ELECTROANALYTICAL STUDIES ON SOME HETEROGENEOUS MEMBRANES AS ION SENSORS

Abstract

Analytical chemists have concerned themselves with the development of instruments and techniques capable of determining the identityand concentration of chemical substances. Ion-selective electrodes (ISEs) or sensors are one of the subjects which has received great attention as these find applications in environmental, industrial, agricultural and medicinal fields, mainly due to the fact that these devices provide a rapid, sensitive, low cost and accurate method of analysis. The technique, generally, does not require sample pre-treatment, is non destructive, adaptable to small sample volumes and can be used for real time analysis. Tremendous efforts by various workers have led to the development oflarge number of sensors for various ions. However, only limited number of sensors for about a dozen ions have been found to be sufficiently selective and, thus, made commercially available. Most of the other sensors reported in literature suffer from poor sensitivity, selectivity and long response time. Hence, further efforts are needed to have better electrodes. This appears possible with the increasing availability of newer materials which show selectivity for specific metals and, therefore, can be used as electroactive component in the membrane to be used as ion sensors. Over the years, selective complexation between charged/neutral ligands and metals has been exploited to advantage for the construction and design of chemical sensors. These ligands (orextractants) are known to behave as ionophores and have the capability to extract ions selectivity from aqueous solutions and transport them across barriers by carrier translocation in membrane phase. Based on this concept plasticized PVC-based membranes of Zn-bis-(2,4,4-trimethylpentyl)dithiophosphinic acid (I) have been investigated for the estimation of Zn2+ while those of di-(2-ethylhexyl)phosphoric acid (DEHPA) and dibutyl butylphosphonate (DBBP) have been tried for the determination of V02+. Macrocycles are the compounds of recent interest for developing highly selective sensors for a number of cations and anions. These compounds provide different donor atoms, ring size and ligand geometry to host ions. The macrocycles 5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraazacyclotetradeca-4,11-diene diperchlorate (II), 3,5,7,7,10,12, 14,14-octamethyl-l,4,8,ll-tetraazacyclotetradeca- 4,11-diene diperchlorate (III), 5,10,15, 20-tetraphenylporphyrin (IV), 5,10,15,20- tetra(4-methylphenyl)porphyrin (V) and the cobalt complex of IV (5,10,15,20- tetraphenylporphyrinatocobalt) (VI) were, hence, tried for developing sensors for Co +, Ni2+ and MoOj". Further, the use of crown ethers and calixarenes as electroactive materials has increased over the recent past owing to their highly selective nature. Efforts were, therefore, also made to develop sensors for some ions using heterogeneous PVC-based membranes incorporating the above mentioned materials as the ion-active phase. The composition of membranes, concentration of equilibrium solution and time ofcontact were first optimized so that stable and reproducible potentials are generated. The membranes exliibiting best performance with regard to working concentration range, slope and response time were chosen for further investigations. Membranes incorporating I, anion excluder, sodium tetraphenylborate (NaTPB) and plasticizer, dibutyl butylphosphonate (DBBP) in PVC matrix in the percent (w/w) ratio 7:59:5:29 (I:PVC:NaTPB:DBBP) exhibited a linear response to Zn2+ in the concentration range 2.8 xlO-5 -1.0x10_1M with a Nernstian slope of 30.1 mV per decade of activity and a fast response time of 15s. Selectivity data showed that the sensor was selective for zinc over a large number of cations with slight interferences from copper only. The sensor worked well in the pH range 2.1-6.9 and could be satisfactorily used in non-aqueous medium having upto 10% (v/v) non-aqueous content. The membrane could be used for a period of six months and its analytical applicability has been demonstrated by using it as an indicator electrode in the potentiometric titration of Zn(II) with EDTA. The sensors reported so far for the estimation of vanadium sense it as vanadate (VO3) only and no report is available on a sensor for the cationic oxometal species, vanadyl (V02+). Membranes incorporating DEHPA or DBBP in PVC matrix showed selective response towards V02+ ions between the concentration range -lO^-lO^M with a non-Nernstian slope and a response time of ahnost 1 min. The performance of the membranes improved after the addition of NaTPB with the best response characteristics being exhibited by the membrane incorporating DBBP and NaTPB in PVC in the ratio 10:7:83. The sensor had a wide working concentration range of7.9 x10"6 -1.0 x10"1 M, slope of 29.1 mV per decade of activity and a fast response time of 15s. The membranes work well over the pH range 1.8-2.5 as well as in partially non-aqueous medium having upto 10% (v/v) ethanol content and could be used for a period ofmore than six months. The sensor shows good selectivity over a number of mono, bi and trivalent cations. The practical utility of the sensor was tested by estimating in vanadium in a sample of waste V205 catalyst. The results obtained showed excellent concurrence with the data obtained from ICP-AES. It was also possible to use the sensor as an indicator electrode in the potentiometric titration of V02+ with EDTA. PVC-based membranes of macrocycles II, III and IV were tried for developing Co2+-sensors and it was observed that the membrane incorporating the ionophore IV, anion excluder, NaTPB and solvent mediator, dibutyl phthalate (DBP) in the composition 15:40:5:40 (IV:PVC:NaTPB:DBP) exhibited good selectivity for Co2+ over a number of other cations. It shows a wide working concentration range of 8.0x IQ"6 -l.Ox10_1M, a Nernstian slope of 29.0 mV per decade of activity, response time of 20s and a useful life time of four months. The working pH range of the membrane sensor was between 2.8-7.3 and it could even be used in partially non-aqueous medium having upto 25% (v/v) non-aqueous content. The sensor has also been used as an indicator electrode in the potentiometric titration of Co2+ with EDTA. The macrocycles IV and V were used as electroactive materials in the PVC-based membranes selective for Ni2+. The best performance was exhibited by the membrane having the composition 7:54:4:35 (V:PVC:NaTPB:DBP). This membrane showed a wide working concentration range of 5.6x 10~6 - l.Ox 10_1M with a Nernstian slope of 30.1 mV perdecade of activity and a fast response time of 20s. The potentiometric data showed good selectivity for Ni2+ over a number of cations and anions. The valid pH range of the sensors was found to be between 2.5-7.4 and the performance in partially non-aqueous medium was also satisfactory with a 30% (v/v) alcohol content having no effect on the working of the sensor. Analytical applicability of the proposed sensor has been demonstrated by using it IV for the detennination of nickel in some Indian brand chocolates and also as indicator electrode in the potentiometric titration of Ni2+ with EDTA. The sensor could be used for a period of eight months without showing any appreciable change in the response characteristics. Membrane having the composition 9:55:36 (VI:PVC:DBP) responds to molybdate (MoO2-) over the concentration range 7.6 xlO-6 -1.0x10_1M, between pH 4.9-11.3, with a slope of 30.1 mV per decade of activity and a fast response time of 15s. The membrane showed excellent selectivity towards Mo04 ions over a number of other anions and exhibited stable response for more than four months. The sensor can be used in non-aqueous medium with no significant change in the value of slope or working concentration range for the estimation of M0O4" in solutions having upto 25% (v/v) non-aqueous content. It was applied for the estimation of molybdenum as molybdate in a sample of molybdenite, a low grade ore. It is reported in literature that Ba2+ forms stable complex with dibenzo-24-crown-8, Sr2+ with 4-/e/-/-butylcalix[8]arene and UO2/ with 4-/ert-butylcalix[6]arene. It was, therefore, thought interesting to use these compounds as carriers in the fabrication of selective sensors for the respective ionic species. The sensor based on dibenzo-24-crown-8 could be used for the estimation of Ba2+ in the concentration range 1.4xl0~5-l.Ox 10-1M between pH 3.5-8.9. The sensor exhibits a slope of 30.0 mV per decade of activity and a fast response time of15s. The electrode can be used for Ba2+ determination in the presence ofa number of ions, though NH4 causes some interference at higher concentrations. Partially non-aqueous medium having upto 30%> (v/v) non-aqueous content has no effect on the performance of the sensor which was used for a period of four months. Plasticized PVC-membranes incorporating 4-fer/-butylcalix[8]arene as the ionophore, NaTPB as the anion excluder and tris-(2-ethylhexyl)phosphoric acid (TEP) as the plasticizer in the percent (w/w) ratio 9:50:5:36 (ionophore:PVC:NaTPB:TEP) responded selectively to Sr2+ in the concentration range 2.0xl0~5-1.0xl0-1M with a near-Nernstian slope of31.3 mV per decade of activity and a response time of 15s for a period of four months. It can be used in partially non-aqueous medium having upto 30% (v/v) non-aqueous content. U02+-selective sensors have been developed using 4-/er/-butylcalix[6]arene as the electroactive material embedded in PVC matrix. The sensor exhibits Nernstian response in the concentration range 3.9 xlO-5 -1.0x10_1M with a fast response time of 10s. The life time of the sensor is five months and it works well in the pH range 2.2-3.2 and in partially non-aqueous medium having upto 20% (v/v) alcohol content.