STUDIES ON ION SELECTIVE ELECTRODES BASED ON CYANOPOLYMERS

Abstract

Ion selective electrodes are mainly membrane-based devices, consisting of permselective membrane, which separates sample solution from inner solution of the electrode. The inner solution contains the ions of interest at constant activity. The overall potential is a function of various potential steps but ion selectivity of the electrode is a function of the membrane properties. The ion selectivity of the membrane is developed by incorporating ionophore with ion specific binding properties and added anion excluder like sodium tetraphenyl borate. The relative amount of electrode ingredients also contributes significantly toward selectivity, sensitivity and working range of the electrodes. The ion selective electrodes based on polyvinyl chloride are reported but electrodes based on polymers other than polyvinyl chloride have not been developed sufficiently. The hydrophobic nature and presence of polar cyano groups in poly (styrene-acrylonitrile) copolymers have been found to be advantageous in improving the selectivity and average lifetime of the electrodes. The cyano groups reduce the anions interference through Donnan exclusion; hence improved selectivity of the electrodes over routinely used cations. Investigations on electrodes composition have been found very fascinating to optimize the composition of electrodes for specific applications. The electrodes with high amount of plasticizer have shown significant deviations in their response. First of all, cyanocopolymers of different molecular weights ranging from 23310-82052 g mol"1 were prepared with acrylonitrile and styrene monomers using benzoyl peroxide as initiator. The composition of the cyanocopolymers was determined by elemental and IR studies. The reported interferences of Ag+ and Hg2+ ions in copper (II) ion selective electrodes have prompted to develop copper electrodes using Cu (ll)-salicylaniline complex (SAL) as Cu2+ ion specific ionophore. The optimized copper (II) electrodes have shown sufficient working range from 2.50x10"7 - 1.12x10"2 mol dm"3 activities of the Cu2+ ions with aNernstian slope of 30.00 ±0.053 mV decade'1 activities of Cu2+ ions. The electrodes have shown response time of 15.00 ±0.002 seconds at 1.0X10"5 mol dm"3 activity of the Cu2+ ions. The addition of tetraphenyl borate (NaTPB) in electrode membranes has shown no significant variation in electrodes response. However, a small amount (1.19x10"3 mol kg"1) of sodium tetraphenylborate (NaTPB) has been added as a precautionary measure. This has clearly indicated that the cyano groups of the polymer matrix have accomplished the role of anions excluder. The SEM studies have confirmed the asymmetric nature of electrode membranes. The Cu2+ ion selective electrodes have shown superior selectivity over number of metal ions, as confirmed from selectivity coefficients determined by mixed solution method (MSM) and separate solution method (SSM). The selectivity, sensitivity, response time, pH range and detection limits have been found to be dependent on electrode ingredients. The selectivity of ionophore is improved due to the cyano groups of the polymer matrix. The equilibrium hysteresis curve and temperature coefficients of electrode potential (AE/AT) and Nernstian slope (AS/AT) have been determined. The electrodes have shown complete absence of anions interference and deviations in electrodes response in non-aqueous phase containing 50% of alcohol. The electrodes have shown analytical applications as indicator electrodes for the titration of the Cu2+ ions with EDTA. II The 8-hydroxyquinoline based electrodes have shown better selectivity for Cd2+ ions in presence of routinely used interfering metal ions. These electrodes have also shown total absence of anions interference and deviations in non-aqueous phase with 50% alcohol. The Cd2+ ion selective electrodes with optimized composition have shown a working range of 4.46x10"5- 1.0x10'1 mol dm"3 activity of Cd2+ ions and Nernstian slope of 29.00 ± 0.033 mV decade"1 activity of Cd2+ ions. The response time was 12.00 ±0.005 seconds at 1.0x10"2 mol dm"3 activity of Cd2+ ions. The electrodes response has been found to be dependent on the amount of Cd (ll)-8-hydroxyquinoline (HYQ), dibutyl phthalate (DBP) and molecular weight of the cyanocopolymers (SAN). The dibutyl phthalate (DBP) has shown better performance in comparison to other plasticizers used in the fabrication of Cd2+ ion selective electrodes. The electrodes have shown useful applications in potentiometric titration of Cd2+ ions using EDTA as a titrant. A sigmoidal nature of titration curve has indicated the absence of interference from the titrant as well as from the Cd2+-EDTA complex. Electrodes have shown interference from Ag+ ions at low activity of Cd2+ions (< 1.0x10"4 mol dm"3). The synthesized Pb(ll)-diphenylmethyl-N-phenylhydroxamic acid complex (DPMNPHA) has shown high selectivity for Pb2+ ions in cyanocopolymer matrix as clear from the values of selectivity coefficient of electrodes obtained in presence of interfering metal ions. SEM studies have shown asymmetric nature of the electrode membranes as observed in other electrodes. The electrodes with optimized composition have shown a super Nernstian slope of 36.75 ± 0.011 mV decade"1 activity of the Pb2+ ions. The addition of different amount of plasticizer (DBP), ionophore (DPMNPHA) has III shown significant variation in electrodes response and selectivity. The electrodes prepared with dibutyl phthalate (DBP) have shown better response in comparison to other plasticizers. The electrodes have shown very small temperature coefficient of potential and slope (AE/AT) within the temperature range of 20°C to 40°C, which is an indicative that electrodes have no temperature dependence for their selectivity and potential. The electrodes have shown their applications as indicator electrodes for the titration of Pb2+ ions with sodium bromide solution. The titration curve has shown symmetric nature with a substantial potential jump of 118 mV at the end point of titration, which is an indicative o,f electrodes sensitivity and selectivity for the lead ions. The limited number of Ni2+ ion selective electrodes has prompted to develop electrodes using cyanocopolymer matrix and Ni(ll)-Nhydroxynaphthaldoxime complex (HYNOX) as ionophore. The electrodes with 14.3X10"3 mol kg"1 activity of HYNOX and 1.18 x10"2 mol dm"3 activity of plasticizer have shown a Nernstian response of 34.00 ±0.001 mV decade"1 activity of Ni2+ ions with a response time of 11.00 ±0.001 seconds. These electrodes have shown aworking range of 5.60x10"8-1.00x10"1 mol dm"3 activity of the Ni2+ ions. The electrodes have shown high selectivity for Ni2+ ions over a number of cations as measured with mixed solution method (MSM) as well as with separate solution method (SSM). The electrodes have shown constant potential within the pH range from 2.5-7.5. The temperature coefficients of electrode potential (AE/AT) and Nernstian slope (AS/AT) were high in comparison to Cu2+ and Cd2+ ion selective electrodes. The electrodes have shown complete absence of deviations in non-aqueous phase and in presence IV of different anions. The electrodes have been used successfully for potentiometric titration of Ni2+ ions with EDTA. Cyanocopolymer based Hg2+ ion selective electrodes fabricated with Hg(ll)-diketohydrindylidene diketohydrindamine complex (DHYDHA) as ionophore, have shown a superior response in comparison to reported Hg ion selective electrodes. The Hg2+ ion selective electrodes with optimized composition have shown a Nernstian slope of 36.80 ± 0.001 mV dacade activity of Hg2+ ions and a response time of 13.00 ±0.020 seconds at 1.0x10 mol dm"3 activity of Hg2+ ions. The electrodes have shown a working range of 1.26x10"7-8.90x10'2 mol dm"3 activity of Hg2+ ions at a solution pH of 6.5 and constant ionic strength (u) of 0.1 mol dm'3. The selectivity of the electrodes over a number of cations has been found to be better as determined by mixed solution method (MSM) and separate solution method (SSM). However, the selectivity, sensitivity and response time have been found to be dependant on the amount of ionophore (DHYDHA) and plasticizer (o-NPPE) used in the fabrication of the electrodes. The polar plasticizers have shown better results in comparison to apolar and semi-polar plasticizer. The response time has been evaluated for 10-fold concentration jump as in other electrodes and has shown dependence on the amount of ionophore and plasticizer. The solution temperature has shown an insignificant effect on electrodes response, as the values of temperature coefficient of the electrode potential (AE/AT) and slope (AS/AT) were very small. The intersection in calibration curves obtained at different temperatures is used to determine the isopotential concentration of Hg2+ ions. Electrodes have shown complete absence of anions interference, which is due to the cyano groups of the polymer matrix. The electrodes have -1 4

been used successfully for the titration of Hg2+ ions with potassium iodide as titrant and obtained a symmetrical titration curve, which is an indicative that electrodes could respond to transient activity of Hg2+ ions during titration process. All electrodes have shown high reproducibility in their response as checked by recording each measurement five times. The standard deviation of mean value is calculated and found to be within the admissible limit of measurements. All electrodes have shown better response in comparison to reported values. The cyano groups of the copolymer have played an important role in controlling the electrodes characteristics.