научная статья по теме SUB-NANOMOLAR DETERMINATION OF BERYLLIUM ION BY A NOVEL BE(II) MICROSENSOR BASED ON 4-NITROBENZO-9-CROWN-3-ETHER Химия

Текст научной статьи на тему «SUB-NANOMOLAR DETERMINATION OF BERYLLIUM ION BY A NOVEL BE(II) MICROSENSOR BASED ON 4-NITROBENZO-9-CROWN-3-ETHER»

ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2008, том 63, № 7, с. 749-755

^=ОРИГИНАЛЬНЫЕ СТАТЬИ

УДК 543

SUB-NANOMOLAR DETERMINATION OF BERYLLIUM ION BY A NOVEL Be(II) MICROSENSOR BASED ON 4-NITROBENZO-9-CROWN-3-ETHER

© 2008 r. M. R. Ganjali*' **, P. Norouzi*' **, R. Dinarvand***, V. Akbar*, A. Moghimi****

*Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran

Tehran, Iran

**Endocrine & Metabolism Research Center, Tehran University of Medical Sciences

Tehran, Iran

***Medical Nanotechnology Research Centre, Tehran University of Medical Sciences Tehran, P.O. Box 14155-6451, Iran ****Department of Chemistry, Imam Hossein University Tehran, Iran Received 01.12.2006, in final form 06.06.2007

In this work, for the first time, we introduce a highly selective and sensitive Be(II) micro-sensor. 4-nitroben-zo-9-crown-3-ether (NBCE) was used as a membrane-active component to prepare a Be(II)-selective polymeric membrane microelectrode. The electrode exhibits a Nernstian response toward Be(II) ions over a very wide concentration range (1.0 x 10-4-1.0 x 10-10 M), with a detection limit of 3.5 x 10-11 M (~350 pg/L). In fact, the ekektrode presents fast response time in the whole concentration range (6 s). The proposed microelectrode can be used for at least six weeks without any considerable divergence in potentials. The proposed membrane sensor revealed selectivity toward Be(II) ions over a wide variety of other metal ions including common alkali, alkaline earth, and rare earth ions. It could be used in the pH range of 3.0-11.5. The microelectrode was successfully used as an indicator electrode for the titration of 20 mL of 1.0 x 10-6 M Be2+ solution with 1.0 x 10-4 M of EDTA. It was also applied to the direct determination of beryllium ions in beryl and binary mixtures.

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 [1, 2].

There are many techniques currently available for the determination of Be(II) at low concentration; micellar electrokinetic's chromatography coupled to capillary electrophoresis [3], spectrophotometry [4], microwave plasma source atomic emission spectrometry [5], and high performance chelating ion chromatography [6]. These methods are usually time consuming, and relatively expensive.

Although the neutral carrier-type ISEs have been successfully used for the determination of a wide variety of metal ions, including the alkali, alkaline-earth, transition, and some other heavy metal ions, there are only a limited number of reports on the development of highly selective electrodes for Be(II) ions [7-14]. These sensors have relatively low detection limits (1.0 x 10-8-1.0 x 10-6 M) and are not free from the effect some interfering ions such as Li+ and Mg2+. In this work we wish to report the first Be(II) microelectrode based on NBCE for the monitoring of pico-molar content of Be2+ ions.

EXPERIMENTAL

Reagents. Potassium tetrakis(4-chlorophenyl)bo-rate (KTpClPB), PVC of high relative molecular weight, o-nitrophenyloctyl ether (NPOE), dibutyl ph-thalate (DBP), oleic acid (OA), acetophenon (AP), tet-rahydrofuran (THF), chloride and nitrate salts of cations were of the highest purity available (from Merck and Aldrich), and were used without further purification. The NBCE (Fig. 1) was synthesized and purified as described in [15]. All aqueous solutions were prepared with deionized distilled water. The pH value of all solutions were adjusted with diluted nitric acid and sodium hydroxide.

Electrode preparation. To prepare the PVC membrane, we used dipping method [16-20]. After thoroughly mixing 20 mg of powdered PVC, 74 mg of NPOE, 1 mg of additive KTpClPB, 4 mg of NBCE, and 3 mL of THF, the resulting mixture was transferred into a glass dish with a 2 cm in diameter. The solvent was

Fig. 1. Structure of NBCE.

E, mV 180

160 140 120 100 80 60 40 20 0

12

10

2 0 -log

—Be2+ -"-Mg2+

—*- Ca2+ —Sr2+ —•- Na+ —i-K+

-Ba2+

—Al3+ —Rb+ —Fe3+

—Zn2+ —*- Cu2+ —•- Co2+ —•— Ni2+ -*- Cs2+ -Cd2+

Fig. 2. Potential response of various metal ion-selective microelectrode sensors based on NBCE.

slowly evaporated until a relative oily concentrate was obtained. The gold electrode was prepared by sealing gold micro-wire (Goodfellow Metals Ltd., UK) into a soft glass capillary. The capillary was then cut perpendicularly to its length to expose the gold wire. Electrical contact was made using silver epoxy (Johnson Matthey Ltd., UK). Before each experiment the electrode surface was polished for 1 min using extra fine carborundum paper and then for 10 min with 0.3 ^m alumina, sonicated in distilled water and dried in air. The polished gold electrode was dipped into the membrane solution mentioned above and the solvent was evaporated. A membrane was formed on the gold surface and the electrode was allowed to set overnight. The electrode was finally conditioned for 48 h by soaking in 1.0 x 10-3 M NBCE.

Apparatus. Potentials were measured with a Corning ion analyzer Model 250-pH/mV meter. pH of the sample solutions was monitored simultaneously with a conventional glass pH electrode.

EMF-Measurement. All emf measurements were carried out with the following assembly: Hg2Cl2, KC1 (satd.) | sample solution | PVC membrane | gold surface.

RESULT AND DISCUSSION

The microelectrode sensor is an asymmetrical sensor without any internal reference electrode and internal solution. The main problem of the symmetric ion selective liquid membrane electrodes is the leaking of the internal solution to the outer surface of the membrane, causing changes in the surface potential. Therefore, the detection limit of this kind of electrode is about 10-6 M. In the case of asymmetric sensors, the wire coated and

the graphite coated detection limit is about 10-8-10-11 M (sinse that there is no leaking of the internal solution). Due to the high sensitivity of the asymmetric microelectrode to the low Be(III) concentration, the selectivity will be drastically improved [21].

Unfortunately, the determination of Be(II) using IS-Es 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% higher than that of Mg2+ and Li+ ions, respectively, making an appropriate ionophore design for Be(II) very difficult [22]. Due to the small size of Be(II), 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 [23].

Considering the crown ether as a neutral ion carriers which has been reported for the construction of selective and sensitive transition and heavy metal ion membrane electrodes [24-26] and regarding the cavity of NBCE, it was expected to act as a suitable ion carrier for very small hard cations such as Li+, Be2+, and Mg2+ in the PVC membranes.

To investigate the possible ions, towards which NBCE is selective, this ion carrier was used during the initial experiments for the construction of membrane microelectrodes for a large number of metal ions. The curves of the best potential responses of the resulting membranes vs. the concentration change of each ion are shown in Fig. 2. These curves revealed that among the transition and representative ions of the other tested groups, the ion of Be(II) presenting the most evident Nernstian response, can be suitable for the determina-

Table 1. Optimization of membrane ingredients

No. PVC, % Plasticizer, % Ionophore, % Additive, % Slope, mV/decade DL

1 20 77, NPOE 3 - 14.0 ± 0.3 5.0 x 10-9

2 20 76, NPOE 4 - 15.5 ± 0.7 3.0 x 10-9

3 20 75, NPOE 5 - 15.0 ± 0.4 3.0 x 10-9

4 20 75, NPOE 4 1 KTpClPB 22.2 ± 0.6 7.0 x 10-11

5 20 74, NPOE 4 2 KTpClPB 25.9 ± 0.3 3.5 x 10-11

6 20 73, NPOE 4 3 KTpClPB 25.4 ± 0.5 3.0 x 10-11

7 20 71, NPOE 4 5 OA 20.1 ± 0.3 1.0 x 10-10

8 20 66, NPOE 4 10 OA 22.7 ± 0.4 4.0 x 10-11

9 20 61, NPOE 4 15 OA 24.1 ± 0.9 6.0 x 10-11

10 20 74, AP 4 2 OA 20.3 ± 0.5 9.0 x 10-11

11 20 74, DBP 4 2 OA 7.1 ± 0.8 8.0 x 10-10

tion with the PVC membrane microelectrode micro-electrode based on NBCE. This is likely due to the highly selective behavior of the NBCE towards the beryllium ion, and the fast exchange kinetics of the resulting beryllium-NBCE complex at the membrane-aqueous phase interface. This kind of behavior has already been observed for small size crown ethers [7-10].

Because the sensitivity and selectivity of any given membrane microelectrode are significantly related to the composition of the ion selective membrane, the nature of the solvent mediators, and the additives used [17-20, 27], we decided to study such effects on the behavior of the proposed microelectrode. The effects of the nature and amount of the plasticizer amount, the amount of PVC, and the additive on the potential response of the proposed microelectrode were investigated, and the results are given in Table 1. These data show that the three different plasticizers were used, o-NPOE, DBP, and AP. o-NPOE is more polar than others, so, in order to enhance to the extraction of the ber-eliyum ion, it was chosen as the solvent mediator.

In addition, from the data in Table 1 it is clear that the optimum amount of the ion carrier is 4% (No. 2), while the slope of the resulting emf vs. the log Be(II) activity plot is about half of the expected Nernstian value (membranes No. 2). However, addition of 2% KTp-ClPB (membranes No. 5) would increase the sensitivity of the electrode response to a great extent. The fact that the presence of lipophilic anions in the composition of cationic-selective membrane microelectrodes not only diminishes the ohmic resistance and enhances the potential behavior and selectivity, but a

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