Fluorimetric determination of samarium(III) and europium(III) in neodymium oxide by separation with a resin column

67

Analytzca Chuwca Acta, 266 (1992) 67-71

Elsewer science F’ubhshers B V , Amsterdam

Fluorimetric determination of samarium(II1) and europium(II1) in neodymium oxide by separation with a resin column Shaorong Lm and Jlan Meng &Jurg Research

Instrtute of Chemcal Enguteenng and Metallurgy, P 0 Box 234, Be9cng 101149 (Chma)

Wenhua Lm BelJmgGenemi Research Instrtute for Non-Ferrous Metals, Betpng (Chuza) (Recewed 14th October 1991, revised manuscnpt recensed 27th February 1992)

When thenoyltnfluoroacetone-phenanthrohne-Trrtn X-100 ISused to detennme samarmm(II1) and europmm(III) fluorunetncally, only a lmnted amount of neodymmm(II1) can be tolerated By usmg an on-lme separation wluch can partmlly separate neodymmm(III) from samanum(III) and europmm(III), a practical and convement method was developed to detect samarmm(111) at concentrations > 0 05% and europmm(II1) at concentrations > 0 005% m neodymmm ogde Keywords Flow mJ_on,

Fkonmetry, Europrum, Neodymmm oxide, Rare earth elements, Samarmm

Neodymium IS one of the most wdely used rare earth elements Neodymmm oxide contams samaruun011) and europnun(III) as unpurrtles which need to be determmed Methods used to determme these lmpuntles mclude atomic enusslon spectrometry [1,2], x-ray fluorescence methods [3] and solid molecular fluornnetrlc methods [4] However, all these procedures are troublesome or the mstruments are not readily available The conventional fluorunetnc method for the determmatlon of samarmm(II1) and europmm(III) usmg thenoylt&uoroacetone (‘ITA)-trroctylphosphme oxide (TOP01 or phenanthrolme

Corraspondence to Shaorong Lu, Department of Chenustry and Bmchenustry, Box 209. Texas Tech Umverslty, Lubbock, TX 79409-1061, USA (present address)

(Phen)-Tnton X-100 1s sensitive and reliable [5], and requrres relatively simple eqmpment When this method IS used to determme samarmm(II1) and europmm(II1) m ores, the detection hnuts are low enough (0 2 and 0 02 pg kg- 1 for samarmm and europmm, respectively) [5] The senatlvsty of this system was further improved by usmg the technique of tune-dlscnmmated laser fluonmetry [61 However, no work on the determmatlon of samarmm and europmm m neodymmm oxide has been reported, because neodymmm seriously quenches the fluorescence of samanurn010 and europmm(II1) complexes Prehmmary tests showed that when the samaruun(III)or europmm(III)-‘ITA-Phen-Tnton X-100 complex system is used, only a llrmted amount of neodymnun(II1) is tolerated If the bulk of the neodymmm(II1) can be removed to

0003-2670/92/$05 00 Q 1992 - Elsevler Sxence Pubhshers B V All nghts reserved

S Lcu et al /Anal

68

below the maxmmm tolerated amount, it 1s posseble to determme samarium and europmm with reasonable rehab&y

TABLE 1 Relative fluorescence ropmm(II1) m the neodymmm(II1) Element

EXPERIMENTAL

Reagents and apparatus Solutum To prepare 0 01 M ‘ITA solution, 0 222 g of TI’A 1s dissolved m a mmunum amount of ethanol and then dduted to 100 ml with 2 5% (v/v) aqueous Trlton X-100 solution For 0 05 M Phen solution, the same procedure as for the preparation of the TTA solution 1s followed, startmg vvlth 0 100 g of Phen A 10 M acetate buffer solution (pH = 4 7) 1s prepared To prepare 1000 mg ml-’ stock solutions of rare earth elements (Sm, Eu, Nd, etc 1, suitable amounts of their oxides are dissolved m 5 ml of HCl Cl+ 1) and then diluted to 100 ml wth dlstllled water P507 rem Thus extractant resm contams monoethylhexyl2-ethylhexyl-phosphonate (75 mesh) Apparatus A flow-mjectlon system was constructed based on a Hitachi Model 850 spectrofluornneter, and a ‘I’ype 752 W spectrophotometer The separation column consIsted of a 60 mm x 3 mm 1 d glass tube packed with Pso7 resin and seaIed with nylon net at the ends Procedure (see Fig 1) The samplmg tube 1s placed m the sample solution and the valve 1s set at the “adsorption” posltlon for 1 mm Durmg this tnne, sample solutlon passes through the column The valve IS then rotated to the “elutlon” posltlon for a further 1 mm, durmg which time eluent passes through the

5

12

Chtm Acta 266 (1992) 67-71

Sm (10 pg) ’ Eu (1 Leg)b

mtensltles presence

of samarmm(II1) and euof various amounts of

Content of Nd (& 0

100

200

500

loo0

1608 1744

14 15 15 26

973 10 88

308 3 37

101 0 87

’ Exatation at 346 2 nm, emission at 648 9 nm b Excitation at 383 9 nm, emlsslon at 614 8 nm

column When the cohunn has been washed, the samplmg tube can be moved to the next sample solutlon and, after elutlon has fmlshed, the above procedure 1s repeated The period of the determination is 2 mm

RESULTS AND DISCUSSION

Table 1 shows the effects of dtierent concentrations of neodymmm(II1) It quenches the fluorescences of samarmm~II1) and europmm011) complexes When the concentration of neodymmm(II1) 1s higher than 1 mg ml-‘, most of the fluorescence of the samarmm(II1) (10 pg ml-‘) or europium(II1) (1 pg ml-l) complexes is quenched, whereas about 20% or more of the fluorescence can be retained if the concentration of neodymmm(II1) 1s 500 pg ml- ’ or less This fluorescence 1s constdered to be adequate for the determmatlon Hence it IS necessary to remove the neodymium from the sample until its concentration IS below 500 pg ml-’

W

Fig 1 Flow mamfold S = Sample, E = eluent, R = reagents, B = buffer solution, P = penstaltrc pump, V = valve, SC = separation column, m,, m2 = nmung cods, D = detector, W = waste

S LUI et aL /Anal

1

002

C&m. Acta 266 (1992) 67-71

0.04

506

0 08

0

10

69

0

0

12

acidby of lfuml (WCLY)

Fig 2 Adsorption behawour of Sm, Eu and Nd on the resm column as a function of acid concentration See text for dwuss~on of the curves

Figure 2 shows the adsorptlon behavlour of (a) europnun(III), (b) samanum(II1) and (c) neodymmm on the resin column as a function of acid concentration The separation efficiencies under these conditions are not high enough for the determmatlon and the separation procedure 1s tune consummg However, when the sample solution 1s drnren by a perlstaltlc pump at 12 ml mm-‘, the separation efficiency improves slgmficantly (Fig 2d) For curve (d), the concentration of neodymmm 1s 4 mg ml- ‘, which corresponds to that m the real sample From this experiment, it can also be seen that, at an acidity of 0 08 M HCl, only about 10% of neodymmm remained and its concentration 1s only about 400 pg ml-‘, which 1s below the maxunum tolerable amount, and negligible amounts of samarium and europium were lost The separation efficiency can be further mproved by controllmg the acidity of the eluent For example, if 0 25 M HCl 1s used, the separation efficiency can be increased several-fold However, the elutlon curve has a long tall (see Fig 31, which seriously decreases the sample throughput If 0 5 M NaCl 1s added to the eluent solution (0 5 M HCl) the elutlon peaks are sharper 171

16

06

24

12

40

Fig 3 Elufion curves of ( 1 Sm, (---_) Eu and Nd as a fun&on of acid concentration Eluent (a) ( -_) 050MHCl+O50MNaCl,(b)O25MHCl+O50MNaCl

Theoretically, sharper elutlon peaks can be obtamed by usmg a slower flow-rate for column washmg This 1s one way to improve the detection hnuts when the sample volume 1s small For a gwen tune period, a higher flow-rate means that more sample passes through the column, thus yleldmg higher sensltlvlty However, as the flowrate mcreases, the pressure mslde the flow system increases, thus mcreasmg the hkehhood of leakage from the system

25.

E20.

B g

15.

E ” 3 10. 0 E 5.

-0

1

2 residence

3 time

4

5

(mtn)

Fig 4 RelationshIp between the residence tme and fluorescence mtenslty of Eu

S Luc et at! /Anal.

70 TABLE 2 Tolerated amounts of other rare earth elements Content of Sm (IA 75

Element

Tolerated amount (mg,

Content of Eu (CLB)

Element

Tolerated amount (m&I)

Nd La ce Pr Gd

125 125 25 10 25

25

Nd La ce Pr Gd

125 125 25 10 25

If higher sensrtrvrty IS requrred, the best approach is to lengthen the time of samplmg The sensmvrty increases farrly raprdly with an mcreased samplmg time (Fig 41, although the sample throughout declmes Under the workmg condrtrons grven m Fig 1, the flow-rate of washing of ca 12 ml mm-’ is suitable When the contents of samarmm and europmm m different samples of neodymmm oxrde vary wrdely (over two orders of magmtude), the separatron effects for those samples wrll change, possrbly resultmg m drfferent fluorescence quenchmg Fortunately, the fluorescence mtensmes of samarmm(II1) and europmm(II1) complexes mamtam a good lmear relatronshrp wrth concentratron m sprte of the drfferent quenchmg, but a standard addrtron method must be used to correct for the effects of the different quenchmg Table 2 shows the tolerated amounts of other possible co-existing rare earth elements m neodymnun oxrde In fact, the tolerable amounts (except that of neodymium) could be even higher however, when the concentration of samarmm IS 40 times that of europmm, there IS about a 10% interference wrth the europmm concentration On

Chun

Acta

266 (1992)

67-71

the other hand, when the europmm concentration IS 0 05 tunes that of samarium, this causes about a 10% mterference wtth the samanum concentration Generally, because the amount of samarium is usually less than 40 times of that of europmm m neodymmm oxrdes, samarmm can be consrdered not to mterfere wrth europmm and then europmm can be determmed duectly by the standard addrtron method Europium usually mterferes with samarium, but fortunately the mterference is Just the contrrbutron of the fluorescence of the europmm complex, so rt can be avoided by simple blank subtraction Hence samarium may be determmed by subtractmg the europmm blank and applymg the standard addltron method Several samples were determmed using the proposed method The results were m good agreement with those obtamed by manual fluorrmetric methods wrth careful preseparatron (Table 31 Compared with other methods, the proposed method 1s convement and practrcal because the separation is done automatrcally and the equrpment 1s relatrvely sunple The determmatton hmits of samarmm and europmm are low enough for most samples The srgmfrcant advantage of thus method is the high rate of determmatron with very good reproducrbrhty However, m some mstances rt is strll troublesome because of the mutual interference of samanum and europmm

This work was supported by the Natronal Nature Science Foundatron of Chma The authors thank Mr Dan Knight for reviewing the manuscript

TABLE 3 Comparison of the results obtamed wth the proposed method and the manual method 111the presence of neodymmm (0 01%) Method

This work ’ Manual method

Sample 3 (25 ml)

Sample 2 (25 ml)

Sample l(25 ml) Eu (ra)

Sm (cL8)

Eu (re)

Sm CcLg)

Eu (I&

Sm CfiLB)

064I*oo1 060*0005

66+03 68*01

073fOOl 067f0005

70*03 69*01

057*001 057*0005

52rtO2 48*01

S LUJet al /Anal

Own. Acta 266 (1992) 67-71

REFERENCES 1 Y-Q Zhu, Fen XI %I Yan %I, 4 (11) (1985) 62 2 0 Yasualu and M Yosiuzo, Fresemus’ Z Anal Chem, 260 (1972) 97 3 LC Chandola and A.N Mohde, In&an Atonuc Energy Comnusslon, Bhabha Atonuc Research Centre, Report BARC-886, 1977, p 6

71 4 N S Poluektov, Ulu Khun Zh, 38 (1972) 367 5 T Taketatsu and A. Sato, Anal Chun Acta, 108 (1979) 429 6 L-Y Dong, K-M Yle and S -R Lm, J Zhong Guo XI Tu Xue Bao, 34) (1985) 59 7 L -Y YI, Zhong Guo Ke Xue B, 2 (1984) 175