ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
THE BEHAVIOR OF POLYANILINE-COATED PVC MEMBRANE BASED ON 7,16-DIDECYL-1,4,10,13-TETRAOXA-7,16-DIAZACYCLOOCTADECANE FOR pH MEASUREMENTS IN HIGHLY ACIDIC MEDIA
© 2014 M. Arvand1, R. Ansari, L. Heydari
Department of Chemistry, Faculty of Science, University of Guilan P.O. Box: 1914, Namjoo Street, Rasht, Iran
1E-mail: arvand@guilan.ac.ir Received 22.04.2011; in final form 27.01.2014
Polyaniline (PANI) chemically coated on Polyvinylchloride (PVC) membrane based on a neutral carrier 7,16-didecyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (Kryptofix 22 DD) as the active component has been developed for determination of pH values ranging from pH 0.1—1. Effect of experimental parameters such as membrane composition, nature and amount of plasticizer, lipophilic additives and thickness of PANI film on the potential response of the pH electrode was investigated. The electrode has an apparent Nernstian response slope of 54.5 ± 0.4 mV/pH (at 20°C). The equilibrium water content of the electrode was determined in pure water and NaCl solution (I = 0.1 mol/kg). The electrode had low electric resistance, good potential stability and reproducibility (±1.5 mV n = 10). It had a rapid potential response to changes of pH (15 s). The excellent performance in terms of linearity, stability and fast response makes this device suitable for pH measurements in highly acidic media.
Keywords: polyaniline, 7,16-didecyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane, polyvinylchloride membrane, hydrogen ion-selective electrod.
DOI: 10.7868/S0044450214090023
Today the pH measurements are generally carried out by use of glass electrodes. These electrodes have become very popular due to their selectivity, reliability and dynamic pH range. Although, pH-sensitive glass electrode has certain setbacks such as high resistance, fragility, instability in hydrofluoric acid or fluoride solutions and unsuitability to serve as a microelectrode for biological applications [1—3]. Classical liquid exchangers have been used for the preparation of hydrogen ion-sensitive polyvinylchloride membranes [4, 5] with limited success. Neutral carriers showed much more promising characteristics for the preparation of PVC pH electrodes and have been studied by several investigators [6—8]. Fouskaki et al. [9] have reported liquid polymeric membranes based on 3-hydroxypi-colinic acid derivatives for measurement of subzero pH values. Hydrogen ion-selective electrodes were also prepared using calixarene derivatives as neutral carriers such as 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetracyanomethoxycalix[4]arene [10], calix[4]-aza-crowns [11], p-tert-butylcalix[4]arene-oxacrown-4 [1], that all showed sensitivity at pH > 2. Some hydrogen ion selective membrane electrodes based on amines as
neutral carriers were also prepared with similar pH sensitivity [12—15].
One approach toward durable ion selective electrodes (ISEs) is to produce all-solid-state electrodes without any internal filling solution. However, high potential stability is a prerequisite for obtaining reliable sensors. Nikolskii and Materova identified three conditions that should be fulfilled in order to obtain a stable electrode potential for ISEs with a solid internal contact [16]: 1) reversibility and equilibrium stability of transition from ionic to electronic conductivity, 2) sufficiently high exchange currents in comparison with the current passed during measurement, 3) absence of side reactions parallel to the main electrode reaction. The potential stability of the all-solid-state electrodes can be improved by using a protecting layer with suitable ion-exchange properties on the ionically conducting ion-selective membrane. A promising approach in this respect is to use an electroactive polymer (conducting polymer or redox polymer) in combination with carrier-based ion-selective membranes [17]. On the basis of their ion-exchange properties, conducting polymers fulfil at least condition 1 presented above [18, 19].
The development of polymer blends resulted in the improvement of their mechanical properties [20] and the application of PANI and polypyrrole in electrochemical devices has been reported [21]. Much work has been devoted to the study of charge transport through polymeric materials containing extended n-conjugated backbones, such as polypyrrole, poly-thiophene and polyaniline [22]. A literature review shows that there are many reports on using of PANI as conducting polymer matrix in the development of electrochemical sensing devices. PANI-coated Pt, Pd and graphite electrodes have been studied [23]. Their potential relationship to pH follows the same principle as the pH glass electrode. The nonprotonated form of polyaniline was used for pH 6.8 to 1.2 and the proto-nated form of polyaniline was used for pH 6.8 to 13 [24]. PANI films can be obtained directly on an ultra-microelectrode through the anodic electropolymer-ization of the monomer through successive potential cycles, at best a linear relationship with pH is observed for 3—9 with a slope of 60 mV per pH unit [23]. The
behavior of a polyaniline solid contact pH selective electrode based on N,N',N,N'-tetrabenzylethanedi-amine ionophore was investigated by Han group [24]. The linear dynamic range of this electrode was pH 3.5—11.9 with a Nernstian slope of 52.1 mV/pH. Han et al. [25] also prepared a hydrogen selective polyaniline solid contact electrode based on diben-zylpyrenemethylamine ionophore for highly acidic solutions (pH = 0.5). They also investigated the presence of other neutral carriers such as N,N'-dialkyl-benzylethylenediamine and tribenzylamine in a PVC membrane with polyaniline solid contact [26, 27]. Zine et al. [28] suggested a neutral conductive polymer, polypyrrole doped with cobalt-te-dicarbollide ions, as an internal solid contact layer for pH measurements with linear response over the pH range 3.5—11 (slope 52.2 mV/pH).
Polyaniline (Scheme 1) is one of the most studied conducting polymers due to its good stability, easy preparation and low cost of reactants.
Polyaniline (emeraldine) salt
■NH-
■NH-
±2 n H®AS
1—N^ ^=N—^—NH
NH
Polyaniline (emeraldine) base
Scheme 1. Polyaniline (emeraldine salt) in acidic media and emeraldine base in alkaline media (HA is an arbitrary acid).
The chemistry of PANI is a little intricate, due to the existence of different acidic functions and oxidation states. There are three oxidation states, each re-dox couple corresponds to a 2e- exchange. The less oxidized state, leucoemeraldine, the intermediate state emeraldine (EB) and the most oxidized state, pernigraline are insulating, the only conductive form is the intermediate emeraldine salt (ES) [23]. Imines sites of the EB forms are easily protonated, with a striking insulator — conductor transition, induced due to the appearance of polarons in the lattice, while the number of n-electrons remains constant. As a consequence, new optical, conducting, and paramagnetic properties appear in the ES. These properties make PANI successfully used as a sensor [29—31].
The surface properties of thin PANI films, such as the hydrophilicity and surface morphology, can improve the properties of various matrices, in particular
the traditional PVC membranes, and make them more useful in the field of analytical chemistry [32].
Due to the pH-dependent PANI (emeraldine) salt — base transition, polyaniline has been successfully used as an additive to the typical ion-selective membrane based on PVC or silicone [32, 33], but none of them have been ever applied at pH < 1. With regard to above-mentioned features, the polyaniline film should act simultaneously as a new additive to the membrane electrode and as a membrane component affecting the ion transport.
In this work, we report the effect of PANI deposition on the potentiometric behavior of a membrane pH electrode based on a neutral carrier 7,16-didecyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (Kryptofix 22 DD, Scheme 2) toward hydrogen ions in highly acidic media (pH < 1). The response time and the lifetime of the electrode and its selectivity against some anions and cations were investigated.
-O O--N N-
Scheme 2. Chemical structure of Kryptofix 22 DD used as ionophore.
EXPERIMENTAL
Reagents and solutions. Aniline and tetrahydrofu-ran (THF) were purified by vacuum distillation. Redistilled water and analytical-reagent grade reagents were used throughout. High-molecular weight PVC, èis-(2-ethylhexyl)sebacate (DOS), dibutyl sebacate (DBS), dibutyl phthalate (DBP), acetophenone (AP), ammonium persulfate, potassium tetrakis-(pp-chorophe-nyl)borate (KTpClPB), oleic acid (OA), 7,16-didecyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (Krypto-fix 22 DD), and tridodecylmethylammonium chloride (TDDMACl) were purchased from Fluka or Merck. The solutions were prepared in order to determine the effect of mono-, di- and trivalent ions on the electrode response by use of fixed interference method and modified separate solution method. A series of calibration solutions was prepared in pH range 0.1—1.0 by dilution of 12 M HCl. During this work, buffer solutions were prepared from HIO3—KIO3 (Ka = 1.7 x 10-1) for the determination of pH sensitivity and potentiometric se-lectivities of the respective electrode.
Membranes. In the present study, 1.2 mL THF was used to dissolve a mixture composed of the active component Kryptofix (5.7%, 3.6 mg), plasticizer (AP 41.2%, 26.2 mg) and PVC (53.1%, 33.8 mg). Solutions of the resultant mixtures were placed on a glass plate, and THF was evaporated at room temperature.
Polyaniline deposition. The deposition of a PANI film was performed by soaking an appropriate PVC membrane in a freshly prepared mixture of aniline hy-drochloride (1 M) and ammonium persulfate (0.5 M) as oxidant. During the aniline polymerization, a thin green PANI film with a thickness of about 100—200 nm was grown on both sides of the plasticized PVC membrane surfa
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