVC(38.5) 2 days in 0.01 mol L−1 Fe(NO3)3 8.3 × 10−8–1.0 × 10−1 2.3 × 10−8 19.5 ± 0.4 10 6 weeks 1.5–6.5
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of L2:NPOE:NaTPB:PVC in the ratio of 3:56:2.5:38.5 (w/w; mg) performs best in terms of all parameters. As we know, the replacement of the inner solution of PME by a solid substance like graphite rod in coated graphite electrode (CGE) inhibits the leakage from the internal solution into the test solution and improves the characteristic properties of the electrode along with its selectivity [30,31]. Therefore, it was decided to prepare a coated graphite electrode using L2 as the ionophore with the membrane composition that is the same as that of the best PME electrode. It can be seen from Fig. 5 that CGE generated a linear potential response to Fe 3 + ion over a wide working concentration range of 8.3 × 10− 8–1.0 × 10− 1 mol L− 1 with a Nernstian slope of 19.5 ± 0.4 mV decade− 1 of activity and a low detection limit of 2.3 × 10− 8 mol L − 1 . The comparison of the performance of two electrodes in Table 3 clearly shows that CGE is superior to PME in terms of all performance parameters i.e., (i) a wide working concentration range, (ii) a lower detection limit, (iii) a slope that tends to be more closer to the Nernstian value and (iv) a wider pH range. Further it is seen from Table 2 that even the selectivity of CGE is better than that of PME.
Table 5 Determination of iron in water and vegetables by the proposed PME and CGE. Sample
Amount of Fe3+ determineda ± SD (ppm) PME
Tap water Waste water River water Potato Brinjal Spinach Apple Potato a
0.65 4.1 1.4 2.39 3.47 3.07 1.05 2.39
CGE ± ± ± ± ± ± ± ±
0.5 0.7 0.6 0.03 0.03 0.03 0.03 0.03
0.77 4.6 1.6 2.41 3.49 3.10 1.08 2.41
AAS ± ± ± ± ± ± ± ±
0.4 0.6 0.3 0.03 0.03 0.03 0.03 0.03
0.81 4.4 1.9 2.43 3.51 3.12 1.11 2.43
± ± ± ± ± ± ± ±
0.6 0.4 0.5 0.03 0.03 0.03 0.03 0.03
Mean value ± standard deviation (three measurements).
4. Analytical applications
The crucible was washed properly with aqua regia and the washing was also collected in the same beaker. Deionized and distilled water were added and the contents were heated for 10 min. After this, the solution was filtered and the filtrate was collected in a 100 mL volumetric flask. Deionized and distilled water were used to make up the volume. The amounts of Fe3+ determined with the help of the PME and CGE electrodes are given in Table 5.
4.1. Titration with EDTA
4.4. Determination of Fe3+ in medicinal plants
A potentiometric titration of 25 mL of 1.0 × 10−4 mol L− 1 Fe3 + ions against 1.0 × 10−3 mol L−1 EDTA at pH 5.0 was carried out using these electrodes. The titration plots (Fig. 6) are found to be sigmoid shaped and the inflection point of the plot corresponds to a 1:1 stoichiometry of Fe3+–EDTA complex.
Leaves of Adhatoda vasica (Arusa), Ocimum sanctum (Tulsi), Withania somnifera (Ashwagandha) and Cassia fistula (Amaltas) were collected from the Haridwar region and dried at 80 °C overnight in an oven. The samples were powdered in an agate mortar. 2.0 g of dried powdered plant parts was digested with a 5:1 mixture of nitric acid (25% v/v) and perchloric acids, followed by a controlled heating until the evolution of gases ceased. After digestion few drops of concentrated HCl were added. The solution was heated and filtered. After the digestion residue was removed by filtration, the filtrate was neutralized by NH4OH and the volume was made up to 100 mL. Fe3+ ion was determined with the help of CGE by direct potentiometry and the results are given in Table 6. It is clear from Tables 4, 5 and 6 that the results obtained by the use of electrode are in good agreement with those obtained by AAS.
4.2. Determination of Fe3+ ion in soil The soil samples (2.0 g) were digested with 10 mL nitric acid. The solution was heated until the evolution of gases stopped. Then a mixture of nitric acid, perchloric acid and conc. hydrofluoric acid (5:3:5) was added followed by controlled heating until evolution of white fumes stopped. After digestion, the residue was removed by filtration and the filtrate was made up to 100 mL. The electrode was used to determine Fe3+ in this solution and the results are compiled in Table 4.
5. Conclusions 4.3. Determination of Fe3+ ion in water and in vegetables The proposed electrode was also used for the monitoring of Fe3 + ions in the Ganga (Roorkee) and Yamuna (Delhi) river waters and also in the waste water taken from the Chemistry Department of the institute. The samples were collected from different locations and treated with 1 M nitric acid and the pH of these samples was adjusted to 5.0. The amounts of Fe3 + determined with the help of the PME and CGE electrodes are given in Table 5. Apple and vegetables (potato, brinjal and spinach) were taken and washed properly with deionized water. These were subsequently cut with a knife and weighed instantly. The samples were placed in crucibles, which were already cleaned and weighed. These crucibles were then placed in a muffle furnace where the temperature was slowly raised from 200 to 500 °C, for 5 h; the process was repeated till a constant weight was achieved. Gray white ash was kept in sample bottles. Subsequently the ash samples were transferred into the 100 mL beaker. Table 4 Determination of iron in soil samples by the proposed PME and CGE. Sample
Soil sample 1 (Haridwar) Soil sample 2 (Rishikesh) a
The investigations on a large number of membranes of L1, L2 and L3 have shown that they can act as Fe3+ ion-selective electrodes. However, of the three ligands, PME having a membrane of L2 with a composition of L2:o-NPOE:NaTPB:PVC in the ratio of 3:56:2.5:38.5 (w/w; mg) performs best as it exhibits widest working concentration range (1.0 × 10− 1 to 6.7 × 10− 7 mol L−1), Nernstian compliance (19.6 ± 0.5 mV decade−1 of activity) and high selectivity for Fe3+ ions. The performance of CGE having a membrane of the same composition was prepared and found better as compared to PME, in all performance parameters. The comparison with reported electrodes (Table 7) shows that the proposed CGE is superior to the reported electrodes as it shows the widest concentration range, the lowest detection limit, a Nernstian slope and a wider pH range. Even the selectivity of the electrode is comparable to the reported electrode overall. Thus the
Table 6 Determination of iron in medicinal plants by the proposed PME and CGE. Sample
PME
Amount of Fe3+ determineda ± SD (mg Kg−1) PME
CGE
AAS
2.19 ± 0.03 2.29 ± 0.03
2.21 ± 0.03 2.32 ± 0.03
2.24 ± 0.02 2.30 ± 0.04
Mean value ± standard deviation (three measurements).
Amount of Fe3+ determineda ± SD (mg Kg−1)
Adhatoda vasica Ocimum sanctum Withania somnifera Cassia fistula a
187 466 485 586
± ± ± ±
CGE 3 2 3 4
188 470 489 587
AAS ± ± ± ±
Mean value ± standard deviation (three measurements).
2 3 4 3
190 473 491 590
± ± ± ±
3 4 3 5
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Table 7 Comparison of response characteristics of proposed Fe3+ ion selective CGE over with reported electrodes. The bold entries indicates the results of present work. Ref. no.
Linear range (mol L−1)
Detection limit (mol L−1)
Slope (mV decade−1 of activity)
pH range
Response time (s)
15 16 17 18 19 17 18 19 20 CGE
3.5 × 10−6–4.0 1.0 × 10−7–1.0 6.3 × 10−6–1.0 1.0 × 10−7–1.0 1.0 × 10−6–1.0 3.0 × 10−7–1.0 1.0 × 10−7–1.0 1.0 × 10−6–1.0 9.1 × 10−6–1.0 8.3 × 10−8–1.0
2.5 × 10−6 5.0 × 10−8 5.0 × 10−6 8.6 × 10−8 5.0 × 10−7 2.0 × 10−7 4.8 × 10−8 6.8 × 10−7 NM 2.30 × 10−8
28.5 ± 0.5 19.9 ± 0.3 20.0 19.6 60 ± 5 20.2 ± 0.8 19.4 ± 0.4 19.4 ± 0.5 21.2 19.5 ± 0.4
4.5–6.5 3.0–6.3 3.5–5.5 1.6–4.3 1.3–3.5 1.8–5.8 2.3–3.4 2.2–4.8 3.2–4.8 1.5–6.5
b15 b12 15 b10 25 b10 b10 b15 20 10
× 10−2 × 10−1 × 10−1 × 10−2 × 10−2 × 10−2 × 10−2 × 10−1 × 10−1 × 10−1
proposed electrode is a useful addition to the family of reported Fe3+ electrodes and can be used to quantify Fe3+ ion in water, soil samples, medicinal plants and vegetables. Acknowledgment Mr. Koteswara Rao Bandi is highly thankful to the Ministry of Human Resource Development (MHRD), New Delhi, India, for their financial support. References [1] N.N. Greenwood, A.E. Haw, Chemistry of the Elements, Pergamum Press, Oxford, 1984. [2] A.F. Oliverra, J.A. Nobrega, O. Fatibello-Filho, Asynchronous merging zones system: spectrophotometric determination of Fe(II) and Fe(III) in pharmaceutical products, Talanta 49 (1999) 505–510. [3] J.M.T. Carneiro, A.C.B. Dias, E.A.G. Zagatto, R.S. Honorato, Spectrophotometric catalytic determination of Fe(III) in estuarine waters using a flow–batch system, Anal. Chim. Acta. 455 (2002) 327–333. [4] A. Safavi, H. Abdollahi, M.R. Hormozi-Nezhad, Simultaneous kinetic determination of Fe(III) and Fe(II) by H-point standard addition method, Talanta 56 (2002) 699–704. [5] B. Nagabhushana, G. Chandrappa, B. Nagappa, N. Nagaraj, Diformylhydrazine as analytical reagent for spectrophotometric determination of iron(II) and iron(III), Anal. Bioanal. Chem. 373 (2002) 299–303. [6] J. Zolgharnein, H. Abdollahi, D. Jaefarifar, G.H. Azimi, Simultaneous determination of Fe(II) and Fe(III) by kinetic spectrophotometric H-point standard addition method, Talanta 57 (2002) 1067–1073. [7] L.S.G. Teixeira, E.S. Santos, L.S. Nunes, Determination of copper, iron, nickel and zinc in ethanol fuel by energy dispersive X-ray fluorescence after pre-concentration on chromatography paper, Anal. Chim. Acta. 722 (2012) 29–33. [8] S. Waheed, S. Rahman, Y. Faiz, N. Siddique, Neutron activation analysis of essential elements in multani mitti clay using miniature neutron source reactor, Appl. Radiat. Isot. 70 (2012) 2362–2369. [9] J.D. Jong, V. Schoemann, D. Lannuzel, J. Tison, N. Mattielli, High-accuracy determination of iron in seawater by isotope dilution multiple collector inductively coupled plasma mass spectrometry (ID-MC-ICP-MS) using nitrilotriacetic acid chelating resin for pre-concentration and matrix separation, Anal. Chim. Acta. 623 (2008) 126–139. [10] İ. Durukan, Ç.A. Şahin, N. Şatıroğlu, S. Bektaş, Determination of iron and copper in food samples by flow injection cloud point extraction flame atomic absorption spectrometry, Microchem. J. 99 (2011) 159–163. [11] M. Cheize, G. Sarthou, P.L. Croot, E. Bucciarelli, A.C. Baudoux, A.R. Baker, Iron organic speciation determination in rainwater using cathodic stripping voltammetry, Anal. Chim. Acta. 736 (2012) 45–54. [12] E. Govender, S.T.L. Harrison, C.G. Bryan, Modification of the ferric chloride assay for the spectrophotometric determination of ferric and total iron in acidic solutions containing high concentrations of copper, Miner. Eng. 35 (2012) 46–48.
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Construction and performance characteristics of polymeric membrane electrode and coated graphite electrode for the selective determination of Fe3 + ion Koteswara Rao Bandi, Ashok K. Singh 1, Anjali Upadhyay Department of Chemistry, Indian Institute of Technology-Roorkee, Roorkee 247667, India
a r t i c l e
i n f o
Article history: Received 21 October 2013 Received in revised form 20 November 2013 Accepted 6 December 2013 Available online 14 December 2013 Keywords: Potentiometric electrodes Polymeric membrane electrode Coated graphite electrode Ion selective electrode
a b s t r a c t Novel Fe3+ ion-selective polymeric membrane electrodes (PMEs) were prepared using three different ionophores N-(4-(dimethylamino)benzylidene)thiazol-2-amine [L1], 5-((3-methylthiophene-2yl) methyleneamino)-1,3,4thiadiazole-2-thiol [L2] and N-((3-methylthiophene-2yl)methylene)thiazol-2-amine [L3] and their potentiometric characteristics were discussed. Effect of various plasticizers and anion excluders was also studied in detail and improved performance was observed. The best performance was obtained for the membrane electrode having a composition of L2:PVC:o-NPOE:NaTPB as 3:38.5:56:2.5 (w/w; mg). A coated graphite electrode (CGE) was also prepared with the same composition and compared. CGE is found to perform better as it shows a wider working concentration range of 8.3 ×10−8–1.0 × 10−1 mol L−1, a lower detection limit of 2.3 × 10−8 mol L−1, and a near Nernstian slope of 19.5 ± 0.4 mV decade−1 of activity with a response time of 10 s. The CGE shows a shelf life of 6 weeks and in view of high selectivity, it can be used to quantify Fe3+ ion in water, soil, vegetable and medicinal plants. It can also be used as an indicator electrode in potentiometric titration of EDTA with Fe3+ ion. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Iron is the fourth most abundant element of the Earth's crust and is widely and uniformly distributed. It generally presents in surface waters as salts containing Fe3+ ion. Most of those salts are insoluble and settled out, thus, the concentration of iron in well-aerated waters is seldom high. Siderite, hematite, limonite and magnetite are the most important commercial iron ores. It finds many applications both in support of animal life and in industrial processes. At trace concentration, iron promotes many important biological processes of human life [1]. However, at higher concentration, it is toxic and causes liver and kidney damages. Tissue damage has also been reported from prolonged consumption of acidic foodstuffs cooked in iron kitchenware. Thus it is important to monitor the concentration of iron in various environmental samples [2–6]. A number of instrumental techniques such as energy dispersive Xray fluorescence [7], neutron activation analysis [8], isotope dilution multiple collector inductively coupled plasma mass spectrometry (IDMC-ICP-MS) [9], flame atomic absorption spectrometry [10], cathodic stripping voltammetry [11] and spectrophotometry [12] are available for its estimation. These methods provide accurate determination of Fe3+ ion content but require large infrastructure backup and support of expertise. Therefore in the search of attaining such requirements, we found the simple potentiometry technique ion-selective electrodes.
1
E-mail address: [email protected] (A.K. Singh). Tel.: +91 9412978289.
0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.12.010
Ion selective electrodes can analyze a large number of samples practically in small time, in the presence of different interferents and can even be adapted to online monitoring. In view of significant advantages of ion selective electrodes (ISEs), a number of good ISE systems have become commercially available for the determination of various chemical species. A number of ISEs for the estimation of Fe3+ concentration have also been recently reported [13–23]. However they exhibit shortcomings in terms of (i) a narrow working concentration range, (ii) a nonNernstian potential response, (iii) a narrow pH range, and (iv) a high response time. These limitations have affected their widespread use in quantification of Fe3+ ion. Thus a more sensitive and selective electrode with good performance parameters for Fe3 + ion quantification is required and needs to be developed. To develop a good ISE, a selective ionophore is required that which has a property of showing strong affinity for a particular ionic species and poor affinity for others. When such a compound is present in the membrane separating two solutions of an electrolyte, the membrane first facilitates the transport of the ions for which the ionophore has high affinity, from high concentration to low concentration and hinders the passage of all other ions. Different groups of compounds such as porphyrins [13], calixarenes [14], Schiff bases [15–17], macrocycles [18] and different groups of compounds [19–23] have been used as ionophore in the construction of ion-selective electrodes. The difficulty in the development of ISE is that good ionophores are not available. However with the advancement of chemical research, novel compounds are being synthesized. Schiff base is another group of compounds which may show selective affinity. We have therefore chosen Schiff bases as ionophores to prepare ion selective electrode. During
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the past years, the lower detection limit and the selectivity behavior of polymeric membrane ion-selective electrodes (ISEs) have been dramatically improved. ISEs with solid internal contact, i.e., without an inner solution, have been reported to produce better values of lower detection limits, selectivity values and smaller size of the conventional electrodes with optimized inner solutions [24,25]. Therefore in the present study, three schiff bases N(4-(dimethylamino)benzylidene)thiazol-2-amine [L 1 ], 5-((3methylthiophene-2yl)methyleneamino)-1,3,4-thiadiazole-2-thiol [L2] and N-((3-methylthiophene-2yl)methylene)thiazol-2-amine [L3] have been synthesized (Fig. 1) for their possible use as ionophores. Preliminary complexion study of these Schiff bases has shown that they form stable complexes only with Fe3 + ion and weak complexes with other metals, thus indicating the strong affinity for Fe3+ ion and the poor affinity for other metals. Thus these Schiff bases are the ideal choice to be taken as ionophore for preparing a selective membrane to be used as an Fe3 + ion-selective electrode. Poly(vinylchloride) based membranes of L1, L2 and L3 were prepared and investigated as ISEs for Fe3+ ion. 2. Experimental 2.1. Reagent Reagent grade sodium tetraphenylborate (NaTPB), dibutylphthalate (DBP), tetrahydrofuran (THF) and high molecular weight poly(vinylchloride) were procured from E. Merck (Germany). Dioctylphthalate (DOP), acetophenone (AP), o-nitrophenyloctyl ether (o-NPOE), 3-methyl-2-thiophenecarboxaldehyde, 4-(dimethylamino) benzaldehyde, 5-amino-1,3,4-thiadiazole-2-thiol and thiazol-2-amine were obtained from Sigma Aldrich. Potassium tetrakis p-(chloro phenyl)borate (KTpClPB) was taken Fluka. 1-Chloronapthalene (1-CN) from LOBA Chemie. The nitrate and chloride salts of all the cations used were of analytical grade and used without any further purification. The solutions of metal salts were prepared in doubly distilled water and standardized whenever necessary. 2.2. Apparatus and instrumentation The potential measurements were carried out using a 5652 digital pH meter/millivoltmeter (ECIL, India) and a Cyberscan 510 bench pH/ion/mV meter (Eutech Instruments, Singapore). Metal concentrations were determined on a Perkin Elmer-3100 atomic absorption spectrophotometer. IR spectra were recorded with a Perkin Elmer FT-IR 1000 spectrophotometer as films between KBr. The UV–Vis spectra of the compounds were recorded on a Specord S600 PC single beam spectrophotometer using a 10 mm path length silica cell. 1H NMR spectra were recorded on a Bruker DRX 500 MHz spectrophotometer. The elemental analyses were performed with a Vario EL III instrument.
2.3. Synthesis of Schiff bases L1, L2 and L3 2.3.1. Synthesis of Schiff base N-(4-(dimethylamino)benzylidene)thiazol2-amine [L1] 0.14919 g (1 mmol) of 4-(dimethylamino)benzaldehyde was dissolved in 15 mL of methanol. To this 15 mL of thiazole-2-amine (0.1001 g, 1 mmol) was added by continuous stirring and the mixture was refluxed for 5–6 h at 60 °C. A yellow colored solution was formed. The product was purified by column chromatography (ethyl acetate/ hexane). A yellow colored solid was obtained. Yield: 44%. Anal. calc. for C12H13N3S: C, 62.31; H, 5.66; N, 18.17; and S, 13.86. Found: C. 63.11, H. 5.02, N. 17.52, and S. 14.35; UV, λmax (nm): 207.4, 256, and 406.7; IR (KBr, cm− 1): 1593; and 1H NMR (CDCl3, 500 MHz): δ 3.08 (s, 6H), 6.71 (d, 3H), 7.74 (d, 3H), and 9.73 (s, 1H). 2.3.2. Synthesis of Schiff base 5-((3-methylthiophene-2yl)methyleneamino)1,3,4-thiadiazole-2-thiol [L2] 0.1332 g (1 mmol) of 5-amino-1,3,4-thiadiazole-2-thiol was dissolved in 15 mL of methanol in a 100 mL round bottom flask, to this 3-methylthiophene-2-carbaldehyde (1 mmol) in 10 mL of methanol was added. The mixture was stirred for 2 h and it was refluxed for 4 h. A yellow colored solution was formed. The product was purified by column chromatography (ethyl acetate/hexane). A yellow colored powder was obtained. Yield: 67%. Anal. calc. for C8H7N3S3: C, 39.81; H, 2.92; N, 17.41; and S, 39.86. Found: C. 38.13, H. 3.47, N. 18.01, and S. 40.39; UV, λmax (nm): 208, 316.6, and 394.5; IR (KBr, cm−1) 3095 (_CH), 1560 (C_N), and 1279 (C\O); and 1 H NMR (CDCl 3 , 500 MHz): δ 8.69 (s, 1 H), 7.66 (d, 1H), 7.02 (d, 1H), 3.54 (s, 3H), and 2.34 (s, 1H). 2.3.3. Synthesis of Schiff base N-((3-methylthiophene-2yl)methylene) thiazol-2-amine [L3] 3-Methyl-2-thiophene carboxaldehyde (1 mmol) was dissolved in 15 mL of methanol. To this 15 mL of thiazole-2-amine (1 mmol) was added by continuous stirring and this solution was refluxed for 5–6 h at 60 °C. A yellow colored solution was formed. The product was purified by column chromatography (ethyl acetate/hexane). A yellow colored liquid was obtained. Yield: 54%. Anal. calc. for C9H8N2S2: C, 51.89; H, 3.87; N, 13.45; and S, 30.79. Found: C. 53.59, H. 4.02, N. 12.52, and S. 29.87; UV, λmax (nm): 207, 267.5, and 361.2; IR (KBr, cm−1): 1574 (\CH_N); and 1H NMR (CDCl3, 500 MHz): δ 2.51 (s, 3H), 6.95 (d, 1H), 7.19 (d, 1H), 7.51 (d, 1H), 7.62 (d, 1H), and 9.24 (S, 1H). 2.4. Electrode preparation It is well known that the performance of an ion selective electrode depends to a great extent on the composition of its membranes. Therefore in order to arrive at an optimum composition for the membranes of L1, L2 and L3, PVC based membranes of different compositions were
Fig. 1. Structure of ligands N-(4-(dimethylamino)benzylidene)thiazol-2-amine [L1], 5-((3-methylthiophene-2yl)methyleneamino)-1,3,4-thiadiazole-2-thiol [L2] and N-((3methylthiophene-2yl)methylene)thiazol-2-amine [L3].
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Fig. 2. SEM images of graphite rod (A) without coating and (B) with coating.
prepared. The ionophore (L1, L2 and L3), PVC, cation excluder (NaTPB/ KTpClPB) and different plasticizers viz. o-NPOE, DBP, 1-CN, DOP and AP were dissolved in different relative amounts in 2 mL of THF. All the ingredients dissolve readily and a slight viscous solution was prepared by continuous stirring. The solution was poured in to an acrylic ring placed on a clean and smooth glass plate. The solution was allowed to stand overnight which resulted in the slow evaporation of THF and resulted in the form of a transparent membrane of 0.1 mm in thickness and 1 cm in diameter. The membrane was removed carefully and attached to end of a Pyrex tube with araldite. In this way membranes of different compositions were prepared and studied. The membranes were equilibrated with 0.01 mol L−1 Fe(NO3)3 solutions for 2 days. To prepare the coated graphite electrodes, spectroscopic grade graphite rods 10 mm long and 3 mm in diameter were used. A shielded copper wire was glued to one end of the graphite rod, and the electrode was sealed into a PVC tube of about the same diameter with epoxy resin. The working surface of the exposed end of the graphite rod was polished with fine alumina slurry on a polishing cloth, sonicated in distilled water and dried in air. The polished graphite electrode was dipped into the membrane solution mentioned above, and the solvent was evaporated. A membrane was formed on the graphite surface, and the electrode was allowed to stabilize overnight. The coating of polymeric membrane electrode on the graphite rod surface was observed by SEM studies (Fig. 2) and it is found that the coating is homogenous throughout the graphite rod. 2.5. Conditioning of membranes and potential measurements
20 mL of 1.0 × 10− 4 mol L− 1 metal ion solution was titrated with a 1.0 × 10− 2 mol L− 1 ligand solution and the conductance of the mixture after each addition of ligand, was measured. The observed conductance was plotted against [L]/[Fe3 +] ratio and shown in Fig. 3 for the titration of Fe3+ ion. It is seen that the conductance decreases in the beginning on the addition of the ligand which shows that Fe3+ ions are getting complexed with the added ligand. After the completion of complexation reaction conductance becomes constant, the break in this plot corresponds to 1:2 (metal to ligand) stoichiometry of the complex. For other metal ions conductance decreases gradually and does not show the clear change. Non-constancy in the conductance shows that the formed complexes are weak. Thus conductometric study also shows that these Schiff bases have strong affinity for Fe3+ ion and poor affinity for other metals. Thus they can act as a good ionophore for Fe3+. The conductance studies of Schiff bases (L1, L2 and L3) against various metal ions revealed that the Schiff bases were showing high affinity for Fe3+ ion and poor affinity for other metal ions. 3.2. Determination of stability constants The stability constants of the resulting 1:2 metal–ionophore complexes were calculated according to the sandwich membrane method [27]. The formation constants of various ion–ionophore complexes were evaluated from the following relation: nR −n EM zI F βILn ¼ LT − T exp RT zI
ð1Þ
The electrode was equilibrated with a 0.01 mol L−1 Fe(NO3)3 solution. The optimized equilibration time was found to be at 2 days for CGE and PME. The following cell setup and the potential measurement were made on an Orion pH meter. The potential of the cell was determined as a function of Fe3+ ion concentration in the range 1.0 × 10−9 to 1.0 × 10−1 mol L−1. Hg=Hg2 Cl2 jKClðsatd:Þjjinternal solution ð0:01molL
−1
FeðNO3 Þ3 Þj
PVC membranejsample solutionjjHg–Hg2 Cl2 ; KClðsatd:ÞðPMEÞ CGEjtest solutionjjHg–Hg2 Cl2 jKCl ðsatd:Þ Activity coefficients were calculated according to the following Debye–Huckel equation [26]. 3. Results and discussion 3.1. Complexation study of Schiff bases L1, L2 and L3 In order to know the complexation property of the Schiff bases with metals, conductometric studies were carried out. In this study
Fig. 3. Variation in the conductance of the Fe3+ solution on the addition of ligands L1, L2 and L3.
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Table 1 Formation constants of metal complexes with ligands L1, L2 & L3. Metal ions
L2
L1 Fe3+ La3+ Cd2+ Ce3 Zn2+ Co2+ Cr3+ Ni2+ a
Metal ions
Formation constant (log βILn)a ± SD sandwich membrane method
5.02 2.84 2.63 2.67 2.51 2.44 2.32 2.21
± ± ± ± ± ± ± ±
0.03 0.04 0.03 0.02 0.02 0.05 0.07 0.06
6.23 3.12 3.05 2.76 2.63 2.56 2.44 2.38
Formation constant (log βILn)a ± SD sandwich membrane method
L3 ± ± ± ± ± ± ± ±
0.01 0.03 0.04 0.03 0.06 0.03 0.02 0.06
5.23 3.01 2.77 2.89 2.59 2.49 2.37 2.31
L1 ± ± ± ± ± ± ± ±
0.04 0.05 0.02 0.04 0.06 0.05 0.04 0.05
Cu2+ Ca2+ Ag+ Pb2+ Mn2+ Al3+ Fe2+
2.02 1.75 1.62 1.52 1.44 1.29 3.04
L2 ± ± ± ± ± ± ±
0.03 0.02 0.03 0.02 0.05 0.02 0.02
2.27 2.11 2.03 1.89 1.73 1.59 3.33
L3 ± ± ± ± ± ± ±
0.02 0.03 0.05 0.03 0.03 0.03 0.03
2.16 1.82 1.71 1.66 1.53 1.45 3.16
± ± ± ± ± ± ±
0.05 0.03 0.06 0.05 0.04 0.02 0.03
Mean value ± standard deviation (three measurements).
where LT is the total concentration of ionophore in the membrane segment, RT is the concentration of lipophilic ionic site additives, n is the ion–ionophore complex stoichiometry, EM is membrane potential, and R, T and F are constants having their usual meaning. zI is the charge on the ion I. The stability constants of different complexes calculated by the sandwich membrane method are given in Table 1. These values show that these ligands form most of the stable complex with Fe3 + ion having stability constants 5.02 ± 0.03, 6.23 ± 0.01 and 5.23 ± 0.04 for L1, L2 and L3 respectively. The stability constant values for other metal complexes are much smaller, indicating their weak stability. The stability constant values also indicate the strongest affinity of Schiff bases for Fe3 + and poor affinity for all other metals listed in Table 1. Therefore Schiff bases are the potential ionophores for preparing Fe3+ ion selective electrodes. 3.3. Potential response study and optimization of membrane composition It is well established that the performance of membrane electrode depends on its composition i.e., relative amounts of various ingredients (PVC, ionophore, plasticizer and ionic additives) present in the membrane phase. Therefore to arrive at an optimum composition of the membrane performing best, first of all potential response of the sensor with blank membranes having only ionophores L1, L2 and L3 was investigated. Potential response of the sensor was then plotted as a function of log a3+ Fe (Fig. 4). These plots were analyzed and it was evaluated that the membrane generates a linear potential response over a narrow range of working concentration range of 3.5 × 10− 5–1.0 × 10− 1,
7.0 × 10−5–1.0 × 10−1 and 1.2 × 10− 4–1.0 × 10−1 mol L− 1, a subNernstian slope of 13.0 ± 0.4, 14.2 ± 0.6 and 16.8 ± 0.6 mV decade−1 of activity and a high detection limit of 1.7 × 10−5, 5.2 × 10−5 and 7.5 × 10−5 mol L−1 for L1, L2 and L3 respectively. In view of a limited concentration range, a sub-Nernstian slope and a high detection limit it is required to improve the performance of the membrane sensor. One way of doing it is to prepare membranes by adding plasticizers. Thus the membranes containing different plasticizers were prepared and their potential response was investigated. Potential response plot clearly indicates that the addition of plasticizer considerably improves performance characteristics. A perusal of potential response plots of all plasticized membranes indicated that the addition of plasticizer to the membrane phase widens the working concentration range in all cases, and increases the slope and lower the detection limit. Of all the 5 plasticizers (DBP, DOP, AP, 1-CN, o-NPOE) studied, the addition of o-NPOE produces the best effects as the membrane sensor produces the widest working concentration range of 1.9 × 10− 6–1.0 × 10− 1, 6.7 × 10− 7 –1.0 × 10 − 1 and 8.1 × 10 − 7 –1.0 × 10 − 1 mol L − 1, a Nernstian slope of 19.9 ± 0.7, 19.6 ± 0.5 and 19.8 ± 0.8 mV decade − 1 of the Fe3 + activity and a lower detection limit of 3.7 × 10− 7, 1.4 × 10− 7, and 2.8 × 10− 7 mol L− 1 for sensors of L1, L 2 and L 3 respectively. Not only this, the effect of other ingredients and the amount of ionophore in the membrane phase were also studied. After carefully evaluating the performance of different membranes it was concluded that the sensor having a membrane with the composition of L2:o-NPOE:NaTPB:PVC as 3:56:2.5:38.5 (w/w; mg) gives the best performance characteristics. 3.4. Dynamic response time behavior of the proposed electrode It is well known that the dynamic response time of an ionselective electrode is one of its most important characteristics. To measure the dynamic response time of the electrode the concentration of the test solution was changed in steps from 1.0 × 10 − 6 mol L− 1 to 1.0 × 10− 1 mol L− 1. The average time required for the electrode to reach a potential response within ± 1.0 mV of the final equilibrium value after successive immersion in Table 2 Selectivity coefficients of various interfering ions for Fe(III) ion-selective electrodes. Metal ions
Selectivity coefficient (KPot A,B) PME
Fig. 4. Calibration plot for Fe3+ selective polymeric membrane electrodes based on L1, L2 and L3.
Co2+ Ni2+ Ce3+ Pb2+ Ca2+ Al3+ Cd2+ Cu2+
4.89 3.89 7.07 1.20 2.18 7.76 9.77 3.16
Metal ions
CGE × × × × × × × ×
10−4 10−4 10−4 10−4 10−4 10−5 10−4 10−4
1.54 7.94 1.73 3.71 5.62 1.86 2.23 6.45
Selectivity coefficient (KPot A,B) PME
× × × × × × × ×
10−4 10−5 10−4 10−5 10−5 10−5 10−4 10−5
Zn2+ K+ Cr3+ Na+ La3+ Ag+ Mn2+ Fe2+
6.16 1.05 4.37 1.48 1.15 1.82 8.70 2.18
CGE × × × × × × × ×
10−4 10−4 10−4 10−4 10−3 10−4 10−5 10−3
1.65 3.16 1.12 4.46 2.45 5.13 2.04 5.27
× × × × × × × ×
10−4 10−5 10−4 10−5 10−4 10−5 10−5 10−4
K.R. Bandi et al. / Materials Science and Engineering C 36 (2014) 187–193
Fig. 5. Calibration characteristics of the electrodes (PME and CGE) based on L2.
a series of Fe3 + ion solutions, each increasing in concentration by a factor of 10, was 13 s for proposed PME and 10 s for CGE. The potentials generated by the electrodes remained stable for about ~ 5 min after which a slow divergence was recorded. 3.5. Effect of soaking time and lifetime It is well established that the loss of membrane components due to chemical process at the membrane sample interface is the main cause for the limited lifetime of neutral carrier based ion selective electrodes [28]. To determine the shelf life of the electrodes, their performance was monitored daily over a period of 12 weeks, their potential response plots were drawn on a daily basis and their performance characteristics measured. No significant change was noticed for 5 weeks for PME and 6 weeks for CGE. After this period, performance of the electrodes started deteriorating with the slope becoming smaller and the detection limit becoming higher. However it is important to mention that the electrodes were kept equilibrated with a 1.0 × 10−2 mol L−1 Fe3+ solution when not in use. 3.6. Effect of pH on electrode performance The pH dependence of the best polymeric membrane electrode and coated graphite electrodes based on ionophore L2 was examined at a 1.0 × 10−3 mol L−1 concentration of Fe3+ ions. The pH of the solution was varied by the small addition of a 0.1 mol L−1 solution of either HCl or NaOH. The potential remains constant over the pH range for proposed PME 2.0–5.0 and 1.5–6.5 for CGE. Therefore, the same was taken as the working pH range of the electrodes. The significant change in potential response observed at a lower pH may be due to the
191
Fig. 6. Potentiometric titration curve of 25 mL of 1.0 × 10−4 mol L−1 solution of Fe3+ ion with 1.0 × 10−3 mol L−1 EDTA at pH 6.0 using PME and CGE as indicator electrode.
interference of hydrogen ions. On the other hand, the observed potential drift at higher pH values could be due to the hydrolysis of Fe3+. 3.7. Effect of interfering ions on electrode performance The most important feature of an ISE is its selectivity which is measured in terms of selectivity coefficient. The potentiometric selectivity coefficient values for these electrodes were determined by the IUPAC recommended fixed interference method (FIM) [29]. For this purpose, a fixed concentration of interfering ion (aB = 1.0 × 10−2 mol L− 1) was added to the primary Fe3+ ion solutions ranging from 1.0 × 10−9 to 1.0 × 10−2 mol L−1 and the potentials were measured. The potential values obtained were plotted versus the activity of the Fe3+ ion. The linear portions of the potential response curve were extrapolated and the value of a3+ Fe was obtained from the intersection point. Potentiometric selectivity coefficients were then calculated using the expression: Pot
KFe3þ ;B ¼
aFe3þ : ðaB ÞZA =ZB
ð2Þ
The potentiometric selectivity coefficient values given in Table 2 indicate that the electrodes are highly selective to Fe3+ over a number of monovalent (Ag+, Na+, K+), divalent (Pb2+, Co2+, Ni2+, Zn2+, Cu2+, Ca2+) and trivalent (Ce3+, Al3+, La3+, Cr3+) ions. 3.8. Comparative performance characteristics of polymeric membrane electrode (PME) and coated graphite electrode (CGE) The investigations on PVC based membranes of three ligands, viz. L1, L2 and L3 have shown that they act as Fe3+ ion-selective electrodes. However, of the three ligands, the electrode with the membrane composition
Table 3 Response characteristics of Fe(III) ion selective PME and CGE. Properties
Optimized membrane composition Soaking time Working concentration range (mol L−1) Detection limit (mol L−1) Slope (mV decade−1 of activity) Response time (s) Life span pH range
Electrode response PME
CGE
L2(3):NaTPB(2.5):o-NPOE(56):PVC(38.5) 2 days in 0.01 mol L−1 Fe(NO3)3 6.7 × 10−7–1.0 × 10−1 1.4 × 10−7 19.6 ± 0.5 13 5 weeks 2.0–5.0
S2(3):NaTPB(2.5):o-NPOE(56):PVC(38.5) 2 days in 0.01 mol L−1 Fe(NO3)3 8.3 × 10−8–1.0 × 10−1 2.3 × 10−8 19.5 ± 0.4 10 6 weeks 1.5–6.5
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of L2:NPOE:NaTPB:PVC in the ratio of 3:56:2.5:38.5 (w/w; mg) performs best in terms of all parameters. As we know, the replacement of the inner solution of PME by a solid substance like graphite rod in coated graphite electrode (CGE) inhibits the leakage from the internal solution into the test solution and improves the characteristic properties of the electrode along with its selectivity [30,31]. Therefore, it was decided to prepare a coated graphite electrode using L2 as the ionophore with the membrane composition that is the same as that of the best PME electrode. It can be seen from Fig. 5 that CGE generated a linear potential response to Fe 3 + ion over a wide working concentration range of 8.3 × 10− 8–1.0 × 10− 1 mol L− 1 with a Nernstian slope of 19.5 ± 0.4 mV decade− 1 of activity and a low detection limit of 2.3 × 10− 8 mol L − 1 . The comparison of the performance of two electrodes in Table 3 clearly shows that CGE is superior to PME in terms of all performance parameters i.e., (i) a wide working concentration range, (ii) a lower detection limit, (iii) a slope that tends to be more closer to the Nernstian value and (iv) a wider pH range. Further it is seen from Table 2 that even the selectivity of CGE is better than that of PME.
Table 5 Determination of iron in water and vegetables by the proposed PME and CGE. Sample
Amount of Fe3+ determineda ± SD (ppm) PME
Tap water Waste water River water Potato Brinjal Spinach Apple Potato a
0.65 4.1 1.4 2.39 3.47 3.07 1.05 2.39
CGE ± ± ± ± ± ± ± ±
0.5 0.7 0.6 0.03 0.03 0.03 0.03 0.03
0.77 4.6 1.6 2.41 3.49 3.10 1.08 2.41
AAS ± ± ± ± ± ± ± ±
0.4 0.6 0.3 0.03 0.03 0.03 0.03 0.03
0.81 4.4 1.9 2.43 3.51 3.12 1.11 2.43
± ± ± ± ± ± ± ±
0.6 0.4 0.5 0.03 0.03 0.03 0.03 0.03
Mean value ± standard deviation (three measurements).
4. Analytical applications
The crucible was washed properly with aqua regia and the washing was also collected in the same beaker. Deionized and distilled water were added and the contents were heated for 10 min. After this, the solution was filtered and the filtrate was collected in a 100 mL volumetric flask. Deionized and distilled water were used to make up the volume. The amounts of Fe3+ determined with the help of the PME and CGE electrodes are given in Table 5.
4.1. Titration with EDTA
4.4. Determination of Fe3+ in medicinal plants
A potentiometric titration of 25 mL of 1.0 × 10−4 mol L− 1 Fe3 + ions against 1.0 × 10−3 mol L−1 EDTA at pH 5.0 was carried out using these electrodes. The titration plots (Fig. 6) are found to be sigmoid shaped and the inflection point of the plot corresponds to a 1:1 stoichiometry of Fe3+–EDTA complex.
Leaves of Adhatoda vasica (Arusa), Ocimum sanctum (Tulsi), Withania somnifera (Ashwagandha) and Cassia fistula (Amaltas) were collected from the Haridwar region and dried at 80 °C overnight in an oven. The samples were powdered in an agate mortar. 2.0 g of dried powdered plant parts was digested with a 5:1 mixture of nitric acid (25% v/v) and perchloric acids, followed by a controlled heating until the evolution of gases ceased. After digestion few drops of concentrated HCl were added. The solution was heated and filtered. After the digestion residue was removed by filtration, the filtrate was neutralized by NH4OH and the volume was made up to 100 mL. Fe3+ ion was determined with the help of CGE by direct potentiometry and the results are given in Table 6. It is clear from Tables 4, 5 and 6 that the results obtained by the use of electrode are in good agreement with those obtained by AAS.
4.2. Determination of Fe3+ ion in soil The soil samples (2.0 g) were digested with 10 mL nitric acid. The solution was heated until the evolution of gases stopped. Then a mixture of nitric acid, perchloric acid and conc. hydrofluoric acid (5:3:5) was added followed by controlled heating until evolution of white fumes stopped. After digestion, the residue was removed by filtration and the filtrate was made up to 100 mL. The electrode was used to determine Fe3+ in this solution and the results are compiled in Table 4.
5. Conclusions 4.3. Determination of Fe3+ ion in water and in vegetables The proposed electrode was also used for the monitoring of Fe3 + ions in the Ganga (Roorkee) and Yamuna (Delhi) river waters and also in the waste water taken from the Chemistry Department of the institute. The samples were collected from different locations and treated with 1 M nitric acid and the pH of these samples was adjusted to 5.0. The amounts of Fe3 + determined with the help of the PME and CGE electrodes are given in Table 5. Apple and vegetables (potato, brinjal and spinach) were taken and washed properly with deionized water. These were subsequently cut with a knife and weighed instantly. The samples were placed in crucibles, which were already cleaned and weighed. These crucibles were then placed in a muffle furnace where the temperature was slowly raised from 200 to 500 °C, for 5 h; the process was repeated till a constant weight was achieved. Gray white ash was kept in sample bottles. Subsequently the ash samples were transferred into the 100 mL beaker. Table 4 Determination of iron in soil samples by the proposed PME and CGE. Sample
Soil sample 1 (Haridwar) Soil sample 2 (Rishikesh) a
The investigations on a large number of membranes of L1, L2 and L3 have shown that they can act as Fe3+ ion-selective electrodes. However, of the three ligands, PME having a membrane of L2 with a composition of L2:o-NPOE:NaTPB:PVC in the ratio of 3:56:2.5:38.5 (w/w; mg) performs best as it exhibits widest working concentration range (1.0 × 10− 1 to 6.7 × 10− 7 mol L−1), Nernstian compliance (19.6 ± 0.5 mV decade−1 of activity) and high selectivity for Fe3+ ions. The performance of CGE having a membrane of the same composition was prepared and found better as compared to PME, in all performance parameters. The comparison with reported electrodes (Table 7) shows that the proposed CGE is superior to the reported electrodes as it shows the widest concentration range, the lowest detection limit, a Nernstian slope and a wider pH range. Even the selectivity of the electrode is comparable to the reported electrode overall. Thus the
Table 6 Determination of iron in medicinal plants by the proposed PME and CGE. Sample
PME
Amount of Fe3+ determineda ± SD (mg Kg−1) PME
CGE
AAS
2.19 ± 0.03 2.29 ± 0.03
2.21 ± 0.03 2.32 ± 0.03
2.24 ± 0.02 2.30 ± 0.04
Mean value ± standard deviation (three measurements).
Amount of Fe3+ determineda ± SD (mg Kg−1)
Adhatoda vasica Ocimum sanctum Withania somnifera Cassia fistula a
187 466 485 586
± ± ± ±
CGE 3 2 3 4
188 470 489 587
AAS ± ± ± ±
Mean value ± standard deviation (three measurements).
2 3 4 3
190 473 491 590
± ± ± ±
3 4 3 5
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193
Table 7 Comparison of response characteristics of proposed Fe3+ ion selective CGE over with reported electrodes. The bold entries indicates the results of present work. Ref. no.
Linear range (mol L−1)
Detection limit (mol L−1)
Slope (mV decade−1 of activity)
pH range
Response time (s)
15 16 17 18 19 17 18 19 20 CGE
3.5 × 10−6–4.0 1.0 × 10−7–1.0 6.3 × 10−6–1.0 1.0 × 10−7–1.0 1.0 × 10−6–1.0 3.0 × 10−7–1.0 1.0 × 10−7–1.0 1.0 × 10−6–1.0 9.1 × 10−6–1.0 8.3 × 10−8–1.0
2.5 × 10−6 5.0 × 10−8 5.0 × 10−6 8.6 × 10−8 5.0 × 10−7 2.0 × 10−7 4.8 × 10−8 6.8 × 10−7 NM 2.30 × 10−8
28.5 ± 0.5 19.9 ± 0.3 20.0 19.6 60 ± 5 20.2 ± 0.8 19.4 ± 0.4 19.4 ± 0.5 21.2 19.5 ± 0.4
4.5–6.5 3.0–6.3 3.5–5.5 1.6–4.3 1.3–3.5 1.8–5.8 2.3–3.4 2.2–4.8 3.2–4.8 1.5–6.5
b15 b12 15 b10 25 b10 b10 b15 20 10
× 10−2 × 10−1 × 10−1 × 10−2 × 10−2 × 10−2 × 10−2 × 10−1 × 10−1 × 10−1
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