ELECTROANALYTICAL STUDIES ON SOME ION SELECTIVE MEMBRANES ELECTRODES

Abstract

Detection of environmental pollutants both organic and inorganic has been a major challenge to analytical chemists. In recent years many instrumental techniques viz., Electro- thermal atomization-Atomic absorption spectroscopy (ETA-AAS), Inductively coupled plasma-Atomic emission spectrometry (ICP-AES), Neutron activation analysis, Ion chromatography, High performance liquid chromatography [HPLC], Flame photometry and Cyclic voltametry have been used for the quantitative analysis of pollutants. All these listed techniques are sophisticated, time consuming and costly and require huge infrastructure and expert handling. On the other hand Ion-selective electrodes (ISEs) provide an analytical procedure that is simple, convinces and fast. Analysis by Ion-selective electrode can be carried out in field and is also adaptable to online monitoring another major advantage of analysis by ISEs i.e. requires minimum chemical manipulation of sample and is applicable to turbid as well coloured solution. Ion-selective electrodes are extensively used for determining concentration of various ions in bloods, biological fluids, eatables, drugs etc. One of the important applications of ion- selective electrodes is the monitoring of toxic metals in effluents and natural waters. Some of the established applications of ISEs are determination of F– in drinking water, Ca2+ in dairy products, determination of promethium in spiked water sample and K+ in fruit juices etc. They are also used to determine clinically important ions viz., Na+, K+, Li+ and Ca2+ A survey of literature reveals that a number of Ion-selective electrodes have been developed for various metals and anions. However the availability of Ion-selective electrode for metal such as molybdenum, promethium, fluoride and mercury is rather limited. These are some reported electrode for these metals but they exhibits certain limitation such as short in body fluids. They have also been used to determine metals in soils, fertilizers and plant products. ii working range, poor selectivity and higher response times. Thus improved Ion-selective electrode for these metals are requested attempts in this direction we initiated by us. To develop good ion-selective electrodes ionophores having high affinity for a particular ionic species is required. Different types of material such as solid electrolytes in other multivalent metals, ion exchange, macrocyclic, calixarene and Schiff base have been used for this purpose. We have used some Schiff base, calixarene and macrocycle to develop ion-selective membrane electrodes for molybdenum, promethium, fluoride and mercury. The result of this investigation is incorporated in the present thesis divided into six chapters which are discussed briefly here. First chapter presents a “General Introduction” which describes the background of the research work, the problem statement and objectives of the present study. The chapter also summarized a review of good ion-selective electrodes for alkali metal ion, alkaline earth metal ion, rare earth metal ion, transition metal and anions. Second Chapter presents; “Theory and Methodology of Ion Selective Membranes Electrodes” This chapter describes the classification of ion-selective electrodes, theory of membrane potentials and parameters of ion-selective electrodes, the terms selectivity used in the study of ion selective electrode. The selectivity of an ion selective membrane is a measure of the selectivity of an electrode for the primary ion in the presence of interfering species. The degree of selectivity of the electrode is given by Eisenman-Nicolsky equation For determination of Selectivity coefficient three methods are used. 1. Separate Solution method 2. Mixed solution method +±=Σ≠BABBzAzBPotBAAAaKaFzRTEE/,0)(log303.2 iii 3. Fixed Interference Method Third Chapter; this chapter reveal a “Promethium ion-selective sensor based on the comparative study of two Schiff base ligands as neutral ionophores” have been developed and effect of various plasticizers have been studied. Two Schiff base ligands (3,5:12.14:21,23-tribenzo-8,9:17,18:26,27-tricyclohexyl-1,6,10,15,19,24-hexaazo-[27]-1,6,10,15,19,24-triene (X1) and 15,17,32,34-tetramethyl-3,29-dioxa-11,12,16,20,21,35-hexaazapentacyclo[29.3.1. 114,18.04,9.023,28]hexatriaconta 1(35),4,6,8,10,14(36),15,17,21,23,25,27,31,33-tetradecaene-13,19-dione (X2)) as neutral ionophores and effect of various plasticizers: 2-nitrophenyl- octylether (o-NPOE), dibutyl phosphonate (DBP), dioctylphthalate (DOP), tri-(2-ethylhexyl) phosphate (TEHP), dibutyl butylphosphonate (DBBP), 1-chloronaphthalene (1-CN) and anion excluders: potassium tetrakis(p-chloropheny1) borate (KTpClPB), sodium tetraphenylborate (NaTPB) and oleic acid (OA) have been studied. The membrane with a composition of ionophore (X1/X2):KTpClPB:PVC:o-NPOE (w/w, %) in the ratio of 5:5:30:60 exhibited best performance. The best responsive membrane sensors (8 and 21) exhibited working concentration range of 4.5 × 10−7 – 1.0 × 10−2 mol L-1 and 3.5 × 10−6 – 1.0 × 10−2 mol L-1 with a detection limits of 3.2 × 10−7 mol L-1 and 2.3×10−6 mol L-1 and Nernstian slopes of 20.0 ± 0.5, 19.5 ± 0.5mV decade−1 The fourth Chapter; “A comparative study on PVC based sensors in determination of the Molybdenum”, deals with Ion-selective electrode based on macrocyclic for selective determination of molybdenum. Three macrocyclic 4,8-diaza-3,3,10,10-tetramethyl-1,2- of activity, respectively. The sensor no. 8 works satisfactorily in partially non-aqueous media up to 10% (v/v) content of methanol, ethanol and acetonitrile. Analytical application of the proposed sensor has been demonstrated in determination of promethium(III) ions in spiked water samples and the electrode could be used successful in water sample. iv dithiacyclodecane(S1), 5,8-diaza-3,3,10,10-tetramethyl-1,2-dithiacyclodecane-N,N”-diacetic acid (S2) and N,N”-bis(2,2-dimethyl-2-mercaptoethyl)ethylenediamine-N,N-diacetic acid (S3) when prepared and preliminary studies shows that macrocyclic neutral ionophores shows high affinity of molybdenum. These three membranes are used to develope molybdenum selective membrane electrode. A number of membrane prepared of three macrocyclic separately, number of PVC base three membrane prepared and investigated, the result have shown the sensor using as three ionophore perform best. The performance of electrodes adding the different plasticizer, It was found that ortho nitro phenyl octylether (o-NPOE) is the best plasticizer and potassium tetrakis(p-chlorophenyl) borate (KTpClPB) best cationic additives. The membrane having composition (w/w, mg%); 5.0(S):30.0(PVC):5.0 (KTpClPB):60.0(o- NPOE), performed best in respect of different performance characteristic. It exhibits a wide working concentration range 2.3 × 10–7 – 1.0 × 10–2 mol L–1 with a detection limit of 1.2 × 10–7 mol L–1 and slope of 11.2 decades-1 The fifth chapter; “A comparative study of fluoride selective PVC based electrochemical sensors”: this chapter reports PVC based membrane electrodes based on four ionophore M of activity. The sensor was found to be sufficient effective for the Molybdenum and it can be used to determine the concentration in different sample. The electrodes have a shelf life time of 2-5 months and dynamic response time of 11s. 1–M4 (Ionophore M1, meso-octamethylcalix[4]pyrrole. Ionophores M2, [7H,23H-34,39-etheno-6,43;24,30 dimethenotribenzo [o,v,g] [1,4,7,11,34] trioxadiazacyclo heptatriacontine-7,23-dione,8,9,10,11,13,14,16,17,19,20,21,22,31,32,33,40,41,42-octadeca hydro-44,45,46,47-tetrahydroxy-32,41-bis(methylene). Ionophore M3, is 7H,23H-34,39- etheno-6,43;24,30-dimethenotribenzo[o,v,g] [1,4,7,11,34] trioxadiazacycloheptatriacontine- 7,23-boropyrene,8,9,10,11,13,14,16,17,19,20,21,22,31,32,33,40,41,42-octadecahydro- 44, 45, 46,47-tetrahydroxy-32,41-bis(methylene) and ionophore M4; dinitrophenyl functionalized trisv (amide)) which have been synthesized and characterized by IR, 1H NMR, spectroscopic investigations indicate good affinity of these ligands for fluoride anion. Different polyvinyl chloride (PVC) based membranes of ligands have been synthesized using different cationic excluders; CTAB, TDMAC, HTAB, ToMACI and plasticizers; DBBP, DBP, o-NPOE, CN, DOP, TEHB and investigated as F− -selective sensors. The best performance is observed by the sensor with a membrane of composition (%, w/w) M1:PVC:o-NPOE:CTAB 3.5:30.0:63.0:3.5. The sensor generates linear potential response over a wide working concentration range of 2.5 × 10−7 to 1.0 × 10−2 mol L-1 with Nernstian slope (59.8 mV decade−1 of activity) over a pH range of 2.5–6.5 with a fast response time of ∼11 s. It shows good selectivity for fluoride anion (F− The final and Sixth chapter “Mercury selective potentiometric sensor based on low rim functionalized thiacalix[4]-arene as a cationic receptor”: this is the last chapter it report the result of potentiometric sensor of mercury prepared by calixarene. PVC membrane of these calixarene where prepared and studied as a mercury selective electrodes. It was found that this membrane shows potential responses to mercury, the effect of plasticizer and performances of membrane electrodes also studies. This describes a novel potentiometric mercury(II) sensor based on the use of cation receptor 5,11,17,23-tetra-tert-butyl-25,27- dihydroxy-26,28-bis(O-methylglycylcarbonylmethoxy)thiacalix[4]-arene in poly(vinyl chlorid -e) (PVC) matrix for detection of Hg ) in preference to many anions. The sensor exhibits a shelf life of two and half months and could be successfully used for the comparative determination of fluoride contents in various samples. The proposed method is faster, cheaper and more accurate in comparison to already used methods. 2+ has been developed. The sensor exhibits best performance with a membrane composition of PVC:o-NPOE:Ionophore:NaTPB of 60:120:5:10 (%, w/w). The sensor selectively used for determination of mercury ions is in the vi concentration range 5.0 × 10−8 – 1.0 × 10−2 mol L-1 with a lower detection range of the order 1.0 × 10−8 mol L-1 and a Nernstian compliance of (29.5) within pH range 6.0 to 7.5 and fast response time of 10 s. it can also be worked satisfactory. Selectivity coefficient of the electrode shows the high affinity to Hg+2 over a number of metal ions. Influence of the membrane composition and possible interference of other ions have also been investigated on the response properties of the sensor, fast and stable response, good reproducibility and longterm stability of the sensor are demonstrated. It has been observed that the developed sensor satisfactorily works in partially non-aqueous media up to 10% (v/v) content of methanol and acetonitrile and could be used for a period of 2.5 months. Selectivity coefficients determined with fixed interference method (FIM) and match potential method (MPM) indicate high selectivity towards mercury(II) ions. The proposed electrode shows fairly good discrimination of mercury from other cations. The developed mercury ion-selective electrode can be successfully employed as an indicator electrode in potentiometric titration with EDTA.