with the generated sulfide ion; or it may interact with thioacetamide directly as well as with sulfide ion. Nickel(II), zinc(II), and cadmium(I1) belong in the first category, being so effectively masked by excess EDTA that no sulfide precipitate is obtained during a 2-hour period at 90 "C. Interestingly, if a small amount of zinc(I1) is added to the solution after the ammonia-thioacetamide reaction has proceeded for some time, a zinc sulfide precipitate forms temporarily, but redissolves quickly as long as excess EDTA remains when the mixture is stirred. When the experiment is repeated with cadrnium(II), the resulting cadmium sulfide precipitate does not redissolve. Copper(II), along with lead@), displays the second kind of behavior, because the copper-EDTA complex does not undergo a direct reaction with thioacetamide and because the precipitation of copper sulfide appears to be controlled by the ammonia-thioacetamide reaction. We established these facts in a semiquantitative way by preparing a reaction mixture initially 0.005M in copper(II), 0.006F in EDTA, and 0.100F in thioacetamide at pZI 10 and by observing that at room temperature no precipitate of copper sulfide resulted from interaction of thioacetamide with the copperEDTA complex. However, when 0.60F ammonia was introduced, precipitation of copper sulfide commenced at a rate comparable to that of lead sulfide under similar conditions.
Inasmuch as the sulfides of mercury(II), silver(I), and bismuth (111) are precipitated rapidly at room temperature in the presence of EDTA, it is evident that a direct reaction occurs between these metal ions and thioacetamide and that these species are members of the third class of reactivity. A number of possible sulfide separations of metal ions can be proposed on the basis of the present study. Mixtures of metal ions, representing each of the three groups of characteristic behavior, should be separable through proper control of the concentrations of EDTA and ammonia and the reaction temperature. As a brief example, one might separate cadmium(II), lead(II), and mercury(I1) by adding an excess of EDTA at pH 10 to complex each of the metal ions and then excess thioacetamide to precipitate mercury sulfide at room temperature, by introducing ammonia and heating the sample at 90°C to form lead sulfide, and by adding excess calcium ion or by acidifying the solution to obtain cadmium sulfide. Future research must include investigations of the kinetics and mechanism of precipitation of other metal sulfides in the presence of EDTA, quantitative tests of the sulfide separations suggested by this work, and examinations of coprecipitation phenomena.
RECEIVED for review August 6, 1969. Accepted October 2, 1969.
K. E. Shuping, G. R. Phillips, and A. A. Moghissil Southeahtern Radiological Health Laboratory, P. 0. Box 61, Montgomery, Ala. 36101 KRYPTON-~Sfound in the atmosphere originates from atmospheric nuclear weapons tests and, more recently, from the operation of nuclear reactors including fuel reprocessing operations. Horrocks (1) first proposed the application of liquid scintillation to the determination of radioactive gases. Setszer et al. (2) and Sax et al. (3)used plastic scintillators for krypton counting. Cohen et al. measured the concentration of * K r dissolved in water using a dioxane-based scintillation solution (4). Curtis et al. (5) reported a liquid scintillation counting technique suitable for environmental 8GKr analysis. It consisted of introduction of krypton with a pressure of 25 mm Hg into a 25-ml evacuated plastic vial and subsequent addition of a toluene-based scintillation solution. This technique suffers from two disadvantages. The amount of krypton which may be introduced into the solution is limited to less than 1 mI (at STP) and the reproducibility of the technique is poor. Dis-
Present address, Southwestern Radiological Health Laboratory, P. 0.Box 15021, Las Vegas, Nev. 89114 ~~
(1) D. L. Horrocks and NI. H. Studier, ANAL.CHEM.,36, 2077 (1964). (2) J. L. Setser, T. C. Rozzell. and B. L. Smith. Radiochiin. Acta. 8, 18 (1967). (3) . . N. I. Sax, J. D. Dennv. and R. R. Reeves. ANAL.CHEM.. , 40., 1915 (1968). (4) J. B. Cohen, J. L. Setser, W. D. Kelley, and S. D. Shearer, Talanta, 15,233 (1968). (5) M. L. Curtis, S. L. Ness, and L. L. Bentz, ANAL.CHEM., 38, 636 (1966). _
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solved krypton escapes from the solution to the void space of the vial and thus changes the counting efficiency. This paper describes a reproducible liquid scintillation technique which is considerably more sensitive than the previously described methods. EXPERIMENTAL
Materials. A Beckman liquid scintillation counting system was used in this investigation. Scintillation grade solutions were prepared by dissolving 7 g of 2,5-diphenyloxazole (PPO) and 1.5 g of p-bis-(0-methylstyry1)-benzene (bis-MSB) per liter of toluene or per liter of p-xylene. The PPO and bis-MSB were received from Pilot Chemicals of Boston, Mass. All other reagents were analytical grade. Counting vials were made of borosilicate glass and were fused to male h e r fittings. The diameter and height of the vials were dictated by the specifications of the counting system. The volume was approximately 25 ml per vial. Stopcocks, couplings, and caps utilizing standard h e r taper fittings were obtained from the Hamilton Company, Whittier, Calif. Pure krypton samples were received from Cryogenic Rare Gas Laboratories, Newark, N. J.; Air Products and Chemicals, Inc., Emmaus, Pa.; and Union Carbide Gorp., kinde Division, East Brunswick, N. J. All samples had a known collection date. Procedure, Handling of krypton was carried out by use of a manifold connected to a closed-end U-lube manometer. The volume of the entire system, excluding the counting vials was approximately 10 ml. Krypton was introduced into the manifold and the attached counting vials to a pressure of 400--600 mm Hg. The pressure of the system was
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
controlled by double stopcocks at the krypton inlet and at the pump connection. The stopcock attached to each vial was closed and the vial was removed from the manifold. The scintillation liquid was deaerated prior to introduction into the counting vial by refluxing for approximately 15 minutes. Subsequently, it was drawn into a 50-ml h e r type syringe through a tube of Teflon (Du Pont). The stopcock attached to the syringe was closed and the solution cooled to room temperature. The vial and syringe were then connected and the stopcocks opened. The liquid was rapidly drawn into the vial and after a few minutes of agitation the entire vial was filled with the scintillation liquid. The stopcock was then removed and the vial capped. Background samples were prepared using the same procedure except no krypton was added. Counting efficiency was determined using a s5Kr standard received from the National Bureau of Standards. Results. The counting efficiency of s6Kr dissolved in scintillation liquid is essentially 100%. The optimum counting conditions, however, are determined by the Y-value defined as the minimum limit of detection at I-minute counting time and 1-u confidence level (6).
Y=
IrB 2.22 E * M
Where B = background in cpm, E = counting efficiency in cpm/dpm, and M = volume of krypton in ml (STP). Optimization of the Y-value resulted in an efficiency of 92 Z and a background of 25 cpm, the Y-value being approximately 0.14 pCi/ml Kr. Under these conditions 1 cpm above background corresponds to 0.025 pCi/ml Kr. The critical point of the procedure was the separation of the vial from the syringe and closing the vial with the cap. The possible loss of krypton at this point was investigated by removing the cap and counting the sample periodically. In the first 10 minutes, no loss of counts was observed and only after an hour could measurable losses be detected. Table I presents the analytical results obtained from 13 dated krypton samples. Each activity shown is the average of at least five individual determinations of the same sample. The standard deviations presented are at the 95 Z confidence level and in each case was less than 6%. The standard deviation is indicative of the total analytical error of the procedure. These values were calculated using a constant concentration of 1.14 ppm of krypton in air. Early in the study both toluene and p-xylene base solutions were evaluated and found to yield equivalent results. All results presented in this paper were obtained using the toluene solution. DISCUSSION
The proposed system combines simplicity, accuracy, and sensitivity for determination of environmental 85Kr. Recount of samples kept for several weeks produced identical results within the expected counting error. The combination of h e r fittings on the vials and plastic valves provided an excellent vacuum seal. Because of the large amount of krypton which may be introduced into the solution, the sensitivity of this method is (6) A. A. Moghissi, H. L. Kelley, J. E. Regnier, and M. W. Carter, Int. J. Appl. Radiat. Isotopes, 20, 145 (1969).
Table I. Krypton-85 Concentration in Air Sample origin Collection date Activity (pCi/ma) Munich, Germany 4/65 10.1 2 z a Essington, Pa. 7/66 9.9 f 4% Essington, Pa. 8/66 9.3 f 3 % Essington, Pa. 9/66 12.1 f 4% Essington, Pa. 10166 11.3 f 4% Essington, Pa. 12/66 8.9 f 4% Essington, Pa. 2/67 11.5 f 5% Essington, Pa. 4/67 11.8 f 3% Lyon, France 12/66 13.9 f 3x Lyon, France 8/68 15.9 i 4% Cleveland, Ohio 8/67 12.0 i 6 z Ontario, Canada 12/67 11.8 i 4% Ontario, Canada 9/68 11.4 f 2z a Standard deviations calculated at the 95 % Confidence level.
*
high. Krypton is remarkably soluble in aromatic solvents to approximately 1 ml Kr/ml solvent (7). Preliminary experiments, however, did not confirm this high degree of solubility. Curtis et al. (5) made a similar observation. Only after deaeration of the solvent could the full solubility of krypton be employed. As expected, the amount of krypton dissolved in the scintillation liquid had no effect on counting efficiency, This was confirmed by the introduction of krypton at pressures ranging from 25 to 600 mm Hg and counting the samples. If the 85Kr concentration in air is sufficiently high, the sample may be introduced directly into the solution and successfully counted. Because of the high beta energy of 8 6 K r , the presence of oxygen does not noticeably affect the counting efficiency. Unfortunately, air is not very soluble in the scintillation solution and a maximum quantity of air corresponding to approximately 150 mm of Hg pressure at room temperature may be introduced into the solvent. The Y-value under these conditions is 0.55 pCi B5Kr/mlof air and 1 cpm above background corresponds to 0.1 pCi ssKr/ml of air. Levels as low as a fraction of the maximum permissible concentration (3 pCi 86Kr/mlof air) are detectable using this technique. Occasionally a small bubble was observed at the top of the counting vial. This, presumably, was caused by a temperature reduction. The counting efficiency was, however, not noticeably affected by the presence of the bubble. The values reported in this paper are in agreement with those reported by Sax et al. (3) for the same geographical locations. In order that an accurate prediction regarding a possible geographical or seasonal variation of can be made, considerably more analytical results are needed.
RECEIVED for review July 10, 1969. Accepted October 6,1969. Mention of several commercial products used in connection with the work reported in this article does not constitute an endorsement by the U. S . Public Health Service. (7) W. F. Linke and A. Seidell, “Solubilities of Inorganic and Metal Organic Compounds,” Vol. 11, 4th ed., American Chemical Society, Washington, D. C., 1965, p 342.
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