Table V. Results of Rock Analyses
Sample Sand,
Weight of sample 1.25 grams
95 keVb
310 keV
Average"
10.7 11.4
... ...
11.10 11.1
11.08 ppm =t 0.12
18.0 16.9
...
19.3 17.6
17.9 ppm f 0.50
... ...
0.964% f 0.016
... ...
1.176% =t 0.028
Sand2
720 mg
55.7 60.2
Xenotimet
2.99 mg
77.1 79.5
0.998 0.934
Xenotimez
9 . 9 mg
83.2 85.4
1.194 1.208
a
Th found 100 keV
55.6 50.9
AEB d
Mean of 4 results obtained on 2 different days.
* The 5 cm3 detector was used for these measurements.
...
0.986 0.938 1.092 1.211
Due to its higher resolving power in the lower energy region (see Experimental)
the 95 and 100 keV peaks could be integrated separately.
Table V shows the results obtained. The long cooling periods after irradiation, necessary in order to obtain accurate results for the xenotime samples, was to be expected because a spectrographic analysis of both samples showed the presence of most of the rare earth elements, some up to a concentration of 1 %. The low error of the results indicate the broad applicability of the method which ranges from very low thorium concentrations to very complicated matrices.
ACKNOWLEDGMENT The authors thank M. Wiernik and N. Lavi for assistance in the preparation of 0.01% thorium in sodium carbonate standards.
RECEIVED for review July 14, 1969. Accepted October 6, 1969. This work is part of an investigation performed by Mrs. M. Mantel in partial fulfillment of the requirements for a Ph.D. degree of the Hebrew University, Jerusalem.
Rapid Spectrophotometric Determination of Uranium in Ores T. NI. Florence and Yvonne J. Farrar Analytical Chemistry Section, Australian Atomic Energy Commission, Lucas Heights, N.S. W . , Australia
THEINTERNATIONAL Atomic Energy Agency recently organized an international comparative analytical survey of spectrophotometric methods for the determination of uranium in four ores having U308contents in the range 0.3 to 0.5 %. Despite the relatively high uranium concentration in these ores, there was considerable disagreement in the results from the various laboratories, and the survey brought to light some difficulties associated with each reagent, particularly dibenzoylmethane. A need exists for an improved, versatile, spectrophotometric method for determining uranium in ores. This method should be rapid, simple, precise, and applicable without modification to all types of ores with U308contents as low as 0.01 %, which is probably the limit of commercial interest. We recently described a highly sensitive spectrophotometric method for uranium ( I ) using the reagent 2-(2-pyridylazo)-5-diethylaminophenol (PADAP) (2). By combining the PADAP method with a preliminary tri-n-octylphosphine oxide (TOPO) extraction of uranium from nitric acid solution ( 3 , 4 ) ,a specific method was developed for the determination of uranium in (1) T. M. Florence, D. A. Johnson, and Y. J. Farrar, ANAL. CHEM., 41, 1652 (1969). (2) S. I. Gusev and L. M. Shchurova, Zhur. Anal. Khim., 21, 1042 ( 1966). (3) J. C White and W. J. Ross, AEC Report NAS-NS 3102, 1961. (4) R. J. Battisberger, ANAL.CHEM., 36, 2369 (1964).
ores. Because of the high sensitivity of the PADAP reagent, small sample weights can be used, thus avoiding long dissolution times. With a 100-mg sample, the sensitivity of the method is 15 ppm of US08 in an ore. The method was applied successfully to a wide variety of ores with U308contents ranging from 0.02 to 0.8 Z. EXPERIMENTAL Reagents. Synthesis of PADAP (zinc complex) has been described previously ( I ) . The reagent solution was 0.05 % Zn-PADAP in ethanol. The mixed complexing solution described earlier ( I ) was used after diluting it 1:2 with water. A triethanolamine buffer was prepared by dissolving 149 grams of triethanolamine in 800 ml of water, neutralizing to pH 8.75 with perchloric acid, and diluting to 1 liter with water. A 0.1M TOPO solution was prepared by dissolving 19.3 grams of tri-n-octylphosphine oxide in 500 ml of cyclohexane. Procedure. Weigh 0.1 gram of ore (minus 200 mesh) into a platinum dish, add 5 ml of 15M HNO, and 5 ml of 40% HF, and evaporate to dryness on a water bath. Add a further 5 ml of 15M HNOBand 5 ml of 40% HF, and again evaporate to dryness on the water bath. Add 2 ml of 72 % HC104 and fume on a hot plate until the volume of HClO, is less than 1 ml. Cool and dissolve the residue in 20 ml of 5M HN03. Filter the solution through a small paper into a 100-ml volumetric flask. Ash the filter paper in the platinum dish, add
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Table I. Analysis of Ores Re1 u308, No. of std Ore proposed deter- deviation, U308,reference no. method, % method, % minations 1 0.867 4 0.6 0.80", 0.8381 2 0.465 2 1 .o 0,4471 3 0.423 4 2.1 0.41" 4 0.252 6 0.9 0.2411 5 0.221 4 1.9 0. 24e 6 0.145 4 0.7 0. 1208,0.1388 7 0.090 4 2.2 0.0889 8 0.085 4 1.2 0,0840 9 0.067 2 1.2 0,070" 10 0.064 4 3.1 0.0658 11 0.040 14 4.3 0.040" 12 0.030 4 4.1 0.029'~ 13 0.024 4 6.5 0.019e,0.035p 14" 0.472 2 1.7 0. 476h, 0 . 46e 15* 0.424 2 0.5 0.419", 0.42e 0.376h,0.36e 16c 0.370 12 1.8 17d 0.316 2 1.0 0.314*, 0.296 a IAEA sample, Torbernite (Australia). * IAEA sample, Carnotite (U. S. A,). c IAEA sample, Uraninite (Australia). d IAEA sample, Torbernite (Spain). e X-ray fluorescence. PADAP spectrophotometric after hexone extraction. 0 Fluorimetric after hexone extraction. Dibenzoylmethanespectrophotometric,mean value from IAEA survey. 0.3 gram of sodium carbonate, and fuse for 5 minutes. Dissolve the melt in 20 ml of 5M H N 0 3 and add to the 100-ml volumetric flask. Dilute to volume with water. If the sodium carbonate fusion step is omitted, add an additional 20 ml of 5M H N 0 3 to the 100-ml flask after filtering the residue from the perchloric acid fuming, then dilute to volume. Pipet an aliquot (10 ml to 100 mi) containing less than 150 pg of uranium (75 to 125 pg optimum for 1-cm cell) into a separating funnel. Add 2.0 ml of 2 % sodium fluoride and 2.0 ml of 5 % ascorbic acid, freshly prepared. Mix, pipet in 5.00 ml of 0.1M TOPO, and extract for 1 minute. Allow the phases to separate and run off the aqueous phase. Pipet 2.00 ml of the organic phase into a dry 2 5 4 volumetric flask. Add 1.00 ml of complexing solution, 2.00 ml of PADAP solution, and 1.00 ml of triethanolamine buffer, in that order. Mix thoroughly after each addition. Stopper the flask and allow to stand for 10 minutes. Add 18.0 ml of reagent grade ethanol, with swirling. Dilute to volume with water. Measure the absorbance at 560 nm in a 1-cm or 4-cm cell against a blank which has been carried through the extraction procedure. RESULTS AND DISCUSSION
TOPO Solvent Extraction Step. In agreement with White and Ross (3)we found that essentially complete (>99%) extraction of uranium was obtained within an acidity range of 0.5 to 3M HN03. A nitric acid concentration of 2 M was chosen in preference to lower acidities because reduction of vanadium(V) occurred more readily at high acidities, and hydrolyzable metals were easily kept in solution. The volume of the aqueous phase was varied between 10 ml and 100 ml, using 5 ml of TOPO-cyclohexane, but this had no significant effect on the per cent extraction. The sodium fluoride and ascorbic acid masking agents did not hinder the extraction of uranium. Cerium(1V) and vanadium(V) extracted appreciably into plus fluoride, whereas the TOPO-cyclohexane from 2M"O3 272
lower valency states did not. Several reducing agents were satisfactory for the reduction of cerium(1V) to (111) in 2 M "03, but ascorbic acid was by far the most efficient for reduction of vanadium(V) to (IV). Using the recommended procedure, the following elements were tested at the levels indicated, and gave less than =t1.5% error (one standard deviation) in the spectrophotometric determination of 98 pg of uranium: Al, Ce(IV), Cr(III), Mo(VI), Nd, Th, Ti(IV), V(V), Zr (10 mg); Fe(II1) (20 mg); Fe(II1) (20 mg) 4- V(V) (10 mg); P04-3(2 mg); 72 % HC104(1 ml). PADAP Spectrophotometric Method. Ethanol was the best of several solvents tried for maintaining a homogeneous solution between the buffer, complexing solution, and 2.00 ml of 0.1M TOPO-cyclohexane. To prevent phase separation the minimum volume of ethanol needed is 19.0-19.5 ml per 25-ml volume. The molar absorptivity for uranium is almost constant with 19.5-20.5 ml of ethanol, but decreases rapidly at higher ethanol concentrations. Constant absorbance readings were obtained when the pH of the buffer was between pH 8.6 and 8.9. Problems of variable sample acidity do not arise, because 0.1M TOPO extracts a constant concentration of acid (approximately 0.1M) fiom 2 M H N 0 3 (3). Since the parameters of the spectrophotometric method were optimized using a 2.00-ml aliquot of TOPO-cyclohexane, it is important that the aliquot size should not be varied. The uranyl-PADAP color developed within 2 minutes, and was unchanged after 21 hours. Beer's law was obeyed up to 2.8 pg of uranium per ml in the final solution and an absorbance of 0.365 (1-cm cell) was obtained when 100 pg of sranium was carried through the complete procedure. Application to Ores. Table I shows results on seventzen different ores which were selected to cover a wide variety of matrix types. Included in Table I are four ores distributed by the International Atomic Energy Agency for international comparison of analytical methods. Four other methods were used to analyze the ores so that results could be cornpared with the proposed new spectrophotometric procedure. These methods were: nondestructive X-ray fluorescence using background scattering techniques to correct for matrix interference (9,fluorimetry after extraction of uranium into hexone from calcium nitrate-EDTA medium (6), dibenzoylmethane spectrophotometric (7), and PADAP spectrophotometric after extraction of uranium into hexone from calcium nitrate-EDTA ( I ) . To decide whether or not a sodium carbonate fusion step is necessary in the dissolution, the ores listed in Table I were reanalyzed with sodium carbonate fusion omitted. Although significant residues remained after the HF-HN03-HC104 acid attack on the samples, only ores No. 12 and 13 gave results lower (0.024%, and 0.014% U,Os, respectively) than those obtained with the fusion step. Clearly, sodium carbonate fusion is unnecessary in most cases, but this fact should be established for each type of ore body before fusion is omitted in routine analyses. Direct attack on the ores by sodium fluoroborate was also successful. The most refractory ores in Table I were fused with sodium fluoroborate (1 g per 100 mg ore) for 5 minutes. The cooled melt was dissolved in nitric acid, and uranium determined by the recommended procedure. No residue remained after the fusion, and the uranium results were in agreement with those obtained previously. Fusion with sodium fluoroborate is a ( 5 ) K. P. Champion, C. E. Matulis, and R. N. Whittem, Australian
At. Energy Comm. Rept. TM/446, 1968. (6) C . J. Rodden, "Analysis of Essential Nuclear Reactor Materials," U. s. At. Energy Comm. (1964). (7) P. Blanquet, Chim.Atial. (Paris), 41, 247 (1959).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 2, FEBRUARY 1970
rapid and convenient dissolution technique, but when a large number of samples must be handled simultaneously the HFHN03method requires less operator time per sample. Precision and Sensitivity. The standard deviation of the spectrophotometric method was determined by carrying ten aliyuots, each containing 100 p g of uranium, through the extraction and spectrophotometric procedures. The relative standard deviation obtained was =t1 . 3 z . The practical molar absorptivity of uraniurn in the final solution was calculated to be 5.45 X 10'. This is lower than the practical
molar absorptivity found for uranium in the normal PADAP method, 6.59 X l o 4( I ) , but the sensitivity is still much higher than that given by dibenzoylmethane or thiocyanate. Limit of detection, calculated from the standard deviation of the blank, was equivalent to a U30, content of 15 ppm in an ore, using a 100-mg sample.
RECEIVED for review August 18, 1969. Accepted October 27, 1969.
Gas Chromatographic Determination of SuHide, Sulfite, and Carbonate in Solidified Salts J. R. Birk, C. M. Larsen, and K. G . Wilbonm' Atomics International, A Division of North American Rockwell Corporation, P. 0. Box 309, Canoga Park, Calif. 91304 SEVERAL TECHNIQUES for the determination of sulfide and sulfite in the presence of one another have been suggested (f-5). For the most part these procedures are quite tedious and time consuming. In addition, erroneous results are caused by undesired alteration of the components during analysis and indirect measurement or measurement by difference. The molten carbonate process for the removal of sulfur dioxide from flue gases (6) has prompted the need for a rapid, sensitive, and reasonably accurate technique for the determination of minor quantities of sulfide and sulfite (1-20z) in the presence of large amounts of carbonate. A gas chromatographic technique was developed which depends upon the evolution of hydrogen sulfide, sulfur dioxide, and carbon dioxide from the sulfide, sulfite, and carbonate, respectively, when the sample is acidified. EXPERIMENTAL
Pure anhydrous sodium sulfide (analytical reagent) was obtained from Research Organic/Inorganic Chemical Co. This and all other chemicals which were also reagent grade were used without further purification. A Loenco Model 15A gas chromatograph in conjunction with a thermal conductivity detector was used for this work. The column of Poropak Q (Waters .4ssociates, Inc.) was 6 ft long and was operated at 100 "C with a helium flow rate of 60 cc/niin. The retention times for air, carbon dioxide, hydrogen sulfide, and sulfur dioxide were 40, 61, 155, and 316 seconds, respectively. Solidified samples and standards were obtained by drawing the fused salt mixture into 5-mm quartz tubes, which were subsequently broken after the melt solidified. The cylin1
Present address, Marquardt Corporation, Van Nuys, Calif.
(1) A. Kurtenacker and E. Goldbach, 2.Anorg. Allgem Chem., 166,
75,"H2S04-
/-------
/TO
ig I=
CHROMATOGRAPH\
I
1
REACTIONVESSEL
Figure 1. Reaction vessel and sampling system drical pieces of salt had a uniform composition and were more easily handled than powdered mixtures. The standard samples were prepared by combining the constituents-i.e., NazS or Na2S03with a eutectic mixture of Na2C03, K2C03, and Li,C03, mp 397 OC-in a quartz vessel and heating to a temperature of 425 "C in an inert atmosphere. Since the specific cation is unimpcrtant in the analysis, the combination of the alkali ions (K, Na, and Li) will be referred to hereafter in this paper as M-Le., MzC03, M2S03,and M2S. The procedure consisted of the following steps. The sample was weighed, placed in the reaction vessel shown in Figure 1, and the system evacuated. Value A was closed and 1.0 ml of a 7 5 z sulfuric acid solution was added to the sample. After dissolution the mixture was heated with a small hemispherical heating mantle at 200 "C for 1 minute. Value B was then closed, A was opened, and a 5.0-ml portion of the total volume (81.9 ml) was placed in the carrier-gas stream by means of a Perkin-Elmer sampling valve (Model 008-0659) and then analyzed.
177 (1927); C.A., 22, 362 (1928).
(2) J. H. Karchmer and J. W. Dunahoe, ANAL. CHEM.,20, 915
RESULTS AND DISCUSSION
(19.18). (3) P. Kivalo, ibid., 27, 1809 (1955). (4) R. Wollak, 2. A n d . Chem., 77, 401 (1929); C.A., 23, 4643 (1929). (5) I. M. Kolthoff and R. Belcher, "Volumetric Analysis," Vol. 111, pp 299-302, Interscience, New York, N. Y.,1957. ( 6 ) L. A. Heredy, D. E. McKenzie, and S. J. Yosim, U. S. Patent 3,438,722 (1969).
The selection of the most appropriate acid for the gas evolution reaction was of considerable importance. Ideally, the acid should be fast reacting witn the salt; unreactive with the evolved gases; nonvolatile to avoid interfering chromatographic peaks and equipment deterioration; and hygroscopic to avoid, during heating, the evolution of water which
ANALYTICAL CHEMISTRY, VOL. 42, NO. 2, FEBRUARY 1970
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