A comparative study of some methods for the spectrofluorimetric determination of terbium in aqueous solutions containing other lanthanides and yttrium

Analytica Chimica Acta, 163 (1983) 229-236 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

A COMPARATIVE STUDY OF SOME METHODS FOR THE SPECTROFLUORIMETRIC DETERMINATION OF TERBIUM IN AQUEOUS SOLUTIONS CONTAINING OTHER LANTHANIDES AND YTTRIUM

SAMUEL J. LYLE* and NIDAL A. ZA’TAR The Chemical Labomtory. (Received

University of Kent at Canterbury,

Kent CT2 7NH (Ct. Britain)

17th May 1983)

SUMMARY Published methods based on the use of water-soluble binary and ternary complexes for the spectrofluorimetric determination of terbium(II1) are compared. The complexes formed by terbium(II1) with (1) ethylenediamine-N,N’-his-o-hydroxyphenylacetic acid, (2) o-hydroxyphenylimlnodiicetic acid, (3) EDTA and sulphosalicylic acid, (4) EDTA and Tiion, and (6) iminodiacetic acid and Tiion were examined. In each system, the characteristic sharp-line emission from terbium(II1) at around 545 nm is measured. On the basis of emission spectra from 400 to 600 nm, fluorescence intensity in relation to variation in terbium/reagent mole ratios, sensitivity and some interference tests involving other lanthanides, yttrium, thorium and dioxouranium(VI) ions, it is concluded that the system represented by (4) is best, followed by (l), (5) and (3) in that order; (2) is least satiafactory on account of the strong dependence of fluorescence intensity on the terbium(III)/ reagent mole ratio.

The analytical problems encountered in the determination of the lanthanides arise from the similarity of chemical properties of these elements. Selective and sensitive reactions are not readily available and it is usually necessary to resort to physical properties of the ions of individual elements. Spectrophotometric methods, using the near-ultraviolet and visible spectral region, are satisfactory only for relatively high concentrations, because molar absorptivities arising from f-electron excitation within the metal ion tend to be low. Spectrofluorimetric methods, where applicable, can give higher sensitivities and are potentially more suitable for the determination of trace amounts. However, it can still be difficult to find conditions whereby interference from other lanthanides is not a seriously confining factor. Among the methods described for the spectrofluorimetric determination of terbium(III), those involving the use of aminopolycarboxylate ligands, which produce stable water-soluble complexes, are attractive in that they tend to give good sensitivity and promise of spectral discrimination against other lanthanides. Ethylenediamine-N,N’-bis(o-hydroxyphecylacetic acid) (EDBHPA) [l] and o-hydroxyphenyliminodiacetic acid (HPIDA) [ 21 have been used as complexing agents for this purpose. However, others have pre0003-2670/83/$03,00

o 1983 Elsevier Science Publishers

B.V.

230

ferred to use mixtures of complexing agents giving ternary complexes. Thus, EDTA with either sulphosalicylic acid [ 31 or disodium 1,2dihydroxybenzene-3,5disulphonate (Tiron) [ 41 and iminodiacetic acid with Tiron [5] have been proposed for terbium determination. Two of the reagent mixtures producing ternary complexes can also be used for the determination of dysprosium [4,5]. These methods are compared here in an attempt to determine which reagent system is most suitable for the determination of terbium in the presence of other lanthanides and yttrium. Absorption and emission spectra, metal ion/reagent ratios and, for ternary complex formation, reagent/reagent ratios, and sensitivity of fluorescent emission at 545 nm were examined. Interferences from other lanthanides and yttrium were also tested for comparison with related published data. The selection of a reagent system suitable for the development of an automated method of analysis was a major objective of the study. EXPERIMENTAL

Reagents and equipment Unless otherwise stated, all chemicals used were of analytical-reagent grade (Fisons Scientific, Loughborough). Those reagents used to form complexes with terbium are listed in Table 1. HPIDA was obtained from the related lactone, 2,3dihydro-2-oxobenzomorpholine4-acetic acid in alkaline solution. The lactone was prepared by the method of Irving and Da Silva [6]. (M.p. 178-179°C (lit. 178-179°C). Required for &H9N04, 58.0% C, 4.4% H, 6.8% N; found 57.5% C, 4.0% H, 6.6% N.) Iminodiacetic acid, Tiron (97% pure; Lancaster Synthesis, Lancaster, England) and EDBHPA (95% pure; Fluka/Fluorochem) were further purified, iminodiacetic acid by recrystallisation from water, Tiron from ethanol and EDBHPA by dissolution in dilute aqueous ammonia followed by precipitation at pH 4 with mineral acid [7]. The aqueous reagent solutions prepared were 10-l M EDTA (disodium salt), 10m3M iminodiacetic acid, low3 M Tiron, and 2.5 X 10m3M EDBHPA. A 10-l M sulphosalicylic acid solution was prepared by dissolving 12.7 g of the dihydrate in 500 ml of water containing 20 ml of buffer solution (see below). The 10m3M HPIDA solution was prepared by dissolving the corresponding lactone in hot water. The buffer solution was prepared by adding 70 ml of diethylamine (laboratory reagent, 99% purity) to 400 ml of water and adjusting the pH to 11.9 with concentrated hydrochloric acid. Standard low3 M terbium(III) solution. Terbium oxide (Tb,O,; 99.9% pure; Rare Earth Products, Widnes, England) was dissolved in the minimum quantity of hydrochloric acid. The solution was evaporated to dryness and the residue dissolved in 1 1 of water. The terbium concentration was determined by EDTA titration [8] ,

231 TABLE

1

Published conditions for spectrofluorimetric Range of lin@arity~ (ng Tb mlF)

Reagent R, added

determinations

Reagent R, added

of terbium

Mole ratiob R,:R,:Tb

RI

Mole

Rl

Mole

l-1600

EDBWA

-

-

1:l

6-3200

EDTA

1.25X 104 1.00x 104 2.50 X 10-6 2.50 X 104 3.75X lo*

SSA'

5.00x 104 2.50 x lo* 2.50X lo* -

1:l:l

>0.008

EDTA

0.004-790000

IDAd

0.01-16

o-HPIDA

Tiron Tiron -

RR range

1:l:l 1:l:l 1:l

7.57.0 11.611.9 11.512.5 12.513.0 ll.O12.0

\Vavelength (nm)

Ref.

h ex

A em

295

545

1

320

545

3

320

546

4

320

546

5

294

544

2

aFor the solution as measured with reagents added. bin the complex undergoing excitation and emission. CSulphosalicylic acid. dhninodiacetic acid.

Solutions of other lanthanides (except cerium) and yttrium were prepared from the corresponding oxides (Rare Earth Products, Widnes). Each oxide was ignited at a dull red heat and cooled, and an appropriate amount was weighed out and dissolved in the minimum quantity of hydrochloric acid. The solution was evaporated to dryness and the residue dissolved in a known volume of water. Cerium(II1) nitrate (Hopkin and Williams) was dissolved in water and standardised with EDTA [8]. Ammonium iron(I1) sulphate, iron(II1) chloride, thorium nitrate, dioxouranium(V1) acetate, sodium chloride, sodium perchlorate, sodium sulphate, sodium oxalate and calcium nitrate were used in the interference tests. Apparatus. Either an Aminco-Bowman or a Perkin-Elmer MPF3 spectrofluorimeter was used with a quartz cell (1 X 1 cm). Absorption spectra were obtained with a Unicam SP800 spectrophotometer. General procedure for the determination of terbium A portion of solution containing an amount of terbium within the range recommended for each method (Table 1) was transferred to a 25-ml beaker. Appropriate amounts (Table 1) of the essential complexing agents were added. The pH was then adjusted by adding dilute hydrochloric acid and sodium hydroxide [l, 21, or buffer solution [3, 41, or 10% (w/v) sodium hydroxide and 4 M sodium perchlorate to adjust the pH and the final ionic strength to 0.2 [ 51. The solution was transferred to a 25-ml volumetric flask and diluted to volume with water. The fluorescence was measured after 10 min. The pH, excitation and emission wavelengths recommended in each method are listed in Table 1.

232 RESULTS

AND DISCUSSION

Absorption spectra of blank solutions The absorptions of terbium-free (i.e., blank) solutions prepared as in the above general procedure, with the solution compositions set out in Table 1, were checked over the range 450-600 nm. Only solutions containing HPIDA absorbed; additional experiments showed that this reagent was unstable in solution above pH 7. The absorbance of the solution containing HPIDA at pH 11.5 was studied at 544 nm (the wavelength of maximum emission of its terbium(II1) complex) as a function of time. The absorbance increase was roughly rectilinear over the first 40 min from preparation of the solution and then remained approximately constant. However, addition of a reducing agent such as hydrazine hydrate prevented formation of the coloured decomposition product and the absorbance at 544 nm remained negligible over a period of at least one month. Fluorescence spectra of terbium-containing solutions The fluorescence spectra of complexes of terbium with the reagents listed in Table 1, prepared as prescribed, were recorded from 400 to 650 nm. Typical spectra are presented in Fig. 1 except for the terbium-EDBHPA system which gave a spectrum very similar to that for terbium-HPIDA. The resolution of the emission peaks depended on the reagents; the best separaloo-

goBO-

4

> 5 2 GOC 8 50; ;L &Os 3070 -

2

II, d

zo-

Ii

\

I, 00 lo;..

A._

1

k.

i

1-m1111,11111111 400

500

600

400 Emlsslo”

600

600

wa”ele”gth.nm

400

500

,;,, 1

600

500

600

Fig. 1. Emission spectra from terbium(II1) with: (1) EDTA and sulphosalicylic acid; (2) iminodiacetic acid and Tiron; (3) EDTA and Tiron; (4) HPIDA. Conditions: 4.61 x lo* M terbium(II1) with 9.16 x lo-’ M EDTA, 9.12 X 10-I M iminodiacetic acid, 8.00 x 10m5M sulphosalicylicacid, 4.61 x loss M Tiron and 4.70 X lo* M HPIDA as appropriate. Excitation wavelengths and pH of the solution are those recommended in Table 1.

233

tion of the peak at around 545 nm was obtained with HPIDA and EDBHPA. The spectra from EDTA/Tiron and iminodiacetic acid/Tiron complexes showed superior resolution of this peak when compared with those of the corresponding sulphosalicylic acid complexes. The spectrum of the terbiumiminodiacetic acid/sulphosalicylic acid complex (not shown) was similar to that of the corresponding EDTA ternary complex. The presence of 10-l M hydrazine did not appear to affect the terbium-HPIDA fluorescence. Effect of reagent concentration Fluorescence intensity from metal complexes of the type considered here often exhibits a dependence on reagent concentrations. Dagnall et al. [3] studied the effect of changing molar ratios of metal ion to reagents in the terbium-EDTA/sulphosalicylic acid system. They found that keeping the concentration of sulphosalicylic acid fixed (Tb:SSA = 1:2000 at 10m6M Tb) and varying the EDTA concentration resulted in an increase in fluorescence intensity up to a terbium/EDTA mole ratio of 1:2; further increase in EDTA relative to metal ion did not affect fluorescence intensity. However, with a fixed mole ratio of terbium to EDTA (lO-‘j M Tb), the fluorescence intensity increased logarithmically with increasing sulphosalicylic acid concentration. Taketatsu and Yoshida [l] examined the effect of changing molar ratios of terbium to EDBHPA for a fixed metal ion concentration (5 X 10” M Tb) and found that fluorescence output was almost constant over terbium/EDBHPA ratios of 1:20 to 1:lOO but further increase in relative reagent concentration decreased the intensity. Observations in the work described here confirmed these results except that a somewhat more marked dependence (quenching) was found for ratios beyond 1:20 (Fig. 2) than in the original work [l] . Other terbium-reagent systems were studied further and representative results are presented in Fig. 2 for fixed terbium concentrations. Fluorescence intensity increases in the terbium-HPIDA system up to a 1:l mole ratio beyond which marked quenching occurs; thus HPIDA is a much less satisfactory reagent than EDBHPA. The system containing EDTA is less dependent on the terbium: Tiron ratio than the corresponding iminodiacetic acid system. Fluorescence intensities were maximum in the terbium/Tiron ranges 1:4 to 1:30 and 1:4 to 1:20 for the EDTA and iminodiacetic acid systems, respectively. A mole ratio of terbium/EDTA or iminodiacetic acid of 1:2 is necessary for maximum fluorescence intensity but further increases in relative EDTA or iminodiacetic acid concentrations, at least to ratios of l:lOO, do not affect the fluorescence yield. Relative intensity of fluorescence and comparison of reagent utility The ranges of linearity (Table 1) of fluorescence intensity as a function of terbium(II1) concentration provide a rough measure of the sensitivity of each system. However, because different instruments each with its own light source were used, the results do not provide a strict comparison of sensitivity of the

234

100,

)

I

I

0

HPlDAlTb 6 I

4

I 20

I

mole ratlo 12

I 40 EDBHPA

20 I

60

’

60

I

100

or T~ronl Tb mole ratlo

Fig. 2. Effect of changing the terbium(III)/reagent mole ratio on the fluorescence intensity of terbium complexed with: (1) HPIDA; (2) EDBHPA; (3) iminodiacetic acid and Tiron; (4) EDTA and Tiion. Conditions: 6.15 x lo+ M terbium(II1) in (1) and 4.61 x 10m6 M terbium(II1) in (2-4); 5.55 x lo+ M EDTA and 5.52 X 10ds M iminodiacetic acid. The excitation and emission wavelengths and reaction conditions are those recommended in Table 1.

methods. Such a comparison was made by using the Perkin-Elmer spectrofluorimeter with a single light source, a fixed terbium concentration, and the prescribed procedure in each case. The results are summarised in Table 2. It can be seen that the terbium-HPIDA system is the most sensitive but unfortunately the strong dependence on reagent/metal concentration ratio greatly detracts from the practical utility of HPIDA. The ternary systems with EDTA and sulphosalicylic acid or Tiron exhibit similar sensitivities but the latter is preferable because of the better-resolved terbium emission peak. The system with iminodiacetic acid and Tiron is somewhat less sensitive than the EDTA/Tiron system although otherwise as attractive for terbium determination. The EDBHPA binary system is also satisfactory in most respects, but the sensitivity is relatively low and, unlike the EDTA/Tiron system, it cannot be used for the fluorimetric determination of dysprosium(II1). Thus, on balance, it can be concluded that the TABLE 2 Relative intensity of fluorescence for the terbium(II1) systems studied (4.61 x 10d M Tb was used; pH, excitation and emission wavelengths are those recommended in Table 1) System E,

R,

HPIDA EDBHPA EDTA EDTA IDA

SSA Tiron Tiron

Mole ratio Tb:R, :R,

Relative intensity

1:l 1:s 1:20:17 1:20:20 1:20:10

100 12 37 35 24

235

ternary system based on EDTA and Tiron is the best of the five systems tested for use in standard commercial spectrofluorimeters. Of the other reagent systems studied, EDBHPA would be second choice followed by iminodiacetic acid/Tiron and EDTA/sulphosalicylic acid. Only HPIDA is unsatisfactory for terbium determinations under the conditions set out in the literature.

Interference from other metal ions and common anions Interference from other lanthanides, scandium, yttrium, thorium or dioxouranium(VI) cations was not detected in the EDTA/sulphosalicylic acid system by Dagnall et al. [3] for at least 50-fold molar amounts of a single metal ion relative to lo* M terbium. The present work confirmed these observations but interferences did occur at somewhat greater relative concentrations. Taketatsu and Yoshida [l] reported that in their method with EDBHPA, of the elements listed above, cerium(III), praesodymium, neodymium, samarium and europium interfered slightly and thorium seriously; adequate quantitative data were not provided to enable a precise comparison to be made. The present work with EDTA/Tiron showed that in the range 0.020.24 pg ml-’ terbium (1.2 X lo- ‘-1.5 X lo* M), up to the 200 pg ml-’ level of a given lanthanide ion, yttrium or a mixture of lanthanides, except for cerium(III), produced <6% quenching. Cerium(II1) at the 150 pg ml-’ level depressed the fluorescence intensity by 5.7%. Uranium(V1) and thorium did not interfere seriously at 200 pg ml-’ and 450 pg ml-‘, respectively. Thus tolerance for these metal ions is relatively good. Quantitative information on interferences by the same metal ions is not available from the literature for the iminodiacetic acid/Tiron system. It was claimed [2] that the presence of lanthanide/terbium ratios up to 1OOO:l did not decrease the fluorescence intensity when HPIDA was used. In view of the conclusions drawn above, interferences on these systems were not tested in the present work. The EDTA/Tiron reagent system should be used only after a group separation of lanthanides and yttrium from other elements present. For example, iron(I1) and iron(II1) give intense colours with Tiron and greater than lo-fold molar amounts of either relative to terbium cannot be tolerated. A 4000-fold molar amount of chloride, perchlorate, sulphate, nitrate or oxalate relative to 8 X lo-’ M terbium does not interfere. Except for nitrate, a 5000-fold molar amount of any of these anions can be tolerated. With the EDTA/Tiron system, equilibrium is attained in reaction with terbium(II1) in the concentration range 1.2 X lo-‘-l.5 X 10e6 M within 1 min, under conditions which should be suitable for automation with the segmented flow technique. One of us (N. Z.) gratefully acknowledges financial assistance from the Arab-British Chamber Charitable Foundation.

236 REFERENCES 1 T. Taketatsu and S. Yoshida, Bull. Chem. Sot. Jpn., 45 (1972) 2921. 2 M.A. Tishchenko, N. S. Poluektov, G. F. Yaroshenko, R. P. Lastovskii, G. L. Gerasimenko, I. I. Zheltvai and L. M. Timakova, Zh. Anal. Kbim., 33 (1978) 2368. 3 R. M. Dagnaii, R. S. Smith and T. S. West, Analyst, 92 (1967) 368. 4 N. S. Poluektov, M. A. Tishchenko and L. A. Alakaeva, Tr. Khim. Khim. Tekhnol., 5 (1973) 104. 5 M. A. Tiichenko, G. I. Gerasimenko and N. 6. Poluektov, Zavod Lab., 40 (1974) 935. 6 H. Irving and J. J. R. F. Da Silva, J. Chem. Sot., (1963) 3308. 7 H. Kroii, M. Kneii, J. Powers and J. Simonian, J. Am. Chem. Sot., 79 (1957) 2024. 8 S. J. Lyle and Md. M. Rahman, Taianta, 10 (1963) 1177.