Materials Science and Engineering C 33 (2013) 626–633

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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec

A comparative study on fabrication of Mn 2 + selective polymeric membrane electrode and coated graphite electrode Ashok Kumar Singh ⁎, Koteswara Rao Bandi, Anjali Upadhyay, A.K. Jain Department of Chemistry, Indian Institute of Technology-Roorkee, Roorkee-247 667, India

a r t i c l e

i n f o

Article history: Received 23 January 2012 Received in revised form 1 September 2012 Accepted 26 October 2012 Available online 2 November 2012 Keywords: Mn2 + ion selective electrodes Potentiometric electrodes Coated graphite electrodes

a b s t r a c t Poly(vinyl chloride)-based membranes of two ligands 2,4-bis(2-acetoxybenzylamino)-6-phenyl-1,3,5-triazine (L1) and N2,N4-di(cyanoethyl)-2,4-bis(2-acetoxybenzylamino)-6-phenyl-1,3,5-triazine (L2) were fabricated and explored as Mn2+ ion selective electrodes. The performance of the polymeric membranes electrodes of ionophores with different plasticizers (dibutylphthalate, benzoic acid, o-nitrophenyloctyl ether, 1-chloronapthalene and tri-nbutylphosphate) and anion excluders (sodium tetraphenylborate and potassium tetrakis p-(chloro phenyl)borate) was looked in to and the better results were obtained with the membrane having composition L2: NaTPB: DBP: PVC as 6: 3: 56: 35 (w/w; mg). The coated graphite electrode (CGE) with same composition was also fabricated and investigated as Mn2+ selective electrode. It was found that CGE showed better response characteristics than PME. The potentiometric response of CGE was independent of pH in the range 3.0–9.0 exhibiting the Nernstian slope 29.5±0.3 mV decade−1 of activity and working concentration range 4.1×10−7–1.0×10−1 mol L−1 with a limit of detection 6.7×10−8 mol L−1. The electrode showed a fast response time of 12 s with a shelf life of 105 days. The proposed CGE could be successfully used for the determination of Mn2+ ions in different water, soil, vegetables and medicinal plants also used as an indicator electrode in potentiometric titration with EDTA. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Manganese is an important metal widely used in steel and alloy industries to improve hardness, stiffness and strength [1]. Its compounds are widely used in dry-cell batteries, matches, fireworks, catalyst in the chlorination of organic compounds and in animal feed, fertilizer, livestock nutritional supplement, glazes, varnishes and ceramics. Manganese is an essential metal promoting many physiological functions viz. lipid, carbohydrate and protein metabolism, immune system functions, blood clotting, osteogenesis in humans and animals. Thus diet containing trace amounts of manganese is considered nutritionally necessary [2–4]. The chronic intake of manganese produces many toxic effects, which includes injury to central nervous system creates Parkinson's disease [5–7]. Some other toxic effects of manganese intake are impotence, loss of libido, adverse effect on eye, hand coordination, reduction in visual effect time and steadiness. The widespread use of manganese causes its presence in environment at sufficient concentration level and therefore its monitoring is essential. A number of techniques such as inductively coupled plasma-mass spectrometry (ICP-MS) [8], anodic stripping voltammetry [9–11], atomic absorption spectroscopy (AAS) [12], X-ray fluorescence [13], ICP-AES, glassy carbon electrodes [14–16] and cathodic stripping voltammetry [17–19] are used for its determination. These techniques are sophisticated, time consuming and require large infrastructure back up and ⁎ Corresponding author. Tel.: +91 9412978289. E-mail address: [email protected] (A.K. Singh). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.10.007

expertise and are therefore cumbersome for the analysis of Mn2+ in large number of samples. Thus there is a critical need for the development of selective, portable, inexpensive diagnostic tools for the determination of Mn2+ to analyze large number of samples in field. Recently ion selective electrodes have proved to be promising analytical tools as they provide procedures for fast, convenient analysis with minimum chemical manipulations and can even be adapted to online measurements. ISEs are thus an ideal choice for the analysis of large number of environmental samples. As such there has been intensive activity in developing ion selective electrodes for many metal ions, but there are very few electrodes reported for Mn2+ determination [20–26]. However, they are not highly sensitive, selective and show many other shortcomings viz. narrow working concentration range, non Nernstian slope. Therefore it is apparent that selective, sensitive ion selective electrodes with better performance characteristics are still to be developed. Efforts in this direction were initiated. To develop an electrode an appropriate ionophore having the property of showing high affinity for a particular metal ion for which the electrode is to be developed and poor affinity for all other metals is required. However such materials are not available in abundance, which has made the development of ISE a slightly difficult job. Nevertheless in recent years some new groups of compounds such as Schiff bases [27–29], porphyrins [30,31], calixaranes [32,33], macrocyclic compounds [34,35] and other group of compounds [36,37] have been developed which have a coordinating cavity and due to which they tend to show high affinity for some particular metals and poor for others. Among these groups, Schiff bases are the item of choice, because it is easy to form them with varying cavity size. Therefore we have opted up for Schiff

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bases as ionophores to attempt to develop Mn2+ selective electrodes. The literature review [38–40] reveals that recently reported some Schiff bases form complexes with large number of metals. Any Schiff base forming complexes with metals is a potential ionophore provided one of the metal complexes has high stability as compared to complexes with other metals. These Schiff bases were looked into and preliminary investigation showed that they don't form complexes of high stability with any particular metal. So they are not good ionophores for ISE. Attempts were then made to modify Schiff bases. Thus Schiff base 2,4-bis(2-hydroxybenzylidenamino)-6-phenyl-1,3,5-triazine (S) was modified by acetylation followed by reduction. The reduced form 2,4bis(2-acetoxybenzylamino)-6-phenyl-1,3,5-triazine (L1) was further modified to obtain a pendent compound N2,N4-di(cyanoethyl)-2,4bis(2-acetoxybenzylamino)-6-phenyl-1,3,5-triazine (L2). The pendent arms are likely to change the cavity size. The reduced form and the pendent form of parent Schiff base (S) are likely to have different cavities and hopefully may form strong complexes with some metals. The preliminary complexometric investigations of L1 and L2 have shown that they form strong complexes with Mn2+ and weak with others. Thus L1 and L2 are the potential ionophores for preparing Mn2+ selective electrode. Polymeric membrane electrode and coated graphite electrodes based on L1 and L2 were prepared and studied as Mn2+ ion selective electrode and the results are presented in the present communication.

2.2.1. Synthesis of 2,4-bis(2-acetoxybenzylamino)-6-phenyl-1,3,5-triazine (L1) The acetylated Schiff base (0.01 mol L −1) was dissolved in 20 mL of anhydrous methanol; the flask was kept in an ice bath, to this sodium borohydride (0.01 mol) was added in small portions with continuous stirring and it was stirred for 8 h. The compound was separated with DCM, washed with water and recrystallized in methanol. Yield: 87%. Anal. Calc. for C27H25N5O4: C, 67.07; H, 5.21; N, 14.48; O, 13.24. Found: C, 68.61; H, 5.79; N, 13.01. IR (KBr, cm −1) 3217 (\NH), 1H NMR (CDCl3, 500 MHz): δ 2.16 (s, 6H), 4.28 (d, 4H), 4.82 (t, 2H), 6.32–8.49 (m, 13H).

2. Experimental

2.3. Fabrication of electrode

2.1. Reagents

It has been reported by various workers that the membrane ingredients (ionophores, PVC, plasticizers and anionic excluder) and composition of membrane ingredients (relative amounts of membrane ingredients) significantly affects the performance of the membrane. Various membrane ingredients viz. ionophores (L1 and L2), PVC, plasticizers (BA, DBP, DOP, 1-CN, TBP and o-NPOE) and anionic excluder (NaTPB and KTpClPB) were used for the fabrication of the membrane. These ingredients were dissolved in various amounts in 3 mL of THF. The ingredients were dissolved and mixed vigorously with a glass rod. By varying the relative amounts of membrane ingredients, a number of membranes with different compositions were prepared and studied. Membrane to membrane reproducibility was assured by carefully following the optimum condition of fabrication. The membrane that gave reproducible results and showed best performance was selected for further studies. The spectroscopic grade graphite electrodes of 5 mm diameter and 15 mm length were polished and a copper wire was glued to one end of the electrode. The other end of the graphite rod was dipped in the concentrated membrane solution, prepared as above and left overnight. The solvent evaporated off and a thin PVC film formed on the graphite electrode surface. The electrode was then sealed in a glass tube with epoxy resin, taking care that the electrode portion having membrane layer remains exposed. A membrane was formed on the graphite surface, and the electrode was allowed to

Reagent grade sodium tetraphenylborate (NaTPB), potassium tetrakis p-(chloro phenyl)borate (KTpClPB), dibutylphthalate (DBP), benzoic acid (BA), o-nitrophenyloctyl ether (o-NPOE), 1-chloronapthalene (1-CN), tri-n-butylphosphate (TBP), tetrahydrofuran (THF) and high molecular weight polyvinylchloride were procured from Merck and used as received. 2,4-diamino-6-phenyl-1,3,5-triazine was procured from SigmaAldrich, salicyladehyde and acrylonitrile from LOBA Chemie. and sodiumborohydride (Laboratory reagent) from Rankem. All the nitrate and chloride salts of cations used were of analytical grade (LOBA Chemie.) and used without further purification. Millipore water was used for the preparation of metal salt solutions of different concentrations by diluting stock solution (0.1 mol L −1).

2.2. Synthesis The ligands 2,4-bis(2-acetoxybenzylamino)-6-phenyl-1,3,5-triazine (L1) and N2,N4-di(cyanoethyl)-2,4-bis(2-acetoxybenzylamino)-6phenyl-1,3,5-triazine (L2) were prepared by using Schiff base 2,4bis(2-hydroxybenzylidenamino)-6-phenyl-1,3,5-triazine (S) (Scheme 1) [41]. The Schiff base was acetylated by general method using acetyl chloride in pyridine [42].

2.2.2. Synthesis of N2,N4-di(cyanoethyl)-2,4-bis(2-acetoxybenzylamino)-6phenyl-1,3,5-triazine (L2) The ligand L1 was dissolved in 20 mL of anhydrous methanol. To this 10 mL of K2CO3 in methanol was added and stirred for 3 h. Acrylonitrile was then added drop by drop and the solution was stirred for 5 h and refluxed for 8 h. The volume of reaction mixture was reduced and then it was kept overnight in refrigerator. The compound was filtered and washed with cold methanol. Yield: 44%. Anal. Calc. for C33H31N7O4: C, 67.22; H, 5.30; N, 16.63; O, 10.85. Found: C, 66.45; H, 5.11; N, 16.04. IR (KBr, cm −1) 3221 (\NH), 2257 (\CN), 1H NMR (CDCl3, 500 MHz): δ 2.20 (s, 6H), 2.66 (t, 4H), 3.42 (t, 4H), 4.31(s, 4H), 6.34–8.52 (m, 13H).

Scheme 1. Synthesis of ligands 2,4-bis(2-acetoxybenzylamino)-6-phenyl-1,3,5-triazine (L1) and N2,N4-di(cyanoethyl)-2,4-bis(2-acetoxybenzylamino)-6-phenyl-1,3,5-triazine (L2).

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stabilize overnight. Preliminary investigations revealed that the CGE prepared with membrane of the same composition that performed best than polymeric membrane electrode, have good results for CGE.

change sharply at the point where the ligand to cation mole ratio is one. It is concluded that Mn 2 + forms 1:1 complex with both the ligands L1 and L2.

2.4. Equilibration of membranes and potential measurements

3.2. Determination of formation constant by conductometric method

The ratio of the membrane ingredients, time of contact, and equilibration time of the electrode gives reproducibility and stability. Three days of equilibration of PME and CGE with 0.01 mol L −1 Mn 2 + solution was good enough to produce reproducible and stable potentials. The potentials have been measured by varying the concentration of Mn(NO3)2 in test solution in the range 1.0 × 10 −9 to 1.0 × 10 −1 mol L −1. The potential measurements of polymeric membrane electrodes and coated graphite electrodes were carried out at 25 ± 1 °C using saturated calomel electrodes (SCE) as reference electrodes with the following cell assembly:

The conductometric data was further used to calculate the stability of complexes [44]. By using a non-linear least-square programs using Genetic Algorithm [45], the formation constants (log Kf) of the complexes were calculated and are given in Table 1. It is seen from the table that the stability of Mn2+ complexes is highest compared to other metals showing that the ligands under consideration show maximum affinity for Mn2+ and can therefore work as a selective ionophores for it.

−1

Internal SCE || Internal solution of 0.1 mol L Mn(NO3)2 | PVC membrane | test solution || External SCE (for PME) Coated graphite electrode | test solution || External SCE (for CGE). Activity coefficients were calculated according to the Debye– Huckel procedure, using the following equation [43]. logγ ¼ −0:511z

2

1=2

μ
 1 þ 1:5μ 2 −0:2μ

! ð1Þ

where, μ is the ionic strength and z is the valency. 3. Results and discussion

3.3. Determination of formation constant by sandwich membrane method The ion–ionophore complex formation constants were also evaluated by a potentiometric method using sandwich membrane method [46]. In this method, a sandwich membrane is prepared by fusing two membranes, with only one containing the ionophore. This membrane electrode is brought in contact with the aqueous metal ion solution of identical concentration on both sides, and the potential measured. The cell potential of another membrane having no ionophore was also measured. As reported, the membrane potential EM is determined by subtracting the cell potential for a membrane without ionophore from that of the sandwich membrane. The formation constant is then calculated by using the following equation:  βILn ¼

LT −

3.1. Complexation study In order to understand the complexation property of the ligands L1 and L2 with different metals, conductometric studies were carried out in an acetonitrile solution, at 25 ± 0.05 °C. In this experiment, 25 mL of 1.0 × 10 −4 mol L −1 cation solution was titrated against a 1.0 × 10 −3 mol L −1 ionophore solution in acetonitrile. The change in conductance of Mn 2+ solution as a function of [L]/[Mn 2+] ratio is shown in Fig. 1. It is obvious from this figure that the addition of the ligand to metal caused a rather large and continuous decrease in the conductance of the solution, indicating the lower mobility of the complexed cations compared to the free ones. It is seen from Fig. 1 that the slopes of the corresponding conductance–mole ratio plots

nRT ZI

−n

  E zF exp M I : RT

ð2Þ

Where LT is the total concentration of ionophore in the membrane segment, RT is the concentration of lipophilic ionic site additives, zI is the charge on the ion I, n is the ion–ionophore complex stoichiometry, and R, T and F are the gas constant, the absolute temperature and the Faraday constant respectively. The stability constants of different complexes calculated by sandwich membrane method are also given in Table 1. It is seen from the table that formation constant determined by both methods are comparable and are in close agreement. The maximum formation constant shown by Mn 2+ complexes with both the ligands is 5.53 and 6.82 for L1 and L2 respectively, whereas the stability constant values for other metal complexes are much smaller, indicating their weak stability. The higher stability of Mn 2+ complexes with L1 and L2 shows the high affinity of these ligands for Mn 2+ ions compared to others. Therefore these ligands

Table 1 Formation constants of ligands L1 and L2. Metal ions

2+

Mn Ni2+ Fe2+ Cd2+ Ce3+ Cr3+ Pb2+ Al3+ Zn2+ Cu2+ Ca2+ Co2+ Fig. 1. Conductometric study of ionophores L1 and L2.

a

Formation constant (log βILn)a ± SD Conductivity method

Formation constant (log βILn)a ± SD Sandwich membrane method

L1

L2

L1

L2

5.58 ± 0.03 4.90 ± 0.07 3.48 ± 0.04 3.32 ± 0.03 2.39 ± 0.02 3.93 ± 0.02 2.13 ± 0.05 3.49 ± 0.07 4.41 ± 0.06 4.56 ± 0.03 2.90 ± 0.08 4.48 ± 0.03

6.74 ± 0.01 5.17 ± 0.06 3.68 ± 0.03 3.79 ± 0.04 2.77 ± 0.03 4.19 ± 0.06 3.27 ± 0.03 3.84 ± 0.02 4.98 ± 0.06 4.72 ± 0.02 3.06 ± 0.06 4.81 ± 0.05

5.73 ± 0.01 4.91 ± 0.09 3.65 ± 0.04 3.68 ± 0.07 2.83 ± 0.06 4.14 ± 0.03 2.27 ± 0.02 3.56 ± 0.04 4.83 ± 0.06 4.68 ± 0.03 2.81 ± 0.05 4.61 ± 0.03

6.82 ± 0.02 5.23 ± 0.05 3.87 ± 0.07 4.01 ± 0.01 3.15 ± 0.07 4.30 ± 0.02 3.39 ± 0.05 3.91 ± 0.02 5.12 ± 0.07 4.78 ± 0.02 3.18 ± 0.09 4.92 ± 0.07

Mean value ± standard deviation (three measurements).

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are used as potential ionophores for preparing Mn 2+ ion selective electrodes. 3.4. Optimization of membrane composition The potential of cell set up with membranes of different compositions of two ionophores L1 and L2 was measured as a function of Mn 2+ ion concentration and plotted against logarithm of Mn 2+ ion activity in Figs. 2 and 3. From these plots performance characteristics i.e., working concentration range (linear potential response concentration range) and slope were evaluated and summarized in Tables 2 and 3. It is seen from plots and Tables 2 and 3 that for electrodes nos. 1 and 13 having membrane of ionophore L1 and L2 respectively, the working concentration range and slope of two electrodes come out to be 5.5 × 10−4–1.0 × 10−1 mol L −1, 34.0 ± 0.4 mV decade−1 of activity and 5.2× 10−4–1.0 × 10−1 mol L −1, 30.3 ±0.6 mV decade−1 of activity, respectively. The working concentration range is narrow and the slope is non Nernstian. Thus the performance of membranes needs to be improved. The attempt was then made to improve the performance by the addition of plasticizers viz. BA, o-NPOE, TBP, DBP and 1-CN to the membrane and the potential plots of membranes having plasticizers are also shown in Figs. 2 and 3 and the performance characteristics also summarized in Tables 2 and 3. It is observed that addition of plasticizers improves the performance of electrode (electrode nos. 2–6 for L1 and electrode nos. 14–18 for L2) in all cases i.e. working concentration range becomes wider and slope become near Nernstian. It is further seen that electrode no. 6 having 1-CN as plasticizer and sensor no. 18 with DBP as plasticizer perform best as they exhibit widest working concentration range but they show non Nernstian slope. Attempts were then made to bring the slope to Nernstian value by the addition of anion excluder viz. NaTPB and KTpClPB. Of the two additives, NaTPB produced better effects, electrode nos. 7 and 19 containing 3% (w/w) NaTPB perform best in terms of wide concentration range with Nernstian slope and low detection limit. The results of Tables 2 and 3 show that electrode no. 7 (Table 2) of composition (L1: NaTPB: 1-CN: PVC) ≡ (5: 3: 55: 37) and electrode no. 19 with membrane composition (L2: NaTPB: DBP: PVC) ≡ (6: 3: 56: 35) give the best performance in terms of working concentration range, slope and lower detection limit. Next the effect amount of ionophore and NaTPB was also seen. The performance characteristics of electrode nos. 9–12 and 21–24 show that the change in amount of ionophore and anion excluder does not in any way improve the performance of the membranes.

Fig. 2. Plots showing variation of membrane potential with the concentration of Mn2+ ion based on L1 with different plasticizers.

Fig. 3. Plots showing variation of membrane potential with the concentration of Mn2+ ion based on L2 with different plasticizers. Calibration characteristics of Mn2+ ion-selective electrode (PME and CGE) based on L2.

3.5. Selectivity The selectivity coefficient values determined by fixed interference method (FIM) [47] are given in Table 4. It is expected that there ought

Table 2 Optimization of membrane composition and their potentiometric response for Mn2+ ion-selective electrode based on ionophore L1. S. no

1 2 3 4 5 6 7 8 9 10 11 12

Membrane composition (w/w; mg) L1

NaTBP

KTpClPB

BA

DBP

o-NPOE

TBP

1-CN

PVC

5 5 5 5 5 5 5 5 5 5 6 4

– – – – – – 3 – 4 2 3 3

– – – – – – – 3 – – – –

– 55 – – – – – – – – – –

– – 55 – – – – – – – – –

– – – 55 – – – – – – – –

– – – – 55 – – – – – – –

– – – – – 55 55 55 54 54 56 54

92 37 37 37 37 37 37 37 37 37 37 37

Working concentration range (mol L−1)

Slope (mV decade−1 of activity)

Detection limit (mol L−1)

5.5 × 10−4–1.0 × 10−1 1.9 × 10−4–1.0 × 10−1 8.3 × 10−4–1.0 × 10−1 1.0 × 10−4–1.0 × 10−1 6.6 × 10−5–1.0 × 10−1 3.9 × 10−5–1.0 × 10−1 7.8 × 10−6–1.0 × 10−1 6.7 × 10−4–1.0 × 10−1 4.4 × 10−5–1.0 × 10−1 3.5 × 10−5–1.0 × 10−1 1.9 × 10−4–1.0 × 10−1 5.1 × 10−5–1.0 × 10−1

34.0 ± 0.4 35.3 ± 0.5 28.7 ± 0.3 36.0 ± 0.4 33.5 ± 0.6 28.2 ± 0.4 29.0 ± 0.3 27.6 ± 0.5 28.8 ± 0.7 31.5 ± 0.5 32.8 ± 0.4 30.6 ± 0.6

1.4 × 10−4 4.3 × 10−5 2.8 × 10−5 5.8 × 10−5 2.6 × 10−5 8.3 × 10−6 2.8 × 10−6 1.9 × 10−4 1.7 × 10−5 9.7 × 10−6 6.2 × 10−5 2.7 × 10−5

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Table 3 Optimization of membrane composition and their potentiometric response for Mn2+ ion-selective electrode based on ionophore L2. S. no

13 14 15 16 17 18 19 20 21 22 23 24

Membrane composition (w/w; mg) L2

NaTPB

KTpClPB

BA

NPOE

TBP

1-CN

DBP

PVC

6 6 6 6 6 6 6 6 6 6 5 7

– – – – – – 3 – 4 2 3 3

– – – – – – – 3 – – – –

– 56 – – – – – – – – – –

– – 56 – – – – – – – – –

– – – 56 – – – – – – – –

– – – – 56 – – – – – – –

– – – – – 56 56 56 55 57 57 55

91 35 35 35 35 35 35 35 35 35 35 35

to be a qualitative relation between formation constants and selective coefficients. All metals which form weaker complexes with ligands (low formation constant values) show less affinity for the ligands as compared to Mn2+ having maximum affinity for the ionophore. Thus the membrane of the ionophore should facilitate the transport of Mn2+ ions in preference to other metal ions for which the ionophore shows less affinity. Thus there should exist an inverse relationship between the formation constant values and their selectivity. In the present case the formation constant values decrease in order (Ni2+ >Zn2+ >Co2+ > Cu2+ >Cr3+ >Cd2+ >Al3+ >Fe2+ >Pb2+ >Ca2+ >Ce3+), whereas the selectivity coefficient values decrease in reverse order (Na+ >K+ > Ce 3 + > Ca 2 + > Pb 2 + > Fe 2 + > Al 3 + > Cd 2 + > Cr 3 + > Cu 2 + > Co 2 + > Zn 2 + > Ni 2 +). The values of selectivity coefficient are much smaller than unity indicating that the electrode is significantly selective over interfering metal ions listed in Table 4. Therefore the electrode could be used to determine the Mn2+ ion in the presence of these ions by direct potentiometry. The utility of the electrode has been successfully demonstrated by determining it in various samples. 3.6. Potential–pH profile of the electrode The effect of pH on the performance of the both PME and CGE was studied at two concentrations, 1.0 × 10 −3 and 1.0 × 10 −4 mol L −1 of Mn 2 + ion. The potential response was measured for PME and CGE in the pH range 1.0–12.0. The potential response plots (Fig. 4) show that the potential response for PME remains constant over pH range 4.0–9.0 and 3.0–9.0 for PME and CGE respectively. The change in potentials at lower pH values can be attributed to the electrode responding to hydrogen ions. On the other hand, the observed potential drift at higher

Working concentration range (mol L−1)

Slope (mV decade−1 of activity)

Detection limit (mol L−1)

5.2 × 10−4–1.0 × 10−1 1.6 × 10−4–1.0 × 10−1 5.4 × 10−5–1.0 × 10−1 3.9 × 10−5–1.0 × 10−1 1.2 × 10−5–1.0 × 10−1 5.1 × 10−6–1.0 × 10−1 8.1 × 10−7–1.0 × 10−1 1.4 × 10−4–1.0 × 10−1 5.1 × 10−4–1.0 × 10−1 3.8 × 10−6–1.0 × 10−1 6.1 × 10−5–1.0 × 10−1 8.6 × 10−4–1.0 × 10−1

30.3 ± 0.6 30.0 ± 0.7 28.0 ± 0.4 33.3 ± 0.5 27.4 ± 0.4 30.7 ± 0.6 29.6 ± 0.5 32.5 ± 0.7 31.7 ± 0.5 27.5 ± 0.4 26.1 ± 0.2 29.1 ± 0.6

8.2 × 10−5 3.7 × 10−5 7.9 × 10−6 5.4 × 10−6 3.1 × 10−6 1.1 × 10−6 2.1 × 10−7 8.1 × 10−5 2.4 × 10−4 1.1 × 10−6 2.9 × 10−5 4.5 × 10−4

pH values appears due to the formation of some hydroxyl complexes of Mn2+ ion in solution. 3.7. The dynamic response time behavior and shelf life time of the proposed electrode Dynamic response time is an important factor for any ion-selective electrode. To measure the dynamic response time of the proposed electrode the concentration of the test solution was successively changed from 1.0 × 10−5 to 1.0 × 10 −1 mol L −1. Dynamic response time is the average time required for the electrodes to reach a potential response within ±1 mV of the final equilibrium value after successive immersion in a series of Mn2+ ion solution, each having a 10-fold difference in concentration of 15 s for PME (Fig. 5) and 12 s for CGE (Fig. 6). Potentials generated by the developed electrodes remain stable for about ~5 min after which a slow drift is recorded. The much higher electrical conductivity of the copper wire (in CGE) than that of the internal solution (in PME) is expected to result in lower response time of the CGE in comparison with the PME. The degradation of the sensitivity in the polymeric membrane may be dependent upon the lipophilicity and chemical stability of the ionophores, which can result in the ionophore bleeding from the membrane. Since Mn2+ chelates of ionophores are the compounds having high lipophilicity, the membranes containing them should provide very low bleeding of the ionophore. The membrane

Table 4 Selectivity coefficients of various interfering ions for Mn2+ ion-selective electrodes. Metal ions

Pot Selectivity coefficient (KA,B )

PME 2+

Ni Zn2+ Co2+ Cu2+ Cr3+ Cd2+ Al3+ Fe2+ Pb2+ Ca2+ Ce3+ K+ Na+

CGE −3

7.9 × 10 6.1 × 10−3 5.6 × 10−3 4.8 × 10−3 3.3 × 10−4 3.0 × 10−4 2.9 × 10−4 2.5 × 10−4 2.0 × 10−4 1.6 × 10−4 9.0 × 10−5 8.1 × 10−5 6.8 × 10−5

7.2 × 10−3 2.5 × 10−3 2.0 × 10−3 1.4 × 10−3 2.5 × 10−4 2.3 × 10−4 2.0 × 10−4 1.7 × 10−4 1.5 × 10−4 1.3 × 10−4 7.9 × 10−5 6.6 × 10−5 5.7 × 10−5

Fig. 4. Influences of pH of the test solution on the potential response of the PME and CGE in the presence of 1.0 × 10−3 mol L−1 and 1.0 × 10−4 mol L−1 Mn2+ ion.

A.K. Singh et al. / Materials Science and Engineering C 33 (2013) 626–633

Fig. 5. Dynamic response time of the proposed PME for step changes in the concentration of Mn2+ ion.

could be used over a period of 75 days for PME (electrode no. 19) and 105 days for CGE. However, it is important to emphasize that electrodes be stored in 0.01 mol L−1 Mn2+ ion solution when not in use.

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Fig. 7. Calibration characteristics of Mn2+ ion-selective electrode (PME and CGE) based on L2.

4. Analytical application The CGE is better than PME, all analytical applications are done with CGE only.

3.8. Comparison of polymeric membrane electrode (PME) and coated graphite electrode (CGE)

4.1. Titration with EDTA

2+

The potentials versus logarithm of Mn ion activity plots for PME and CGE are shown in Fig. 7 and their performance is evaluated and summarized in Table 5. It is seen from Table 5, that the performance of CGE is better than PME in all respects i.e., wide working concentration range (4.1 × 10 −7–1.0 × 10 −1 mol L −1), low detection limit (6.7 × 10 −8 mol L −1), low response time (12 s), and longer shelf life (105 days), wider pH range (3.0–9.0) and better selectivity. The improved performance characteristics of the CGE over those of PME presumably originate from the coated graphite technology, where an internal 0.1 mol L −1 Mn 2+ ion solution, in case of PME, has been replaced by a copper wire of much higher electrical conductivity, in the case of CGE. Coated graphite electrodes show significantly the improved performance characteristics as compared to polymeric membrane electrodes. It is well known that the higher limit of detection of PME than that of CGE is mainly due to some leakage of the internal solution into the test solution via polymeric membrane [48].

The proposed Mn2+ ion-selective electrode (CGE) was successfully used as an indicator electrode in potentiometric titration of Mn2+ ion with EDTA. A 1.0 × 10−2 mol L −1 solution of Mn2+ ion (10.0 mL) was titrated with a 2.5 × 10 −2 mol L−1solution of EDTA at pH 6 using CGE. The titration plot (Fig. 8) is of sigmoid shape and the inflection point of the plot corresponds to 1:1 stoichiometry of Mn2+–EDTA complex. The sigmoid shape of the titration plot is an indication of high selectivity of the CGE for Mn2+ ion. 4.2. Determination of Mn 2+ ion in soil and water samples Two samples of soil and water were collected from different locations and analyzed on AAS. The water samples were collected by routine technique and preserved with HNO3, stored in plastic containers and analyzed within 12 h of collection. As the samples contain particulate matter, they were centrifuged and the potentials were measured after

Table 5 Response characteristics of Mn2+ ion selective electrodes based on PME and CGE. Properties

Optimized membrane composition Soaking time

Fig. 6. Dynamic response time of the proposed CGE for step changes in the concentration of Mn2+ ion.

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

S2(6): NaTPB(3): DBP(56): PVC(35)

S2(6): NaTPB(3): DBP(56): PVC(35)

3 days in 0.01 mol L−1 Mn(NO3)2 8.1 × 10−7–1.0 × 10−1

3 days in 0.01 mol L−1 Mn(NO3)2 4.1 × 10−7–1.0 × 10−1

2.1 × 10−7

6.7 × 10−8

29.6 ± 0.5

29.5 ± 0.3

15 75 days 4.0–9.0

12 105 days 3.0–9.0

632

A.K. Singh et al. / Materials Science and Engineering C 33 (2013) 626–633 Table 8 Determination of Mn2+ ions in vegetables by proposed CGE. Samples

ISEa ± SD (mg kg−1)

AASa ± SD (mg kg−1)

Tomato (Solanum lycopersicum) Potato (Solanum tuberosum) Carrot (Daucus carota) Onion (Allium cepa) Garlic (Allium sativum)

23.7 ± 0.5 08.1 ± 0.9 07.0 ± 0.4 14.6 ± 0.6 07.4 ± 0.3

23.9 ± 0.8 08.3 ± 0.5 07.1 ± 0.7 14.5 ± 0.3 07.6 ± 0.9

a

Mean value ± standard deviation (three measurements).

distilled water. By using the proposed electrode, Mn2+ was determined and the values were recorded in Table 8.

4.4. Determination of Mn 2+ ion in medicinal plants

Fig. 8. Potentiometric titration curve of 10 mL 1.0 × 10−2 mol L−1 of solution of Mn2+ ion with 2.5 × 10−2 mol L−1 EDTA at pH 6.0 using CGE as indicator electrode.

adjusting the pH to 5.5. The amount of Mn2+ ion determined with electrode is given in Table 6. The soil samples were digested in a cleaned Teflon beaker by treating weighed 2.0 g of soil samples with 10 mL nitric acid. The solution was heated until the evolution of gasses was completed. Then mixture of nitric acid, perchloric acid and hydrofluoric acid (5:3:5) was added followed by controlled heating until white fumes evolved. The solution was filtered and diluted with deionized water to make a final volume of 25 mL in a volumetric flask. The pH of the samples was adjusted to 5.5 and determined by the electrode. Determined values (Table 7) by the electrode are found to be in close agreement with those determined by AAS. 4.3. Determination of Mn 2+ ion in vegetables Fresh vegetables were washed with double distilled water thoroughly to remove soil from the surface and dried. These vegetables (2 g) are taken and digested with 1:1 (v/v) HNO3 and HCl solution and heated to 100 °C for 12 h. After digestion, the resulting solution was cooled to room temperature and filtered. pH was adjusted to 5.5 by using 10% NaOH and the mixture was finally diluted to 100 mL using double

Table 6 Potentiometric determination of Mn2+ ions in different water samples by CGE. Samples Haridwar Roorkee a

Water sample Water sample Water sample Water sample

1 2 1 2

ISEa ± SD (mg mL−1)

AASa ± SD (mg mL−1)

2.3 ± 0.6 2.6 ± 0.4 1.9 ± 0.5 1.7 ± 0.4

2.2 ± 0.5 2.5 ± 0.2 1.8 ± 0.8 1.6 ± 0.2

Mean value ± standard deviation (three measurements).

Table 7 Potentiometric determination of Mn2+ ions in soil samples by proposed CGE. Samples Haridwar Roorkee a

Soil Soil Soil Soil

sample sample sample sample

1 2 1 2

ISEa ± SD (mg kg−1)

AASa ± SD (mg kg−1)

265.7 ± 0.6 267.2 ± 0.4 240.2 ± 0.5 238.5 ± 0.3

264.6 ± 0.4 268.4 ± 0.6 242.6 ± 0.4 239.8 ± 0.5

Mean value ± standard deviation (three measurements).

The CGE was also used successfully for the determination of Mn2+ ion in some medicinal plants collected from Haridwar region. The leaves of some medicinal plants viz., Tulsi (Ocimum sanctum), Amaltas (Cassia fistula) and Ashwagandha (Withania somnifera) were collected and dried. 2.0 g of dried powdered plant leaves was digested with a 5:1 (v/v) mixture of nitric acid and perchloric acid, followed by controlled heating until the evolution of gasses ceased. After digestion residue was removed by filtration and the filtrate was neutralized by NH4OH and the volume made up to 100 mL. Mn2+ ion was determined with the help of the electrode by direct potentiometry. The results were tabulated in Table 9. It is seen from Tables 6–9 that the Mn 2+ determined by the electrode is in close agreement with that obtained with AAS, thereby reflecting the utility of this electrode for the determination of Mn 2+ in real samples.

5. Conclusion A study of membranes of various compositions has shown that the PME (electrode no. 19) of membrane of composition L2: NaTPB: DBP: PVC≡ 6: 3: 56: 35 (w/w; mg) shows best performance in all parameters. The CGE of the same composition and PME perform best as they exhibit wide working concentration range 8.1 × 10−7–1.0 × 10 −1 mol L−1 and 4.1× 10−7–1.0 × 10−1 mol L −1, near Nernstian slope of 29.6 ± 0.5 mV decade −1 of activity and 29.5 ± 0.3 mV decade−1 of activity, lowest detection limit 2.1 × 10−7 mol L −1 and 6.7 × 10−8 mol L −1 and low response time 15 s and 12 s, respectively. Of the two electrodes CGE performs better than PME with respect to all membrane characteristics. As a result CGE was used successfully as an indicator electrode for the determination of Mn2+ ion by potentiometric titration against EDTA and also to quantify Mn2+ ion in soil, water, vegetables and medicinal samples. That quantification based on this electrode is found to be in close agreement with that determined by AAS showing successful applications of electrode to determine Mn2+. Comparison of this electrode with reported electrodes (Table 10) shows that it is better than the reported electrodes in terms of wide working concentration range, Nernstian slope, low detection limit and wider pH range. It's selectivity overall is comparable, though it is superior in selectivity towards Mn 2+, Pb 2+, Cd 2+, Zn 2+, Cr 2+, Na 2+ and Fe 2+ than the reported electrodes.

Table 9 Potentiometric determination of Mn2+ ions in medicinal plants by proposed CGE. Samples

ISEa ± SD (mg kg−1) AASa ± SD (mg kg−1)

Ocimum sanctum (Tulsi) 55.6 ± 3.1 Cassia fistula (Amaltas) 41.6 ± 1.8 Withania somnifera (Ashwagandha) 71.2 ± 4.4 a

Mean value ± standard deviation (three measurements).

55.1 ± 4.6 41.4 ± 0.8 70.6 ± 5.3

A.K. Singh et al. / Materials Science and Engineering C 33 (2013) 626–633

633

Table 10 Comparison of response characteristic of Mn2+ ion selective electrode with previous reported electrodes. Ref. no.

Linear range (mol L−1)

Detection limit (mol L−1)

Slope (mV decade−1 of activity)

pH range

Response time (s)

Selectivity coefficient FIM (−logKA,B )

[20]

5.0 × 10−6–1.0 × 10−1

NM

30.0

3.0–6.5

10

[21]

1.0 × 10−5–1.0 × 10−1

8.0 × 10−6

Ni2+ 3.09, Pb2+ 3.52, Cu2+ 4.0, Cd2+ 3.09, K+ 3.69, Zn2+ 3.88, Cr3+ 3.18, Na+ 3.37 Ni2+ 0.65, Pb2+ 1.16, Cu2+ 1.0, Cd2+ 1.0, K+ 2.03, Zn2+ 0.49, Cr3+ 1.12, Na+ 2.28 Ni2+ 0.69, Pb2+ 1.04, Cu2+ 0.61, Cd2+ 1.95, K+ 2.14, Zn2+ 0.55, Cr3+ 1.62, Na+ 2.28 Ni2+ 1.8, Pb2+ 2.2, Cu2+ 2.5, Cd2+ 2.2, K+ 1.9, Zn2+ 1.9, Cr3+ 4.2, Na+ 2.3, Co2+ 2.3 Ni2+ 1.88, Pb2+ 1.79, Cu2+ 2.20, Cd2+ 2.02, K+ 1.37, Zn2+ 2.23, Cr3+ 2.16, Na+ 2.58 Ni2+ 2.86, Cd2+ 3.25, Cu2+ 2.89,K+ 4.25, Zn2+ 2.98, Cr3+ 3.85, Na+ 4.14, Co2+ 2.40, Fe2+ 2.44 Ni2+ 0.01, Pb2+ 0.5, Cd2+ 0.05, Zn2+ 0, Co2+ 0.5, Fe2+ 0.05 Ni2+ 2.51, Pb2+ 3.60, Cu2+ 3.81, Cd2+ 3.75, K+ 4.10, Zn2+ 3.87, Cr3+ 4.18, Na+ 4.24, Co2+ 2.14, Fe2+ 2.68

−5

–1.0 × 10

−1

29.3 ± 0.5

4.0–9.0

≤15

−5

29.5

3.0–8.0

20

[22]

1.2 × 10

[23]

4.0 × 10−7–1.8 × 10−2

1.0 × 10−7

30.1

4.5–7.5

∼10

[24]

1.0 × 10−8–1.0 × 10−1

NM

33.14

2.5–5.5

15

29.6 ± 0.5

4.0–8.0

b15

1.0 × 10

−6

[26]

1.0 × 10

−6

This work

4.1 × 10−7–1.0 × 10−1

[25]

1.2 × 10

–1.0 × 10

−1

−7

4.0 × 10

–1.0 × 10

−1

NM

NM

2.3–8.8

35

6.7 × 10−8

29.5 ± 0.3

3.0–9.0

12

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