cessive extractions were made with 20-ml aliquots of dithizone solution until the organic layer was pale red, indicating the end of the extraction. All of the organic layers were combined, and the aqueous layer was discarded. The tartaric acid served as a masking agent fcr possible remnants of any interferences not removed by the earlier acid extractions with dithizone. The extraction with dithizone was done at pH 13-14 because of the stability of cadmium dithizonate and instability of zinc dithizonate in very basic solution. Separation of Cadmium from Zinc. The collected organic layers were transferred to a 250-ml separatory funnel and washed with 30 ml of distilled water to remove any traces of entrained 24N2. The cadmium and zinc were backextracted into the aqueous phase with 10 ml of 1 M HCI. Four grams of NaSCN were dissolved into the solution containing the cadmium and zinc, and zinc was separated from the cadmium by extraction into a 1:4 mixture of isoamyl alcohol and anhydrous diethyl ether. The extraction of the zinc into etheral isoamyl alcohol was done three times, using 20-ml, 10-ml, and 10-ml aliquots. One molar HC1 back-extracts cadmium and zinc dithizonates rapidly and quantitatively. Cadmium was separated frolm the zinc by the preferential solubility of what is theorized to be zinc thiocyanate in the etheral isoamyl alcohol. Preparation of Standards and Processed Samples for Counting. Standards were prepared for counting by rinsing the irradiated vial 4-5 times with 0.1N HC1 and decanting each time carefully into a counting vial. The volume of the decantate in the counting vial was brought up to 25 ml. The processed tissue samples were prepared for counting by transferring the aqueous solution (which contained the radiochemically pure 11Cd) from a separatory funnel to a counting vial and diluting up to a final volume of 25 ml. Measurement of the ll5Cd Activity. The l W d activities in the samples and standards were determined by counting the combined 0.53-0.49-MeV gamma-ray photopeak. Counting
times ranged from 10 to 30 minutes. The only gamma rays present in the sample were due to 115Cd-11jInmand 109Cd. No other radionuclides were observed. Determination of Chemical Yield. The chemical yield of the cadmium was determined by comparing the net activities of the 88 keV gamma-ray photopeak of the lo9Cdstandard and that of the lo9Cdtracer that had been introduced into the irradiated tissue sample at the beginning of the radiochemical separation procedure. Chemical yields of the cadmium ranged between 50-65 %. Cadmium-109 was a suitable radionuclide to serve as a chemical yield indicator because of its long half-life (453 days) and because it emits a gamma-ray at an energy not in conflict with any gamma-ray emission characteristics of lljCd. In addition, the level of lo9Cdactivity introduced to the sample is high enough so that the induced I W d activity does not interfere with lo9Cdmeasurement. RESULTS
Twelve tissue samples from the same postmortem were analyzed for cadmium; specifically, five thigh-muscle, four kidney, and three lung samples. The five thigh muscle samples gave results of 0.35 f 0.03, 0.35 f 0.02, 1.00 f 0.06, 0.27 f 0.01, and 0.37 5 0.01 ppm cadmium (dry weight basis). The four kidney samples contained 450 i 10, 185 =t 5, 270 f 5, and 460 10 ppm cadmium; and the three lung samples 4.7 =t 0.4, 3.5 f 0.3, and 4.9 =t1.1 ppm cadmium. The associated errors are calculated from counting statistics only. The expected accuracy is about 5%. A practical lower limit of detection of the technique is 50 ppb cadmium when using a neutron flux of 1 X 1013m/cm2 sec, an irradiation period of 100 hours, and a decay period of 4 days. RECEIVED for review August 12, 1969. Accepted November 7,1969.
Neutron Activation Analysis of Thorium in Rocks and Ores by Multiple y-Ray Peak Ratio Determination Mariana Mantel, Propai Sung-Tung,’ and Saadia Amiel Nuclear Chemistry Department, Soreq Nuclear Research Centre, Yavne, Israel ACTIVATION ANALYSIS of thorium is mostly based on the nuclear reaction : P
232Th (n,y)Z33Th 9 233Pa 22.4min
6 27.0d
The determination of trace amounts of thorium as found in nature in rocks and ores usually involves chemical processing of the irradiated samples, with subsequent measurement of *33Th (1-3) or 233Pa(4-6) by beta counting or gamma scintillation spectrometry. However, in view of the inconvenience 1 Present address, Office of Atomic Energy for Peace, Bangkok, Thailand.
(1) E. N. Jen!cins, Analyst (London), 80, 301 (1955). Appl. . Radiat. Isotopes, (2) G. W. Smith and D. M. Morgan, Int. .I 16, 81 (1965). (3) H. Stark and C. Turkowsky, Radiochim. Acta, 5, 16 (1966). (4) Y.C. Schiltz and C. Coquema, BUN. Soc. Fr. Mitieral Crist., 87, 156 (1964); Anal. Abstr., 12, 2131 (1965). (5) G. L. Bate, Y . R. Huizenga, and H. A. Portratz, Geockim. Cosmochim. Acta, 16,88 (1969). (6) Wakita Hiroshi and Kigoshi Kunihiko, J. Chem. Soc. Japan, Pure Chem. Sect., 85, 476 (1964); Atial. Absfr., 12, 6589 (1965).
of performing chemical separations [the common procedures are paper chromatography ( I , 4, 7), solvent extraction (5,6) or ion exchange (8, 2 , 311 in rock analysis, especially when handling many samples, an instrumental analysis would be of great advantage. Nondestructive activation analysis techniques reported so far are delayed neutron counting from thorium fission by Amiel (9), and high resolution lithium-drifted-germanium y-ray spectrometry as mentioned recently by Gordon et a/. (10) and Olin and Sayre (11). (7) R. Coulomb and Y . C. Schiltz, “Radiochemical Methods of Analysis,” Vol. 11, Vienna, 1965, 177. (8) Y . W. Morgan and Y . F. Lovering, Atzal. Chim. Acta, 28, 405 (1963). (9) S. Amiel, A N ~ LCHEM., . 34, 1683 (1962). (10) E. G. Gordon, K. Randle, E. G. Coles, B. Y . Corliss, H. M. Beeson, and S. S . Oxley, Geoclzim. Cosmochim. Acta, 32, 369 (1968). (11) S. J. O h and V. E. Sayre, International Conference of Modern Trends in Activation Analysis, National Bureau of Standards, Gaithersburg, Md., October 1968, pp 207-21 3.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 2, FEBRUARY 1970
267
~
~~~~
~~
~
~
~~~~
Table I. Photopeak Area Ratios of 2 3 3 P a ~ 110/300 keV 3001310 keV 100/310 keV 1001340 keV 100/300keV l00jllO keV 3101340 keV 3.40i0.12 2.23=tOo.07 21.90zk0.67 0.18i0.007 9.823~0.29 3.57+0.09 12.15+00.31 The results are the average of 10 separate determinations, using the 15 cm3 Ge(Li) detector. The error limits given are one standard deviation calculated from 10 determinations. 0
"U I ' ~ ~ " ' " 60" " " " "
I03
150
'
"
I
203
'
"
I
250 '
11'4
L
"
300
ChonW Nmter
Fignte 1. Gamma ray spectrum of 2a8Pa obtained by the irradiation of Specpure Tho2 15 cc Ge(Li) detector, 3.99 mg Th, 5 min irradiation, 40 min counting time, 29.48 days AEB
In the present work a n attempt is reported to develop a method for the assay of Z33Pa, by calculating a series of selfconsistent gamma peak ratios (12). The method offers high specificity and accuracy even in very complex matrices. EXPERIMENTAL
Irradiation. The irradiations were carried out in the pneumatic tube or in the core of the IRR-1 reactor a t neutron fluxes of 4 x l O l 2 and neutrons/cm2 sec, respectively. For irradiations in the pneumatic tube, samples containing thorium of the order of 100 pg were weighed into a small polyethylene bag, sealed, introduced into a polyethylene vial, and irradiated for 20 minutes. For irradiations in the core, samples containing 1-10 pg thorium were wrapped in aluminum foil, introduced into a quartz ampoule, and irradiated for 6 hours. Counting. Two Ge(Li) detectors were used: a 15 cm3 coaxial detector (Ortec USA) and a planar 5 cm3 detector (Yissum, Israel). The output signals from one of the detectors were passed through a preamplifier (Ortec Model 118A) and a linear amplifier (Ortec Model 410). The resulting pulses were analyzed by a 400 channel analyzer. The resolution of the system for the 136 keV and 1333 keV gamma rays of "Co and 6oCowas 3 keV and 4.2 keV (FWHM), respectively, for the 15 cm3 detector and 2.5 keV and 4.0 keV for the 5 cm3 detector. Standards. For samples of high thorium concentration (more than 100 ppm) thorium oxide was used as a standard (Johnson Mathey Specpure reagent). For low thorium concentrations (from 0.1-100 ppm) a standard of 0.01% Th in calcium carbonate was prepared by vacuum drying of a fine suspension of solid CaC03 in a thorium nitrate solution containing 0.01 thorium. (12) M. Mantel, J. Gilat, and S. Amiel, J. Radioanal. Chem., 2, 395 (1969). 268
Procedure. The samples were homogenized, weighed, irradiated, and counted as described above. A suitable quantity of the appropriate standard was irradiated together with each sample and counted. The intensities of the Z3"aa peaks were found by subtracting the background and the Compton contribution of the higher energy peaks from the integrated number of counts under each of the photopeaks concerned. A procedure proposed by Guzzi et al. (13) which has already been applied successfully by us (12) was used. The lower edge of the peaki.e., the first channel included in the integration-was chosen as the first channel whose neighboring (higher) channel had a number of counts exceeding its own by more than the statistical counting error. Similarly, the upper edge was taken as the last channel in which the number of counts was less than in the preceding channel by more than the statistical counting error. The ratios between the integrated areas of the different photopeaks were calculated and compared with those obtained for a pure thorium standard. The thorium concentration in the sample was calculated from the photopeaks whose ratios corresponded to those of pure thorium. RESULTS AND DISCUSSION
Selection of Gamma Rays. The relatively high thermal neutron activation cross section ( r = 7.4 barns) of the radiative capture reaction and the 100% abundance of the naturally occurring 232Thisotope make it possible to obtain high sensitivities in the determination of thorium by neutron activation. Furthermore, because of the high resolving power of the lithium-drifted-germanium detectors, at least 10 different gamma-ray peaks can be well resolved in the spectrum of 233Pa. Figure 1 shows the spectrum obtained from a n irradiated sample of Specpure Tho*. The areas of the 5 principal photopeaks (100, 110, 300, 310, and 340 keV) were integrated and their ratios calculated. Table I summarizes the results obtained for the 7 ratios chosen. In a complex spectrum, the gamma ray peaks whose energies correspond to those shown in Figure 1 will belong to *33Pa if their ratios are equal (within the error limits) to those calculated for the thorium standard. The advantage of this method of ratio determinations is that it allows one to decide immediately-from a single measurement-if a particular photopeak corresponds to 233Paand is free of interferences, without resorting to the usual method of half-life determination which in the case of 233Pa ( t l j n = 27.0 days) would be time consuming. When many interfering peaks are present, the relatively long half-life of 233Pa permits variation of the delay between the irradiation and counting of the sample until agreement is found between all or most of the photopeak ratios calculated for the sample and the standard. In this way up to 10 independent results for the concentration of thorium in a given sample may be obtained from a single experiment (if all 10 photopeaks are con(13) G. Guzzi, Y.Pauly, F. Girardi, and B. Dorpema, Euratom Report EUR-346@, 1967.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 2, FEBRUARY 1970
sidered). Thus the accuracy of the method is much increased, and at the same time an immediate check on the results is possible. It is obvious that the number of results depend upon the number of photopeaks which may be integrated. Interferences. The present work was concerned primarily with the determination of thorium in complex matrices such as ores. Because the number of additional elements which could be present in such matrices is as many as the periodic table can offer, it was impossible to study the interference of every separate element. But because thorium and uranium occur together in most ores, and in fact are sought for together, the influence of uranium and its fission products on the determination of thorium was studied. Three samples of different uranium-to-thorium ratios (1 :1, 2 :1, and 1 :2) were prepared through mixing of appropriate quantities of ThOs and U308in a mortar. The samples were irradiated and counted as described above, and the photopeak area ratios calculated and compared with those obtained for the T h o 2standard. According to the results obtained and as seen from Figure 2, three days after irradiation thorium may be determined in a mixture containing up to 1 : l UjTh, from the 300, 310, and 340 keV photopeaks. The 100 and 110 photopeaks are partially covered by the strong 105 and 115 keV peaks of zs9Np produced from uranium by neutron activation (12). Only after a delay of 18-20 days after irradiation can thorium be determined according to all 5 principal photopeaks of 233Pa. As the ratio UjTh increases, the interference of the 239Np photopeaks is enhanced and the delay after irradiation, necessary in order to obtain accurate results, increases. Thus for a
1IL"
P
239
VP
228 I
239Nc
r
i
c
A
N UUI
.
c Channel ndrnber
Figure 2. Comparison of gamma-ray spectra of a mixture containing 1 :1 UjTh measured at different times after irradiation 5 cc Ge(Li) detector, 2.5 mg sample, 1 min irradiation, 40 min counting time A = 3 days AEB; B = 9 days AEB; C = 20 days AEB
Table 11. Photopeak Area Ratios for Thorium Ore Samples Compared with Those for Pure T h o z l00/llO keV ThOi
3.57i0.09 Th Z
AEB
1.0
9d 1Od 15d 16d 20d 21 d 22d 9d 14d 16d 21d 22d 7d 14d 16d 20d 22d
0.1
0.02
100/300 keV ThOs 12.15f0.31
100/310 keV ThOs 2.23i0.07
100/340 keV ThOi 21.90rt0.67
Th ore
Th ore
Th ore
Th ore
3.21 f 0.08 3.30 f 0.08 3.72 f 0.08 3.48 i 0.08
8.78 f 0.40 10.65 i 0.30 11.67 f 0.30 11.97 f 0.30
1.67 f 0.06 2.10 f 0.08 2.18 f 0.08 2.30 rt 0.08
15.45 rt 0.5 18.86 f 0.6 22.03 f 1.0 21.40 f 1.1
4.35 f 0.11 3.63 f 0.08 3.43 f 0.08
13.05 f 0.30 11.39 f 0.30 11.50 f 0.30
2.79 rt 0.09 2.37 f 0.08 2.30 =!= 0.07
21.40 & 1.0 21.31 1.1 22.07 5 1.0
4.38 f 0.11 3.36 f 0.08 3.69 f 0.09
11.80 f 0.30 11.82 f 0.30 12.36 f 0.30
3.23 f 0.12 2.37 f 0.08 2.27 f 0.09
27.9 f 0.9 22.69 f 0 . 9 22.08 + 1.0
110/300 keV ThOz 3.40f0.12
110/310 keV ThOz 0.66rt0.02
300/340 keV ThOz 1.72 f 0.07
Th ore 2.64 zk 0.11 2.67 f 0.11 2.85 f 0.11 2.73 rt 0.11 3.24 & 0.13 3.26 f 0.13 3.35 f 0.12 2.63 f 0.11 3.40 =!= 0.14 3.10 & 0.12 3.28 f 0.13 3.53 f 0.14 1.89 =!= 0.07 2.95 f 0.12 3.42 f 0.14 3.52 f 0.14 3.60 rt 0.14
Th ore 0.49 rt 0.01 0.47 f 0.01 0.58 f 0.02 0.59 f 0.02 0.61 f 0.02 0.66 =!= 0.02 0.65 f 0.02 0.49 f 0.02 0.69 f 0.02 0.64 f 0.02 0.70 f 0.02 0.69 f 0.02 0.35 f 0.01 0.71 f 0.02 0.73 f 0.02 0.70 f 0.02 0.62 =!= 0.02
Th ore 1.49 f 0.09 1.80 f 0.12 1.79 rt 0.12 1.76 i 0.12 1.77 rf 0.14 1.80 f 0.14 1.70 i 0.13 1.52 f 0.12 1.67 i 0.13 1.64 f 0.12 1.83 f 0.12 1.79 f 0.13 1.24 f 0.11 1.91 f 0.13 1.79 f 0.11 1.80 f 0.12 1.92 f 0.13
Table 111. Thorium Concentrations in Standard Ores Th Weight of Weight specified sample mg of Th pg 1. 0
9.7 20.1
97 201
0.1
100. 1 500
100
0.02
100
100
keV 94 194 112 106
Weight of Th found ( p g ) 110 300 310 keV keV keV 104 205 94 99
99 214 103 105
102 211 99 97
340 keV
Average (pg)
107 201 116 103
101 2.2 207rt 5.0 105 =!= 4.1 1 0 2 5 1.7
*
Z Th found
Result obtained by delayed neutrons Th)
*
1.09 f 0.05
1.041 0.02 1.032 f 0.035 0.105 =!= 0.004 0.0204f 0.0003
(x
0.112 f 0.011
ANALYTICAL CHEMISTRY, VOL. 42, NO. 2, FEBRUARY 1970
269
Table IV. Precision and Accuracy of the Method Precision, Accuracy, Th, pg AEB d Resulta p g 99.6 23.9 102.0 12 . 5 12.45 f2.41 100.8 20.6 101.9 f 1.4 f 1 . 3 7 +1.09 9 9 . 5 f 2.2 99.9 27.9 f2.22 -0.4 100.3 35.2 100.8 i 2.1 12.09 fO. 5 99.9 27.9 101.73~ 1.9 =k1.87 +1.8 99.9 20.6 101.4 3Z 2.3 iz2.27 f1.5 10.3 -i. 0 . 3 1 2 . 9 1 10.1 35.2 +2.0 10.0 23.9 9 . 8 7 i 0.3 13.04 -1.3
I35
d
i
Weight of
z
z
Mean of 5 values obtained from 5 separate photopeaks. The error limits are the mean standard deviation of all 5 values calculated according to:
n(n
Figure 3. Comparison of gamma-ray spectra of thorium ore standards (0.1% Th) measured at different times after irradiation 15 cc Ge(Li) detector, 100 mg thorium ore, 10 min irradiation, 100
min counting time Upper = 9.3 days AEB
Lower = 22.37 days AEB
UjTh ratio of 2 :1 the 100 keV peak cannot be used even after 19 days, the 110 keV peak is free of interferences after 18 days, and the 300, 310, and 340 keV peaks may be used after about 4 days. Determination of Thorium in Standard Ores. Three different standard thorium ores obtained from New Brunswick Laboratories (Atomic Energy Commission, Oak Ridge) containing 1.0, 0.1, and 0.2% thorium, were analyzed along with Specpure thorium oxide as a standard. The spectra of the irradiated samples were recorded daily until all the 7 chosen ratios of the integrated areas of the 5 principal 233Pa photopeaks agreed with those obtained for pure thorium oxide. Table I1 summarizes the results obtained. Up till 9 days after the irradiation it was impossible to distinguish the photopeaks corresponding to 233Pa,these being covered by the peaks of the neutron activation products of uranium and the rare earths present in the ores. Complete agreement between the 7 ratios involving 5 photopeaks chosen was obtained 20 days after irradiation. Thus after a cooling period of 20 days the thorium concentration of the ores can be found from the integrated area of each of the 5 photopeaks, giving 5 independent results from one single spectrum. If quick results are at a premium the thorium concentration can be calculated from the 300 and 340 keV peaks already 10 days after irradiation with a certain sacrifice in accuracy. The thorium concentrations in the ores, calculated from the spectra obtained after a cooling period of 20 days, are summarized in Table 111. Figure 3 shows a comparison between the spectrum obtained for one of the ores 9 and 22 days after irradiation. The standard ores were also analyzed for comparison by the method of delayed neutron counting (9). Very good agreement was found between the two methods, as seen from Table 111. 270
where r
=
individual result, m
- ') number of results.
=
mean result, n =
Precision and Accuracy. In order to check the reproducibility of the method and determine its accuracy, the 0.1% thorium standard was analyzed repeatedly and the thorium concentration calculated from the spectra obtained after different cooling periods. Table IV shows the results obtained. It is seen that the precision and accuracy are good, independent of the cooling time or the quantity of the sample analyzed. A question arises as to the effect of self-shadowing of epithermal neutrons, which could represent a serious source of error due to the high resonance peaks in the thorium neutron capture cross section curve between 20-300 eV. As seen from the good agreement between the observed and known values, this did not influence the accuracy of the measurements. By irradiating increasing quantities (up to 600 mg) of thorium ore containing 0.02% thorium, no selfshielding effect exceeding the total *2% error of the method, could be calculated. In the case of the low concentration standards, the question of self-shielding does not arise, because it was possible to match well the quantities (volumes) of sample and standard. The overall precision of 1.2.5 % for a single measurement is most satisfactory for an activation analysis method. The values reported in the literature for thorium determinations are all higher than 15 % (4,5,2)except for the method of delayed neutron counting (9). Even in a very recent work (14) on the determination of thorium in ultramafic rocks by activation analysis, 10% is regarded as excellent after tedious radiochemical separations. Determination of Thorium in Rock Samples. In view of the satisfactory results obtained for standard thorium ores, the method was applied to the analysis of different types of rocks with unknown thorium content, namely two xenotime (YP04)and two sand samples taken at random. The samples together with an appropriate standard were irradiated for 6 hours in the core of the reactor. Two different standards were used, the thorium standard ores mentioned above, and the 0.01 % Th solid standard prepared as described above. The thorium concentration was calculated after the photopeak area ratios of the sample spectra reached the values for pure thorium.
(14) E. V. Gangadharam, Radiochirn. Acta, 10, 65 (1968).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 2, FEBRUARY 1970
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
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
... ...
0.964% f 0.016
... ...
1.176% =t 0.028
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 U 3 0 8contents 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 U 3 0 8contents 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 U 3 0 8contents 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
ANALYTICAL CHEMISTRY, VOL. 42, NO. 2, FEBRUARY 1970
271
