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Novel sensor fabrication for the determination of nanomolar concentrations ofCe3+ in aqueous solutions

Hasan Bagheri,*a Abbas Afkhami,b Mohammad Saber-Tehrani,a Ali Shirzadmehr,b Seyed Waqif Husain,a

Hosein Khoshsafarb and Masoumeh Tabatabaeec

Received 4th January 2012, Accepted 6th April 2012

DOI: 10.1039/c2ay00005a

A new Ce3+ carbon paste electrode based on a nanocomposite containing multi-walled carbon

nanotubes (MWCNTs), nanosilica, room temperature ionic liquid (1-butyl-3-methylimidazolium

hexafluorophosphate), and 4-(4-methylbenzylideneamino)-5-methyl-2H-1,2,4-triazole-3(4H)-thione

(L) as an efficient ionophore was prepared. This sensor responds to cerium ions in a wide linear

dynamic range of 2.5� 10�8 to 1.0� 10�1 mol L�1 with Nernstian slope of 19.32� 0.10 mV per decade.

The detection limit of 7.0 � 10�9 mol L�1 was obtained at pH range of 3.0 to 8.0. It has a fast response

with response time of about 5 s, and can be used for at least 11 weeks without any considerable

divergence in the potentials. The proposed sensor displays an excellent selectivity for Ce3+ ions with

respect to a large number of alkali, alkaline earth, transition and heavy metal ions. This sensor was

successfully applied in the determination of cerium ions in aqueous samples.

Introduction

Cerium is an important element in the lanthanum group and the

most abundant of them. It is found in monazite, ceric bastnaesite

and silicate rocks. It is widely used in production of ductile iron,

cast iron and aluminium alloys and some stainless steels.1 Cerium

is mostly dangerous in the working environment due to the fact

that fumes and gases can be inhaled with air. This can cause lung

embolisms, especially during long-term exposure. Cerium can be

a threat to liver when it accumulates in the body. It is noteworthy

that despite the urgent need for a cerium(III) sensor for the

potentiometric monitoring of cerium in many industrial,

geological and chemical samples,2,3 there are only a few reports

on cerium(III)-selective electrodes in the literature.4–7

Instrumental techniques which have been applied to cerium

determination include X-ray fluorescence,8 inductively coupled

plasma atomic emission spectrometry (ICP-AES),9 spectropho-

tometry,10 neutron activation analysis (NAA).11 However, these

methods are time consuming, involving sample manipulations,

require large infrastructure and are too expensive for many

analytical laboratories. Thus, the development of a convenient

and direct method for the assay of cerium ions in different

samples is urgently needed. Potentiometric sensors based on

carbon paste electrodes offer several advantages such as ease of

preparation, simple instrumentation, fast response, wide

aDepartment of Chemistry, Science and Research Branch, Islamic AzadUniversity, Tehran, Iran. E-mail: h.bagheri@srbiau.ac.irbFaculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran. E-mail:afkhami@basu.ac.ircDepartment of Chemistry, Yazd Branch, Islamic Azad University, Yazd,Iran. E-mail: tabatabaee45m@yahoo.com

This journal is ª The Royal Society of Chemistry 2012

dynamic range, reasonable selectivity and low cost.12–22 This led

us to construction and design a new potentiometric carbon paste

sensor for the determination of Ce(III) in aqueous solutions.

Carbon paste electrodes (CPEs) have attracted attention as ion

selective electrodes mainly due to their advantages over

membrane electrodes, such as renewability, stable response, low

ohmic resistance and no need for an internal solution. The

carbon paste usually consists of graphite powder dispersed in

a non-conductive mineral oil.22–27 Incorporation of mineral oil

gives CPEs some disadvantages. Mineral oil is not component-

fixed since it is involved in various refining of petroleum and

processing of crude oil, and some unaccounted ingredients may

engender unpredictable influences on detection and analysis.28

Due to the unique properties of carbon nanotubes such as

ordered structure with high aspect ratio, ultra-light weight, high

mechanical strength, high electrical and thermal conductivity,

the presence of mobile electrons on the surfaces of the nanotubes,

metallic or semi-metallic behavior and high surface area, they

have been widely used in the development of chemically modified

electrodes. The combination of these characteristics makes CNTs

unique materials with the capability to improve sensitivity in

electrochemistry, and thus they are widely used to prepare

modified electrodes. Moreover, extensive efforts have been

devoted to design novel CNTs modified electrodes to improve

the determination of various species. In comparison to the

conventional CPEs, carbon nanotube paste electrodes have

shown considerable enhancement in electrochemical signals

leading to improvement of the detection limit in the potentio-

metric measurements.29,30

In this work we applied MWCNTs, nanosilica particles, room

temperature ionic liquid (BMIM-PF6) (Fig. 1a), and (Fig. 1b)

Anal. Methods, 2012, 4, 1753–1758 | 1753

4-(4-methylbenzylideneamino)-5-methyl-2H-1,2,4-triazole-3(4H)-

thione (L) as an ionophore into the structure of CPE. The

advantages of MWCNTs and ionic liquids are reported in the

literature.12,13 Also, silica-based materials are robust inorganic

solids displaying both high specific surface area (200–1500 m2

g�1) and a three-dimensional structure made of highly open

spaces interconnected to each other. This imparts high diffusion

rates of selected target analytes to a large number of accessible

binding sites, which constitutes a definite key factor in the design

of sensor devices with high sensitivity.29,30 The combination of

above mentioned characteristics makes modified CPEs unique as

a sensor with the potential for the diverse applications.

Experimental

Apparatus

The glass cell in which Ce3+ carbon paste electrode was placed

contained a single junction saturated Ag/AgCl reference elec-

trode (Azar electrode, Iran) as the reference electrode. The Ce3+

prepared CPE was used as the working electrode. Both electrodes

were connected to a digital milli-voltmeter (HIOKI 3256.50,

Japan). A Metrohm pH meter (Crison glp 22, Switzerland) was

used to control pH, and a stirrer (Heidolph, MR 2000, Germany)

was used to stir the solutions. The Labtam (Australia) plasma

scan model 710 sequential inductively coupled plasma-atomic

emission spectrometer (ICP-AES) with plasma scan multitasking

computer and peristaltic pump was used for cerium determina-

tion under optimum working conditions: radio frequency (RF)

27.12 MHz; incident power 2000 W; Labtam GMK nebulizer;

RF power 5 W; observation height 14 mm; argon coolant flow

rate 10 dm3 min�1; argon carrier flow rate 1 dm3 min�1; inter-

graph period 10 s; resolution 0.004 nm; peristaltic pump flow rate

1 cm3 min�1; wavelength – cerium 413.77 nm.

Reagents and materials

The graphite powder with a <50 mm particle size (Merck,

Darmstadt, Germany, www.merck.de), 2.2 g cm�3 density and

about 200–300 g L�1 bulk density,Vtotal 0.35 cm3 g�1 and SBET 4.5

m2 g�1 and high-purity paraffin oil (Aldrich, USA) were used for

the preparation of the carbon pastes. The ionic liquid 1-butyl-3-

methylimidazolium hexafluorophosphate and chloride and

nitrate salts of the cations were purchased from Merck. The

multi-walled carbon nanotubes (MWCNTs) with 10–40 nm

diameters, 1–25 mm length, core diameter: 5–10 nm, SBET: 40–600

m2 g�1, Vtotal: 0.9 cm3 g�1, bulk density 0.1 g cm�3, true density 2.1

Fig. 1 Chemical structure of 1-butyl-3-methylimidazolium hexa-

fluorophosphate (a) and 4-(4-methylbenzylideneamino)-5-methyl-2H-

1,2,4-triazole-3(4H)-thione (b).

1754 | Anal. Methods, 2012, 4, 1753–1758

g cm�3 and with 95% purity were purchased from Plasmachem

GmbH (Germany, www.plasmachem.com). The nanosilica used

is Wacker HDK� H20 with specific surface area of 170–230 m2

g�1, Vtotal: 0.81 cm3 g�1 and tamped density 40 g L�1. Deionized

water was used throughout all experiments. L as an ionophore

was synthesized and purified according to our literature.31

Electrode preparation

The general procedure to prepare the carbon paste electrode was

as follows: different amounts of the ionophore along with an

appropriate amount of graphite powder, ionic liquid, nanosilica

and MWCNTs were thoroughly mixed. The resulting mixture

was transferred into an insulin syringe with internal diameter of

2.5 mm and a height of 3 cm as a electrode body. After the

homogenization of the mixture, the prepared paste was carefully

packed into a tube tip to avoid possible air gaps, which can

enhance electrode resistance. A copper wire was inserted into the

opposite end of the CPE to establish electrical contact. The

external surface of the carbon paste was smoothed with soft

paper. A new surface was produced by scraping out the old

surface and replacing the new carbon paste. Finally, the electrode

was conditioned for 24 h by soaking it in a 1.0 � 10�3 mol L�1

cerium nitrate solution.

Emf measurements

The electrochemical cell can be represented as follows: Ag,

AgCl(s), KCl (3 mol L�1) | sample solution | CPE.

Calibration graph was drawn by plotting the potential, E,

versus the logarithm of the cerium ion concentration.

Results and discussion

The results obtained from some experimental works revealed

that the performance of Ce3+ carbon paste potentiometric sensor

can be highly improved by using RTIL instead of mineral oil and

MWCNTs and nanosilica. For this purpose, the potentiometric

responses of the unmodified CPE and modified CPE towards

target metal ions were studied in terms of selectivity coefficients,

response time, Nernstian slope, linear range, and response

stability which are important in characterization of every ion

selective electrode.

Electrode composition and modification

It is well known that the selectivity of an ion-selective sensor is

closely related to the ionophore used as a sensing material.32–45

Unmodified CPEs were prepared by mixing 80% of graphite

powder with 20% of paraffin oil with a mortar and pestle. A

modified paste was prepared in a similar fashion, except that the

graphite powder was mixed with a desired weight of ionophore,

MWCNTs, BMIM-PF6 and nanosilica to get different compo-

sition as shown in Table 1. According to this table, the electrode

composed of 20% BMIM-PF6, 17% ionophore, 50% graphite

powder, 10% MWCNT, 3% nanosilica (no. 13) was found to be

optimal for the Ce3+ electrode.

This new nanocomposition was selected for further examina-

tion. As it can be seen from Table 1, using RTILs instead of

paraffin oil in the carbon paste yields more efficient extraction of

This journal is ª The Royal Society of Chemistry 2012

Table 1 Optimization of the carbon paste compositions

Electrodeno Binder Ionophore Graphite powder MWCNTs Nanosilica Slope (mv decade�1) R2

1 20%-paraffin 0.0% 80% 0.0% 0.0% 1.30 � 0.20 0.9942 20%-paraffin 5.0% 75% 0.0% 0.0% 6.90 � 0.20 0.9893 20%-paraffin 10.0% 70% 0.0% 0.0% 9.80 � 0.30 0.9924 20%-paraffin 15.0% 65% 0.0% 0.0% 14.50 � 0.22 0.9945 20%-paraffin 17.0% 63% 0.0% 0.0% 17.25 � 0.30 0.9906 20%-paraffin 18.0% 62% 0.0% 0.0% 16.93 � 0.20 0.9917 20%-[BMIM] PF6 17.0% 63% 0.0% 0.0% 17.86 � 0.22 0.9958 25%-[BMIM] PF6 17.0% 58% 0.0% 0.0% 17.55 � 0.33 0.9989 20%-[BMIM] PF6 17.0% 58% 5.0% 0.0% 18.20 � 0.31 0.99110 20%-[BMIM] PF6 17.0% 53% 10.0% 0.0% 18.74 � 0.23 0.99211 20%-[BMIM] PF6 17.0% 48% 15.0% 0.0% 18. 60 � 0.40 0.99012 20%-[BMIM] PF6 17.0% 51% 10.0% 2.0% 19.20 � 0.21 0.99713 20%-[BMIM] PF6 17.0% 50% 10.0% 3.0% 19.32 � 0.10 0.99914 20%-[BMIM] PF6 17.0% 49% 10.0% 4.0% 19.25 � 0.32 0.996

Fig. 2 Schematic diagram of electrode response to various cations

(electrode no. 13).

Fig. 3 Calibration curve of the Ce3+ modified carbon paste electrode.

Fig. 4 Effect of pH on cell potential of the modified electrode at 1.0 �10�3 and 1.0 � 10�6 mol L�1 Ce3+ solutions.

This journal is ª The Royal Society of Chemistry 2012

Ce3+ into the CPE. This is probably due to the much higher

dielectric constant, high ionic conductivity and good electro-

chemical and thermal stability of RTIL, and that they may be

a better binder compare to paraffin oil.

UsingMWCNTs in the carbon paste improves the conductivity

and, therefore, conversion of the chemical signal to an electrical

one. Carbon nanotubes have many properties that make them

ideal as components in electrical circuits, including their unique

dimensions and their unusual current conduction mechanism. By

increasing the conductivity, the dynamic working range and

response time of the sensor improve. If the transduction property

of the sensor increases, the potential response of the sensor

improves to Nernstian values. Also, application of nanosilica in

the composition of the carbon paste can also improve the

response of the electrode. Nanosilica is a filler compound which

has high specific surface area. It has a hydrophobic property that

helps extraction of the metal ions into the surface of the CPE.

Also, it enhances the mechanical resistance of the electrode.

Response of the electrode to various cations

In preliminary experiments, the optimal modified Ce3+ carbon

paste sensor (no. 13) was tested for wide variety of metal ions,

including alkali, alkaline earth, transition, and heavy metal ions

and some lanthanide ions. The potential obtained for the most

sensitive ion-selective electrodes based on L are shown in Fig. 2.

The cerium selective electrode exhibited a linear response to the

logarithm of the activity of Ce3+ ions within the concentration

range of 2.5 � 10�8 to 1.0 � 10�1 mol L�1 of cerium nitrate with

Nernstian slope of 19.32 � 0.10 (mV) decade�1 at 25 � 1 �C.

Measuring range and detection limit

Linear curve fitting using IUPAC method as used for the

determination of ISE characteristics. The electrode shows

a linear response with calibration equation, y ¼ 19.32x + 320.7

and correlation coefficient, r¼ 0.999 to the activity of Ce3+ ion in

the range of 2.5 � 10�8 to 1.0 � 10�1 mol L�1 (Fig. 3). The slope

of the sensor is 19.32 � 0.10 mV decade�1. The standard devia-

tion for ten replicates is 0.1 mV. By extrapolating the linear parts

of the Ce3+ modified carbon paste sensor calibration curve,

detection limit can be calculated. In this work, the detection limit

Anal. Methods, 2012, 4, 1753–1758 | 1755

Table 2 Selectivity coefficients of cerium selective sensor for various interfering cations

Interference(j)

KCe,j

13 49 50 This work (MPM)

La3+ 4.50 � 10�4 3.49 � 10�2 5.00 � 10�1 9.35 � 10�5

Gd3+ 2.10 � 10�5 — — 1.50 � 10�5

Cd2+ 1.65 � 10�4 5.01 � 10�3 — 2.50 � 10�4

Pb2+ 3.40 � 10�4 6.21 � 10�4 1.99 � 10�1 8.35 � 10�5

Co2+ 4.30 � 10�4 5.70 � 10�4 — 2.55 � 10�4

Mn2+ 1.80 � 10�3 6.57 � 10�3 — 3.60 � 10�4

Ni2+ 4.80 � 10�3 4.88 � 10�3 — 3.75 � 10�3

Al3+ 4.00 � 10�5 — — 2.50 � 10�5

Zn2+ 2.90 � 10�3 2.65 � 10�3 — 1.25 � 10�4

Sr2+ 7.00 � 10�4 5.59 � 10�3 — 5.10 � 10�4

Cr3+ 6.50 � 10�4 — — 5.95 � 10�4

Fe2+ — — — 6.50 � 10�4

Fe3+ 3.40 � 10�3 2.9 � 10�3 5.00 � 10�1 2.46 � 10�3

Cu2+ 5.50 � 10�4 4.51 � 10�3 2.51 � 10�1 3.55 � 10�4

Hg2+ — 7.52 � 10�3 1.58 � 10�1 6.45 � 10�4

Ca2+ 4.35 � 10�5 — 2.51 � 10�1 2.30 � 10�5

Ba2+ 5.50 � 10�4 — — 7.50 � 10�4

Mg2+ 4.50 � 10�4 6.75 � 10�3 3.98 � 10�1 8.55 � 10�3

Ag+ 1.20 � 10�5 2.90 � 10�4 6.31 � 10�2 1.50 � 10�5

Na+ 1.50 � 10�4 1.33 � 10�4 — 1.20 � 10�4

Cs+ — — — 1.50 � 10�4

Tl+ 2.10 � 10�3 4.92 � 10�3 — 9.15 � 10�4

K+ 1.10 � 10�4 — 1.99 � 10�1 1.54 � 10�5

Fig. 5 The lifetime of the Ce3+ nanocomposite electrode.

of the modified Ce3+ modified carbon paste sensor was 7.0 �10�9 mol L�1, which was calculated by extrapolating the two

segment of the calibration curve in Fig. 3.

Effect of pH on the electrode response

In order to investigate the pH effect on the potential response of

the electrode, the potentials were measured for two concentra-

tion of Ce3+ solutions (1.0 � 10�3 and 1.0 � 10�6 mol L�1) having

different pH values. The pH varied from (0.5–10.5) by addition

of 1.0 mol L�1 HCl or 1.0 mol L�1 NaOH. The potential variation

as a function of pH is plotted in Fig. 4.

The composition of the electrode was kept constant during all

experiments. The results showed the potential of electrode is

Table 3 Comparison of some characteristics of the proposed electrode with

DL (mol L�1) Linear range (mol L�1) Slope (mV

7.00 � 10�9 2.50 � 10�8 to 1.00 � 10�1 19.323.60 � 10�7 8.00 � 10�7 to 1.00 � 10�1 19.908.91 � 10�8 1.41 � 10�7 to 1.00 � 10�2 20.003.00 � 10�5 1.00 � 10�5 to 1.00 � 10�1 19.00

1756 | Anal. Methods, 2012, 4, 1753–1758

constant between pH (3.0–8.0). Thus, the electrode works satis-

factorily in the pH range 3.0–8.0, as no interference from H+ or

OH� is observed in the range. The decrease in potential values

above the pH value of 8.0 might be justified by the formation of

the soluble and insoluble Ce3+ ion hydroxyl complexes in the

solution. Also, the fluctuations at the pH values <3.0 were

attributed to the partial protonation of the employed ligand.

Response time of the electrode

The response time of an ion-selective electrode is also an

important factor for any analytical application. In the case of all

electrodes, the average response time was defined as the required

time for the electrodes to reach a cell potential of 90% of the final

equilibrium values after successive immersions in a series of

solutions each having a ten-fold concentration difference.46 The

response time of the electrode was evaluated (according to

IUPAC definition) by measuring the time required to achieve

a 90% value of steady potential for a cerium solution. A response

time of 5 s was obtained for this electrode.

Reversibility of the electrode

To evaluate the reversibility of the electrode, the practical

potential response of the modified electrode was recorded

by changing solutions with different Ce3+ concentrations from

previous reported ISEs for the determination of Ce3+

decade�1) Response time (s) Ref.

�5 Proposed electrode10 13<10 4920 50

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Table 4 Determination of Ce3+ by the proposed electrode in waste water and synthesized Ce(III)-polluted water samples (mol L�1)

Samples Ce3+ added Ce3+ found Recovery (%) Ce3+ found by ICP-AES

Waste water (1) — 1.25 � 10�6(�0.20) — 1.34 � 10�6

1.50 � 10�6 2.68 � 10�6(�0.44) 95.3 3.02 � 10�6

3.00 � 10�6 4.15 � 10�6(�0.21) 96.7 4.20 � 10�6

5.00 � 10�6 6.33 � 10�6(�0.44) 101.6 6.33 � 10�6

Waste water (2) — 1.46 � 10�6(�0.38) — 1.51 � 10�6

1.50 � 10�6 2.94 � 10�6(�0.60) 98.6 3.50 � 10�6

3.00 � 10�6 4.44 � 10�6(�0.30) 99.3 4.49 � 10�6

5.00 � 10�6 6.60 � 10�6(�0 14) 102.8 6.72 � 10�6

Synthesized Ce(III)-polluted water — 2.20 � 10�6(�0.25) — 2.54 � 10�6

1.50 � 10�6 3.64 � 10�6(�1.05) 96.0 3.61 � 10�6

3.00 � 10�6 5.15 � 10�6(�0.54) 98.3 5.11 � 10�6

5.00 � 10�6 7.26 � 10�6(�0.32) 101.2 7.22 � 10�6

1.0 � 10�3 to 1.0 � 10�4 mol L�1. The results showed that the

potentiometric response of the electrode was reversible; although

the time needed to reach equilibrium was longer than that when

the solution sequence was reversed.

Selectivity and interference

Selectivity, which describes ion selective electrode specificity

toward the target ion in the presence of interfering ions, is the

most important characteristics of these devices. The potentio-

metric selectivity coefficients of the Ce(III) sensor were evaluated

by the matched potential method (MPM).12,13,47,48 The resulting

values of the selectivity coefficients are given in Table 2.

According to this table, all 23 cations would not affect the

selectivity of the present sensor, and have a very small value of

selectivity coefficient in most cases compared with the previously

reported electrodes for cerium determination.

Lifetime of the electrode

The average lifetime for most ion selective sensors ranges from 4–

9 weeks. After this time the slope of the sensor decreases, and the

detection limit increases. The lifetime of the proposed nano-

composite Ce3+ sensor was evaluated for a period of 17 weeks,

during which the sensor was used two hours per day. The

obtained results showed that the proposed sensor can be used for

at least 11 weeks. After this time, a slight gradual decrease in the

slope from 19.32 to 18.90 mV decade�1 is observed (Fig. 5).

Response characteristics

The response characteristics of the present electrode are

compared with some recently reported cerium electrodes in Table

3. As it is shown, the proposed electrode represents a wider

dynamic range compared to most of the sensors and near-

Nernstian slope compared to the sensors referred to; also it has

a very short response time in comparison to some other sensors.

Precision and accuracy of the method

The precision of the method was checked by the analysis of four

replicates of the sample, expressed by R.S.D.% at the limit of

quantification range, which was <1%. Also, the accuracy was

expressed in terms of percentage deviation of the measured

This journal is ª The Royal Society of Chemistry 2012

concentration from the actual concentration. The obtained

results are within the acceptance range of <1%.

Ruggedness

The ruggedness of the potentiometric method was evaluated by

carrying out an analysis using a standard working solution, the

same electrodes, and the same conditions on different days. The

R.S.D. of <1% was observed for repetitive experiments in 3 day

time periods. The result indicates that the method is capable of

producing results with high precision on different days.

Analytical applications

The new cerium selective sensor was successfully applied to

obtain recoveries of cerium in waste water samples from Razi

Petrochemical Co., Mahshahr, Iran. For sampling and the

preparation of these samples, first, the pHs of real samples were

adjusted to pH 7, and then the samples were filtered using 0.2 mm

pore-size, polycarbonate membrane filters. The analysis was

performed by the standard addition technique. The results are

given in Table 4. The sensor was also successfully applied for the

determination of Ce3+ spiked in a synthesized Ce(III)-polluted

water samples [synthesized by: 2.00 � 10�6 mol L�1 Ce(III);

1.50� 10�2 mol L�1 Na(I), Ca(II) andMg(II); 8.50� 10�5 mol L�1

Li(I) and K(I); 2.50 � 10�5 mol L�1 Mn(II), Ni(II), Cu(II) and

Co(II); 5.00 � 10�6 mol L�1 La(III) and Gd(III)]. The results are

given in Table 4. According to the results, the obtained results are

comparable with those obtained by ICP-AES. Thus the sensor

provides a good alternative for the determination of Ce3+ in real

samples.

Conclusions

This work demonstrates that a new nanocomposite carbon paste

electrode fabricated from L, as an ionophore, MWCNTs,

nanosilica, and room temperature ionic liquid, BMIM-PF6, is

a novel sensor for the quantification of cerium ions. The

proposed electrode exhibits excellent potentiometric perfor-

mance. It shows wider working activity range (2.5 � 10�8 to

1.0 � 10�1 mol L�1), Nernstian slope (19.32 mV decade�1 of

activity), low detection limit (7.0 � 10�9 mol L�1), low response

time (�5 s) and better selectivity. These values are better or very

similar than those previously published ones in the bibliography

Anal. Methods, 2012, 4, 1753–1758 | 1757

for other cerium potentiometric sensors. The potentiometric

response of this electrode is independent of the pH of test solu-

tion in the pH range 3.0–8.0. The sensor could be successfully

applied for the determination of Ce3+ in diverse samples.

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