New Selenite Ion-Selective Electrodes Based on 5,10,15,20-Tetrakis-(4-Methoxyphenyl)-21H,23H-Porphyrin-Co(II)

Journal of Hazardous Materials 181 (2010) 857–867 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat New selenite ion-selective electrodes based on 5,10,15,20-tetrakis-(4-methoxyphenyl)-21H,23H-porphyrin-Co(II) H. Ibrahim, Y.M. Issa ∗, Ola R. Shehab Chemistry Department, Faculty of Science, Cairo University, Gamma Street, Giza, Cairo 12613, Egypt article info abstract Article history: New polymeric membrane (PME), modified carbon paste (MCPE), and coated wire (CWE) selen- Received 11 January 2010 ite ion-selective electrodes based on 5,10,15,20-tetrakis-(4-methoxyphenyl)-21H,23H-porphyrin-Co(II) Received in revised form 29 March 2010 (CoTMeOPP) are reported. The best composition was the electrode containing 2% CoTMeOPP as the active Accepted 19 May 2010 material and 49% TCP as plasticizer. The electrodes reveal a Nernstian behavior over a concentration range Available online 26 May 2010 of 5.5 × 10−5 to 1.1 × 10−2 M for PME, 5.2 × 10−5 to 1.2 × 10−2 M for MCPE and 1.2 × 10−4 to 4.4 × 10−3 M for CWE. The potentiometric response is pH dependent, since selenous acid is a diprotic acid. The slope Keywords: of the selenite PVC electrode was −57.0 mV for the monovalent anion at pH 6.47, and −26.0 mV for the Selenite × −5 × −5 Graphite divalent anion at pH 11.00. The detection limits were 3.4 10 and 4.7 10 M at pH values 6.47 and Membrane 11.00, respectively. The electrodes manifest advantages of low resistance, very short response time (15 s), Coated wires and most importantly good selectivities relative to a wide variety of other anions. In fact, the proposed Porphyrin selenite ion-selective electrodes show a great improvement compared to previously reported electrodes for selenite ion. The electrode was used for the determination of selenite in selenite/selenate mixture, in sodium selenite raw material powder, and in VitaFit Selenium ACE antioxidant tablets with recovery ranges of 90.0–103.3%. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Selenium occurs naturally in a number of inorganic forms, including selenide, selenite and selenate. Selenium is an essential In the recent years, there has been increasing interest in the trace trace element for plants, animals and humans. The biological func- determination of selenium. Apart from natural sources (mainly tion of selenium shows dual characteristics, it can cause disease by metal–sulfur minerals), selenium compounds are widely spread its deficiency but is toxic at level relatively close to those required out from the combustion of fossil fuels, and used in the glass, elec- for health [2]. Elemental selenium and most metallic selenides have tronic industries and in agriculture [1]. The toxicity and essential relatively low toxicities because of their low bioavailability. By con- nature of selenium depends on its chemical form. Selenium nat- trast, selenite and selenate are very toxic and have modes of action urally exists in several oxidation states in inorganic and organic similar to that of arsenic. Speciation of selenium in environmental forms. The inorganic species most frequently found in water and and biological samples is necessary to understand the biochemical 2− 2− soil are selenite (SeO3 ) and selenate (SeO4 ). cycle, mobility and uptake of selenium, as well as its toxicity. The development of reliable techniques to study the speciation Hydride generation coupled to atomic absorption spectrome- of selenium in environmental and biological samples is necessary try (AAS), inductively coupled plasma (ICP) or atomic fluorescence to understand the biological cycle, mobility, and uptake of sele- spectrometry (AFS) are the techniques that most commonly used nium, as well as its toxicity. Many problems in selenium speciation for the speciation of inorganic selenium [3–6]. Detection limits analysis are associated with the low concentration of each species were 0.1–0.4 ␮g/l. Since this reaction is specific for Se(IV), these to be determined. Significant errors may also arise by loses of techniques can be applied for total inorganic selenium (the sum of volatile selenium compounds, instability of its chemical species Se(IV) and Se(VI)) after quantitative reduction of selenate to selen- specially in analytical methods involving many chemical treat- ite. The content of selenate is obtained as the difference between ments [1]. two determinations. The disadvantages of these techniques are the complexity and cost of the instrument. The use of ion-selective electrodes has gained importance because of their ease of handling and their selectivities on spe- ∗ Corresponding author. Tel.: +20 2 35868094; fax: +20 2 35728843. cific ions. There are only a few investigations regarding selenite E-mail address: [email protected] (Y.M. Issa). ion-selective electrodes. Ion-selective electrodes for the determi- 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.05.092 858 H. Ibrahim et al. / Journal of Hazardous Materials 181 (2010) 857–867 as the outer reference electrode. Millipore Elix S (Automatic Sani- tization Module, ASM) was used for obtaining the deionized water. CARY 50 Probe UV–Visible spectrophotometer, Varian was used for the spectrophotometric measurements. Anhydrous sodium selenite (Na2SeO3) was purchased from Loba Chemie PVT LTD., India. 5,10,15,20-tetrakis-(4-methoxyphenyl)- 21H,23H-porphyrin-Co(II) [C48H36CoN4O4], poly(vinyl chloride) (PVC) of high relative molecular weight, graphite powder, tetrahy- drofuran (THF), AgNO3, tris(hydroxymethyl)-aminomethane and the plasticizers, o-nitrophenyloctyl ether (o-NPOE), dibutyl phthalate (DBP), dioctyl phthalate (DOP), dibutyl-butyl phosphonate (DBBP), and tricresyl phosphate (TCP) were purchased from Aldrich chemical company, USA. 2.2. Solutions Stock solution of 0.1 M sodium selenite was prepared by dissolving 1.73 g Na2SeO3 in 100 ml deionized water. The solution was kept in refrigerator. Acetate buffer solution of pH 5.89 was prepared by mixing 95 ml of 0.1 M sodium acetate solution with 5 ml 0.1 M acetic acid solution. Phosphate buffer solutions of pH values, 5.59, 6.47, and 7.38 Fig. 1. Structure of (CoTMeOPP). were prepared by mixing the appropriate volumes 0.01 M KH2PO4 and 0.01 M Na2HPO4 solutions [14]. Tris–HCl buffer solution pH 8.5 was prepared by the addition of 0.01 M HCl to 50 ml 0.01 M tris nation of selenite were constructed by compressing either HgSeO3 solution [15]. Ammoniacal buffer solution of pH 9.5 was prepared or a mixture of HgSeO3 and Hg2Cl2 [7]. Cai et al. [8] had developed selenite ion-selective electrode using 4,6-dibromopiaselenole as by the addition of the required volume of 0.01 M NH4OH to 50 ml active material, PVC as membrane matrix and dibutyl phthalate as 0.01 M NH4Cl solution then completed to 100 ml using deionized plasticizer. Three different selenite-selective electrodes were fab- water. Sodium chloride–sodium hydroxide solution of pH 10.2 and 11.0 were prepared by the addition of the required volume of 0.01 M ricated, the first was developed using solid salts of Ag2Se and Cu2S [9]. Piaselenol (PIS) was used in the form of 1,2-phenylenediamine NaOH to 50 ml 0.01 M NaCl solution then completed to 100 ml using selen complex as the active material for ion-selective electrode deionized water. To investigate the selectivity of the proposed electrodes, 0.5 M membrane sensitive to selenite ion [10]. Silicon rubber [11] was − − − used as the active membrane matrix for selenite-selective electrode sodium salt solution of each of the following ions, Cl ,Br ,NO3 , 2− consisted of 4% dialkyldimethylammonium chloride, and QAD 86 SO4 , and 0.1 M sodium salt solution of each of the following ions, 2− − 2− 3− 2− S ,F ,SO3 ,PO4 , and 0.1 M potassium salt solution of SeO4 , PI. A self-made ion-selective electrode was made using Ag2Se as − electroactive material [12], and used for the determination of sele- SCN were prepared in deionized water. nium in biological materials. It can be observed that in all papers reported in the literature, there is no mention about seleneous acid 2.3. Preparation of the electrodes species as a function of pH. It was, therefore, felt worthwhile to develop a better sensor 2.3.1. Preparation of PVC-membrane electrodes for selenite ions using 5,10,15,20-tetrakis-(4-methoxyphenyl)- Membranes of different compositions and plasticizers were pre- 21H,23H-porphyrin-Co(II) (CoTMeOPP) (Fig. 1) as an electroactive pared. The membrane was prepared by dissolving the required phase in PVC matrix or carbon paste. amount of PVC and plasticizer in 5 ml THF. The calculated amount of An unusual feature of porphyrin chelate is that the metal ion ionophore was dissolved in THF and mixed with the PVC/plasticizer is bonded to four nitrogen atoms, which are themselves linked solution in Petridish (5.0 cm diameter). The total weight of con- together in a conjugated system. This imparts a high degree of stituents in each batch is fixed at 0.2 g. The membranes were left mesomerism whereby a substitution anywhere in the porphyrin to dry in air (not less than 24 h) to obtain homogeneous and uni- nucleus can relay its electron donating and attracting tendency to form thickness. The membranes formed as a film in the bottom all the four coordinating atoms that makes metalloporphyrins so are taken out carefully and glued to a homemade electrode bod- sensitive to the influence of substituents [13]. ies with an adhesive of PVC dissolved in THF. The electrodes were PVC membranes, coated wires, and modified carbon paste elec- filled with 10−1 M NaCl and 10−3 M sodium selenite solution and trodes based on CoTMeOPP as ionophore for selenite ions were preconditioned by soaking in 10−3 M selenite solution for 24 h.

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