cobalt-58 and cobalt-60 isotopes enable a correction to be applied from the amount of nickel determined previously. The results for cobalt were confirmed by the isolation and determination of cobalt with nitroso-R salt (disodium salt of l-nitroso-2-naphthol-3,6-disulfonic acid) using the method of ion exchange on alumina (3). CONCLUSIONS
Activation analysis is adequate for the determination of manganese, iron, and titanium in aluminum alloys following the semiroutine procedure as systematized at the Oak Ridge National Laboratory. The positive bias initially exhibited by the results for copper and nickel were eliminated by a silicon removal step in the nickel procedure and a chemical separation of the copper activity; only samples of high copper content exhibit a bias. The proper choice of counting conditions is pointed up by the error in the nickel determination, which was revealed only by the statistical treatment of the results. Beta absorption and decay measurements would not indicate the presence of silicon-31 in the separated nickel-65 fraction. Thus, the more discriminating method of gamma counting, whenever applicable,
is favored over the use of beta counting with its inherent lack of specificity (9). The accuracy of the activation analysis method appears to be better than lo%, generally 5 to 7%, for the major constituents of aluminum alloys. No attempt was made to verify the results for antimony, silver, or zirconium. They are given only as a n indication of the sensitivity of activation analysis The foregoing results and discussion may be used to decide on the feasibility of the method of activation analysis for the estimation of a given element in a n aluminum matrix. Rarely would activation analysis be considered a substitute for conventional methods of analysis for aluminum alloys, except possibly when only minute amounts of sample were available, such as inclusion, shavings, surface boundary studies, and similar problems. For these problems, the method proves invaluable.
LITERATURE CITED
(1) Boyd, G. E., ANAL.CHEX 2 1 , 335 (1949). (2) Brooksbank, IT. ii., Leddicotte, G.
W., Mahlman, H. A., J . Phys. Chem. 5 7 , 815 (1953). (3) Dean, J. A., ANAL.CHEM. 2 5 , 1096 (1953) \-
--/.
(4) Hollander, J. M., Perlman, I., Seaborg, G. T., Revs. Modern Phys. 2 5 , 469 (1953).
( 5 ) Hughes, D. J., Harvey, J. A., U. S.
Atomic Energy Comm., Rept. BNL-325
(1955). (6) Jenkins, E. N., Smales, A. A,, Quart. Reus. (London) 10, 83 (1956). (7) Leddicotte, G. W.,Reynolds, S. A., hucle~nics8, pu’o. 3, 62 (1951). (8) hlorrison, G. H., Cosgrove, J. F., ANAL.CHEJI. 2 7 , 810 (1955). (9) Plumb, R. C., Len-is, J. E., A-ucleonics 13, No. 8, 42 (1955). (10) Reynolds, S. A., Recoid Chenz. Progr. (Kresge-Hooker Sci. Lzb.) 16, 99 (1955). (11) Smales, A. A., Mapper, D., Wood, A. J., Analyst 82, 7 5 (1957). (12) Taylor, T. I., Havens, JV. W., iyucleonics 6 , No. 4, 54 (1950). (13) Youden, W.J., “Statistical Methods for Chemists.” DD. 40-9. Wilev, ?;em York, 1951. I
ACKNOWLEDGMENT
Thanks are due 31. T. Kelley for permission to use the facilities of the Analytical Chemistry division, to J. A. Cox, reactor superintendent, and to N. F. Sharp for her assistance in the counting room.
- 1
.
I
RECEIVED for review December 6, 1957. Accepted June 25, 1958. Taken from a thesis submitted by William A. Brooksbank, Jr., t o the Graduate School of the University of Tennessee in partial fulfillment of the requirements for the degree of master of science. 1956.
Spectrophotometric Determination of Uranium with 1-(ZPyridylazo)-Zna pht hol HAROLD H. GILL, R.
F. ROLF,
and G.
The Dow Chemical Co., Midland,
From 40 to 400 y of uranium can b e determined b y exttaction of uranyl nitrate into a tributyl phosphatechloroform solvent and subsequent color development in the organic extract with 1 -(2-pyridylazo)-2-naphtho1 (PAN) in the presence of pyridine. The disodium salt of (ethylenedinitril0)tetraacetic acid increased the selectivity of uranium extracticn b y chelating zirconium, bismuth, and manganese(l1). Thorium is removed as thorium oxalate prior to extraction. Synthetic solutions containing uranium and 37 diverse ions were analyzed b y the PAN and fluorornettic procedures. The PAN procedure gave an average and an uranium recovery of 99% average deviation of f 2 % . The fluorometric procedure gave an average recovery of 98y0 and an avAnalyses erage deviation of f4%.
1788
ANALYTICAL CHEMISTRY
W. ARMSTRONG
Mich. of alloys, ores, and organic extracts by the PAN and fluorometric methods are reported.
M
quantities of uranium usually are determined by a fluorometric method (1, 4, I S ) . In the authors’ laboratory, fluorometry is used for the estimation of up to 100 y, with a n average relative error of *47,. Data from the literature indicate the same order of precision. For determining somewhat larger quantities, it would be desirable to have a spectrophotometric procedure. This should be applicable to a wide variety of samples, require a minimum of manipulation, and show better accuracy and precision than the fluorometric method. Sumerous chromogenic reagents have been proposed for uranium determination (2, 7-9, 11, 15, 17, 18). DibenzoylICROGRAM
methane, 1- ( 0 - arsonophenylazo) - 2 naphthol-3,6-disulfonic acid (thoron), and 1-(2-p yridylazo) -2-napht hol (PAN) offer high sensitivity, but they can be applied only to solutions from n hich interfering ions have been removed. The dibenzoylmethane procedure of Yoe, Kill, and Black (18) required four ether extractions, removal of the ether by evaporation, and a filtration prior to color development. -4danls and 1Iaeck ( 1 ) and Francois (12) have reported modifications of the dibenzoylmethane procedure to shorten the determination time. The thoron procedure of Foreman, Riley, and Smith (11) requires the extraction of interfering ions iyith cupferron, followed by extraction of the uranium as the carbamate in chloroform, back-extraction into ammonium carbonate solution, evaporation to dryness, and reduction
of uraniuni(V1) to uranium(1V) prior t o color development. Cheng and Bray (6) investigated the use of P A S as an indicator for titrations of multivalent ions using the disodium salt of (ethylenedmitri1o)tetraacetic acid (EDTA). They reported that within varying p H ranges zinc, copper, nickel, cadmium, iron(II), iron(III), lead, thorium, thallium(I), silver, mercury(II), uraniuni(T’I), cobalt, and bismuth give pink or red precipitates v i t h PAN and that most of these precipitates are soluble in organic solvents. The PAXuranium chelate is reported to detect as little as 2 y of uranium. Cheng (5) reported that P A S could be used quantitatively to determine from 95 y to 119 nig. of uranium. He states that in an alkaline solution containing strong coinplexing agents only uranium precipitates upon the addition of PAX. The precipitated uranium is extracted into an organic solvent and the uranium determined spectrophotometrically at 570 mp. The color is reported to be stable and to follow Beer’s law over a concentration range from 0.095 to 119 mg. Eberle and Lerner (9) proposed the use of 8-quinolinol in chloroform as the chromogenic reagent for the determination of microgram quantities of uranium. Their procedure involves a tributyi phosphate extraction of uranium(VI), followed by an ammonium acetate stripping of the tributyl phosphate extract, extraction of the uraniuni(V1j from the ammonium acetate into chloroform containing 8-quinolinol, and spectrophotometric determination of the resulting uranium(V1)-quinolinol complex. Although the method is highly specific, it requires from nine to fourteen extractions, and a 2.7% loss of uranium is reported for each of the four nitric acid washings of the tributyl phosphate extract. Because of the nonspecific nature of the proposed chromogenic reagents, a preliminary separation of uranium is necessary. Various techniques involving the use of cellulose columns (S), ion exchange resins (IO, 16),and solvent extractions (7) have been proposed for the separation of uranium salts from complex mixtures. Eberle and Lerner (9) give an excellent literature review of the solvent extraction procedures used for the separation of uranium from complex mixtures. Solvent extraction appeared to be the most attractive process. After a consideration of the degree of extraction, selectivity, and availability of various solvents, tributyl phosphate vias decided upon for this work. APPARATUS AND REAGENTS
All spectrophotonietric measurements were made with a Beckman Model B spectrophotometer with 1-cm. cells.
of uranium per ml. were prepared by diluting the standard solution. PREPARATION OF STANDARD CURVE 0‘ 8
O
F
600 WAVE L E N G T H ,
mp
Figure 1. Absorbance of PAN-uranium(IV) complex 0
A W
PAN, chloroform blank PAN-uranium(VI), PAN blank PAN-uranium(VI), chloroform blank
The p H measurements were made n-ith a Beckman Model G pH meter. Reagent grade chemicals were used unless otherwise stated. EDTA (obtained from Fisher Scientific Co., Pittsburgh, Pa.) and potassium fluoride (obtained from Baker and .4damson, New York, I S.Y.) were used as received. d4LVTMINU?vf NITRATE SSLTING SOLUTION. This solution was prepared by dissolving 1800 grams of Mallinckrodt aluminum nitrate nonahydrate in 920 ml. of water. When 20 nil. are diluted with 10 ml. of distilled water, this solution should have an apparent p H of 0.0 to 0.3. If the diluted aluminum nitrate solution has a pH outside the 0.0 to 0.3 range, adjustment is made with concentrated nitric acid or ammonium hydroxide. TRIBUTYLPHOSPHATE 9% EXTRACTION SOLUTION.A 10-ml. portion of Eastman white label tri-n-butyl phosphate was added to 100 ml. of reagent grade chloroform. 1-(2-PYRtDYLAZO)-2-NAPHTHOL SOLU-
A 0.05% solution was prepared by dissolving 0.100 gram of 1,Zpyridylazo-2-naphthol (Eastman Kodak 7192) in dry methanol (conforming to ACS specifications), filtering through glass wool, and diluting to 200 ml. This solution is stable for several weeks if stored in an amber bottle. The reagent mas used as received. When a new lot of reagent was used, the standard curve was checked, as different bottles of reagent gave slight differences in absorption. STAXDARD URA4h-ICM SOLUTIOh’. Uranyl sulfate trihydrate, c.P., obtained from Amend Drug and Chemical Co., was assayed and found to have a purity of 99.9%. A standard uranium solution containing 1.000 mg. per ml. was prepared by dissolving 1.7655 grams of the salt in 100 ml. of water containing 10 ml. of nitric acid, and the solution was then diluted to 1 liter. Solutions containing 5.0, 10.0, 25.0, and 50.0 TION.
Transfer 0.0, 25.0, 50.0, 150.0, 200.0, 250.0, 300.0, and 400.0 y of uranium, in 10 ml. or less of solution, to a 150-ml. separatory funnel. Add 0.2 gram of potassium fluoride and 0.2 gram of EDTA, swirl t o dissolve, and dilute to 10 ml. Add 2 drops of methyl orange, and neutralize with 1 t o 1 ammonium hydroxide or 1 to 1 nitric acid to the indicator change or until a permanent precipitate is obtained such as when aluminum or iron are present. Add 20 ml. of the aluminum nitrate salting solution and 10 ml. of tributyl phosphate extraction solution, and shake for 2 minutes. To ensure complete removal of wher, filter the organic extract through absorbent cotton into a 25-ml. volumetric flask. Repeat the extraction using 5 ml. of tributyl phosphate solution, and then wash the aqueous phase with 3 ml. of chloroform. Add 5.0 ml. of the 0.05% PAX‘ solution and 0.5 ml. of pyridine to the combined filtered extracts, and dilute to volume with chloroform. Mix the flasks thoroughly, and after 15 minutes measure the absorbance a t 560 mp using the zero standard as a reference solution. Plot absorbance us. concentration of uranium on Cartesian coordinate paper. Fluorometric Comparison. After the absorbance oi the PAN-uranium complex has been measured, the uranium content can be checked independently by a fluorometric method. A 0.10-ml. aliquot from the 25-m1. volumetric flask is pipetted into a 1-em. platinum disk and then evaporated, To the residue in the dish, 0.4 gram of carbonate flux is added:, and a 5-minute fusion is made a t 610 C. After cooling 30 minutes, the fused buttons are removed from the dishes, and their fluorescence is measured on a fluorometer such as the GalvanekMorrison (Jarrell-Ash Co., Newtonville, Mass.). I n this work, the flux was a mixture of 45.5% sodium carbonate, 45.5% potassium carbonate, 8.98% sodium fluoride, and 0.02% lithium fluoride. PROCEDURE
Unknown uranium samples are prepared for analysis by the appropriate procedure. Then, for samples known t o be free of cerium(1V) and thorium, a suitable aliquot (1 to 10 nil.) containing from 40 to 400 y of uranium is treated as in preparation of standard curve. Samples containing thorium or c.eriuni(IV) require further treatment. Sample Preparation. AQUEOUS SoAqueous samples can be pipetted directly for extraction when all the uranium is known t o be present as uranium(T’1) and the solution is acid to methyl orange. To ensure complete oxidation t o uranium(VI), pipet a suitable aliquot into a 150-ml. beaker, neutralize with nitric acid and add 5 ml. in excess, add 15 ml. of hydrochloric acid, and boil LUTIOXS.
VOL. 30, NO. 1 1 , NOVEMBER 1958
0
1789
.
Table I. pH (apparent)
U present U found PAN Fluorometric
Effect of Acidity on Extraction of Uranium
2.2
100
1.4 100
l.3 100
0.8 100
o.5 100
100
47 77
105 96
104 103
99 96
99 95
101 100
until oxidation is complete. Cool, transfer to a suitable volumetric flask (50 or 100 ml.), and dilute to volume. ORGANICEXTRACTS.Pipet a 5- or 10-ml. sample into a 250-ml. Erlenmeyer flask, add several boiling chips and 5 ml. of concentrated sulfuric acid, and char on a hot plate for about 10 minutes. Cool, and add 10 ml. of concentrated nitric acid. Evaporate to fumes of sulfur trioxide, and repeat the nitric acid additions until the sample has a light yellow-brown appearance. Add 10 ml. of nitric acid and 1ml. of 70% perchloric acid, and heat to fumes of sulfur trioxide. If the solution retains a brown cast, repeat the nitric acid-perchloric acid treatment. When the destruction of organic matter is complete, cool, transfer to a suitable volumetric flask, and dilute to volume. ALLOYS. Weigh out a sample calculated to contain from 0.2 to 2.0 mg. of uranium and transfer to a 400-ml. beaker containing 10 ml. of water. For each gram of sample, add 10 ml. of nitric acid, 10 ml. of hydrochloric acid, and heat till dissolution of the sample. Evaporate this solution to a volume of 20 to 30 ml., cool, transfer the sample to a 50 to 100 ml. volumetric flask, and dilute to volume. This procedure is not suitable for alloys containing large amounts of refractory metals, such as zirconium. ORES. To 0.5 t o 2.0 grams of the crushed sample, add 15 ml. of hydrochloric acid, 5 ml. of nitric acid, and boiling chips. Boil until the oxidation
(2NHNo3)
100
the uranium determination is carried through as described in Preparation of Standard Curve, This procedure is designed for samples containing up to 20 mg. of thorium.
66 90
is complete. Dilute to approximately 50 ml., and filter through medium porosity paper. Ash the filter paper containing the insoluble residue and fuse with 4 to 8 grams of sodium carbonate. Dissolve the fused residue in dilute nitric acid, filter, and then combine with the original filtrate. Boil the combined solution 10 to 15 minutes, cool, transfer to a 100-ml. volumetric flask, and dilute to volume. Samples Containing Cerium(1V). Cerium(1V) is extracted by the tributyl phosphate extraction solution and forms a colored complex with PAN. The addition of an iron(I1) solution reduces cerium(1V) to cerium(II1) which is not extracted. To a n aliquot of an acidified solution containing 40 to 400 y of uranium, add 1 ml. of 0.1N iron(I1) sulfate. This addition reduces approximately 14 mg. of cerium(1V). Determine uranium as described in Preparation of Standard Curve. Samples Containing Thorium. When thorium is suspected of being present, the original sample solution, containing 2 ml. of nitric acid per 100 ml. of solution, is treated with 5 ml. of a saturated oxalic acid solution and brought to a boil. The solution is cooled, transferred to a 50- or 100-ml. volumetric flask, and diluted to volume. A portion of the sample is centrifuged for 5 minutes at 1500 r.p.m. An aliquot of 10 ml. or less is pipetted from the centrifuge tube into a 150-m!. separatory funnel, and
EXPERIMENTAL
Absorption Curves. The absorption curves of PAN, and PAN-uranium complex, and the PAN-uranium complex us. PAN were determined over the spectral range from 500 to 600 mp. These curves, shown in Figure 1, $yere obtained by measuring the absorbance of 250 y of uranium in 25 ml. of solution in 1.0-em. cells, using chloroform as a blank. The solutions were prepared as described under Preparation of Standard Curve. The maximum absorbance of the PANuranium complex us. PAN occurs a t 560 nw, The calibration plot for 40 to 400 y of uranium followed Beer's law. Cheng and Bray (6) in previous work using P A S as an indicator and Cheng ( 5 ) using PAN as a reagent for uranium had not reported the nature of the colored complex of PAN with uranium. Job's method (14) of continuous variation was used to determine the nature of the PAX-uranium complex. The experimental results are shown graphically in Figure 2; 2 moles of PAN will complex with 1 mole of uranium. Effect of Acidity on Extraction. The extraction of uranium with tributyl phosphate is reported over a wide range of nitric acid concentrations, from a p H of 3.0 to a normality of 4.7. Either too low or too high acidity caused low results for the uranium. At first it was thought t h a t the uranium was not being extracted completely from solutions of high acidity, but difference b e h e e n the authors' results and literature reports
I
I
I
Fiaure 2. Maximum absorbance vs. mole fraction of uranium
I
I
/
\
1
A. Absorbance of PAN-uranium complex A=. Absorbance of PAN Cm. Concentration of uranium, pmoles Cz. Concentration of PAN, pmoler Cm Cz = 2.00 pmoler
+
Cm
At maximum, C, -k C, = 0.325 *' cz = 2.08
C,
1790
0
ANALYTICAL CHEMISTRY
0
40 60 TIME (MINUTES)
20
80
IO0
Figure 3. Effect of fluoride on stability of PAN-uranium complex 0 W
Fluoride present Fluoride absent
of extraction at high aridity was finally found to be caused by incomplete color development in the organic phase. n hich contained too much acid. Further color de\elopmcnt could be obtained by adding more pyridine. However, to have a reproducible procedure requiring the same amount of reagents, the proper acidity is obtained by initial sample neutralization and control of the apparent pH of the aluminum nitrate salting solution. Table I shows the effect of varying apparent pH during extraction on the subsequent color development. This was determined by treating 10-ml. samples containing 100 y of uranium m-ith 0.2 gram of potassium fluoride, 0.2 gram of EDTA, and 20 ml. of the salting solution, and adjusting the resulting solutions to varying apparent pH’s by the addition of ammonium hydroxide and nitric acid. The uranium in these solutions was then determined by the usual procedure. Effect of EDTA and Fluoride. The possible use of fluoride as a complexing agent was investigated. The fluoride failed as a complexing agent, but it stabilized the PAK-uranium complex. The exact nature of the stabilization of color resulting from fluoride has not been determined. The effect of fluoride on color stability is shonn in Figure 3. Zirconium and manganese(I1) were extracted and gave an increased color with PA?;. EDTA m s used successfully t o complex these ions and prevent their extraction. Bismuth, although not extracted, causes considerable difficulty when present in large amounts by precipitating as bismuthyl nitrate (BiOKOs), which produces heavy emulsions. EDTA prevents the bismuth from precipitating by forming a soluble complex in the salt solution. I n the presence of EDTA, aliquots for extraction containing 50 y of thorium gave no interference, while 100 y gave a positive interference of 2070. Effect of Pyridine. I n the absence of pyridine, standard uranium samples gave erratic b u t stable absorbance. This was attributed t o varying concentrations of acidity in the chloroform-tributyl phosphate extracts. I n aqueous media the PAN-uranium complex forms in a neutral or basic solution. The addition of 0.5 ml. of pyridine gives a reproducible absorbance. Increasing amounts of pyridine cause a slight gradual increase in the absorbance. Diluents. Several diluents for the tributyl phosphate extraction of uranium were studied. Uranium was extracted n i t h iso-octane and ethyl acetate solutions of tributyl phosphate, but the PAN-uranium color could not be successfully developed for a colorimetric determination. As chloroform is convenient for multiple
extractions, there was no advantage in using another solvent. Stability. After the addition of pyridine, the absorbance of the PANuranium complex increased with time for approximately 15 minutes, a t which time full color development was attained (Figure 3). Absorbance measurements after 24 hours gave identical results. As samples are usually read shortly after full color development, no attempt was made to determine color stability over a long interval.
Table II.
(100
Effect of Diverse Ions -i
uranium added)
,
Ion Concn., Uranium MgJlO 111. Found 2 0 97 2 4 96 96 4 0 96 2 0 96 2 0 2 0 98 99 2 0 98 1 0
Ion
2 2 2 2 2 2 2 2 2 23 120 2 20 2 2 2 100 2 2 2 2 2 2 15 15 15 15 2 2
RESULTS
Zr
Standard quantities of a uranium(V1) solution were treated with many diverse ions, and the resulting solutions were analyzed for uranium. Solutions containing cerium(1V) or thorium were analyzed by the modified procedures, All other solutions were treated by the general procedure used for preparing the calibration curve. KO attempt was made to determine maximum allonable amounts of diverse ions. The results are shown in Table 11. The precision of the method was determined by analyzing eight synthetic solutions containing 100 y of uranium, These results showed a 95% confidence limit of *4.770 of the uranium present, After their absorbances n-ere measured, the above solutions were analyzed by
Table 111.
Ce +4 CeT--
Th Pllg
Ni
Sa
V as NaVO3
hIn+T
co
Bi
Sd Pr La
cu
Fe-+T C*O4--
c1so4 - -
PO, - 103MnOaCrOl--
1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
100 105 103 103 102
99
102 102
98
97 101
99 98 9;
101 100 103 103
0
0 0 0 0 0 0 0 0 0 0 0 0
99 97
102 100
97 98
101 100 99 103
99
Comparative Precision of PAN and Fluorometric Procedures (100-7 uranium standards)
U Found
Run 1 102
P F 97 P. P.4N procedure
2 101 108
3 103
96
4
97
101
5
100
96
6
97 97
7
8
97 95
102
97
1u = 2 . 4
9570 confidence = 4 . 7 F. Fluorometric procedure 1u = 4 . 3 95T0confidence = 8 . 6
the fluorometric procedure and a confidence limit of A8.670 was found. Results are shown in Table 111. Ores, alloys, organic extracts, and aqueous solutions containing unknown quantities of uranium were analyzed by the PAN and fluorometric procedures (Table IV). DISCUSSION
This method was not compared with other spectrophotometric procedures, as the existing methods are too timeconsuming or not applicable for the quantities of uranium being determined. The proposed method, in spite of the
modifications required for samples containing thorium or cerium(IV), is applicable to a wide variety of materials. The accuracy of the method a t the 957, confidence limit is t o =t4.7% compared to 1 8 . 6 7 , for the fluorometric method. After samples are in solution, one technician can analyze 40 samples in an 8hour day. Samples giving a lo^ absorbancebe analyzed i.e., 0.05 or less-can fluorometrically by using a portion of the remaining chloroform extract. Thus, microgram quantities of uranium can be determined spectrophotometrically. If insufficient color is developed, a fluorometric procedure can be directly VOL. 30, NO. 11, NOVEMBER 1958
e
1791
Table IV. Comparison of PAN and Fluorometric Procedures on Various Types of Materials
Type
yG Uranium
of
Material
PAN
Ores Aqueous Organic
Fluorometric
0,0024
0.002s
0.0100 1.55 1.10 1.28 0.0050 0.097 0.071 0.074 0.095
0.0110 1.52 0.90 1.30 0.0050 0.091 0.075 0,078 0.093
0 0046 0 0023
0,0043 0.0021
n ,057
n. O M
applied for determining submicrogram quantities. Considering the advantages of the tributyl phosphate extraction of uranium, the procedure might be modified to extract milligram quantities of uranium. The uranium could then be back-extracted into an aqueous phase, followed by reduction and subsequent volumetric determinatipn. The feasibility of the above approach is under investigation. LITERATURE CITED
(1) Adams, J. A. S., hfaeck, IT. J., ANAL. CHEhZ. 26, 1635 (1954). (2) Almassy, G., Nagy, Z., Straub, J., Magyar Tudomdnyos &ad. Kem. Tudmdnyok Osztdlyanak, Kozlemdnyei 5 , 257 (1954). (3) Burstall, F. H., Wells, R. A,, Analyst 76, 396 (1951). (4) Byrne, J. T., AXAL. CHEni. 29, 1408 (1957). (5) Cheng, K. L., Ibid., 30, 1027 (1958). (6) Cheng, K. L., Bray, R. H., Ibid., 27, 782 (1955).
(7) Crouthamel, C. E., Johnson, C. E., Ibid.,24, 1780 (1952). (8) Currah, J. E., Beamish, F. E., Zhid., 19, 609 (1947). (9) Eberle, A. R., Lerner, 11. K., Zbzd., 29, 1134 (1957). (10) Fbher, S., Kunin, R., Ibid., 29, 400 (1951). (11) Foreman, J. K., Riley, C. J., Smith, T. D., Analyst 82, 89 (1957). (12) Francois, C. A , , ANAL. CHEM.30, 51 (1958). (13) Grimaldi, F. S., Fletcher, M.,
Titcomb, J., U. S. Geol. Survey Bull.
1006 (1954). (14) Job, P., Ann. chzm. 9,,113 (1928). (15) Rodden, C. J., “Analytical Chemistry of the Manhattan Project,” AIcGrawHill, New York, 1950. (16) Seim, H. G., Morris, R. J., Frew, D. W.,AKAL.CHEM.29, 443 (1957). (17) Thomason, P. F., Xutting, L. A,,
Koskela, U., Bverly, W. 31.) U. S. A-itomic Energy Conimission ORNL-1641 (1955). (18) Yoe, J. H., Kill, F., Black, R. A., A N A L . CHEhf. 25, 1200 (1953). RECEIVED for review January 10, 1958. Accepted May 27, 1958.
Separation and Determination of Small Amounts of Rare Earths in Uranium CHARLES V. BANKS, JAMES A. THOMPSON, and JEROME W. O’LAUGHLIN lnsfitute for Afomic Research and Department of Chemistry, Iowa State College, Ames, Iowa
A spectrophotometric method for the determination of total rare earths in the range 100 to 200 y is presented. The chromophoric agent is 2-( 1,8dihydroxy 3,6 disulfo 2 naphthylazo)-benzenearsonic acid (arsenazo). The absorbance is measured a t 570 mp and p H 7.0. The main objective of this investigation was to develop a method of determining rare earths in the presence of large amounts of uranium and small amounts of iron. As both metals interfere in the proposed spectrophotometric method for rare earths, an ion exchange method of separation is presented which is based on the absorption of uranium(V1) sulfate and iron(ll1) thiocyanate on a quaternary ammonium anion exchange resin.
-
-
-
S
-
of rare earths in uranium by metallurgy groups of the Ames Laboratory necessitated an analytical method for the determination of small amounts of rare earths in uranium. The relatively low solubility of rare earths in uranium and the accuracy desired indicated that a spectrophotometric method would be satisfactory. OLUBILITY studies
1792
ANALYTICAL CHEMISTRY
Rare earths have been separated from uranium by gravimetric (d), extraction ( I % ) , and ion exchange (6) methods. Ion exchange seemed to lend itself readily t o this problem and a rapid method for separating small amounts of rare earths from uranium is reported. DETERMINATION OF RARE EARTHS
Many color-forming reactions including both group reactions for all the rare earths and specific reactions for individual rare earths, have been mentioned in the literature. Among the group reagents are murexide (3), 8quinolinol and substituted 8-quinolinol (16), and naphthazarin (20). Some methods for specific rare earths are those for cerium based on the oxidizing and chromophoric action of cerium(1V) ( I S ) , for ytterbium based on the reducing action of ytterbium(I1) (4), and for terbium based on the fluorescence of terbium when irradiated with ultraviolet light (8). Many of the rare earths have characteristic absorption spectra, but methods based on individual rare earth absorption bands are not very sensitive. +Hydroxyazo compounds, having an
arsono group in the second ortho position to the azo group, are capable of complexing many metals with a simultaneous sharp change in color (1, 6). Kuznetzov (17) illustrated the influence of the arsono group and also the necessary condition that it be ortho t o the azo group. He (18) also reported that rare earths form colored solutions in neutral or weakly acidic media vcith the compound 2-( 1,8-dihydroxy-3,6-disulfo2-naphthylazo)-benzenearsonic acid, which n-as called arsenazo. This compound has been named by several numbering systems, but the authors believe the original system used by Kuznetzov (18) is the simplest. The common names neothorin ( 7 ) and 2-(0arsonopheny1azo)-chroniotropic acid (14, 15) have also been used for this compound. Recently it has been used for the colorimetric determination of zirconium (19), thorium (14, 15), and fluoride ( 7 ) . A colorimetric titration procedure for determining rare earths with arsenazo has been outlined in the literature (18). Apparatus. Absorbance measurements were made with a Beckman, Model DU, quartz spectrophotometer, using matched 1-em. Corex or silica
