shaken by hand, it is better to shake them at intervals of 15 minutes for a period of 1 hour.
the ORNL Chemistry Division who kindly supplied the standard solution of potassium pertechnetate.
ACKNOWLEDGMENT
LITERATURE CITED
The authors are indebted to R. E. Biggers for the many machine calculations made in determining the aborbances of the various species and to J. M. Chilton for his help in attempting to estimate the equilibrium constants. The authors also thank Q. V. Larson of
Extraction in Analytical Chemistry,” pp. 22, 180, 219, 240, Wiley, h e w York, 1957. (6) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., pp. 130, 565, Interscience, New York, 1m.
(1) Boyd, G. E., J . Chem. Ed. 36, 39 ( 1959). (2) Clark, R. E. D., Keville, R. G., Zbid.,
36, 390 (1959). (3) Gilbert, T. W., Sandell, E. B., J . Am. Chem. SOC.82, 1087 (1960). (4) Miller, F. J., Thomason, P. F., ANAL.CHEM.32, 1429 (1960). (5) Morrison, G. H., Freiser, H., “Solvent
( 7 ) United
Kingdom Atomic Energy Authority, PG Rept. : 2 5 ( s ) , September 4, 1959.
RECEIVED for review July 27, 1960.
Accepted November 14, 19G0. Oak Ridge National Laboratory is operated b.y Union Carbide Gorp. for the C. S. Atomic Energy Commission.
Determination of Dissolved Radium A. S. GOLDIN’ Radiological Health Research Activities, Division of Radiological Health, Robert Cincinnafi, Ohio
b Radium carried on barium sulfate is determined by alpha counting. The radium is concentrated from solution by coprecipitation with mixed barium and lead sulfates. The lead and barium carriers are added to a solution containing alkaline citrate, which prevents precipitation until complete interchange has been accomplished. Sulfuric acid is then used to precipitate the carriers and the radium. The barium sulfate is purified by nitric acid washes and is reprecipitated from EDTA solution by treatment with acetic acid. The EDTA masking serves to purify from other naturally occurring alpha emitters and from the lead carrier. Decontamination from other alpha emitters is shown and the problems involved in specific determination of any of the radium isotopes are discussed. Isotopic nature of the radium is determined by measurement of ingrowth or decay. Growth and decay curves for radium-226, radium224, and radium-223 are presented.
T
HE DETERYIN.4TION
Of
radium-226
at levels of a few micromicrocuries (or picocuries) which is a unit of curie has become a matter of considerable public health interest. The occupational maximum permissible concentration (MPC) of this nuclide in drinking water is lo-’ pc. per ml. (6), or 100 picocuries (pc.) per liter, which is lower than that of any other radioisotope. For application to the general public, levels as low as 1 to 10% of this are of interest (7). Radium is present in uranium ores to an extent which varies somewhat, but is roughly comPresent address, New York University Medical Center, New York 16, N. Y .
406
ANALYTICAL CHEMISTRY
A. Tuff Sanitary Engineering Center,
parable to the equilibrium amount of 300 mc. per ton of uranium. Considering the extent of our uranium industry, the introduction of any appreciable fraction of this radium into public water supplies could be of great importance. For this reason, methods for the determination of radium which would be suitable for use by state and other public health agencies have been investigated. The method would have to be sufficiently sensitive to permit determination of radium at a level of 5% of the M P C or less-i.e., 5 X pc. per ml. ( 5 pc. per liter)-without interference from larger quantities of other natural alpha emitters. It should be applicable to radium in waters of a wide variety of mineral content, particularly those with a high sulfate content, and should be simple to permit analysis by technicians familiar with equipment commonly found in chemical laboratories and measurement by routine counting equipment. A method which did not require long periods for ingrowth or decay of radioactive materials would be preferable. Two methods are commonly used for radium determination-measurement of the gaseous radon daughter or coprecipitation with barium salts, especially sulfate. Determination through the radon daughter, although a method of choice under suitable conditions, was ruled out because a rather long waiting period is required unless it is known that radioactive equilibrium has been attained. I n addition, specialized equipment is required for handling the counting radon gas. Several problems must be considered in the determination of radium on barium sulfate. Naturally occurring sulfate ion may cause premature pre-
cipitation and thus prevent interchange of the radium and barium. Although radium can be adsorbed on barium sulfate, this adsorption is hindered by substances which might be present in many waters (6). Unless the barium carrier is restricted to a very small quantity, excessive self-absorption losses in alpha counting will occur. Finally, since very small quantities of radium may have to be determined in the presence of much larger quantities of other alflha emitters, very good decontamination is required. REAGENTS
Ba(NOq).. 0.10N Pb(N0jj;j 1N Citric Acid, H3C~H607,1J1 (should contain 0.1% phenol to prevent biological growth). PROCEDURE
Add about 5 ml. of citric acid to the sample and make it alkaline with ammonium hydroxide. Add 2 ml. of lead carrier and 1 ml. of barium carrier. Heat t o boiling and add sufficient sulfuric acid (1 to 1 by volume) to precipitate substantially all the lead (This may be done by adjusting to p H 1 with a meter, by adding about 0.25 ml. in excess after neutralizing to methyl orange or methyl red, or simply visually by the amount of precipitate.) Collect the precipitate; wash it twice with concentrated nitric acid. Dissolve the precipitate in alkaline EDTA (disodium salt, 0.25111) and reprecipitate the barium sulfate by dropwise addition of glacial acetic acid in excess. When necessary, repurify the barium sulfate by a second solution in alkaline EDTA and reprecipitation with acetic acid. In this case, add a little ammonium sulfate to the EDTA solution to ensure complete precipitation. Wash the barium sulfate precipitate and transfer to a planchet for counting.
EXPERIMENTAL
Concentration. Initial attempts to concentrate 1 mmole of lead and 0.05 mmole of barium from a liter of sample by precipitate as carbonate or oxalate failed because of premature precipitation, presumably by sulfates present in the water. Complexing by ammoniacal E D T A , followed by demasking and precipitation through p H adjustment, was next tried. Although barium could be satisfactorily recovered by this technique, no conditions adequate for the precipitation of lead as oxalate, sulfate, chromate, or carbonate could be found. Citrate proved a satisfactory masking agent. Xeither lead nor barium precipitated in alkaline citrate solution, while both could be precipitated a t p H 3 as sulfate, chromate, or oxalate. Sulfate was chosen as precipitant because it is less sensitive to p H and more specific. The mixed lead and barium sulfates, precipitated from hot solution, settled extremely well and were satisfactorily separated from the bulk of the solution by decantation. Only traces of lead could be found in the decanted supernatant (sulfide test); none in filtered supernatant. Washing. It was originally planned to convert the precipitated sulfates to nitrates and reprecipitate in strong nitric acid. However, since the sulfates did not dissolve in reasonable quantities of 8N nitric acid, a concentrated nitric acid wash was substituted. KO lead or barium was lost since their nitrates, if partially formed, are insoluble in concentrated nitric acid. This wash was found useful in decontamination, particularly from polonium. Reprecipitation. The washed lead and barium sulfates did not dissolve in water, indicating nonconversion to nitrates, b u t did dissolve when E D T A and ammonia were added. On addition of excess acetic acid to this ammoniacal E D T A solution, the barium sulfate was reprecipitated, while the lead remained in solution. The solution and reprecipitation of the barium sulfate by the ammoniacal EDTAacetic acid cycle can be repeated as often as needed for further decontamination. I n all such cycles after the first, additional sulfate should be present to ensure quantitative reprecipitation of barium. Counting. The final barium sulfate precipitate is washed with water, transferred to a planchet, dried, flamed, weighed, and alpha counted. The best procedure found for obtaining a uniform deposit was to transfer the precipitate, with 15 to 25 ml. of water, to an Atomlab-Ekstein centrifuge tube, let i t settle for 10 to 20 minutes, and then centrifuge the barium sulfate into the planchet. EVALUATION
Decontamination. Since the maximum permissible concentration of radium-226 in water is much lower
BECAUSE OF INSTRU-
b Figure 1. Growth of alpha activity in radium226 Curves calculated 0- Experimental points
0
1
2
3
4
5
6
7
8
9
10
II
12
1314
TIME, DAYS
than t h a t of any other alpha emitters with which it may be associated (6), determination of small quantities of this nuclide in the presence of a large quantity of other alpha emitters is of considerable interest. Consequently, one of the prime requisites of any radium method is complete elimination of other radioactive interferences. The removal of the natural alpha emitters uranium, thorium, and polonium was tested by carrying out the procedure in solutions containing these materials but no radium. Table I shows the effective decontamination from these nuclides. Other natural elements that must be removed are radon and lead. The effectiveness of removal of radon is shown by the rate of ingrowth of activity in a Bas04 precipitate, compared with the theoretical values for pure radium and for radium contaminated with different amounts of radon, as shown in Figure 1. The data indicate a contamination certainly not greater than 0.5%. (Since this work,
Table 1.
Element U(nat)
Th-234
Po-210
Ratio
Activity Added, C.P.M.
decontamination factors on the order of a t least 10,000 have been demonstrated in field samples where radon was present in great excess.) Removal of lead was checked chemically and a decontamination factor on the order of 20,000 to 50,000 was found. The radioactive elements protactinium and actinium have long-lived isotopes which either emit alpha particles or have alpha-emitting daughters. Although decontamination from these two elements was not tested, their general chemical behavior is such that they would be expected to remain in solution as EDTA complexes in acetic acid. Furthermore, both of these occur only in the minor uranium-235 series and would not be expected to be a major contaminant of most environmental samples. They could also be distinguished from radium-226 by growth and decay characteristics. The remaining radioactive elements-francium, astatine, bismuth, and thalliumhave no long-lived isotopes which either
Decontamination from Other Elements
Activity Recovered in Bas04 % C.P.M. 78OOb 0.7 0.009 0.7 0.009 78005 39,000b 1.2 0.003 1.2 0.003 39, oooc 2.4 0.024 10,OOOb 10,oooc 5.6 0.056 50,O0Ob 2.9 0.006 10.2 0.02 50,000" 78,5OOb 1446 1.84 78, 500b 14 0.02 78,500d 13 0.02 78,500d 19 0.02 of contaminant added to contaminant recovered. barium sulfate washed once with nitric acid. barium sulfate not washed with nitric acid. barium sulfate washed twice with nitric acid.
Decontamination Factora 11,000 11,000
.
32,000 32,000 4200 1800 17,000 4900 54 5600 6000 4100
* Lead + Lead + * Lead +
c
VOL. 33, NO. 3, MARCH 1961
407
to isolate additional radium from the various supernatant liquids in these runs failed. A counting efficiency of 33% rather than the 51 % characteristic of weightless radium samples is not entirely unreasonable since the sample weight was on the order of 2 mg. per sq. em.
L !
DISCUSSION
0
There are four naturally-occurring radium isotopes. Radium-226 occurs in the uranium-238 series, radium-223 in the uranium-235 series, and radium-228 and radium-224 in the thorium-232 series. Radium228 is a beta emitter 11-ith a 6.7-year half life. Its first daughter, actinium228 with a 6-hour half life, is also a beta emitter, and its second daughter, thorium-228 with a 1.9-year half life, is an alpha emitter. Because of the 1.Q-year half life of thorium-228, the ingrowth of alpha activity into radium228 samples is extremely slow and radium-228 may, therefore, be disregarded as a contributor to the measured radium alpha activity. The other three naturally occurring radium isotopesradium-226, radium-224, and radium223-are alpha emitters and each gives rise to a family of relatively short-lived daughter products including three more alpha emitters. The series of descendants of each of the radium isotopes is shown in Figure 2. Because of the different half lives involved in these series, the radium isotopes can be identified by the rate of ingrowth of their daughters in the barium sulfate precipitate. The activity of radium-226 increases a t a rate governed by the 3.8-day half life of radon-222; the activity of radium-223 increases very rapidly so that it is complete by the time the precipitate can be prepared for counting and then decays with a half life of 11.7 days; the Isotopic Purity.
Figure 2. ters
Radium isotopes and daugh-
emit alpha particles or have alphaemitting daughters. An exception to this rule is bismuth-210 (a beta emitter with a 5-day half life) whose daughter is the alpha emitter, polonium-210. I n this case, however, the polonium-210 activity is less than that of the bismuth210, approximately in the inverse ratio of their half lives. Satisfactory decontamination from all the significant naturally-occurring alpha-emitting nuclides is, therefore, assured. Recovery. Recovery of added radium was checked by the use of standardized radium solution obtained from the Kational Bureau of Standards. Liter volumes containing from 44 to 6700 d.p.m. of radium were analyzed by the above procedure. Radium recovery is shown in Table 11. The average recovery for the 17 runs, expressed as counts per minute per disintegration per minute was 0.33 =k 0.04. The difference between this and the expected value of 0.51 is probably due to self-absorption losses rather than to chemical losses since repeated efforts
Table II.
Radium Taken,
Radium Recovered, C.P.M .
.
79 135 385 1062 502 538 516 5.4 19.0 28 41 107 546 1090 490; 496 1098; 1150 2360; 2440
D.P.M. 220 440 1110 3330 2220 2220 2220 44 90 180 -~ 220 330 1650 3300 1650 3440 6700
0
Recovery of Added Radium
Chem. Yield @a), %
...
... ... ...
80 93 84 41 60 57 62 99 100 92 72 87 94
Corrected t o basis of 100% recovery of Ba carrier.
408
ANALYTICAL CHEMISTRY
Radium Recovery, Corrected5 C.p.m. C.p.m./D.p.m.
628 578 614 13.2 31.7 49 66 108 546 1185 683 1260 2520
0.36 0.31 0.35 0.32 0.28 0.26 0.28 0.30 0.35 0.27 0.30 0.33 0.33 0.36 0.41 0.37 0.37
1
I
,
I
2
3
1
I
*no
I
JY-226
I 4
.
6
7
TIME, H Q S
I
:t
r!
Figure 3. Growth of alpha activity into pure radium isotopes
activity of radium-224 and its first two daughter elements is substantially complete in a few minutes and the activity then increases slowly as lead-212 grows in with a half life of 10.6 hours, in opposition to the 3.6-day decay of the radium-224 itself. The validity of such an isotopic determination Kill, of course, depend on the complexity of the gron thdecay measurements. The growth curves of the three radium isotopes are shown in Figure 3. Features. The method presented here, which gives in detail a procedure previously outlined briefly ( d ) , includes three major factors which constitute improvements over previously used procedures (1, 3) : First, premature precipitation is prevented by the use of alkaline citrate; secondly, lead is used as a simultaneous cocarrier with barium: and thirdly, (ethylenedinitri1o)tetraacetic acid (EDTA) is used as a masking agent in decontamination. Since preparation of this manuscript, a method (2) has appeared which includes the masking by EDTA but not the use of citrate acid or the simultaneous use of lead and barium. The alkaline citrate solution complexes lead and barium, even in the presence of sulfate which may be present in natural waters, until they are thoroughly mixed into the solution and so have become interchanged with the radium. This ensures that the barium sulfate, when precipitated, will carry the radium quantitatively, without the uncertainties of depending on an adsorptive take-up of the radium. The use of a double carrier (lead plus barium) simplifies the concentration procedure. A total of over 300 mg. of precipitate are obtained, so that losses on the order of a milligram or so in the decantation procedure are negligible.
Furthermore, since barium sulfate is much less soluble than lead sulfate, it is completely precipitated (along with radium sulfate) when conditions are such that most of the lead is precipitated. ii secondary benefit is that the heavy lead sulfate settles very well, making it possible to decant large volumes of liquid with no significant loss of precipitate. The large quantity of lead sulfate carrier, which is desirable in precipitating from a large volume, is eliminated later in the procedure by formation of the EDTA-Pb complex, and so does not interfere in the counting procedure, as a similar quantity of barium would do. The purification of the barium sulfate by its solution in alkaline E D T A and reprecipitation by addition of acetic acid has proved extremely satisfactory. Coprecipitation of such ions as lead, thorium, and uranyl has been very small. This is presumably because the very large complex ions formed by these metals with E D T A cannot fit into the barium sulfate crystal lattice, while the small metallic ions, even those whose sulfates are soluble. can replace barium in the crystal to come extent. Furthermore, E D T S solution is a very simple method for dissolving the otherwise insoluble barium sulfate, without recourse to such tediouq procedures as metathesis. \171iile the method refers to dissolved
radium, it is recognized that some radium in suspension will be carried down with the barium sulfate precipitate and Bill eventually appear in the final counting specimen unless the sample is first filtered. The term dissolved radium was used t o indicate that this method does not necessarily recover all radium present. For example, it is possible that some radium may be present in a silicate material in such a form that it is not accessible to the reagents used, and, therefore, will be discarded. For a rigorous measure of truly dissolved radium, the sample must be filtered before analysis, care being taken that this is done in such a way t h a t there will be no losses by adsorption or otherwise. Similarly for a rigorous measure of total radium, it will be necessary t o recover the insoluble radium and place it in solution by some rigorous treatment, such as carbonate fusion. I n practice the method has been applied t o natural waters, to mill effluents, and to samples resulting from the leaching of soils and the ashing of biological materials. The lowest limit of sensitivity is limited by the small amounts of radium-226 to be found in reagent grade chemicals and also by the length of time permissible in counting. In our case, this has usually been 100 minutes, using instrumentation with an alpha efficiency of 50%
at a background of approximately 0.25 c.p.m. Sensitivity of the method is about 1 pc. gram) of radium and reproducibility (when this is not limited by counting statics) is about =k10%. LITERATURE CITED
(1) DeSesa, M. A., U. S. Atomic Energy Comm. Doc. WIN-I01 (December 1958). ( 2 ) Ebersole, E. R., Harbertson, A.,
Flygare, J. K., Jr., Sill, C. W., “Determination of Radium-226 and Thorium-230 in Mill Effluents.” u. S. Atomic Energy Comm. internal doc., Idaho Falls, Idaho. (3) Jenkins, E. N., Sneddon, ,G. W., U. K. Atomic Energy Authority Doc. AERE C/R 2385 (Xovember 1953). (.4,) Kahn. B.. Goldin. A. S.. J . Am. Water Work; Assoc. 49, 767 (1957). (5) Khlopin, V. G., Merkulova, M. S. Izvest. Akad. Nauk S. S. S.R. Otdel. Khim. A-auk 5 , 461 (1949); Nuclear Sci. Abstr. 4, 1061 (1950). (6) National Committee on Radiation Protection and Measurements, National Bureau of Standards Handbook 69, U. S. Government Printing Office (June 1959). ( 7 ) Tsivoglou, E. C., Shearer, S. D., Jones, J. D., Sponagle, C. E., Pahren, H. R., Anderson, J. B., Clark, D. A., Survey of Interstate Pollution of the Animae River (Colorado-New Mexico), U.S. Department of Health, Education, and Welfare (Public Health Service), Robert A. Taft Sanitary Engineering Center, Cincinnati, Ohio (May 1959). RECEIVED for review June 20, 1960. -4ccepted January 9, 1961.
Determination of Tritium in Water and Urine Liquid Scintillation Counting a n d Rate-of-Drift Determination FRANK E. BUTLER Savannah River Plant,
E. 1.
du Pont de Nemoors & Co., Inc., Aiken, S. C.
b Two methods are presented for the accurate determination of tritium in urine and water. One method, liquid scintillation counting, utilizes an improved scintillation mixture and disposable, low background, polyethylene vials. Untreated urine samples are assayed a t tritium levels of 1 p c . per liter after a 1-minute count. The lower limit of detection of tritium in water i s 0.005 pc. per liter and 0.05 pc. per liter can b e determined with a relative standard deviation of less than 10% after a 30-minute counting period. Results are comparable with the second method of tritium determination, using a vibrating reed electrometer rateof-drift determination. The vibrating reed procedure requires less expensive equipment and i s suitable for analyzing 4 to 6 samples per day.
T
is present in air and surface maters as a naturally occurring nuclide (8). It is also a product of nuclear industry and may become a health problem wherever heavy water is used as a reactor moderator. Tl’ater and urine samples must be analyzed frequently to evaluate possible biological influences of tritium. Urinalysis sensitivity of 1 pc. of tritium per liter is considered adequate for a radiological health program. The maximum permissible body burden for tritium recommended by the National Committee on Radiological Protection is 1000 pc. (9). Neilson summarized current methods of urinalysis used at atomic energy installations in the United States (6). Water samples must be analyzed for tritium in amounts lower than 1 fit. per liter for adequate regional RITIUM
monitoring in the vicinity of heavy water moderated reactors. Tritium in mater is also determined by methods reported by Neilson (6). Brown and Grummitt (2) concentrated tritium in natural water by electrolysis, prior to analysis. Day and Attix (4) reported accurate determination of minute currents in a vibrating reed electrometer. By means of a rate-of-drift current measurement, currents as lorn as lo-“ ampere were measured. Electrolysis, followed by rate-of-drift determination, was used a t the Savannah River Plant during initial investigations t o assay tritium in water a t levels below 1 pc. per liter. These procedures are suitable for high sensitivity determinations of a limited number of samples. Liquid scintillation counting has most often been used to determine low energy nuclides in compounds soluble VOL. 33, NO. 3, MARCH 1961
409
