Potentiometric Sensor for Gadolinium(III) Ion Based on Zirconium

ISSN: 0973-4945; CODEN ECJHAO

E-Journal of Chemistry

http://www.e-journals.net 2009, 6(4), 1139-1149

Potentiometric Sensor for Gadolinium(III) Ion

Based on Zirconium[IV] Tungstophosphate

as an Electroactive Material

HARISH K. SHARMA* and NADEEM SHARMA

Department of Chemistry, M. M. University,

Mullana, Ambala-133203, Haryana, India.

[email protected]

Received 10 March 2009; Accepted 5 April 2009

Abstract: 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 1x10-5 to 1x10-1M of Gd(III) ion with an over- Nernstian

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 10-1 M and 3.0-7.0 for 10-2 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.

Keywords: Inorganic ion exchanger, Gd(III), Selectivity, Potentiometric titration.

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

1140 HARISH K. SHARMA et al.

metallurgical applications and as polishing compounds and for carbon arcs used in movie

projectors and search lights1.

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 wastes2 to the separation

of trace elements3 and from catalysis of organic reactions

4 to the decontamination of water in

cooling systems of nuclear reactors5. 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 accepted6 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 elements7. 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 ion-

exchangers 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-exchangers8-11

.

A new direction was given to the field of inorganic ion-exchangers when Clearfield and

Stynes12

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 ion-

exchange characteristics like phosphate, tungstates, titanates, heteropoly acid salts and

layered compounds including double hydroxides.

Some zirconium13

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 ions14-16

and are excited to explore these versatile

compounds for making new ion sensors.

Experimental 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

Potentiometric Sensor for Gadolinium(III) Ion 1141

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 (Kd) 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 PbNO3). Then the distribution

coefficient was determined by using the formula-

I-F 20 Kd =

I X

0.2

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 /m2 at least for 24 hours to get

fine, smooth and thin ion selective membrane.


Table 1. The Kd value for various metal ions.

S.No. Metal ion Kd(distribution coefficient)

1 Gd(III) 35.0

2 Pr(III) 5 .0

3 Nd(III) 20.0

4 Sm(III) 10.0

5 La(III) 5.0

6 Ce(III) 10.0

7 Cu(III) 25.0

8 Dy(III) 20.0

9 Tb(III) 5.0

10 Y(III) 30.0

Table 2. Composition of the ingredients for the preparation of membranes

S.No. Ion selective

Membrane, %

Quantity of the

exchanger, g

Quantity of the Epoxy

resin, g

1 40 0.40 0.60

2 50 0.50 0.50

3 60 0.60 0.40

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 3/4th

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-Hg2Cl2(s), KCl (sat.) 0.1M Gd+3

membrane test solution | KCl (sat.), Hg2Cl2-Hg

Results and Discussion 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 recommendations17-18

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 common19-20

. 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.

log a

-9 -8 -7 -6 -5 -4 -3 -2 -1 0

EM

F (

mV

)

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

10-1

M10

-2 M

10-3

M

Figure 1(a). Curves for Gd(III) selective electrode (40% membrane) based on ZrWP in

epoxy resin and also the effect of internal solution.

10-2

10-3

log aGd3+

-9 -8 -7 -6 -5 -4 -3 -2 -1 0

EM

F(m

V)

-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

10-1

M

10-2

M

10-3

M

Figure 1(b). Curves for Gd(III) selective electrode (50% membrane) based on ZrWP in

epoxy resin and also the effect of internal solution.

log a Gd3+

EM

F,

mV

E

MF

, m

V


log aGd

3+

-9 -8 -7 -6 -5 -4 -3 -2 -1 0

EM

F (

mV

)

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

10-1

M10

-2 M

10-3

M

Figure 1(c). Curves for Gd(III) selective electrode (60% membrane) based on ZrWP in

epoxy resin and also the effect of internal solution

log aGd3+

-9 -8 -7 -6 -5 -4 -3 -2 -1 0

EM

F (

mV

)

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

10-1

M

Figure 1 (d). Calibration curve for Gd(III) ion selective electrode based on ZrWP.

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 HNO3 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.

log a Gd

3+

EM

F,

mV

E

MF

, m

V


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.

Table 3. Optimization of membrane ingredients for ZrWP based membrane.

Electrode

Conc. of the

internal

solution, M

Slope

mV/decade

Measuring

range

Response

time

sec

10-1

30 1.0 x 10-5

15

10-2

30 1.0 x 10-5

20

40%

10-3

29 1.0 x 10-5

30

10-1

32 1.0 x 10-5

30

10-2

26 1.0 x 10-4

20

50%

10-3

29 1.0 x 10-4

15

10-1

25 1.0 x 10-5

17

10-2

27 1.0 x 10-5

20

60%

10-3

31 1.0 x 10-4

20

pH

0 2 4 6 8 10 12

EM

F (

mV

)

-0.032

-0.030

-0.028

-0.026

-0.024

-0.022

-0.020

Figure 2 Effect of pH on the response of Gd(III) selective electrode based on ZrWP

taking10-2

M as external solution.

Potentiometric titration

Potentiometric titrations were performed by using the proposed electrode as an indicator

electrode for the titration of 10 mL of Gd3+

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.

EM

F,

mV


Volume of oxalic acid added (mL)

0 1 2 3 4 5 6 7

EM

F (

mV

)

-0.038

-0.036

-0.034

-0.032

-0.030

-0.028

-0.026

Figure 3(a). Potentiometric titration curve of 1.0 x 10

-2 M Gd(III) solution with 5 x 10

-2 M

of oxalic acid

Volume of EDTA added (mL)

0 1 2 3 4 5 6

EM

F (

mV

)

-0.038

-0.037

-0.036

-0.035

-0.034

-0.033

-0.032

-0.031

-0.030

Figure 3(b). Potentiometric titration curve of 1.0 x10

-2 M Gd(III) solution with 5 x 10

-2 M of EDTA

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. Table 4 shows 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 Gd3+

ions in the

Volume of oxalic acid added, mL

Volume of EDTA added, mL

Pot

EM

F,

mV

EM

F,

mV


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.

Table 4. Selectivity coefficient values for Gd(III) ion selective electrode based on matched

potential method and fixed interference method.

Selectivity coefficient value (FIM) Selectivity coefficient value

( MPM)

Interfering

ions

At interfering ion conc. 10-3

M

Sm(III) 5.0 X 10-3

4.0 X 10-3

Y(III) 2.5 X 10-2

-

Cu(III) 2.86 X 10-3

1.15 X 10-4

Ce(III) 2.5 X 10-2

3.16 X 10-2

Pb(III) - 1.2 X 10-2

Fe(III) - 2.45 X 10-2

Ca(III) 2.87 X 10-3

-

Nd(III) 3.16 X 10-2

5.25 X 10-2

Dy(III) 1 X 10-2

2.35 X 10-2

La(III) 7.94 X 10-3 1 X 10-4

Pr(III) 6.3 X 10-2 1.86 X 10-3

Na(III) 1.15 X 10-4 2.17 X 10-3

Tb(III) 7.94 X 10-2 -

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).

Table 5. Effect of non-aqueous solvent.

Non-

aqueous

solvent

Solvent content

% v/v

Slope

mV/decade

Working conc.

Range, M

Response

Time

sec

0 30 10-5

15

10 25 10-3

15

20 20 10-4

5

Methanol

30 15 10-4

5

0 30 10-5

15

10 35 10-5

15

20 32 10-5

20

Ethanol

30 31 10-4

10

0 30 10-5

15

10 30 10-4

20

20 25 10-4

5

Acetone

30 20 10-3

10


log aGd

3+

-9 -8 -7 -6 -5 -4 -3 -2 -1 0

EM

F (m

V)

-0.08

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

0.00

0.00

0.01

10% methanol

20% methanol

30% methanol

Figure 4(a). Effect of non- aqueous solvent-methanol.

log aGd

3+

-9 -8 -7 -6 -5 -4 -3 -2 -1 0

EM

F (m

V)

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

10% ethanol20% ethanol

30% ethanol

Figure 4(b). Effect of non- aqueous solvent-ethanol.

Figure 4(c). Effect of non- aqueous solvent-acetone.

- 9 - 8 - 7 - 6 - 5 - 4 - 3 - 2 - 1 0

-0 . 0 9

-0 . 0 8

-0 . 0 7

-0 . 0 6

-0 . 0 5

-0 . 0 4

-0 . 0 3

-0 . 0 2

0 . 0 0

1 0 % a c e t o n e

2 0 % a c e t o n e

3 0 % a c e t o n e

log a Gd

3+

log a Gd

3+

log a Gd

3+

EM

F,

mV

E

MF

, m

V

EM

F,

mV


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Potentiometric Sensor for Gadolinium(III) Ion Based on Zirconium

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Transcript of Potentiometric Sensor for Gadolinium(III) Ion Based on Zirconium