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INTRODUCTION

An Ion-selective electrode (ISE) is a transducer (sensor) that converts the activity of a specific ion dissolved in a solution into an electrical potential that can be measured by a voltmeter or pH meter. The voltage is theoretically dependent on logarithm of the ionic activity, according to Nernst equation.

The general expression for the electrode potential of a metal M in contact with Mn+ ions, involving the electrode reaction

Mn+ (aq) + ne- [[Image:]] M(s)

may be written as

Eel = Eoel + RT ln [Mn+]

nF

= Eoel + 0.0591 log [Mn+]

n

The above equation gives the effect of the concentration of Mn+ ion on the potential of Mn+ , and is known as the Nernst equation.

The sensing part of the electrode is usually made as an ion-selective membrane, along with a reference electrode. Ion-selective electrodes are used in biochemical and biophysical research, where measurements of ionic concentration in an aqueous solution are required, usually on a real time basis. It measures the potential of a specific ion in solution. This potential is measured against a stable reference electrode of constant potential. The potential difference between two electrodes will depend upon the activity of the specific ion in solution. This activity is related to the concentration of specific ion, therefore following the end-user to make an analytical measurement of specific ion. Several types of sensing electrodes are commercially available. They are classified by the nature of membrane material used to construct the electrode. It is this difference in membrane construction that makes electrodes selective for a particular ion. The most important specification for ion selective electrodes is the type of ion the user needs to detect. Other important factors are the concentration range and accuracy, pH range through which the electrodes can operate and the response time, which is typically given as the time needed to reach 95% of the final value. Lindner et al. (1) build pH-sensitive, ion-selective electrodes and looked into the bulk of solvent polymeric membranes during potentiometric measurements (spectropotentiometry) and image concentration profiles in situ with high spatial & temporal resolution.

Ion selective electrodes may be half-cell models that require a separate reference electrode, or combination models composed of two parts, the measuring electrode and the reference electrode.

Samples studied by ion selective electrodes may be aqueous, non-aqueous, or dry, and the electrodes may have many different types of membranes. Each type of membrane has its own unique characteristics that make it the best choice for a particular application.

An electrochemical sensor is based on thin films or selective membranes as recognition elements, and an electrochemical half-cell equivalent to other half-cells. The ion-selective electrode must be used in conjunction with a reference electrode (i.e. ‘outer’ or ‘external’ reference electrode) to form a complete electrochemical cell. The measured potential differences (ion-selective electrode vs. outer reference electrode potentials) are linearly dependent on logarithm of the activity of a given ion in solution.

The accuracy (how close the result is to the true value) and precision (reproducibility; i.e. how close are a series of measurements on the same sample to each other) of ISE measurements can be highly variable and are dependent on several factors. Apart from the accuracy and precision of the digital potentiometer, the most important factors in achieving the most precise results is controlling the electrode drift and hysteresis (or memory), and limiting the variability in Liquid Junction Potential of the reference electrode, so that the measured voltage is reproducible. The amount of drift can vary significantly between different ions and different electrode types, with crystal membranes being generally more stable than PVC.

M.SAK-BOSNAR et al. (2) described electrochemical sensors in biotechnology. They can be classified as follows: -

  1. Potentiometric surfactant sensors: The most popular sensors in this group are the coated wire-type, liquid membrane and ISFET (ion selective field effect transistor) sensors. The sensors are suitable for determination of ionic and non-ionic surfactants. They can be used continuously in flow injection analysis (FIA) mode. 

Gavach et al. (3) were probably the first to apply liquid membrane-based ISE for titration of long chain alkyl methyl ammonium salts with sodium tetraphenyl borate. Birch et al. (4) were also among the first researchers to use liquid ion-exchange electrodes responsive to ionic surfactants. Fogg et al. (5) used silicone rubber surfactant electrode. Cotrell (6) used a coated wire polyvinyl chloride (PVC) electrode for determination of low concentration of surfactants.

Coated wire & liquid membrane surfactant sensors: Both types operate with ISE membranes. Liquid membrane can be a misleading term because these membranes are usually solids, mainly for practical reasons. However, the mobile sites are complexing agents that are dissolved in a suitable solvent and usually trapped in a matrix gel.

The determination of the concentration of anionic and cationic surfactants by titration with an ion-selective electrode as end-point titration sensor has been described. An all-solid-state surfactant sensitive electrode has been prepared, based on a teflonised graphite conducting substrate coated with plasticized PVC membrane containing a new synthesized tetrahexyldecyl-ammonium-do-decyl sulphate as anionic surfactant sensing material. The electrode exhibited Nernstian response (58.1 mV per decade) for dodecylbenzene sulphonate and near-Nernstian response (64.2 mV per decade) for dodecyl sulphate. The electrode was used as end-point indicator for potentiometric surfactant titrations. Several commercial surfactants have also been titrated. The electrode enables the titration of sorter hydrocarbon chain anionic surfactants as well.

  1. Amperometric surfactant sensors: Among amperometric surfactant sensors, there are several true surfactant biosensors that employ microorganisms or cells. 

A range of modified PVC membranes based on their permeability and selectivity to electrochemically active species has been characterized. The species are detected at an amperometric (two-electrode) cell in phosphate buffer solution. Amperometric sensor study on the selectivity of PVC membranes plasticized with surfactants and liquid crystals was reported.

  1. Conductometric surfactant sensors: M.SAK-BOSNAR et al. (2) have reported the type of surfactant sensors for on-line monitoring of the evolution of a number of particles in the emulsion polymerization by conductivity measurements. The model developed was built on the assumption that surfactant is partitioned among three principal phases of the polymerizing latex. A soft-sensor strategy was then proposed for monitoring the number of polymer particles by combining the conductivity model with the available conversion, temperature, and conductivity signals. The main objective was to validate the conductivity model in a broader range of operation conditions and to follow the evolution of N-P (nucleation/coagulation) on-line under different reaction conditions. Results showed that the model was able to perform accurate predictions of N-P even when disturbances of ± 2oC in temperature and ± 0.03 % in monomer conversion took place during the polymerization process. 

  1. Impedimetric surfactant sensors: Electrochemical sensors based on electrochemical impedance spectroscopy and cyclic voltammetry were utilized to detect water leaks and continuously monitor the time-dependent dynamics of water-oil interactions following the injection of water into industrial lubricant. Immediately following the injection, water molecule interacts with the oil 

additives forming a water-in-oil emulsion based on the inverse micelles. Emulsification was followed by gradual loss of water from the solution evaporation and electrolysis. On-line data were used to characterize the dynamics of water micellisation, evaporation and electrolysis. The value of kinetic rate constants and diffusion coefficients for the components of the water or oil system were determined.

Among various classes of chemical sensors, ion-selective electrodes (ISE) are one of the most frequently used potentiometric sensors during laboratory analysis as well as in industry, process control, physiological measurements, and environmental monitoring. The use of ISEs in environmental analysis offers several advantages over other methods of analysis. They are cheap instruments, which are easy to use and maintain, and can be easily miniaturised making them ideal for in-situ real time measurements that can not be achieved using other analysis techniques. ISEs can be used very rapidly and easily, under favourable conditions, when measuring ions in relatively dilute aqueous solutions. Furthermore, they are unaffected by sample colour or turbidity and are suitable for on-line continuous analysis. Giannetto et al. (7) have proposed new membrane electrodes based on a functionalized tetraphenylborate covalently bound to the polymeric backbone. Ebdon and coworkers (8) reported about realization of nitrate selective electrodes based on tetraalkylammonium exchanger units bound to the copolymer by means of radicalic linking. More recently, they have found practical applications in the analysis of environmental samples, usually where in-situ determinations are not practical with other methods. Mittal et al. (9) have proposed Samarium (III) selective membrane sensor based on Tin (IV) Boratophosphate which revealed good selectivities with respect to alkali, alkaline earth, some transition & rare earth metal ions and can be used in the pH range of 4-10. Nolan et al. (10) have analyzed micro-fabricated Iridium ultra microelectrodes in chloride media. They had used several different types of materials such as gold, platinum, indium and carbon as the electrode substrate. Most of these sensors consist of an electro active area (working electrode) which is connected to a bonding pad by an interconnect trace. Herdan et al. (11) have performed a field evaluation of an electrochemical probe for in situ screening of Heavy metals in groundwater. Adam et al. (12) have suggested a new heavy metal biosensor based on interaction of heavy metal ions (Cd2+ and Zn2+) with phytochelatin, which was adsorbed on the surface of the hanging mercury drop electrode, using adsorptive transfer stripping differential pulse voltammetry. On the basis of the obtained results, they proposed that the suggested technique offers simple, rapid and low-cost detection of heavy metals in environmental, biological and medical samples. Gupta et al. (13) determined strength of Vanadium, Zirconium and Molybdenum by PVC membrane based Alizarin sensor.

In this report, Styrene-acrylonitrile coplymeric membrane—Pb(II) ion selective electrode has been fabricated.

Contents [hide]

   * 1 Basic principle of ion selective electrode
   * 2 EXPERIMENT
   * 3 Determination of functional properties of co-polymeric membrane
   * 4 Preparation of electrode
         o 4.1 Equilibration of Membrane
         o 4.2 Potential measurement
         o 4.3 Estimation of lead ions
         o 4.4 Effect of pH
         o 4.5 Effect of solvent
   * 5 Response Time and Selectivity
         o 5.1 Effect of Detergent
   * 6 Effect of Surfactant
   * 7 Titrations

[edit] Basic principle of ion selective electrode

The principle of ion-selective electrode is quite well investigated and understood. An ion-selective membrane is the key component of all potentiometric ion sensors. It establishes the preference with which the sensor responds to the analyte in the presence of various interfering ions from the sample. If ions can penetrate the boundary between two phases, then electrochemical equilibrium will be reached, in which different potentials in the two phases are formed. If only one type of an ion can be exchanged between the two phases, then the potential difference formed between the phases is governed only by the activities of this target ion in these phases. When the membrane separates two solutions of different ionic activities (a1 and a2) and the membrane is only permeable to this single type of ion, the potential difference (E) across the membrane is described by the Nernst equation:

E = RT / zF · ln (a2/a1)

In practice the potential difference i.e. the electromotive force is measured between an ion selective electrode and a reference electrode, placed in the sample solution. An exemplary set-up for the measurement of electromotive force is presented in figure 1. It is important to note that this is a measurement at zero current i.e. under equilibrium conditions. Equilibrium means that the transfer of ions from the membrane into solution is equal to the transfer from the solution to the membrane.

[[Image:]]

Fig. 1: Measurement set-up of ion selective electrode (ISE)

Using a series of calibrating solutions, the response curve of an ion-selective electrode can be measured and plotted as the signal (electromotive force) versus the activity of the analyte. The linear range of the calibration curve is usually applied to determine the activity of the target ion in any unknown solution. However, it should be pointed out that only at constant ionic strength a linear relationship between the signal measured and the concentration of the analyte is maintained. The cell notation can be expressed in the following form :

External Reference Test solution Internal solution Internal Reference

(2) (1)

Saturated calomel Membrane Saturated calomel

Electrode (SCE) EL(2) EL(1) electrode (SCE) [edit] EXPERIMENT

Preparation of co-polymer

Styrene-acrylonitrile coplymeric membrane membrane is prepared by bulk polymerization on using benzoyl peroxide in a free radical reaction mechanism carried out at 60 oC.

[edit] Determination of functional properties of co-polymeric membrane

The behavior of membrane is closely linked to its structure. The membrane shows ion selective character due to the diffusion of ions. The first pre-requisite for understanding the performance of an ion exchange membrane is its complete physico-chemical characterization. The process involves the determination of those parameters which affect the electrochemical properties of the membrane i.e. water content, porosity and electrolyte absorption.

  1. Water content 

Membrane was kept immersed in a solution of 1M concentration of electrolyte (NaCl) for overnight. The membrane was then washed several times with distilled water. The membranes then weighed and dried to a constant mass in vacuum desiccator. The difference in two weighings divided by the mass of wet membranes was taken as the water content.

  1. Porosity 

It was estimated by the method followed by Mizutani and Nishimura and was calculated using the formula-

Porosity == water content / A x L x D

Where: A is the area of membrane,

L is the thickness

D is the density of water.

  1. Electrolyte absorbance 

The membrane after attaining equilibrium in electrolyte solution was wiped free of adhering electrolyte and dipped in 20 ml of distilled water. It was intermittently shaken and left as such for a few hours. The solution was transferred to a 100 ml measuring flask. The whole process was repeated three to four times and the entire solution was collected in a measuring flask. It was finally filled up to the mark with distilled water and the strength was measured conductometrically.

The functional properties of Styrene-acrylonitrile copolymer membrane are compiled in Table 1.

Table 1: Functional Properties of Styrene-acrylonitrile Co-Polymeric membrane

Membrane Water content per gram

wet membrane

(g) Porosity Amount of KCl absorbed per gram of membrane 1 0.22 0.051 3.5 x 10-2 [edit] Preparation of electrode

The membrane is prepared, detached and fixed to one end of a hollow tube with the help of araldite. The film was left undisturbed for over night to get it fixed on glass tube. The tube is filled with the reference solution (0.1 M lead nitrate) and is immersed in beaker containing test solution of different concentrations (1x10-1 – 1x10-8 M). [edit] Equilibration of Membrane

Membrane was equilibrated by partially immersing it in 0.1 M lead nitrate solution for 2-3 days. The potentials obtained after this equilibrium period were quite stable and reproducible. Membrane equilibrated for shorter durations did not develop stable potentials. The electrodes were stored in 0.1 M Pb2+ ion solution when not in use to avoid any change in metal ion concentration in the membrane phase.

[edit] Potential measurement

The potential measurement of co-polymeric membrane was determined by forming an electrochemical cell. The tube was filled with the reference solution (0.1 M lead nitrate) and immersed in a Pyrex beaker containing test solutions of different concentrations (1x10-1-1x10-8 M). The potential was then measured at different concentrations. Saturated calomel electrode was used as reference electrode. All the potential measurements were carried out using the following cell assembly:

Hg - Hg2Cl2 (s), KCl (sat.) | 0.1 M Pb2+ || Membrane || test solution | KCl (sat.), Hg2Cl2 – Hg

A digital potentiometer was used for the potential measurement at 25 ± 0.1 oC. Test solutions of Pb2+ were obtained by gradual dilution of 0.1 M Pb2+ solution and their potential measurements were made in unbuffered solution.

The variation of potential with concentration of solution is given in Table 2.

Table 2: Variation of potential with concentration of Pb2+ in solution

S.No. Concentrations (in M)

Potential (in mV) 1 1 X 10-1 0.016 2 5 X 10-2 0.023 3 1 X 10-2 0.037 4 5 X 10-3 0.047 5 1 X 10-3 0.062 6 5 X 10-4 0.071 7 1 X 10-4 0.079 8 5 X 10-5 0.083 9 1 X 10-5 0.089 10 5 X 10-6 0.091 11 1 X 10-6 0.088 12 5 X 10-7 0.086 13 1 X 10-7 0.085 [edit] Estimation of lead ions

The membrane exhibits nearly Nernstian response with Pb2+ ions having a slope of 25 mV/decade of concentrations in a wide range of 1x10-1-1x10-8 M (Figure 2), which is considered to be effective working range of the electrode. The membrane electrode can thus be used to estimate lead ion in the above-mentioned range of concentrations.

[[Image:]]

Figure 2: Variation of membrane potential with conc. of lead ions [edit] Effect of pH

The pH range, in which this sensor can be used, has also been determined. Potentials do not change in pH range of 4-8 as shown in Figure 3, which may be taken as the working pH of this membrane. The change of potential with pH is given in Table 3.

Table 3: Effect of pH on potential of Pb2+ ions in solution

S.No. PH of sample (5 x 10-6 M)

Potential (in mV) 1 1 0.019 2 2 0.024 3 3 0.034 4 4 0.039 5 5 0.041 6 6 0.040 7 7 0.041 8 8 0.041 9 9 0.044 10 10 0.046 11 11 0.049 12 12 0.051

[[Image:]]

Figure 3: Variation of membrane potential with pH of solution [edit] Effect of solvent

The electrode assembly can also be used to measure lead ion concentration in partially non-aqueous media, upto a maximum of 50% non-aqueous content. The range of concentration for Pb2+ selective electrode was observed to be approximately same as observed in pure aqueous solutions. However, the slope changes significantly in non-aqueous solutions but the functioning of the electrode system remains unaffected. The effect of partially non-aqueous media on the working of Pb2+ selective electrode is shown in Table 4.

Table 4: Effect of partially non-aqueous media on working of Pb2+ selective electrode

SolventPercentage (%)Slope (mV)

2528

Ethanol3530

5031

[edit] Response Time and Selectivity

The response time of electrode has been measured at various concentrations of salt solutions and at all dilutions it is found to be almost same, i.e., one minute. Besides this, the potentials stay constant for more than five minutes. The sensing behavior of the membrane remains unchanged when the potentials are measured either from low to high or high to low concentrations.

Selectivity is one of the most important characteristics of an electrode, which defines the nature of the device and extent to which it may be employed in the determination of a particular ion in the presence of other interfering ions. Fixed Interference Method (FIM) at 1x10-4 M ions concentration evaluates selectivity coefficients of Pb2+ membrane electrode for various salts. According to this method, a calibration curve is drawn for the primary ion with a constant interfering ion. The linear portion of the curve is extrapolated so as to make it intersect with the second linear part of the curve in the low concentration region. The selectivity coefficients were calculated from the two extrapolated linear segments of the calibrating curve.

KpotA,B = aA / aB ZB/ZA

It is observed that monovalent cations interfere significantly, whereas bivalent and polyvalent cations do not interfere. The selectivity co-efficient values for monovalent cations in case of lead selective electrodes are of the order of 0.1-.05, whereas bivalent and polyvalent cations values are 10-2 and 10-3 respectively. The low value of selectivity co-efficient indicates the poor interference of bivalent and polyvalent cations even if present in equivalent amounts. The variation in membrane potential with concentration of Pb2+ solution in the presence of Al3+ ions is given in Table 5 and its plot is shown in Figure 5. The value of selectivity coefficient K pot A,B is calculated from the given formula where charge on Al & Pb is 3+ & 2+, respectively and a signifies the activity which is given by their respective axes.

Table 5: Selectivity EMF measurement for Pb2+ ions selective electrode in presence of Al3+ ions

Sno. Conc. of 20 ml Pb(NO3)2 taken

(M) Volume of 10-4 M Al3+ soln. added

(ml) Potential

(mV) 1 1x10-1 5 -0.049 2 5x10-2 5 -0.056 3 1x10-2 5 -0.077 4 5x10-3 5 -0.089 5 1x10-3 5 -0.109 6 5x10-4 5 -0.114 7 1x10-4 5 -0.147 8 5x10-5 5 -0.153 9 1x10-5 5 -0.154 10 5x10-6 5 -0.143 11 1x10-6 5 -0.143

[[Image:]]

Figure 5: Variation of membrane potential with Conc. of Pb2+ solution in presence of Al3+ ions

KpotA,B = aA / aB ZB/ZA

Where A = reference solution, i.e. Pb2+ solution; ZA = 2+

B = Al3+ solution; ZB = 3+

aA = value on x-axis

aB = value on y-axis

On substituting the values, KpotA,B for Al3+ ions was calculated to be 3.0x 10-3.

Similarly, the values of selectivity coefficients for other ions were determined and are given in Table 6.

Table 6: Selectivity EMF measurement value for Pb2+ ions selective electrode as calculated by FIM

S.No. Interfering ion

Selectivity co-efficient values (K pot A,B 1 K+ 2.0 x 10-1 2 Na+ 3.0 x 10-1 3 Sr2+ 2.2 x 10-2 4 Ni2+ 2.5 x 10-2 5 Cu2+ 2.6 x 10-2 6 Mn2+ 4.4 x 10-2 7 Ba2+ 4.5 x 10-2 8 Al3+ 3.0 x 10-3 9 Fe3+ 3.5 x 10-3 10 Co3+ 5.0 x 10-3 [edit] Effect of Detergent

The effect of detergent, on the measurement of lead ions concentration, has been reported in Table 7.

The values of potential decreases with increase in concentration of detergent. The effect of three detergent i.e. Ariel oxy-blu, Active Wheel gold and Surf, on potential was found to be nearly same as shown in Table 7.

Table 7: Effect of detergent on working of Pb2+ ions selective electrode

Sno. Volume of sample Potential

(Surf excel) Potential

(Ariel) Potential

(Wheel) 1 20 ml + 5 ml 1% detergent 38 35 29 2 20 ml + 5 ml 2% detergent 34 32 25 3 20 ml + 5 ml 3% detergent 29 31 26 4 20 ml + 5 ml 4% detergent 28 27 26 5 20 ml + 5 ml 5% detergent 24 28 25 [edit] Effect of Surfactant

The effect of surfactant on the measurement of lead ion concentration, have also been investigated. It was found that potential remains the same even in the presence of appreciable amount of surfactant (sodium lauryl sulphate). [edit] Titrations

The membrane sensor has also been tried as end point indicator in potentiometric titrations involving lead ions. The titration plot of 20 ml of 10-3 M lead nitrate solution with 10-2 M H2SO4 is shown in Table 8. The inflection point in titration curve corresponds to the stoichiometric ratio of H2SO4 required to remove the lead ions.

Table 8: Potentiometric titrations of Pb(NO3)2 solution with H2SO4

Sno. Volume of Pb(NO3)2 taken

(ml) Volume of H2SO4 added

(ml) Potential

(mV) 1 25 0.5 37 2 25 1.0 49 3 25 1.5 60 4 25 2.0 60 5 25 2.5 58

[[Image:]]

Figure 6: Variation of membrane potential with Volume of H2SO4 added in Pb2+ solution

  1. E. Lindner et al.; A glance into the bulk of solvent polymeric pH membranes; Pure Appl. Chem., 2001, Vol. 73, No. 1, pp 17. 

  1. M. SAK-BONSAR et al.; Electrochemical sensors in biotechnology, Food Technol. Biotechnol. ; 42 (3), 197, 2004. 

  1. Gavach et al. 

  1. Birch et al. 

  1. Fogg et al. 

  1. Cotrell 

  1. M. Giannetto et al.; New membrane electrodes based on a fictionalized tetraphenylborate covalently bound to the polymeric backbone; Sensors and Actuators B 133, 2008, 235. 

  1. Ebdon and coworkers 

  1. Susheel K. Mittal, Harish K. Sharma, Ashok S. K. Kumar; Samarium (III) Selective membrane sensor based on Tin (IV) Boratophosphate; Sensors, 2004, 4, 125 

  1. M.A. Nolan, S.P. Kounaves; Failure analysis of micro-fabricated Iridium ultra microelectrodes in chloride media; Sensors and Actuators B; 50(1998); 117-124. 

  1. J. Herdan et al.; Field evaluation of an electrochemical probe for in situ screening of heavy metals in groundwater; Environmental Sci. Technol.; 1998, 32, 131. 

  1. V. Adam, J. Zehnalek et al.; Phytochelatin modified electrode surface as a sensitive heavy-metal ion biosensor; Sensors, 2005, 5, 70-84. 

  1. V.K. Gupta, R.N. Goyal and R.A. Sharma; Novel PVC membrane based Alizarin sensor and its applications : Determination of strength of Vanadium, Zirconium and Molybdenum; Int. J. Electrochem. Sci., 2004, 4, 156-172.