Sensors and Actuators B 100 (2004) 315–319
Nano levels detection of beryllium by a novel beryllium PVC-based membrane sensor based on 2,3,5,6,8,9-hexahydro-1,4,7,10-benzotetra oxacyclododecine-12-carbaldehyde-12-(2,4-dinitrophenyl)hy Mohammad Reza Ganjali a,∗ , Maryam Ghorbani a , Parviz Norouzi a , Azadeh Daftari a , Magid Faal-Rastegar a , Abolghasem Moghimi b a
Department of Chemistry, Tehran University, P.O. Box 14155 6455, Tehran, Iran b Department of Chemistry, Imam Hossein University, Tehran, Iran
Received 20 September 2003; received in revised form 19 December 2003; accepted 6 January 2004 Available online 8 April 2004
Abstract A novel poly(vinyl chloride)-based 2,3,5,6,8,9-hexahydro-1,4,7,10-benzotetra oxacyclododecine-12-carbaldehyde-12-(2,4-dinitrophenyl)hy (PBC) with sodium tetraphenyl borate (NaTPB) as an anion excluder, benzyl acetate (BA), acetophenon (AP) and o-nitrophenyloctyl ether (NPOE) as plasticizing solvent mediators was prepared and investigated as a beryllium selective sensor. The best performance was observed with the membrane having the PVC–NaTPB–NPOE–PBC composition 30%:3%:62%:5%, which worked well over a very wide concentration range (1.0×10−7 M to 1.0×10−1 M). The sensor exhibits a Nernstian slope of 29.9 mV per decade of Be2+ activity. The detection limit of the sensor is 7.0 × 10−8 M (∼630 ppt). The proposed electrode shows excellent discriminating ability toward Be2+ ion with regard to alkali, alkaline earth, transition and heavy metal ions. It was successfully applied to the determination of beryllium in a mineral sample. © 2004 Elsevier B.V. All rights reserved. Keywords: Beryllium-selective electrode; PVC membrane; Potentiometry
1. Introduction Macrocyclic ligands have the ability to form selective and stable complexes with metal ions of compatible dimensions [1,2]. They can potentially be applied to their separation and determination [3,4]. Thus, these ligands have been widely used as suitable neutral carriers for constructing membrane-selective electrodes for metal ion [5]. In particular because of the remarkable ability of macrocyclic polyethers to form complexes with alkali and alkaline earth cations, these ligands have been used as suitable neutral carriers for lithium, sodium, potassium, magnesium, calcium, and barium [5]. Utility of ion-selective electrodes (ISEs) is being increasingly realized by analytical chemists in view of the rapid growth of industry and technology over the world as they represent a rapid, accurate and low-cost method of analysis. Moreover, analysis by ISEs could be nondestructive and adaptable to small sample volumes.
∗ Corresponding author. Fax: +98-21-6112788. E-mail address:
[email protected] (M.R. Ganjali).
0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.01.021
Among alkali and alkaline earth metal ions, beryllium has received less attention in spite of its wide industrial uses. Beryllium has found many applications in aerospace, nuclear, telecommunication, and computer industries. Because of its high toxicity and debated carcinogenicity, analysis of this element is necessary in the vicinity of ore processing plants and their disposal sites as well as in the industry using beryllium products [6,7]. Unfortunately, the interest in Be2+ ISEs has been dampened by the fact that this ion, owing to its very small size and high charge density, is very strongly hydrated. Its Gibbs free energy of hydration is 31 and 400% larger than that of Mg2+ and Li+ ions, respectively, making an appropriate ionophore design for Be2+ very difficult [8]. Because of the small size of Be2+ , which limits its maximum coordination number, and the high required complex stabilities, it has been suggested that the search for ISEs for this analyte ion might be hopeless [9]. A further reason for the lack of interest in Be2+ ISEs is probably that the high toxicity of beryllium compounds required, determination at concentrations that are too low for conventional direct measurements with phosphate ester ionophores [10–12].
316
M.R. Ganjali et al. / Sensors and Actuators B 100 (2004) 315–319
O2N
N
NH
CH
O O O
NO2
O
Scheme 1.
Recently, we reported the beryllium membrane sensors based on various ionophore including benzo-9-crown-3 [13], 3,4-di[2-(2-tetrahydro-2H-pyranoxy)]ethoxy styrene–styrene copolymer [14], naphthalen 9-crown-3 [15], and 2,4dinitrophenylhydrazine benzo-9-crown-3 [16,17]. These electrodes revealed good selectivity and sensitivity. In this work, we report a highly selective and sensitive beryllium membrane electrode for fast monitoring of ppt level of Be2+ ion based on (PBC) (Scheme 1).
2. Experimental section 2.1. Reagents Reagent grade acetophenon (AP), benzyl acetate (BA), o-nitrophenyloctyl ether (NPOE), high relative molecular weight PVC, sodium tetraphenyl borate (NaTPB) and tetrahydrofuran (THF) were purchased from Aldrich and used as received. Beryllium chloride, nitrate and the nitrate of other cations used (all from Merck) were of the highest purity available and used without any further purification except for vacuum drying over P2 O5 . The PBC was synthesized and purified by usual method as described before [17]. Triply distilled deionized water was used throughout. 2.2. Electrode preparation The general procedure to prepare the PVC membrane was to mix thoroughly 30 mg of powdered PVC, 62 mg of NPOE, 3 mg of NaTPB, and 5 mg of PBC in 3 ml of THF. To this mixture was added 5 mg of PBC and the solution was mixed well. The resulting mixture was transferred into a glass dish of 2 cm diameter. The solvent was evaporated slowly until an oily concentrated mixture was obtained. A Pyrex tube (3–5 mm i.d.) was dipped into the mixture for about 10 s so that a nontransparent membrane of about 0.3 mm thickness was formed [19–22]. The tube was then pulled out from the mixture and kept at room temperature for about 12 h. The tube was then filled with an internal filling solution (1.0×10−3 M BeCl2 ). The electrode was finally conditioned for 12 h by soaking in 1.0 × 10−2 M BeCl2 . A silver/silver chloride coated wire was used as an internal reference electrode. 2.3. emf measurements All emf measurements were carried out with the following assembly: Ag-AgCl/internal solution (1.0 × 10−3 M
BeCl2 )/PVC membrane/test solution/Hg2 Cl2 , KCl (saturated). A Corning ion analyzer 250 pH/mV meter was used for the potential measurements at 25.0 ± 0.1 ◦ C. The emf observations were made relative to a double junction saturated calomel electrode (SCE, Philips) with the outer chamber filled with an ammonium nitrate solution.
3. Results and discussion At first, the PBC was used to prepare the plasticized PVC-membrane electrodes for a large number of metal ions including, common alkali, alkaline earth, transition and heavy metal ions in order to test its suitability as a selective and sensitive ion-carrier for given cations. The potential responses of these electrodes, prepared under the same experimental conditions (except for 24 h conditioning in a 1.0 × 10−3 M of the corresponding cations), revealed that except Be2+ ion with the most sensitive response, in all other cases the slopes of the corresponding emf versus pMn + are much lower than the expected slopes(59, 29, and 19 mV per decade for mono, di and trivalent cations, respectively). The presence of four oxygen atoms as the hard coordination sites as well as the rigid PBC molecule together with a more convenient cavity size seem to generate great affinity of PBC toward beryllium ions. This kind of behavior was already observed for beryllium sensors [13–16]. It is well understood that the sensitivity, linearity and selectivity obtained for a given ionophore depend significantly on the membrane composition and nature of plasticizer and additive used [8–12,18–22]. Thus, several membranes of various plasticizer PVC/ionophore/additive ratios were tested, and the results are summarized in Table 1. As can be seen from Table 1, among three different solvent mediators used, NPOE with much polar than BA and AP, revealed the highest sensitivity for the proposed electrode (no. 6). Moreover, it is obvious that, in the presence of NPOE as plasticizer, the increased amount of ionophore, up to a value of 5%, resulted in the best sensitivity, although
Table 1 Optimization of membrane ingredients No.
1 2 3 4 5 6 7 8 9
Slope (mV decade−1 )
Composition % PBC
PVC
NaTPB
Plasticizer
– 3 5 6 5 5 5 5 5
30 30 30 30 30 30 30 30 30
– – – – 2 3 4 3 3
70, 67, 65, 64, 63, 62, 61, 62, 62,
NPOE NPOE NPOE NPOE NPOE NPOE NPOE AP BA
∼2.3 9.8 ± 12.2 ± 15.3 ± 21.7 ± 29.9 ± 24.4 ± 18.6 ± 17.9 ±
0.3 0.2 0.4 0.4 0.5 0.3 0.1 0.6
M.R. Ganjali et al. / Sensors and Actuators B 100 (2004) 315–319
60 40 20
E(mV)
0 -20 -40
10 0 -10
E (mV)
the slope of emf versus log concentration plot in this case is about two-third of the expected Nernstian value (no. 3). However, in the presence of 62% NPOE, 5% PBC, and 30% PVC, addition of 3% sodium tetraphenyl borate as a suitable additive will increase the sensitivity of the electrode response considerably, so that the membrane electrodes demonstrate a Nernstian behavior (no. 6). It is well known that the presence of lipophilic anions in cation-selective membrane sensors not only diminished the ohmic resistance and enhances the response behavior and selectivity, but also, in cases where the extraction capability is poor, increases the sensitivity of the membrane electrode [23–25]. The proposed sensor was also examined at various concentrations of the inner reference solution, in the range of 1.0 × 10−3 M to 1.0 × 10−5 M. The results showed that the variation of the concentration of the internal solution does not cause any significant difference in the corresponding potential response, except for an expected change in the intercept of the resulting Nernstian plots. A 1.0 × 10−3 M concentration of the reference solution is quite appropriate for smooth functioning of the electrode membrane. The optimum equilibration time for the membrane electrode was 12 h, after which it generates stable potentials when placed in contact with Be2+ solution. The emf response of the membrane at varying concentration of beryllium ion (Fig. 1) indicates a rectilinear range from 1.0 × 10−7 M to 1.0 × 10−1 M. The slope of the calibration curve was 29.9±0.5 mV decade−1 of Be2+ activity. The detection limit of the sensor, as determined from the intersection of the two extrapolated segments of the calibration graph, was 7.0 × 10−8 M (∼630 pg/ml). The standard deviation of 10 identical measurements with 10 electrodes at several concentrations of Be2+ ion was found to be in the range of ≤0.6 mV. For investigation of stability and life time of the proposed sensor, two electrodes was checked over a period of 120 days. During this time, the response of the vsensors was evaluated daily over the extended period of time (1 h per
-20 -30 -40 -50 -60 0
-80 -120 -140 8
7
6
5
4
pBe
3
2
1
2+
Fig. 1. Calibration curve for Be(II) sensor based on PBC.
0
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Fig. 2. Effect of the pH of the test solution on the potential response of the Be2+ ion-selective electrode.
day, and after using, the sensors were dried and stored). The results showed that a very slight change in the slopes of the sensors was observed (from 29.9 ± 0.5 mV decade−1 to 27.7 ± 0.3 mV decade−1 ). Dynamic response time is an important factor for any ion-selective electrode. In this study, the practical response time was recorded by immediate changing of beryllium ion concentration from (1.0 × 10−7 M to 1.0 × 10−2 M). The results showed that in whole concentration range, the sensor reaches to the equilibrium response in a very short time (
