LITERATURE CITED
( 5 ) McDonald, I. R. C., Nature 174, 703
(1) Ahlers, N. H. E., Brett, R. A., McTaggart, N. G., J . A p p l . Chem. 3, 433 (1953). (2) ~~~~i~~~ petroleum Research project 44, Carnegie Institute of Technology. (3) Hartman, L., Shorland, F. B., Cleverley, B., Biochem. J. 69, 1 (1958). ( 4 ) Hartman, L., Shorland, F. B., McDonald, I. R. C., Ibid., 61, 603 (1955).
(1954). (6) SePhtOni H. H.1 Sutton, D. A.1 Chem.
& Znd. (London) 1953, 667. ( 7 ) Shreve, 0. D., Heether, A I . R., Knight, H. B., Swern, D., i l s a ~CHEX . 22, 1261 (1950).
( 8 ) Sinclair, R. D., McKay, A. F., >Iyers, G. S., Jones, R. IC.,J. Am. Chem. SOC. 74, 2578 (1952).
(9) Vandenbelt, J. &I.,Henrich, C., A p p l . Spectroscopy 7, 171 (1953).
BARRY CLEVERLEY Department of Scientific & Industrial Research '"*lingtonJ x e w Zealand. RECEIVED for review June 2 3 , 1959. Acc e p t d Octolier ?eti, 195!1.
Determination of Carbon in Organic Substances by an Oxygen-Flask Method SIR: Gotte, Kretz, and Baddenhausen (2) have used the Hempel-Schoniger oxygen-flask method (3, 6) for the determination of carbon-14 in organic materials. This method may also be used to determine the total carbon content in solid organic substances. The method is comparable in accuracy to the Pregl dry combustion method (4) and is much faster than the classical method. KO expensive apparatus is required. The sample in milligram amounts is placed on a glass-wool pad and is ignited electrically in a n atmosphere of oxygen. The carbon dioxide evolved is absorbed in aqueous sodium hydroxide and determined acidimetrically. A complete determination, excluding the time required to weigh the sample, may be made in less than 20 minutes. EXPERIMENTAL
The apparatus used is shown in Figure 1. Two platinum wires (B. & s. gage No. 22) sealed in two 6-inch lengths of 4-mm. glass tubing are used to make electrical contact with the ignition coil, and two other short platinum wires
Table I.
fused at the ends of the tubing are used to support the glass-wool sample holder. A 15-cm. length of nickel-chromium resistance wire (B. 8: s. gage KO.32) with a resistance of ea. 5 ohms is made into a coil by winding around a wire 1 nim. in diameter. A few windings of the ends of the coil around the platinum conducting wires make satisfactory electrical contact, and no soldering is required. A 1 x 1.5 inch pad of Corning KO. 7220 borosilicate glass wool. thick enough to look ofaque to light, is placed betn-een the ignition coil and the supporting mires. A weighed amount of sample, preferably from 3 t o 25 mg., is transferred to the glass-wool pad close to the ignition coil by a Tveighing tube. The glass-wool sample holder is folded in the middle and the tvr-o halves are held together by the supporting wires. A 500-ml. thick-walled Erlenmeyer flask containing about 25 ml. of carbonate-free 0.5N sodium hydroxide solution is flushed with a rapid flow of oxygen for 2 or 3 minutes. Complete replacement of air is required for good results. The rubber stopper with the sample assembly is inserted, and the leads are connected to a Tariac. Although the optimuni
Analyses of Known Compounds
Sample Wt ., Substance Benzoic acido
Formula C~HGOI
Vanillin Dextrose Cholesterol 6-Nitrocholesteryl acetate Acetanilidea Azobenzene 2-Saphthyl phenyl sulfide N-benzenesulfonylN-carbethoxymethyltert-Butyl mesidine Cystinea Benzeneboronic anhydridepyridine complex Potassium acid phthalatea a Sational Bureau of Standards samples.
130
ANALYTICAL CHEMISTRY
% Carbon
Absolute Error,
hIg. 3.478 7.763 11.385 12 042 6.400 17.310 24.975 11,120 9.390 13,940 12.460 4.458
Calcd.
Found
70
68.84
68.43 69.28 68.42 68.84 63.47 63 26 40.31 84.07 73.84 71.31 79.91 80,50
-0.41 +O 44 -0.42 0.00 + O , 32 so.11 +0.31 +o. 20 +0.31 +o . 2 2 + O . 82 -0.81
14 400 i. 710 20.132
66.15 29.99
66.19 .. ~. 30.56 30.13
$0.04 +O. 57 $0.14
8.123 17.185
TO. 68
i1.00 46.61
+O. 32 -0.44
63.15
40.00 83. 87 73.53 71.09 79.09 81.31
47.05
Figure 1.
Combustion apparatus
applied voltage required for a good combustion varies somewhat n i t h the conibustibility of the sample, 16 volts, which provides a current of about 3 amperes, is usually suitable. As soon as the sample begins to burn, the flask must immediately be inverted. I'io soot or snioldering occurs in a complete combustion. After combustion is complete, the flask is shaken vigorously for about 5 minutes. Most of the excess alkali may be neutralized by addition of about 10 nil. of 1 s hydrochloric acid solution. The solution is then brought to the phenolphthalein end point by careful addition of dilute hydrochloric acid. Thymol blue or cresol red-thymol blue mixed indicator maybe used, if preferred. The titration with 0.1A- hydrochloric acid is continued with a 10-ml. microburet to the methyl orange end point. A methyl orange-indigo carmine indicator has also been recommended for this titration (1). The hydrochloric acid is standardized against a neighed amount of sodium carbonate. Comparison color standards are recommended for both end points, and correction for the blank must be made. Carbonate-free water is used for all preparations. The carbon content is calculated from the equivalents of acid consumed between the tn-o end points by the equation: 1.201
%
=
x
103iv.v.
mg. of sample
where Sa is the normality of the acid, and T', is the milliliters of acid. corrected
for the blank, required between the first and second end points.
2-iodobenzoic acid. Coniplete combustion was not achieved n-ith sodium oxalate.
RESULTS AND DISCUSSIONS
ACKNOWLEDGMENT
The results obtained for a variety of compounds are shon n in Table I. A mean absolute deviation of 0.3y0from the true result n-as found. I n general. organic compounds containing nitrogen, sulfur, boron. and alkali metals are readily analyzed. ~ ~ results were obtained for halogen compounds such as 2-chlorol~enzoicacid and
The authors thank Josef IYemeth for providing most of the pure samples used in this work.
-
LITERATURE CITED
FIT.J., Pvleloche, I-.\V.,~“Ele~ ~ Analysis,” mentary Quantitative p. 358, Row, Peterson &- Co., Evanston, Ill., 1957.
(1) Blaedel,
( 2 ) Gotte, H., Kretz, R., Baddenhausen H., AnQeW. CheVL. 6 9 , 561 (1957).
(3) Hempel, W., Ibid., 5 , 393 $11892). (4) Niederl, J. B., Niederl, V., Quantita-
tive Organic Analysis,” 2nd ed., p. 101, Wiley, New York, 1952. ( 5 ) Schoniger, JV., Alikrochim. Acta 1955, 123; 1956, 869. R . S. JUVET JENCHIIT Noyes Chemical Laboratory University of Illinois Urbana, 111. i ~ f RECEIVED for review September 25, 1959 Accepted October 29, 1959.
Gas-Proportional Counting of Carbon-14 and Tritium, and the Dry Combustion of Organic Compounds SIR: I n a previous report, the lower limit of activit’y detection and the precision with which it could be accomplished by gas-proportional counting were discussed ( 5 ) . Since that time, a n anticoincidence counter of conventional design, made to operate with a ring of proportional counters and n-ith two proportional sample counting channels, has been installed in this laboratory. By this means, both the precision and the lon-cr limit of activity detection have been significantly improved. The unit is siniilar to those used for carbon-I4 dating by gas-proport ional counting ( 6 ) , t’sc(’Ljt that no intwury shield is employed. Thc ordinary P y r m $740 or Kinible KG-33 glass 100-cc. counters ( 1 ) have shown backgrounds generally between 60 and $0 counts per minute inside a 2inch lcad shicld. The same counters in the anticoincmidrnce circuit have backgrounds betn-een about 16 and 25 c.p.ni., with a spread over several weeks’ time of about f1 c.p.ni. About 10 t o 15 c.p.m. of this background appears to be due, to radioactivity in the glass itself, as metal and quartz counters of similar size have backgrounds n-hich are lower by about that amount. However, the backgrounds of tubes made from recently purcliasetl samples of t’hese glasses have brcn consistently verj- much higher than those made before 1958, and they have bern very erratic. Different picxes of glass from the same shipment have recently yielded counters with widely different backgrounds, some having run as high as 200 c.p.m. Because the counters have been made othrrwise identical t o those used preriously, one can only conc.lude that the amount of radioactivity in these borosilicate glasses has rccently increased appreciably, and that the amount of activity now present can vary considerably from batch to
T. STOPCOCK
O 2 IASCARITE-ANHYDROhE SUPPLY
ABSORBER
COPPER T U B E W I T H B R A S S B A L L JOINTS
Figure 1. Top view of oxygeninlet system
batch. Glass intended for use in counters should now be checked for activity level before large numbers of counters are made from it, and older stocks of tubing should be used n h e n possible. Assuming that this effect is due to contamination by fallout of the alkali used in making borosilicate glasses, it might also prove fruitful to investigate glasses made in the southern hemisphere, where the amount of fallout is presumably less. Metal counters of a design similar to the ordinary glass counters can be made easily, either with glass ends sealed to Kovar or with soldered metal bottoms and Teflon connectors a t the upper end. An example of this type of counter has been described recently (IO). While Kovar itself is noticeably better than even the older borosilicate glass tubes with regard t o background, it is not as good as steel or nickel (and perhaps other metals) in this r e s p c t . Counters whose active volume is made of optical grade quartz, with the usual silver coating, also have escellent background characteristics. These have been made with a standard-taper ground on each end of the center section, with borosilicate ends sealed on by means of Apiezon n ax; the general construction is othern ise similar to the ordinary
Bernstein-Ballentine tubes ( 1 ) . These counters, when made with a total volume of about 100 cc., show backgrounds in a 2-inch lead shield of about 45 t o 50 c.p.m., and with the anticoincidence circuit about 5 to 7 c.p.m. I n the latter case, the background is seldom seen to vary more than about i 0 . 5 c.p.m. on a single filling, so long as the electronic circuits are norking properly. These tubes must be calibrated b j counting known amounts of activity several times in each tube, because the usual mcthod of filling n i t h toluene from a buret (8)is inapplicable. K i t h this counting system, assuming a background of 5 c.p.m. to be counted for 400 minutes, the standard deviation of the background is calculated to be A0.11 c.p.m. (as against 1 0 . 4 3 c.1i.m. for a similar tube in a 2-inch lead pig) ( 5 ) . For a sample having 10 c.p.m. (about 12 disintegrations per minute), counted for 180 minutes, the standard deviation is 1 0 . 3 0 c.p.m. and the net count is 10.0 10.32 c.p.m. Under the previous conditions, the deviation was calculated as 1 0 . 8 0 c.p.m. ( 5 ) . For a sample having 2.0 c.p.m., similar calculations indicate a net count of 2.0 1 0.23 (as against a standard deviation of 10.80 without the anticoincidence circuitry) ( 5 ) . Khereas about 2 c.p.m. was previously considered the 1013-er detection limit, it is obvious that 1 c.p.m. can be detected with certainty in a reasonable time when simple anticoincidence circuitry is used, and t h a t even 0.5 c.p.m. should be detectable if somewhat longer counting times are used. With gas-proportional counting, the lower detection limit and the precision with n hich tritium can be counted are the same as for carbon-14, because the counting efficiency is essentially the same for the tn o isotopes (6). Suitable VOL. 32, NO. 1,
JANUARY 1960
131
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