Construction of a Novel Potentiometric Terbium ( III ) Membrane Sensor and its Application for the Determination of Terbium Ion in Binary Mixture and Fluoride Ion in a Mouth Wash Preparation

Um novo sensor potenciométrico constituído por uma membrana de PVC, altamente seletivo a íons Tb(III) foi preparado usando-se 4-amino-3-{2-[4-amino-6-metil-5-oxo-4,5-diidro-1,2,4triazina-3(2H)-ilidina] hidrazono}-6-metil-3,4-diidro-1,2,4-triazina-5(2H)-ona (ATO) como um carregador adequado. O sensor trabalha satisfatoriamente no intervalo de concentração de 1,0 × 10 a 1,0 × 10 mol L (limite de detecção 8,6 × 10 mol L) com uma inclinação Nernstiniana de 19,4 ± 0.5 mV per década de atividade. O eletrodo mostrou rápido tempo de resposta (~15 s) e foi usado por um período superior a 2 meses. Os valores do coeficiente de seletividade potenciométrica (para cátions mono-, di-, e trivalentes) como determinado pelo método match potential (MPM) indicou boa resposta para Tb na preseça de íons interferentes. A membrana sensora foi usada como um eletrodo indicador na titulação potenciométrica de íons Tb(III) com EDTA. Foi também aplicado na determinação de íons fluoreto em amostras de preparados para enxágue bucal e na recuperação de Tb de várias misturas binárias.


Introduction
The rare-earth industry is growing and the average increase over the past years has been 5-15%.The rare earths are considered only slightly toxic according to the Hodge-Sterner classification system and thus can be handled safely with ordinary care.Similar to, the other lanthanides, terbium compounds are low to moderate toxicity, although their toxicity has not been investigated in detail. 1 When rare-earth vapors or dust are inhaled, they are somewhat more toxic but tend to remain in the lungs and are only slowly absorb into the body. 2 Terbium has no known biological role.High purity individual lanthanides are used increasingly as major components in laser, phosphors, magnetic bubble memory films, refractive index lenses, fiber optics and superconductors. 3The main applications include; firstly, the use of mixed rare-earth as gasoline-cracking catalysts, and as starting materials for making misch metal, secondly, the use of rare-earth silicides for various metallurgical applications as polishing compounds, and for carbon arcs used in movie projectors and searchlights. 1any techniques have been used for the determination of terbium.Most of them belong to spectroscopic methods such as ICP-MS, electron spin resonance, high resolution γ-spectroscopy atomic emission spectroscopy, spectrophotofluorimetric, laser-based multi step resonance ionization and some nucleic methods.But all of these methods are either time consuming, involving multiple sample manipulations or they are too expensive for most analytical laboratories.As a result, it is valuable to find an inexpensive and simple technique, which can provide the selectivity that is necessary to determine terbium in the presence of other lanthanides that are very similar to terbium.However, a simple method that offers great advantages such as speed and eases of preparation and procedures, relatively short response times, reasonable selectivity, wide linear dynamic ranges, and low cost, is the potentiometric detections which is based on ion-selective sensors.The above mentioned may be the reason for the interest that has led to the development of electrodes for several ionic species.Therefore the list of the available sensors has hence, grown substantially over the past years. 4here have been many studies on ion-selective electrodes for transition and heavy metal ions. 5][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] There is just one report on the determination of terbium using ion selective electrode with a narrow dynamic range (from 1.0 × 10 -5 to 0.10 mol L -1 ) and high detection limit (7.0 × 10 -6 mol L -1 ). 18We have recently reported a number of sensors for some ions such as Zn(II), 24,25 Ca(II), 26 Cu(II), 27,28 SCN -, 29 Cr(III), 30 Ba(II). 31In this work, we report a Tb(III) membrane sensor based on 4-Amino-3-{2-[4-amino-6methyl-5-oxo-4,5-dihydro-1,2,4-triazin-3(2H)-yliden] hydrazono}-6-methyl-3,4-dihydro-1,2,4-triazin-5(2H)one (Figure 1) with a Nernstian response over a relatively wide working range.

Reagents and materials
Reagent grade 2-nitrophenyl octyl ether (NPOE), dibutyl phthalate (DBP), nitrobenzene (NB), sodium tetraphenyl borate (NaTPB), tetrahydrofuran (THF) and high relative molecular weight PVC were purchased from Merck and Aldrich, used as received.The nitrate and chloride salts of all cations used (all from Merck and Aldrich) were of the highest purity available and used without any further purification except for vacuum drying over P 2 O 5 .Triply distilled de-ionized water was used throughout the experiment.
A Corning ion analyzer 250 pH/mV meter was used for potential measurements at 25.0 ± 0.1 °C.Activities were calculated according to Debye-Huckel procedure. 32

Preparation of the electrode
Membrane solutions were prepared by thoroughly dissolving 5.5 mg of the ATO ionophore, 30 mg of powdered PVC, 3.5 mg of additive NaTPB and 61 mg of plasticizer DBP in 3 mL of THF.The resulting clear mixture was evaporated slowly until an oily concentrated mixture was obtained.A Pyrex tube (5 mm o.d.) was dipped into the mixture for ~10 s, so that a transparent membrane 0.3 mm thick was formed.23][24][25][26][27]33,34 The tube was then pulled out from the mixture and kept at room temperature for 12 h.The tube was then filled with an internal solution (1.0 ×10 -3 mol L -1 TbCl 3 ).The electrode was finally conditioned for 36 h by soaking in a 1.0 × 10 -2 mol L -1 terbium chloride solution.A silver/silver chloride electrode or a silver/silver chloride-coated wire was used as the internal reference electrode.

Results and Discussion
Potential response of the electrode based on ATO It was found that the plasticized PVC-based membrane containing ATO (with ten nitrogen and two oxygen donor atoms in its structure) as a neutral ionophore, generated stable potentials in solutions containing the terbium ion, after proper conditioning in a 1.0 × 10 -2 mol L -1 solution of TbCl 3 .The membranes showed remarkable selectivity for Tb 3+ compared with the most common metal ions.For this purpose, in preliminary experiments, ATO was used as a neutral carrier to prepare plasticized polymeric membrane for a variety of alkali, alkaline earth, transition and heavy metal ions.The potential responses of some of the most sensitive electrodes based on ATO are shown in Figure 2 (a, b).As it is obvious from Figure 2, among different cations tested, Tb(III) with the most sensitive response seems to be suitably determined with the electrode in the concentration range of 1.0 × 10 -6 -1.0 × 10 -1 mol L -1 .This is due to the selective behavior of the PVC membrane system against Tb(III) in comparison with other metal ions.Due to the existence of a semi cavity with four donor nitrogen atoms in the structure of ATO, it can form relatively strong complex with transition metal ions, and especially lanthanide metal ions.Among the lanthanide metal ions with different size, charge density and hydration energy, terbium ion in the middle of this group, forms more stable complex with ATO.

The membrane composition effect on the potential response of the Tb(III) sensor
][26][27][28][29][30][31] Thus, the influences of the membrane composition, the nature and the amount of plasticizer and the amount of sodium tetraphenylborate as lipophilic additives on the potential response of the terbium electrodes were investigated.The corresponding results are summarized in Table 1.It is seen that the membrane [No.8] with the optimized compositions of 5.5% ionophore and 61% DBP in the presence of 30% PVC and 3.5% NaTPB results in the best sensitivity with Nernstian slopes of 19.4   ± 0.5 mV decade -1 over wide dynamic ranges.As it is obvious from Table 1, the presence of 3.5% NaTPB, gave the provided the electrode with Nernstian potential responses.][37] For the investigation of the potential response of the sensor to the common anions, a number of soluble salts of sodium ion including NaCl, NaNO 3 , NaCH 3 COO, Na 2 SO4, NaHCO 3 , NaH 2 PO 4 , NaBr, NaI and NaClO 4 were chosen and their potential responses were measured.The results showed that the responses of these common anions were very weak.

Response time
Dynamic response time is an important factor for any ion selective electrodes.In this study, the practical response time of the sensor was recorded by changing the solution with different Tb 3+ concentrations from 1.0 × 10 -6 to 1.0 × 10 -1 mol L -1 .The actual potential versus time trace for the electrode based on ATO is shown in Figure 3.As it can be seen, the electrode reaches its equilibrium response in a short time of about 15 s.This is most probably due to the fast exchange kinetics of complexation-decomplexation of Tb 3+ ion with the ionophore at the test solution membrane interface.

plot and statistical data
The potential responses of the proposed sensors at varying concentration of Tb(III) ion show a linear response to the concentration of Tb 3+ ions in the range of 1.0 × 10 -6 -1.0 × 10 -1 mol L -1 (Figure 4).The slopes of the calibration plots were 19.4 ± 0.5 mV decade -1 of the Tb 3+ ions activity.The detection limit as determined from the intersection of the two extrapolated segments of the calibration plots were 8.6 × 10 -7 mol L -1 .The standard deviation of 9 replicate measurements was ± 0.4 mV.The membrane electrodes prepared could be used for at least 2 months without any measurable divergence.

pH effect
The pH influence of the test solution (containing 1.0 × 10 -3 and 1.0 × 10 -2 mol L -1 of Tb 3+ ions) on the potential responses of the three membrane sensors was tested in the pH range of 1.5 -11.0 and the results summarized in Figure 5. Evidently, potentials remain constant over a pH range of 3.8 -8.2.At the lower and higher pH values, the potential changes sharply.The observed drifts at lower and higher pH values could be attributed to the protonation of the ionophore and the formation of some hydroxyl complexes of Tb 3+ ions in solution, respectively.

Potentiometric selectivity
The potentiometric selectivity coefficients, which reflect the relative response of the membrane sensor for the primary ion over other ions that are present in solution, are perhaps the most important characteristics of an ion-selective  electrode.In this work, the potential response of the proposed Tb 3+ ion-selective sensors compared with most common metal ions was investigated by the match potential method (MPM). 38According to the MPM, the selectivity coefficient is defined as the activity ratio of the primary ion (A) and the interfering ion (B) that gives the some potential change in a reference solution.The resulting values are shown in Table 2.As it is seen, for all diverse ions used, the selectivity coefficients are in the order of 10 -3 or smaller, indicating they would not significantly disturb the function, of the proposed Tb(III) membrane sensor.
In Table 3, the major interfering ions (just a gross relative comparison), dynamic linear range, the detection limit and response time of the proposed electrode are compared with the best data of the previously reported Tb(III) selective membrane electrode.It is immediately obvious that the proposed Tb(III) sensor in terms of selectivity coefficients, dynamic linear range, detection limit and response time is superior to the previously Tb(III) ion-selective electrode. 18

Analytical applications
The proposed sensor works well under laboratory conditions and can be successfully employed as an indicator electrode in the potentiometric titrations of Tb 3+ (1.0 × 10 -4 mol L -1 ) with a standard EDTA solution (1.0 × 10 -2 mol L -1 ).The respective titration curve is shown in Figure 6.Obviously, the amount of Tb 3+ ions in solution can be determined with the electrode.1.0 ×10 -5 -1.0 ×10 -1 1.0 ×10 -6 -1.0 ×10 -1 Limit of detection / (mol L -1 ) 7.0 × 10 -6 8.6 × 10 -7 The membrane sensor was used for the fluoride ion determination in a mouth wash preparation sample (Aquafresh, Brentford, U.K.).1.0 g of each sample was taken and diluted with distilled water in a 100 mL flask and titrated with a Tb 3+ solution (1.0 × 10 -3 mol L -1 ) and the results of the triplicate measurements are summarized in Table 4.As it is seen, there is a satisfactory agreement among the declared fluoride content, the determined values by the sensor and the commercial solid fluoride sensor (ELIT 101).
Because of the high selectivity and the low detection limit of the proposed Tb(III) sensor, it was additionally, applied for the monitoring of the Tb(III) ions concentration of in various binary mixtures.These results are listed in Table 5, demonstrating that the recoveries of the Tb(III) ions in all mixtures are acceptable and in the range of 98.6-102.3%.

Figure 5 .
Figure 5.The effect of the pH of the test solution (1.0 × 10 -3 mol L -1 of Tb 3+ ) on the potential response of the terbium sensor based on ATO (membrane No. 8).

Table 1 .
Optimization of membrane ingredients

Table 2 .
Selectivity coefficients of the electrode

Table 3 .
Comparison of the selectivity coefficients, linearity range, detection limit and response time of proposed Tb(III) electrode and the previously reported Tb(III) PVC-membrane sensors

Table 5 .
Recovery of Tb(III) ions from binary mixtures by the proposed Tb(III) ion-selective electrode a Results are based on three measurements.