ANALYTICAL CHEMISTRY
904 95% for a range of 0.10 to 0.70 mg. as shown in Table I. As the loss is small and remains constant and the method is standardized empirically, this fact is of little practical consequence. Various means of improving recovery have been tried, such as increasing the amount of hypophosphorous acid, varying the amounts of hydrochloric acid, heating under a reflux condenser, and adding a trace of mercuric chloride to catalyze the reduction (7, 10); however, none of these expedients has resulted in an increased recovery. APPLICATION OF METHOD TO COMMERCIAL AND BUREAU OF STANDARDS SAMPLES
Table I1 lists results obtained for several common arsenical copper-base alloys as well as for a lead-base and a tin-base alloy and for a sample of open hearth iron. In general, results obtained by the method described in this paper are slightly higher than those obtained by methods involving a distillation separation. This supports the author’s contention as well as that of Evans (6), that distillation recoveries of arsenic are likely to be somewhat low. Although the method described in this paper was developed primarily for application to copper and copper-base alloys, the results for the white metal alloys and the open hearth iron show that it is of rather general application. The lead- and tin-base alloys were dissolved in 8 ml. of aqua regia, evaporated to dryness on a steam plate, treated with 5 ml. of hydrochloric acid, and again evaporated to dryness, and the analysis was completed as usual. Cuprous chloride (0.5 gram) was added to all samples that did not contain a substantial amount of copper, as copper salts apparently catalyze the reduction to metallic arsenic ( 6 ) . The lead-base sample had to be kept hot during filtration to prevent crystallization of lead chloride. Further details on the analysis of white metal and ferrous alloys are given by Anderson ( 2 ) and Evans (6).
The successful results for the sample of open hearth iron indicate that extremely small amounts of arsenic in certain materials (such as electrolytic copper) may be gathered from a 25gram or larger sample by coprecipitation with ferric hydroxide and determined in the presence of the iron. ACKNOW LEDG-MENTS
Thanks are due to G. A. Reihl who made many of the conventional arsenic determinations, to J. A. Crane who analyzed the firerefined copper, and to R. P. Nevers who contributed suggestions and assisted in the preparation of this paper. LITERATURE CITED
Am. SOC. Testing Materials, “A.S.T.M. Methods of Chemical Analysis of Metals,” p. 216, 1946. Anderson, C. W., IND. ENQ.CHEM.,ANAL.ED.,9, 569 (1937). Boltr, D. F., with Mellon, M. G., Ibid., 19, 873 (1947). Brandt, Chem.-Ztg., 37, 1445, 1471, 1496 (1913); 38, 295, 461. 474 (1914).
Engel and Bernard, Compt. rend., 122, 390 (1896). Evans, B. S., Analyst, 54, 523 (1929); 57,492 (1932). King, B. W., and Brown, F. E., J. Am. Chem. SOC.,61, 968 (1939).
Kolthoff, I. M.,and Amdur, E., IND.ENQ.CHEM.,ANAL.ED.. 12, 177 (1940).
Luke, C. L., Bell Telephone Laboratories, Analytical Proceduw 200,449, Bell Telephone Laboratories,.Murray Hill, N. J. Robinson, R. G., Analyst, 65, 159 (1940). Rodden, C. J., J . Research Natl. Bur. Standards, 24, 7 (1940). Sloviter, H. A., et al., ISD. ENQ.CHEW, ANAL. ED., 14, 516, (1942).
Thiele. Ann., 263, 361 (1890) ; 265, 55 (1891). RECEIVEDMarch 17, 1948. Presented before the Third Symposium on Analytical Chemistry, Analytical Division, Pittsburgh Section, AMERICAN CHEMICAL SOCIETY, February 12 and 13, 1948.
Measurement of Carbon 14 JOHN D. ROBERTS, WINIFRED BENNETT, E. W. HOLROYD, AND C. H. FUGITT Department of Chemistry and Laboratory f o r Nuclear Science and Engineering. Massachusetts Institute of Technology, Cambridge, Mass.
A procedure for the measurement of radioactivity from C14 is described. The samples are converted to barium carbonate and the activity is measured with a modified Lauritsen electroscope.
A
NUMBER of sensitive and accurate assay methods for C14 involve Geiger-l\IJler counters (1-4, 8-11, 13, 14) or electrometers (4-6, 12) and employ carbon dioxide or barium carbonate as the sample material. Excellent summaries of the properties and problems associated with the measurement of 0 4 have been provided by Kamen ( 7 ) and by Reid, Weil, and Dunning (10). Most of the published procedures for assay of C14 are primarily intended for biological research where dilution factors are large and high sensitivity is required to obviate prohibitive activity levels. For chemical work the dilution factors are usually small, and it is often possible to sacrifice sensitivity for simplicity of apparatus and convenience of procedure. The method for the determination of C14in organic compounds described in this paper makes use of a combustion train to convert the organic materials to carbon dioxide. The carbon dioxide is precipitated as barium carbonate and subjected to radioactive analysis with a Lauritsen electroscope as modified by Henriques, Kistiakowsky, Margnetti, and Schneider (4) and by Seligman (12). To eliminate necessity for determining the thickness of the precipitates, the original sample weights were chosen so as to give sufficient barium carbonate for samples of thickness greater than the range of the C146-particles in barium carbonate (10).
EXPERIMENTAL
The apparatus for the precipitation of the radioactive carbon as barium carbonate is shown in Figure 1. The sintered-glass filters were prepared by the method of Henriques, Kistiakowsky, Margnetti, and Schneider (4). The filter paper was S and S Blue Ribbon (Schleicher and Schuell Co., Xew York, X. Y.), cut into 2.6-cm circles. The Lauritsen electroscope (Fred Henson Co., Pasadena, Calif.) was modified so that the samples could be placed within the electroscope chamber directly below the quartz fiber (4, I d ) . The air in the electroscope chamber was kept dry by a tray of anhydrous magnesium perchlorate covered with lens paper. The combustion train was of the conventional semimicro design. PROCEDURE
The precipitation apparatus was flushed thoroughly with carbon dioxide-free oxygen or nitrogen and 4 ml. of 0.5 N carbonate-free (3) sodium hydroxide solution were run into the absorption tube, which was connected to the combustion train. The weight of the sample of organic material was determined by the area of the barium carbonate filter and the range (17 to 19 mg. per sq. cm.) of the C14 @-particlesin solid barium carbonate. In the authors’ work most of the precipitates had an area of 2.9 sq. cm. and a sample large enough to give 50 to 55 mg. of barium carbonate was required. After the combustion was complete, the flow of gas was continued for 20 minutes to make sure that all the carbon dioxide was swept into the absorber. Four milliliters of a
V O L U M E 20, NO. 10, O C T O B E R 1 9 4 8
905 Table I.
Variation of Measured Activity with Thickness of Barium Carbonate Precipitate
Sample BaCOa, Weight, M g . Mg./Sq. Cm. Activity” 40.1 13.9 5.62 f 0.06 16.4 44.7 6.56 f 0.05 17.4 50.3 6 . 7 1 i= 0.06 51.3 17.7 6.79 f 0 . 0 8 55.1 19.0 6.76 f 0.04 Electroscope scale divisions per minute corrected for background with etandard deviations. Q
Table 11. Analyses of Mixtures of Radioactive and Inactive Barium Carbonate Standard Sample,
Total Bctivity Activity Sample, Calcd., Found, Weight, h l g Activity‘ % % 55.1 6 . 7 5 f 0.06 100.0 100.0 55.8 5.45 f 0.05 79.0 80.7 55.3 4.96 + 0.04 73.5 78.1 55.0 4.43 0.08 65.6 66.2 56.0 3.95 1 0 . 0 4 58.5 58.0 39.6 56.6 2.66 I 0.04 39.4 55.0 1.97 I 0 . 0 2 29.2 29.8 55.6 1.67 i 0 . 0 2 24.7 25.9 0.674 f 0.009 10.0 55.9 10.0 5 Electroscope scale divisions per minute corrected for background with standard deviations. hlg. 55.1 44.1 43.2 36.4 32.5 22 .o 16.4 14.4 5.6
v
-Filter
Figure 1.
Flask Connector
Apparatus for Precipitation of Radioactive Carbon
solution of 0.5 iV barium chloride and 0.4 S ammonium chloride (3) were placed in the precipitation chamber with the suction ;turned off and the sodium hydroxide-carbonate solution was run in. The gas absorption spiral and tube were washed with three 5-ml. portions of boiling distilled water and the washings were added to the precipitation chamber. The precipitate was then filtered, using suction, and washed with five 5-ml portions of boiling distilled water. With the suction on, the springs holding the filter together were removed, the precipitation chamber was raised, and the filter paper was removed from the sintered glass disk and placed in the brass holder (Figure 2). The sample was dried under an infrared lamp a t a distance of 15 em. (6 inches) for 10 minutes and then allowed to cool over anhydrous magnesium perchlorate in a desiccator. The precipitates were even, and if handled and dried carefully showed little tendency to develop serious cracks. The sensitivity of the electroscope was checked witha uranium oxide source, after taking background measurements, just before the sample and holder were placed within the electroscope chamber. The electroscope used gave a fairly linear sensitivity curve from 10 to 40 scale divisions, and all measurements were made over this part of the scale. Reproducible activity readings mere obtained only after the sample had been in the electroscope for 5 minutes. Thereafter, the time required for the fiber to travel from 10 to 40 on the scale was recorded.
*
Table I shows that the measured activity of a standard sample is independent of precipitate thickness with more than 17 mg. per sq. em. of barium carbonate. These results were obtained by acidifying weighed samples of active barium carbonate and passing the resulting carbon dioxide into the precipitation a p paratus with a stream of nitrogen. The precision possible with this assay method is illustrated by the measured activities of samples obtained by diluting a standard barium carbonate sample with inactive material (Table 11). The activity of the standard barium carbonate was 2046 counts per minute per milligram of barium carbonate as measured in a carbon dioxide gas counter ( 2 ) . The method clearly has adequate precision for virtually all ’ chemical mechanism studies where sufficiently active material can be used. ACKh-OWLEDGMENT
The authors are indebted to J. W.Irvine and \I7. ITr. Miller for many helpful suggestions, and to J. W. Reese for the determination of the activity of the standard barium carbonate sample in the gas counter. LITERATURE CITED
(1) Boyd, G. E., and Leslie, W.B., U. S.Atomic Energy Commiasion Rept., P B 67,957 (1947). (2) Brown, S.C., and Miller, W.W., Rev. Sci. Instruments, 18, 496 (1947).
(3) Dauben, SV. G., Reid, J. C., and Yankwich, P. E., ANAL,
CHEM.,19, 828 (1947). (4) Henriques, F. C., Kistiakowsky, G. B., Margnetti, C., and Schneider, TV. G., Ibid., 18, 349 (1946). ( 5 ) Henriques, F. C., and Margnetti, C., Ibid., 18,417 (1946). (6) Janney, C. D., and Moyer, B. J., U.S. Btomic Energy Commia-
sion Rept., PB 67,956 (1947). (7) Kamen, M. D., “Radioactive Tracers in Biology,” Chap. VIII, New York, Academic Press, 1947. (8) Libby, W.F., ANAL. CHEY.,19, 2 (1947). (9) Miller. W. W.. Science. 105. 123 11947). (io5 Reid, 14. F., TS7eil, -4.’S.,‘and Dunning, J. R., ANAL. CHEW, 19. - - , 824 (1947). (11) Ruben, S., and Kamen, hI. D., Phys. Rew., 59,349 (1941). (12) Seligman, A. M., J . Clin. Invest., 22, 281 (1943). (13) Wilson, D. W., Nier, A. 0. C., and Reirnann, S.P., “Preparation ~~
‘PRECIPITATE FILTER PAPER
~
Figure 2.
”,I
O.D.
4
Sample Holder
%
~
and Measurement of Isotopic Tracers,” Ann Arbor, Mich., J ST. Edwards, 1946. (14) Yankwiirh, P. E., Rollefson, G. K., and ?;orris, T. H., J . Chem. Phys., 14, 131 (1946)
RECEIVED April 2, 1948. Assisted by the joint program of the Offiae of Nava: Research and the Atomic Energy Commission.
