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A New Hg(ІІ) potentiometric sensor based on synthetic Schiff Base N,N-bis(salicylidene)-naphthylene-1,8-diamineH. BagheriSadeghi,M. Hosseini,H. Afifinia,N. Almasian and M. Salavati Niasari Islamic Azad University, Tehran Central Branch,Tehran, Iran. Islamic Azad University,Savadkooh Branch&Young Researchrs Club, Iran.Faculty of Chemistry, University of Kashan, Kashan, Iran.Corresponding Author E-mail: [email protected]


A PVC membrane incorporating a synthetic Schiff Base (N,N-bis(salicylidene)-naphthylene-1,8-diamine) as neutral ionophore was prepared and used for the determination of mercury (ІІ) ions. The electrode based on this ionophore showed an excellent potentiometric response for mercury (ІІ) ions over a wide concentration range of 5.0×10-8 - 1.0×10-1M with a Nernstian slope of 29.5 mV per decade. The detection limit of the electrode was 2.0×10-8M and the electrode worked well in the pH range of 3.5-8.0. The electrode showed a short response time of less than 15s. The electrode also showed better selectivity for mercury (ІІ) ions over many of the alkaline-earth; and heavy metal ions. Also, sharp end points were obtained when the sensor was used as an indicator electrode for the potentiometric titration of mercury (ІІ) ions with iodide ions, and it was successfully employed for direct determination of mercury content of amalgam alloy and water samples.

Key words: mercury(ІІ), potentiometric sensor, Schiff Base.

determined with the PVC membrane based on NBSND and the emf responses obtained for all other cation-selective electrodes are much lower than that predicted by the Nernst equation. Therefore, the response mechanism of the electrode suggests that an interaction between the N and O atoms in complex and Hg2+ may take place [6].

The membrane, containing NaTPB, exhibits a Nernstian response (No. 8 with a slope of 29.5mV per decade).

Plasticizers play an important role in the behavior of polymeric liquid membrane ion-selective.

Noticeably, It has been clearly illustrated that the presence of lipophilic additives in ion-selective electrodes is necessary to induce permselectivity, so that without these additives the electrodes do not respond properly [9]. The presence of such additives not only reduces the ohmicresistance and improves the response behavior and selectivity, but it also increases the sensitivity of the membrane electrode when the extraction capability of an ionophore is poor .[10]

Fig. 6.Dynamic response of the NBSND membrane electrode for step changes in concentration: a)10-7M, b)10-6M, c)10-5M, d)10-4M, e)10-3M, f)10-2M, g)10-1M.

3.6. Reversibility of the electrode response

To evaluate the reversibility of the electrode, a similar procedure with opposite direction was adopted. The measurements were performed in the sequence of high-to-low sample concentrations, and the results are shown in Fig.7. It is noteworthy, that in the case of high-to-low concentrations, the time needed to attain a stable potential is somewhat 60 times larger than that required for the case of low-to-high concentrations (for a 10 times change in the cation concentration) [10].

Table. 2.

1. Introduction

The mercury (ІІ) ion is a type of poison, which can damage the kidney and the gastrointestinal tract. Because of its serious hazardous effect on human health, there is a strong need to develop new methods for determination of Hg2+ ions in samples. The most widely used techniques for quantification of Hg2+ are based on cold vapor atomic absorption spectrometry[1] and cold vapor atomic fluorescence spectrometry[2] other published methods include inductively coupled plasma mass spectrometry[3], However, these methods are either time-consuming, involving multiple sample manipulations, or are too expensive for most analytical laboratories to afford. Among the available analytical techniques, the application of carrier based ion-selective electrodes (ISEs), has become a well-established routine analytical technique. Potentiometric sensors based on ion-selective electrodes are specially suited for such determinations because they offer advantages such as selectivity, sensitivity, good precision, simplicity and low cost [4].

Schiff Base derivatives have been used as building blocks for the construction of artificial molecules in the preparation of metal sensors [5].

In this work we used a Schiff Base (L) (figure 1) containing π-coordinated groups in construction of PVC based Hg2+ ion-selective electrode for sensitive and selective determination of mercury ions in different samples.

Fig. 2. Potential response of ion-selective electrode based on NBSND (composition No. 8) for various metal ions.

3.1. Complex study

In order to determine the stoichiometry and stability of the resulting NBSND complex with mercury ion in acetonitrile solution, the spectra of a series of solutions containing a constant concentration of ligand (5.0×10−5 M) at 25 °C and varying amounts of the metal ion were obtained and the result is shown in Fig. 3. As can be seen, the complexation was accompanied by decreasing in the absorption band of the NBSND at 335 nm, and increasing in the absorption band at about 290 nm. The absorbance–mole ratio plot revealed a level off at [Hg2+]/[NBSND) molar ratio of 1, emphasizing the formation of 1:1 (metal to ligand) complex in solution. Also, the complexation of NBSND with a number of metal ions was investigated. The formation constants of the resulting 1:1 complexes are listed in Table 1. As it can be seen, the NBSND with the most stable complex with Hg(II) ion is expected to act as a selective ionophore for preparation of Hg(II) ion-selective membrane electrodes.

The formation constants of the resulting complexes between cations and NBSND were evaluated by computer fitting of the corresponding mole ratio data to a previously derived equation [7] using a non-linear curve-fitting program, KINFIT [8]. Absorbance spectra were recorded using a spectrophotometer (PerkinElmer's LAMBDA 25) equipped with a thermostated bath (Huber polystat cc1). For complexation studies, the temperature of the cell holder was maintained at 25±0.1 °C.

Fig. 7.Dynamic response characteristics of the Hg(II)-electrode for several high-to-low sample cycles.

3.3. Calibration graph and statistical data

The EMF versus pHg‏ plot for optimal membrane ingredients indicates that it has a Nernstian behavior over a broad concentration range of Hg2+ ions (Fig. 4). The slope and the linear range of the resulting calibration graph were 29.5±0.4mV per decade and 5.0×10-8M-1.0×10-1M , respectively. The limit of detection (LOD), defined as the Hg2+ ion concentration obtained when the linear region of the calibration graph was extrapolated to the base line potential, was 2.0×10-8M.

This certain electrode was satisfactorily stable, and it could be used for at least 6 weeks without observing any change in its response characteristics.

3.7. Potentiometric selectivity

The most important characteristic of any ion-selective sensor is its relative response for the primary ion over other ions present in solution, which is expressed in terms of potentiometric selectivity coefficients. Potentiometric selectivity coefficients, , describing the preference by the membrane for an interfering ion M n+ relative to Hg2+, were determined by the mixed solution method [11,12]from potential measurements of the solutions containing a fixed amount of Hg2+ ion (1.0×10-5 M) and varying amounts of the interfering ion M n+ , according to

where E1 and E2 are the electrode potentials for the solution of Hg2+ ion alone and for the solution containing interfering ions and mercury ions, respectively. According to Eq. (1), the values can be evaluated from the slope of the graph of


The resulting selectivity coefficients obtained for the optimized membrane composition (No.8) are summarized in Table 3.

The selectivity coefficients presented in Table 3 clearly indicate that the electrodes are highly selective to Hg2+ over other cations tested.

Fig. 1.N,N-bis(salicylidene)-naphthylene-1,8-diamine(NBSND).

2. Experimental

2.1 Reagents

All of the chemicals used were of analytical reagent grade (Merckor Fluka). Doubly distilled deionized water was used throughout. Dibutylphetalate(DBP), benzylacetate(BA), nitrobenzene(NB), acetophenon(AP), sodium tetraphenylborate (NaTPB), tetrahydrofuran(THF) and relatively high molecular weight polyvinyl chloride (PVC). Also, the nitrate salts of all cations used (from Merck or Fluka) were of highest purity available.

2.2 synthesis of ligand L :

The Schiff Base was synthesized by the following procedure:

To an ethanolic solution (50 ml) of salicylaldehyde (3.09g, 25.28 mmol) was added an ethanolic solution (20 ml) of 1,8-diaminonaphthalene (2 g, 12.64 mmol) drop by drop with stirring. A yellow compound separated out during mixing at room temperature. The mixture was heated under reflux on a water bath for 30 min and was cooled to room temperature. The yellow precipitates separated were suction filtered, washed with ethanol and dried under vacuum. F.W = 366.33, Yield 54٪, color green, mp = 161 °C (decomposed). Selective IR bands (cm-1), KBr pellets, (O-H, C=N, C-O), 3355, 1607 (s), 1249. Anal. calcd for C24H18N2O2: C, 78.64; H, 4.93; N, 7.62. Found: C, 78.45; H, 4.84; N, 7.94. 1H NMR (400 MHz) chemical shift (δ ppm), 14.80 (s, 2H, O-H), 9.56 (s, 2H, CH=N), 6.43-8.12 (m, 14H, H-aromatic). 13C NMR (400 MHz) chemical shift (δ ppm) 112 (2), 114 (2), 117 (2), 122, 126 (2), 127, 128 (2), 136 (3), 145,161, 156.

2.3 preparations of the electrode:

Polyvinylchloride-based Hg2+ membrane sensors were prepared by thoroughly mixing 6mg of ionophore (NBSND), 2mg of additive (NaTPB), 30mg of powdered PVC and 62 mg of plasticizer NB in a glass dish of 2.5cm diameter. The mixture was then completely dissolved in 5mg of fresh THF. A Pyrex tube (3-5mm i.d) was dipped into the mixture for 5s so that a membrane was formed. The tube was then pulled out from the mixture and kept at room temperature for 12h. The tube was then filled with internal filling solution of 1.0×10-3M of Hg (ІІ) and left for 24h to be conditioned.

Fig. 4. Calibration graph for mercury(II) ion selective electrode based on NBSND (composition No. 8).

3.4. Effect of pH

The influence of pH on the response of the proposed membrane electrode to 1.0×l0-4 M Hg2+ concentration over the pH range 1–12was investigated. EMF measurements were studied and the results are shown in Fig. 5. As can be seen, the membrane electrode can be suitably used in the pH range 3.5–8. However, the observed changes below and above this pH range may be due to the increased protonation of the NBSND at nitrogen atoms of the molecule and the decreased amount of Hg2+ in solution due to the complexation with OH-, respectively.

Table. 3.

Fig. 3. Electronic absorption spectra of ligand (NBSND) in acetonitrile (5.0×10-5 M) in the presence of increasing concentration of mercury(ІІ) ion.

3.8. Analytical application

3.8.1. Potentiometric titration

The practical utility of the proposed membrane sensor was checked by its use as an indicator electrode for the titration of 25.0 ml of 1.0×10-4 M Hg2+ ions with a 1.0×10-2 M Potassium iodide and the results are shown in Fig. 8.

Table 1.

2.4 EMF measurements:

The potential measurements were performed by means of a Corning ion analyzer with a 250 pH mV-1 meter at 25.0±0.1°C.

Moreover, a cell assembly of:

Hg│Hg2Cl2│KCl (satd.)│test solution││PVC membrane││1.0×10-3M Hg(NO3)2│KCl(satd.)│ Hg2Cl2│ Hg

was used to carry out all the needed potential measurements.

Fig. 8. Potentiometric titration curve of 25 mL of a 1.0×10-4 mol L-1 solution of Hg2+with 1.0×10-2 mol L-1 of KI.

Fig. 5. Effect of pH on the response of the mercury ion-selective electrode (composition No. 8).

3.8.2. Determination of mercury in water samples and amalgam alloy

The proposed Hg2+- selective electrode was found to work well under laboratory conditions. It was successfully applied to the determination of mercury content of amalgam alloy and spiked waters samples. The results were shown in table 4.

3. Results and discussion

Due to its sufficient insolubility in water and the presence of donor atoms (N, O) in its structure, ligand NBSND was expected to act as a suitable ion carrier in the PVC membranes with respect to special transition and heavy metal ions of proper size and charge. Thus, in preliminary experiments, it was used as a neutral carrier to prepare PVC-based membrane electrodes for a variety of metal ions. The potential responses of the most sensitive electrodes, prepared under the same experimental conditions (except for 24 h conditioning in a 0.001 mol L−1 of the corresponding cations) are shown in Fig.2. As it can be seen, among different tested cations, mercury(II) with the most sensitive response seems to be suitably

3.2. Effect of electrode composition

It is clear that some important features of the PVC based membranes, such as the nature and amount of ionophore, the properties of the plasticizer, the plasticizer/PVC ratio and the nature of additives used, significantly influence the sensitivity and selectivity of the ion-selective electrodes. Thus, different aspects of membrane preparation based on NBSND were optimized and the results are given in table 2. It was seen that the membrane number 8 with the PVC:NB:L:NaTPB ratio of 30:62:6:2 exhibits a Nernstian behavior for an extensive concentration range of the Hg2+ ions.

3. 5. Dynamic response time

Dynamic response time is an important factor for any ion-selective electrode. In this work, the practical response time was recorded by immediate and successive Hg2+ concentration changes from 1.0×10-7M to 1.0×10-1M. As shown in Fig.6, the electrode reaches to its equilibrium response in a short time (<15 s) in all concentrations.

Table. 4.

Note: Values in parentheses are SDs based on four replicate analysis.


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