LITERATURE CITED
(1) Frost, A. A., Pearson, R. G., “Kinetics and Mechanism,” 2nd ed., p. 140, Wiley, New York, 1961. (2) Hanna, J. G., Siggia, S., ANAL. CHEM.37, 690 (1965). (3) Hodgman, C. D., “Handbook of Chemistry and Physics,” 42nd ed., p. 2513-22, The Chemical Rubber Publishing Co., Cleveland, Ohio, 1961.
(4) Hogg, J. S., Lohmann, D. H., Russell, K. E., Can. J. Chem. 39, 1588 (1961). (5) McGowan, J. C., Powell, T., J. Chem. Soe., 2106 (1961). (6) McGowan, J. C., Powell, T., Raw, R., Ibid., 3103 (1959). ( 7 ) Papariello, G. J., Janish, M. A. M., ANAL.CHEM.37, 899 (1966). (8) Venker, P., Herzmann, H., Naturwissenschuften 47, 133 (1960).
(9) Washburn, E. W., “International Critical Tables of Numerical Data, Physics, Chemistry and Technology,” Vol. 6, p. 102, McGraw-Hill, New York, 1929. (10) Wilkinson, R. W., Chem. & Ind. (London) 1961, 1395. RECEIVEDfor review July 27, 1965. Accepted December 2, 1965.
Spectrofluorometric Trace Determination of Trivalent Samarium, Europium, Terbium, and Dysprosium in Sodium Tungstate Solutions GlULlO ALBERT1 and MARIA A. MASSUCCI laboraforio di Chimica delle Radiazioni e Chimica Nucleare del CNEN, lstifufo di Chimica Generale ed Inorganica, Universifi di Roma, Italy Sodium tungstate acts as a specific reagent for enhancing the fluorescence intensity of samarium, europium, terbium, and dysprosium in aqueous solutions. Maximum fluorescence intensity is obtained by irradiating at 265-270 mp lanthanides dissolved in 0.6M sodium tungstate solution with a final pH of 9 and a temperature of 2 0 ” c. All measurements are related to the fluorescence intensity of a quinine sulfate solution. Correction for quenching by inorganic ions and conditions for decreasing positive interferences are given, Sensitivity is 10-1-1 0-3 Clg./ml.
I
that the analytical problems encountered in the determination of the lanthanide ions, especially as traces in solutions, arise from their very similar chemical properties. Therefore, it is extremely difficult to find specific reactions for the individual ions. Spectrophotometric methods produce satisfactory results for only high lanthanide ion concentrations, since the molar absorptivities of the individual ions are rather low (8). Better results are obtained using optical emission spectra (9,13) or activation analysis (6),but it is still difficult to determine traces of individual lanthanides in complex mixtures. Fluorometry has been only partially successful because the fluorescent intensity of solutions of common soluble salts of the lanthanides-e.g., sulfates, chlorides, and nitrates-are fairly low (10, 12, 14). A previous paper (4) described certain characteristic fluorescence tests for lanthanide ions on filter paper. It was possible to identify with sodium tungstate, 10-2 pg. of Samarium, 2 X T IS WELL KXOWN
214
ANALYTICAL CHEMISTRY
pg, of dysprosium, and pg. of europium; with sodium oxalate, pg. of terbium; and with sodium tetraborate, 5 pg. of cerium (111). The specificity of these reactions and the ease of separation of lanthanide ions on ion exchange paper make these tests fairly useful for qualitative and semiquantitative analyses of traces of these ions, but not for quantitative estimations. We reported (3) that in sodium tungstate solution the molar absorptivities of all the lanthanides in the ultraviolet range (260-270 mp) were considerably increased (about lo4 times). These studies showed that samarium, europium, and dysprosium in sodium tungstate solutions are from 104 to 105 times more fluorescent than in mineral acid solutions. It seemed appropriate, therefore, to investigate the possibilities of determining these ions in tungstate solutions by spectrofluorometry. THEORY
The transfer mechanism of electronic excitation has been extensively studied in recent years. It is well established that excitation transfer may occur not only between different molecules, but also between separate electronic systems of the same molecule (11). The excitation transfer between different molecules may occur if the absorption spectrum of the acceptor overlaps the fluorescence spectrum of the sensitizer (resonance transfer). Since the individual trivalent lanthanide ions have different but characteristic absorption spectra, it is possible to transfer selectively the absorbed energy only to a given ion. This is achieved by the choice of a suitable sensitizer whose fluorescence spectrum overlaps solely,
or more completely than any other ion, the absorption spectrum of the lanthanide ion to be determined. Crosby ( 7 ) has established that the requirement for the intramolecular energy transfer in a given lanthanide complex is that the lowest triplet state energy level of the complex must be nearly equal to or must lie above the resonance level of the lanthanide. In this case, too, there is the possibility of transferring the energy selectively to certain lanthanide ions by the suitable choice of a complexing agent, so that the lowest triplet state will be in a position favorable only to the resonance level of such ions. Since the energy transferred to the lanthanide ions may be emitted again as fluorescence in the visible region, this transfer of energy can be employed to cause selective fluorescence of given lanthanide ions. Since the molar absorptivities of lanthanide complexes are much greater than those of the simple ions, it should be possible to obtain also in these cases high fluorescence intensities. Such a possibility is of particular importance for the estimation of traces of single lanthanide ions in mixtures. The selective increase in the fluorescence intensity of samarium, europium, and dysprosium in scdium tungstate solutions, and of terbium in oxalate solutions (1, 3) must be connected with ionic association or chelate formation of lanthanides which absorb in the ultraviolet region and with energy transfer to the lanthanide ions of that complex. Since this energy cannot be transferred to the gadolinium or cerium ions, these ions, although highly fluoresc ent in chloride solutions, do not show appreciable fluorescence in aqueous tungstate or oxalate solutions.
EXPERIMENTAL
All the fluorescent measurements were made with an Aminco-Bowman (Cat. No. 4-8106) spectrofluorometer. The cells used had quartz optically flat surfaces (square base, 10.5 mm.) suitable for 1 ml. of working solution. An RCA1P21 photomultiplier (p.m.j was used for measurements in the visible region, while for the red part of the spectrum the Aminco accessory with an RCA-7102 p.m. was employed. The cooling mixture used for this phototube was dry ice-acetone. Materials. Standard chloride solutions of lanthanides were prepared by dissolving the corresponding oxides in hydrochloric acid solution. A11 lanthanide oxides were a t least 99.9% (Johnson-Matthey Co., London). For samarium, europium, and gadolinium, specpure oxides of 99.99570 purity were used. All other reagents were ERBA R P products. Sodium Tungstate Solution. This solution was prepared by dissolution of 25 g. of NazWO4.2H20in about 70-80 ml. of distilled water, followed by 1.13 g. of Ka2B40i~10H20.Finally 5 ml. of 0.531 KaOH were added and the resulting solution was diluted to 100 ml. with distilled water. Apparatus.
RESULTS A N D DISCUSSION
Samarium, europium, terbium, and dyspro:ium are the only lanthanide ions appreciably fluorescent in sodium tungstate solutions. The fluorescence spectra of these ions in sodium tungstate solutions are reported in (3). The fluorescence intensity of lanthanide ions in sodium tungstate solutions depends upon the wavelength of exciting light, the sodium tungstate concentration, and the temperature, pH, and aging of the solution. This must be connected with the ability of tungstate ions in aqueous solutions to form polytungstates, absorbing strongly in the excitation region of fluorescence and thus performing as internal filters ( 2 ) . The optimum conditions t o achieve the maximum fluorescence intensity (2) are obtained by irradiating a t 265-270 mp lanthanides dissolved in 0.6.11 sodium tungstate solution (final pH of solution = 9 ; temp. = 20" (2.). The relative fluorescence intensity values (R.F.I.) are corrected for the contribution due to the blank. A11 measurements vere related to the fluorescence intensity of a quinine sulfate solution (0.1 pg./ml,, A, = 250 mp; 1, = 450 mp). The intensity of the fluorescence increases with concentration of lanthanide ion until about 10-12 pg./ml. For concentrations greater than this, the fluorescence decreases in intensity. This phenomenon is linked with internal filter quenching because of the high absorbance a t 265 mp of lanthanides when in sodium tungstate solution.
Slight variations in the emitted intensity may occur in the first few minutes of mixing the sodium tungstate and chloride solutions containing the lanthanide ions. It is best to carry out the fluorescence measurement a t least 5 minutes after mixing. The fluorescence intensity is fairly stable for some hours, decreasing only slowly with time ( 2 ) . For samarium, europium, terbium, and dysprosium linear calibration curves are obtained for concentrations lower than 1 pg./ml. The standard deviation for the relative error, calculated on 15 solutions each containing 1 pg./ml. of europium, was 0.7y0with the 1P21 and 2.5y0 with the 7102 photomultipliers. The corresponding deviations for samarium, terbium, and dysprosium solutions containing 1 pg./ml. of lanthanide were 0.75, 1, and 1.1y0, respectively (with the 1P21 photomultiplier). However, the error in the determination increases with decreasing lanthanide ion concentration. In Table I, the lowest concentrations of samarium, europium, terbium, and dysprosium that it is possible to determine (uith a 5oyOerror) are given with the excitation wavelength (A,), the wavelength used to determine fluorescence intensity (A,j, and the photomultiplier used. Because of the remarkable separation between A, and A, scattering by the sodium tungstate solution is very low, Therefore it is possible to decrease markedly the sensitivity limits reported in Table I by increasing the intensity of the excitation light a t 265 mp. Interference by Various Substances on the Fluorescent Intensity. Various
inorganic ions, including some rare earths, markedly quench the fluorescent intensity of samarium, europium, terbium, and dysprosium in sodium tungstate solutions. The main reason for fluorescent quenching is the presence of substances or ions which, in such solutions, absorb strongly a t 265 mp (excitation wavelength of lanthanides in sodium tungstate solutions). All the rare earths, absorbing a t such a wavelength may behave as quenching agents if present in fairly high concentrations (Table 11). Since the main reason for quenching is the absorption of ultraviolet light a t the excitation wavelength, it is possible to calculate the percentage quenching from the solution absorbance a t 265 mp. It has been demonstrated ( 2 ) that the relation between absorbance and per cent quenching is:
Yo quenching
=
100
100
- __ (1 - 10-0) (1) 2.3a
where a is the absorbance of the solution
Table 1. Minimum Concentrations of Sm(lll), Eu(lll), Tb(lll), and Dy(lll) Determinable in 0.6M Sodium Tungstate Solution with a 50% Error
pH
= 9; A, =
265 mp h4in.
Sm(II1) Sm(II1) ELI(III) ELI(III) Tb(II1)
P.m. XI 1P21 560 mp 7102 595 mp 1P21 590 mp 7102 695 mp lP21 545 mu
concn. determinable, pg./ml. 5 X 10-1 5 X 10-1 5
x
10-3
10- 2 10-1 ~~
Table II. Per Cent Quenching b y Rare Earths on Fluorescence of 1 pg./ml. of Dy(lll) in 0.6M Sodium Tungstate Solution (pH = 9)
Instrumental parameters: Table 111, column b Quenching, ';c 1 a.
3 pg.
5 pg.
LalIII) 11 26 3~. 9 CeiIIIj 13 28 40 Pr(II1) 16 28 40.5 Nd(II1) 12 27 41 11.5 27 Sm(II1) 40.5 Eu(II1) 11 26 40.5 Gd(111) 9.5 38 27.5 Tb(III)a 10 28 39 , ' J : Ho(II1) 7 37 Er(II1) 8 24 31 Tm(II1) 9 19 29 Yb(II1) 17 7.5 25 Lu(II1) 8 13 24 Y(II1) 14 34 3 Corrected for mutual enhancenieiits (Table 111). I
relative to water. This relatiomhip may be employed to correct the fluorescence intensity for the internal quenching action of the solution. Another method which, although less precise, may be employed for the determination of percentage quenching, is the addition method which is commonly used in the fluorometric determination of uranium. Besides the quenching or negative interference, it is possible t o have positive interferences or enhancements due to the presence of more than one fluorescent lanthanide in the mixture t o be determined. To decrease the enhancement effects, it is convenient to determine dysprosium a t 480 mp using the 1P21 (pm.), terbium a t 545 mp using the 1P21 (pm.), europium a t 695 mp using the 7102 (p.m.), and samarium a t 560 mp with the 1P21 (p.m.), or a t 640 mp with the 7102 (p.m.), (3). Table I11 shows the R.F.I. values of 2 pg./ml. concentrations of samarium, europium, terbium, and dysprosium a t the wavelengths mentioned above. The reported R.F.I. values are corrected for the quenching due to the solution absorbance by means of Equation 1. Table I11 shows that no enhancements take place in the europium deterVOL. 38, NO. 2, FEBRUARY 1966
b
215
mp with the 1P21 p.m., and strong enhancements take place in the determination of samarium in the presence of europium and dysprosium even when very narrow slits are employed. From the sensitivity limits reported in Table I and from the positive interference reported in Table 111, the lanthanide determination in sodium tungstate solution is fairly satisfactory for traces of europium and dysprosium. Positive errors for the determination of terbium in the presence of large concentrations of dysprosium, but mainly for samarium in- the presence of euro-
mination a t 695 mp with the 7102 p.m., only a slight interference by terbium occurs in the determination of dysprosium a t 480 mp, with the 1P21 p.m.
R’F’l’ Of Tb R.F.I. of Dy
= 0.07; samarium and
,
europium do not interfere medium. interference by dysprosium
R’F’l*Of Dy = 0.16 R.F.I. of T b
\-
one by europium
R.’F.I. of EU
=
o.03)
(R.F.I. of T b in the determination of terbium a t 545 Table 111.
Mutual Enhancements among Various Fluorescent Lanthanide Ions in 0.6M Sodium Tungstate Solution a t p H 9
(a) (b) Lanthanide R.F.I. at R.F.I. a t ion concn., XI = 695 mp, XI = 480 mp, 2 pg./ml. p.m. i102 p.m. 1P21
