this technique proved to be less sensitive than the slow drop method and was utilized only in the exploratory phases of the study. All the experimental evidence gained thus far indicates that the method could be used for the analytical determina-
tion of tin(1V) in concentrations ranging from 0.1 m M t o 3 m M with acceptable accuracy.
RECEIVED for review November 14, 1969. Accepted January 19,1970.
Determination of Oxygen in Sodium Photon Activation AnaIysis G . J. Lutz National Bureau of Standards, Washington, D . C. 20234 THEvery desirable thermal and nuclear properties of liquid sodium make it useful as a heat transfer medium for nuclear power plants. However, trace amounts of carbon, oxygen, and hydrogen in the sodium may cause carburization, corrosion, or embrittlement of structural components with which the sodium comes in contact. F o r control of these elements, accurate analyses are neczssary. Pepkowitz and Judd ( I ) have described a procedure for the determination of oxygen in sodium. Their method depends on the physical separation of sodium from oxygen compounds of sodium by repeated extractions with mercury. The sodium oxide is insoluble in the amalgam and floats o n the top. After extraction, the residue is dissolved in water and titrated with acid or the sodium determined by flame photometry. White et al. ( 2 ) have published a method based o n the fact that alkyl halides react with sodium to form neutral salts, but d o not react with sodium monoxide. After the reaction, the residue is dissolved in water and the resulting sodium hydroxide is titrated. deBruin (3) has described a modification in which the sodium monoxide is metathesized to sodium carbonate which is determined by infrared spectrophotometry. Other methods include distilling the sodium from the oxide ( 4 ) and a n on-line method, the plugging meter (5),which depends on the formation of a saturated solution of the oxide in the metal at a known temperature. This paper describes the determination of oxygen in sodium by high-energy photon activation and a rapid separation of the resulting radioactive oxygen. Etching of the sample after irradiation, but before separation and counting, assures removal of surface contamination. The separation involves dissolution of the sodium sample in a dilute sodium hydroxide solution under nitrogen. The radioactive oxygen, present as N a 2 0 , is converted to N a O H and exchanges with the water. A portion of the water is rapidly distilled from the mixture and counted by coincidence spectrometry. Nuclear Considerations. Oxygen has three stable isotopes, l6O, ''0, and l8O with abundances of 9 9 . 7 6 z , O.O37z, and 0.204 %, respectively. The nuclear reactions of these isotopes (1) L. P. Pepkowitz and W. C. Judd, ANAL.CHEM.,22, 1283 (1950). ( 2 ) J. C. White, W. J. Ross, and R. Rowan Jr., ibid., 26, 210 (1954). (3) H. J. deBruin, ibid., 32, 360 (1960). (4) J. Humphreys Jr., Chem. Eng. Progr. Symp. Ser., 53, (20) 8
(1957).
(5) I. L. Gray, R. L. Neal, and B. G. Voorhees, Nucleonics, 14, (lo), 34 (1956).
Table I. Nuclear Reactions of Oxygen of Potential Use in Activation Analysis
Reaction '80(n,y) '60(n,p) '6N lQ(aHe,p)l8F W(p, n) l8F '60(d,n) 17F 1 8 0 ( ~ ,PI 1"
IOO(y,n)1 5 0
Product half-life 29 sec 7.4 sec 110 min llOmin 66 sec 4.4 sec 124 sec
Cross section 0.0002 b for thermal neutrons 0.05 b a t 14 MeV 0 . 4 b a t 7 . 5 MeV 0.5bat6.8MeV 0.06 b a t 3 . 2 MeV 0.15 MeV-b integrated to 25 MeV 0.31 MeV-b integrated to 24-MeV
of potential use in activation analysis are summarized in Table I. can be rejected because of the The reaction l80(n, low abundance and low capture cross section of I 8 0 . The reaction 16O(n, p)l6N, induced by 14-MeV neutrons, has been used for the determination of oxygen in a large variety of materials including sodium (6). Its sensitivity is about 100 micrograms. The charged particle reactions, 160(He3,n)18F,l8O(p, n)'8F, and '60(d, n)"F have sensitivities in the submicrogram region, but charged particles are not very penetrating and activation of the surface of the sample of a reactive metal like sodium may not give an analysis representative of the bulk. The reaction 18O(y, p)"N is very attractive as the product nucleus decays by neutron emission, making its detection in even a complex sample quite simple. However, the low abundance of l8O reduces the sensitivity of this reaction by about that of the l6O(n,p)'6N reaction. The reaction used in this work was l6O(y,n)I5O. Although the intrinsic sensitivity for the determination of oxygen by this reaction is in the region of a few hundred nanograms, it is difficult to exploit this sensitivity because of the short half-life of the product nucleus and its non-unique mode of decay; l60 decays by positron emission as do the (y, n) products of all of the elements between carbon and the rare earths. Holm and Sanders (7) and Engelmann and Loeuillet (8) have used this reaction to determine oxygen in sodium a t levels of a few tens of ppm without separation utilizing sophisticated counting techniques. (6) I. Johnson and H. M. Feder, AEC Report ANL-6725, Argonne, Ill. (1963). (7) D. M. Holm and W. M. Sanders, AEC Report LA-DC-7931, 1964. (8) C. Engelmann and M. Loeuillet, Bull. SOC. Chim. Fr., 680, 2 , (1969). ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
531
TO TR6P AN0 V6CUUM PUMP
Table 11. Oxygen in Sodium Results Reagent grade sodium90, 77, 82 ppm Lot I Reagent grade sodiumLot I1 51, 42, 47 ppm Reagent grade sodiumLot I11 100, 105, 110, 100 pprn Sodium corrosion loop experiment 29, 22, 20 ppm for irradiation and decay time and the ratio of activities calculated.
Figure 1. Apparatus for separating
150
activity
EXPERIMENTAL
Samples of sodium, weighing 1-2 grams, are encapsulated in aluminum rabbits and irradiated for 4 minutes with bremsstrahlung in the 45-degree facility at the NBS electron linear accelerator. The convertor target and pneumatic transfer system have been previously described (9). The accelerator electron energy is 35 MeV and the beam current is about 30 PA. The apparatus for the separation of the oxygen activity is shown in Figure 1. The irradiated sodium sample is etched in three separate mixtures of ethyl alcohol containing 10% water. Approximately 25% of the sample is dissolved in this way to assure removal of surface contamination. With joint ( J ) disconnected, stopcock S1is opened to allow nitrogen t o flush the distilling flisk ( A ) which contains 25 ml of 6 M N a O H and a few boiling chips. The sodium sample is dropped into the flask for dissolution. Dilute NaOH is used rather than water t o assure that the dissolution reaction does not proceed too vigorously. When the sodium has dissolved, joint ( J ) is connected, SI is closed, stopcock Sz is opened to vacuum, and the flask is heated with a Meker burner. Water distills into trap ( B ) which contains 0.5 rr1 of a saturated sodium hydroxide solution. The trap is cooled in a dewar flask of liquid nitrogen. After approximately one-half of the water has been distilled, the liquid nitrogen dewar is removed and the distillate melted and boiled under vacuum for a few seconds. The NaOH in the trap assures that volatile activities, such as 13N as ammonia, which may have distilled with the water will be removed from the condenser during boiling. The distillate is then transferred to a graduated cylinder for yield determination, diluted to a constant volume, and placed between two 4-inch by 4-inch NaI(T1) detectors and the positron activity determined by coincidence counting of the accompanying annihilation radiation. The complete separation requires about four minutes. The residue of the distillation flask ( A ) is diluted t o a constant volume and the activity of 22Nainduced by the reaction 23Na(y,n)22Na is counted. This activity is used as a n internal standard. The ratio of oxygen to sodium activity is subsequently calculated and corrected for irradiation and decay times and for yield. Because samples may be irregular in size and shape and because a flux gradient exceeding 20 over the length of the sample may exist, the internal standard method has been used. This is the most convenient method of comparing the relative irradiation of sample and standard. About one gram of sodium hydroxide is used as a standard. After irradiation, the standard is dissolved in water. An aliquot of the solution is taken and the 1 5 0 activity counted in the same manner as the sample. Subsequently the activity from the sodium is obtained in the same way as the sample. Oxygen and sodium activities are also corrected (9) F. A. Lundgren and G . J. Lutz, Trans. Amer. NucI. SOC.,10 89 (1967). 532
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
RESULTS AND DISCUSSION
Using the described method, the oxygen content of a sample may be calculated using the expression
X = lo6 X K X R I X Rz where X = ppm of oxygen; R1= ratio of activities of oxygen and sodium in the sample corrected for irradiation time, decay, and yield; R2 = ratio of activities of sodium and oxygen in the standard corrected for irradiation time, decay, and yield; and K = ratio on a weight basis of oxygen to sodium in the standard. Results of samples of reagent grade sodium and sodium from corrosion experiments are shown in Table 11. The spread of results is not only due to random errors in the analysis, but also, to a n unknown extent, to inhomogeneity of the oxygen content of the sodium samples. Inhomogeneity may occur because of oxygen segregation during solidification of a molten sample. In general, no particular care is required in handling samples. Some of the samples used in this work had large amounts of sodium hydroxide and sodium carbonate o n their surface. N o purification of reagents is required. Any surface contamination is removed in the etching process. It is not necessary to weigh either sample or standard. A small amount of radioactivity is entrained during the rapid distillation. From an examination of the gamma-ray spectrum of photon irradiated sodium, it was ascertained that this activity is primarily due to 18F(half-life 110 m) produced uiu the reaction Z3Na (y, crn)l8F and to 22Na(half-life 2.6 yr.) produced Diu the reaction 23Na(y,n)2ZNa. These isotopes are sufficiently long lived relative to 1 5 0 that they may be considered a background to be evaluated for each sample. A typical value of this background is 3 counts/min. Utilizing a method described by Currie (ZO), it was calculated that a n activity of half-life of 2.1 min would require an initial counting rate of about 9 counts/min to be detected in the presence of this background. One microgram of oxygen, irradiated for three half-lives under the conditions described, with a 50% chemical separation yield and counted four minutes after irradiation gives an initial counting rate of about 3 counts/min. Thus about 3 micrograms can be detected. Samples of two grams in mass can be analyzed and therefore a sensitivity of less than 2 ppm at the available beam current is obtained. ACKNOWLEDGMENT
The author thanks the NBS LINAC operators for the very fine services provided. Special thanks are due to Miss D. M. Setlock for her very capable assistance in the experiments performed. RECEIVEDfor review October 30, 1969. Accepted February 11. 1970. (10) L. A. Currie, ANAL.CHEM.,40, 586 (1968).
