1509
V O L U M E 26, NO. 9, S E P T E M B E R 1 9 5 4 Table VI. Comparison of Results Obtained by Stannous Chloride Reduction and by a Fluorimetric Method (rraniurn by stannous chloride method estimated by correcting for separately determined iron) Stannous Chloride Reduction, Grams/Liter
USOS
Sample so.
Fe
1
2
1
0.074 0.061 0,059 0.054 0.061 0.057 0 068 0.073
6 63 6 57 6 61 6.59 5.96 6.04 5.99 6 39
6 63 6.59 6.63 6.63 5.98 6.02
2 3 ‘I
5
; 8
Table VII.
6:39
Fluorometer, G./Liter UaOa 6.64 i 0.04 6.56 i 0.04 6 65 & 0 . 0 6 6 . 5 0 i. 0 . 0 8 5 93 f 0 . 0 7 6.0 i0.1 5.97 0.09 6.4 0.2
*
Precision of Stannous Chloride >lethod
(I-ranium estimated by correcting ior separately determined Iron) barnple so. 1 2 3
4 5 6
A
0.02.V Dichromate. 111. Iron Uianirirri titer 1 2
correction 0.36 0.27 0.34 0.23 0.30 0.29 0.17 0.16
55.25 54 84 56.22 54.33 54.46 54 27 51.86 53 91
5.5 35 54.79 56.27 51.48 54.41 53.87 52.16 53.97
milliliters of 0.02.V dichromate titrant, the error amounted, on the average, to k0.004 ml. This error is insignificant compared with errors associated with estimating the end point and reading the buret. Consequently, Table VI1 gives a fair measure of the precision which may be expected from the stannous chloride method. A 10-ml. aliquot (containing approximately 60 mg. of uranium oxide equivalent to 20 ml. of 0.02N dichromate) was used to obtain each value shown in Table VI. The average titer subtracted to correct for the iron was 0.56 ml.; the range was 0.48 to 0.66 ml. The samples in Table VI1 were similar in content to those of Table VI, except that they contained magnesium and calcium in addition to the other substances. The samples contained, on the average, 0.15% of iron, equivalent to 0.27 ml. of 0.02‘V dichromate. The correction applied to each titration, in addition to the uranium titer, is shown in Table VII.
VaOs. %
LITERATURE CITED
1
2
77.5 76.9 78.9 76.2 76.4 76.1 72 8 75.6
77.6 76.9 78.9 76.4 76.3 75.6 73.2 757
ferences. The uranium in these samples was estimated by correcting for the iron by the ferron colorimetric method. The duplicates for the iron determination did not deviate by more than 3 ~ 1 % ;the average agreement was =k0.48%. In terms of
(1) Hodgman, C. D., ed., “Handbook of Physics and Chemistry,” 31st ed., p. 1442, Cleveland, Ohio, Chemical Rubber Publishing Co., 1949. (2) Kern, E. F., J . Am. Chem. Soc., 23, 605-9 (1901). (3) Kolthoff, I. lI.,and Lingane, J. J., I b i d . , 55, 1871 (1933). (4) Rodden, C. J., “Analytical Chemistry of the Manhattan Proj-
ect,” New York, McGraw-Hill Book Co., 1950. (5) Sarver, L. A, and Kolthoff, I. RI., J . A m . Chem. Suc., 53, 2906-9 (1931). (6) Swank, H. W., and hlellon, M. G., IND.ENQ.CHEX.,~ ~ N A LED., . 9, 406 (1937). (7) Zimmerman, J. B., Rahhitts, F. T., and Kornelson, E. D., Can. Dept. Mines and Tech. Surveys, >lines Branch, Tech. Paper 6 (1953). RECEITED f o r relieu. August 18, 1953
Accepted May 6, 1954
Determination of the Short-lived Decay Products Of Radon in Natural Waters P. K. K U R O D A D e p a r t m e n t o f Chemistry, University o f Arkansas, Fayetteville, A r k .
YUJl Y O K O Y A M A University o f Tokyo, Tokyo, Japan
I
T HAS long been established that the major portion of
natural radioactivity of rain water is caused by the decay products of radon, but not by radon itself (3, 7 , 9, 21, id). I n other words, in fresh rain water there is an excess of decay products the radioactivity of which has an “apparent” half life of some 30 minutes. This is in sharp contrast with the fact that fieah spring waters usually show a deficiency of decay products (6, 8 ) . Although many papers on the determination of radon and radium have been published, practically nothing has been done on the quantitative determination of the short-lived decay products. The general impression has been that the equilibrium hetween radon and its short-lived decay products always should be established in water, because of the short half lives of the decay products. RIethods for use of short-lived radioactive elements in natural waters as tracers in nuclear-geochemical studies have been developed by the authors during the past several years, and some of the results have been published in preliminary reports (6-9). The present paper describes the analytical procedures used by the authors in the studies of natural waters both in Japan and in the I‘nited States.
EXPERIMENTAL
Apparatus. A Lauritsen-type electroscope (8, IO) of the Institute of Scientific Research, Tokyo, was used for the determination of the decay products, especially in the field work. Measurements were also made with a loop-line windowless Geiger counter ( 1 ) and a 2.9 mg. per cm.2 end-window Geiger counter ( 3 ) in the study of the radioactivity of rainfall. An 1.M.-type fontactoscope ( 5 )of the Institute of Scientific Research was used for the determination of radon in water. Procedure. A simple evaporation method was used except when the concentration of the decay products in the sample of water was low. The natural radioactivity of rain water varies between l o w 9and 5 , l O - S curie per liter, and the radon content of most spring waters varies between lo-” and 10-6 curie per liter. The evaporation method is usable with concentrations higher than curie per liter. When the concentration was lower than 10-9 curie per liter, the decay products were coprecipitated with lead sulfide. For the determination of radium B, C in rain water, the water sample (1 to 10 ml.) was collected as quickly as possible, the volume was measured, and the water was transferred to a small dish (surface area 19.6 sq. cm.) and evaporated to dryness. The time consumed in each step was recorded and the radioactivity measurement was begun immediately after evaporation. As the salt content of rain water is low (up to a few micrograms per milli-
1510
ANALYTICAL CHEMISTRY
Table I.
Calibration of Lauritsen-Type Electroscope by Evaporation Method
Timeafterevaporation minutes 20 40 60 90 120 200 260 Observed, rsdioactivit; ( l a b s . ) , div./min. 11.2 9 . 7 7 . 8 5 . 5 4 . 2 3 . 7 3.7
Table 11. Agreement between Determinations by Evaporation and Fontactoscope Methods Method Evaporation Fontactoscope
(10-8 Curie per liter) Radon Standard 1 2 46 U. S. A. 56 Japan 46.9 54.2
3 64 54.5
4
53 54.4
5 46 43.1
liter), the absorption of alpha rays by the thin film of residue can be disregarded. In the case of spring water, such as -449 Masutomi in Japan, which contained more than 10-6 curie of radon per liter, a 2-ml. sample of water was taken quickly with a syringe and the time of sampling was recorded. After various intervals of time (6 = a few minutes to 3 hours), a 0.25-ml. portion of the sample water was evaporated to dryness and the alpha radioactivity was nieasured with the Lauritsen-type electroscope. The residue was approximately 0.1 mg. em. --z When the radon content of the sample water was to 10-7 curie per liter, t$e sample was taken in several 5-ml. or 10-nil. syringes, the time of sampling was recorded, and after various intervals of time (6 = a few minutes to 3 hours), a 5-ml. or 10-ml. portion of the sample water was evaporated to dryness and the alpha radioactivity \\-as measured. When the radon content of the sample water was 10-9 curie per liter or less, the sample was collected in several 500-ml. bottles, without leaving air space. and the time of sampling was recorded. After various intervals of time (0 = a few minutes to 3 hours) the sample water was acidified with 10 ml. of concentrated hydrochloric acid, the lead carrier (100 mg. as lead nitrate) was added, and lead sulfide xvas precipitated with 20 ml. of 5% sodium sulfide solution. The lead sulfide precipitate was filtered through a rapid-filtering paper and dried quickly. The alpha radioactivity of the dried lead sulfide was measured and the values of radioactivity of 20 minutes or 30 minutes after the filtering procedure (Iobsd.20 or Iobsd.SO)were recorded. The instruments were calibrated against the standard radon solutions prepared n-ith the radium st’andard solutions obtained from the United States Bureau of Standards and the Scientific Research Institute, Tokyo. -4I-ml. portion of the radium standgram of radium per milliliter), which ard solution (1.00 X had been kept in a sealed bottle for 1 month, was evaporated to dryness in a small dish and the alpha or beta radioactivity was measured over a period of a few hours. The radioactivit,y was found to decrease gradually for the first 3 hours ( I = radioactivity from the short-lived decay products of radon) and finally reaches the const’ant value (IL = radioactivity from radium and the long-lived decay products of radon). In most natural water, in which there is a large excess of radon to radium, the radioactivity approaches zero after approximately 3 hours ( I S >>IL). When the decay products of thoron are present, the radioactivity of ThB,C (10.6 hour) should be subtracted from the total activity to obtain Is.
The radon content of several radon solutions was determined by the evaporation method and then by the fontactoscope method in order to check the accuracy of the former method. The agreement between the values obtained by the two different methods was satisfactory (Table 11). Experimental results obtained with rain water collected at Fayetteville, Ark., are presented in Table 111. The sampling of 5 ml. of rain water and the evaporation took 10 minutes and 8 minutes, respectively. The radioactivity measurement w m started 17 minutes after the midpoint of the sampling procedure. The value of ZobdPO was obtained by averaging the intensity of radioactivity from 17 minutes to 23 minutes after the mid-point of the sampling procedure. The radioactivity approached zero, indicating a low concentration of the long-lived elements. The RaC,C’ content of the sample water was calculated as follows:
Kzo X 1.09 X
1000
7= 29 X
10-9 curie RaC,C’ per liter
(2)
The experiments were carried out changing the length of time Table I V shows the relationship between the time (6) and the radioactivity 20 minutes after the evaporation ( L b d 20). The values in Table IV indicate that there is a large excess of decay products in rain water. The apparent half life of the radioactivity of rain m t e r is approximately 30 minutes. ( 8 ) between the sampling and the evaporation.
Table 111. Radioactivity of Rain Water (Fayetteville, Ark., 1:30 P . M . , December 19, 1952. Sample: 5 ml.) Time, minutes 20 40 60 90 150 180 360 Radioactivity, l o b s d . , div./min. 1.09 0.80 0.60 0 . 2 4 0 . 0 7 0.02 0.00
Table IV. 8 , minutes Zobsd.to, div./min.
Relationship between 0 and lobsd.20 30
0.97
60 0.42
90 0.20
120 0.07
180
0.00
The radon content of rain water was measured with the 1 . X type fontactoscope and was found to be lo-” curie per liter or less. In Table V there is summarized the radioactivity of rain n-atrr as observed at several locations in Japan and in the United States. The content of the decay products of thoron was usually less than 10-10 curie per liter in these samples. However, larger amounts of ThB and ThC,C’ could be expected a t other locations and corrections n-ould be needed (4). The radioactivity of rain water resulting from fission products caused by atomic explosions n ill be reported elsewhere.
Table V.
Natural Radioactivity of Rainfall Observed in Japan and United States
Location
Data
so.of Samples
Rn RaC,C’ (10-9 curie/liter)
RESULTS AND DISCUSSION
The observed radioactivity, when 1 ml. of radium standard solution (1.00 x 10-9 gram of radium per milliliter) in which there is equilibrium between radium and radon was evaporated to dryness and the alpha radioactivity was measured with the Lauritsen-type electroscope through 0.75 mg. per cm.2aluminum foil window, is presented in Table I. The sensitivity constant, K20, of the instrument was obtained from the value of I S 20 minutes after the evaporation (Is.20). Because the half life of Ra.4 is 3.05 minutes, 99% of the radioactivity from RaA vi11 be lost 20 minutes after the evaporation. As the branching ratio of RaC is 99.96% beta and 0.04% alpha and the half life of RaC‘ is extremely short (1.5 X second), the Is.Z0 results almost entirely from the alpha rays from RaC’.
K z= ~ 0.133 X
curie per division per minute
(1)
A deficiency of the short-lived decay products was observed in most spring waters (immediately after issue) in contrast to an excess of decay products in rain vmter. Experimental results obtained with the vater of spring h49, Masutomi, Japan, which contains 1.3 to 2.7 x 10-6 curie of radon per liter, are presented in Table VI. h increase of radioactivity (I’obsd 2o = value O f radioactivity 20 minutes after the evaporation) with time 0 (time between the sampling and evaporation) indicates that the concentrations of the short-lived decay products of radon in that spring are extremely low in relation to the equilibrium amount of radon.
1511
V O L U M E 26, NO. 9, S E P T E M B E R 1 9 5 4
8, Min. 3 5 20 34 47 84
I’nobsd.zo,
d/m 0.53 1.07 2.88 485 5,70 9.40
I ~ ~ R I’obsd.za/~ c d/m Irnasc ., 0.041
. ...
... ...
Ilk
4
1
0.082
8
0.373 0.449
36 44
...
13107
Ash.
..
21
.. .
I
ICaalcd.
Azd,
d/m 0.60
d/m -0.07
4.90 6.13
-0.05 -0.43
..
..
Value of alpha radioactivity 20 minutes after evaporation. b Value of alpha radioactivity from RaC,C’, which are in equilibriup with radon in sample water, calculated from value of I’obsd.na a t 8 = 84 minutes. C Calculated value from Equations 4, 5 , 6 and 7 = 2 minutes. a
d A I = 7ob.d.
-
Zoalod.
The separation of radon from its decay products (either the precipitation of decay products or the dissolution of radon) must have occurred under the surface of the ground shortly before the issue of the spring water. The time ( T ) which elapsed between the separation of radon from its decay products under the earth’s surface and the arrival of the nater a t the orifice may be calculated as follows:
7 = T - e
(3)
where e ia the time betn een the sampling of water a t the orifice and evaporation, and T is the time between the first separation of radon from its decay products under the surface of the earth (dissolution of radon or precipitation of the decay products) and the second separation of radon from its decay products (evaporation of the sample watei). schematic diagram of the process is presented in Figure 1. The radium .i,radium B, and radium C contents of the sample water were calculated from the value of T and from Equations 3, 4, 5, and G [Raa] = 1
e - XAT - ( X C - X AXBXC )(XB - XA) AAXB XAXC e - XBT e - XCT ( X A - X C ) ( h B - XC) (Xc - X B ) ( X A - A B )
[RaC] = 1
Radioactivity of Water of Spring A49, Masutomi, Japan
Table VI.
(6) where XA, AB. and Xc are the deea!- constants of radium A, radium B, and radium C, respectively. The radium A, radium B, and radium C contents of the water of Spring A49, Masutomi, ale presented in Table VII, as are the amounts of other radioactive elements found i n thij water. The contents of the thoron decay products in mineral waters are usually small, except in some mineral waters whose radioactivity is predominantly due to thoron, such as BY, Masutomi (6). In determining the decay products in the waters of Hot Springe National Park, Ark. (10-9 to 10-8 curie of radon per liter), the lead sulfide precipitation method was used. The values obtained for T varied from I 1 minutes to 84 minutes. The water from the reservoir at the Sational Park .;ldministration Building gave the 03 value for r (which means equilibrium between radon and its decay products) because the water is stagnant. Water from drill core S o . 1 a t the C. Wilson uranium prospect area, Wilson Springs, Ark.! shoved a slight escew of decay products over radon content. The excess of decay products probably resulted from the escape of radon when the water was agitated at the time of sampling.
Table VII. Radioactive Elements in the Water of Spring A49, Masutomi (10-9 Curie per liter) Po2:s (Ra.k) Pb214 (RaB) Bi2’4(RaC) Razz6
450
13
Table \ . I l l .
- e--AT
(4)
0.3
bIetliod Lead sulfide Fontaotoscope
I
PI
Radon Standard U. S. A . Japan
1340
0.39
0.02
C u r x ;?rr !i:rr
3
1 1.0 1.6fi
2
1.7 1.55
3 8.3 8.70
4
5
7.5 7.55
0.1 6.Gi
The radioactivity of the lead sulfide precipitate as found to be proportional to the radon content of the spring mater, when the sample had stood for approximately 3 hours to establish equilibrium between the radon and its short-lived decay products. The lead sulfide method may be used for the determination of radon in mineral waters (Table VIII). LITERATURE CITED
I Surfoce o f the Earth-+
Razz4 Rn222 PoP’J Pb’” 0.006
.igrcement between Drterniinations by Lead 5itlfitle and Fontactoscopc J I c t hods IO
Q
0.008
D a m o n , P. E., Rev. Sci. Instr., 22, 587 (1961). D a m o n , P. E., a n d K u r o d a , P. K., Nucleonics, 11, Yo. 12, 59 (1953). D a m o n , P. E., a n d K u r o d a , P. K., paper presented a t a n n u a l meeting of American Geophysical Union, Washington, D. C., M a y 1953. Hess, V. F., J . .4tm. and Terrest. Phys., 3, 172 (1953). K u r o d a , P. K., a n d Y o k o y a m a , Y., ASAL. CHEJI.,25, 832 (19.53). , K u r o d a , P. K., and Yokoyama, Y., Bull. Chem. SOC.Japan, 21, 52 (1 94s). IbId., p. 58. K u r o d a , P. K . , a n d Yokoyania, Y . , Citemica2 Reseamhes ( J a p a n ) , 3 , 29 (1948). K u r o d a , P. K . , a n d Yokoyama, Y . , J . Chem. SOC. Japan, 69, \ -
79 (1948).
Figure 1. Time Sequence of Experiments to Determine the Short-Lived Decay Products of Radon in Spring Waters
Lauritaen, C. C., a n d Lauritsen, T., Rev. Sci. I 7 ~ s t r . .8, 438 (1937). Priebsch, J. A,, Gerlands Boztr. Geophys., 35, 304 (1932). Wilson, C. T. R., Proc. Cambridge Phil. SOC.,11, 428 (1902). RECEITED f o r review July 23, 1953. Acoepted April 30, 1954.
