Modi�cation of Nanoclino�tilolite �eolite �ith Hexadecyltrimethylammonium Surfactant as an Active Ingredient of Chromate-Selective Membrane Electrode

A novel chromate selective PVCmembrane electrode was constructed using a hexadecyltrimethylammonium-surfactant modi�ed zeolite (SMZ) . Nano particles of an Iranian clinoptilolite zeolite were prepared by a ball-mill mechanical method and characterized by FT-IR, SEM and XRD. Best performance was exhibited at the membrane composition of SMZ : PVC : dioctyl phthalate (DOP) in the ratio of 4 : 32 : 64.is membrane worked well over a wide concentration range from 5× 10 to 1.0 × 10mol L of CrO4 2− with a Nernstian slope of −29.38 ± 0.29mv per decade of CrO4 2− concentration with detection limit of 2 × 10mol L in a wide pH range of 6.8–10.7.e response time of the sensor is 5–10 s over a period of 2 months with good reproducibility.e selectivity coefficients of the proposed electrode towards various interfering ions were determined by FIM and SSMmethods.e sensor was successfully used as indicator electrode in the potentiometric titration of CrO4 2− against Pb(II) and determination of chromium in an electroplating wastewater sample.


Introduction
Ion-selective electrodes (ISEs), because of their great interests regarding to their advantages, are used in a wide range of applications for the determination of ions and pharmaceuticals in aqueous environments.Comparing with other analytical methodologies, ion selective electrodes have some advantages such as: ease of preparation, relatively inexpensive, robust, durable, rapid response time, continuous monitoring, measure activity rather than concentration, do not affected by turbidity or color of sample and ideal for their use in the environmental studies [1].As an important advantage, ISEs are known to be one of the few techniques that can measure both positive and negative ions depending on the nature of the ionophore.
Chromium is an element widely used in industrial activities such as chrome plating and electroplating, metal smelting and metallurgy to manufacture alloys, dyestuff industries, leather tanning, and wood treatment [2,3].It exists as two major species, Cr (VI) and Cr (III) in natural waters and soils.Cr (III) is a stable and slightly soluble cation considered as an essential microelement [4], while Cr (VI) (as chromate) is highly water soluble and mobile anion, known as a toxic pollutant, with mutagenic and carcinogenic effects [5].Due to the different toxicities of these two species, it is essential to determine hexavalent chromium rather than the total chromium concentration using a rapid and simple method that can be applied in industrial and environmental �elds.
Zeolites, clinoptilolite is the most abundant of their natural category, are crystalline microporous aluminosilicates with very well-de�ned structures that consist of a framework formed by tetrahedra of SiO 4 and AlO 4 .e isomorphous substitution of Al 3+ for Si 4+ in the tetrahedra results in a negative charge on the zeolite framework that can be balanced by exchangeable cations.Hence, zeolites can exchange cations but not anions.It has been demonstrated that the cationic surfactants have a great affinity to this negative charge.is property has been used to modify the external surface of the zeolites by adsorbing a cationic surfactant to improve its anion exchange capacity.e cationic surfactants that are most commonly used to modify zeolites are usually long alkyl chains with a quaternary ammonium group at one end of the chain such as hexadecyltrimethylammonium (HDTMA) bromide [12].e sorption of a cationic surfactant on the external surface of a zeolite can be governed mainly by cationic exchange and hydrophobic interactions [13].At a low surfactant loading, the surfactant cations are exchanged with the exchangeable cations of the zeolite until a monolayer of surfactant cations is formed at the external surface.At concentrations above the critical micelle concentration, a bilayer of surfactant molecules (admicelle) is attached to the external surface, where the outer layer of surfactant molecules is bound by hydrophobic interactions [14].In this case a surfactant modi�ed zeolite (SMZ) has produced whit changing the external surface charge of the zeolite from negative into positive which now displays anion exchange capacity.
Different properties were observed when immobilizing HDTMA on different substrate.Li et al. immobilized HDTMA onto the clinoptilolite for sorption of oxyanions [15].Natural mordenite (NM) was used for adsorption of As (V) by Chutia et al. [16], and also zeolite NaY was used as substrate for ethylene adsorption by Patdhanagul et al. [17].Zeolites have different cation exchange capacities (CECs) and external cation exchange capacities (ECECs) depending on their structures and Si/Al ratio, where the latter case affects on the properties of the obtained SMZ.In addition, presence of bilayer on SMZ acts as an organic medium which affects in the hydrophilicity of obtained SMZ.Hence, we believe that the support on different SMZs has a major role in their adsorption behavior.Regarding different behaviors of HDTMA on different supports, we have constructed different selective electrodes based on SMZ modi�er towards potentiometric determination of anions [18][19][20][21].
In this work, an Iranian natural clinoptilolite as an abundant zeolitic material in Iran was pretreated and changed to nanoparticles by mechanical ball milling process.Aer puri�cation of the solid by re�ux method, the puri�ed nanoparticles were ion exchanged in HDTMA solution to yield SMZ. en, the obtained SMZ was used as an active ingredient to modify PVC membrane.Constructing the electrode by this membrane shows better Nernstian behavior towards chromate anions rather than the other investigated anions.Aer the investigation of some experimental factors which affect the behavior of the proposed electrode, the electrode was used as indicator electrode in the potentiometric determination of chromate in a precipitation titration by Pb (II) solution.e statistical "t" and "g" tests were used to prove the validity of the results.

Experimental
2.1.Reagents.e natural zeolite used in this study originated from Semnan Province, in the north east of Iran.For membrane preparation, poly(vinyl chloride), dioctyl phthalate (DOP), tetrahydrofuran (THF), and hexadecyltrimethylammonium bromide (HDTMABr) were purchased from Aldrich chemicals and were used without further puri�cation.All salts were purchased from Merck and used as received.Double-distilled deionized water was used throughout the experiments.e pH of the solutions was appropriately adjusted with NaOH or HCl solution as appropriate.

Preparation of the Nanozeolite and the SMZ.
Natural clinoptilolite zeolite was mechanically pretreated, by crushing in an agate mortar and sieving in analytical sieves for the separation of <100 m particle-size fractions.Nanoclinoptilolite was prepared using a planetary ball mill (PM100; Retsch Corporation).e milling can be down in two states including dry milling and wet milling.e milling conditions including the dry milling speed, the dry milling time, the ballto-powder ratio for dry milling and the wet milling speed, the wet milling time, the balls-to-powders ratio, and the waterto-powder ratio for wet milling should be optimized to reach size reduction and crystallinity retention.In this study, dry milling was applied.e milling speed, milling duration, and balls-to-powder ratio were 600 rpm, 6 h, and 4.0, respectively.
In order to remove the water-soluble impurities and also paramagnetic impurities especially, one entered during the ball milling step, the obtained nanoparticles were re�uxed at 70 ∘ C for 8 h in distilled water while the suspension was stirred by a magnetic stirrer continuously.is was repeated �ve times to reach complete puri�cation by observing the elimination of the adsorbed magnetic particles on magnet.In order to reach �xed water content, aer �ltration, washing, and drying of the powder, the puri�ed material was stored in a desiccator over saturated sodium chloride solution for 2 weeks.
Surfactant modi�cation of the zeolite was carried out using the surfactant, hexadecyltrimethylammonium bromide (HDTMABr), a cationic surfactant.A preweighed quantity of nanoclinoptilolite sample was mixed with HDTMABr solution in a 125 mL polyethylene bottles in a 1 : 100 (solid : liquid) ratio.e concentrations of HDTMABr solutions used for preparation of SMZ-1, SMZ-2, and SMZ-3 were 50, 100, and 200 mM, respectively.e solution was agitated for 24 h at 150 rpm on an orbital shaker and a temperature of 25 ∘ C, a period shown sufficient to obtain sorption equilibrium.e mixture was then centrifuged at 3000 rpm for 15 min.e solution was then �ltered, and the solid residue was washed with double-distilled water and airdried for 24 h.e synthesized SMZ was then mechanically ground with a mortar and pestle to a �ne particle size.

Characterization.
Nano-particles of clinoptilolite were prepared using a planetary ball mill (PM100; Retsch Corporation).e zeolites were characterized by X-ray diffraction (XRD) by using a diffractometer Bruker, D8ADVANCE, X-ray tube anode: Cu, wavelength: 1.5406 Å (Cu K radiation), Filter: Ni. e Fourier transform infrared (FT-IR) spectra of the samples were recorded at room temperature in KBr pellets using a Perkin Elmer FT-IR (Spectrum 65) spectrometer between 4000 and 400 cm −1 wavenumbers.To study probable changes of morphology for ground powders aer ball milling, samples were subjected to SEM investigation (Seron technology, model: AIS2100).
2.4.Preparation of Electrodes.e general procedure to prepare the SMZ-PVC membrane was mixing of various amounts of the ionophore (SMZ) together with appropriate quantities of DOP (as plasticizer) and PVC to give a total mass of 100 mg in 1.5 mL of THF.e optimum composition of the membrane was 32 mg of powdered PVC, 64 mg of DOP plasticizer, and 4 mg of SMZ.ese compounds were dissolved in 1.5 mL of THF.e resulting mixture was poured into a glass dish of 2 cm diameter.e solvent was slowly evaporated until an oily concentrated mixture was obtained.A Pyrex tube of 4 mm i. d. was dipped into the mixture for about 5 s, so that a nontransparent membrane of 0.3 mm thickness was formed.e tube was then pulled out from the mixture and kept at room temperature for 24 h.en it was �lled with internal �lling solution 1.0 × 10 −2 M Na 2 CrO 4 ⋅4H 2 O. e prepared electrodes were �nally conditioned for 2 h by soaking in a 1.0 × 10 −2 M Na 2 CrO 4 ⋅4H 2 O solution.A silver-silver chloride wire was used as the internal reference electrode.(1) A small magnetic stirring bar and 20 mL of chromate solution were placed in a 50 mL beaker.e test solutions were constantly stirred by a magnetic stirrer.e electrodes were conditioned with chromate and thoroughly rinsed with double-distilled deionized water, and the surface of the ISE was washed between measurements to minimize electrode fouling.e chromate selective electrode and reference electrode were then inserted into the solution, and the potential difference was measured.

X-Ray Diffraction Studies. e X-ray diffraction (XRD)
patterns of reference clinoptilolite and prepared nanoclinoptilolite are shown in Figures 1(a) and 1(b), respectively.e XRD pattern of the reference clinoptilolite sample was selected from the manual library of the instrument which is in accordance with the JCPDS no.39-1383.e lines and position remained intact that show a good agreement with the data of clinoptilolite in the literature [22].Good agreement between the results shows that the original natural zeolite includes clinoptilolite structure as major component.XRD pattern also showed that the used zeolite tuffs include slight amounts of quartz (3.5%) and cristobalite (7.8%) phases as impurities.Presence of the broad lines in the XRD pattern of the prepared sample agrees with the formation of nanoparticles during the ball-milling process.So, we determined the average size of this sample using , the excess of width line of the diffraction peak in radians, and , the Bragg angle in degrees, and using the Debye-Scherrer formula: where  is the mean crystallite diameter,  is the X-ray wavelength,  is the Scherer's constant (0.89),  1/2 is the full width at half maximum (FWHM) of the diffraction peaks and  is the diffraction angle.e average particle sizes of prepared nanoclinoptilolite lays between 20 and 28 nm.

Surface Morphology
Studies.Some of the SEM images of nanoclinoptilolite and the obtained SMZ are shown in Figure 3. SEM images indicated that almost in all samples, zeolite powder with particles size less than 100 nm can be recognized as a separated particle or in the form of larger agglomerates.Moreover most particles have lost their initial layered shape and converted into spherical, elliptical, or irregular shapes.By careful considerations of SEM images, some crystals with sharp edges and clean surfaces were observed; which are about 100-200 nm in size.erefore, it can be concluded that careful selection of milling conditions may result in production of nanoclinoptilolite zeolite with desirable crystalline structure.Nevertheless, mechanical production of zeolitic nanoparticles by means of planetary ball mills may also reduce the zeolite crystallinity.

eoretical and Membrane Composition Investigations.
Anionic properties can easily be imparted on the zeolitic surface using the surface modi�cation by surfactants.�hen the initial concentration of the sorbed surfactant molecules is less than its critical micelle concentration (CMC), a monolayer of surfactant molecules forms on the zeolite surface.At concentration greater than the CMC and when enough surfactant exists in the system, the sorbed surfactant molecules form bilayers which induces positively charged functional groups to orient toward the solution; thus creating sorption sites for anions was performed [23][24][25][26][27][28].e surfactant molecules form bilayers on zeolite external surfaces with the lower layer held by electrostatic interaction between the negatively charged zeolite surface and the positively charged surfactant head groups while the upper layer is bound to the lower layer by hydrophobic forces between the surfactant tail groups in both layers [29].Sorption of inorganic anions on cationic surfactant modi�ed zeolite has been attributed to the formation of a surface-anion complex [30].e positively charged head groups are balanced by counter ions, and the sorption or exchange of other anionic constituents involves the replacement of weakly held counter ions by more strongly held counter ions.Preliminary experiments showed that when SMZ was mixed with a solution containing chromate anion, the concentration of chromate in the remaining solution was decreased.ese investigations con�rm that the double layers of surfactant on the surface of zeolite have been formed and SMZ can adsorb anions.Adsorption of colored anions such as chromate, dichromate, and permanganate onto the surface of the SMZ causes colored surfaces and this color is stable even by rinsing the surface for several times while if the raw zeolite was added to this colored anions solution, the anion adsorption separated with rinse easily and color intensity on surface of zeolite decreases extremely.is method was reported as the easiest and fastest way to con�rmation for produce of SMZ in previous works [21,31].
In this work, we were interested in investigating the possibility of the use of surfactant modi�ed zeolite, formulated as SMZ, as an ionophore in PVC-based membranes for chromate ion.us, in preliminary studies, SMZ was used as an ionophore to prepare PVC membrane ion-selective electrodes for a wide variety of anions.e potential responses of various anion-selective electrodes based on SMZ used are shown in Figure 4 and Table 1.As seen, with the exception of chromate ion, all anions tested show negligible responses in the concentration range of 1.0 × 10 −7 -1.0 × 10 −1 mol L −1 .Noteworthy, among these anions, chromate shows a higher potential response at higher concentrations.Hence, the modi�ed nanoclinoptilolite with HDTMA was used as an ionophore for construction of a potentiometric sensor for this anion.Since CrO 4 2− is the predominant form of chromium species at the conditions used in this work, the sorption mechanism can be shown by the following reaction: where, R 4 N + represents the anion-exchange sites of HDTMA surfactant on SMZ.us, when the SMZ-PVC membrane electrode is immersed in the chromate solutions, an anion exchange reaction occurs between the anion on the electrode surface (Br − ) and CrO 4 2− in the solution.So, the electrode was investigated in further detail.Secondly, the amount of SMZ modi�er was optimized.e responses of the electrodes to several SMZ compositions are presented in Table 2.It is clear that only the electrode containing 4% SMZ prepared from clinoptilolite at 100 mM HDTMA solution shows Nernstian behavior.is electrode also showed good linear response within the concentration range of 1.0 × 10 − to 1.0 × 10 −2 mol L −1 chromate.In the 50 mmol L −1 HDTMA solution, a bilayer may be formed partially at the zeolite surface, and the obtained SMZ is not suitable for CrO 4 2− adsorption (Table 2A).e electrode C in Table 2, prepared in a 200 mmol L −1 HDTMA solution, does not exhibit Nernstian behavior.In this case, a partial bilayer may be loaded on the zeolite surface.Consequently, the obtained SMZ does not respond appropriately to CrO 4 2− .ese observations are in accordance with our previous works [18][19][20][21] and also with results for antimonate sorption [32].Wingenfelder et al. found when the external CEC was almost twofold, the zeolite absorbed >98% of the initial HDTMA in solution.When the applied HDTMA concentrations were more than twice the concentration of the external CEC, the zeolite absorbed 84% of the initial HDTMA concentration.Sullivan et al. upon the FT-Raman spectroscopy and batch sorption methods showed the orientation of surfactant depends on the surfactant's amount that was adsorbed onto surface of zeolites (Figure 5).For example, if concentration of surfactant is lower than the certain value, interaction between surfactant and zeolite will be horizontal; it means that, at lower loadings, the sorbed HDTMA tail groups tended to have more disorder, similar to solution monomers shown in Figures 5(a) and 5(b), but if concentration of surfactant is appropriate, interaction of them will be vertical, and in this situation we have the highest order, shown in Figure 5(c) [33].
e response properties of ion-selective electrodes based on ionophore are strongly in�uenced by the membrane composition, especially ionic sites [34,35].In the case of ISEs based on neutral carriers, ionic sites with a charge sign opposite to that of the primary ions are necessary for obtaining a Nernstian response, to decrease the membrane resistance, reduce the counter-ion interferences, and improve the detection limit and optimization of selectivity.In ISEs based on the electrically charged carriers, on the other hand, the use of ionic sites with the same charge sign as the primary ions can signi�cantly improve the response properties and selectivity of the sensor.us, the in�uence of the membrane composition and amount of SMZ as an ionophore on the potential response of the chromate sensor was investigated.e effect of the membrane composition on the potentiometric response of the electrodes was investigated by varying the proportions of the ionophore (SMZ), plasticizer (DOP), and PVC.Table 3 presents the compositions of several typical membranes, along with their potentiometric response characteristics.e potential response of all the SMZ-PVC electrodes was studied in the concentration covering the range of 1 × 10 −7 -1 × 10 −1 mol L −1 chromate.e results indicate that the electrode comprised of 4% SMZ, 32% PVC, and 64% DOP is suitable and the electrode exhibited a Nernstian response with a small standard deviation (membrane no. 2, Table 3).us, this membrane composition (optimized electrode) was used in the next experiments.
e performance characteristics of the investigated electrodes were studied as a function of conditioning times.For this purpose, the electrode was soaked in 1.0 × 10 −2 mol L −1 chromate solution for different intervals, and then the effect of soaking on the calibration graph slope was measured.Table 4 presents the results of investigating conditioning times.e results show that the best Nernstian response, combined with a dynamic range, was obtained at 2 h.Hence, this time was employed as the conditioning time in later studies.).e distribution of Cr (VI) species is dependent on both the total concentration of Cr (VI) and pH of the equilibrium solution.e following equations describe the distribution of chromium species in aqueous solution [36]: potential response of the proposed electrode in the pH range of 1-12 was tested, and the results are shown in Figure 6.As can be seen, the potential response remains almost constant over the pH range of 6.8-10.7.pH values decrease in the measurable concentration of chromate and formation of dichromate and polychromate [38], to which the sensor does not appear to respond in Nernstian manner.For studying the effect of pH, another method was used as follows: different series of solutions covering the concentration range from 1×10 − to 1×10 −2 mol L −1 were made, and the pH of each set was kept constant value.Results which are shown in Table 5 show the good Nernstian behavior for pH = 8.Subsequently, in our investigations, the solution pH values were adjusted to 8 when required.More acidic pH values affected the potential response probably due to partial destruction of the zeolite lattice, which occurs in acidic media [39], and interference from Cl − .Zeolite minerals decompose in acids and form gels or insoluble silica depending on the structure of the zeolite and Si/Al ratio [21].Conversely, in high pH media, the interference by a high concentration of OH − ions induces an increased electrode response [21].In addition to the dealumination of zeolites in acidic media, many of the zeolites, although stable under the particular conditions of their synthesis, may undergo conversion to different species with time [21].In these conditions, SMZ may decompose due to the different external CEC values of clinoptilolite and the converted materials.In highly acidic and basic media, the electrode may respond to monovalent Cl − and OH − anion additions to the chromate, and due to the their different charges and hence different slopes (according to Nernst equation for monovalent Cl − and OH − anions) with chromate, a potential change occurs.In addition to these problems, chromate protonation that occurs with a decrease in pH must be taken into consideration.erefore, a pH of 8 was selected as the optimal value.[21] and may also be related to surfactant desorption from the SMZ and destruction of the electrode surface at higher temperatures.

Response Time.
Generally, dynamic response time is an important factor for any sensor.e critical EMF response of the electrode was assessed according to IUPAC recommendations [42].e average time required for the mentioned electrode to reach a potential within ±1 mV of the �nal equilibrium value aer successive immersion of chromate ion solutions, each having a 10-fold difference in concentration, was investigated.e measurements of potential versus time were carried out with chromate solutions from lower 1.0 × 10 − mol L −1 to higher 1.0 × 10 −1 mol L −1 concentrations (Figure 7).e electrode yielded steady potentials within 5-10 s. e potentials remained constant for about 150 s. e response time increased to 10 s when the concentration was lowered to 1.0 × 10 − mol L −1 because of the longer equilibration time, whereas higher concentrations have response times of less than 5 s.

Potentiometric Selectivity.
Interfering of concomitants on the response of the analytical methods is the most limiting factor of these methods.e most important characteristic of any ion-selective electrode is its response for the primary ion in the presence of other anions in solution, which is expressed in terms of the potentiometric selectivity coefficient ( A,B pot ).Potentiometric selectivity coefficients were determined by the �xed interference method (FIM) at �xed concentration of interfering ions and varying amounts of the primary ion [43].is method measured the potential of the electrode in a series of solutions containing a �xed activity of the interfering anion  B (0.1 mol L −1 ) and varied concentrations of chromate in the range of 5 × 10 −6 to 1 × 10 −2 mol L −1 .e potential values obtained are plotted versus the activity of the primary ion.e intersection of the extrapolation of the linear portions of this curve will indicate the value of  A which is to be used to calculate  A,B pot from the equation (Figure 8): where both  A and  B have the same signs, positive or negative, and are the valence of primary and interfering ions, respectively.e selectivities of electrodes were also investigated by the separate solution method [43] ( A   B  10 −2 mol L −1 ) in the way that the potential of a cell comprising an ion-selective electrode and a reference electrode is measured with each of two separate solutions, one containing the ion A at the activity  A (but no B) and the other containing the ion B at the same activity  B   A (but no A).If the measured values are  1 and  2 , respectively, the value of  A,B pot can be calculated from the equation: In Table 7, the selectivity coefficients and response characteristics of the SMZ-PVC electrode are compared with the other chromate selective electrodes.According to the results, selectivity shows deviation from Hofmeister series.e reason the selectivity coefficients of this electrode do not comply with the Hofmeister series, and show an anti-Hofmeister behavior, is thought to be due to the possible interaction of the anions with HDTMA [44].Depending on the mode of interaction with cationic surfactants, anions are classi�ed into four classes [44].Most "Hofmeister" anions do not bind strongly on the head groups and behave as typical counter ions (OH − , CH 3 COO − , and HCOO − ).Class II ions are usually large complex ions with large polarizabilities, which form water-insoluble ion pairs with the amine-based surfactants (I − , SCN − , and ClO 4 − ).Class III consists of complex anions containing transition metal ions, which form covalent bonds with the amine-head groups and also between themselves through oxygen bridges.Finally, class IV consists of a number of hydrophobic organic anions (benzoate, salicylate, tosylate), which partly dissolve in the palisade layer of the surfactant.Hofmeister anions also affect the size and shape of micelles of cationic surfactants and their phase diagrams regarding their hydrated sizes [44].It oen happens that several different anions compete for a cationic micellar surface.
According to the previous discussion, different anions have different interactions with head group and the chain of HDTMA surfactant, which in turn cause a deviation from Hofmeister series.solvent polymeric membranes depends on the distribution coefficient of the ionophore and the plasticizer between the aqueous and membrane phases [43].Hence, the lifetime of electrodes must depend on the components of the solution and the measured specimens with electrodes.Durability of the membrane was tested by monitoring the change in the slope and linear response range with time.e lifetime of the electrode was determined by reading its potentials and plotting the calibration curves for a period of 2 months.e results of the studies revealed a very slight gradual decrease in the slope of the electrode aer 2 months.is negative effect of soaking is attributed to the leaching of the ionophore and plasticizer to the bathing solution which is related to the distribution equilibria and diffusion rates.
Another explanation can be related to the penetration of the water molecules from the bathing solution to the membrane and consequently slow solvation of the lipophilic salts in situ, so they are slowly leached out and limit the electrode life [45].

Response Characteristics of the Electrode.
Using the optimized membrane composition and conditions described previosly, the critical response characteristics of the electrode were evaluated according to IUPAC recommendations [42].e EMF response of the electrode to different concentrations of chromate samples demonstrated a linear range from 5.0 × 10 −6 to 1.0 × 10 −2 ( 2 = 0.9954) (Figure 9).Inset of Figure 8 shows the linear segment of the response of the electrode.e slope of calibration curve was 29.4 ± 0.3 mV per decade of CrO 4 2− concentration.To study the effect of modi�cation and also the effect of nanoparticles on the potentiometric behavior of the proposed electrode, the potentiometric responses of some different membranes containing PVC alone, PVC + nanoclinoptilolite (NCP), PVC + surfactant, PVC + SMZ-clinoptilolite (SMZ-CP), and the proposed modi�ed electrode (SMZ-NCP), were studied and the obtained results are shown in Figure 10.As results show, the membranes containing the raw PVC, nanozeolite alone, and also surfactant alone do not show a Nernstian response towards chromate.Comparing the results con�rms the active component in the membranes is SMZ.According to the results, both prepared SMZs using microand nano-sized clinoptilolite show a Nernstian response towards chromate.But the obtained linear rang decreased when surfactant modi�ed micro-sized clinoptilolite was used (2.0 × 10 −5 to 5.0 × 10 −3 ,  2 = 0.9918, slope = 30.36mV per decade).
e lower and higher detection limits were 2.0 × 10 −7 and 2.0 × 10 −2 mol L −1 , respectively.e relative standard deviation (RSD) of a single electrode (within electrode variation) was also funded out ( = 10).e RSD was small (1.01%); the good electrode repeatability indicates that the membrane is homogeneous.
e reproducibility of the potentiometric response of the optimized electrode was also investigated by measuring the response of �ve independent electrodes with the same composition.ese measurements were also carried out in 4 replicates.e RSD (between electrode variations) was less than 2.5 % (Table 8).e validity of the results was examined by "" value statistical testing, or a -test [40], using the following equation: where  2  is the variance of each class of determinations.e calculated g value was 0.4261, which is less than the critical   value at a 95% con�dence interval ( 0.0,,4  0.6287,  replicates per 4 classes) [40].
As shown in Table 7, the selectivity coefficient and dynamic linear range obtained for the proposed electrode are superior in some cases and comparable to those reported previously for other chromate-selective membrane electrodes based on a variety of different ionophores.Notably, in all cases in Table 7, response time, conditioning time, Nernstian behavior (slope values), and electrode response of the independent pH range for the proposed SMZ-PVC electrode are improved compared with other reported electrodes.Additionally, the short conditioning period of the proposed electrode saves time.e RSD values obtained from analyzing the reproducibility and repeatability of the SMZ-PVC electrode con�rmed that the SMZ-PVC electrode has good precision.

Analytical Application
4.1.Analysis of Electroplating Waste.e proposed chromate sensor was also used for determining chromium (as chromate) in wastes discharged from electroplating industry.Sample from a local electroplating unit was collected, �ltered, and stored without any further pretreatment.Suitable aliquots of sample solution were analyzed, aer neutralizing with NaOH within pH range 7.0-8.0.e analyses were performed by direct potentiometry using calibration plot, and the results showed the chromate content in the wastewater obtained from measurement with the sensor was found to be in agreement with that determined by atomic absorption spectrometry (AAS) (Table 9).

Conclusions
e results of this wor� showed that the surfactant-modi�ed zeolite when incorporated in polymeric matrix membrane preserved the anions adsorption manner.e amount of the modi�er used in the membrane affected hydrophilicity and hydrophobicity properties de�nitely and this behavior will affect membrane response to different anions.According to this information the electrode with a composition SMZ : PVC : DOP in the ratio 4 : 32 : 64 made useful quali-�cation for chromate potentiometric determination among anions such as benzoate, perchlorate, chlorate, oxalate, nitrate, iodide, and cyanide.e main advantage of the potentiometric sensor is its simplicity of preparation, short conditioning time, fast response time, Nernstian behavior, and improved good selectivity.e practical utility of membrane sensor was demonstrated by its use as an indicator electrode in the potentiometric titration of CrO 4 2− with Pb 2+ and was successfully applied to the determination of chromate in real samples.

2. 5 .
Potential Measurements.e potential measurements were carried out at 25 ± 1 • C with a Metrohm pH meter 691.To prevent any transfer of ions to the measuring solution, the external reference electrode used was an Ag/AgCl electrode shielded by an intermediate salt bridge compartment containing the background electrolyte.e cells for potential (mV) measurement were of the following con�guration: Ag/AgCl, KCl (3M) | internal solution chromate 1.0× 10 −2 M | electrode membrane | test solution | Ag/AgCl, KCl (3M) .

F 1 :
XRD patterns of (a) the reference pattern of clinoptilolite and (b) prepared zeolite nanoclinoptilolite.

3. 1 . 2 .F 2 :
FT-IR Studies.e FT-IR spectra of natural and modi�ed zeolite samples covering the range from 400 to 4000 cm −1 were taken.FT-IR spectra obtained using KBr pellets of the unmodi�ed clinoptilolite zeolite, the HDTMA surfactant, and the SMZ are presented in Figures2(a), 2(b), and 2(c), respectively.Comparing the IR spectrum of the SMZ sample with others (Figure 2(c)) shows characteristic peaks at wavenumbers of 3016, 2919, 2849, and 1483 cm −1 FT-IR patterns for (a) the prepared zeolite nanoclinoptilolite, (b) HDTMA and (c) SMZ.which indicate the incorporation of HDTMA onto the zeolite surface.Although there are some slight shis in the peaks position at each wavenumber in spectra of (a) and (c) in Figure2, both of the patterns are nearly the same, showing unchanging of the zeolite structure during the HDTMA loading.Particularly, the presence of peaks at 3016, 2919, 2849 and 1483 cm −1 in the SMZ spectrum (related to C-H, C-C and N-C vibrations in the surfactants, resp.), that are absent in the clinoptilolite spectrum, con�rms the loading of HDTMA onto the zeolite surface.

T 1 :F 3 :T 2 :F 4 :
Potentiometric response of the SMZ-PVC electrode toward different anions (  ) with each electrode conditioned in a 0.01 M solution of the corresponding anion for 2 h at 25 (membrane includes 4% SMZ).SEM images of nano-clinoptilolite (a-c) and SMZ (d-f).Optimization of the modi�er percentage in chromate-selective electrode based on SMZ at 25 ∘ C (  ).ElectrodeModi�er percentage Slope (mV⋅decade −1 ) (  ) A 4% (nanoclinoptilolite +50 mM HDTMA) A schematic diagram of the electrode response to various anions.

F 5 :
Various orientations of surfactant molecules on zeolite surface zeolite surface (a) lower coverage monolayer, (b) midcoverage monolayer, and (c) higher coverage monolayer.

T 6 :
Effect of temperature on the optimized SMZ-PVC electrode behavior (  ).

3. 2 . 6 . 5 F 8 :
Life-Time Study.Lifetime or durability of ISEs is one of the important factors in deciding the applicability of ISE.e lifetime of electrodes based on ionophores in A schematic diagram of the electrode response to various interferences with FIM method.

F 9 :
Potentiometric response of the proposed electrodes towards chromate ion (inset: linear segment (calibration curve) of the electrode response).

4. 2 . 2 − 2 −
Potentiometric Titration.e analytical applicability of the proposed sensor has been tested by using it as an indicator electrode to �nd out the end point in potentiometric titration of CrO 4 2− with Pb 2+ .10 mL of 1.0 × 10 −2 mol L −1 CrO 4 solution was brought to pH 8.0 with NaOH and then titrated with 1.0 × 10 −2 mol L −1 Pb 2+ solutions.Addition of Pb 2+ to the solution precipitates Pb-CrO 4 which causes an increase in potentials as a result of decrease in the concentration of CrO 4 in the solution.e end point obtained from the potentiometric curve is sharp and corresponds to the 1 : 1 stoichiometry of Pb 2+ : CrO 4 2− (Figure 11).Inset of Figure 11 shows the �rst-derivation titration curve of electrode.erefore, the sensor assembly can be successfully used as an indicator electrode.
T 3: Effect of SMZ-PVC membrane composition on the potentiometric behavior of the chromate-selective electrode (  ) at 25 ∘ C.
[40]0,35,40,45, of pH on Nernstian behavior of the chromateselective electrode (−log  = 5-2;   ) at 25 ∘ C. To study the thermal stability of the sensors, they constructed at different test solution temperatures of25,30,35,40,45, 50, and 60 ∘ C. e results which are shown in Table6show the electrode exhibited good Nernstian behavior in the range of 25-40 ∘ C. e Nernstian and experimental slope values at each temperature were compared statistically by -test[40], showing conformity of the slopes for 5 replicates.On the other hand, the smaller experimental -values than its critical value in the 25-40 ∘ C temperature range show systematic errors affect the response the proposed electrode.e results prove the validation of the obtained results with the expected values in the temperature range of 25-40 ∘ C.However, at temperatures higher than 40 ∘ C the slopes show a signi�cant deviation from the theoretical values, because of the presence of the systematic errors in the responses.e main factors for this deviation can be considered as the phase boundary equilibrium at the gel layer test solution interface which is disturbed by the thermal agitation of the solution F 6: In�uence of pH on the potential response of the chromateselective electrode to 1.0 × 10 −4 mol L −1 chromate.3.2.3.Effect of Temperature.
T 7: Comparison of some potentiometric characteristics of the chromate ISE described in this paper with previously published reports.
a Salicylate.b e pH range of the electrode response is independent of pH.
T 8: Potentiometric response of four independent optimized SMZ-PVC membrane electrodes.