PVC-Membrane Potentiometric Electrochemical Sensor Based on 2-(4-Oxopentan-2- ylideneamino)isoindoline-1,3-dione for Selective Determination of Holmium(III)

2-(4-Oxopentan-2-ylideneamino) isoindoline-1,3-dione (OID) was found to be a suitable neutral ionophore in the fabrication of a highly selective Ho membrane sensor. The electrode has a near-Nernstian slope of 19.6±0.5 mV per decade with a wide concentration range between 1.0×10 and 1.0×10 mol/L in the pH range of 3.5–8.8, having a fast response time (~5 s) and a detection limit of 5.8×10 mol/L. This electrode presented very good selectivity and sensitivity towards the Ho ions over a wide variety of cations, including alkali, alkaline earth, transition and heavy metal ions. The practical utility of the electrode has been demonstrated by its use as an indicator electrode for the potentiometric titration of a Ho solution with EDTA and for the determination of Ho ions concentration in mixtures of two and three different ions.


Introduction
The rare earths are considered only slightly toxic according to the Hodge-Sterner classification system and thus can be handled safely with ordinary care.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.High purity individual lanthanides are used increasingly as major components in laser, phosphors, magnetic bubble memory films, refractive index lenses, fiber optics and superconductors.Many techniques have been used for the determination of holmium such as: spectrophotometry, x-ray fluorescence spectrometry, inductively coupled plasma atomic emission spectroscopy (ICP-AES), inductively-coupled plasma mass spectrometry (ICP-MS), isotope dilution mass spectrometry.But all of these methods are either time consuming, involving multiple sample manipulations or they are too expensive for most analytical laboratories.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.Literature survey shows that there are only a limited number of reports on selective determination of Ho 3+ ions in the presence of other rare earth elements by electrochemical method [1][2][3] .Recently, several studies concerning the selective and sensitive PVC-membrane ion-selective electrodes have been reported for some ions [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] .In this study, another ion selective sensor has been introduced for the potentiometric Ho 3+ determination, based on 2-(4-oxopentan-2-ylideneamino) isoindoline-1,3-dione (OID) (Figure 1).

Experimental
Nitrate and chloride salts of all cations were purchased from the Merck and the Fluka Co. and dibutyl phthalate (DBP), nitrobenzene (NB), acetophenone (AP), benzyl acetate (BA), sodium tetraphenyl borate (NaTPB), tetrahydrofuran (THF) and high relative molecular weight PVC were reagent grade.All reagents were used without any modification.As far as the nitrate and chloride salts of all employed cations are concerned, they were of the highest available purity and were P 2 O 5-vacuum dried.During the experiments, triply distilled deionized water was used.

Electrode preparation
The pentane-2, 4-dione (1 mmol, 0.1 g) was solved in hot ethanol, after that the 2-aminoisoindoline-1, 3-Dione (1 mmol, 0.162 g) and catalytic amount of acetic acid were added to the solution.The mixture of reaction was refluxed for 1 h.Then the solid product was crystallized in solution of acetone and ethanol (1:1).
The membrane solution preparation involved the total dissolution of the following compounds; 3 mg of the OID ionophore, 30 mg of the powdered PVC, 3 mg of the additive NaTPB and 64 mg of the AP plasticizer in 3 mL of THF.The resulting clear mixture was evaporated slowly up to the point that an oily concentrated mixture was obtained.A Pyrex tube (5-mm o.d.) was dipped into the mixture for ~10 s, in order to achieve a transparent membrane formation of 0.3 mm in thickness [15][16][17][18][19][20] .The tube was then removed from the mixture, kept at room temperature for 24 h and filled with an internal solution (1.0×10 -3 mol/L HoCl 3 ).The electrode was finally conditioned for 36 h by soaking in a 1.0×10 -2 mol/L HoCl 3 solution.A silver/silver chloride electrode was used as the internal reference electrode.

The EMF measurements
The equipment for the emf (electromotive force) measurements consisted of; a) Ag-AgCl | internal solution, 1.0×10 -3 mol/L HoCl 3 | PVC membrane | sample solution | Hg-Hg 2 Cl 2 , KC1 (satd.) and b) a Corning ion analyzer with a 250 pH/mV meter for the potential measurements at 25.0±0.1 0 C. The activities were calculated according to the Debye-Huckel procedure 24 .

Results and Discussion
The OID suitability as an ion carrier for different metal ions was checked by its utilization in the fabrication of the PVC-membrane-ISEs in preliminary experiments for a wide variety of cations, including alkali, alkaline earth, transition and heavy metal ions.The OID-based membrane displays a Nernstian response towards the Ho 3+ ions concentration in a broad concentration range.This is likely due to the quick exchange kinetics of the resulting Ho 3+ -OID complex as well as the high ionophore selectivity for holmium ions, compared with the other metal ions.Therefore, the ionophore (OID) was selected as a suitable sensing material for Ho 3+ ions in the PVC matrix.

Effect of membrane composition
Some important features of the PVC membranes, such as the properties of the plasticizer, the plasticizer/PVC ratio, the nature and the amount of the ionophore and, especially, the nature and the amount of the used additives, are reported to significantly influence the ISEs sensitivity and selectivity [25][26][27][28] .Thus, the influence of the membrane composition, the nature and the amount of the plasticizer along with the NaTPB amount (as a suitable lipophilic additive) was investigated on the potential response of the Ho 3+ sensor.The corresponding results are provided in Table 1.Obviously from Table 1, the best electrode response sensitivity is reached with about 3% of OID.However, the NaTPB addition of 3% will increase the electrode response sensitivity considerably, so that the membrane electrode demonstrates a Nernstian behavior.It should be noted that the presence of lipophilic anions in the cation-selective membrane electrodes not only diminishes the ohmic resistance 25 , but it also enhances their response behavior and selectivity 27 , Table 1 displays that the optimum response characteristics were obtained with a membrane composition of 30% PVC, 64% AP, 3% OID and 3% NaTPB (no.8).

Calibration curve
The potential sensor response at various Ho 3+ concentrations is shown in Figure 2, covering a linear range from 1.0×10 −6 to 0.01 mol/L.The slope of the calibration graph was 19.6± 0.5 mV per decade of Ho 3+ concentration.The detection limit (derived from the intersection of the two extrapolated segments of the calibration graph) was 5.8×10 −7 mol/L.The potential electrode response was considered in the pH range of 2.0-11.0(the pH was adjusted with the use of concentrated NaOH or HCl) and the results are depicted in Figure 3. Clearly, the potential response of the sensor remains constant in the pH range of 3.5-8.8.At pH values lower than 3.5, a potential increase was observed.This increase was caused by the membrane response to hydronium ion (protonation of nitrogen atoms in acidic media).At pH values higher than 8.8, a potential decrease, due to the insoluble holmium hydroxide formation, was observed.

Response time
The average time was measured which was required for the membrane electrode to reach a ±1 mV potential of the final equilibrium value, after immersion into Ho 3+ solutions each having a 10-fold concentration difference.The actual potential versus time traces plot in Figure 4 depicts that the electrode reaches its equilibrium response in a short time (~5 s) in the entire concentration range (1.0×10 -6 to 1.0×10 -2 mol/L).

S102 HASSAN ALI ZAMANI
For all diverse ions, the selectivity coefficients of the electrode are in the order of 8.6×10 -3 or smaller, indicating they would not significantly disturb the function of the Ho 3+ selective membrane sensor.It is also worth noticing that the response of the Ho 3+ sensor was found to be insensitive to the nature of the tested anions.Also, Table 2 compares the selectivity coefficients, detection limit, linear range and response time of the proposed Ho 3+ sensor with those of the holmium membrane electrodes previously reported [1][2][3] .As it is obvious, the proposed electrode is superior to the previously reported holmium sensor.

Analytical application
The suggested Ho(III) PVC-membrane sensor was found to work well under the laboratory conditions.The selective holmium membrane sensor was used as an indicator electrode in the titration of a 1.0×10 -4 mol L -1 holmium ion solution with a standard 1.0×10 -2 mol L -1 EDTA.The resulting titration curve is shown in Figure 5.According to this figure, the sensor is capable of monitoring the amount of holmium ions.Because of high selectivity and low detection limit of the developed Ho 3+ sensor (membrane no.8), it was also applied for the determination of Ho 3+ ions in mixtures of two and three different ions.The corresponding results in Table 3 reveal that the recovery of Ho 3+ ions in all mixtures is acceptable.

Conclusion
In the present study, using of 2-(4-oxopentan-2-ylideneamino) isoindoline-1,3-dione (OID) in poly vinyl chloride membrane sensor as a selective ionophore with AP as plasticizer creates a Ho(III)-PVC membrane electrode.The sensor showed a Nernstian response (slope of 19.6±0.5 mV/decade), low limit of detection (5.8×10 -7 M), applicable pH range of 3.5-8.8,fast response time (~5 s) and wide linear range (1.0×10 −6 -1.0×10 −2 M) and very low interference from common alkali, alkaline earth, transition and heavy metal ions.The practical utility of the sensor were demonstrated by using it as an indicator electrode in the potentiometric titration of Ho 3+ ions with EDTA and the determination of Ho 3+ ions concentration in mixtures of two and three different ions.

Figure 2 .
Figure 2. Calibration curve of holmium electrode based on OID

Figure 3 .
Figure 3. pH effect of the test solution (1.0×10 -3 mol/L of Ho 3+ ) on the Ho 3+ sensor based on OID

Figure 5 .
Figure 5. Potential titration curve of 25.0 mL from a 1.0×10 -4 mol L -1 Ho 3+ solution with 1.0×10 -2 mol L -1 of EDTA.Because of high selectivity and low detection limit of the developed Ho 3+ sensor (membrane no.8), it was also applied for the determination of Ho 3+ ions in mixtures of two and three different ions.The corresponding results in Table3reveal that the recovery of Ho 3+ ions in all mixtures is acceptable.Table 3. Determination of Ho 3+ ions in mixtures of different ions Observed content mol/L Composition S. No.

Table 1 .
Optimization of the membrane ingredients