Nano-level Monitoring of Yttrium by a Novel PVC-membrane Sensor Based on 2 , 9-dihydroxy-1 , 10-diphenoxy-4 , 7-dithiadecane

A poly (vinyl chloride)-based membrane of 2,9-dihydroxy-1,10-diphenoxy-4,7-dithiadecane (C20H26O4S2) as a neutral carrier was prepared and investigated as an Y-selective electrode. Effects of various plasticizers and anion excluders were studied in detail and improved performance was observed. The best performance was obtained for the membrane sensor having a composition of L: PVC: NPOE: PA in the ratio of 2:30:62:6 (mg). The performance of the membrane was found to be the following: A Nernstian slope of 20.0 ± 0.2 mV per decade across a broad range (1.0 × 10 to 1.0 × 10 mol dm); a detection limit of 2.14 × 10 mol dm between the pH = 4.5 and 9.0; additionally, the response time was about 15 s; good Y selectivity over a wide variety of other metal ions. The membrane sensor was applied as an indicator electrode in potentiometric titration of fluoride ion and also used for determination of F ion in tap water and toothpaste samples. (doi: 10.5562/cca1937)


INTRODUCTION
Ion selective electrodes (ISEs) for different cations have been widely used with polymeric membranes containing appropriate carriers (i.e., ionophores).These ionophores have been examined so that they could be incorporated to form complexes with metal ion within the membrane.The quest for the new ligands capable of specific and effective molecular recognition of metal ions in carrier assisted membranes or polymeric membranes based on ion selective electrodes (ISEs) are a topic of current interest.Macrocycles are a favoured class of compounds in this area as their complexes have high stability constants, lipophilicity to remain in the membrane phase and sufficient conformational flexibility for rapid ion exchange. 13][4][5][6] The chemical properties of macrocyclic complexes can be tuned to force metal ions to adopt unusual coordination geometry.Currently a great deal of attention is being focussed on macrocyclic ligands because they play an important role in many aspects of chemistry, medicine and the chemical industry.
The rare-earth elements (REEs) are distributed in low concentration throughout the earth's crust and are considered slightly toxic.REEs are being increasingly used as an important component in lasers, phosphors, magnetic bubble memory films, refractive index lenses, fiber optics, superconductors, high-intensity lightning, coloured glasses, refining industry and nuclear technology. 7In recent years, the monitoring and evaluation of REEs in some biological materials have received increasing attention, from both nutritional and toxicological point of view. 8,9ttrium is an important member of rare-earth family and widely used for various applications.The most important use of yttrium is in making phosphors, such as in the red phosphors in colour TV tubes and in LEDs. 10 Other uses include the production of electrodes, electronic filters, lasers, superconductors, computer monitors, trichromatic fluorescent lights, temperature sensors, X-ray intensifying screens and various medical applications and also as traces in various materials to enhance their properties.Yttrium is an important element used in atomic reactors for control rods.It is also used in manufacturing of glass, ceramics and in microwave communication equipments.It is used for the production of labeled monoclonal antibodies for tumor therapy studies. 11,12][15][16][17][18][19] Croat.Chem.Acta 85 (2012) 131.
The solvent extraction techniques for the extraction of this metal cation are scarce. 20Yttrium(III) cation can not be determined by direct atomic absorption or plasma atomic emission methods, since the ionization causes low response and it is reduced by the presence of mineral acids.Also the flame spectrometric determination of yttrium is not sensitive. 21otentiometric sensors can offer an inexpensive and convenient analysis method of rare-earth ions in solution, provided that acceptable sensitivity and selectivity are achieved.3][24] Most of the previous studies include some disadvantages such as high detection limits, narrow dynamic range and serious interfaces.In this work we report a highly selective and sensitive Y(III) sensor based on 2,9-dihydroxy-1,10-diphenoxy-4,7dithiadecane (Scheme 1) for fast monitoring of nanomolar concentration of Y(III) ions.Reagent grade dibutyl phthalate (DBP), dioctylphthalate (DOP), o-nitrophenyl octyl ether (NPOE), nithrobenzene (NB), oleic acid (OA), palmitic acid (PA), sodium tetraphenyl borate (NaTPB), ionic liquid (1-ethyl-3-methylimidazolium hexafluorophosphate) and high relative molecular weight PVC (all from Fluka) were used as received.All metal-ion solutions were prepared in doubly distilled water and solutions of different concentrations were made by diluting 0.1 mol dm -3 stock solutions.

Synthesis of 2,9-dihydroxy-1,10-diphenoxy-4,7-dithiadecane
Synthetic route for the preparation of acyclic poly ethers is described in Scheme 2. The β,β'-dihydroxydithioethers were prepared by the action of two mole equivalents of epoxides with deporotonated dimercaptoethane which was formed by proton abstracting ofcarbonate anion under reflux condition and vigorous stirring.The method used here is a simple, efficientand environmentally friendly procedure with excellent yields, high regioselectivity and need not any organic solvents either for reaction medium or extracting the products.Therefore, the work follows the basic aims which are important to green chemistry. 25

General Procedure for Ring Opening of Epoxides with Dimercaptoethane
The ring opening of the starting epoxides was region specific by nucleophilic attack on the terminal carbon atoms affording a secondary diols.The water solubility of epoxides are decreased in the presence of high concentrations of potassium carbonate lead to decrease epoxide ring opening percentage by the protic solvent.Dimercaptoethan is deprotonated by carbonate anion (rather than water deprotonation) lead to formation of water soluble dimercaptid anion, which can react with epoxides in the boundary surface of aqueous and organic phase.This is a suitable condition of the S N 2 mechanism resulted nucleophilic attack on low substituted site.Obviously, the ring opening of epoxides afforded corresponding dihydroxy dithioethers as a mixture of isomeric diastreomers. 29It was notable that in low concentrations of potassium carbonate, the formation of mono substituted ethandiols instead of corresponding β,β'dihydroxy-dithioethers, was increased.This was obvious when the results were checked by TLC and sodium metaperiodate-benzidine test of 1,2-diols.

Apparatus and Potential Measurement
All potential measurements were carried out using the following cell assembly: Ag-AgCl/KCl (sat)/internal solution 1.0 × 10 −3 mol dm -3 Y(NO 3 ) 3 /PVC membrane/test solution//Ag-AgCl/KCl (sat) All the potential measurements were carried out with a digital pH/Ion meter, model 692 Metrohm, at 25.0 ± 0.1 °C.The activities were calculated according to the Debye-Huckel procedure. 27Standard Y(NO 3 ) 3 solutions were obtained by gradual dilution of 0.1 mol dm -3 Y(NO 3 ) 3 solution.The solutions were stirred and potential readings recorded when they reached a steady state values.A glass Ag-AgCl combination electrode was used for pH measurements.The electronic absorption spectra of lanthanum, ligand and the formed complex were recorded in acetonitrile solvent in the region of 200-700 nm using an Agilent UV-Vis spectrophotometer and a quartz cell of 1.0 cm path length.

Electrode Preparation
The general procedure to prepare the PVC membrane was to mix thoroughly 30 mg of powdered PVC, 62 mg of plasticizer NPOE, 6 mg of additive PA, and 2 mg of ionophore in a glass dish of 2 cm diameter.The mixture was then completely dissolved in 3 mL of THF.The solvent was evaporated slowly until an oily concentrated mixture was obtained.A Pyrex tube (2.5 mm o.d.) was dipped into the mixture for 10s so that a transparent membrane was formed.The tube was then pulled out from the mixture and kept at room temperature for 24 h.The tube was filled with internal filling solution (1.0 × 10 -3 mol dm -3 Y(NO 3 ) 3 ).The electrode was finally conditioned for 7 h in a 1.0 × 10 -3 mol dm -3 solution of Y(NO 3 ) 3 .

Effect of Membrane Composition
It is well known that some important features of the PVC-based membranes, such as the nature and amount of the ionophore, the properties of the plasticizer, the plasticizer/ PVC ratio and especially the nature of the additives used, significantly influence the sensitivity and selectivity of the ion-selective electrodes. 28,29Thus, different aspects of preparation of membranes based on C 20 H 26 O 4 S 2 were optimized and the results are given in Table 1.In order to improve the performance of the membrane, different plasticizers (i.e., DBP, DOP, o-NPOE and NB) and additives (i.e., NaTPB, PA, OA and an ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate) were studied.It is reported that the selectivity and working concentration range of membrane sensors are affected by the nature and amount of the plasticizer used.This is due to the influence of the plasticizer on the dielectric constant of the membrane phase, the mobility of the ionophore molecules and the state of ligands.As is seen from Table 1, among the four different used, NPOE resulted in the best sensitivity.
.31The use of ionic additives such as different tetraphenylborate salts and its more lipophilic derivative, also fatty acids such as oleic acid as lipophilic additives is widely reported in the preparation of different ion-selective electrodes.In this study the effect of palmetic acid, oleic acid, sodium tetraphenyl borate, and ionic liquid (1-ethyl-3-methylimidazolium hexafluorophosphate), as an additive on the response of membrane were investigated.From the data given in Table 1, it is immediately obvious that the nature and amount of additive influences the performance characteristics of the membrane sensor significantly.As shown in this table, the slopes and the linear range become better in the presence of palmitic acid (a long-chain fatty acid) additive.The palmitic acid has been used for construction of some potentiometric biosensors, 32 but in this work, we used it as a very suitable additive in PVC matrix of the ion selective membrane electrode.Palmitic acid is probably interposed between the matrix (62 % NPOE, 30 % PVC) and C 20 H 26 O 4 S 2 to facilitate more effective binding and it may also prevent that the active site of the ionophore to be located in deep position of the membrane.
The effect of relative amounts of 2,9-dihydroxy-1,10-diphenoxy-4,7-dithiadecane on the response function of membrane was investigated (Table 1). 2 mg of the ionophore was chosen as the optimum amount of ionophore in construction of the PVC membrane electrode.Further addition of ionophore, however, resulted  in some decreases in the response of the electrode, most probably due to inhomogenity and possible saturation of the membrane.
Optimum conditioning time for the membrane sensor in a 1.0 × 10 -3 mol dm -3 Y 3+ solution was obtained to be 7 h.Then, the electrode generates stable potentials when placed in contact with Y 3+ solution.

Calibration Curve and Statistical Data
The plot of EMF vs -log a(Y 3+ ) shown in Figure 2, indicates that the sensor has a Nernstian behavior over a wide concentration ranges from 1.0 × 10 -9 to 1.0 × 10 mol dm -3 of Yttrium(III) cation.The respective slopes of the resulting calibration graphs for electrode are 20.0 ± 0.2 mV decade -1 and limit of detection (LOD) was found to be 2.14 × 10 -10 .

Effect of pH
In order to study the effect of pH on the performance of the sensor, the potentials were determined at two concentrations (1.0 × 10 -4 and 1.0 × 10 -3 mol dm -3 ) of Y 3+ as a function of pH.The pH of solutions was adjusted by the addition of NaOH and HNO 3 .The obtained results shown in Figure 3 indicate that the potential remains approximately constant over pH = 4.5 to pH = 9.At higher pH values, the potential decreased due to the formation of Yttrium hydroxide in solution; and at lower pH values, the potential increased, indicating that the electrode also responds to hydrogen ion.

Static and Dynamic Response Times of the Electrode
Response time is one of the most important factors for analytical applications of selective electrodes.In order to evaluate the practical static response time of the electrode, the average time required to achieve a potential within ±1 mV of the final steady-state potential was measured by recording the potential-time plots of three different concentrations of Y 3+ and the results are shown in Figure 4a.The results clearly indicate that, in all cases, the electrode exhibits a constant and stable potential within 15 s.Moreover, the practical dynamic re-   sponse time of the electrode was recorded by immediate changing of Y 3+ concentration from low-to-high over a concentration range from 1.0 × 10 -9 to 1.0 × 10 -1 mol dm -3 and the results are shown in Figure 4b.As it can be seen, by an increase in the concentration of the analyte, the potential changes very rapidly (< 10 s) and the electrode reaches its equilibrium response and remains stable with an elapse of time.

Evaluation of Selectivity Coefficients
The potentiometric selectivity coefficients, which reflect the relative response of the membrane sensor toward the primary ion over the ions present in the solutions, perhaps are the most important characteristics of an ion-selective electrode.To investigate the selectivity of the proposed membrane electrode, its potential responses were investigated in the presence of various interfering foreign cations using the separate solution method (SSM).
In the SSM method, the potential of a cell comprising a reference electrode and an ion-selective electrode is measured with each of two separate solutions, one containing the ion of interest i with the activity of a i (but not j) and the other containing an interfering ion j with the activity of aj (but not i) at the same activity of a i = a j .If the measured values are E i and E j , the value of selectivity coefficient Pot , i j K can be calculated as: 33,34 Pot , ( ) log (1 ) log 2.303 where z i and z j are the charges on ions i and j, respectively.It should be noted that this method is recommended if the electrode possesses a Nernstian response.The resulting Pot , i j K values thus obtained for the proposed Y 3+ -selective electrode are summarized in Table 2.As seen, the alkali, alkaline earth and transition metal ions do not significantly disturb the functioning of the proposed Y 3+ ion selective membrane electrode.

Analytical Applications
The proposed Y 3+ selective electrode was found to work well under laboratory conditions.A typical potentiometric titration curve for titration of yttrium(III) cation (20 cm 3 of 1 × 10 -3 mol dm -3 ) with a solution of sodium flouride (0.01 mol dm -3 ) is shown in Figure 5.The end point of the titration and the concentration of yttrium(III) cation in solution can be determined potentiometrically by using this ion selective electrode.The present electrode has been successfully used for the determination of fluoride ion in aqueous solutions including tap water and in the pharmaceutical preparations such as toothpaste.In each case, the pH value was adjusted to 5.0 (using a TISAB solution) and a successful titration was carried out.The determination of fluoride concentration was performed by the standard addition method.In this work, before the titration, the potential of the ISE for 10 cm 3 of the sample is measured.Then the standard solutions of fluoride ion are added and the potential is measured.The electrode potential is related to the logarithm of the concentration of the fluoride ion by the Nernst equation.We also determined the fluoride ion concentration in a sample solution with a fluoride ion selective electrode as a reference method.
The results are compared in Table 3.As is evident in this Table, there is a good agreement between the results obtained with these two ion selective electrodes.

CONCLUSION
A o-NPOE mediated PVC-membrane containing 2,9dihydroxy-1,10-diphenoxy-4,7-dithiadecane (C 20 H 26 O 4 S 2 ), as a suitable ionophore, revealed the best response characteristics with a Nernstian behavior over a wide concentration range from 1.0 × 10 -9 to 1.0 × 10 -1 mol dm -3 for the Y 3+ , with a fast response time of 10 s.The results showed that palmitic acid is a suitable lipophilic additive for the electrode construction.The main advantages of this constructed yttrium(III) cation electrode are the simplicity of its preparation, short conditioning time, fast response time, wide dynamic range, low detection limit, low cost, Nernstian behavior, and fairly good selectivity.Another major advantage of the present ptentiometric sensor, concerns its application.The electrode permits the measurement of the fluoride ion in different real samples such as pharmaceutical products and water without prior separation steps.

Figure 1 .
Figure 1.Molar conductance-mole ratio plots for the complexation of C 20 H 26 O 4 S 2 with Yttrium cation in acetonitrile at 25°C.

Figure 2 .
Figure 2. Calibration curve of Yttrium electrode based on C 20 H 26 O 4 S 2 .

Table 1 .
Optimization of membrance ingredients