modification of clinoptilolite nano-particles with hexadecylpyridynium bromide surfactant as an...

Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx

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JIEC-1283; No. of Pages 8

Modification of clinoptilolite nano-particles withhexadecylpyridynium bromide surfactant as an activecomponent of Cr(VI) selective electrode

Alireza Nezamzadeh-Ejhieh *, Mehri Shahanshahi

Department of Chemistry, Shahreza Branch, Islamic Azad University, P.O. Box 311-86145, Shahreza, Isfahan, Iran

A R T I C L E I N F O

Article history:

Received 2 February 2013

Received in revised form 13 March 2013

Accepted 18 March 2013

Available online xxx

Keywords:

Potentiometric sensor

Dichromate-selective electrode

Surfactant-modified zeolite

Hexadecylpyridynium bromide

Nano particles

Clinoptilolite

A B S T R A C T

Clinoptilolite was pretreated by mechanical ball-milling method to obtain nano particles and it was

modified by hexadecylpyridynium bromide (HDPBr) surfactant to prepare a surfactant modified zeolite

(SMZ) as an effective anion exchanger. The obtained SMZ was used as the active ingredient of a PVC

membrane electrode which showed the best Nernstian response toward dichromate in the used

conditions. The proposed electrode showed linear response in the concentration range of 1.0 � 10�5 to

5.0 � 10�2 mol L�1 dichromate with a detection limit of 5.0 � 10�6 mol L�1 at pH = 3–6 and a Nernstian

slope of 29.9 � 0.9 mV per decade of dichromate concentration.

� 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

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Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

Ion-selective electrodes (ISE) are useful devices for determiningthe different cationic and anionic species because of high speed,low cost, simple preparation, a wide dynamic range and finally norequirement for the sample pretreatment [1–4]. In addition ofusing ISEs for direct determination of the investigated species theycan be also used for indirect determination of other species [5]. Inrecent decade, considerable efforts have been made on thedevelopment of surfactant-sensitive electrodes [6–16].

Metallic environmental pollutants, unlike most organic pollu-tants, are non-biodegradable and can gather in living tissues, andhence become concentrated throughout the food chain. Hence theyare particularly troublesome [17]. One of the most important of themetallic pollutant in aqueous solutions is Cr(VI). Chromium is ahighly reactive metal and exists in various oxidation states inaqueous solutions (II, III, VI). The threshold value for Cr(VI) hasbeen reported 1 mg L�1. While the drinking water usually containslow level of chromium, but contaminated well water maybecontain the dangerous Cr(VI) [18]. Cr(VI) is more toxic than Cr(III),because of its higher solubility in almost the whole pH range andalso greater mobility than Cr(III) [19]. Hence, Cr(VI) affects on thebiological functions and is extremely carcinogenic [20]. Because of

* Corresponding author. Tel.: +98 321 3292515; fax: +98 321 3213213.

E-mail address: [email protected] (A. Nezamzadeh-Ejhieh).

Please cite this article in press as: A. Nezamzadeh-Ejhieh, M. Shaj.jiec.2013.03.018

1226-086X/$ – see front matter � 2013 The Korean Society of Industrial and Engineer

http://dx.doi.org/10.1016/j.jiec.2013.03.018

the different toxicities of Cr(VI), its determination rather than thetotal chromium concentration is necessary using a quick andsimple method which is applicable in the industrial andenvironmental fields. Due to this importance, in addition of newmethods for removal and selective extraction of Cr(VI) from waterand wastewater samples [21,19], in recent decade chromium(VI)-potentiometric sensors based on different ionophores have beeninvestigated [22–31].

Recently, modification of electrode surfaces with zeolites hasbeen extensively investigated, due to their precisely uniformcrystal lattice with pores of molecular dimension into which guestmolecules can penetrate, possess outstanding potentialities for usein electro analytical measurements [32]. Due to the presence ofpermanent negative charge of zeolites, they act as good cationexchangers. But due to their definite pores size, when they ionexchange in a solution containing large cationic surfactants such ashexadecyltrimethylammonium (HDTMA), ethylhexadecyldi-methyl ammonium (EHDDMA), octadecyltrimethyl-ammonium(ODTMA) and hexadecylpyridinium bromide (HDP), the surfaceactive sites of the zeolite occupied by these large cations [33–36].When the concentration of surfactant will be above its criticalmicelle concentration, a double layer of surfactant can be formonto the zeolite surface which causes the charge reversal of thezeolite surface from negative to positive. In this case, the obtainedsurfactant modified zeolite (SMZ) can be used as an effective anionexchanger [33–36].

hanshahi, J. Ind. Eng. Chem. (2013), http://dx.doi.org/10.1016/

ing Chemistry. Published by Elsevier B.V. All rights reserved.


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Fig. 1. XRD pattern of the nano-zeolite clinoptilolite.

A. Nezamzadeh-Ejhieh, M. Shahanshahi / Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx2

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Due to the above discussion, we used modified zeolites withHDTMA in the potentiometric determination of anions and ourresults showed that HDTMA on the different zeolites and alsodifferent matrixes including carbon paste and PVC has Nernistianresponses to different anions [36–39]. In this work, in order toshow the role of surfactant on the behavior of ISEs, modification ofan Iranian natural clinoptilolite nano-particles, as an abundantzeolite in Iran, with hexadecylpyridinium (HDP) cationic surfac-tant was investigated and the obtained results showed goodNernstian behavior toward dichromate anion. The effect of somekey experimental factors on the behavior of the proposed electrodewas investigated and the validity of the obtained results wasconfirmed by ‘t’ test.

2. Experimental

2.1. Reagents

Natural clinoptilolite-rich tuffs were obtained from Semnanregion in the north-east of Iran. All solutions were prepared indouble-distilled deionized water. High relative molecular weightpolyvinyl chloride (PVC) was used for preparing the polymericmatrix. Reagent grade dioctylphthalate (DOP), tetrahydrofuran(THF) and hexadecylpridinium bromide (HDPBr) were purchasedfrom Fluka Chemie AG (Buch, Switzerland) and were used asreceived. All other salts were obtained from Merck with the highestpurity and were used without any further purification. The pH ofthe solutions was appropriately adjusted with NaOH or HCl.

2.2. Apparatus

A pH/Ion Meter model 691 (Metrohm) was used for potentio-metric experiments. The pH measurements were performed usinga corning model 125 pH meter equipped with a combinedelectrode. Infrared spectra were recorded on a FT-IR Spectrum65 Spectrophotometer and potassium bromide (KBr) pelletmethod. The TG/DTG studies were performed by thermogravi-metric analyzer (TGA) model Ned 2007, Setaram. XRD patternswere obtained with a Bruker diffractometer (D8 Advance) with Ni-filtered Cu Ka radiation (l = 1.5406 A).

2.3. Modification of nano-clinoptilolite zeolite

Clinoptilolite particles were crushed by mechanical method toobtain micro sized particles. The nano particles of zeolite wereobtained by mechanical ball-milling of the obtained powder. Thenano-particles were heated at 70 8C for 8 h to remove solubleimpurities (3 times) while it was stirred by magnetic stirrer toeliminate magnetic impurities. For preparation of SMZ, the naon-clinoptilolite particles (2 g) were mixed with 100 mL of 50, 100 and200 mmol L�1 HDP-Br solutions in separate bottles, and stirred for24 h on a magnetic stirrer. The mixture was then centrifuged at3500 rpm for 15 min and the resulting SMZ was dried in air.

2.4. Electrode preparation

The experiment was carried out on a number of membranesprepared by varying the concentration of PVC, DOP and SMZ. Themaster membrane was fabricated by dissolving 33 mg ofpowdered PVC, 62 mg of DOP plasticizer and 5 mg of SMZ in2 mL of dry freshly distilled THF. The resulting mixture wastransferred into a glass dish with a 2 cm diameter. The THFevaporated slowly until an oily, concentrated mixture wasobtained. A Pyrex tube (1 mm o.d.) was dipped into the mixturefor approximately 10 s, so that a membrane of approximately0.5 mm thickness was formed. The tube was then withdrawn from


the mixture and maintained at room temperature for approxi-mately 2 h. A 1.0 � 10�2 mol L�1 K2Cr2O7 ion solution was used asthe internal reference solution for the electrode. The prepared PVCelectrode was conditioned in 1.0 � 10�2 mol L�1 K2Cr2O7 ionsolution for 12 h and stored in air when not in use.

2.5. EMF measurements

All potential measurements were recorded with the followingcell assembly at (24 � 1 8C): double-junction Ag/AgCl/internalsolution (1.0 � 10�2 mol L�1 K2Cr2O7/modified PVC membrane/sam-ple solution/SCE). The performance of the electrode was investigatedby measuring its potential in dichromate solutions prepared in theconcentration range of 1.0 � 10�10 to 3.0 � 10�1 mol L�1 by serialdilution. By adding an appropriate amount of 1.0 mol L�1 HCl or1.0 mol L�1 NaOH the pH of the test solutions was adjusted. Thesolutions were stirred and potential readings were recorded whenthey became stable. The data were plotted as the observed EMF versusthe logarithm of the dichromate concentration. The selectivitycoefficients (Kpot

a;b ) were measured using the fixed interference(FIM) and separate solution methods (SSM) [40].

3. Results and discussion

3.1. Characterization of the SMZ

3.1.1. XRD results

The used nano-clinoptilolite sample was analyzed by XRDmethod. The relative intensity of indexed lines was compared withthe existence referenced sample in the instrument memory. Thecharacteristic lines located at 2u values of 108, 11.48, 17.48, 238, 268,28.28, 30.28 and 328 were observed from XRD pattern (Fig. 1). Theobserved reflection peaks can be indexed to clinoptilolitecrystalline structure data in the library of the instrument [JCPDSNo. 39-1383, pattern (a) in Fig. 1]. In Fig. 1, the indexed lines(pattern ‘‘a’’) are related to referenced sample within the XRDinstrument memory and peaks (pattern ‘‘b’’) are related to the usednano-clinoptilolite sample. A good agreement between twopatterns shows that the used zeolite sample is clinoptilolite asmajor phase and the peak broadening confirms producing of nanoparticles during ball milling process. XRD pattern also showed thatthe used zeolite tuffs includes slight amounts of quartz (3.5%) andcristobalite (7.8%) phases as impurities.

For determining the size of particles by XRD pattern thefollowing Sherrer equation was used [41].

d ¼ Klb cos u




Fig. 2. FT-IR spectra of (a) natural zeolite clinoptilolite (NZ), (b) HDP and (c) SMZ.

Table 1Potentiometric response of the SMZ-PVC electrode toward different anions (n = 5).

Anion Linear range (�log C) S (mV/decade) r2

Cr2O72� 1–6 28.9 � 0.2 0.9981

CN� 3–5 22.1 � 1.0 0.9578

Cl� 3–5 42.4 � 0.9 0.9946

Br� 2–5 34.8 � 0.6 0.9759

NO3� 3–5 28.8 � 0.7 0.9872

C2O42� 3–5 8.6 � 0.8 0.9981

C6H5O7� 3–5 8.4 � 0.8 0.9711

SO42� 2–5 25.0 � 1.5 0.9896

S2O32� 3–5 26.1 � 1.7 0.9832

A. Nezamzadeh-Ejhieh, M. Shahanshahi / Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx 3

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where d is the average diameter of particles in nm, b is the width ofpeak cross half of the height in radian, k is the fixed numberbetween 0. 6 up to 1.2, l is the wavelength of X-ray in Angstromand u is the dispersion angle in degree. The average size for nano-clinoptilolite particles was estimated about 20–65 nm.

3.1.2. FT-IR spectroscopy

FT-IR spectra were obtained using KBr pellets of HDP surfactant,nano-clinoptilolite zeolite (NCP) and SMZ-HDP as shown in Fig. 2.In Fig. 2a the observed peaks located at 470, 613, 780, 1086, 1636and 3503 cm�1 are in agreement with IR spectral data reported forclinoptilolite [42]. The IR pattern of SMZ-HDP (Fig. 2c) showscharacteristic peaks at 1486, 2851, 2916 and 3391 cm�1 with slightshift in peak positions, which some peaks located at 1486, 2851and 3391 cm�1 related to C–H, C–C and N–C vibrations in thesurfactants, respectively. Comparing the spectra of SMZ and rawmaterials shows the absence of these peaks in the raw material.These observations confirm loading of HDP on zeolite structure.

3.1.3. Thermogravimetric analysis

With the help of thermal analysis we can get some informationabout the separation of adsorbed molecules on the surface andholes of zeolites. Water is a molecule that exists in the most zeoliticstructures. The thermogravimetric analysis curves for the raw andmodified samples are shown in Fig. 3. In the thermal analysis graph

Fig. 3. Thermogravimetric analysis curves for NZ, HDP and SMZ.


of raw nano-clinoptilolite in 50–100 8C, the loose of weight is seenwhich is related to physical characteristics of adsorbed water onthe surface of zeolite with mass-loss was about 9.7% [43]. Thewater found within the zeolite structure is known as zeolitic waterand remove in the temperature range of 100–200 8C. The results ofTG-DTG agree with the reported TG-DTG curves for clinoptiloliteby Gottardi and Galli [43].

In TG/DTG curves of the SMZ in Fig. 3, two peaks can beobserved at temperatures of 100 and 250 8C which the first peak isdue to the removal of free water. The second and also sharp peakoccurred at a temperature of 250 8C is due to the decomposition ofthe surfactant adsorbed on the SMZ with the mass-loss of 46.6%. Onthe other hand, by comparing the thermal analysis graphs of rawclinoptilolite, surfactant and surfactant modified clinoptilolite wecan come to conclusion that the observed peak of losing weight in200–300 8C is because of evaporation of HDP surfactant within itsboiling point from the surface of zeolite. This in turn confirms asufficient loading of surfactant on the surface of zeolite [19].

3.2. Potentiometric measurements

3.2.1. Optimization of the electrode composition

It is well known that the sensitivity, linear range and selectivityof the ISEs depend on the nature of the used carrier [36,38,44]. Inpreliminary studies, some experiments concerning the potentio-metric response toward some mono- and divalent anions wereperformed. According to the results, the best potentiometricresponse, Ecell versus log concentration, was obtained fordichromate (see Table 1 and Fig. 4). The charge has a fundamentalrole in ionic property of anions especially for Hofmeister anions.For ions of the same charge, their size, e.g. the Pauling radius, is themost fundamental property. On the other hand the polarizability ofanion plays an important role on its interaction with head group ofthe SMZ. The quantum electronic levels of the ions are alsofundamentally important, since they determine their reactivity.Finally, there exist mixed properties, such as the free energy or the

-logC1 2 3 4 5 6 7 8

E(m

V)

-100

0

100

200

300

400

Cr2O72-

CN-

Cl-

Br-

NO3-

C2O42-

C6H5O73-

SO42-

S2O32-

Fig. 4. A schematic diagram of the electrode response to various anions, SMZ = 4%

(n = 10), at 25 8C.




Table 2Effect of the concentration of HDP for preparing SMZs on the electrode response.

Electrode CHDP (mM) S (mV/decade) texp

A 50 40.9 � 1.3 26.9

B 100 30.5 � 1.6 0.4

C 200 35.9 � 0.9 11.5

Note: t0.05,4 = 2.78 [52].

Table 4Effect of temperature on the optimized SMZ-CPE behavior (n = 5).

Temp. (8C) Sn (mV decade�1) Sexp (mV decade�1) texp

10 28.5 26.2 � 0.5 14.8

20 29.1 28.0 � 0.7 2.5

25 29.6 29.1 � 0.9 0.9

30 30.1 30.4 � 0.4 2.5

35 30.6 32.1 � 1.2 2.6

40 31.1 32.8 � 1.6 2.1

50 32.1 35.9 � 0.6 11.5

60 33.0 40.9 � 0.5 26.9

70 34.0 46.0 � 1.0 22.1

Note: t0.05,4 = 2.78 [52].


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entropy of hydration, which provide indirect measures of theinteraction of ions with water [45,46]. Depending on the mode ofinteraction with cationic surfactants, anions are classified into fourclasses [45,46]. Most ‘Hofmeister’ anions do not bind strongly onthe head groups and behave as typical counter ions (OH�,CH3COO� and HCOO�). Class II ions usually form large complexions with large polarizabilities, which form water-insoluble ionpairs with the amine-based surfactants (I�, SCN� and ClO4

�). ClassIII consists of complex anions containing transition metal ions,which form covalent bonds with the amine-head groups and alsobetween themselves through oxygen bridges. Finally, class IVconsists of a number of hydrophobic organic anions (benzoate,salicylate, tosylate), which partly dissolve in the palisade layer ofthe surfactant. Hofmeister anions also affect on the size and shapeof micelles of cationic surfactants and on their phase diagramsregarding to their hydrated sizes. It often happens that severaldifferent anions compete for a cationic micellar surface. Accordingto above discussion, different anions have different interactionswith head group and the chain of surfactant. Hence, in the usedconditions, dichromate has the best interaction with surfactanthead group.

Thus, the electrode was investigated in further details. First, theeffect of the surfactant concentration in ion-exchange solution wasexamined. The obtained results are summarized in Table 2, whichshows the best potentiometric response for the membrane containingthe prepared SMZ in 100 mmol L�1 HDP solution. In other cases, theslopes and working ranges of the electrodes were not very satisfactoryfor dichromate. In a 50 mmol L�1 HDP solution, a partial bilayer maybe formed on the zeolite surface, and the obtained SMZ is not suitablefor dichromate adsorption. The prepared electrode with a200 mmol L�1 HDP solution also does not indicate Nernstianbehavior. Statistical comparing of the results shows the response ofthe membrane B is only affected by random errors while in the othercases systematic errors limit the electrode responses [47]. Second,several membranes were prepared with different compositions withthis SMZ by varying the ratio of PVC to plasticizer. According toobtained results which are collected in Table 3, the electrodecontaining 5% SMZ, prepared from nano-clinoptilolite at100 mmol L�1 HDP, 33% PVC and 62% DOP was found more suitableand the electrode exhibited a Nernstian response with a smallstandard deviation and good linear response within the concentrationrange of 1.0 � 10�5 to 5.0 � 10�5 M dichromate. Thus, this membranecomposition (optimized electrode) was used in the next experiments.

3.2.2. Internal standard solution and conditioning time

The concentration of internal solution may affect the electroderesponse when the membrane’s inner diffusion potential is notable

Table 3Effect of membrane composition on the behavior of the electrode (n = 5).

No. %PVC %SMZ %DOP S (mV/decade) texp

1 32.5 2.0 65.5 44.4 � 1.5 6.4

2 32.0 4.0 6.4 27.4 � 1.2 2.9

3 33.0 5.0 62.0 29.0 � 0.2 1.1

4 31.5 6.0 62.5 31.3 � 0.7 4.2

5 30.0 10.0 60.0 41.6 � 0.4 9.7

Note: t0.05,4 = 2.78 [52].


[48,49]. So the effect of inner solution concentration was studiedwith the change of concentration from 1 � 10�4 to1 � 10�1 mmol L�1 dichromate. The results show that by changingin the concentration of internal solution, it does not contain anotable difference in electrode response (Nernstian slopes) and itslightly change the intercept of the curves. Because of theexpanded concentration scope and appropriate electrode slopein 1 � 10�2 mmol L�1 dichromate concentration it was used innext measurement as the inner solution.

The effect of conditioning time on the electrode response wasstudied at the time interval covering the range from 2 to 48 h. Thebest Nernstian response, combined with a dynamic range, wasobtained at 2 h. Hence, this time was employed as the conditioningtime in later studies.

3.2.3. Effect of temperature

To investigate the thermal stability of the electrode, calibrationcurves were constructed at different temperatures covering therange 10–70 8C. The electrode exhibited good Nernstian behaviorin the range of 20–40 8C. The results are shown in Table 4. TheNernstian and experimental slope values at each temperature werecompared statistically, showing conformity of the slopes for 5replicates. The results prove the validation of the obtained resultswith the expected values in the temperature range of 20–40 8C.However, at temperatures higher than 40 8C the slopes show asignificant deviation from the theoretical values. This deviationmay be related to desorption of the surfactant from SMZ anddestruction of the electrode surface.

3.2.4. Effect of pH

The pH dependence of the membrane electrode was tested overthe pH range of 1–13 and the results are shown in Fig. 5. As shown,the variation in potentials is negligible for 0.01 mmol L�1

dichromate with respect to a 0.001 mmol L�1 dichromate solution.On the other hand, at higher concentrations of dichromate theinterference from protons and hydroxyl ions in the strong acidicand basic conditions, respectively, is small. The experimentalslopes as a function of solution pH are collected in Table 5. ANernstian behavior is seen in the pH rang of 3–6. According to theinset of Fig. 5, which shows distribution of Cr(VI) as a function ofsolution pH, predominant forms of Cr(VI) species in pH � 6 areCr2O7

2� and HCrO4� [50]. Although HCrO4

� is more popular formwith respect to Cr2O7

2� in these conditions, but the electroderespond to Cr2O7

2� species due to its higher charge. We concludethis from the observed Nernstian slopes for the electrode (in the pHrang of 3–6) as shown in Table 5. At very strongest pH conditions,interferences from chloride anions (due to adding HCl) [51] andalso partially destroying of zeolite matrix and hence decomposi-tion of the SMZ [36–39] are another reasons to deviate theelectrode from its Nerstian response.

At pH higher than 8, a Nernstian behavior was also observed forproposed electrode. In these conditions chromate anions are




Fig. 5. The influence of pH on the potential response of the optimized PVC-SMZ electrode for a 1.0 � 10�2 and 1.0 � 10�3 mol L�1 dichromate solution at 25 8C.

Time (s)0 50 10 0 15 0 20 0

E (

mV

)

50

100

150

200

250

300

350

SMZ-NCP

SMZ-MCP

Fig. 6. The response times of the optimized PVC-SMZ electrode for stepwise changes

in the concentration of dichromate ranging from 1.0 � 10�6 to 1.0 � 10�1 mol L�1

dichromate at 25 8C (from top to bottom respectively), SMZ = 5% (n = 5), at 25 8C.

Table 6Response time of the optimized SMZ-PVC electrode behavior (n = 5), LR (�log C = 2–

6).


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predominant Cr(VI) species in the solution and the electroderespond to these anions in the pH range of 8–10. At pH � 10interference from hydroxyl anions deviates the electrode responsefrom its Nernstian response [36–39].

3.2.5. Response time and reversibility

The response time of the prepared electrode was determined bymeasuring the time required for the achievement of a stabilepotential by changing the dichromate ion concentration from1 � 10�6 to 1 � 10�1 mol L�1. To study the effect of the particlessize of natural clinoptilolite, the same experiments wereperformed by the surfactant modified clinoptilolite with microsized dimensions (SMZ-MCP) and also the nano sized SMZ (SMZ-NCP). The results which are presented in Fig. 6 confirm that therequired time to reach a constant response is longer for SMZ-MCPmembrane. This is due to the higher homogeneity and alsoeffective surface area of the SMZ-NCP due to smaller particle sizesof nano particles.

Long-term stability of the electrode was also investigated bytaking the response of the optimized electrode in a period of 60days. The results which are summarized in Table 6 confirm that theelectrode retains its Nernstian behavior at least for 60 days. Thesame studies were studied for the constructed electrode by SMZ-MCP (the results not shown) and show a Nernstian response for 50days.

3.2.6. Response characteristics of the electrode

The EMF response of the electrode to various concentrations ofthe dichromate species revealed a linear range from 1.0 � 10�5 to0.5 mol L�1 dichromate anion (r2 = 0.9997) with slope of calibra-tion curve 29.9 � 0.9 mV per decade of dichromate concentration

Table 5Effect of pH on the electrode response in measuring dichromate (n = 5).

pH Slope (mV decade�1) R2 texp

2 26.2 � 0.5 0.9873 9.2

3 29.2 � 0.8 0.9967 1.0

5 29.1 � 0.9 0.9874 1.1

6 30.1 � 0.5 0.9899 2.23

8 30.4 � 0.4 0.9972 1.5

9 28.5 � 1.4 0.9892 1.8

10 28.3 � 1.2 0.9882 2.4

11 39.7 � 0.7 0.9934 6.0

Note: t0.05,4 = 2.78 [52].


(n = 20) and a lower detection limits of 5.0 � 10�6 mol L�1 dichro-mate (Fig. 7).

The potentiometric behaviors of the SMZ-MCP-PVC electrodeand the membranes constructed using unmodified nano-zeolitealone and also surfactant alone were also studied and the obtainedresults are shown in Fig. 8. As shown, presence of raw zeolite andalso surfactant alone in the membrane do not cause a suitable

Time S (mV decade�1) RSD texp

1 h 29.7 � 0.4 1.4 0.5

3 h 29.5 � 0.4 3.3 0.4

7 h 29.8 � 0.6 2.1 0.6

10 h 28.5 � 0.6 1.3 2.8

16 h 30.3 � 0.5 1.9 2.0

1 d 29.1 � 0.6 2.3 1.2

2 d 29.2 � 0.5 2.0 1.0

3vd 30.1 � 0.61 2.0 1.5

1 w 29.5 � 0.5 1.7 0.1

2 w 29.7 � 0.3 1.2 0.8

3 w 30.7 � 0.7 2.3 2.6

1 m 30.0 � 0.5 1.7 1.5

2 m 31.0 � 1.0 3.4 1.3

Note: t0.05,4 = 2.78 [52].




-log C0 2 4 6 8

E(m

V)

160

180

200

220

240

260

280

-log C1 2 3 4 5 6

E(m

V)

160180200220240260280300

Fig. 7. Calibration curve of the optimized PVC-SMZ electrode for dichromate

determination.

log α(d ichromate)

123456

E (

mV

)

220

240

260

280

300

320

340

Br-

CO3

2-

CN-

Cl-

HPO4

-

NO3

-

C2O4

2-

PO4

3-

SO4

2-

I-

Fig. 9. Schematic FIM results for determination of selectivity coefficients.


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response toward dichromate species. In the case of surfactantmodified micro sized zeolite, in addition to decreasing of linearrange (1.0 � 10�4 to 5.0 � 10�2 mol L�1) the sensitivity of theelectrode decreases from 0.765 mV per mg dichromate (in the caseof SMZ-NCP) to 0.536 mV per mg dichromate. Using SMZ-MCPmodified electrode, detection limit increases from5.0 � 10�6 mol L�1 (in the case of SMZ-NCP) to 8.0 � 10�5 mol L�1.�1. The average experimental slope and also regression of thecalibration curve in case of SMZ-MCP electrode was poor withrespect to SMZ-NCP electrode (S = 28.9 � 1.7 mV per decade ofdichromate concentration, r2 = 0.9878 � 0.0531 for n = 20). Theseconfirm that with reducing the particles size of clinoptilolite, theeffective surface area increases which causes to loading higheramounts of surfactant molecules onto zeolite surface and alsohomogeneity of the prepared membrane. Hence higher active centersof SMZ-NCP will provide to respond to dichromate anions.

The repeatability of both SMZ-MCP and SMZ-NCP membraneelectrodes was studied by 15 replicate measurements and thestandard deviations of slopes were determined. The relativestandard deviations of 3.2% and 1.7% were respectively obtainedfor SMZ-MCP and SMZ-NCP membrane electrodes, indicating highrepeatability of both electrodes. In this study the SMZ-NCP has alsoa higher repeatability. The reproducibility of the potentiometricresponse of both SMZ-MCP and SMZ-NCP membrane electrodeswas also investigated by carrying the measurements on fourindependent electrodes with the same composition. Thesemeasurements were also performed in 5 replicates. The resultswere investigated by g-test [52]. The calculated g values were 0.28and 0.47 for SMZ-NCP and SMZ-MCP, respectively, which aresmaller than the critical g value at 95% confidence interval,

-log (C)

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

E (

mV

)

-10 0

-80

-60

-40

-20

0

20

40

60

80

SMZ-MCP

SMZ-NCP

PVC-NCP

PVC-Surf

Fig. 8. Response of the electrode based on different modified and unmodified

membranes.


g0.05,5,4 = 0.6287 (4 class, 5 replicate in each class) [52] indicatingthat there is no significant difference between the behavior of thefour independent electrodes. Also, in this study the SMZ-NCP hasalso a higher reproducibility.

3.2.7. Selectivity of the electrode

The most important feature of any ion-selective electrode is itsselectivity against the relevant analyst ion over other ions presentin the solution, which is usually described in terms of thepotentiometric selectivity coefficient kpot

A;B [40]. These coefficientsfor Cr2O7

2� selective electrode were determined by the FixInterference Method (FIM) from potential measurements of asolution prepared with a fixed concentration of the interfering ions(0.01 mol L�1) and by various concentrations of Cr2O7

2�. Thepotential values gained are plotted versus the logarithm of theactivity of the primary ion (Fig. 9). The intersection of theextrapolation of the linear portions of this plot indicates the valueof Cr2O7

2�which will be used to calculate kpotcr2o7

from the Nikolsky–Eisenman equation [40]:

KpotA;B ¼

aAðPABÞZA=ZB

aZA=ZB

B � 100

The selectivity pattern of the electrode for these interferinganions goes along with the Hoffmeister series as follows:

CN� > Br� > Cl� > I� > SO42� > CO3

2� > NO�3> C2O42�

> HPO42� > PO4

3� (Table 7)The potentiometric selectivity of selective dichromate elec-

trode was also determined by separate solutions method (SSM)using 0.01 mol L�1 dichromate and interfering anions. The resultsare also shown in Table 7 indicate these electrodes have a good

Table 7Comparison of some potentiometric characteristics of the dichromate ISE described

in this work.

Interfering ions �log k

SSM FIM

CN� 1.52 1.25

Br� 2.13 1.52

NO3� 2.78 2.57

SO42� 2.30 1.60

PO43� 2.85 2.57

CO32� 2.52 2.46

HPO42� 2.37 2.54

C2O42� 2.43 2.50

Cl� 2.13 1.55

I� 1.82 1.57




V (mL)

0 2 4 6 8 10 12

E (

mV

)

180

190

200

210

220

230

V (mL)

0 2 4 6 8 10 12

δE/ δ

V

0

2

4

6

8

10

12

14

Fig. 10. A potentiometric titration curve for 10 mL of a 8.3 � 10�4 mol L�1 K2Cr2O7

solution (pH = 3) with 0.01 mol L�1 Fe2+ using the proposed sensor as an indicator

electrode at 25 8C, SMZ = 5%.


G Model


selectivity compared to common mineral anions. Selectivitypattern of electrode is distinct from the pattern of lipid basedHofmeister [53]. According to these results, electrodesreaction is based on dichromate interaction with positivesurfactant (HDP).

3.2.8. Application of the SMZ-PVC electrode

To evaluate the applicability of the proposed electrode in theindirect potentiometry, the electrode was used as indicatorelectrode in the titration of a solution of 8.3 � 10�4 mol L�1

(pH = 3) potassium dichromate by 0.01 mol L�1 Fe2+ (as Ferroam-munium sulfate salt) based on a reduction-oxidation reaction[54,55]. Fig. 10 shows a typical titration curve for this titration thatthe inset of figure shows differential curve for the determination ofend point. The end point of titration was calculated by differenti-ating 4 replicate titration curves. Based on the used concentrationthe end point should appear in 5 mL titrant. The averageexperimental end point was obtained in 4.85 (�0.13) mL(RSD% = 2.7) which is a satisfactory result. Due to the lowconcentration of free dichromate ions in the solution, the potentialresponse after the endpoint was almost constant. Statisticalcomparison of the theoretical and experimental endpoints demon-strated that there was no significant difference at a 95% confidencelimit between experimental and theoretical values (texp = 2.31,t0.05,3 = 3.18). Hence, the proposed electrode can be used as indicatorelectrode in the titration of dichromate in an oxidation-reductionprocess.

The proposed method was also used for the determination ofdichromate in a wastewater of Behrad Electrolysis Company inIsfahan, Iran. To remove insoluble impurities, the waste samplewas filtered and then used for determination of its dichromatecontent by the proposed method and also atomic absorptionspectroscopy. To avoid matrix interferences, standard additionmethod was used. The obtained results which are summarized inTable 8 confirm the validity of the proposed method for realsamples studies.

Table 8Results of dichromate determination in an electrolysis wastewater (n = 5), the

results belong to 200 times diluted wastewater sample.

Method Cr2O72� (mg L�1)

Potentiometry 45.8 � 0.5

AAS 46.1 � 0.3


4. Conclusion

Based on the results, it concluded that SMZs retain their anionicadsorption behavior when they enter in a PVC matrix. Bothmodified electrodes with SMZ-MCP and SMZ-NCP showed goodpotentiometric responses toward dichromate anion, but in thepresence of nano-particles of clinoptilolite a wider linear range,better sensitivity, smaller response time and finally betterrepeatability and reproducibility were obtained due to increasingin the surface area and hence more active centers in the membraneby nano particles of zeolite. The results showed that the electrodebehavior is extremely pH dependent, so the electrode respond todichromate anion in the pH range of 3–6, while it respond tochromate anions in the pH range of 8–10.

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