Molybdate Anion Recognition through a Cationic Crowned ... · Molybdate Anion Recognition Through A Cationic Crowned Ionopore Based Electrochemical Sensor: Detection Of An Environmental

INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 1, No 6, 2011

© Copyright 2010 All rights reserved Integrated Publishing Association

Research article ISSN 0976 – 4402

Received on January, 2011 Published on April 2011 1361

Molybdate Anion Recognition through a Cationic Crowned Ionopore Based Electrochemical Sensor: Detection of an Environmental Pollutant

Sethi.B 1 , Kumar.S 1 , Singh.R 2 , Gupta.V.K 3 ,Singh.L.P 4 1 M.K.P. (P.G.) College, Dehradun, Uttarakhand, India 2 D.B.S Degree College, Dehradun, Uttarakhand, India

3 Indian Institute of Technology, Roorkee, Uttarakhand, India 4 Central Building Research Institute, Roorkee, Uttarakhand, India

[email protected]

ABSTRACT

In the view of the complexities involved in the environmental cycling of hazardous metal species, attempts have been made to develop a poly (vinyl chloride) (PVC) based sensor for detection of MoO4

2 ions using positively charged diaza crown ether (18crown6). The influence of membrane compositions on the potentiometric response of the electrodes have been found to substantially improve the performance characteristics of the membrane. Optimum performance was observed with the membrane having IPVCTBABDBP in the ratio 1:33:1:65 (w/w). The sensor shows a linear potential response for MoO4

2 over a wide concentration range 2.5x10 5 1.0x10 1 M with Nernstian compliance 31.2 mVdecade 1 of activity within pH range 5.8 to 10.9 and a fast response time of 30 s. The sensor has been found to work satisfactorily in partially nonaqueous media up to 15% (v/v) content of methanol, ethanol. It works over a period of 45 days with good reproducibility. The analytical usefulness of the proposed electrode has been evaluated by its application in the determination of molybdate ions in corrosion inhibitor samples.

Keywords: Metal ion pollution, Diaza crown ether; Molybdateselective electrode; Sensors; PVC membrane; Potentiometry

1. Introduction

1.1 Molybdenum and environment

Anthropogenic activities have long been identified to be the main contributor to the perturbation of metallic cycle. However, relatively little is known about how such perturbations interact or influence between different environmental media. In the view of the complexities involved in the environmental cycling of hazardous metal species, issues related to their pollution have nowadays often been treated as one of the major issues in the environmental societies and agencies. The ability to rapidly detect environmentally significant metallic components is hence important to account for various environmental processes regulating their cycle. Molybdenum metal is one such metal which along with its alloys find applications in various industries including electrical and electronic devices, materials processing, glass manufacturing, high temperature furnaces and equipment, aerospace & defense applications. High melting point, high thermal and electrical conductivity, thermal expansion, hightemperature strength, vapor pressure, environmental stability and resistance to abrasion and wear are some properties that make molybdenum metal and its alloys the materials of choice for various industrial processes. Release of molybdenum into the environment can also occur through weathering and agricultural uses of

Molybdate Anion Recognition Through A Cationic Crowned Ionopore Based Electrochemical Sensor: Detection Of An Environmental Pollutant

Sethi.B, Kumar.S, Singh.R, Gupta.V.K, Singh.L.P International Journal of Environmental Sciences Volume 1 No.6, 2011

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molybdenum compounds. The combustion of fossil fuels is a constant source of molybdenum. Molybdenum does not exist naturally in the pure metallic form and of the 5 oxidation states (2–6) the predominant species are Mo(IV) and Mo(VI). It rapidly polymerizes to a wide variety of complex polymolybdate compounds in solution. It is the only metal of second transition series which is equally important in biological processes also. For the general population, the diet is the most important source of molybdenum and concentrations in water and air usually are negligible. The average daily dietary intake is about 0.1–0.5 mg (Barceloux, 1999). In animals, it is an indispensable cofactor for several enzymes, such as xanthine oxidase and sulfite oxidase (Holzinger et al., 1998; Mendel, 2005). Molybdenosis (teart) is a form of molybdenum toxicity that produces a disease in ruminants similar to copperdeficiency. In plants, although present in low levels, molybdenum is an essential micronutrient, and is involved in biochemical processes related to the fixing of the N2 of atmosphere by bacteria. It appears to be toxic when its concentration in plants is larger than 5 μg/g (Jabbari and Shamsipur, 1993). Though little data is available on the human toxicity of molybdenum, a goutlike syndrome and pneumoconiosis have been found to be associated with excessive concentrations of molybdenum. Thus the determination of molybdenum at trace levels is of special interest (Marczenko and Lobinski, 1991). Several analytical techniques, such as inductively coupled plasma–atomic emission spectrometry (ICPAES), inductively coupled plasma–mass spectrometry (ICPMS), atomic absorption spectrometry, spectrophotometry, voltammetry and flowinjection analysis (FIA), have been used for molybdenum assay in various samples. These methods usually involve complicated methodology, sample pretreatment and are cost ineffective. A technique which permits rapid, accurate and low cost analysis is the ideal choice and such a situation is met to a great extent by ion sensors. These sensors offer several advantages over other analytical techniques particularly in regard to speed, simplicity and cost of operating systems. Often no sampling, dilution or reagent addition is required, and changes in the analyte concentration or activity can be displayed in real time. Thus, these sensors allow a wide range of environmental pollutants to be detected and quantified rapidly, sensitively and selectively.

1.2 Choice of ionophore

Few ISEs have been reported for the determination of molybdate ions. Malik et al., 1982 reported Polystyrene based membranes of zirconium molybdate gel for determination of molybdate ions. But the electrode exhibited a nonNernstian response and suffered serious interference from vanadate, halides, thiocyanate and nitrate. Zirconium oxide was explored as the membrane phase embedded in polystyrene matrix for the determination of molybdate ions by Srivastava et al., 1983. Some liquid membrane electrodes based on trioctyl ammonium aspartate (Sergeev et al., 1983) and MoNPhenylbenzohydroxamic acid chelate (Shpigun et al., 1985) has also been reported. Lee and Chen (1990) used insoluble lead salts for the above purpose. Gupta et al. reported two kinds of cobalt porphyrin complexes based ISEs for a molybdate assay with improved response characteristics compared to the aforementioned ISEs (Gupta et al., 1999, 2002). In 2006, Fang et al. explored several metalloporphyrin complexes as electroactive materials for the preparation of molybdatesensitive electrodes. These electrodes showed narrow working concentration ranges and interference from various anions. It was, therefore, felt worthwhile to develop a new sensor for molybdate ions.

Research on the supramolecular chemistry of anions (selective recognition of guest molecules by synthetic receptors) has accelerated in recent years. The overwhelming majority of host compounds of this group are based on cationic nitrogen compounds. Thus, different azamacrocyclic compounds viz. azacrown ethers and cryptands have been explored as



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ionophores for anionic species. The binding of an anion by crown ether can be improved by introduction of positive charges in the ligand. Positive charge can be introduced onto neutral crown ethers in two ways: either by protonation of nitrogen donor or complexation by metal ions. In this paper we employed cationic diazacrown ether, I (figure 1) as a suitable ion carrier to construct a PVCbased membrane electrode which exhibits significantly high selectivity to molybdate ions over other anions.

Figure 1: Structure of Cationic Diazacrown ether (18crown6) (I).

2. Experimental

2.1 Reagents and Apparatus

All reagents used were of analytical reagent grade and were used without further purification. High molecular weight poly(vinyl chloride) (PVC) were obtained from Aldrich, USA; tetrabutyl ammonium bromide (TBAB), BDH, England; dibutylphthalate (DBP); and dioctylphthalate (DOP), Reidel India; dibutyl (butyl) phosphonate (DBBP), Mobil, USA, 1 chloronaphthalene (CN), E.Merck, Germany and tetrahydrofuran (THF) was obtained from Ranbaxy, India. Bicarbonate, bromide, chloride, iodide, fluoride, acetate, sulphate, sulphite, nitrate, nitrite, phosphate, thiocyanate, molybdate, dichromate, permanganate etc. solutions were prepared from sodium or potassium salts and were obtained from Ranbaxy, India. Potentiometric measurements were carried out on a Mettler Toledo pH/ion analyser (model MA235).

2.2 Fabrication of electrodes

The PVCbased membranes were prepared by adding THF (510 ml) to 1% of ionophore, I and PVC (33%), cation excluder, tetrabutyl ammonium bromide (TBAB) (1%), Solvent mediators (DBP, DOP, DBBP and CN) (65%) were further added to obtain membranes of different compositions (Table 1). The optimum composition of the membranes was obtained after a good deal of experimentation. After complete dissolution of all the components and thorough mixing, the homogeneous mixture was poured into polyacrylate rings placed on a smooth glass plate. THF was allowed to evaporate at room temperature. The viscosity of the solution and solvent evaporation were carefully controlled to avoid drastic variation in the morphology and thickness of the membranes, which ultimately affected the sensor response. After 24 h, transparent membranes of 0.5 mm thickness were obtained. A 5 mm diameter piece was cut out and glued to one end of a Pyrex glass tube (Katsu et al., 2002).

2.3 Conditioning of membranes and potential measurements

Proper equilibration of a membrane, to be used as a sensor, is essential for generating stable and reproducible potentials and to avoid long response time. It is necessary to optimize the concentration of the contacting solution and the time required for complete equilibration.



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The membranes were found to be equilibrated within 23 days in 1.0 M MoO4 2 solution and

the potentials were measured at 25 ± 0.1 0 C using PVC matrix membranes in conjunction with saturated calomel electrodes (SCE) by setting up the following cell assembly:

A fixed concentration of MoO4 2 solution was taken as internal solution (0.1 M) and a

saturated calomel electrode was used as a reference electrode. The performance characteristics of the electrodes was examined by measuring potentials of the primary ion solutions with a concentration range of 10 6 to 10 1 M. Selectivity values ( Pot

B A K , ) were evaluated using the ‘Fixed Interference Method’ (Umezawa et al., 1995). The initial concentration for the interfering ions in FIM was 10 2 M.

3. Results and discussion

3.1 Initial evaluation of ISE responses

In order to investigate the suitability of the ionophore for comparison purposes, ionophore was used as to prepare several PVC membrane ion selective electrodes under identical conditions for a variety of anions. It has been observed from figure 2 that except for the MoO4

2 ion for all other anions, the slope of the corresponding potential plot is much different than the expected Nernstian slopes of 59, 29.5 and 20 mV decade 1 of activity for the univalent, bivalent and trivalent anions respectively, although over limited concentration range. As is obvious from the obtained results, the molybdate ion with the most Nernstian response over a wide concentration range seems to be suitably determined with the PVC membrane electrode based on ionophore I. Thus, in the next step, the ionophore was used as a potential carrier in construction of PVC membrane ionselective electrodes for molybdate ion.

Figure 2: Potentiometric response curves of PVCbased electrodes containing I as ionophore towards various anions.

3.2 Optimization of membrane composition

The sensitivity and performance of an ionophore significantly depends on the membrane composition and the nature of solvent mediators. Generally, a good solvent mediator should exhibit high lipophilicity, high molecular weight, low exudation from polymeric matrix and

Internal reference electrode

Internal solution (0.1M MoO4

2 )

PVC Membrane

Test solution External reference electrode



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high capacity to dissolve the substrate and other additives present in the membrane. It is documented that the addition of the plasticizers not only improves the workability of the membranes but also contributes significantly towards the improvement in the working concentration range, stability and shelf life of the sensor [Hogberg, 1980; Jadhav and Bakker, 2001]. This is achieved by decreasing the zerocurrent ion refluxes from the membrane into the solution through judiciously optimizing the membrane ingredients and inner electrolyte composition.

Five membranes with different compositions (Table 1) have been prepared and their response characteristics were evaluated according to the IUPAC recommendation (Guilbault et al., 1976). Further, the effect of addition of tetrabutyl ammonium bromide (TBAB), cation excluder was also observed. A perusal of data presented in Table 1 shows that the sensor based on the membrane of the ionophore (without plasticizer) exhibited a narrow working concentration range of 1.6x10 4 to1.0x10 1 with a slope 26.7 mV decade 1 of activity (Figure 3). Addition of plasticizers to the membrane enhanced sensitivity of the sensors, as the membranes fabricated using DBP showed a potential response for MoO4

2 over a wide concentration range of 2.5x10 5 1.0x10 1 M with a nearNernstian slope of 31.2 mV decade 1 (Figure 3). Addition of other plasticizers viz. DOP, DBBP and CN also improved performance of the membranes. Further, it was observed that the addition of tetrabutyl ammonium bromide (TBAB) increases the sensitivity of the membrane as it reduces cationic interference. The composition of the membranes showing the best results (wide working concentration, Nernstian/nearNernstian slope, fast response time) are presented in Table 1. The best performance characteristics are obtained with the membrane having DBP as plasticizer (sensor no. 2). This sensor exhibits the maximum working concentration range of 2.5x10 5 1.0x10 1 Mwith a slope of 31.2 mVdecade −1 of activity.

Figure 3: Variation of cell potential with activity of MoO4 2 ions of PVC based membranes

of I with different plasticizers. (i: CN; ii: DBBP; iii: without plasticizer; iv: DOP; v: DBP).



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Table 1: Composition and response characteristics of the PVCbased Cationic diaza Crown ether (I) membrane sensor for MoO4

2– .

3.3 pH and nonaqueous effect

The molybdate species in aqueous solutions depend on the molybdenum concentration and pH of the solution. In alkaline and neutral solutions molybdates are present as the monomeric [MoO4] 2 ion. As the pH is lowered polymerisation condensation occurs giving at pH 56 the heptamolybdate ion [Mo7O24] 6 and at pH 35 the octamolybdate ion [Mo8O26] 4 . Both ions are built up from linked MoO6 octahedra species. At pH 0.9 MoO3 precipitates and in more acidic solutions the [MoO2] 2+ ion is formed [Mitchell, 1990; Busey and Keller, 1964].

Figure 4: Effect of pH on the potential response of the optimized MoO4 2 selective electrode

The effect of pH on the sensor potential shows that the useful pH range where the potentials remain constant is 5.8 to 10.9 (figure 4). pH of aqueous 1.0x10 3 and 1.0x10 4 M MoO4

2

solution was altered by dilute nitric acid or sodium hydroxide and the potential of the solutions was monitored. As pH value decreases below 5.8, the potential values are drastically changed. This is due to decrease in the molybdate concentration and formation of para, tri and/or tetramolybdate ionic forms.

The performance of the sensor was further assessed in partial nonaqueous media, i.e. methanol–water and ethanol–water mixture. The results obtained are compiled in Table 2 and show that up to 15 % nonaqueous content no significant change occurs in the slope and working concentration of the sensor. However, above 15 % nonaqueous content, the working

Me mbr ane No.

% Composition (w/w) of various components in membranes

Working Concentration

Range

Slope (mV per decade of activity)

Response time (s)

I(%) PVC TBAB DBP DOP DBBP CN

1 1 99 1.6 x 10 4 1.0 x 10 1 26.7 52

2 1 33 1 65 2.5 x 10 5 1.0 x 10 1 31.2 30

3 1 33 1 65 5.0 x 10 5 1.0 x 10 1 32.0 38

4 1 33 1 65 7.9 x 10 5 1.0 x 10 1 33.0 45

5 1 33 1 65 3.2 x 10 4 1.0 x 10 1 34.0 41



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concentration of the sensor is significantly reduced, and thus the sensor can onlybe utilized in mixtures containing up to 15 % nonaqueous content.

Table 2: Performance of electrode system in alcoholwater mixtures

Alcohol composition %(v/v)

Slope (mV/decade of concentration)

Working Concentration range (M)

Nil 31.2 2.5x10 5 1.0x10 1

Ethanol 10 31.2 2.5 x10 5 1.0 x10 1

15 31.5 3.0 x10 5 1.0x10 1

25 33.2 4.4 x10 5 1.0x10 1 30 34.6 8.2 x10 4 1.0x10 1

Methanol 10 31.2 2.5 x10 5 1.0x10 1 15 31.8 3.6 x10 5 1.0x10 1

25 32.7 5.5 x 10 5 1.0x10 1

30 34.3 8.7 x10 4 1.0x10 1

3.4 Response time and lifetime

The practical response time of the sensor was calculated by measuring the time required to achieve 95% of the equilibrium potential from the moment of addition of 1.0x10 5 M MoO4

2

solution. Figure 5 gives the plot of EMF against time and it was found that the practical response time was 30 s. The practical reversibility required for the MoO4

2 sensor to reach a potential within ±1 mV of the final equilibrium value was measured by successive immersion in a series of the molybdate ion solutions, each having a 10fold difference in concentration. This dynamic response is plotted as EMF versus time and is shown as Figure 6. The potentials remained constant for about 1.5 min. The sensing behaviour of the membrane remained unchanged when potentials were recorded either from low to high concentrations or vice versa.

The membranes were used over a period of 45 days without significant change in potentials. Whenever a drift in potential was observed, membranes were reequilibrated with 1.0 M MoO4

2 for 23 days. The membranes were stored in 0.1 M MoO4 2 solution when not in use.

Figure 5: Practical response time of the sensor from the moment of addition of MoO4 2

(1x10 5 M) solution.



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Figure 6: Dynamic response time of the sensor for reversibility with step changes in concentration of MoO4

2 (1x 10 5 to 1x10 1 M).

3.5 Potentiometric selectivity

Selectivity is perhaps the single most important characteristic of any sensor which defines the nature of device and the extent to which it may be employed in the determination of a particular ion in the presence of other interfering ions. This is measured in terms of potentiometric selectivity coefficients which has been evaluated using fixed interference method at 1.0x10 2 M concentration of interfering ions. The potentiometric selectivity coefficient measures the response of the electrode for the primary ion in the presence of foreign ions. Table 2 shows potentiometric selectivity coefficient data of cationic diazacrown ether based ion selective electrode for interfering anions relative to MoO4

2 . The selectivity pattern for MoO4

2 clearly shows a deviation from conventional Hoffmeister anion response pattern for highly lipophilic anions such as SCN , I NO3

.The deviation from Hoffmeister series resulted from the unique interactions between the ionophore and anions, rather than hydration free energy of the anions. As the value of the selectivity coefficient is less than 1.0, the sensor is selective over all anions mentioned in Table 3. Thus these anions would not cause any interference in the estimation of Mo(VI).

Table 3: Selectivity coefficient values for molybdate selective membrane electrode for various interfering ions (B) using the Fixed Interference Method (FIM)

Interfering Ions (B)

Selectivity coefficient ( Pot

B MoO K

, 2 4

− )

log Pot B MoO

K , 2

4 −

Cr2O7 2 2.1 x 10 1 0.67

PO4 3 1.9 x 10 1 0.72

I 7.3 x 10 2 1.13 SCN 6.2 x 10 2 1.20 NO3

3.7 x 10 2 1.43 NO2

2.3 x 10 2 1.63 VO3

8.0 x 10 2 1.09 SO3

2 7.4 x 10 2 1.13 SO4

2 7.1 x 10 2 1.14 Br 5.7 x 10 3 2.24 HCO3

4.8 x 10 3 2.31 Cl 4.2 x 10 3 2.37 MnO4

5.4 x10 4 3.26



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3.6 Interference of common anions

SO4 2 , Cl are commonly present anions in aqueous systems. Therefore, in order to determine

the optimum tolerance levels of these anions, working concentration range for MoO4 2 ions

was determined in simulated mixtures containing high concentration of these interfering anions (Jain et al., 1997, Singh and Vardhan, 1995; Singh and Bhattnagar, 2003, 2004). The MoO4

2 ion concentration was varied between 1.0x10 6 and 1.0x10 1 M while fixed concentration SO4

2 and Cl ions, i.e. 1.0x10 3 , 1.0x10 4 and 5.0x10 5 M was maintained in synthetic mixtures (Figures 7,8 ).It is evident from the figure 7 that Cl ions present in concentrations ≤ 5.0 x 10 5 M can be tolerated over the whole working concentration range of the sensor. However, at higher concentrations, a divergence from the potential versus MoO4

2 activity plot was observed. Thus, the sensor can be used for estimation of MoO4 2

over reduced concentration ranges 7.9x10 5 to 1.0 x 10 1 and 1.5x10 4 to 1.0x10 1 M in the presence of 1.0x10 4 and 1.0x10 3 M Cl ions respectively. Similarly, SO4

2 can be tolerated at ≤ 5.0x10 5 M over the entire concentration range (figure 8) and the working concentration range of the sensor reduces to 6.3x10 5 to 1.0x10 1 and 1.3x 10 4 to 1.0x10 1 M SO4

2 ions, respectively. Therefore, these studies reveal that the developed sensor can tolerate a high concentration of interfering ions and, therefore, can be stated as selective over the commonly present interfering ions.

Figure 7: Interference by Cl ions: Variation of potential with activity of MoO4 2 at different

concentration levels of Cl ions.

Figure 8: Interference by SO4 2 ions: Variation of potential with activity of MoO4

2 at different concentration levels of SO4

2 ions.



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3.7 Preliminary Analytical application

Molybdates due to their very low toxicity and being less aggressive oxidants toward organic additives are often used in corrosion inhibiting formulations. They have been used for decades as a substitute for chromates for the inhibition of corrosion in mild steels over a wide range of pH.

In order to assess the practical utility of the developed MoO4 2 sensor to real samples, an

attempt was made to determine the content of MoO4 2 in corrosion inhibitor samples. The

molybdate content was determined directly by using the proposed electrode at pH of 6.2 by adding 10 ml of buffer solution to 2 ml of the sample. It was found that the content of MoO4

2– , as determined by the electrode, was in good agreement with the actual content in corrosion inhibitor samples (Table 4). Thus, the cationic diazacrown ether based electrode seems to be useful for the determination of MoO4

2– in actual samples.

Table 4: Determination of MoO4 2– in corrosion inhibitor samples using the proposed sensor

Sample Content of the sample/M

Proposed ISE/M Relative error, %

1. 0.86 x 10 3 0.84 x 10 –3 2.35 2. 1.07 x10 –4 1.12 x 10 –4 2.80

4. Conclusions

The membrane assembly prepared using positively charged diaza crown ether (18crown6) as membrane ingredient with plasticizers (DBP) and anion excluder (TBAB) exhibited linearity over a wide concentration range (2.5x10 5 1.0x10 1 M) with Nernstian slope (31.2 mVdecade 1 of activity), fast response time (30s), long lifetime (45 days) and exhibits good selectivity over a number of anions. The analytical usefulness of the proposed electrode has been evaluated by its application in the determination of molybdate ions in corrosion inhibitor samples.

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Molybdate Anion Recognition through a Cationic Crowned ... · Molybdate Anion Recognition Through A Cationic Crowned Ionopore Based Electrochemical Sensor: Detection Of An Environmental