tities greater than 20% of the weight of P u interfere with the coulometric determination of 239Pu in sulfuric acid medium ( 3 ) . T h e coulometric titration in hydrochloric acid applies to 23sPu in the presence of a n equal amount of Am. The coulometric determination of 239Pu in sulfuric acid requires a separate analysis of the sample for Fe to correct for the amount of Fe(I1) titrated. Titration in hydrochloric acid, however, shares with the coulometric determination of 239Pu in HC1 or HC104 media a relative lack of sensitivity to Fe, and quantities of Fe of u p to 1% of the total P u do not interfere. Elements such as Ir and Rh, which routinely foul Pt electrodes when present in microgram amounts, are deposited as metals on the P b reductor and are effectively separated from the P u solution to be analyzed. These elements may be present in quantities of up to 1% of the total weight of P u . The P b column must be replaced every 2 days or whenever significant quantities of these contaminants restrict flow through the column. Large excesses of U can be tolerated by most coulometric determinations of 239Pu, as the U(V1) is not reduced. When the sample is passed through a P b reductor, U(1V) is produced which is oxidized coulometrically to U(V1) a t f0.90 V us. SCE. The U remains as U(V1) when the plutonium is reduced a t +0..54 V L’S. SCE and does not interfere in the subsequent reoxidation a t
+0.90 V us. SCE. A change of the standard procedure to a n oxidation-reduction-reoxidation cycle is necessary if significant quantities of U are present. At U concentrations greater than 10% of the P u content, the time required for preoxidation of U becomes unreasonably long, allowing time for buildup of radiolytic interferences in the titration cell. The same problem is encountered with N p in concentrations greater t h a n 1% of total Pu. The presence of even small quantities of Nb, Hf, or M n causes low results. It is believed this is caused by hydrolysis of the ionic species and subsequent fouling of the Pt electrode. This effect has been reported for Zr (6) also. Although Ru is partially removed by the P b reductor, separation is not complete, and extended titration times and electrode fouling are observed. The effects of nitrate and perchlorate ions also were investigated. Even small quantities of either anion produced solutions which were unstable to radiolytic oxidation or reduction of the 238Pu. The sulfate anion was avoided because of the prereduction step through a P b reductor. Received for review October 24, 1972. Accepted January 8, 1973. This work was performed under the auspices of the U.S. Atomic Energy Commission.
Determination of Trivalent Europium Concentration in Yttrium Oxysulfide Phosphor Lyuji Ozawa and Harvey Forest Zenith Radio Corporation, Chicago, ///. 60639
Yttrium oxysulfide activated with trivalent europium, Y202S:Eu, has become a n important red phosphor in color TV picture tubes ( I ) because it has high brightness, short decay time, and exhibits long term stability in the polyvinyl alcohol slurry. The luminescent color of Y202S:Eu phosphor changes markedly from whitish orange to red with increasing europium concentration as a consequence of the different concentration quenching of the luminescence from the 5Dz, 5D1, and 5Do emitting levels of Eu3t (2). The europium concentration in commercial Y202S:Eu phosphor is about 3.6 mol 70;a t this concentration, the 5Dz luminescence (which falls in the bluegreen spectral region) does not play a n important role in the luminescent color. The difference in the luminescent color of various commercial phosphors of the same nominal concentration is due to the small differences in the active europium concentration in Yz02S which affects the relative amounts of 5D1 luminescence (green-orange spectral region) and 5Do luminescence (yellow-red spectral region). Active E u concentration refers to the E u which occupies Y202S lattice sites. Consequently, accurate determination (less than 0.1 mol 90error) of the active trivalent europium concentration in commercial Yz02S: E u is important in order to maintain reproducibility of the luminescent color. (1) M. R . Royce, U.S. Patent 3,418,246 (1968). (2) L. Ozawaand P. M . Jaffe, J . Electrochem. SOC.,118, 1678 (1971)
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, MAY 1973
Analytical methods, such as atomic absorption spectrometry, are not sufficient for the present purpose because the total europium concentration is determined, including europium in the unreacted starting material. Spectral methods using selected luminescent line intensities under photon ( 3 ) , cathode-ray (4, 5 ) , or X-ray (6) excitation are adequate for determining rare-earth trace impurities in yttrium oxide, but are not good for yttrium oxysulfide because of the variation of absolute luminescent line intensities with preparation technique. In this note, it is shown t h a t the active trivalent europium concentration for a given Y202S:Eu phosphor can be easily and accurately determined, however, from the luminescence intensity ratio of selected 5D1 and 5Do luminescent lines. The variation in the absolute luminescent intensity due to synthesis procedure remains, but is compensated for by measuring line ratios. The accuracy of this procedure is within &0.05 mol 70error.
EXPERIMENTAL The yttrium oxide used throughout the study was 99.9999% pure (Shin-Etsu Chemical Industry Co., Ltd., Japan) and the europium oxide was 99.99% pure. The Y202S:Eu phosphors actiL. Ozawaand T. Toryu.AnaL Chem.. 40, 187 (1968) K . A. Wickersheim. R . A . Buchanan. and L. E. Sobon, Anal. Chem., 40, 807 (1968). G. Urbain, Ann. Chim. Phys., 8ESer., 18, (1909). R. C . Linares, J. B. Schroeder, and L. A. Huribut, Spectrochim. Acta, 21, 1915 (1965).
1.
.-0
e
2 ' 8
\6 !
0
Figure 1. Luminescent spectrum of Y202S: Eu phosphor ranging from 580 to 590 nm. The excitation was made with 365-nm radiation from a Hg discharge lamp 1
1
4 5 Eu mole '10
6
1
vated with known concentrations of europium were prepared from oxides by a flux technique (7): the yttrium oxide and the europium oxide were dissolved in hot nitric acid and co-precipitated with oxalic acid. The mixed oxalate was heated a t 900 "C for 2 hr to convert to the oxide. A mixture of the oxide, sulfur, sodium carbonate, and sodium metaphosphate in the molar ratio 1:3:1:0.3, respectively, was fired a t 1100 "C for 2 hr in a nitrogen atmosphere. After firing, the samples were washed with hot deionized water and dried. Europium concentration is denoted X mol 70corresponding to ( Y I - ~ E U ~ ) ~ The O ~ Sintensities . of the selected lines were determined from the luminescent spectrum which was measured with a Jarrell-Ash grating monochromator (1500 lines/mm) having a 100-pm slit. The luminescent spectrum used in this study is shown in Figure 1. Since the luminescence line width was narrow, good reproducibility was obtained only with slow scanning of the monochromator (less than 10 n m per minute). The excitation of the phosphor was made with 365-nm radiation from a high pressure mercury discharge lamp plus a Corning filter No. 7-54. No difference in the luminescent spectrum was found with other excitation wavelengths, for example, with the 253.7-nm radiation from a low pressure mercury lamp.
RESULTS AND DISCUSSION Selection of Luminescent Lines. The luminescence of trivalent europium in 'Y202S lattice under ultraviolet, cathode-ray, or X-ray excitation consists of many narrow characteristic lines occurring in the visible spectral region ( 8 ) . The'se luminescent lines have been identified by Sovers and Yoshioka (9) as electronic transitions originating from 5Dj excited levels to 7Fj levels. Two luminescent lines located a t 583 and 587 nm belonging to ODo to 7Fo and the 5D1 to 7F3 transitions, respectively, were selected among the 10-15 luminescent lines to calculate the 5D1 to 5Do luminescent intensity ratio (LIR). These lines were selected because they were close in wavelength, had comparable intensities a t 3.6 mol % europium, and did not overlap any lines from other transitions. Another luminescent line a t 589 nm also belongs to the 5D1to 7F3 transitions and can be used. Calibration Curve. The line intensity-ratio, LIR, which is a function of the europium concentration (shown in Figure 2) is determined by the ratio of the 5D1 to 5Do luminescence. The curve in Figure 1 was obtained under 365-nm ultraviolet excitation. Since the LIR depends
Figure 2. Luminescence intensity ratio (5D1 to 5 D ~ )of Y a 0 2 S :Eu phosphors having different europium concentrations
upon the type of' excitation (10) and the phosphor temperature ( I I ) , the calibration curve and the determination of europium concentration must be made a t the same phosphor temperature and with the same excitation. The LIR for samples made with the same initial europium concentration depended upon the phosphor preparation. This was found to be due to the incomplete conversion of Y203:Eu to Y202S:Eu. The unconverted oxide can be detected by the luminescent line a t 611 nm using the short UV (or cathode-ray) excitation. The LIR of Y202S:Eu phosphor containing unreacted oxide was usually smaller than the ratio of the pure Y202S:Eu phosphor. When the Y203:Eu was completely converted to Y202S:Eu by firing in nitrogen atmosphere with excess sulfur, however, the LIR was found to be reproducible *3%. Forest (12) has reported the n-y color coordinates and LIR of Y202S:Eu phosphor for different europium concentrations. He indicates t h a t the luminescent color coordinates of Y202S:Eu phosphor are also accurately determined by the LIR. In conclusion, the active trivalent europium concentration and luminescent color of Y202S:Eu phosphor are accurately determined by the luminescence intensity ratio of the 5D1 to 5Do lines.
ACKNOWLEDGMENT The authors wish to thank H. Stanczyk and K. Wolejko for their assistance in preparation of samples and in measurements. Received for review November 10, 1972. Accepted January 10, 1973. (10) L.
(7) L. Ozawaand P. M .Jaffe, J. Electrochem. SOC.,117, 1297 (1970). (8) L. Ozawa, Oyo Butsuri (Appl. Phys.), 37, 80 (1968) (in Japanese). (9) 0. J. Sovers and T. Yoshioka, J. Chem. Phys., 49, 4945 (1968).
I
3
Ozawa, H . Forest, P. M. Jaffe, and G. Ban, J. Electrochem.
188,482 (1971). (11) W. H. Fonger and C. W. Struck, J. Chem. Phys., 52,6364 (1970). (12) H.Fored, to be published in J. Electrochem. SOC. SOC.,
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