V O L U M E 25, NO. 10, O C T O B E R 1 9 5 3 of the area. Fx this r c ~ a s O 1 lamong , others, it is desirable in spectrochemical analysis to usc slits in both spectrograph and densitometer which are no smaller than necessary to obtain sufficient resolution. The linearity of the density scale depends upon the precision with which the logarithmic apertures are machined. Each blade is carefully hand filed down to a tool-hardened master. The departure from linearity can be measured and corrected for, if the highest precision is required. This is done by carefully marking two points on density steps differing by about 0.15 and measuring this density difference all along the scale as the zero setting is changed. The results of this measurement on one instrument are shown in Figure 8. Cumulative density error (departure from linearity) is plotted. The average reading is assumed to be the true density difference of the test steps, which automatically makes the cumulative error a t density 2.00 vanish. For any differential density reading B - A ( B being more dense), the rorrection to be added is Ea - EB. The light scattered into the 13-micron slit n-hen it is obscured
1477 by a 100-micron wire is less than 1 %-that sity greater than 2 is obtained.
is, a reading of den-
ACKNOWLEDGMENT
The authors would like to express appreciation to C. G. Bearce, Langdon C. Hedrick, and D. E. Williamson for assisting with some of the details of the mechanical and electronic design and the construction of the modpl. LITERATURE CITED
(1) Baird, W. S., J . Opt. SOC.Amer., 31,179 (1941) (2) Carpenter, R. O’B., Ibid., 36, 676 (1946). (3) Jones, L. A , and Higgins, G. C., Zbid., 36, 203 (1946). (4) ;\lorriaon, C.d.,Ibid., 42,90 (1952). (5) Radio Corp. of America, “RCA Handbook.” (6) Sweet, LI.H . , J . O p t . SOC.Amer., 37,432 (1947); Eldronics, 19, 105 (1946).
RECEIVED for re\iew May 6, 1953. Accepted July
10, 1953. Presented a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectrosr o p y . Jfarch 2 to 6, 1953.
Estimating the Tritium Content of Tritiated Water WILMER A. JENKINS’ Department of C h e m i s t r y , California I n s t i t u t e of Technology, Pasadena, Calif. Present methods for the radioactive assay of tritiated water are somewhat laborious and require specialized equipment. For this reason, a simpler method, involving the use of a windowless flow counter, would be desirable. I t was found that a method, based on measuring the activity of solid ammonium chloride which had been rendered radioactive by exchange with tritiated water, could be developed to give reproducible results. The accuracy of the method is at present limited to 15%. In its present state, this method should be particularly useful for the rapid determination of the approximate activity of tritiated water samples in experiments where an accurate figure for the activity is.not necessary.
T
H E advent of solid phase tritium counting techniques ( 2 , 3 ) has opened up new possibilities for the rapid radioactive a m y of tritiated compounds. For experiments which involve tritiated water (HTO) and in which an accurate value for its activity is not necessary, it would be useful to have a method by which its tritium content could be rapidly estimated with fair accuracy. Such a situation might arise, for example, if an investigator were measuring the rate of exchange of tritium between some hydrogen-containing solute and tritiated water and u-ished to know the tritiated water activity only accurately enough to make the correct dilutions of his stock solution so that the solute activities which he was measuring would be in the optimum range for counting. For such an assay method, one would choose a hydrogen-containing compound which was solid a t room temperature and which could be easily made radioactive. Then the solid would be rendered radioactive t o a known extent by treating it in an appropriate way with tritiated water of known activity. The activity of the solid could then be measured in a windowless flow counter, thereby determining the ratio
One could then measure the activity of sample of unknonn activity by treating the solid i n the same way with the unknown and measuring the resulting activity of the solid in the flow coun1 Present address, Pigments Department, E. I. du Pont de Neinours 8Co., Wilmington, Del.
ter; the activity of the unknown could then be calculated from the known value of S. In this work, ammonium chloride was the hydrogen-containing solid used. (Preliminary experiments were also carried out with copper sulfate and magnesium perchlorate. The resulting hydrates were found to be unsatisfactory; a steady loss of activity was observed when they were counted, because of volatilization of tritiated water from the solid.) Although the experiments were not as extensive or as complete as might be desired, they served to outline the difficulties and point the m y toward a more complete development of the method as an accurate analytical tool. EXPERIMENTAL
Preparation of Materials. Tritium was obtained from the Argonrie Xational Laboratories in the form of hydrogen gas containing 2.6 curies of tritium. This gas mixture waa converted to water by diluting it with tank hydrogen and passing it slowly over copper oxide at 350’ C. The resulting tritiated water vapor was trapped with liquid air, diluted with inactive water, and distilled several times in an all-borosilicate glass still. An aliquot portion of this water was then completely converted to hydrogen by passing it over magnesium turnings a t 450“ C. (4). The absolute activity of the resulting hydrogen-tritium mixture wa8 measured in an ionization chamber ( 1 ) . This assay method is accurate t o about 10%. The major source of uncertainty in this measurement wm the capacitance of the ionization chamber. It is hoped that in the near future, the capacitance can be measured to about 1 or 2%, thereby reducing the error in the determination of tritiated water activity t o 2 or 3%. Baker’s Analyzed ammonium chloride, dried at room tempera-
ANALYTICAL CHEMISTRY
1478 ture in vacuo over magnesium perchlorate, was used without further purification. Exchange and Counting Procedure. hmmonium chloride was rendered radioactive by exchange with standard tritiated water by placing accurately weighed bortions of ammonium chloride in the bottom section of the still (Figure 1). A known amount of standard tritiated water was pipetted in. The resulting solution was allowed to stand a t room temperature for 10 minutes to ensure exchange equilibrium (although equilibrium is probably reached in a very few seconds), and the tritiated water was then distilled off in a hood, using the apparatus shown in Figure 1. .4fter the still had been gently warmed with a Bunsen burner, it was taken apart, and the bottom section which contained most of the tritiated ammonium chloride (NHsTCI) was replaced on the hot plate. ~
Figure 1. Apparatus for Distilling HTO from HTO-NHITCI Solutions A. B. C. D. E.
Bottom section of still Top section of still Flanged ground joint Ears for washbottle springs Hot plate
Two procedures were used t o remove the last traces of adsorbed water from the tritiated ammonium chloride. In the first method, the solid material was heated for about 10 to 15 minutes on the hot plate until about 50% of it had sublimed away. The solid was then judged to be dry. The second procedure involved heating the solid tritiated ammonium chloride until dense white fumes of vapor began to appear. At this point, the bottom section of the still containing the solid was removed from the hot plate and allowed t o cool. The salt was then scraped out of the still, ground up well, and placed in a crystallizing dish. The dish was covered with a watch glass containing cold water and placed on the hot plate. I n about 20 minutes, all of the tritiated ammonium chloride had sublimed and condensed on the watch glass: the watch glass was then removed and the material scraped off into a mortar. Before counting, the tritiated ammonium chloride was ground and mixed up well. For counting, about 0.75 gram of the ground-up salt (enough to ensure infinite thickness) was placed in an aluminum planchet 7.30 sq. cm. in area. The salt surface was packed down and smoothed out with a spatula. The planchet was then placed in the planchet holder of a windowless, &-gas (99% helium, 1% butane) flow, Geiger-Muller counter (Nuclear Instrument and Chemical Co., Model D-46A) and counted in the usual way. Pulses from the counter were detected with a Berkeley Decimal Scaler, Model 1000. Smaller amounts of the salt can be used t o fill the planchet. However, care must be taken t o ensure that a t all points in the planchet, the thickness of the salt layer is greater than infinite thickness, which is about 1 mg. per sq. cm. The counter was operated at 1200 volts, in the center of a plateau which was about 250 volts long and which had a slope of about 0.04% per volt a t 1200 volts. Counter and scaler were checked from time to time with a cobalt-60 source of known activity. After counting, the salt was removed from the planchet, mixed up well, repacked in the planchet, and recounted. In many cases, 10 or 15 refillings and recountings (referred to below
as countings) were made in oidei to obtain a reliable value for the activity. All samples were counted for times long enough t o give a standard deviation equal to or less than 1%. Coincidence corrections were made on all counting rates above 1600 counts per minute. RESULTS
Preliminary experiments Jvere carried out to examine the counting characteristics of tritiated ammonium chloride. A sample was prepared as described above, dried by the first method, and counted. I t was found that the counter did not become contaminated with tritiated water or tritiated ammonium chloride vapor and that the active salt could be left in the open overnight without its activity decreasing to any significant extent. Furthermore, successive samples of tritiated ammonium chloride, prepared from the same weight of ammonium chloride and the same volume of tritiated water, gave the same counting rate within aliout 1 or 2%. In the next group of experiments, a dilution curve was made from data obtained by rendering successive portions of ammonium chloride active by exchange with tritiated water samples of varying activity. A plot of the observed tritiated ammonium chloride activity us. the tritiated water activity should be a straight line which passes through the origin, showing that the S is a true constant. These experiments were carried out with samples which had been made up by diluting standard tritiated water with various amounts of distilled water. 411 exchange solutions were prepared by dissolving 2.86 grams of ammonium chloride in 10 ml. of tritiated water. The tritiated ammonium chloride was dried by the first method. A plot of the results obtained showed that under these experimental conditions, S was not a true constant and that some tritiated water may have been left adsorbed on the salt even after the drastic drying procedure used. A second dilution curve was then made from data obtained using the same concentration of ammonium chloride as in the previous euperiments, hut using the second method to dry the ammonium chloride. Thc initial concentration of ammonium chloride in the ammonium chlori le-tritiated water equilibration mixture is represented by (NH
