than 1 mg of coextracted elements such as niobium result in smearing of the sample on the glass disc, and erratic count rates. As much as 100 mg of elements such as cerium, neodymium, lithium, sodium, potassium, calcium, strontium, magnesium, manganese, or nickel, which are not extracted and do not have overlapping x-rays, do not interfere. The effect of larger amounts was not studied. Interference was not observed in the presence of 50 mg of zirconium; 30 mg of titanium; 5 mg of rhodium or platinum; 2 mg of tungsten; 1 mg of uranium, vanadium, or chromium; 250 pg of molybdenum; 200 pg of copper, 50 pg of iron, or 10 pg of cobalt. High recoveries observed when extractions were performed from greater than 50 mg of zirconium, 30 mg of titanium, 2 mg of tungsten, or 1 mg of niobium probably were caused by residual tantalum in the metals. Although the method applies specifically to the determination of microgram amounts of tantalum in silver, it can be
readily applied to other materials using the hydrochlorichydrofluoric acid system, or adapted to other types of samples by using nitric-hydrofluoric acid or sulfuric-hydrofluoric acid solvents. One analyst can extract and measure approximately 30 samples per day. ACKNOWLEDGMENTS
The authors gratefully acknowledge the helpful suggestions and assistance of Dr. C. F. Metz, under whose supervision this work was performed, and the assistance of Miss H. L. Barker and W. G . Baughman for performing some of the X-ray measurements and extractions. The Los Alamos Scientific Laboratory is operated under the auspices of the Atomic Energy Commission. RECEIVED for review December 11, 1967. Accepted February 2, 1968.
Determination of Carbon in Sodium by Photon Activation Analysis G . J. Lutz and D. A. De Soete National Bureau of Standards, Washington, D. C. 20234 LIQUIDSODIUM has a number of thermodynamic and nuclear properties which make it very useful as a heat transfer medium for nuclear power plants. These properties include thermal stability, high boiling temperature, a high heat transfer coefficient and a fairly low thermal neutron cross section ( I ) . The main disadvantage of sodium is its ability to carburize or decarburize metals with which it comes in contact, causing subsequent deleterious effects (2). These reactions are generally interpreted in terms of carbon going to a state of lower thermodynamic activity. Thus, it is necessary to have an accurate knowledge of the carbon content in the sodium. Stoffer and Phillips (3) have described a procedure for the determination of carbon in sodium-potassium alloy. This technique involves heating the sample in a combustion tube to a temperature of 950" C and absorbing the liberated COZin an ascarite-drierite absorption bulb. Modifications using manometric (4) or gas chromatographic (5) measurements have been described. Pepkowitz and Porter (6) have published a method for the determination of carbon in which the sodium sample is dissolved in water, the solution neutralized, evaporated to dryness, and the carbon oxidized with the Van Slyke reagent. Kallman and Liu (7) have described a method whereby the total carbon content of sodium metal is determined by lowtemperature ignition of the sample in an oxygen-argon mixture followed by treatment with dilute sulfuric acid and con-
ductimetric determination of the liberated COz. The carbonate content is measured by steam decomposition of the sodium sample and subsequent acid liberation of the COZ. In sodium combustion analysis, apparatus and reagent blanks may be quite significant, thus limiting the sensitivity of the method. In addition, the possibility of surface contamination between the time of sampling and of measurement exists. In activation analysis, however, the possibly contaminated surface can be removed after irradiation, and an analysis, representative of only the bulk of the sample, is obtained. This technique is free from a blank and the sensitivity is only limited by the radioactivity induced in the sample and the detector background. NUCLEAR CONSIDERATIONS
(1) Marshall Sitting, "Sodium: Its Manufacture, Properties and Uses," Reinhold Publishing Company, New York, 1956. ( 2 ) R. W. Lockhart and G. Billuris, IMD Special Report Series NO. 12, Nuclear Metallurgy AIME IX, 1963. (3) K. G. Stoffer and J. H. Phillips, ANAL.CHEM., 27,773 (1955). (4) J. Herrington, U. K. Atomic Energy Authority, AWRE Report
Natural carbon has two isotopes, 12C and I C , with isotopic abundances of 98.89z and 1.11 respectively. There are several nuclear reactions which can be used in an activation analysis. The reaction 13C(n,y)14C can be rejected because of the low abundance and low capture cross section of l3C and the very long half-life of 14C. The fast neutron reactions, 12C(n, p)12B, lC(n,p)l3B and lC(n,cr)lOBe, have half lives either too short or too long for activation analysis. The reaction lC(n,2n)11C is difficult to induce because its threshold is 18.7 MeV. Albert and coworkers (8) have used the reaction '2C(d,n) 1aN for the determination of carbon in iron. This would not appear suitable for the present problem, as charged particles do not penetrate deeply into the sample, and it is intended to remove the sample surface after irradiation but before separation and counting. The nuclear reaction chosen for this work was '2C(y,n)l1C.
( 5 ) T. G. Mungall, J. H. Mitchen, and D. G. Johnson, ANAL.CHEM., 36, 70 (1964) (6) L. P. Pepkowitz and J. T. Porter, Zbid., 28, 1606 (1956). (7) S . Kallmann and R. L u . Zbid., 36, 590 (1964).
(8) Ph. Albert, G. Chaudron, and P. Sue, Bull. SOC.Chim. France, 1953, 97.
z
0-62162
820
ANALYTICAL CHEMISTRY
Table I. Photon and Neutron Induced Reactions in Carbon and Sodium, Yielding Active Isotopes Integrated Threshold cross section Reaction Half-life energy (MeV) (MeV-barn) Decay mode 18.72 0.026 p+ (100%) -0.511 MeV ann 20.5 min ‘ZC(y,n)”C 2.58 yr 13.43 0.081 p+, EC (100%) -0.511 MeV ann 3Na(y,n) 2Na - 1.28 MeV gamma 110 min 23 ... 23Na(y,na)18F /3+, EC (100%) - 0.511 MeV ann 15 hr 23Na(n,y)l4Na ... 0.53” p- (100%) - 2 . 7 5 MeV gamma - 1.38 MeV gamma a Thermal neutron activation cross section.
This reaction has been used by Englemann for the nondestructive determination of carbon in beryllium (9). Nuclear reactions of interest in the photon activation analysis of carbon in sodium are summarized in Table I. The last reaction indicated is induced by neutrons which are produced in and around the sample. It is evident that a n effort t o measure the 11C activity nondestructively would be greatly complicated by the activity of I S F . One could avoid the formation of this isotope by utilizing bremsstrahlung with a maximum energy below the threshold of the Z3Na(y,n cr)l*F reaction. Because the 12c (y,n) 11C reaction has a high threshold, this would place severe limitations on the sensitivity of the method. Thus, a separation of the carbon is required. Our procedure is based on that of Kallmann and Liu (7). Either the 22Naor the 24Na radioactivity induced in the sample is suitable for use as an internal standard. EXPERIMENTAL PROCEDURE
The samples are irradiated for 20 minutes with bremsstrahlung in the 45-degree facility at the NBS linear electron accelerator. The converter target and the pneumatic transfer system have been previously described (10). The accelerator electron energy is 35 MeV and a beam current of about 20 pA is utilized. Samples of sodium, weighing about 1 gram, are packaged in polyethylene vials, which are encapsulated in aluminum rabbits. The apparatus for the radiochemical separation of the carbon 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. This assures removal of any surface contamination. After etching, the bulk of the alcoholate residue is scraped from the sample to prevent an excessively vigorous combustion. The sample is then placed with 100 mg of inactive sodium carbonate in a quartz crucible ( B ) . This is introduced in a Vycor combustion flask ( A ) and the apparatus is assembled. With stopcock SI closed and Sz and S a open, oxygen at atmospheric pressure is allowed t o flow through the system. The correct setting of S4 compensates for the differential hydrostatic pressure in the absorption bulbs. The sample i s cautiously ignited with a Meker burner utilizing a n air-propane mixture. During the combustion, S3 is closed, preventing sodium oxide vapors from escaping from the combustion flask. After combustion, the flask is cooled, stopcock S 3 is opened and about 80 ml of 4 M HzS04is added slowly by means of S1and Sz. This liberates the Con. After the addition is completed, SI and S q are closed, the flask is heated and the solution is degassed by boiling under
Table 11. Separation and Absorption of COz. Two COZAbsorption Bulbs Are Used in Series 17Cactivity left in llC activity 11C activity combustion flask absorbed in absorbed Experiment ( %) bulb I (%) in bulb I1 (%) 1
0.35 0.47 0.37
2 3
98.96 99.19 99.27
0.76 0.48 0.53
vacuum for about five minutes. Subsequently, S1is opened and air is allowed to bubble through the apparatus for about one minute. The decomposition of the inactive Na2C03 assists in flushing the radioactive COPfrom the system. The liberated COz is trapped in 100 ml of 20% NaOH solution (D). Absorption bulb (C) contains a 2z solution in 4 M HzS04for removal of any radiofluorine as a fluoboric acid complex. The completeness of this trapping was demonstrated with 18F tracer. The separation and COz collection were checked by tracer experiments using 100 rng of irradiated Na2C03,and a 1-gram sample of inactive sodium. Two COz absorption bulbs were mounted in series. The results are shown in Table 11. It is evident that the separation and the collection are reproducible and quantitative. The complete separation required about 15 minutes. The radioactive COz solution is diluted t o 250 ml in a volumetric flask, and the 51 1 keV positron annihilation radiation is counted for 20 minutes. A 4-inch by 5-inch NaI crystal and a gamma-ray spectrometer are used. The background is 3000 counts per 20 minutes. Radiochemical purity was demonstrated by analysis of the gamma spectrum and the decay curve. A typical sample yielded about 40
0 OXYGEN
TO WATER A S P I RATOR
(9) Ch. Englernann, Colloque sur les Mkthodes Radiochemiques d’Analyse, Salzborg, Oct. 1964. ( I O ) F. A. Lundgren and G. J. Lutz, Trans. Am. Nucl. Soc., San Diego, June 1967, p 89.
Figure 1. Apparatus for the separation of carbon from sodium VOL 40, NO. 4, APRIL 1968
821
is insensitive to the chemical nature of the standard, beam Table 111. 24Na and 22Na to llC Specific Activity Ratios, Corrected for Irradiation and Decays K X Ra for K X R2for 22Na Standard Z4Na, X 10-2 3.46 1,37 Na2C03 3.01 1.20 Na2C03 3.22 1.29 Na2C03 3.34 1.37 Na2C03 2.28 1.32 NaGOa 3.14 1.42 Na2C03* 3.34 1.49 Na2C204* 3.28 1.26 Na2C03C 3.21 1.47 Na2C204c AV = 3.25 AV = 1.35 Stddev = 0.13(4%) Stddev = 0.09(6.5%) a See Equation 1. * Irradiation at half beam intensity. Beam position moved inch upward.
Table IV. Sample
Carbon Determination in Sodium ppm Carbon
Series 1 reagent grade sodium 51.1 62.2 49.3
1 2 3
Series 2 Argonne Nat. Lab. 1
AV Std dev
= =
54.2 7.0(13x)
44.5 46.7 Av = 47.6 54.2 Std dev = 4.9 (10%) 42.7 49.9 (146.4) Not included in average
3 4 5 6
Series 3 Argonne Nat. Lab. 1 2 3 4 5
5.69 = 4.03 4.23 AV 3.40 Std dev = 1.35 (3373 4.72 2.11
counts per minute per microgram of carbon at the end of separation. The residue of the combustion flask ( A ) is also diluted to 250 ml and the activity of the 1.28 MeV photopeak of 22Na or the 2.75 MeV photopeak of 24Na is counted. This activity is used as an internal standard. The ratio of carbon to sodium activity is subsequently calculated and corrected for irradiation and decay. Because an unknown quantity of sodium is removed during the etching procedure and the samples and standards are irradiated separately, an internal standard method is utilized. About 1.5 grams of sodium carbonate or 1 gram of sodium oxalate is used as a standard. After irradiation, the standard is dissolved in water and diluted to 250 ml in a volumetric flask. After allowing the 2.1-minute ' 5 0 to decay, the standard is counted several times in a manner identical to the samples. The carbon activity is obtained from a decay curve analysis of the activity in the annihilation radiation photopeak. The activity from the sodium in the standard is obtained in the same way as in the samples. Carbon and sodium activities are also corrected for irradiation and decay time and the ratio of activities is calculated. Some typical results for these ratios for different materials and differing irradiation conditions are given in Table 111. The data show that the ratio of specific activities of the two elements 822
ANALYTICAL CHEMISTRY
intensity, and slight changes in the beam position. These data were collected over a period of seven hours.
RESULTS AND DISCUSSION Using the described method, the carbon content of a sample may be calculated using the expression:
X where X
=
=
10' X K X RI X R2
(1)
ppm of carbon,
RI = ratio of activities of carbon and sodium in the sample corrected for irradiation time and decay, R P = ratio of acthities of sodium and carbon in the standard corrected for irradiation time and decay, and K = ratio on a weight basis of carbon to sodium in the standard. One series of samples of reagent grade sodium and two series of samples obtained from Dr. C. Luner of the Argonne National Laboratory were analyzed. The results are shown in Table IV. The spread of results in Table IV is not only due to random errors in the analysis, but also to an unknown extent, due to inhomogeneity of the carbon concentration of the sodium samples. There is no explanation of the 146 ppm value in the second series, but one may presume that there might have been a small crack in the sample which became contaminated with atmospheric COz but was not cleaned in the alcohol etching operation. It is necessary to be careful in the sampling operation to assure that smooth surfaces are obtained. 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 on their surface. No purification of reagents is required and the method is free from blanks. Any surface contamination is removed in the etching process. The elimination of these sources of errors contributes to the reliability of the determination. During the separation, however, it is important that no sodium oxide leave the combustion area and settle in other parts of the apparatus where it may trap the carbon dioxide. Any stoichiometric sodium-carbon compound can be used as a standard. It is not necessary to weigh either samples or standards. Under our irradiation and counting conditions the detection limit, calculated according to the method described by Currie ( I I ) , can be set at 0.5 pg of carbon, the errors of the first and second kind being 5 % each. If desired, samples of about 3 grams of sodium can be accommodated. The precision observed is typical for most photon activation analyses. ACKNOWLEDGMENT
The authors thank the NBS Linac operators for the very fine services provided. Special thanks are due to C. Luner of the Argonne National Laboratory for helpful discussions and for providing sodium samples.
RECEIVED for review December 4, 1967. Accepted January 31. 1968.
(11) L. A. Currie, ANAL.CHEM. 40, 586 (1968).
