Table VI. Effect of Foreign Ions Ions tolerated at 100 ppm:
Aluminum Ammonium
Arsenic Barium Bismuth Calcium Lithium Magnesium Potassium
Strontium Tin Zinc Acetate Bromide Bromate Chloride Citrate Fluoride
Iodide Nitrate Perchlorate Periodate Phosphate Pyrophosphate Sulfate Tartrate Tetraborate
Ions toIerated at lower concentrations (ppm tolerated):
Chromium (10) Cobalt (2) Copper (50)
Manganese (50) Nickel (2)
Oxalate (50) Vanadium (2)
ml of the chloroform-PDT solution, the concentration of iron can be as low as 2pg/l. in the acid solution. On applying the recommended procedure to determine iron in reagent grade acids, the results in Table V were obtained. Duplicate determinations show good agreement. Recovery of iron, within 2% experimental error, was confirmed in each case by use of the method of standard additions. Acids not listed for which recovery of iron was unsatisfactory are dichloro- and trichloroacetic acids. These acids inhibit complete formation of the iron(I1)-PDT complex in chloroform because of their extractability and strength. Results of interference studies are given in Table VI. An error of 3% or greater occurs in the presence of 100 ppm of any of the seven ions listed in the bottom section of the
table; however, lower concentrations (noted in parentheses in ppm) are tolerated. The presence of these ions a t their respective interference levels is not ordinarily expected for reagent grade acids. The most severely offending ions are cobalt(II), nickel(II), and vanadium(IV); 5 ppm of these cause positive errors for iron of 5, 19, and 1070, respectively. Interference by 100 ppm of oxalate is unusual in that it is time-dependent and can be avoided by using longer shaking times of 60 sec or more. Studies of the effects of time revealed no serious sources of error. Sample solutions treated with the ascorbic acidthiocyanate solution could stand for a t least 1 hour prior to extraction without adverse effect. Extraction was complete for a shaking time of 15 sec, but a minimum of 30 sec is recommended. No change in absorbance was detectable on storing the final chloroform extracts in stoppered flasks for 3 days. The usual safety precautions should be observed when handling concentrated acids, particularly to guard against excessive heat and splattering on diluting them with water to the concentrations recommended for this procedure. It is also important to cool the diluted acid solution prior to adding the ascorbic acid-thiocyanate solution, because hot solutions of strong acids promote decomposition of thiocyanate to carbonyl sulfide and ammonia. Noteworthy advantages of the recommended procedure are its speed, simplicity, and sensitivity. Moreover, there is no necessity for a reagent blank determination because the reagents are easily freed of iron contamination prior to USe.
Received for review January 11, 1974. Accepted March 7, 1974.
Analytical Applications of X-Ray Excited 0ptical Luminescence Direct Determination of Rare Earth Nuclear Poisons in Zirconia Arthur
P. D’Silva and Velmer A. Fassel
Ames Laboratory-USAEC and Department of Chemistry, l o w a State University, Ames, lowa 50010
An X-ray excited optical luminescence technique for the direct quantitative determination of fractional ppm levels of rare earth “neutron poisons” in ZrOz and Zircaloys is described. A blend of Zr02, KzCO3, Sr(N03)2, and WOa is heated to 1050 “C for 2 hours to yield a quaternary oxide phosphor host with a composition of KzOs2SrO2Zr02.3W03. The irradiation of this pho.sphor by Xrays causes the emission of optical line luminescence of the rare earth impurities. When the phosphor sample is irradiated at 1 5 0 “C, the intense host band luminescence is quenched yielding improved signal to noise ratios for Sm. Dy. Eu, Pr, and Tb, which is the internal reference element. The Gd luminescence, which is quenched at 1 5 0 “C, is observed at ambient temperatures. The detection limits under these conditions are: 0.05 ppm for Gd, Sm, Pr; 0.1 for Eu; and 0.02 pprn for Dy. 996
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO.
8, JULY 1974
Zircaloy 2 and 4 are preferred alloy structural materials in nuclear power and breeder reactors. These alloys meet the criteria of low absorption coefficients for thermal neutrons as well as strength and corrosion resistance a t elevated temperatures ( I ) . The rare earths Gd, Sm, Eu, and Dy are the most potent “neutron poisons” that may occur in these alloys as residual impurities derived from ZrOz, the base material for the preparation of these alloys. As a consequence, the presence of the above rare earth impurities in ZrOz a t levels greater than 1 ppm is considered undesirable (2). The processing of ZrOz to Zr metal and the H . H. Klepfer. “Proceedings of the USAEC Symposium on Zirconium Alloy Development,” GEAP-4089, Vallecitos Atomic Laboratory, General Electric Company, San Jose, Calif. 1962. L. G. Wisnyi, KAPL-3322, Koolls Atomic Power Laboratory, General Electric Company, Schenectady, N . Y . . 1967.
alloys usually reduces the rare earth content to acceptable levels of 0.1 ppm or less. T h e certification that the above criteria are met for the primary Zr or ZrOz used in the preparation of the alloys involves laborious analytical techniques (3, 4 ) . Because direct determination of the rare earths a t the fractional part per million level is not feasible, chemical preconcentration of the rare earths in a Yz03 or La203 carrier followed by spectrographic analysis of the concentrate has been recommended (5). A very sensitive analytical technique for the determination of the rare earths in non-nuclear and nuclear materials (6-18) utilizes the principle of X-ray excited optical luminescence (XEOL) of the rare earths (19-21). Zirconia by itself is not a n acceptable host for the detection of the rare earth activators a t the required low concentrations. T o circumvent this problem, the rare earths were separated from Zr and concentrated in a Tho2 matrix (22) in which their XEOL is observable at the fractional part per million level (15). Another procedure involved the preparation a t high temperature of a mixed oxide host of ZrOz and ThOz (10:l respectively) (23). Apparently, the rare earth impurities diffused from ZrOz into ThOz during the heat treatment of the mixed oxides a t 1000-1500 "C (24). The detfirt,ion limita r e n n r t d fm tho "nontmn nniann" rare earths were 0.5 pprn for Gd, 30 ppm for Sm, i0 ppm for Dy, and 0.1 ppm for Eu (23).These detection limits do not meet the purity criteria for nuclear grade ZrOz. T h e required powers of detection can, however, he achieved if the ZrO2 is incorporated into ternary or quaternary oxide host systems (16, 25). This communication elahorates on these earlier observations and discusses the recent developments which have resulted in a procedure for the direct determination of Gd, Sm, Dy, Eu, and PIin ZrOz a t fractional part per million levels.
( 3 ) M. W. Lerner in "Analysis of EsSenlial Nuclear Reaclor Materials'' J. Rodden. Ed., U.S. Government Printing Office. Washington, C. J D.C., 1964, pp 719.900, ( 4 ) C. G. Goldbeck. Ref. 3. pp959-986. ( 5 ) H. J. Hellel and V. A. Fassel. Anal. Chem., 27 131 1 ( 1 9 5 5 ) . (6) R. C. Linares. J. B. Schroeder, and L. A. Hurlburt, Spectrochim Acta.21. 1 9 1 5 ( 1 9 6 5 ) . ( 7 ) E. L. DeKalb. V. A. Fassel, T. Taniguchi and T. R. Saranathan. 40.2082 (,1 ~~~, 968). Anal. Chem... 40. ~.2082 119681. ( 8 ) J. F. Cosgrove. 0. W. Dblas, R. M. Wallers, and D. J. Bracco. Electrochem. Technol., 6. 137 ( 1 9 6 s ) . ( 9 ) W. E. Burke and 0. L. Wood. Advan. X-Ray Anal., 11, 204-213 119fiR1 , ~~. (10) R. J. Jaworowski, J. F. Cosgrove. D. J. Bracco. and R. M. Wallers. Spectrochim. Acta, Part 8,23, 751 (196s) ). ( 1 1 ) N.Sasaki, BunsekiKagaku, 17,1387 ( 1 9 6 8 ) . ( 1 2 ) W. A. Shand. J. Mater. Sci., 3 , 344 ( 1 9 6 8 ) . ( 1 3 ) T. Nakajima. H. Kawaguchi. K. Takashima, and Y. Ouchi. Bunko Kenkvu. 1 6 , 2 1 0 119691. ( 1 4 ) K. Tikashima, T. Nakajima. H . Kawaguchi, and Y. Ouchi, Bunko Kenkyo, 1 8 , 2 6 2 ( 1 9 6 9 ) . (15) T. R. Saranafhan. V. A. Fassel. and E. L. DeKalb, Anal. Chem., 42, 325 119701 (161 E. L: DeKalb, A. P. D'Silv~.and V. A. Fassel. Anal Chem.. 42. 1246 ( 1 9 7 0 ) . M. Sam. H . Malsui, and M. Tadao, Jap. Anal., 20, 70 (1971) H. Ratinen in Acta Polylechniea Scandinavica, "Chemistry including Metallurgy SerieS No. 107" The Finnish Academy of Technical Science*. Helsinki, 1971, Chap. 107. J. Makovsky, W. Low,andS. Yatsiv, Phys. Lelt., 2, 186 ( 1 9 6 2 ) . V. E. Derr and J. J. Gallagher. "Ouantum Electronics-Paris 1963 Conference," Columbia University Press. New York, N.Y., 1964. p 817. W. Low, J. Makovsky, and S . Yatsiv. Ref. 20. p655. .. . .. .. . . . . . . . lZZ1 I . NaKallma, Y . UUCOI, H . Kawaguchl, and K. Takashima Jap. Anal., 19, 1183 (1970). 123) H. Kawaguchi, T. Nakajima, K. T. Takashima, and Y. Ouchi. Bunkc Kenkw 18, 299 (1969). ( 2 4 ) A. K: rmfimov. Boll. Acad. Soi, USSR, Phys Series, 21, 754 (1957). (25) A. P. 0"Silva. E. L. DeKalb, and V. A. Fassel. Anal. Chem., 42, 1846 ( l !370).
ri.
-
'
ples (A) Phosphor Sample in AI planchef. (SI Resislance healer. (C) Brass slide, ( 0 )Lavile insert
EXPERIMENTAL Apparatus. The instrumentation utilized in this investigation has heen described (7, 26). To provide eapahility of heating phosphor samples to 200 "C during excitation and observation of the luminescence, a miniature resistance heater, shown in Figure 1, was incarporated into the vee-way mounting block, helow the mating sample holder. The temperature was monitored by a copper-constantan thermocouple positioned in close proximity to the AI planchet containing the phosphor. The temperature was eantrolled by an onloff temperature controller which could be set to yield temperatures accurate to f2 "C. The heater assembly as well as the phosphor sample slide was partly made of Lavite, a refractory material (American Lava Corporation, Chattanooga, Tenn.), to minimize heat transfer to the other parts of the sample irradiation chamber. Preparation of Calibrating Standards and Samples. Commercial samples of high purity ZrOz contain appreciable amounts of rare earth impurities and are not suitable as base material far the preparation of calihrating standards. The ZrOz base material utilized in our study was obtained from electron beam melted Zr metal specially prepared by the Ames Laboratory Metallurgy Group. Electron beam zone refined Zr metal, analyzed to contain less than 0.5 ppm rare earths, is now available commercially (Materials Research Corporation, Orangehurg, N.Y. 10962). This may be a suitable base material. This Zr metal was converted to the nitrate through the sulfate and hydroxide. The nitrate master solution was assayed for Zr02 content. Appropriate amounts of the rare earth nitrate solutions were then added to aliquots of the master solution. The resulting. solutions were evaporated to dryness and ignited at 1000 "C for 3 hours to yield a graduated seriesof standards. The quaternary oxide phosphor host K*O.ZSr0.2Zr02.3W03 was prepared from reagent grade KzC03, S r ( N 0 3 ) ~CP grade W08, and the standard or sample ZrOz blended in mole ratios of 1:2:3:2, respectively. Since a number of ZrOz samples initially examined contained Er as a residual impurity, Tb was selected as the external reference element (16). The Tb was added to the quanternary oxide host system with the Sr(N0& component of the phosphor base. For this purpose, 10 pg of Tb was added to 2.11 grams of %(NO& in solution, which was then evaporated to dryness at 110 "C. In practice, the phosphor base mixture of KzC03, Sr(NO&, and WOa in mole ratios of 1 2 3 is blended in hatches large enough to suffice for many analyses. For the analysis of a single sample, 0.123 gram of ZrO2 is blended with 0.839 gram of the phosphor base mixture in an agate mortar for a few minutes and heated in a platinum crucible for 1050 "C for an hour. The phasphar is reground and fired at the same temperature far an additional hour. The luminescence of the product is then examined at ambient temperatures, with the 3000-A blaze grating in position, and thereafter at 150 "C with the 6000-8,blaze grating in position. Phosphor Genesis. As early as 1942, it wag demonstrated that ultraviolet (W) excited optical luminescence of relatively large concentrations of Sm could be observed in fluorite structured ZrOz (27). Later, the luminescence of rare earths present at 1% levels in NazZrSiOs was studied using UV, electron heam, X-ray and y-excitation (28). In our investigations on the XEOL of rare ( 2 6 ) A. P. DSilvaand V. A. Fassel. Anal. Chem., 43,1406 ( 1 9 7 1 ) . ( 2 7 ) R. Tornaschek. Ergeb. Exakt, Natorw., 20,268 (1942) ( 2 8 ) B. V. Shul'gin, F. F. Gavrilov. V. K. Parshin. and V. G. Chuklantsev, I z v . Vumv. Fizika, 7 , 122 (1967)
A N A L Y T I C A L C H E M I S T R Y , VOL. 4 6 , NO. 8 , J U L Y 1974
997
‘“I 8-
P -
-Y -
&
6-
24c
0
-
2 2-
-
nZr 02
Z r 0 2 Si02
SrO.ZrOp3WO3
K20 2SrO,ZZr%3W03
3OOppm Srn in
IOOpprn Srn
IO ppm Srn
1 ppm Srn
in
in
ill
Zr 02
Zr02
Zr 02
Zr02
Figure 2. X-Ray excited optical luminescence of Sm in Z r 0 2
hosts
Cs
K
Rb
No
LI
1ji J
OL
Figure 3. Effect of changing alkali ion in R2+’0.2SrO.2Zr023w03 on XEOL of G d 31 18 A
PI
I
3330
Wavelength
€000
6600
(1)
Figure 5. XEOL spectra of rare earths in K20.2Sr0.2Zr02. 3wo3 with the phosphor sample at 150 “C and the spectra recorded at optimum instrumental gain. The rare earth contents in the ZrO2 were: Gd 0.4 p p m ; S m , 0 2 pprn; Eu, 1.0 pprn; Dy, 0.2 ppm; and Pr, 0.2 ppm
earths in various zirconium hosts, we observed the luminescence of Sm present in excess of 100 ppm in ZrOz, and ppm level rare earths in a mixed host ZrOz-SiOz prepared by heating an equimolar mixture of ZrOz and Si02 at 1400 “C for several hours. However, the limits of detection and preparation procedures were not considered analytically useful. Subsequently we showed that quaternary oxide host systems that incorporated ZrOz provided superior powers of detection (25). The enhancement in Sm line intensities as the host composition is changed from the simple oxide and then to binary, ternary, and quaternary oxide host systems is clearly shown in Figure 2. The process of optimizing the composition of the quaternary oxide host, aR& +0.bRZ+0.cR4+02.dW03 followed the procedure outlined in our earlier communication (26). A set of typical observations on the effect of changing the alkali ion on the Gd line intensity is shown in Figure 3. This process of optimization led to the selection of the KzO.2Sr0-2Zr02.3W03 host. This host could be reproducibly prepared by simple, solid-state reactions and satisfied our criteria for an analytically useful host. It has not been characterized as a single phase crystalline material though the spectra have the unmistakable characteristics of those obtained in a scheelite (CaW04) type host.
RESULTS AND DISCUSSION
vu Pr
1
3100
Wavelength
(A)
Figure 4. XEOL spectra of rare earth impurities and the host in K20-2SrO.2ZrO2.3WO3 w h e n the phosphor samples are
heated at different temperatures 998
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 8, J U L Y 1974
Luminescence Spectra. The XEOL spectrum of fractional ppm levels of rare earths in ZrOz as observed in the quaternary oxide host is presented in Figure 4. One distinct feature observed in this system is the intense host band luminescence in comparison to that observed in the UOz containing host (25). This host luminescence increases in intensity as the ZrOz purity increases and imposes a severe limitation on the optimization of instrumental settings necessary to assess the limits of detection. This limitation can be circumvented by taking advantage of the dissimilar spectral characteristics of the host band and rare earth line luminescence. In an inorganic, rare earth-activated phosphor system, the broad-band host luminescence is due to electronic transitions involving outermost valence electrons in one or all of the host ions but the rare earth line spectra arise from electronic transitions within the shielded 4f subshell. In oxide base hosts, these differences often result in the two spectral emissions having significant differences in their luminescence lifetimes. For example, the blue band emission in C a w 0 4 is known sec, but the lifetime of Nd to have a lifetime of 5 X line fluorescence in CaW04.Nd3+ is of the order of sec. A phosphor system exhibiting mixed transient phenomenon can often be investigated using time resolved spectrophotometric techniques (29). In such a system, it is possible to observe weak rare earth line luminescence un(29) K Doc and A Toshinai. AppL O p t , 9, 2762 (1970)
Table I. Analytical D a t a Ln3
Analytical line pair,
+
Gd
A
Gd 3118 T b 5442 Sm 5957 T b 5442 Eu 6153 T b 5442 Dy 5722 T b 5442 Pr 4850 T b 5442
Sm
Eu
DY Pr
Grating blaze,
Phosphor temperature, OC
Ai
Lowest detectable concn, ppm
Concn range in ZrOr,ppm
3000
ambient
0.05
0.1-2.0
6000
150
0.05
0.1-2
6000
150
0.10
0.2-4.0
6000
150
0.02
0.05-2.0
6000
150
0.05
0.1-2.0
.o
T a b l e 11. Analytical R e s u l t s of Doped Zircaloy 2 Samples Ln3+ added, ppm
Ln3- found, ppm
Sample
Gd
Sm
Eu
DY
Pr
Gd
Sm
Eu
1 2 3
0.2 0.5 1.0
0.2 0.5 1.0
0.5 1.0 2.0
0.2 0.5 1.0
0.2 0.5 1.0
0.17 0.54 0.90
0.23 0.46 1.16
0.44 1.12 1.81
affected by the intense host band luminescence (30). Such an approach was not used in our study, because extensive modifications in our XEOL system would have been necessary. An alternate approach was to utilize the dissimilar temperature quenching behavior of the two types of luminescence. In an extensive investigation on energy transfer in Sm activated tungstate or molybdate compounds, it has been observed that, on excitation by cathode rays, the quantum efficiency of Sm line luminescence reached a maximum and the wo4 band luminescence a minimum a t a phosphor temperature of about 127 "C (31). The results of a similar study performed during our investigation are depicted in Figure 4. It is seen that Gd line luminescence a t 3118 8, is completely quenched, for reasons not clear to us, on raising the temperature to 50 "C and that the host band luminescence degrades gradually with temperature. The diminution of the luminescence of Sm, Eu, Dy, and Pr is not so pronounced and even at 150 " C there is appreciable luminescence. It is evident from the spectra presented in Figure 4, that the host band luminescence changes by a factor of 3 for a change in temperature of 25 "C ( i e . . 25 to 50 "C). Hence controlling the temperature to *2 "C is necessary. The advantages of observing the phosphor luminescence at 150 "C at increased gain is obvious in Figure 5 , which was recorded at an increased in-
(30) G E Peterson and P M Bridenbaugh, J Opt SOC Amer 1129 (1963) (31) Th P J Botden, Philips Res Rep 6 , 425 (1951)
53,
-
DY
0.17 0.45 0.92
Pr
0.22 0.56 1.17
strumental gain that could not be utilized for obtaining the spectra shown in Figure 4. The standard experimental conditions, the limits of detection, and the concentration range covered by linear calibration curves are summarized in Table I. Direct Analysis of Zircaloys. Zircaloy 2 and 4 are alloys of Zr containing Sn (1.2-1.7%), Fe (0.07-0.20%), and Cr (0.05-0.15%). In addition Zircaloy 2 contains from 0.03 to 0.08% Ni. Since Zircaloy 2 contains a large amount of four different alloying elements, a sample of this alloy was chosen to study the effect of these extraneous elements on the luminescence of fractional ppm level of rare earths. The sample of Zircaloy 2 was dissolved in HzSO4, doped with fractional ppm level rare earths, and converted to ZrOz. This sample was coverted to the quaternary oxide phosphor and the XEOL spectra of the rare earths were examined. The XEOL spectrum indicated a reduction of the host band luminescence by a factor of two with no quenching in the rare earth line luminescence. The host band luminescence in this phosphor could be completely quenched a t 100 "C as compared to 200 "C for the pure ZrOz containing phosphor. The data from these experiments are tabulated in Table II. These experiments confirm our earlier observation (26) that these quaternary oxide phosphor hosts (25) can tolerate appreciable variations in the sample composition without affecting the analytical results. Received for review October 3, 1973. Accepted March 4, 1974.
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