Comparative study of fluoride selective PVC based electrochemical sensors

Abstract

Four ligands (M₁–M₄) 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 concentration range of 2.5 × 10⁻⁷ to 1.0 × 10⁻² M 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⁻) 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 in different Indian tea samples. The proposed method is faster, cheaper, and more accurate in comparison to already used methods.

Introduction

Fluoride occurs in all water bodies in small quantity and its content depends on the origin of water sample [1]. It has been found that it varies from 0.01 ppm (fresh water) to 1.5 ppm (sea water) [2]. Fluorine being highly reactive, a large number of its compounds both of metals and nonmetals are prepared which find diverse application in domestic and industrial process.

Fluorine-containing compounds range from potent toxins such as sarin [3] to life-saving pharmaceuticals such as efavirenz [4], and from inert materials such as calcium fluoride to the highly reactive sulfur tetrafluoride. The range of fluorine-containing compounds is considerable as fluorine is capable of forming compounds with all the elements except helium and neon [5], [6].

The applications of fluoride and its compounds are many viz. synthesis of organic and inorganic compounds, enzyme inhibition, nuclear fuel reprocessing, coating of non-sticky fry-pans as a PTFE, polymers (teflon), drugs and fluoridation of drinking water, etc. As a result of its varied applications, the fluoride gets access to the human body causing harm after a certain level of deposition. At higher concentrations, soluble fluoride salts are toxic. Referring to a common salt of fluoride, NaF, the lethal dose is estimated at 1–10 g for adults [7]. A lethal dose is approximately 28 mg/kg of body mass [8]. When the fluoride is ingested via food contamination, it initially acts locally on the intestinal mucosa, where it forms hydrofluoric acid in the stomach. Thereafter it binds with the calcium and interferes with the activity of various enzymes. Some of the chronic toxicities of fluoride ingestion above the lethal limit are dental fluorosis, skeletal fluorosis, rheumatoid arthritis [9].

Due to large applications of fluoride containing inorganic and organic compounds, a number of natural water bodies are contaminated with fluoride. Therefore its determination is very crucial for health purposes. The techniques available for the determination of fluoride in different samples are atomic absorption spectroscopy (AAS) [10], [11], flow-injection [12], gas chromatography (GC) [13], high resolution nuclear magnetic resonance (NMR) spectrometry [14], capillary zone electrophoresis (CZE) [15], radioanalysis [16], colorimetric [17], fluorometric [18], and ion-selective electrode (ISE) [19]. The last decade has seen a rise in the study of ion selective electrode techniques [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63] for such purposes. The current research is focused on the ion-selective electrode for the quantitative determination of fluoride content in different natural samples. However there has been some work already done on fluoride selective sensors. Still many improvements are also necessary for better application ranges. Considering this fact, We have used meso-octamethylcalix[4]pyrrole, tripodal amide receptor and macrocyclic neutral complexes of boron as ionophores in comparative analysis of PVC based membrane sensors in the determination of fluoride in different natural samples with improved working range and detection limit.