Application of Pyridine-2-carbaldehyde-2-( 4-methyl-1 , 3-benzo thiazol-2-yl ) hydrazone as a Neutral Ionophore in the Construction of a Novel Er ( III ) Sensor

A piridina-2-carbaldeído-2-(4-metil-1,3-benzotiazol-2-il) hidrazona (PCMHT) pode ser usada como um ionóforo neutro apropriado à preparação de uma membrana sensora a Er(III), com alta seletividade. A composição ótima encontrada foi PCMHT-PVC-KTpClPB-BA com as razões de 6,0:30,0:5,0:59,0, respectivamente. A resposta Nernstiniana foi de 21,8 ± 0,5 mV década para a atividade de Er(III). O intervalo linear encontrado para o sensor foi relativamente grande (de 1,0×10 a 1,0 ×10 mol L). Sua funcionalidade no intervalo de pH entre 2,5 e 12,0 o torna adequado para aplicações analíticas. O limite de detecção mais baixo do sensor foi 5,0 ×10 mol L. A excelente seletividade a Er(III) do sensor proposto, frente aos íons metálicos mais comuns e, especialmente íons lantanídeos, é uma outra vantagem. O sensor foi usado com sucesso como eletrodo indicador na titulação de 25 mL de 1,0 × 10 mol L de íons Er(III) com 1,0 × 10 mol L de EDTA. O sensor proposto foi usado na monitoração direta de Er(III) em misturas binárias e na determinação indireta de íons fluoreto em duas preparações de enxágüe bucal.


Introduction
It is estimated that the erbium concentration of the earth's crust reaches 24 ppm.Its commercial sources are monazite and bastnasite.Although the conclusion of the investigators is to classify the rare earths as having low acute toxicity rating, the studies of toxicity of various erbium compounds show that when inhaled, taken orally, or injected into the blood stream, erbium salts can cause serious problems. 1ltammetry has been used for the determination of trace of erbium in alkali halides.Secondary ion mass spectroscopy and Rutherford back-scattering techniques were used for the analysis of erbium profiles in lithium niobate crystals.Higher order derivative spectrometric procedures were developed for the determination of erbium in selective rare earth mixtures.][4][5][6][7][8] The utility of ion-selective electrodes (ISEs) is being increasingly realized by analytical chemists in view of the rapid growth of industry and technology all over the world as they represent a rapid, accurate and low-cost method of analysis.

Reagents
Reagent grade benzyl acetate (BA), nitrobenzene (NB), o-nitrophenyloctyl ether (NPOE), high relative molecular weight poly vinyl chloride (PVC), potassium tetrakis(pchlorophenyl) borate (KTpClPB), and tetrahydrofurane (THF) were purchased from Aldrich and used as received.Chloride and nitrate salts of the cations used (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 deionized water was used through out.

Electrode preparation
The viscous solution that was used for the formation of the membrane was prepared by the mixing 30 mg of powdered PVC, 59 mg of BA and 5 mg of additive KTpClPB in 5 mL of THF, and 6mg of PCMHT.The resulting low-viscosity mixture was thoroughly mixed and transferred into a glass dish of 2 cm diameter and then its solvent was slowly evaporated to gain an oily concentrated mixture.The membrane was then formed on the tip of a Pyrex tube of (3-5 mm o.d.) and by dipping the tube into the mixture for about 10 s, a transparent membrane of about 0.3 mm thickness was formed.Before being filled with an internal filling solution (1.0×10 -3 mol L -1 ErCl 3 ), the tube was pulled out and given enough time to dry at room temperature for about 10 h.The final step was to condition the electrode for 24 h by soaking in a 1.0×10 -3 mol L -1 Er(NO 3 ) 3 solution.A silver/silver chloride coated wire was used as an internal reference electrode.
All potential measurements were carried out by means of a Corning ion analyzer 250 pH/mV meter at 25.0 ± 0.1 ºC.
PCMHT, it was expected to act as a suitable ion carrier for special transition and heavy metal ions (specially, higher charge density) in the PVC membranes.
Many experimental and theoretical investigations have been carried out to understand better the fundamental interaction between metal ions and neutral molecules and their relationship to molecular recognition.7][28][29] In order to have a clear picture about the selectivity of ligand for various metal ions, in this work, we investigated its binding to Ce 3+ , Th 3+ , Eu 3+ , Sm 3+ , Gd 3+ , Pb 2+ , Cu 2+ and Er 3+ ions by using the Extended Hückel semi-empirical calculations.Because system contains atom (Er) which have not been assigned any basis function in ab initio calculation, Extended Hückel semi-empirical calculations have been used.The influence of the nature, size and charge of metal ions on the complexation reaction with the neutral ligand is explained on the basis of the calculation of gas-phase binding energies.
The binding energy of the uncomplexed ligand and its complexes with Er 3+ and other metal ions were carried out using Hyper Chem software (Version 6.01).The binding energy (ΔE) was calculated using equation 1: where, ΔE complex , ΔE ligand and ΔE cation are the total energies of the complex, uncomplexed ligand and metal ion, respectively.Table 1 summarizes the theoretical data relating the stability of the ligand compelexes with Ce 3+ , Th 3+ , Eu 3+ , Sm 3+ , Gd 3+ , Pb 2+ , Cu 2+ and Er 3+ ions.Inspection of Table 1 reveals that the cation binding energy with ligand shows a pronounced dependence on the nature of metal ions used.Thus, based on the previous ab initio calculation results, ionophore could possibly be used as a suitable ionophore in preparation of an erbium ion-selective membrane electrode.The optimized results revealed that the erbium ion was coordinated with four donor atoms (N and S atoms in the five members ring, N of prydinum ring, and the N in neighbor of NH group).
This behavior may be due to the result of the selective tendency of the ion carrier against erbium(III) (with relatively high charge density and hydration energy), in comparison to other metal ions and the rapid exchange kinetics of the resulting PCMHT -erbium(III) complex.
Due to the radii of lanthanum ions (from Ce 3+ to Lu 3+ with the range of 1.02-0.80Å, respectively) these elements have different properties such as charge densities and size and hydration energy (from Ce 3+ to Lu 3+ with the range of 3370-3760 kJ mol -1 ). 30Thus, it is possible that by using a suitable ionophore which having the semi-cavity and relatively high flexibility, construct a highly selective lanthanide ion sensors.
To investigate the possible ions towards which PCMHT is selective, during the next experiments this carrier was used in the construction of membrane sensors for Er 3+ , La 3+ , Pr 3+ , Sm 3+ , Gd 3+ , Dy 3+ , Ho 3+ , Tm 3+ , Na + , K + , Ca 2+ , Mg 2+ , Pb 2+ , and Cu 2+ ions.The best potential responses of the resulting membranes towards the mentioned ions are shown in Figure 2 (a, b).As it is obvious from these figures, among the lanthanide ions tested, Er(III), with the most Nerstian response, can be suitably determined with the PVC membrane sensor based on PCMHT.
This behavior may be considered to be the result of the selective tendency of the ionophore against Er(III) (with relatively low charge density and hydration energy), in comparison to other metal ions, and the rapid exchange kinetics of the resulting PCMHT-Er(III) complex.
The fact that the sensitivity and selectivity obtained for a given ionophore is significantly related to and dependent on the membrane composition and the nature of the solvent mediators and additives used 31,32  to investigate such effects on the behavior of the proposed sensor.To investigate these effects, the nature and amount of the plasticizer and the additive on the potential response of the proposed Er(III) sensor were investigated and the results are summarized in Table 2.These data revealed that the three different plasticizers used, BA, NB, and o-NPOE have almost the same results if the optimum composition is used.However, as NB, and o-NPOE are very polar and lead to the extraction of polar interfering ions such as other rare earth ions, which may have negative effects on the selectivity behavior of the sensor, BA was chosen as the solvent mediator.This may seem to lead a poor extraction of Er(III) ion, having a very high charge density by the average-polar solvent, but this seemed to be compensated by the selective complexation of PCMHT with the Er(III) ions.This way, not only was the slope of the sensor response maintained, but its selectivity pattern was also improved.
In addition, one can understand from the data in Table 2 that increasing the amount of the ion carrier up to a value of 6%, the sensitivity increases, while the slope of the resulting emf vs. log Er(III) concentration plot is about twothirds of the expected Nernstian value (membrane No. 2).However, addition of 5% KTpClPB (membrane No. 5) will 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 sensors not only diminishes the ohmic resistance and enhances the potential behavior and selectivity, but also, in poor extraction capacities, increases the sensitivity of the membrane electrodes has long been known. 33,34he potential response of the sensor was evaluated using the optimum composition of all ingredients, while lacking the ionophore and it was found that under these conditions the response of the sensors falls to very low values of 5.0 mV decade -1 , indicating that the sensitivity of the sensor is mainly due to the complexing behavior of PCMHT.
The optimum equilibration time for the membrane electrode, after which it generates stable potentials when placed in contact with Er(III) solutions, was found to be 24 h.The critical response characteristics of the Er(III) sensor were assessed according to IUPAC recommendations. 35The potential response of the membrane at varying concentration of Er(III) ions (Figure 3) indicates a rectilinear range from 1.0×10 -5 -1.0×10 -2 mol L -1 .The slopes of the calibration curves were 21.8 ± 0.5 mV decade -1 of Er(III) concentration.
The limit of detection, as determined from the intersection of the two extrapolated segments of the calibration graph, was 5.0×10 -6 mol L -1 .
The influence of the pH on the response of the proposed membrane sensor to a 1.0×10 -4 mol L -1 Er(III) solution was studied over a pH range from 1.5 to 13.0 and the results are shown in Figure 4.It is obvious that the potential remained constant from pH 2.5 to 12.0, beyond which drastic drifts were observed.The observed drift at higher pH values could be due to the formation of insoluble Er(OH) 3 ion in the solution.It is noteworthy that, in such acidic solution, the ionophore used could be protonated to some extent, which results in an improper functioning of the membrane electrode to the Er(III) ion concentration.
One of the most important characteristics of any membrane sensor is its relative response for the primary ion over other ions present in solution, usually expressed in terms of potentiometric selectivity coefficients.In this work, matched potential methods 36 was used for determination of selectivity coefficients of the sensor.According to MPM, a specified activity (concentration) of the primary ions (A, 5.0 × 10 -4 -1.0×10 -3 mol L -1 of Er(III) ion) is added to a reference solution (1.0×10 -5 mol L -1 of Er(III) ion) and the potential is measured.In a separate experiment, interfering ions (B, 1.0 × 10 -3 -1.0×10 -2 mol L -1 ) are successively added to an identical reference solution, until the measured potential matches the one obtained before by adding primary ions.The matched potential method selectivity coefficient, K MPM , is then given by the resulting primary ion to interfering ion activity (concentration) ratio, K MPM = a A /a B .
The resulting selectivity coefficients values are given in Table 3.It is immediately obvious from these data, that the proposed Er(III) sensor is highly selective with respect to the most of cations.In the case of lanthanide ions (except for Sm(III), and La(III) with the selectivity coefficient of 3.0×10 -1 and 2.0×10 -2 ) the selectivity coefficients are in the order of 2.1×10 -3 or smaller, which seems to indicate that the Er(III) ions can be determined in the presence of other lanthanide ions.The selectivity coefficients for other cations (Na + , K + , Mg 2+ , Ca 2+ , Cu 2+ , and Pb 2+ ) are smaller than 3.1×10 -3 and they can not disturb the functioning of the Er(III) membrane electrode.The surprisingly high selectivity of the membrane electrode for Er(III) ions over other cations used most probably arises from the strong tendency of the carrier molecule for Er(III) ions.
The dynamic response time of the sensor, being another very important parameter in the evaluation of a sensor, was studied by varying the concentration of a solution Conditions: The concentration of the primary ions (A): 5.0×10 -4 -1.0×10 -3 mol L -1 of Er(III) ion; and the concentration of the interfering ions (B): 1.0×10 -3 -1.0×10 -2 mol L -1 .from 1.0×10 -5 mol L -1 to 1.0×10 -2 mol L -1 and recording the times needed for the sensor to reach ±1 mV of its equilibrium potential (Figure 5).The response time of the sensor was found to be between 5-8 seconds, depending on the concentration of the test solution.
The proposed Er(III) membrane electrode was found to work well under laboratory conditions.It was applied as an indicator electrode for the titration of 20.0 mL of 1.0×10 -3 mol L -1 of Er(III) solution with 1.0×10 -1 mol L -1 of standard EDTA and the resulting titration curve is shown in Figure 6.As it can be seen from Figure 6, the amount of Er(III) ions can be determined with good accuracy.
The proposed Er(III) membrane sensor was also successfully used to the determination of fluoride ion concentration in three mouth wash samples.1.0 g of each sample was taken and diluted with distilled water in a   Er(III) solution, with a 1.0×10 -4 mol L -1 EDTA using the proposed membrane sensor as an indicator electrode.
100 mL flask and titrated with a Er 3+ solution (1.0×10 -3 mol L -1 ) and the results of triplicate measurements are shown in Table 4.As it can be seen from Table 4, there is a good agreement between the declared fluoride content and the determined values.

Figure 3 .
Figure 3. Calibration curve of the Er(III) sensor of membrane.

Figure 4 .
Figure 4. Effect of the pH of test solution (1.0×10 -4 mol L -1 ) on the potential response of the Er(III) ion-selective electrode.

Figure 5 .
Figure 5. Dynamic response time of the electrode for step changes in the concentration of Er(III) solution.

Table 1 .
urged us Interaction energy between metal ions-ligand Figure 1.Structure of PCMHT.

Table 2 .
Optimization of membrane ingredients

Table 3 .
Selectivity coefficients of various interfering cations for the membrane with the optimum composition

Table 4 .
The results of the determination of fluoride ion concentration in mouth wash solution samples