Potentiometric Sensor for Gadolinium(III) Ion Based on Zirconium[IV] Tungstophosphate as an Electroactive Material

A new inorganic ion exchanger has been synthesized namely Zirconium(IV) tungstophosphate [ZrWP]. The synthesized exchanger was characterized using ion exchange capacity and distribution coefficient (Kd). For further studies, exchanger with 0.35 meq/g ion-exchange capacity was selected. Electrochemical studies were carried out on the ion exchange membranes using epoxy resin as a binder. In case of ZrWP, the membrane having the composition; Zirconium(IV) tugstophosphate (40%) and epoxy resin (60%) exhibits best performance. The membrane works well over a wide range of concentration from 1x10 to 1x10M of Gd(III) ion with an overNernstian slope of 30 mv/ decade. The response time of the sensor is 15 seconds. For this membrane, effect of internal solution has been studied and the electrode was successfully used in partially non-aqueous media too. Fixed interference method and matched potential method has been used for determining selectivity coefficient with respect to alkali, alkaline earth, some transition and rare earth metal ions that are normally present along with Gd(III) in its ores. The electrode can be used in the pH range 4.0-10.0 for 10 M and 3.0-7.0 for 10 M concentration of target ion. These sensors have been used as indicator electrodes in the potentiometric titration of Gd(III) ion against EDTA and oxalic acid.


Introduction
The rare earth industry is growing steadily and an average increase over past several years has been 5-15%.Main applications involve the use of mixed rare earths as gasoline-cracking catalysts, as starting materials for making "Misch Metal" (Common commercial alloy of cerium containing around 45-65% Ce), the use of rare earth silicides for various metallurgical applications and as polishing compounds and for carbon arcs used in movie projectors and search lights 1 .
In recent years, the ion exchange process has excelled its applications in widely divergent fields such as chemistry, nuclear engineering, biology and medicine.The ion exchangers are accomplishing tasks that range from the recovery of metals from industrial wastes 2 to the separation of trace elements 3 and from catalysis of organic reactions 4 to the decontamination of water in cooling systems of nuclear reactors 5 .Uses of resins in ulcer therapy, edema therapy, as artificial kidneys, bacterial adsorbents, catalysts etc testify to the widespread applications of these materials.
Rare earth elements have been traditionally defined to include the 4f block elements with atomic numbers 57-71 as well as the elements yttrium and scandium, which behave chemically similar to the lanthanide elements.Their electronic configurations differ only in the low-lying 4f orbital.The electronic configuration of the free atoms of most of the lanthanides series is generally accepted 6 to be [Xe] 4f n 5d 0 6s 2 .The difference in chemical reactivity of the lanthanides, then, is likely to be influenced not by the configuration of these valence electrons but by the trend in decreasing atomic radii with increasing atomic number.Studies of solution chemistry of the rare elements are complicated by experimental uncertainties in the determination of coordination number and stereochemistry.Many techniques have been applied to the study of the coordination chemistry of the rare earth elements 7 .Highly selective exchangers are required which are not only stable at high temperature but also have ion-exchange properties unaffected by the acidity and high radiation levels.Organic ion-exchange resins are not suitable for such applications, as change in capacity and selectivity take place on exposure to radiation.
The inorganic ion-exchangers exhibit high selectivities for specific ions resulting in separation factors much larger than those exhibited by organic resins.The inorganic ionexchangers unlike organic ion-exchangers have rigid structures and do not undergo appreciable dimensional change during the ion-exchange reactions.It was soon discovered that hydrous oxides combined with anions such as phosphates, vanadates, molybdates and antimonates produced superior ion-exchangers [8][9][10][11] .
A new direction was given to the field of inorganic ion-exchangers when Clearfield and Stynes 12 demonstrated that zirconium phosphate could be crystallized.The availability of crystals allowed the structure of this polymorph of zirconium phosphate to be determined and with this knowledge; the observed behavior could be explained in structural terms.It is generally understood that a very large number of inorganic compounds possess ionexchange characteristics like phosphate, tungstates, titanates, heteropoly acid salts and layered compounds including double hydroxides.Some zirconium 13 based ion exchangers have shown selectivity towards rare earth metal ions.These ion exchangers possess good ion-exchange characteristics and have been identified as electro-active materials.Such ion-exchangers can be used as sensing materials to prepare ion selective membranes with inert binder such as epoxy resins.We have prepared some sensors for rare earth metal ions [14][15][16] and are excited to explore these versatile compounds for making new ion sensors.

Reagents
All the chemicals were of analytical grade.Hydrochloric acid used for activation of the exchanger, sodium chloride and sodium hydroxide used for the determination of the ion exchange capacity of the exchanger were procured from NICE chemical, India.EDTA, xylenol orange, hexamine buffer and ethyl alcohol were of CDH brand.Oxalic acid and various metal ion solutions were prepared by either direct weighing of AR grade reagent or by indirect standardization.Distilled water was prepared with the help of double distillation plant etc.

Instrumentation
Digital potentiometer (Microsil) was used to measure the emf.pH measurement was done with the help of pH meter (Microsil, LIC 196).Balance Electronic Top Pan (Endeavour) was used for all the weighing.

Preparation of zirconium(IV) tungstophosphate
Zironium(IV) tungstophosphate was prepared by adding zirconyl oxychloride (0.1M, containing 12 mL/L hydrofluoric acid) to a continuously stirred equimolar mixture of orthophosphoric acid and sodium tungstate at 60 °C in a volume ratio of 2:1:1.Gelatinous white precipitates were obtained and the pH of the gel was adjusted to 1.0 by adding either HCl or NaOH solution.Precipitates were filtered, washed until free from halides and dried at 40 °C.The dried product broke down into small granules when immersed in water.The material was converted into the H + form by keeping it in HCl (0.1 M) for 24 hours with intermittent changing the acid and finally dried at 40 °C.The product was washed with DMW to remove excess acid.

Determination of ion exchange capacity
Ion exchange capacity was determined by taking 0.5 g of the exchanger over a bed of glass wool taken in a glass column having an internal diameter ~ 1 cm.Then 400 mL of 1 M NaCl solution was passed as eluent at rate of 8-10 drops per minutes.The eluted solution was titrated with the standard (0.1M) NaOH solution.The volume of NaOH used gave the strength of the H + ions given out by the exchanger, which in turn tells the ion exchange capacity of the exchanger in meq g -1

Regeneration of the ion exchanger
Used exchanger was regenerated by keeping it overnight in hydrochloric acid (0.1 M) and then it was washed with double distilled water, till neutral.The exchange capacity was determined and the procedure was repeated four times.

Determination of distribution coefficient
Distribution coefficients (K d ) for the various metal ions were determined by keeping 2 mL of 0.1 M (standardized solution) metal ion solutions, 18 mL of distilled water and 0.2 g of synthesized exchanger, overnight in a titration flask.Meanwhile intermittent shaking was done to attain the equilibrium.The strength of the exchanged metal ion solution was obtained by titrating against 0.1M EDTA (standardized with PbNO 3 ).Then the distribution coefficient was determined by using the formula- Where, I is the initial volume and F is the final volume of EDTA (0.1M).The procedure was repeated at least for 10 metal ion solutions to get their distribution coefficient for the various metal ions and the results are given in Table 1.

Preparation of ion selective membrane
Ion selective membranes are prepared by mixing various amounts of the finely powdered exchanger with appropriate quantity of the adhesive (epoxy resin) as given in the Table 2. Variable quantities of the ingredients as given in the Table 2 were mixed with epoxy resin and a homogeneous mixture was prepared.Then mixture was kept undisturbed between two fine, smooth surfaced glass plates, under a weight of 2 Kg /m 2 at least for 24 hours to get fine, smooth and thin ion selective membrane.

Activation of the membrane
The membranes were fixed to one end of the glass tube of 1.8 cm (internal diameter) using epoxy resin as an adhesive.These electrodes were then equilibrated with Gd +3 ion solution (0.1 M) for 24 hours.Now the membrane is ready to sense the metal ion in the external solution.Further regeneration is essential at least for 2 hours whenever these are to be used and whenever not in use, these are kept in distilled water.

EMF Measurements
The tube was filled 3/4 th with Gd +3 solution (0.1 M) and immersed in a beaker containing test solution of varying concentrations.All the EMF measurements were carried out using the following cell assembly: Hg-Hg 2 Cl 2 (s), KCl (sat.)0.1M Gd +3   membrane  test solution | KCl (sat.),Hg 2 Cl 2 -Hg

Selection of the metal ion
Distribution coefficients for the various metal ions were found out which show that best exchanged ion is Gd(III) ion.The distribution coefficient for Gd(III) ion is 35 where as for the other metal ions it is very low and hence, the synthesized exchanger (ZrWP) acts as a good sensor for the Gd(III) ion and it can easily detect the Gd(III) ion in the external solution.

Calibration curve
A series of solutions were prepared by using 0.1 M solution of Gd(III) ion.Potential measurements were made on the selected electrodes for different concentrations of Gd(III) ion solutions.EMFs were plotted against log of activities of the Gd(III) ions.
Experiments were repeated four times to check the reproducibility of the electrode system.Standard deviation of ±0.005 mV was observed.Representative curves are shown in Figure 1(a), 1(b), 1(c) and calibration curve is shown by Figure 1(d).Membrane composition ZrWP: epoxy resin as 40%:60% shows linearity in the concentration range 10 -5 M to 10 -1 M with slope of 30 mV/ decade, taking 10 -1 M solution as external solution.The limit of detection was calculated according to IUPAC recommendations [17][18] from the intersection of the two extrapolated linear portions of the curves.Various results are shown in Table 3. ZrWP based electrode showed over-Nernstian response which is common [19][20] .The reason for the non-Nernstian behavior of the electrode may be the possible discrepancy between ion activities in the bulk and at the phase boundary, i.e. the uptake of the ions by the membranes results in a depletion zone of the analyte ions from the Nernst diffusion layer.Response time of the electrodes was less than 15 seconds.It is still lower for the relatively concentrated solution.
Potential behavior of the membranes remains unchanged when the potentials are measured either from low to high or high to low concentrations.These membranes could be used without any measurable divergence.The electrodes were stored in Gd(III) ion solution (0.1 M) when not in use to avoid any change in metal ion concentration in the membrane phase.

Effect of pH
The effect of pH on the potential response of the electrodes was studied using a Gd(III) concentration of 1.0 x 10 -1 M and 1.0 x 10 -2 M for ZrWP based electrode.Experiments were conducted for a number of Gd +3 solutions, pH of which were adjusted between 2 to 12 by using suitable amounts of NaOH or HNO 3 solution.
Figure 2 shows the variation of emf with pH, taking 1.0x10 -2 M Gd(III) concentration as external solution.For ZrWP based electrode, the workable pH range for 1.0 x 10 -1 M Gd(III) ions concentration is 4.0 to 10.0 and for 1.0 x 10 -2 M ions concentration is 3.0 to 7.0.Thus these ranges may be chosen as the working pH ranges for the electrode systems.The variation in this range may be due to formation of Gd(OH) 3 and protonation of oxygen atoms of metal oxide or P=O type groups in the exchangers, due their tendency to hydrolyze at higher pH ranges.

Potentiometric titration
Potentiometric titrations were performed by using the proposed electrode as an indicator electrode for the titration of 10 mL of Gd 3+ ions (10 -2 M) against EDTA (5 x 10 -2 M) and oxalic acid (5 x 10 -2 M).Titration curves are shown in Figure 3(a) and 3(b).Each curve shows a sharp inflexion point at the titrant volume corresponding to the formation of 1:1 complex of gadolinium ions with EDTA and oxalic acid.

EMF, mV
Volume of oxalic acid added (mL)

Selectivity coefficient and analytical properties of Gd(III) selective electrode
Selectivity is the single most important characteristics of any electrode, which defines the nature of device and extent to which it may be employed in the determination of a particular ion in presence of other interfering ions.Potentiometric selectivity coefficients of the Gadolinium membrane electrode were evaluated by the fixed interference method (FIM) at 1x10 -3 M concentration of the interfering ions and matched potential method (MPM) at 1x10 -3 M interfering ion concentration for the two electrodes.MPM is recommended by IUPAC to overcome the difficulties associated with the method based on the Nicolsky-Eisenman equation.According to this method, the specific activity of the primary ion (A) was added to a reference ion (B) and successively added to an identical reference (containing primary ion) solution until the measured potential matches to that obtained only with the primary ions.presence of certain interfering ions.According to FIM a calibration curve was drawn for the varying primary ion concentration in a constant background of the interfering ion.The linear response curve of the electrode was a function of the primary ion activity and is extrapolated until at the lower detection, it intersects with the observed potential for the background linear segment of the calibration curve.

Effect of partially non-aqueous medium on the working of Gd(III) electrode
The proposed sensor based on ZrWP was investigated in partially non-aqueous media using ethanol, methanol and acetone mixtures with water.Table 5 indicates that the slopes remain unaltered with the addition of non-aqueous medium.Plots of EMF vs. activity of Gd(III) ions for partially non-aqueous media are shown in figures 4 (a), 4 (b) and 4 (c).

Figure 1 (
Figure 1(a).Curves for Gd(III) selective electrode (40% membrane) based on ZrWP in epoxy resin and also the effect of internal solution.

Figure 1 (Figure 1 (Figure 1 (
Figure 1(b).Curves for Gd(III) selective electrode (50% membrane) based on ZrWP in epoxy resin and also the effect of internal solution.

Table 3 .Figure 2
Figure 2 Effect of pH on the response of Gd(III) selective electrode based on ZrWP taking10 -2 M as external solution.

Table 1 .
The K d value for various metal ions.

Table 2 .
Composition of the ingredients for the preparation of membranes Table 4shows potentiometric selectivity coefficients of Gadolinium selective electrode.The selectivity data indicate that KGd, M values are of the order 10 -2 for the trivalent ions.Therefore the electrode can be used for the determination of Gd 3+ ions in the

Table 4 .
Selectivity coefficient values for Gd(III) ion selective electrode based on matched potential method and fixed interference method.

Table 5 .
Effect of non-aqueous solvent.Effect of non-aqueous solvent-methanol.