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Performance evaluation of copper ion selective electrode based on cyanocopolymers
Sensors and Actuators B 62 Ž2000. 171–176 www.elsevier.nlrlocatersensorb
Performance evaluation of copper ion selective electrode based on cyanocopolymers K.C. Gupta ) , Mujawamariya Jeanne D’Arc Polymer Research Laboratory, Department of Chemistry, UniÕersity of Roorkee, Roorkee-247 667 [U.P.], India Received 23 April 1999; accepted 20 September 1999
Abstract A copperŽII. ion selective electrode based on copperŽII. salicylaniline Schiff’s base complex in styrene-co-acrylonitrile copolymer ŽSAN. has been developed. The SAN-based membrane electrode containing copperŽII. –Schiff’s base complex, dioctylphthalate as plasticizer and sodium tetraphenylborate as an anion excluder exhibited a linear response with a Nerstian slope of 30 mV decadey1 within the concentration range of 10y6 –10y2 mol dmy3 of Cu2q ions. The prepared electrode has an average response time of 15 s to achieve 95% steady potential for Cu2q concentration ranging from 10y4 to 10y2 mol dmy3. The electrode has shown a detection limit of 10y7 mol dmy3 of Cu2q ion with an average lifetime of 6 months. The selectivity of electrode for Cu2q ion has been found to be better in comparison to other various interfering ions. The electrode is suitable for use within the pH range of 2.0–7.0 at 1.0 = 10y3 mol dmy3 of Cu2q ion. The prepared electrode can be used successfully as an indicator electrode for the potentiometric titration of the Cu2q ion using EDTA. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Copper; Selectivity; Electrode; Cyanocopolymer; Potentiometric; Sensor
1. Introduction Ion selective electrodes respond selectively to a particular ion and the potential of the electrode depends on the concentration of the ion following Nernst equation w1,2x. However, the potential and concentration relationship found to be valid upto a certain range of concentration of ions, pH of the medium, temperature w3x and the type of the polymer matrix used in the fabrication of the electrode membrane. In solid-state ion selective electrodes, the limit of detection of the ion depends upon the solubility of the ionophores w4x, whereas in polymer-based electrodes, the limit of detection is controlled by the activity of the ionophores. The ion selective electrodes for cations w5,6x and anions w7x are normally fabricated with polyŽvinyl chloride. homopolymer employing appropriate ionophores. The carboxylated polyŽvinyl chloride. polymer has been used to fabricate the Hq selective electrode w8x. The ion selective electrodes for alkali and alkaline earth metals w9x have been prepared employing teflon as membrane mate-
) Corresponding author. [email protected]
Fax:
q91-1332-73560;
e-m ail:
rial. Moody et al. w10x and Egorov and Lushchik w11x have reported various types of ion selective electrodes based on various types of polymers. The polypyrrole grafted with u-vinylpyridine has been used in the fabrication of the electrode w12x. Ion selective electrodes based on cyanocopolymers with high dipole moment Ž3.0 = 10y3 0 C m. in comparison to polyŽvinyl chloride. Ž1.1 = 10y3 0 C m. polymer, the most widely used material for ion selective electrodes, can hardly be seen in literature. The styrene-co-acrylonitrile copolymer ŽSAN. is most important thermoplastic system whose properties are directly related to the amount of the highly polar acrylonitrile group incorporated in the polymer. The SAN copolymer seems to be an ideal substitute for the routinely used polyŽvinyl chloride. in fabricating the ion selective electrodes. The copperŽII. selective electrodes using neutral ionophores w13,14x have been reported. The copper selective electrode has been tried for titration in organic solvents w15x. The reported copperŽII. electrodes usually show significant interference from Agq and Hgq ions. The design and development of copperŽII. electrode with no interference from Agq and Hg 2q would be of immense importance. The copperŽII. selective electrodes based on macrocyclic polythiaethers w16x, dithiacarbamates w17x, are
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also reported but show interference from Agq and Hg 2q ions. The poor sensitivity, high response time and significant interference from Agq and Hg 2q ions of the reported copperŽII. electrodes have prompted the development of copperŽII. electrode using Schiff’s base ionophore with SAN. The salicylaniline Schiff’s base forms complex with copperŽII. selectively and shows better sensitivity for Cu2q ions in the presence of Agq ions.
2. Experimental 2.1. Reagents and chemicals All the chemicals used in the preparation of the salicylaniline Schiff’s base ionophore and cyanocopolymer were of analytical reagent grade. Aniline and salicylaldehyde were obtained from E-Merck, India and used after vacuum distillation. Sodium tetraphenylborate ŽFluka, Switzerland. and dioctylphthalate ŽGSC, India. were used as obtained. 2.2. Preparation of cyanocopolymers The SAN was synthesized with purified monomer using benzoyl peroxide initiator in benzene. The mole percentages of acrylonitrile and styrene in SAN copolymer were determined with IR absorbance which shows 28% acrylonitrile and 72% styrene in the synthesized SAN. The effective dipole moment of the copolymer was estimated by applying the Kirkwood equation w18x and found to be 3.0 = 10y3 0 C m. The glass transition temperature ŽTg . and melting point of the copolymer were determined using DSC and DTA techniques and found to be 1128C and 4218C, respectively. 2.3. Preparation of Schiff’s base ionophore A mixture of 3.0 g of aniline and 4.0 g of salicylaldehyde in 20 ml of ethanol was refluxed on a water bath at 808C for about 2 h. The yellow crystals thus obtained on cooling were recrystallised with ethanol. The melting point of this yellow compound was found to be 528C. A solution of 0.4 g of this yellow compound in 20 ml of chlorobenzene was shaken with four 30 ml portions of 0.1 mol dmy3 solution of CuŽNO 3 . 2 prepared in a sodium acetate buffer and subsequently washed with distilled water until no copper ions are seen in the water phase. This organic phase containing the electroactive copperŽII. complexŽI. was used for the preparation of the membrane electrode.
2.4. Preparation of electroactiÕe membrane based on SAN To prepare SAN-based electrode membrane, 0.3 g of organic phase containing copperŽII. complex-I, 0.2 g of polymer ŽSAN., 5 ml of chlorobenzene, 0.02 g of dioctylphthalate, and 0.01 g of sodium tetraphenylborate were mixed together and finally poured on a Pyrex glass plate after degassing the trapped air bubbles from viscous solution. The solvent was allowed to evaporate for about 24 h under controlled temperature of 308C. The reproducibility of the membrane characteristics assured by carefully mixing the ingredients of the membrane casting solution and having control on the rate of solvent evaporation and thickness of the membrane. The prepared asymmetric selective membrane was removed from the glass plate and circular pieces of membrane of 1.25 cm diameter were cut and mounted on grounded end of the Pyrex glass membrane holder with Araldite ŽCiba-Geigy, India.. The membrane mounted electrode body was kept in 0.1 mol dmy3 solution of cupric nitrate for 4 days for equilibration. The membrane equilibrated for shorter duration did not develop stable potential. Finally, membrane was washed with distilled water before recording the potential measurements. In order to test the suitability of the membrane for the selective electrode, the membrane characteristics such as water content, porosity, swelling and electrolyte absorption were determined by routinely used methods. 2.5. Potential measurements The membrane potential measurements were carried out by immersing Pyrex glass membrane holder in a 50-ml beaker containing the test solution of different concentrations of cupric nitrate in 0.1-mol dmy3 solution of sodium nitrate. The membrane holder tube was filled with the solution of cupric nitrate of known concentration. The pH of the solution was maintained with the sodium acetate buffer. The solutions of the salts were prepared in doubly distilled water to avoid the presence of the inorganic and organic interference during potential measurements. The following cell assembly was used to determine the electrode potential with the help of the digital microvoltmeter ŽTestronix, India. at 25 " 0.18C. External Reference Electrode ŽSCE.
Test solutions with varying wCu2q x o
Membrane
Internal solution with wCu2q x I s 0.1 M
Internal Reference Electrode ŽSCE.
3. Results and discussion In order to test the performance of the membrane characteristics, various operation parameters viz., selectivity, response time, sensitivity, lifetime and working range of the electrode at different concentrations, of the metal
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Table 1 Specifications of the copperŽII. selective electrode based on SAN Properties
Valuesrrange
Ionophore Polymer mol. wt. Glass transition temperature ŽTg. and melting point Žm.p.. of polymer Detection limit Slope Response time pH range Water content Porosity Swelling in thickness Electrolyte absorption capacity Average life of electrode
Copper ŽII. –salicylaniline complexŽI. 59 000 1128C, 4218C
)10y7 mol dmy3 of Cu2q 30 mV decadey1 15 s at )10y5 mol dmy3 of Cu2q 3.0–7.0 0.15 g H 2 O per g of the membrane 0.17 0.002 mm 4.6=10y2 m mol of NaCl per g of membrane 6 months
ion and pH of the medium are usually determined. The characteristic parameters of the prepared membranes are summarized in Table 1. The observed water content, porosity and degree of swelling of the membrane are optimum as given in Table 1.
Fig. 1. Potential response of the electrode at various concentration of the Cu2q ions pH s6.0, Temp.s 258C, wDioctylphthalatex ŽA. 0.02 mol dmy3 , ŽB. 0.01 mol dmy3 wNaNO 3 x s 0.1 mol dmy3 , wCu2q x I s 0.1 mol dmy3 .
3.1. Concentration range and slope The potential response of the SAN-based membrane is determined as a function of Cu2q ions concentration employing solutions of cupric nitrate in 0.1 mol dmy3 solution of sodium nitrate at pH 6.0. The membrane has shown a linear response in the concentration range of 10y7 –10y2 mol dmy3 of Cu2q ions with a slope of 30 mV decadey1 ŽFig. 1A.. The membrane prepared with low amount of dioctylphthalate Ž- 0.02 mol dmy3 . has shown low sensitivity and slope value is reduced to 17 mV decadey1 ŽFig. 1.. The presence of plasticizer increases the ionic mobility in the membrane, hence enhances the sensitivity of the electrodes. The PVC-based ion-selective electrodes show variation in their lifetime due to the loss of the plasticizer or the electroactive species from the membrane phase. As the sensitivity of the prepared electrodes does not change after 6 months, hence the leaching of the plasticizer is not a problem with the SAN-based membrane electrode. The high dipole moment of the cyano group in the SAN may attribute a similar mechanism of ionic transport as proposed in electrodes based on ionic polymers w19x. A response time of 15 s was observed to achieve a 95% steady potential at concentration higher than 10y4 mol dmy3 of Cu2q ions ŽFig. 2.. However, at lower concentration of Cu2q ions, the response time was delayed for about 5 s ŽFig. 2.. The response time and the slope of the calibration curve have clearly indicated that the sensitivity and selectivity of the membrane are superior. The higher value of the slope Ž30 mV decadey1 . has also indicated for the
absence of anion effect w20x, which is commonly observed in copper electrodes w21x based on PVC. It may be assumed that the anion mobility in SAN membrane is reduced due to the cyano group attached to the copolymer. The anion excluder sodium tetraphenylborate seems to be more effective in presence of cyano group in comparison to the chloro group present in PVC-based membrane elec-
Fig. 2. Potential response of the electrode as a function of response time pH s6.0, Temp.s 258C, wDioctylphthalatex s 0.02 mol dmy3 , wNaNO 3 x s 0.1 mol dmy3 wCu2q x I s 0.1 mol dmy3 .
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K.C. Gupta, M.J. D’Arc r Sensors and Actuators B 62 (2000) 171–176
trode. The lower water content and porosity of the membrane also contribute further to enhance the selectivity of the electrode due to the controlled and selective moment of the cations in the membrane phase. The response of the membrane was constant and reproducible under identical experimental conditions. 3.2. Effect of pH on electrode response The response of the electrode for Cu2q ions is tested over a pH range of 2.0–7.5 at various concentrations of Cu2q ions ranging from 10y6 –10y2 mol dmy3 ŽFig. 3.. A substantial increasing trend in the potential at low pH may be considered due to the interference from the hydrogen ions, which are more at low pH in comparison to the Cu2q ions. At higher pH ŽG 7.0., the potential drop is observed and may be assumed due to the hydrolysis of the Cu2q ions. At higher pH, turbidity was observed and activity of Cu2q ions was decreased due to the formation of the hydroxo-complex. It is clear in Fig. 3 that the potential remains almost constant during pH variation from 3.0 to 7.0 at higher concentration of Cu2q ions, i.e., above 10y5 mol dmy3 . However, the working range is reduced on taking Cu2q ions less than 10y5 mol dmy3 . 3.3. Potentiometric selectiÕity The selectivity characteristics of the electrode make their use attractive in tough analytical situations which depend on the types of the ionophore, polymer used w22,23x in the preparation of electrode. Therefore, in order to evaluate the selectivity of the prepared membrane, the response of the electrode is investigated by mixed solution method w24x with a fixed concentration of the interfering ions ŽB., i.e., 10y2 mol dmy3. The values of selectivity pot . coefficient Ž K Cu, B for various cations have been evalu-
Fig. 3. Potential response of the electrode as function of the pH of the medium Temp.s 258C, wDioctylphthalatex s 0.02 mol dmy3 , wNaNO 3 x s 0.1 mol dmy3 , wCu2q x s 0.1 mol dmy3 .
pot . 2q Fig. 4. Selectivity coefficient Ž K Cu ion in the presence , B for the Cu of the fixed concentration of the interfering ions ŽB. of 2.0=10y2 mol dmy3 , wCu2q x ŽA. 1.0=10y4 , ŽB. 1.0=10y3 mol dmy3 , Temp.s 258C, pH s6.0 wDioctylphthalatex s 0.02 wNaNO 3 x s 0.1 mol dmy3 , wCu2q x I s 0.1 mol dmy3 .
ated at two different concentrations of Cu2q ions, i.e., 10y4 and 10y3 mol dmy3 and shown in Fig. 4. The pot . calculated values of the selectivity coefficient Ž K Cu, B for the studied cations found to be in the range of 1.0 = 10y6 – 1.0 = 10y2 mol dmy3 and suggested better selectivity for the Cu2q ions. The comparison of the values of the selectivity coefficient Fig. 4 obtained at two different concentrations of the Cu2q ions has clearly indicated that as the concentration of Cu2q ions decreases, the value of selectivity coefficient has increased. This has suggested that at lower concentration of Cu2q ions, the interference of foreign cations becomes substantial as they are at higher concentration Ž) 10y2 mol dmy3 .. However, the magnitude of interference from these external cations is significantly low to reduce the sensitivity of the electrode at low concentration of Cu2q ions ŽFig. 5.. The interference from Hg 2q ions appears to be significant at lower concentration of Cu2q ions Ži.e., - 1.0 = 10y5 mol dmy3 . as shown in Fig. 5. At higher concentration of the Cu2q ions, the interference from Hg 2q ions was not significant ŽFigs. 4 and 5.. The analysis of the values of the selectivity coeffipot . cient Ž K Cu, B shown in Fig. 4 has clearly indicated a systematically decreasing trend on going from Liq to La3q. This indicates that the rate of diffusion of interfering
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cations in membrane phase might be related to the radius of the cations used for the investigations. As the magnitude of selectivity coefficient for all the studied foreign ions is low except Hg 2q ions, hence the prepared membrane is assumed to be sufficiently selective for the Cu2q ions. This has suggested that the prepared electrode is suitable for the selective determination of the Cu2q ions in the presence of the various interfering ions. 3.4. Analytical applications To evaluate the applicability of the prepared electrode as an indicator electrode in the potentiometric titration of Cu2q ions, the titration of the Cu2q ions has been conducted using EDTA as a suitable titrant in absence of the transition metal ions. However, EDTA has not been found to be a suitable titrant in the presence of the other transition metal ions as it forms more stable complex with the transition metal ions in comparison to the Cu2q ions. The potentiometric titration of Cu2q ions with EDTA was performed at pH 6.0 taking 20 ml of 1.0 = 10y3 mol dmy3 solution of CuŽNO 3 . 2 in a 50-ml Pyrex glass beaker and titrating it with 1.0 = 10y3 mol dmy3 solution of EDTA. The potential was measured at each addition of 2.5 ml in the beaker containing Cu2q ions solution. The potential of the membrane has decreased linearly upto a certain volume of the EDTA and beyond that membrane
Fig. 6. Potentiometric titration of Cu2q ions with EDTA. Temp.s 258C, pH s6.0, wCu2q x 20 ml of 1.0=10y3 mol dmy3 , wEDTAx 1.0=10y3 mol dmy3 , wCu2q x I s 0.1 mol dmy3 .
potential has become almost constant. The break point Žend point. in the titration curve is corresponding to the 1:1 stoichiometry of the CuŽII. –EDTA complex. The absence of sigmoid shape in the titration curve ŽFig. 6. has clearly indicated the interference from the CuŽII. –EDTA complex. The mixed silver sulphide–copper sulphide electrodes show anomalous behavior in presence of EDTA and other polycarboxylic acids w25x. Triamines like TRIEN and TETREN are required for titration but the accuracy of the results with the prepared electrode has suggested that it can be used for the quantitative determinations of the Cu2q ions using EDTA as a suitable titrant.
4. Conclusion A new sensitive and selective electrode for Cu2q ions based on SAN has been developed. The selectivity coefficients for all cations except Hg 2q ions are significantly low. The membrane show poor response for the anions as often observed with copper electrode. The response time for the steady potential is significantly low, hence it may be used for Cu2q ions determination in dynamic conditions the electrode has shown reproducible results upto a period of 6 months.
Acknowledgements Fig. 5. Effect of concentration of Hg 2q ions on the potential of the electrode wHg 2q x ŽA. 1.0=10y2 mol dmy3 , ŽB. 1.0=10y3 mol dmy3 , ŽC. 1.0=10y4 mol dmy3 , ŽD. 1.0=10y5 mol dmy3 , ŽE. Blank, Temp.s 258C, pH s6.0, wDioctylphthalatex s 0.02 mol dmy3 , wNaNO 3 x s 0.1 mol dmy3 , wCu2q xI s 0.1 mol dmy3 .
Financial assistance from AICTE, New Delhi is thankfully acknowledged. One of the authors ŽM.J. D’Arc. is thankful to Ministry of Education, Government of Rwanda, for providing the research fellowship.
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References w1x E. Pungor, How to understand the response mechanism of ion selective electrodes, Talanta 44 Ž1997. 1505. w2x R.A. Durst ŽEd.., Ion Selective Electrode, Special Publication, 3, 4, National Bureau of Standard, Washington, DC, 1969. w3x M.T.S.D. Vasconcelos, A.A.S.C. Machado, R.F. Port, Usefulness of hysteresis curves for studying temperature effects on ion selective electrode responses, Anal. Lett. 21 Ž1988. 1987. w4x J. Kontoyannakos, G.J. Moody, J.D.R. Thomas, The detection limit of the Orion iodidersilver ion selective electrodes, Anal. Chim. Acta 85 Ž1976. 47. w5x D. Siswanta, M. Kir, H. Hisamoto, K. Suzuki, Novel Hg 2q-Ionophores based on N-hydroxylamide derivatives as a sensor molecule for an ion selective electrode, Chem. Lett. 11 Ž1996. 1011. w6x M.A.F. Elmosallamy, Potentiometric micro determination of some metal ions using chloranilate PVC matrix membrane electrodes, Anal. Lett. 30 Ž1997. 475. w7x D. Lee, K.L. Chang, An anion selective membrane electrode based on a mixture of insoluble lead salts, Talanta 37 Ž1990. 901. w8x E. Lindner, V.V. Cosofret, R.P. Kusy, R.P. Buck, T.R.U. Schaller, W. Simon, Responses of Hq selective solvent polymeric membrane electrodes fabricated from modified PVC membranes, Talanta 40 Ž1993. 957. w9x K.H. Lee, D.H. Cho, S.S. Jeung, Ion selectivity of polymeric membrane based electrodes — A preliminary study, Bull. Electro. Chem. 11 Ž1995. 4384. w10x G.J. Moody, B.D. Sadd, J.D.R. Thomas, The development of polymer matrix membranes for ion selective electrodes, Sel. Electrode Rev. 10 Ž1988. 7. w11x V.V. Egorov, Y.F. Lushchik, Hq-selective electrodes based on neutral carriers: specific features in behavior and quantitative description of the electrode response, Talanta 37 Ž1990. 461. w12x M. Arca, E. Arca, A. Yildiz, O. Guven, Preparation of an electroactive copolymer by radiation induced grafting of N-vinyl 4-pyridine onto polypyrrole, Radiat. Phys. Chem. 3 Ž1988. 647. w13x R. Krzysztof, A liquid state copperŽII. ion selective electrode con-
w14x
w15x
w16x
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w18x w19x w20x
w21x
w22x
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taining a complex of CuŽII. with salicylaniline, Talanta 36 Ž1989. 767. Z. Brzozka, Transition metal ion selective membrane electrodes based on complexing compounds with heteroatoms-Part I, Analyst 113 Ž1988. 891. P. Fabry, J.P. Gross, J.F. Million Brodaz, M. Kleitz, Nasicon — An ionic conductor for solid state sodium Žq1. selective electrode, Sensors and Actuators 15 Ž1988. 33. S. Kamata, K. Yamasaki, M. Higo, A. Bhale, Y. Fukunaga, CopperŽII. selective electrode based on macrocyclic polythiaethers, Analyst 113 Ž1988. 45. S. Kamata, H. Murata, Y. Kubo, A. Bhale, CopperŽII. selective membrane electrodes based on O-xylylene bisŽdithiocarbamates. as a neutral carrier, Analyst 114 Ž1989. 1029. J.G.J. Kirkwood, The dielectric polarization of polar liquids, J. Chem. Phys. 7 Ž1939. 911. L. Holliday, in: L. Holliday ŽEd.., Ionic Polymers, Applied Science Publication, Essex, England, 1975, pp. 30–35. W.E. Morf, G. Kahr, W. Simon, Reduction of the anions interference in Neutral carrier liquid membrane electrode responsive to cations, Analyst Lett. 7 Ž1974. 9. A. Lewenstam, T. Sokalski, A. Hulaniski, Anionic interference with copper ion-selective electrodes, chloride and bromide interferences, Talanta 32 Ž1985. 531. R. Zhou, K.E. Geckelor, W. Goepal, Functional Polymers for Chemical Sensors, in: R. Arshady ŽEd.., Desk Reference of Functional Polymers, American Chemical Society, Washington, 1997, pp. 601– 620. M. Trojanowicz, V. Krawczyk, T. Krawczynski, P.W. Alexander, Organic conducting polymers as active materials in electrochemical chemosensors and biosensors, Chem. Anal. 42 Ž1997. 199. G.G. Guilbault, IUPAC-Commission on Analytical Nomenclature, Ion Selective Electrode Rev. 1 Ž1979. 139. W.E. Van der Linden, G.J.M. Heijne, Formation of mixed copperŽII. sulphide-silverŽI. sulphide membranes for copperŽII.-selective electrodes. III. Electrode response in the presence of complexing agents, Anal. Chim. Acta 96 Ž1978. 13.
Performance evaluation of copper ion selective electrode based on cyanocopolymers K.C. Gupta ) , Mujawamariya Jeanne D’Arc Polymer Research Laboratory, Department of Chemistry, UniÕersity of Roorkee, Roorkee-247 667 [U.P.], India Received 23 April 1999; accepted 20 September 1999
Abstract A copperŽII. ion selective electrode based on copperŽII. salicylaniline Schiff’s base complex in styrene-co-acrylonitrile copolymer ŽSAN. has been developed. The SAN-based membrane electrode containing copperŽII. –Schiff’s base complex, dioctylphthalate as plasticizer and sodium tetraphenylborate as an anion excluder exhibited a linear response with a Nerstian slope of 30 mV decadey1 within the concentration range of 10y6 –10y2 mol dmy3 of Cu2q ions. The prepared electrode has an average response time of 15 s to achieve 95% steady potential for Cu2q concentration ranging from 10y4 to 10y2 mol dmy3. The electrode has shown a detection limit of 10y7 mol dmy3 of Cu2q ion with an average lifetime of 6 months. The selectivity of electrode for Cu2q ion has been found to be better in comparison to other various interfering ions. The electrode is suitable for use within the pH range of 2.0–7.0 at 1.0 = 10y3 mol dmy3 of Cu2q ion. The prepared electrode can be used successfully as an indicator electrode for the potentiometric titration of the Cu2q ion using EDTA. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Copper; Selectivity; Electrode; Cyanocopolymer; Potentiometric; Sensor
1. Introduction Ion selective electrodes respond selectively to a particular ion and the potential of the electrode depends on the concentration of the ion following Nernst equation w1,2x. However, the potential and concentration relationship found to be valid upto a certain range of concentration of ions, pH of the medium, temperature w3x and the type of the polymer matrix used in the fabrication of the electrode membrane. In solid-state ion selective electrodes, the limit of detection of the ion depends upon the solubility of the ionophores w4x, whereas in polymer-based electrodes, the limit of detection is controlled by the activity of the ionophores. The ion selective electrodes for cations w5,6x and anions w7x are normally fabricated with polyŽvinyl chloride. homopolymer employing appropriate ionophores. The carboxylated polyŽvinyl chloride. polymer has been used to fabricate the Hq selective electrode w8x. The ion selective electrodes for alkali and alkaline earth metals w9x have been prepared employing teflon as membrane mate-
) Corresponding author. [email protected]
Fax:
q91-1332-73560;
e-m ail:
rial. Moody et al. w10x and Egorov and Lushchik w11x have reported various types of ion selective electrodes based on various types of polymers. The polypyrrole grafted with u-vinylpyridine has been used in the fabrication of the electrode w12x. Ion selective electrodes based on cyanocopolymers with high dipole moment Ž3.0 = 10y3 0 C m. in comparison to polyŽvinyl chloride. Ž1.1 = 10y3 0 C m. polymer, the most widely used material for ion selective electrodes, can hardly be seen in literature. The styrene-co-acrylonitrile copolymer ŽSAN. is most important thermoplastic system whose properties are directly related to the amount of the highly polar acrylonitrile group incorporated in the polymer. The SAN copolymer seems to be an ideal substitute for the routinely used polyŽvinyl chloride. in fabricating the ion selective electrodes. The copperŽII. selective electrodes using neutral ionophores w13,14x have been reported. The copper selective electrode has been tried for titration in organic solvents w15x. The reported copperŽII. electrodes usually show significant interference from Agq and Hgq ions. The design and development of copperŽII. electrode with no interference from Agq and Hg 2q would be of immense importance. The copperŽII. selective electrodes based on macrocyclic polythiaethers w16x, dithiacarbamates w17x, are
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 3 6 2 - 7
172
K.C. Gupta, M.J. D’Arc r Sensors and Actuators B 62 (2000) 171–176
also reported but show interference from Agq and Hg 2q ions. The poor sensitivity, high response time and significant interference from Agq and Hg 2q ions of the reported copperŽII. electrodes have prompted the development of copperŽII. electrode using Schiff’s base ionophore with SAN. The salicylaniline Schiff’s base forms complex with copperŽII. selectively and shows better sensitivity for Cu2q ions in the presence of Agq ions.
2. Experimental 2.1. Reagents and chemicals All the chemicals used in the preparation of the salicylaniline Schiff’s base ionophore and cyanocopolymer were of analytical reagent grade. Aniline and salicylaldehyde were obtained from E-Merck, India and used after vacuum distillation. Sodium tetraphenylborate ŽFluka, Switzerland. and dioctylphthalate ŽGSC, India. were used as obtained. 2.2. Preparation of cyanocopolymers The SAN was synthesized with purified monomer using benzoyl peroxide initiator in benzene. The mole percentages of acrylonitrile and styrene in SAN copolymer were determined with IR absorbance which shows 28% acrylonitrile and 72% styrene in the synthesized SAN. The effective dipole moment of the copolymer was estimated by applying the Kirkwood equation w18x and found to be 3.0 = 10y3 0 C m. The glass transition temperature ŽTg . and melting point of the copolymer were determined using DSC and DTA techniques and found to be 1128C and 4218C, respectively. 2.3. Preparation of Schiff’s base ionophore A mixture of 3.0 g of aniline and 4.0 g of salicylaldehyde in 20 ml of ethanol was refluxed on a water bath at 808C for about 2 h. The yellow crystals thus obtained on cooling were recrystallised with ethanol. The melting point of this yellow compound was found to be 528C. A solution of 0.4 g of this yellow compound in 20 ml of chlorobenzene was shaken with four 30 ml portions of 0.1 mol dmy3 solution of CuŽNO 3 . 2 prepared in a sodium acetate buffer and subsequently washed with distilled water until no copper ions are seen in the water phase. This organic phase containing the electroactive copperŽII. complexŽI. was used for the preparation of the membrane electrode.
2.4. Preparation of electroactiÕe membrane based on SAN To prepare SAN-based electrode membrane, 0.3 g of organic phase containing copperŽII. complex-I, 0.2 g of polymer ŽSAN., 5 ml of chlorobenzene, 0.02 g of dioctylphthalate, and 0.01 g of sodium tetraphenylborate were mixed together and finally poured on a Pyrex glass plate after degassing the trapped air bubbles from viscous solution. The solvent was allowed to evaporate for about 24 h under controlled temperature of 308C. The reproducibility of the membrane characteristics assured by carefully mixing the ingredients of the membrane casting solution and having control on the rate of solvent evaporation and thickness of the membrane. The prepared asymmetric selective membrane was removed from the glass plate and circular pieces of membrane of 1.25 cm diameter were cut and mounted on grounded end of the Pyrex glass membrane holder with Araldite ŽCiba-Geigy, India.. The membrane mounted electrode body was kept in 0.1 mol dmy3 solution of cupric nitrate for 4 days for equilibration. The membrane equilibrated for shorter duration did not develop stable potential. Finally, membrane was washed with distilled water before recording the potential measurements. In order to test the suitability of the membrane for the selective electrode, the membrane characteristics such as water content, porosity, swelling and electrolyte absorption were determined by routinely used methods. 2.5. Potential measurements The membrane potential measurements were carried out by immersing Pyrex glass membrane holder in a 50-ml beaker containing the test solution of different concentrations of cupric nitrate in 0.1-mol dmy3 solution of sodium nitrate. The membrane holder tube was filled with the solution of cupric nitrate of known concentration. The pH of the solution was maintained with the sodium acetate buffer. The solutions of the salts were prepared in doubly distilled water to avoid the presence of the inorganic and organic interference during potential measurements. The following cell assembly was used to determine the electrode potential with the help of the digital microvoltmeter ŽTestronix, India. at 25 " 0.18C. External Reference Electrode ŽSCE.
Test solutions with varying wCu2q x o
Membrane
Internal solution with wCu2q x I s 0.1 M
Internal Reference Electrode ŽSCE.
3. Results and discussion In order to test the performance of the membrane characteristics, various operation parameters viz., selectivity, response time, sensitivity, lifetime and working range of the electrode at different concentrations, of the metal
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Table 1 Specifications of the copperŽII. selective electrode based on SAN Properties
Valuesrrange
Ionophore Polymer mol. wt. Glass transition temperature ŽTg. and melting point Žm.p.. of polymer Detection limit Slope Response time pH range Water content Porosity Swelling in thickness Electrolyte absorption capacity Average life of electrode
Copper ŽII. –salicylaniline complexŽI. 59 000 1128C, 4218C
)10y7 mol dmy3 of Cu2q 30 mV decadey1 15 s at )10y5 mol dmy3 of Cu2q 3.0–7.0 0.15 g H 2 O per g of the membrane 0.17 0.002 mm 4.6=10y2 m mol of NaCl per g of membrane 6 months
ion and pH of the medium are usually determined. The characteristic parameters of the prepared membranes are summarized in Table 1. The observed water content, porosity and degree of swelling of the membrane are optimum as given in Table 1.
Fig. 1. Potential response of the electrode at various concentration of the Cu2q ions pH s6.0, Temp.s 258C, wDioctylphthalatex ŽA. 0.02 mol dmy3 , ŽB. 0.01 mol dmy3 wNaNO 3 x s 0.1 mol dmy3 , wCu2q x I s 0.1 mol dmy3 .
3.1. Concentration range and slope The potential response of the SAN-based membrane is determined as a function of Cu2q ions concentration employing solutions of cupric nitrate in 0.1 mol dmy3 solution of sodium nitrate at pH 6.0. The membrane has shown a linear response in the concentration range of 10y7 –10y2 mol dmy3 of Cu2q ions with a slope of 30 mV decadey1 ŽFig. 1A.. The membrane prepared with low amount of dioctylphthalate Ž- 0.02 mol dmy3 . has shown low sensitivity and slope value is reduced to 17 mV decadey1 ŽFig. 1.. The presence of plasticizer increases the ionic mobility in the membrane, hence enhances the sensitivity of the electrodes. The PVC-based ion-selective electrodes show variation in their lifetime due to the loss of the plasticizer or the electroactive species from the membrane phase. As the sensitivity of the prepared electrodes does not change after 6 months, hence the leaching of the plasticizer is not a problem with the SAN-based membrane electrode. The high dipole moment of the cyano group in the SAN may attribute a similar mechanism of ionic transport as proposed in electrodes based on ionic polymers w19x. A response time of 15 s was observed to achieve a 95% steady potential at concentration higher than 10y4 mol dmy3 of Cu2q ions ŽFig. 2.. However, at lower concentration of Cu2q ions, the response time was delayed for about 5 s ŽFig. 2.. The response time and the slope of the calibration curve have clearly indicated that the sensitivity and selectivity of the membrane are superior. The higher value of the slope Ž30 mV decadey1 . has also indicated for the
absence of anion effect w20x, which is commonly observed in copper electrodes w21x based on PVC. It may be assumed that the anion mobility in SAN membrane is reduced due to the cyano group attached to the copolymer. The anion excluder sodium tetraphenylborate seems to be more effective in presence of cyano group in comparison to the chloro group present in PVC-based membrane elec-
Fig. 2. Potential response of the electrode as a function of response time pH s6.0, Temp.s 258C, wDioctylphthalatex s 0.02 mol dmy3 , wNaNO 3 x s 0.1 mol dmy3 wCu2q x I s 0.1 mol dmy3 .
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trode. The lower water content and porosity of the membrane also contribute further to enhance the selectivity of the electrode due to the controlled and selective moment of the cations in the membrane phase. The response of the membrane was constant and reproducible under identical experimental conditions. 3.2. Effect of pH on electrode response The response of the electrode for Cu2q ions is tested over a pH range of 2.0–7.5 at various concentrations of Cu2q ions ranging from 10y6 –10y2 mol dmy3 ŽFig. 3.. A substantial increasing trend in the potential at low pH may be considered due to the interference from the hydrogen ions, which are more at low pH in comparison to the Cu2q ions. At higher pH ŽG 7.0., the potential drop is observed and may be assumed due to the hydrolysis of the Cu2q ions. At higher pH, turbidity was observed and activity of Cu2q ions was decreased due to the formation of the hydroxo-complex. It is clear in Fig. 3 that the potential remains almost constant during pH variation from 3.0 to 7.0 at higher concentration of Cu2q ions, i.e., above 10y5 mol dmy3 . However, the working range is reduced on taking Cu2q ions less than 10y5 mol dmy3 . 3.3. Potentiometric selectiÕity The selectivity characteristics of the electrode make their use attractive in tough analytical situations which depend on the types of the ionophore, polymer used w22,23x in the preparation of electrode. Therefore, in order to evaluate the selectivity of the prepared membrane, the response of the electrode is investigated by mixed solution method w24x with a fixed concentration of the interfering ions ŽB., i.e., 10y2 mol dmy3. The values of selectivity pot . coefficient Ž K Cu, B for various cations have been evalu-
Fig. 3. Potential response of the electrode as function of the pH of the medium Temp.s 258C, wDioctylphthalatex s 0.02 mol dmy3 , wNaNO 3 x s 0.1 mol dmy3 , wCu2q x s 0.1 mol dmy3 .
pot . 2q Fig. 4. Selectivity coefficient Ž K Cu ion in the presence , B for the Cu of the fixed concentration of the interfering ions ŽB. of 2.0=10y2 mol dmy3 , wCu2q x ŽA. 1.0=10y4 , ŽB. 1.0=10y3 mol dmy3 , Temp.s 258C, pH s6.0 wDioctylphthalatex s 0.02 wNaNO 3 x s 0.1 mol dmy3 , wCu2q x I s 0.1 mol dmy3 .
ated at two different concentrations of Cu2q ions, i.e., 10y4 and 10y3 mol dmy3 and shown in Fig. 4. The pot . calculated values of the selectivity coefficient Ž K Cu, B for the studied cations found to be in the range of 1.0 = 10y6 – 1.0 = 10y2 mol dmy3 and suggested better selectivity for the Cu2q ions. The comparison of the values of the selectivity coefficient Fig. 4 obtained at two different concentrations of the Cu2q ions has clearly indicated that as the concentration of Cu2q ions decreases, the value of selectivity coefficient has increased. This has suggested that at lower concentration of Cu2q ions, the interference of foreign cations becomes substantial as they are at higher concentration Ž) 10y2 mol dmy3 .. However, the magnitude of interference from these external cations is significantly low to reduce the sensitivity of the electrode at low concentration of Cu2q ions ŽFig. 5.. The interference from Hg 2q ions appears to be significant at lower concentration of Cu2q ions Ži.e., - 1.0 = 10y5 mol dmy3 . as shown in Fig. 5. At higher concentration of the Cu2q ions, the interference from Hg 2q ions was not significant ŽFigs. 4 and 5.. The analysis of the values of the selectivity coeffipot . cient Ž K Cu, B shown in Fig. 4 has clearly indicated a systematically decreasing trend on going from Liq to La3q. This indicates that the rate of diffusion of interfering
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cations in membrane phase might be related to the radius of the cations used for the investigations. As the magnitude of selectivity coefficient for all the studied foreign ions is low except Hg 2q ions, hence the prepared membrane is assumed to be sufficiently selective for the Cu2q ions. This has suggested that the prepared electrode is suitable for the selective determination of the Cu2q ions in the presence of the various interfering ions. 3.4. Analytical applications To evaluate the applicability of the prepared electrode as an indicator electrode in the potentiometric titration of Cu2q ions, the titration of the Cu2q ions has been conducted using EDTA as a suitable titrant in absence of the transition metal ions. However, EDTA has not been found to be a suitable titrant in the presence of the other transition metal ions as it forms more stable complex with the transition metal ions in comparison to the Cu2q ions. The potentiometric titration of Cu2q ions with EDTA was performed at pH 6.0 taking 20 ml of 1.0 = 10y3 mol dmy3 solution of CuŽNO 3 . 2 in a 50-ml Pyrex glass beaker and titrating it with 1.0 = 10y3 mol dmy3 solution of EDTA. The potential was measured at each addition of 2.5 ml in the beaker containing Cu2q ions solution. The potential of the membrane has decreased linearly upto a certain volume of the EDTA and beyond that membrane
Fig. 6. Potentiometric titration of Cu2q ions with EDTA. Temp.s 258C, pH s6.0, wCu2q x 20 ml of 1.0=10y3 mol dmy3 , wEDTAx 1.0=10y3 mol dmy3 , wCu2q x I s 0.1 mol dmy3 .
potential has become almost constant. The break point Žend point. in the titration curve is corresponding to the 1:1 stoichiometry of the CuŽII. –EDTA complex. The absence of sigmoid shape in the titration curve ŽFig. 6. has clearly indicated the interference from the CuŽII. –EDTA complex. The mixed silver sulphide–copper sulphide electrodes show anomalous behavior in presence of EDTA and other polycarboxylic acids w25x. Triamines like TRIEN and TETREN are required for titration but the accuracy of the results with the prepared electrode has suggested that it can be used for the quantitative determinations of the Cu2q ions using EDTA as a suitable titrant.
4. Conclusion A new sensitive and selective electrode for Cu2q ions based on SAN has been developed. The selectivity coefficients for all cations except Hg 2q ions are significantly low. The membrane show poor response for the anions as often observed with copper electrode. The response time for the steady potential is significantly low, hence it may be used for Cu2q ions determination in dynamic conditions the electrode has shown reproducible results upto a period of 6 months.
Acknowledgements Fig. 5. Effect of concentration of Hg 2q ions on the potential of the electrode wHg 2q x ŽA. 1.0=10y2 mol dmy3 , ŽB. 1.0=10y3 mol dmy3 , ŽC. 1.0=10y4 mol dmy3 , ŽD. 1.0=10y5 mol dmy3 , ŽE. Blank, Temp.s 258C, pH s6.0, wDioctylphthalatex s 0.02 mol dmy3 , wNaNO 3 x s 0.1 mol dmy3 , wCu2q xI s 0.1 mol dmy3 .
Financial assistance from AICTE, New Delhi is thankfully acknowledged. One of the authors ŽM.J. D’Arc. is thankful to Ministry of Education, Government of Rwanda, for providing the research fellowship.
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