Over these concen1,ration ranges the intensity of the copper-carbamate complex is 1.6 times that (of the neocuproine complex. The least squares lines were used to predict the copper values for each standard solution. These predicted values were then compared to the known values and the standard deviations were calculated (Tables I and 11). Effect of Iron. I n studies of the mechanism of solution and transfer of copper in mineral oil (?), the oil samples contained disso.ved iron as well as copper. Consequently, experiments rrere conducted t o determine whether dissolved iron interfered with the copper determination. A nbmber of standard copper solutions containing known concentrations of iron (added as metallo organic iron) were run by each method. No modification of the neocuproine method was necessary to correct for the presence of iron, but the following addition was used with the carbamate method. The combined ac>tic acid extractions were brought to pH 9 or greater with ammonium hydroxide (1:l) and 1.0 gram of citric acid was added
before the addition of the carbamate solution (6). The effect of iron is shown in Table
Table HI. Effect of Dissolved Iron on Copper Determination
111.
CARBAMATE METHOD SUMMARY
Dissolved copper in the range of 2 to 200 pg. per gram of mineral oil can be determined with high accuracy (u e 1 pg. of Cu) by either the neocuproine or carbamate method. The procedures for these methods are much less complex than those required by earlier methods. The neocuproine method is the simpler and quicker of the two. The carbamate method offers an advantage a t low concentrations since the copper-carbamate complex has a greater absorbance than the copper-neocuproine complex. LITERATURE CITED
(1) American Society for Testing and Materials, Philadelphia, Pa., “Part 5,
Fuels, Petroleum, Aromatlfc Hydrocarbons, Engine Antifreezes, A.S.T.M.
D-810-48. 1952. ( 2 ) Buchwald, H.9 wood, G., A N A L . CHEM. 25, 664 (1953). ( 3 ) Hackett, C. E. S., Anal. Chim. Acta 12, 358 (1955). (41 . , Kreulen. D. J. W.. J. Inst. Petroleum 38, 449 (1952).
Copper added.
Iron added.
39.0
40.0 110.0
a.
54.4
rg.
Copper ReDifcovered. ference. rg. 38.1 57.2
rg.
-0.9 +2.8
NEOCUPROINE METHOD
57.1 102.1
58.6 104.8
56.2 101.3
+0.9 -0.8
( 5 ) Massey, L., Ibid., 38, 281 (1952). ( 6 ) Sandell, E. B., “Colorimetric,, Determination of Traces of Metals, 3rd ed., Vol. 3, p. 444, Interscience, New York, 1959. ( 7 ) Spauschue, H. O., ASHRAE J., in
press.
(8) Steinle, H., Kaltetechnik 7, No. 4, 101 (1955). ( 9 ) Steinle, H., Seeman, W., Ibid., 3, KO. 8. 194 (19511. (10) Thompson, C. N., J . Inst. Petroleum 44, 295 (1958). (11). Zall, D. M., McMichael, R. E., Fisher, D. W., ANAL. CHEM. 29, 88 (1957).
RECEIVED for review December 13, 1962. Accepted March 28, 1963.
Quantitative Radiochemical Procedure for Analysis of Polonium-210 and Lead-212 in Minerals HUGH T. MILLARD, Jr.l Division of Geological Sciences, California Institute of Technology, Pasadena, Calif.
b A method for the analysis of polonium-2 10 (1 38.4-day half life) and lead-21 2 ( 1 0.6-ho~’r half life) in zircons and other natural systems has been developed. The procedure employs spontaneous elcctrodeposition on silver for the isolatior of polonium-2 10 and controlled-potential electrogravimetric separation of lead-2 1 2 plus added lead carrier. A diethyldithiocarbamate extraction procedure for the lead was developed to be used prior to plating in the! presence of ions which precipitate at the pH used for lead deposition. Th,? amounts of the deposited nuclides arl? then determined by alpha-counting. The effects of temperature, volume, and inhibiting ions on the yield and rate of deposition of polonium wore also studied. The procedure was calibrated using minerals whose lead-21 0 and thorium2 3 2 contents had been determined by other methods; it was tested on a composite uraninitle-thorite mixture. Finally, it was applied in the analysis of four zircon samples and one uranothorite sample.
A
of the degree of disequilibrium in the radioactive decay chains in natural materials is of value in explaining geochronological inconsistencies and in more general studies of the geochemistry of the members of these chains. Rosholt (20-22, 94, 26) has reported methods for the detailed study and classification of the various types of radioactive disequilibria found in geological samples. He divides the chains into groups, each having a relatively long-lived parent which can be assumed to be in radioactive equilibrium with its immediate daughters. The uranium group contains two parent nuclides: 4.5 X 109-year uranium-238 and 7.1 X 108-year uranium-235. The parents of the other groups in the uranium-238 series are: 8.0 X 104-year thorium-230, 1622-year radium-226, 3.8dag emanation-222 (radon group), and 22-year lead-210 (lead group). The parent of the only other group in the uranium-235 series is 34,300-year protactinium-231 (protactinium group), All of the daughters in the thorium series have sufficiently short half lives KNOWLEDGE
to provide only one group in this series, the thorium group. The objective of this study was to find alternative procedures for analyses of the lead and thorium groups and to apply these procedures to mineral separates with particular attention t o uranium and thorium systems in nature. Assuming that the thorium series is ordinarily in radioactive equilibrium in geological samples, quantitative analysis for any member of the thorium group allows us to find the thorium-232 content. In the uranium-238 series the loss of lead group members from a sample can only produce short-term disequilibrium due to the short half life of lead-210 (22 years). However, because this group comes a t the end of the chain, the knowledge of ita amount relative to uranium is helpful in evaluating the state of radioactive equilibrium farther up the chain. This state of equilibrium may be affected by the gain or loss of Present address, Department of Cheniistry, University of California, San Diego, La Jolla, Calif.
VOL 35, NO. 8, JULY 1963
1017
uranium-234, thorium-230 (ionium), radium-226, or emanation-222 (radon), and is of special importance when evaluating age data which is based upon the lead-206 content. Electroplating procedures naturally suggested themselves as alternatives to the precipitation separations used by Rosholt because they require a minimum amount of manipulation of the sample solution and yield radionuclides in a form convenient for counting. All of the nuclides in the lead group may be electroplated but since alpha-counting was to be used, polonium-210 (138.4day half life) was chosen for the determination. Lead-212 (10.6-hour half life) and bismuth-212 (60.5-minute half life) in the thorium group have combinations of half-life and decomposition potential xhich make them suitable for the determination of this group. Bagnall (S) has reviewed the literature pertaining to the electrochemical deposition of polonium up to 1967. In recent years, the spontaneous deposition of polonium on less noble metals (silver, copper, and nickel) has been used to separate its isotopes from lead and bismuth targets (18, SO) and from lead-210 and bismuth-210 (sa), and for its analysis in biological materials (4, 16, 29, S I ) and radioactive minerals ( 2 ) . Extensive studies of the spontaneous deposition of polonium on silver (4, 9,26, 29, 34) have lead to the conclusion that elevated temperatures are needed for high yield and smooth deposition. Significant quantities of ferric iron and the ions of metals more noble than polonium (gold, platinum, tellurium, and mercury) interfere with its deposition, but these can be reduced prior to plating ( I S , 54). Recently used methods for separating radioactive lead isotopes include precipitation (IO), dithizone extraction ( 1 , 11, S S ) , ion exchange (6-8, 1.2, 15), and electrochemical (14, $8)procedures. EXPERIMENTAL
Apparatus. PoLoNIunz PLATING APPARATUS. The apparatus used t o plate poloniunl consisted of a beaker (ordinarily a 100-ml. beaker for solution volumes less t h a n 75 ml.) covered by a watch glass through which a hole had been bored. A silver-foil disk (punched from Matheson Reagent Grade Silver foil, 0.005 inch thick), 1.25 inches in diameter with a 0.13inch diameter center hole, was suspended by means of a glass rod passing through the center hole in the disk and up through the hole in the watch glass. The back and edges of the silver-foil disks were coated with Glyptal to prevent plating on these surfaces and this gave a plating area of about 7.0 sq. em. The solution was stirred with a magnetic stirring bar 1 inch in length. The stirrer also served to support the beaker. The stirring motor Varlac was set at half scale and the mag1018
ANALYTICAL CHEMISTR?
1
STIRRING MOTOR
/-
MERCURY - POOL CONTACT
RE MO VA B LE L U C I T E CATHODE
CONTACT
\I
I/
-PLATINUM
Figure 1. of lead
SCREW
ANODE
Apparatus for the controlled-potential deposition
netic stirrer rotated at about 500 r.p.m. LEAD PLATINGAPPARBTUS. The apparatus used for the controlledpotential electrodeposition of lead is shown in Figure 1. The parts were mounted on a Sargent Electroanalyzer to provide the stirring motor. A transistorized potentiostat was designed to control the potential of the working electrode to A20 mv. when the cell current was in the range 0 to 200 ma. The saturated calomel electrode arm could be rotated to allow removal of the Lucite cathode piece. The silverfoil disk used as cathode was identical to the disks used to plate polonium except that a glass-disk backing with a thin coating of grease as a seal was used instead of Glyptal to prevent plating on the rear surface. COUNTIXG APPARATUS. The scintillation detector used to count the alpha-radiation consisted of a zinc sulfide screen applied with rubber cement to the face of an RCA 5819 or DuMont 6292 multiplier phototube. The detector was mounted inside a lighttight box into which the sample could be introduced for counting. The multiplier phototube was coupled to a conventional electronic scaler and highvoltage power supply. Solutions and Sources. Except where noted, reagent grade chemicals and reagents were used throughout this work. POLONIUM SOLUTION. -~~~ A stock polonium-210 solution was prepared from a radium-D-E-F solution. Thia was done by spontaneous deposition of the polonium-210 from a 0.5N HC1 medium nnto a silver-plated platinum ba5e at %so C. The silver and polonium were then dissolved in nitric acid. The silver was precipitated as >ill-cr
chloride and separated by filtration, and the solution evaporated to dryness. The residue was then taken up in 0.5N HCl. These plating and precipitation steps were repeated to purify the polonium-210 and the final solution was diluted to 250.00 ml. in a volumetric flask t o give a stock polonium-210 solution (0.5N in HCl). When prepared, this solution contained about 1700 alpha-c.p.m./5 ml. It was used over a period of 2.5 years to prepare working polonium-210 solutions which contained about 70 alpha-c.p.m./5 ml. The specific activity of the polonium-210 in these working solutions was determined from time to time by evaporating 1-mi. aliquot portions to dryness on silver-foil disks and counting the alpha-activity on these. LE.4D CARRIER SOLUTION. Lead chloride (0.6712 gram, Baker’s Reagent Grade) was dissolved in about 200 ml. of hot distilled mater and the p H adjusted to about 4 with very dilute HC1. This solution was then diluted to 250.00 ml. in a volumetric flask giving 10.00 mg. of lead/5 ml. Blank runs were made by carrying aliquot portions of this solution through the plating procedure. The alpha-activity due to the lead carrier was found to be negligible. KATANGA PITCHBLENDE ORE,MS-OR. This ore contains 44.967, uranium and the lead group is 94.57c of the equilibrium amount, or 42.57, cquivalent uranium ( 2 2 ) . -2 50.00-ml. working holution n a s prepared from 0.0199 gram of this ore using aqua regia for its disiolution. HAPPYJ ~ C Kt TORE.GS-641 ~ 31. This ore contains 72.8y0 uranium :md tlit 1c:rtl g r o ~ p15 94.9% of the equilibrium ainount, or 69.17, equiv:tlent uranium ( 2 3 ) .
~
THORIUM NITRATE [BAKER’S REAGRADE, T H ( N O ~ ) ? . ~ H Z O IA. 100.00-ml. working solution was prepared from 0.4020 grain of this material using concentrated HC1 for its dissolution. The thorium series in this material was not in radioactive equilibrium. K ~ v uTHORIIXORE. This ore contains 53.67, thorium and 0.1% uranium (I ‘ 1 ) . Samples used for analysis ranged in >ize from 5 to 10 mg., and these were di~solvedM ith aqua regia. LTR.4NIUM-THORIUM SOLUTION. -4 250.00-ml. working solution was prepared from 0.0259 gram of the Happy Jack uraninite and 0.1223 gram of the Kivu thorite ore using aqua regia for their dissolution. dnalysis for the uranium content of this solution by the isotope dilution method indicated that thwe were 372 + 4 pg, of uranium per 5 ml. of the solution as compared to the expected value of 379 pg. of uranium per 5 ml. calculated on the basis of the uranium content of the uraninite and thorite. P~corm~ CANPOX i ZIIWOS. This zircon contains 208.2 f 1.0 p.p.ni. uranium and 63.0 f 1.0 p.p.m. thorium (28). .I 0.0958-gram sampll: was fused with 0.5 gram sodium peroxide and the entire sample analyzed. A 100.00-ml. working solution was prepared by fusion of 1.8478 grams of zircon with 10 grams of sodium peroxide and 5.00-, 10.00-, or 15.00-ml. aliquot portions used for analysis. ai 500.00-ml. w0rkir.g solution was prepared by boras fusion of 0.5529 gram of zircon, the silica separated by H F treatment, and a 180.00-ml. aliquot portion used for analysis. .I 500.00-ml. w0rkin.g solution was prepared by peroxide fusion of 0.6564 gram of zircon, the sili8:a separated by H F treatment, and 1513.00-ml. aliquot portions were used for analysis. SIERRAAKCH.~ 4 ZIF:COS. This zircon contains 1807 i 18 p.p.m. uranium (27‘). A 250.00-nil. working solution was prepared from 0.0556 gram of this material using a total of 11 grams of SazOLand 2 grams of KaOH for decomposition. Twenty-five-milliliter aliquot portions were used for each . -2nalyris for the uranium of this solution by the isotope dilut’ion method showed 11.97 f 0.12 pg. of uranium per 25 ml. of solution rather than the 10.05 pg. per 25 ml. expected on the basis of the uranium cont’ent of the zircon. SIERRA dKCH.4 1-6 ZIRCOK. This zircon contains 6486 f 65 p.p.m. uranium ( 2 7 ) . A 250.00-ml. working solution was prepared from 0.0247 gram of this material using 2 grams of ?YTa20zand 0.5 gram of h-aOH for decomposition. Twenty-five-milliliter aliquot portions mere used for each analysis. Analysis for the uranium content of this solution by the isotope dilution method showed 16.41 f 0.16 pg. of uranium per 25 ml. of solution rather than the 16.02 pg. per 25 ml. expected on the basis of the uraiiium contcnt of the zircon. HOPI I3 Zrltco:;. This zircoii GEwr
contains 3421 & 34 1i.p.m. uranium and These washings are allowed to flow TOO0 =t2000 p.p.m. thorium ( 2 7 ) . The into the plating solution. At the conthorium value was calculated from the clusion of plating, remove the silverlead isotopic compositional and alphafoil disk, rinse with 0.514’ HCl, and aircounting data. A 250.00-ml. workirig dry. The polonium-210 alpha-activity solution m-as prepared from 0.0332 gram may be counted when convenient. The volume of the solution remaining of this material using 2 grams of Sa2?* from the polonium deposition is reduced and 0.5 gram of XaOH for decompoaito about 30 ml. and 0.60 gram of tion. Twenty-five-milliliter aliquot por’ tions were used for each anal\ sodium tartrate dihydrate added. The MARBLEMOUNTAIN URANOTHORITE. pH of this solution is adjusted t o between 4.5 and 5.0 by the dropwise This uranothorite contains 3.29% i addition of 5-V NaOH. The lead 0.03 uranium and 30% + 5 thorium ( 2 7 ) . A 249.9-m1. working qolution carrier is then deposited electrochemwa? prepared from 0.1057 gram of thiy ically for 30 minutes a t room temperamaterial and 5.00-ml. aliquot portions ture on a second, previously weighed used for each analysis. Analysis for silver-foil disk. The cathode potential the uranium content of this solution during this deposition is controlled a t by the isotope dilution method showed -0.70 volt us. the saturated calomel 72.4 =t0.7 pg. of uranium per 5 ml. reference electrode and the solution is of solution. Uranium (73.6 kg.) per stirred. Electrolysis is concluded by 5 ml. was expected on the basis of the quickly lowering the beaker of solution uranium content of the uranothrite. (while the current and stirring motor The flux used to decompose the remain on) and flushing the deposit zircons normally consisted of 2 grams free of electrolyte with distilled water. The silver-foil disk is cleaned of grease of Ka202plus 0.2 gram of NaOH. A sample of this flux was analyzed for its with benzene, rinsed with alcohol, and uranium content by the isotope dilution air-dried. The yield of lead carrier is method and for its polonium-210 condetermined by reweighing the disk plus tent according to the recommended the deposit of lead and subtracting the procedure. The results indicated 0.07 initial weight of the disk. After waiting about 5 hours for the alpha-activity Po-210 c.p.m. [or 0.2 pg.e (microgram equivalent) U]/2 grams of Na202 supported by lead-212 to reach transient - 0.2 gram of NaOH and 2.0 pg. equilibrium, the alpha-activity decaying C/2 grams of NazOz - 0.2 gram of with a half life of 10.6 hours is observed NaOH. for several half lives and the activity Recommended Procedure. Ordicorresponding to zero decay time (connarily, a solution of the sample should clusion of deposition) is calculated. be prepared from which aliquot porREDUCTION PROCEDURE. If the surface of the silver-foil disk is darkened tions may be taken for analysis. The sample should be decomposed either during the deposition of polonium (due with mineral acids and evaporated to to the presence of ferric ion or the ions dryness or by fusing with the sodium of gold, platinum, tellurium, or merperoxide flux for 15 minutes in a cury), the following reduction step nickel crucible. Slurry the residue should be performed prior to the separaor solidified melt with 0.1N HC1 and tion of polonium: The aliquot portion separate a n y silica by centrifugation. of the sample solution is evaporated to Transfer the silica precipitate to a incipient dryness and the residue disTeflon beaker, add 12 grams of solved in as small a volume of 7N perchloric acid and 25 grams of H F , HC1 as possible. Add the amount of and evaporate to dryness. Dissolve hydrazine dihydrochloride which would normally be used (the solution should the residue in 0.5-VHCl, combine with the main solution, and dilute to the be saturated with hydrazine dihydrodesired volume in a volumetric flask chloride a t this point), cover with a while maintaining the HC1 concenwatch glass, and maintain the temperatration a t 0.5X. With some zircon samture just below the boiling point of the ples, the HC1 concentration may have solution for 30 minutes. Cool the solution slowly, dilute it with distilled to be as high as 2,V to prevent precipiwater until the concentration of HCl tation. is 0.5S, and separate any precipitate Remove an aliquot portion of this which has formed. If the solution is sample solution for analysis (preferably yellow a t this point, due to the presence 25 ml. or less, but i t may be as much of ferric ion, warm until colorless and as 150 ml.). Add 5.00 ml. of the then deposit the polonium. lead carrier solution and dilute to 25 EXTRACTION OF LEAD. If the sample ml. with 0.5N HCI if the solution volis zircon or if some other element is ume is less than this. Add 0.26 gram present which will precipitate when the of hydrazine dihydrochloride for each pH is adjusted to about 4.7, then the 25 ml. of solution. Deposit the pololead-212 plus lead carrier must be nium on a silver-foil disk a t room temextracted with diethyldithiocarbamate perature for a length of time determined prior to plating. The HCI concentraby the volume of the solution. Overtion of the solution remaining after night (12 hours) is a sufficient length deposition of polonium is raised to 1 N of time if the solution volume iq 150 and the lead extracted with two 10-ml. ml. or less. The solution should be portions of carbon tetrachloride, each covered with a watch glass and stirred of which contain3 0.25 gram of diethylduring deposition. Use small quantitiei ammonium diethyldithiocarbamate. of 0.5.V HCl to a a i h the watch glass The yellow carbon tetrachloride solution and sides of the beaker after about twoturns brown during extraction. Sepathirds of the plating time has elaijsed. rate the carbon tetrachloride phase and VOL. 35, NO. 8, JULY 1963
0
1019
O W I-
I
I
I
I
I
I
1
TO 30" C
VOLUME 20 TO 2 3 m l
(3
z za
."\
I
W
K
v 5 0 mI
\
5- 0.10-
z
3
0 a
\80° C
L
l-
001 L LL
L
10
20
-
30 40 50 TIME, MINUTES
L
60
Figure 2. Fraction of polonium remaining unplnted after various periods of deposition as a function of the solution temperature Each solution initially contoined 130 c.p.m. of polonium-21 0 alpha activity
evaporate t o dryness. Destroy the organic material by adding 5 ml. of concentrated nitric acid and then 5 ml. of 60% perchloric acid. The solution is heated until dense white fumes appear, then a n additional 5 ml. of 60% perchloric acid is added and the solution is evaporated t o dryness. The small residue is treated with 2 ml. of 6 J HCl and 25 ml. 0.5N HC1 is added. If any residue remains at this point, the solution is warmed until all material has dissolved. Finally, 260 mg. of hydrazine dihydrochloride and 600 mg. of sodium tartrate dihydrate are added, the p H is adjusted to about 4.7 with KaOH, and the lead deposited for 1 hour. Zero decay time now corresponds to the time of extraction rather than to the conclusion of deposition.
Table 1.
I
4 0.01' p: LL
i
I
I
I
4
5
6
I 7
TIME, HOURS
Eoch soluticn initially ccntained activity
TREATMENT OF COUNTINGDATA. The corrections to be applied t o the counting data are: Background (5 to 20 c.p.h. for the polonium-210 plating but may be higher for the lead-212 plating, presumably due t o the codeposition of some polonium-210 with the lead and the growth of lead-210 supported polonium-210) Decay since removal from radioactive equilibrium-i.e., since deposition or extraction Polonium-210 from the sodium peroxide flux (0.07 c.p.m./2 grams of XatOt - 0.2 gram of NaOH) Plating yield (in the case of lead) Standardization of the Procedure. A thick alpha-source was prepared as a counting standard by diluting 0.0328 gram of the Happy Jack uraninite
Calibration of the Procedure Using the Katango Pitchblende and the Kivu Thorite
ANALYTICAL CHEMISTRY
I
3
Figure 3. Fraction of polonium remaining unplated after various periods of deposition as a function of the solution volume
Corrected Specific Weight sample, Wt. equiv. activity, activity, mg. u, mg. c.p.m. c.p.m./pg.e G Katanga pitchblende, MS-OR,42.5% equiv. U at Pbz10 1.99 0.846 , 249.4 0.2947 1.99 0.846 242.0 0.2560 1.99 0.546 242.0 0.2560 1.99 0.546 235.9 0.2524 1.99 0.846 266.2 0.3026 1.99 0.846 255.7 0.3021 hv. and std. dev. 0.2923 It 0.0058 Specific activity, Wt. equiv. Th, mg. c.p.rn./pg.e Th Kivu thorite, 56.3y0 Th 5.6 3.16 279.1 0.0556 5.6 3.15 285.4 0.0906 9.3 5.24 478.4 0.0913 Av. and std. dev. 0.0902 5 0.0014
1020
I
2
130 c.p.m. of polonium-21 0 a!pha-
with 0.9948 gram of ferric oxide and grinding the powder t o ensure adequate mixing. The diameter of this standard was the same as t h a t of t h e silver-foil disks so t h a t i t had the same counting geometry as t h e disks. All counting data were referred t o this standard alpha-source and it in turn was calibrated against polonium210 and bismuth-21 2-polonium-2 10 alpha-sources, prepared by carrying standard materials through the recommended procedure. Thus, these standard materials served to calibrate the procedure and all counting data for unknown samples were ultimately referred to them. The material used to calibrate the polonium-210 procedure was the Katanga pitchblende, while that used for the lead-212 procedure was the Kivu thorite. These materials were chosen because the state of radioactive equilibrium existing in them had already been determined by established procedures in other laboratories. The counting data obtained for these materials are presented in Table I where pg.e represents microgram equivalents. This unit is used to indicate the weight in micrograms of chain parent (uranium or thorium) required for the radioactive equilibrium support of the amount of daughter nuclide found t o be present in a sample (2@. RESULTS AND DISCUSSION
Polonium-210 Procedure. The results of experiments performed to determine the minimum plating time for polonium for various combinations of volume and temperature are shown in Figures 2 and 3. The two platings at 24' C. and 20 ml. in each figure provide a n indication of the reproducibility of these data. The fraction
of polonium remaining unplated in each solution was calculated from the amount finally plrtted after a long period of time and not from the amount initially present in the solution. The yields based on tl-e amount initially present ranged from 93 to 95%. It is evident from these graphs that relatively high yields of polonium can be obtained within rertsonable lengths of time from solutions kept at room temperature, and thxefore the temperatures of the solutions are not elevated in the recommended procedure. An attempt was made to estimate the degree to which lead or bismuth codeposit with polonium. The results indicated that only 0.5010 or less of the available lead-212 or bismuth-212 is deposited on the platings in the absence of lead carrier and only 0.27’c or less when 10 mg. of Lad carrier is used. Thus, the codeposition of lead-212 and bismuth-212 becomes a problem only if the ratio of the activity of the thorium group to that of the lead group is very large, as is the case for thorite ores. When these conditions prevail in a sample, any alpha-activity supported by the lead-212 may be allowed to decay to negligible levels before determining the polonium-210 activity. Since ferric ion and the ions of mercury, tellurium, piatinum, and gold interfere with the deposition of polonium, provisions were made in the procedure for eliminating these interferences when they occur. Most of the methods devised to deal with .;hese substances involve their reduction to a less troublesome oxidation state. A series of experiments was pepformed to devise a method for their removal by the use of hydrazine alone. The results of these experiments are summarized in Table 11. The absence of yidlow color in the solution was accepted as an indication that the iron had heen reduced to the ferrous state. Solutions 1-3 and 7 demonstrate that complete reduction of the ferric ion can ts,ke place only after dilution of the 7AT H C1 to 0 . W . If this procedure is followed, approximately 2
Table
It.
0
Y-
0.25
0.2c
3 7.0 sq.cm
0.15
i- I IO
I I I 20 30 40 50 WEIGHT LEAD, mg
I
60
I
Figure 4. Average fractional solid angle, fa, subtended by the detector as a function of the weight of lead for various deposit areas The open circles a r e the points obtained from the plating experiment
the controlled potential electrogravimetric method for the analysis of lead which was developed by Lingane (17). The fractional attenuation of the bismuth-212 and polonium-212 alpharadiation due to its passage through the lead carrier was determined as a function of the weight of lead carrier plated because this weight varied from plating to plating. An experiment was performed in which the alpha-activity from a polonium-210 deposit mas determined after each successive plating of lead carrier over it. The results are shown in Figure 4 as open circles. The curves show the average fractional solid angle, f a , subtended by the detector. These were calculated for polonium-210 alpharadiation as a function of the weight of lead through which it must pass for various deposit areas. The source-todetector distance, S, is 0.35 cm. and the radius of the circular alpha-detector, D, is 2.22 cm. Since the actual fractional solid angle for the counting geometry
mg. of ferric ion per 30 ml. of solution ( 10-3M) can be tolerated. Solutions 4-6 indicate that a mixture of 1 mg. or more of mercury, tellurium, platinum, and gold definitely inhibits polonium deposition even though the reduction procedure in 7 N HC1 is followed. In solution 7 , the amounts of these ions were reduced to 0.2 mg. and the reduction procedure in 7147 HC1 appeared to be adequate. This amount for any of these elements is much higher than that anticipated in zircon and most other materials to which this procedure was applied. I n any event, whenever low yields of polonium were obtained, the silver surface was blackened; thus the criterion used to establish whether the hydrazine reduction step was required in the subsequent analysis of portions of the sample m-as the presence of darkening of the silver surface. Lead-212 Procedure. The recornmended procedure for the determinntion of lead-212 is an adaptation of
Removal of Interference b y Mercury, Tellurium, Platinum, Gold, and Ferric Ion in the Deposition of Polonium-2 10
Weights of reagents, mg. Solution
cm Cln
Hg
Te
Pt
All
HydraFe+a azine
HCl concentration during reduction
Heating time during reduction, min.
7F 0.5F 0.5F
60 10 10
Solution color after reduction Yellow Colorless Colorless
No redurtion step 0.5F 30 7F 50 0.5F in 7F 60 Yellow 7 0.2 0.2 0.2 0.2 2 260 20 Colorless 0.5F Solutions 2-6: 5.00 ril. Poz10working soln., 5.00 ml. Th-232 soln., 10 mg. Pb carrier. Solutions 1, 7 : 5.00 ml. Poz10working soh.
Total Po Fractional deposition recovery time, hr. of Po*IO 3 0.8 2.5 3 I9
0.21 0.92 0.98 0.18 0.26
2
0.92
2.8
0.96
VOL. 35, NO. 8, JULY 1963
1021
was only known to about =t0.05and the detection efficiency for the zinc sulfide screen was unknown, the experimental points were fitted to the curve whose slope over the linear portion most closely matched their own. This turned out to be the curve for a deposit area of 5.0 sq. cm. which is less than the geometric area of 7.0 sq. cm. presumably because of graininess in the deposit. Figure 5 shows the 5.0 sq. em. curve calculated for the weighted average of bismuth-212 and polonium-212 alpharadiation It is evident that even for the maximum weight of lead carrier deposited (10 mg.), the reduction in the bismuth-212 and polonium-212 alpha-
Table 111.
activity is of the order of only 2%. These curves represent geometries in which the activity lies at the bottom of the layer of lead, while in the actual situation the activity is distributed throughout the lead. Thus, the attenuation of bismuth-212 and polonium212 alpha-radiation for lead deposits obtained from the procedure is only about 1% and no attenuation correction was applied to the counting data. The fact that zirconium precipitates when the p H of the solution is raised prior to the deposition of lead necessitated the separation of lead from the solution prior to the p H adjustment \$-hen zircons were to be analyzed
Results of Analyses Performed on the Uranium-Thorium Solution
Time since solution prepared, days 6
Corrected activity, c.p.m. 82.4 87.0 85.1 88.9 99.6 95.9 95.5 96.9
Wt. equiv. U Found, f i g . Taken, fig. Po-210 281 358 12 297 358 29 291 358 3nn 304 358 ... 400 340 358 475 328 358 750 336 358 820 341 358 Wt. equiv. Th Found, p g . Taken, fig. 88.4 980 1377 Pb-212 6 12 79.8 885 1377 29 73.1 811 1377 300 112.9 1252 1377 1301 400 117.3 1377 475 126.9 1407 1377 750 103.3 1178 1377 820 119.4 1361 1377 Uranium-thorium solution (500 ml.) was used for each run. Table IV.
Po-210
Time since solution Corrected Solu- prepared, activity, tion days c.p.m. 1 20 4.02 2 7.86 3.86 80 11.32
Wt. equiv. U Equilihrium Found, value,
a.
fig.
13 .S 26.9
19.9 38.4 19.2 57.6
13.2
38.7
0.949 0.916 0.938 0.952
Ratio 0.712 0.643 0.589 0.909 0.945 1.022 0.855 0.988
1
2 4
20 80 150 1
2 357
ANALYTICAL CHEMISTRY
Fraction of equilibrium value 0,694 0.701 0.693 0.675
3.34 11.4 12.3 0.927 Av. and st,d. dev. (for solution 4) 0.954 f.0.018
fig.
1022 *
n ,848
Results of Analyses Performed on the Pacoima Canyon Zircon
357
Pb-212
Ratio 0.787 0.836 0.813
fig.
6.0 u ,567 11.6 0.612 5.8 0.690 17.5 0.680 17.5 0 .709 22.0 12.4 1.77 1.10 12.2 12.4 0 .ns 0.41 4.7 3.7 1.27 Av. s n d std. dev. (tor solution 4) 1.34 & 0.40 0.30 0.64 0.36 1.07 1.11 1.98
3.4 7.1 4.0 11.9 12.4
0.401
-
1
\
0.35
1
I‘
Ld IO
20
30
WEIGHT LEAD, mg Figure 5. Average fractional solid angle, fa, subtended by the detector as a function of the weight of lead for a deposit area of 5.0 sq. cm. for bismuth2 12-polonium-2 12 alpha-radiation
Extraction was considered to be the most convenient method of separation and the procedure using diethyldithiocarbamate was worked out using the data of Bode and Neumann (5). Analyses of Samples. Each sample solution was analyzed over long periods of time t o confirm t h a t the state of equilibrium in the uranium series had not been distributed during preparation of the solution. The criterion used was the absence of change for the polonium-210 analyses. This test is good only if no lead-210 is lost during preparation. The composite uranium-thorium solution prepared from portions of the Happy Jack uraninite and the Kivu thorite provided high activities of polonium-210 and lead-212. Precipitates formed in a 0.5N HCl solution during the dissolution of these samples, and these precipitates were separated by filtration. The separate analysis for uranium in this solution by the isotope dilution method indicated that no uranium had been lost. However, as shown in Table 111, the results of analyses performed on aliquot portions of this solution over a period of 820 days indicate that neither polonium210 nor lead-212 was in radioactive equilibrium at the time these solutions rrere prepared and required relatively long periods to return to equilibrium. Thus, it is important that the entire amount of any sample be dissolved. ilttempts to analyze for polonium210 in the Pacoima Canyon zircon illustrate the need in the procedure for the recovery of ions adsorbed on the silica precipitate and the importance of using a sodium peroxide flux rather than a borax flux. The results are shown in Table IV. The first two solutions of this material were prepared using sodium peroxide but the silica which resulted upon acidification was simply separated by filtration and no attempt was made to recover adsorbed decay chain members. The recovery of polonium-210 was only about 70% and remained con-
staut over a period of 150 days. The third solution was prepared using a borax flux. I n addition, a recovery of material adsorbed 011 the silica was made by evaporating silicon as the tetrafluoride. The rixovery of polonium-210 from a n aliquot portion of this solution was only about 12y0 of the equilibrium value indicating that polonium mas probably being lost during the boras fusion which was conducted a t 1100” C. for several hours. Finally a fourth solution of zircon was prepared using a sodium peroxide flux but still recovering material adsorbed on the silica by treatment with HF. The results of the analyses for polonium210 in aliquot portions of this solution indicate that all of the polonium-210 n a s recovered and that the lead group in this zircon is almost in radioactive equilibrium nith tho uranium. The low thorium content made analysis for lead-212 by this procelure difficult until the diethyldithiocarbamate extraction iiroccdure was applied. Hon ever, el-en when the extraction procedure mas used, a large standard deviation was still observed due t o poor counting statistics. The polonium-210 m d lead-212 in the Sierra Ancha 4 zircon the Sierra Ancha 1-6 zircon, the Hopi 13uttes zircon, and the Marble Mountain uranothorite were analyzed according to the recommended procedure with the results shown in Table V. The U/aliquot content for the zircon solutions is based on the analyzed uranium coitents of the zircons rather than the isotope dilution analyses of the soluticns themselves because the additional uranium in these solutions presumably originates in the sodium peroxide flux. Unfortunately, the lead-212 data for these zircons cannot be interpreted w th respect to the btate of equilibrium because the thorium contents are not arcurately known. The lead-212 content found in the Sierra Ancha 4 zircon presently provides tile best value for its thorium content. ?‘he polonium-210 in this zircon apIicara to be close to the equilibrium amount. On the clther hand, the polonium-210 contenl s found for the Sierra Ancha 1-6 zircon, the Hopi Buttes zircon, and the Marble Mountain uranothorite depart widely from the anticipated equilibrium values. The rate of growth of the polonium-210 activity in the Sierra Ancha 1-6 zircon solution corresponds to the polonium210 half life. The maximum polonium210 content t o be erpected from this growth to equilibrium was computed by a least squares fit to the data and is shown in Table V in place of the average for the analytical resLlts.
Table V.
Analytical Results for the Remaining Samples
Sample Sierra Ancha 4 zircon (10.05 pg. U/aliquot portion)
Time since solution prepared, days 47 71 95 128
Av. and std. dcv. Fraction of equilibrium value Sierra ilnrha 1-6 zircon 48 70 (16.02 pg. U/aliquot portion) 96 129 203 Av . Anticipated equilibrium value and std. dev. Fraction of equilibrium value Hopi Buttes zircon (11.36 pg. 5 U/aliquot portion) Fraction of equilibrium value hlarble Mountain uranothorite 54 (6.95 pg. U/aliquot portion 73 142 634 pg. Th/aliquot portion) Av. and std. dev. Fraction of equilibrium value
Cor-
rected activity, r.p.m. 2.52 2.57 2.61 2 58
1. i 8
1.72 1.86 1.99 2.17
2.65 7.77 7.72 8.10
LITERATURE CITED
~
I
.
‘3.33 0.821
(1) Alberti, G., Bettinali, C., Salvetti, F., Ann. Chim. Ronza 49, 193 (1959); C.A. 53, 16421e (1959). (2) Ancarani. L.. Riva. L.. Comit. A’azl. Ric. Nucl.‘ CNG-31 i1959); C.A. 54, 10666f. (3) Bagnall, K. W.,“Chemistry of the Rare Radioelements,” Academic Press, Sew York, 1957. (4) Black. S. C.. U.S . At. Enerav - “ Coinm.. Rept. UR-463,’ 1956. (5) Bode, H., Neumann, F., 2. Anal. Chem. 172, l(1960). (6) Chen, Y., Wong, C., J . Chinese Chem. SOC.,Ser. IZ 6, 55 (1959). (7) . , Dedek. W.. Monatsber. Deut. Bkud. TViss. (derlik) 1, 502 (1959); C.A. 55, 16206.1’(1961). (8) Dedek, W.,2. Anal. Chem. 173, 399 (1960); C.A. 54, 17151b (1960). (9) Feldman. I.. Frisch. M.. ANAL.CHEM. ‘ 28,2024 (1956). (10) Godt, K. J., Sommermeyer, K., Stomkernenergie 5, 282 (1960); C.A. 55, 13% (1961). (11) Gorsuch, T. T., Analyst 84, 170 (1959). (12) Zbid., 85, 225 (1960). (13) Haissinsky, ?*I.,J . Chinz. Phys. 33, 97 (1936). (14) Harrison, A. D. R.,Lindsey, A. J., Phillips, R., Anal. Chim. Acta 13, 459 (1955). (15) Kahn, M., Langhorst, A. L., J . Inorg. Nucl. Chem 15, 384 (1960). (16) Krebs, C. A., Whipple, G. H., U.S . -4t. Energy Comnz., Rept. UR-501, 1957. (17) Lingane, J. J., “Electroanalytical Chemistry,” Interseience, New York, 1 c).w (18) Pauly, J., Bull. SOC. Chin!. France 1960, 2022; C.A. 55, 11233i (1061). (19) Poulacrt, G., Chimia (Aarau) 12, 116 (1958). ,
Pb-212 Corrected Wt. equiv. activity, Wt. equiv. u,a. c.p.m. Th, rg. 8 .87 9.04 9.18 0.117 1.33 9.08 0.0‘3% 1.05 9.04 i 0 . 1 3 1.19 0.900 6.26 6.05 4.43 6.55 0,388 7.00 0.523 5.97 7.64 5.20 8.97 i 0.18 0.560
analytical data, and advice generously provided.
\
Po-210
__
27.3 27.2 28.5 27.7 0 . 7 0.399
58.8 64.8
671 740 706 1.114
(20) Rosholt, J. K.,Jr., “Advances in Kuclear Engineering,” Vol. 11, Part 2, p. 300, Pergamon, Sew York, 1957. (21) Rosholt, J. N., Jr., AKAL.CHEM.26, 1307-11 f 1954). (22) Zbid., 29, 1398 (1957). (23) Rosholt, J. K., Jr., U. S.Geological Survey, Denver, Colo.,. private communiiation, mi. (24) Rosholt, J. K.,Jr., “Proceedings of the Second U.N. Conference in the Peaceful Uses of htomic Energy,” Vol. 2, p. 230, United Nations, Geneva, 1958. (25) Rosholt, J. S . , Jr., U. S. Geol. Surv. Bull. 1084-A,(1959). (26) Samartseva, A. G., Trudy Gosudarst. Radiev. Inst. 4, 253 (1938); English translation by Rabinomitch, E., AECtr-730; C.A. 33, 45123 (1939). (27) Silver, L. T., Division of Geological Sciences, California Institute of Technology, Pasadena, private communication, 1962. (28) Silver, L. T., hlcKinney, C. R., Deutsch, S., Bolinger, J., J . Geol. 71, 196 (1963). (29) Smith, F. A., Della Rosa, R. J., Casarett, L. J., A E P Rept. UR-305 f lg55). (30) Templeton, D. H., Howland, J. J., Perlman, I., Phys. Rev. 72, 758 (1947). \ - - - - I -
(31) U.K.A.E.A. Rept. ZGO-AMIW-167
(1958). (32) Verbersik, V., 2. Anal. Chem. 175, 405 (1960); C.A. 55, 5189s (1961). (33) Von Gunten, H. R., Buser, IT., Houtermans, F. G., “Proceedings of the Second U. N. International Conference on the Peaceful Uses of Atomic Energy,” Vol. 2, p. 239, United Xations, Geneva, 1958. (34) Whitaker, hl. D., Bjorksted, W., Mitchell, A. C. G., Phys. Rev. 46, 629 (1934).
-.,-.I.
ACKNOWLEDGMENl
The :ruthor esprclsses his appreciation to Dr. L. T. Silver for the samples,
RECEIVEDfor review March 25, 1063. Accepted April 26, 1963. This work was supported by the Atomic Energy Commission under Contract AT(04-3)-427. VOL. 35, NO. 8, JULY 1963
1023
