Studies on monoxide emission spectrometry of rare-earth elements. IV. Simultaneous determination of Sm, Eu, Gd in SmEuGd concentrates by the dual wavelength method

ANALYTICA CHIMICA ACTA ELSEVIER

Analytica Chimica Acta 350 (1997) 365-369

Studies on monoxide emission spectrometry of rare-earth elements. IV. Simultaneous determination of Sm, Eu, Gd in Sm-Eu-Gd concentrates by the dual wavelength method Zaizheng

Zhang”‘*, Jie Zhangb, Ying Chenb, Chuanhong

Tub, Jidian Zhengb

aDepament of Chemistry, Fuzhou University, Fuzhou 350002, China bCentral Lab. Fuzhou University, Fuzhou 350002, China

Received 30 January 1997; accepted 10 March 1997

Abstract This paper reports the determination of samarium, europium and gadolinium by Sm and Gd monoxide emission and Eu atomic emission. Tbe enhancement effect of other rare earth elements on Sm, Eu and Gd emission was studied. Large amounts of lanthanum (La) were used as enhancing reagents and chemical interference inhibitors and the dual wavelength method was introduced to eliminate spectral interference, in which the analytical wavelengths used were Sm 65 1.O nm, Eu 459.4 nm and Gd 461.6 nm and the reference wavelengths were Sm 651.4 nm, Eu 459.8 nm and Gd 461.2 nm. The method has been applied to the assay of Sm, Eu and Gd in synthetic Sm-Eu-Gd concentrates with satisfactory results. Keywords: Samarium; Europium; Gadolinium; Lanthanum; Enhancement effect; Monoxide vapor emission spectrometry; Dual wavelength method

1. Introduction Parts l-3 [l-3] of this series have thoroughly described the emission character of rare-earth elements in the air-acetylene flame and the application of Sm, Y and SC monoxide vapor emission character on the assay of Sm, Y and SC in rare-earth mixture concentrates. A Sm-Eu-Gd concentrate is an important rare-earth material produced in China. It is obtained by extracting bastnaesite-monazite mixed ores and is used for extraction of Sm, Eu and Gd. In such Sm-Eu-Gd concentrates the total amount of rare-earth oxides (REO) is not less than 92%, and the ratios of *Corresponding author. 0003-2670/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved SOOO3-2670(97)00232-8

PII

Sm203, Eu203 and Gd203 to the total amount of REO are 30-55%, 7-12% and IO-20%, respectively. Determination of Sm, Eu and Gd in the concentrates is usually by chemical analysis or X-ray fluorescence spectrometry, which are either tedious or expensive, respectively. This paper reports the analytical conditions for measuring Sm and Gd monoxide molecular emissions, Eu atomic emission and the enhancement effect on them by other rare-earth elements. A new method for simultaneous determination of Sm, Eu and Gd in a Sm, Eu, Gd-La-HC104 system has been established which has rapidity, convenience and accuracy. The method was used to the assay of Sm, Eu and Gd in synthetic Sm-Eu-Gd concentrates with satisfactory results.

Z. Zhang et al. /Analytica

366 Table 1 Analytical

conditions

SpeCtld

bandpass

(nm)

0.2

Chimica Acta 3.50 (1997) 365-369

for Sm, Eu and Gd” measurements

Detection height (cm) 0.3

Flow rate (1 min-‘) Air

C2H2

14.2

2.0

Spray rate (ml min-‘) 1.8

aThe analytical wavelengths used were Sm 65 1.O nm, Eu 459.4 nm and Gd 461.6 nm the references wavelengths were Sm 651.4 nm, Eu 459.8 nm, Gd 461.2 nm.

2. Experimental

Fig. 1. Emission spectra. (2) 1000 Bg ml-’ La.

(1) lOpgml_’

Eu+lOOOpgml-

La;

2.1. Apparatus A Perkin-Elmer model 2380 atomic absorption/ emission spectrometer equipped with a RlOOA recorder was used. The analytical conditions are given in Table 1. 2.2. Reagents Accurately weigh a certain amount of each rareearth oxide (99.99%, heated at 850°C for 2 h) and transfer it into a 100 ml beaker. Add a small volume of distilled water to moisten it, then add cont. hydrochloric acid (CeOz was treated with HN03) and heat until it dissolves. Evaporate nearly to dryness, cool and add 1% (V/V) HCl until the residue dissolves. Transfer the solution to a calibrated flask, then dilute it to volume with 1% HCl. The other reagents were of analytical reagent grade. Water distilled from an allquartz apparatus was used. 2.3. Procedure Standard Sm, Eu solution were added order. The resulting optimum analytical

and Gd solutions, HC104 and La to a 10.0 ml standard flask in that solution was analysed under the conditions mentioned above.

analytical wavelength of 651 .O nm, and reference wavelength of 65 1.4 nm. The linear calibration equation was AZ=O.OO18C+O.O02, r=0.9993, n=lO (C: OSm), and the detection limit was 200 pg ml-’ 0.75 pg ml-’ Sm. Therefore, this paper focused on Eu and Gd. 3.1. Emission spectra of Eu and GdO In an air-acetylene flame, Eu atomic spectral lines can be observed at 459.4, 462.7 and 466.2 nm. The most sensitive line, 459.4 nm, was selected as the analytical wavelength and the 459.8 nm as reference wavelength (Fig. 1). There are no atomic emission spectral lines of Gd in an air-acetylene flame. Often GdO vapor emission spectral bands in an air-acetylene flame, the peak at 461.6 nm was the strongest, so was selected as the analytical wavelength. When selecting the reference wavelength for the assay of Gd, we found that there was a strong emission line of Sr at 460.7 nm. To avoid the spectral interference of Sr, we selected 461.2 nm as the reference wavelength (Fig. 2). AZmeasured = I analytical- Ireference (1 = intensity). 3.2. Effect of inorganic acid

3. Results Part 1 of the series reported SmO emission. A new method for determining Sm in a Sm-1000 pg ml - * La-l% (v/v) HC104 system was established with an

The effects of the concentration of nitric, hydrochloric and perchloric acids on AZ for Eu ad GdO were investigated in the presence and absence of La. The results are shown in Figs. 3 and 4. They show that whether La is present or not, maximum AZ is obtained

Z. Zhang et al. /Analytica

Fig. 2. Emission spectra. (1) 1 pg ml-’ 100pg ml-’ Gd+lOOOpg ml-’ La; (3)

367

Chimica Acta 350 (1997) 365-369

Sr+lOOOpg ml-’ La; (2) 1000pg ml-’ La.

Fig. 4. Influence of inorganic acid on GdO (1) HCIO,; (2) HCl; (3) HN03 in the presence of 1000 pg ml-’ La; (4) HClO,; (5) HCl; (6) HN03 in the absence of La.

3.3. Enhancement efSect of other rare-earth elements on Eu and Gd We investigated the emission background of all the other rare-earth elements except Sm, Eu, Gd and radioactivity rare-earth element promethium (Pm) at 455-470 nm. The results are shown in Figs. 5 and 6. They show that they all had enhancement effects on Eu and GdO emissions but La had the greatest effect. Although the maximum AI for Eu and GdO could be obtained when more than 400 ug ml-’ La was added, we selected 1000 ug ml-’ La as enhancing reagent in order to determine Sm, Eu and Gd, simultaneously.

Fig. 3. Influence of inorganic acid on Eu (1) HClO,; (2) HCI; (3) HN03 in the presence of 1000 pg ml-’ La; (4) HClO,; (5) HCI; (6) HNO3 in the absence of La.

in a perchloric acid medium. With 1000 ug ml-’ La when the perchloric acid concentration is in the range 0.5 to 3% maximum AI for Eu is obtained, and when the perchloric acid concentration is in the range 0.5 to 5% maximum AZ for GdO can be obtained. Therefore, 1% HC104 was selected for use.

Fig. 5. Enhancement effect of other rare-earth emission (1) La; (2) Pr; (3) Dy; (4) Nd.

elements

on Eu

368

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Fig. 6. Enhancement effect of other rare-earth emission (1) La; (2) Pr; (3) Er; (4) Yb.

elements

3.4. Calibration

limit

graph and detection

Chimica Acta 350 (1997) 365-369

on GdO Fig. 8. Relationship between concentration and AI for GdO (1) Gd; (2) Gd+5OCpg ml-‘La; (3) Gd+lOOO pg ml-‘La.

In 1% (V/V) HC104 medium calibration linearity experiments were conducted in the presence and absence of La. The results were shown in Figs. 7 and 8. The results show that in the absence of La, the linear range for Eu is O-60 pg ml-‘. When 1000 ug ml-’ La is present, the linear range for Eu is only O20 pg ml-‘, and the linear calibration equation is AZ=-O.OOl+O.O089C (r=0.9997, n=lO), and the detection limit is 0.2 pg ml-’ Eu. In the absence of La, the calibration curve for Gd was sigmoid. However, the linear range increases with increased addition of enhancing reagent. When 1000 vg ml-’ La is present, the linear range of Gd is O-300 ug ml-‘, and the linear calibration equation is AZ=-0.001 +O.OOl C

Fig. 7. Relationship between concentration and AI for Eu (1) Eu; (2) Eui500 pg ml-‘La; (3) Eu+lOOO ~g ml-‘La.

(r=0.9993, 1.2 ug ml-’ 3.5. Influence

n=lO) Gd.

and

the

of co-existing

detection

limit

is

ions

One of the predominant interference factors in flame emission spectrometry is spectral band overlapping or background emission. Our experiments Table 2 Spectra1 interference of co-existing presence of 1000 Kg ml-’ La)

elements

Ion

intensity (I)

Fe Al Si Mg Sf Ca K Na Ce Pr Nd Sm Gd Tb DY Ho Er Tm Yb Lu Y

Concentration peg ml-’ 100 50 40 50 50 80 250 250 1000 200 100 150 150 200 200 100 100 100 200 100 1000

Emission I459.4 nm

1459.8

0.005 0.000 0.006 0.001 0.016 0.010 0.006 0.008 0.025 0.009 0.009 0.020 0.005 0.015 0.013 0.005 0.010 0.008 0.023 0.005 0.130

0.005 0.000 0.006 0.001 0.014 0.010 0.006 0.008 0.025 0.009 0.009 0.020 0.005 0.014 0.013 0.006 0.007 0.008 0.024 0.005 0.130

with Eu (in the

nm

Al 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000 -0.001 0.003 0.000 -0.001 0.000 0.000

Z. Zhang et al. /Analytica Table 3 Spectral interference of co-existing presence of 1000 pg ml-’ La)

elements

Chimica Acta 350 (1997) 365-369

with Gd (in the

Table 4 Analytical

results for a synthetic

Composition Ion

Fe Al Si Mg Sr Ca K Na Ce Pr Nd Sm Eu Tb DY Ho Er Tm Yb LU Y

Concentration pg ml-’ 500 500 200 500 20 500 500 500 1000 500 300 1000 500 500 150 500 500 500 500 500 500

Emission intensity

369

sample (n=6)

@g ml-‘)

(I)

Results

RSD%

018 ml-‘)

I 461.6 llm

I461.2nm

AI

0.020 0.002 0.02 1 0.007 0.006 0.040 0.012 0.015 0.025 0.020 0.025 0.166 0.052 0.03 1 0.008 0.015 0.037 0.03 1 0.050 0.020 0.080

0.02 1 0.002 0.021 0.007 0.010 0.040 0.012 0.015 0.025 0.020 0.021 0.166 0.052 0.03 1 0.011 0.015 0.036 0.03 1 0.050 0.020 0.080

-0.001 0.000 0.000 0.000 -0.004 0.000 0.000 0.000 0.000 0.000 0.004 0.000 0.000 0.000 -0.003 0.000 0.001 0.000 0.000 0.000 0.000

show that various elements have emissions at the peak wavelengths of Eu and GdO. Thus some interference will occur. Thus we used AZ for measurement in determining Eu and Gd and applied the simple dual wavelength method instead of the classical scanning method. As a result, spectral interference was overcome or weakened. The results are shown in Tables 2 and 3. (AZ for 1Opg ml-’ Eu is 0.088 and for 100 c(g ml-’ Gd is 0.100 under the same conditions.) However, the dual wavelength method cannot overcome chemical interference. It was indicated by our experiments that in the absence of La, interference of some co-existing ions is serious. The allowed amounts of co-existing ions increases with increase of the

SmzOs 330, Eu203 100, GdsOs 130, LaaOs 50, CeOa 50, Pr~Ott 30, Nd203 50 Tb407 10, DyzOs 30, Ho203 20, ErsOs 20, TmzOs 20, Yb203 30, LuzOs 10, FeaOs 15 YzOs 50, A1203 15, SiOz 10, MgO 15, CaO 15

added paper, Tables of Eu

Sm20s=332

1.0

EusOs= 100

0.7

GdzOs=131

0.6

amounts of La. In the system established in this the amounts of co-exist ions mentioned in 3 and 4 do not influence the determination and Gd.

3.6. Analysis

of samples

The samples were prepared according to the composition of Sm-Eu-Gd concentrates and Sm, Eu, Gd determined. The results are shown in Table 4.

Acknowledgements Financial support from the Fujian Science Foundation of China is gratefully acknowledged.

References 111Zhang Zaizheng, Zhang Jie, Tu Chuanbong,

Ke Meisheng, J. Chinese Rare Earth Society, submitted. 121Zhang Jie, Zhang Zaizheng, Chen Ying, Chen Huazhang, Anal. Chim. Acta, 344 (1997) 291. [31 Zhang Jie, Zhang Zaizheng, Zheng Ming, Chen Ying, Anal. Lab., submitted.