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
in organic solvents. Analysis of aqueous samples is less sensitive due to limited solubility in scintillation mixtures and quenching. Quenching, or the attenuation of pulses, causes appreciable reduction in absolute counting efficiency, Furst, a l l m a n , and Brown (6) introduced naphthalene as a secondary '(solvent" and found that counting efficiency for water samples was greatly restored. Summaries of liquid scintillation techniques are available (1, 3, 10, 12). EXPERIMENTAL
Liquid Scintillation Counting. SCINCOUNTER. Samples were counted in a Tri-Carb liquid scintillation spectrometer equipped with an automatic sample changer, and a n automatic readout and 8-channel tape printout system (Model 314, Packard Instrument Co., La Grange, Ill.). The freezer, containing samples, multiplier phototubes, and preamplifiers, was adjusted to 4' C. The high voltage was set a t 1130 volts to obtain the maximum counting rate for a tritium standard. Discriminators were adjusted a t 5 to 72 volts (analyzer channel). REAGENTS. p-Dioxane was used as the solvent (Eastman Organic Chemicals, Rochester, N. Y., m.p. 10.5'11.0' (3.). The primary and secondary TILLATION
I ~
I
I
I
c
L
HOURS A F T E R EXD3SU7E
Figure 1.
50
25
I9
410
r ' I
I I
,
1
'
1;
Figure 2. ANALYTICAL CHEMISTRY
LlGYT
Effect of sunlight and fluoi,escent light on counting rates of blank samples
I G R A M S OF N A P T H A L E N E
-C
ijr '
20
P P O A N D P O P O P A D D E D TO I O 0 0 m i O F D I O X A N E
Efficiencies
of scintillation mixtures
23
24
25
26
27
2'
scintillators, PPO (2,5-diphenyloxazole) and POPOP (p-bis [2-(5-phenyloxazolyl) ] benzene 1, were scintillation grade (Pilot Chemicals, Watertown, Mass.). Naphthalene was used as a secondary solvent (recrystallized from alcohol by Eastman Organic Chemicals). The optimum scintillation mixture was composed of 4.00 grams of PPO, 0.05 gram of POPOP, and 120 grams of naphthalene, all added to 1000 ml. of p-dioxane. SELECTION OF SAMPLE VIAL. The ideal sample container should be inexpensive, an efficient transmitter of light, and free of radioactive contamination. Polyethylene, which showed a blank sample count only slightly higher than that of the empty xell, was chosen over other containers (see Figure 1). Each vial, which costs about $0.04, is used once, eliminating cross contamination.
and scintillation mixture volumes were 3 and 12 ml., respectively.
OPTIMUMVOLUMESOF SAMPLES AND SCINTILLATION MIXTURE. The volumes of scintillation mixture and samples were varied within the 17-ml. capacity of the polyethylene vials. Counting results are shown in Figure 3. The urine sample was spiked a t 35 pc. of HTO per liter and the water sample contained 76 pc. of HTO per liter, The maximum counting rates were obtained with 1 ml. of urine added to 15 ml. of scintillation mixture and 3 ml. of water combined with 13 ml. of scintillation mixture.
INTERNALSPIKINQ OF URINE SAMPLES. The scintillation counting
efficiencies of individual urine samples were inconsistent due to variation in EFFECTOF SLXLIGHT AND FLUORES- solids content and color. Okita et al. CENT LIGHT, Figure 1 shoas increases (11) solved this problem by decolorizin counting rates after exposure of ing each urine sample prior to counting. samples to light. Distilled water and An internal tritium standard was added scintillation mixture, contained in polyand the urine was recounted, thus ethylene, low potassium-40 glass, and determining the counting efficiency for quartz vials, w r e coolcd and counted, each sample. Werbin et al. (IS) obprior to a 10-second exposure to direct tained good counting efficiency by sunlight. The counting rate increased distilling urine and other body fluids to several thousand counts per minute with benzene, prior to analysis. for each sample. Phosphorescence At the Savannah River Plant undiminished with an initial half life of treated urine was accurately assayed 1 to 2 minutes and more than 6 hours for tritium by counting before and n ere required to attain the original after the addition of a small volume of counting rates. One polyethylene tritium spike. For example, a mixture sample vial required an hour to esof 1 ml. of urine sample and 15 ml. of titblish the original counting rate after scintillation mixture counted 514 c.p.m. exposure to bright fluorescent light for One hundred microliters of distilled 3 minutes. water, containing 0.01 pc. of tritium, was These results were similar to those added, and the mixture counted 2814 obtained by Davidson and Feigelson c.p.m.; the spike was equivalent to ( 8 ) for glass vials. They attributed the 10 pc. of HTO per liter of urine. The slow decay of phosphorescence to glass average count for blank urine is 54 rather than to scintillation solution and c.p.m. By using the following formulas, reported that phosphorescence is lower i t was determined that the urine con11ith incandrscent light. tained 2 pc, of HTO per liter. OPTIhlUM
SCINTILL.4TION
MIXTURE.
The ingredients (3,5 ) usually contained in scintillation mixtures for aqueous samples are p-dioxane, PPO, POPOP, and naphthalene. Screening tests. shown in Figure 2, mere made to determine the mivture of these ingredients capable of producing the highest counting efficiency with a spike of 76 pc. of HTO (hydrogen-tritium oxide) per liter. Three commercially available pdioxane solvents were tested. Eastman dioxane (map. 10.5°-110 C.) resulted in a fourfold increase in absolute counting efficiency over the other solvents, using comparable mixtures. This solvent probably contained less of the substances classified as quenchers (1, 3). An increase in naphthalene up to 120 grams was most effective in increasing counting efficiency, while an increase in PPO to 8 grams resulted in further improvement. Mixture number 21 was chosen as optimum, considering efficiency and cost per batch. All samples
$4
LIL
-:-S
:-a
I T_._T
5
Id X T U R E
Figure 3. Optimum volumes of water, urine, and scintillation mixture
the influence of spike volume on the final urine assay was determined. A urine sample which contained 36 pc. of HTO per liter was counted. Internal spiking was accomplished with 0.01 pc. of HTO added in volumes of 50, 100, 150, and 200 pl. The final assays are shown in Table I. On the basis of these tests, the internal spike
C.p.m. with spike - original c.p.m. = c.p.m. due to spike C.p.m. due to spike - c.p.m./pc. per liter due to spike pc. HTO per liter equivalent Original c.p.m. - blank c.p.m c.p.m./pc. per liter due to spike = pc. HTO per liter of sample Since the internal tritium spike is itself an aqueous solution which quenches,
Table 1.
should be in the minimum volume practical for routine analysis.
Effect of Volume of Internal Spike
Quench Factor,c With Effect of Urine,* Spike, Spike, C.P.M./pc./ Assa.y,d pc./ PI. C.P.M. C.P.M. C.P.M. Liter Liter 50 5406 6877 146.1 37.0 1461 100 5466 6912 144.6 37.8 1446 137.0 39.5 150 5412 6782 1370 200 5299 6582 128.3 41.3 1283 All spikes were 0.01 pc. total. All assays in triplicate. All samples, 36 pc. HTO per liter. 0 The internal spike increased the original 1 mi. of urine 10 pc. HTO per liter. 4 Initial urine count + quench factor. Volume of Spike,O
VOL. 33, NO. 3, MARCH 1961
41 1
EFFECTS OF CONTAMINATING NUCLIDESAND ACIDITY. The presence of microcurie quantities of fission products in a urine sample is very unlikely. At the Savannah River Plant no fission products have been detected in urine at levels equivalent to the 1 pc. of HTO per liter level. Contaminating nuclides may interfere in tritium analysis of low-level effluent-water samples. While it would be ideal to distill all samples prior to analysis, distillation of large numbers of samples is time-consuming for routine work. Only a small fraction of water samples assayed for tritium have contained fission products in sufficient quantities to interfere. Most of these were identified by comparison of the ratio of counts in the analyzer and monitor channels of the counter with the ratio obtained for pure HTO standards. Table I1 shows the counting efficiencies and ratios of analyzer and monitor counts for fission products and induced activities. These samples were prepared with 3 ml. of sample and 13
Table II.
Scintillation Counting Efficiencies and Ratios for Radionuclides
Nuclide Tritium Ni-63 C-14 ca-45
Energy, M.E.V.
Sr-89 1-131
8, 1.463
T1-204
ml. of scintillation mixture. Unless otherwise indicated, all m-ere less than 0.1N acid solutions. Most of the tritium scintillations are registered in the analyzer channel, as indicated in Table 11. No voltage setting was found that would eliminate fission product interference without appreciably reducing tritium counting efficiency. The presence of fission products is readily detected, however, by comparing the ratio of counts in the analyzer and monitor channels. If the upper voltage control is set a t infinity, other radionuclides are detected. The ratios (Table 11) obtained for pure HTO and a mixture composed of 90% HTO-10% mixed fission products were 14.0 : 1 and 2.3 : 1, respectively, when the analyzer channel was set a t 5 to 72 volts and the monitor at 72 t o a. RTith the analyzer set a t 5 t o 72 volts and the monitor a t 72 t o 100 volts, the ratios were 22.0:l for pure HTO and 13.9:l for the 90% HTO-10% mixed fission products. Table I1 also demonstrates the effect
8, 0.018 8, 0.067 8, 0.155 8, 0.254
6, 0.765
8, 0.61 (87%)
Ratio of C.P.U. Analyzer: Monitor Channels 5-72 volts: 72-100 5-72 volts: volts 72-m volts 22.0 14.0 5.5 2.4 1.1 3.3 0.30 2.4 0.10 2.7 0.05 2.3 2.6 0.15
Absolute Counting Efficiency, % 5-72 5- m volts volts 9.0 9.6 24 33 50.2 93 21 85 12 100 4 94 9 88
Y,0.4 (80%)
CS-Ba-137 CO-60 Zr-Nb-95 Ru-Rh-106 Ce-Pr-144 Sr-Y-90
Fe-59
Zn-65 Cr-51
Sr-85
6, 0.306
731.2 8,0.4&0.16 Y, 0.75 & 0.76 8, 0.04 & 3.5 Y, 1 . 0 8, 0 . 3 & 3 . 0 y, 0.03 & 3 . 0 6, 0.61 & 2.18 8, 0.27-1.56 y , 0.19-1.29 P, 0 . 3 (2.5%) Y, 1 . 1 (45%) p, None y,0.32 8, None
0.51 Pu-239 a, 5.2 HTO (90yo) & mixed F.P. Sr-Y-90, 8N acjd Sr-Y-90, 2N acid Sr-Y-90, 0.5N acid Sr-Y-90, 0.16N acid HTO, 8N acid HTO, 0.5-47 acid
2.7
0.15
11
81
2.5
0.25
24
100
3.3
0.60
33
85
5.5
0.15
9
68
2.8
0.25
21
100
2.3 3.2
0.08 0.47
7 23
97 69
16.5
1.0
4
8
11.4
1.44
2
3
10.1
1.19
12
22
10 13.9 5.6 4.2 3.2 2.3
0.10
11
100
y,
412
*
ANALYTICAL CHEMISTRY
24 *
2.3 2.2 0.80
0.30 0.10
...
18
16 23 24 56 23 83 a 91 cackground Count 1 1
of acid in destroying the scintillation mixture. Counting efficiency was reduced for samples prepared in greater than 0.1N hydrochloric or nitric acid. The interference of strontium-yttriuni90 is greater in acid solutions because of a downward shift in counting spectra. For example, the counting efficiency of strontium-yttrium-90 in the analyzer channel increases from 7.1 to 23.8% when the fission product is prepared in 2-V acid. Addition of sodium hydroxide showed little effect in counting efficiency. FIXAL PROCEDURE. The liquid scintillation procedure for determining tritium in water and urine samples is as follows: Dispense 3 ml. of 11-ater sample and 13 ml. of scintillation mixture into a polyethylene bottle. Eliminate all direct sunlight and use a minimum of artificial lighting during sample preparation. Place the sample in position in the automatic changer table. Let the sample cool for a t least 30 minutes and count the sample for 30 minutes or more, depending on the required sensitivity level. Use distilled water blanks and tritium standards with each group of samples. After subtracting the blank counts per minute from the sample, calculate the tritium content by comparing the remaining counts per minute with that of the standard. Urine is analyzed by adding 1 ml. of sample and 15 ml. of scintillation mixture to a polyethylene bottle. I n routine IT ork, precooling the scintillation mixture is desirable. One-minute counting is adequate for assaying tritium in urine a t levels of 1 pc. per liter and higher. The internal spiking technique is used to assess the degree of quenching in individual urine samples. One hundred microliters of tritium spike (0.Oi pc. a t the Savannah River Plant) is added to the sample after the initial count. On recounting for 0.5 minute, the additional counts due to the tritium added (70 kc. per liter) are applied in determining tritium in the original sample (see the section on Internal Spiking of Crine Samples). Internal spiking is applied to all urine samples with initial counts greater than 800 c.p.m. The mean quench factor, or counts per minute per microcurie per liter value, obtained from previous analyses. is applied to samples Ion-er than 800 c.p.m. Analyzer-to-monitor ratios for all samples are compared to those of tritium standards (see the section on Effects of Contaminating Nuclides and Acidity). When the ratio is lower than that for the 90% tritium sample (Table 11), the original water sample is distilled and reassayed. Certain 101~level n-ater samples are routinely distilled before analysis to obtain maximum sensitivity. No fission product con-
Figure 4. Apparatus for rate-of-drift tritium determination
taminations were detected in assaying urine samples a t the 1 pc. of HTO per liter level. RESULTS AND DISCUSSION
Liquid Scintillation Counting. The liquid scintillation procedure has been used for routine tritium analyses of water and urine samples for 2 years. This method is unmatched by other methods for accuracy, sensitivity, low cost per sample, and rapid determination. Urine blanks and spikes have varied due to erratic potassium-40 contents and quenching. Over the 2-year period, blank and spiked urine have counted 54 f 20 c.p.m. and 230 =k 30 c.p.m./pc. per liter, respectively. The procedure compared favorably, however, with the previous method of analysis. Sixtyeight urine samples, each assaying higher than 10 pc. of HTO per liter, were analyzed by the potential drop vibrating reed electrometer (V.R.E.) method (6)and by scintillation countiig. Triplicate V.R.E. analyses for each sample resulted in an over-all average of 20.0 pc. of HTO per liter, compared to 19.8 pc. of HTO per liter obtained by averaging 68 analyses by the scintillation method. Comparison a t these levels is reliable, although V.R.E. determinations are relatively inaccurate for tritium concentrations near 1 pc. per liter. Greater sensitivity than 1 pc. per liter is possible without distilling or decolorizing urine. Since the average individual urine counted 230 c.p.m. for each microcurie of HTO per liter, sensitivity to 0.1 pc. of HTO per liter may be attained by counting longer than 1 minute and internally spiking each urine sample. The precision in analysis of a single urine sample is
demonstrated by results obtained before and after spiking the sample. Twenty blank counts were 37.4 d= 5.1 c.p.m. and twenty spike counts were 246.1 + 15.2 c.p.m. per pc. per liter. Water blank and spike counts have counted 25 c.p.m. and 550 c.p.m. per pc. per liter, respectively, during the 2-year period. Table I11 shows the precision and sensitivity obtained for replicate samples of distilled water and tritium spike. Tritium is detected a t 0.005 pc. per liter (1.5 X lo-' pc. per 3-ml. sample). Twenty 0.052-pc. per liter samples counted with a relative standard deviation of less than 10%. Kinard (7) introduced a system of comparing the sensitivities of scintillation procedures, taking into account both the volume of aqueous sample introduced and the counting efficiency. I n this system merit is the product of the percentage of water in a sample and the counting efficiency. Werbin et al. (IS)compared their system with that of others, findiug the merit of their system to be about twice that of the next best system. Although the merit of the Savannah River Plant system is equivalent to that of Werbin, use of the polyethylene vials resulted in greater sensitivity. An additional advantage is that the scintillation mixture costs about 40% less, primarily due to use of a smaller amount of PPO.
Figure 5.
Rate-of-drift tritium determinations
To prevent moisture from entering the ion chamber, the reaction flask was cooled in an ice bath and the time of gas generation was regulated to 15 minutes. Even with these precautions, some moisture escaped from the reaction flask and was trapped in silica gel contained in the U-shaped drying tube. The ion chamber was shielded by lead to reduce the background radiation to one half the unshielded va,lue.
EXPERIMENTAL
Rate-of-Drift Method. APPARATUS. Figure 4 shows the apparatus used for generating tritium gas and determining tritium by rate-of-drift current measurement. The gas generating apparatus is similar to that used for the potential drop vibrating reed analysis previously used a t the Savannah River Plant ( 6 ) . Day and Attix described modifications to the vibrating reed electrometer necessary for rate-of-drift current measurement (4). The ion chamber is surrounded by a lead shield (Technical Associates Burbank, Calif., Model ALlfA). PROCEDURE. The system is evacuated with a Cenco Hy-Vac pump, and then filled with hydrogen and tritium gas (HT) generated by the reaction of water or urine with calcium metal turnings (6). The rate-ofdrift assay is performed by measuring the time (in seconds) required to collect a given cliarge (I00 mv.) on the center electrode of the ion chamber. Tritium content of samples was determined by comparing current drift time with that of standard samples. Several improvements in both the gas generating technique and drift measurement were rewired for additional sensitivity. The closed manometer was used for
RESULTS AND DISCUSSION
Rate-of-Drift Method. The calibration curve, shown in Figure 5, was prepared by analysis of tritium stand-
Table Ill. Scintillation Counting of Tritium in Water Somples" NumHTO, ber
.-.".-~-
Counb C.P.M.
~ n g (5-72V.)
Time, MinUtes
pc. per hter
of Samples
61.2 6.12 0.612
1
1
1 1 1
1
0.052
20
3 in 20
0.025 0.005 0.003 Blank
2 3 3 20
in 10 10 20
0.122
Minus C.P.M. Blank /we./ C.P.M. Liter
33,400 3,420 335
546
559 547 69 569 26.9 555
i2.7
14.5 560 2.9 571 2 . 4 800
28.9 i2.6
Sample and scintillation mixture volumes were 3 and 12 ml., respectively; scintillation mixture Contained 110 erama of naphthalene. Using optimum vdumea of 3 and 13 ml. and mixture containing 120 grams of naphthalene, the counting efficiencyon one instrument in routine use was 11.3%.
in atmospheric pressure, VOL 33, NO. 3. MARCH 1961
413
ards containing 0 to 0.2 pc. of HTO per liter. Thirteen separate gas generations and rate-of-drift measurements of a 0.052 pc. of HTO per liter standard required a drift time of 3535 ==I 194 seconds per 100 mv. The 0.01 pc. of HTO per liter sample, after gas generation, was detected in the amount of 18 d.p.m. of tritium in the 1-liter chamber. This represents a current measurement of 10-17 ampere above background for the instrument used in this analysis.
strument is required. Control samples, containing small quantities of tritium, were analyzed after every four to six unknown samples. Considering the evacuation and gas generation time, and the number of control samples required, one instrument was used t o analyze four to six samples per day. Since the initial expense of the rate-of-draft equipment is small when compared with the Tri-Carb equipment, the rate-of-drift method is desirable for small loads. ELECTROLYSIS
Table IV.
Sample 1 2 3
4
Accuracy by the Rate-ofDrift Method
pc. per Liter Experimental Actual value value 0.022 0,019 0.031 0.024 0.045 0.045 0.062 0,062
Four unknown, low-level samples were analyzed by the rate-of-drift procedure. Drift times were interpolated onto a standard curve to determine tritium. These results are shown in Table IV. Frequent recalibration of the in-
Water and urine samples were electrolyzed by a procedure similar to that of Brown and Grummitt '$). A cast iron cell of 300-ml. capacity was used as the cathode. A nickel screen inside of the cell served as the anode. Samples of water, containing 6% potassium carbonate, were poured into the cell. The solution was cooled by a coil containing tap water. After 36 hours of electrolysis at 25 amperes, the volume was reduced to 10 to 12 ml., with a 12to 14-fold increase in tritium concentration. An automatic cutoff device, based on current conductivity, was installed. Tritium at levels of 5 X IOp4 pc. of HTO per liter was determined within a 2-day period by electrolysis, followed by either scintillation counting or rate-of-drift measurement.
LITERATURE CITED
(1) Bell, C. G., Jr., Hiyes, F. N., North-
western University Conference, August 1957, Pergamon Press, New York, 1958. (2) Brown, R. M., Grummitt, W. E., Can. J . Chem. 34,220-6 (1956). (3) Davidson, J. D., Feigelson, P., Intern. J. Appl. Radiation and Isotopes 2, 1-18 (1957). (4) Day, F. H., Attix, F. H., Natl. Bur. Standards (U. S.) No. 2080 (December 1952). (5) Furst, M., Kallman, H., Brown, F. H., Nucleonics 13, 58 (1955). (6) Hursh, J. E., Ed., "Chyyical Methods for Routine Bioassay, 38-58, AECU-4024, University of 'Kochester, Rochester, N. Y., November 1958. (7) Kinard. F. E.. Rev. Sci. Instr. 28, 293 (1957). (8) Libby, W. F., Phys. Rev. 69, 671 (1946). (9) Natl. Bur. Standards (U. S.), Handbook 69. (10) New England Xuclear Corp., Boston, Mass., Proc. Symposium on Advances ~
in
Tracer Applications of
(October 1958).
Tritium
(11) Okita, G. T., Spratt, J., LeRoy, G. V., Nucleonics 14, 76 (1956).
(12) Packard Instrument Co., Inc., Operation Manual, Tri-Carb Liquid Scintillation Spectrometer Model 314, La Grange, Ill., 1958. (13) Werbin, H., Chaikoff, I. L., Miles, R. I., Espt!. Biol. Med. 102, 8 (1959). RECEIVEDfor review July 28, 1960. Accepted November 4, 1960. Information contained in this article was developed during the course of work under Contract AT(O7-2)-1 with the U. S. Atomic Energy Commission.
Radiotracer Method for Determination of Adsorption of Surfacta nts on Cop per Pht haIocya nine WILLIAM SEAMAN and GEORGE 1. ROBERTS Organic Chemicals Division, American Cyanamid Co., Bound Brook,
b A radiotracer method is reported for determining the adsorption of sodium stearate and cetyltrimethylammonium bromide on copper phthalocyanine b y the use of the carbon-14tagged compounds. By means of an indirect calculation the method solves the difficulty posed b y inability to obtain a supernatant liquor free of dispersed solids with an ordinary laboratory centrifuge. Differences in adsorption values, and surface areas calculated therefrom, are found between undried samples and samples dried b y heating. The significance of these differences is discussed in relation to the surface areas found b y other methods of measurement and to the Langmuir adsorption equilibrium constants. 414
ANALYTICAL CHEMISTRY
T
N. J.
determination of surfactant adsorbed from solution onto a suspended solid may be simple for systems in which complete settling of the solid adsorbent may be effected by filtration, gravity, or centrifuging. This may permit the determination to be made by difference in concentration before and after adsorption. For systems containing suspended solids which are difficult to remove a t equilibrium a less direct approach must be adopted. The need for such a method arises not merely for the determination of adsorption isotherms, but also for the determination of surface areas and derived physical-chemical information. For example, as has been found in the work reported, the surface area may be changed by the very process of drying HE
in preparation for determining it by adsorption of nitrogen. A determination by adsorption from an aqueous medium sometimes leads to a better surface area value. The method reported here is based upon the radiometric determination of surfactants-in this work, sodium stearate and cetyltrimethylammonium bromide. Carbon-14-tagged compounds served as tracers. With copper phthalocyanine dried at 60" C., and with wet press-cake samples of copper phthalocyanine at some low concentrations of surfactant, a negligible amount of pigment remains in suspension after equilibration and centrifuging. A direct approach to the determination is therefore possible. With wet press-cake samples at higher surfactant concentra-
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