Techniques in the Use of Carbon 14 WILL1 1RI G . D 1UBES, J.I\IES C. REID, AND PETER E. YANKWICH, Radiation Laboratory and Department of Chemistry, University of California, Berkeley, Calif,
.i method is described for the incorporation of isotopic carbon diolide into organic compounds bj the Grignard reaction. The jields obtained var) f r o p 80 to 95%, based on carbon dioxide. A detailed description is gi\en for the coiirersion of organic substance into a form suitable for radioactirity determination and the cotistriic.tion and ube of equipment employed to determine relatire radioactirity of such sample+.
I
S T I I E ; ~ ~ o u r 01' w a
i.iuti:,- ( Z j oi' t h v mc~rliaiiisiiiof some The deterrniiiation of the relative amount of carbon 14 in a n organic rcactioiis using a radioactive isotope of carbon, organic compound is usually accomplished by converting the (,artion 14, it \\-as nee ry to c,mploy certain well-founded techcompound to barium carbonate and measuring its radioactivity. iiiqut's of tracer chemistry. Although iiunierous investigators An amount of 50 to 100 nig. of barium carbonate, corresponding have used thc radioactive and heavy isotopes as tracers, it \\-as to 5 to 10 mg. of an organic. ubstance, is normally required for difficult or irnpossiblc to locate such techniques adequately dcsuch activity determinations \\-hen the methods described below scribed in the literature. Furthermow, in many cases, the proarc used. Conscquentlj-, thc combustion is executed on this scale cedures described xerc applicable only to the specific problem in with equipment identical n-ith that employed in a Liehig-Pregl that particular research. In vien- of this situation, studies were microanalysis, except that a jpecial type of carbon dioside made in three important phases of tracer work: the ineorporaabsorber containing "carbciiiate-free'' sodium hydroxide is intion of the tracer atom into the organic molecule, the conversion serted. Many of the methods dejcrilicd in the literature for the of an organic substance to a form suitable for radioactivity deterpreparation of carbonate-free sodium hydroxide were found inmination, and the construction and usc of equipment employed adequate, since the carbon dioxide content of thrx base vias t o determine the relative ra(1ioactivit.y of saniplcs encountered in significant. The method of Kuster (8,14, 1 7 ) , using metallic experimentation. sodium and ethanol, gave a reagent of high purity. \Then comOf the many reactions that arc capablt: of forriiiiig a ncw carbustions v w e carried out, using alkali prepared by this probon-carbon bond in an organic moleculr., the carboilation of a ccdurc, 100 * 0 . 5 7 of the theoretical aniount of barium carGrignard reagent (1, 11, 15, 1 6 ) has been most widely eniployed. bonate was obtained. Thus, this method serves both to convert In this reaction, the manncr in which the reagents are added tlie organic compound to a form suitable for radioactivity deterdetermines to a large cxtent the importance of certain side reaerninations and as an analysis for carbon. tions. I n order to reduce the amount of kctonc or tcrtiary alcohol 1Iany methods are described in the literature (5, 13) for the formed as side reaction products, it is cust,oniary t,o add the quantitative deterniinat ion of radioactive isqt,opes. The princiGrignard reagent to a large exc(:ss of carbon dioxide. Such a pal difficulty associated with the routine use of radioactive isoprocedure is not possible in tracer chcmistry, bince the amount of topes like carbon 14) which decay tiy emission of beta-particles of isotopic carbon dioxide is the limiting factor. To circumvent this relatively low energy, is that these radiations are appreciably absorbed in the sample material being counted, in the air, and in difficult,y, various schemes have been devised employing the addition of excess Grignard reagent to a reservoir of gaseous isothe window of tho CkigPr-1ICiller tube. Part of this difficulty has topic carbon dioxidc (12). Thih procedure suffers from the disbeen reduced by thc use of a hell-typr, tube (4,5 , 6, 18) with a advantage that large rcservoirs must be used to hold large thin mica sindon-. By the insertion of a grid to support the niira, a tuhc may be constructed with a large xindow and still volumes of gaseous carbon dioxide and that agitation of such reservoirs is cumbersome and inefficient. The use of solid carbon dioxide would obviate som(~ of thcsc difficulties but a t the same time introduce others. However, high yields in thv synthesis of acetic acid (1, 11) have been rcported when gaseous carbon dioxide was rapidly 2 m m SLANT added tb a Grignard reagent. This method also BORE has the disadvantage that all the gaseous carbon uc TlON ST IR RER dioxide is usually generated before tlie reaction is allomd to proceed and that manual agitation is employed. c It has been found possible to generate the carbon dioxide gradually from barium carbonate in ihe reaction system and obtain a high yield of the desired acid if the Grignard reagent is agitated by efficient internal st,irring. This procedure places no limitations on the size of the reaction to be run and all manipulations are easily executed. The pure acids have bcen obtained in yields of 80 t o 95% (based on carbon dioxide) from the aromatic halides such as bromobenzene, benzyl chloride, and p-bromoanisole, and from long-chain aliphatic bromides such as ciecyl bromide. There is no appreciable isotopic dilution during this carbonation. Figure 1. Grigriard Carbonation :Ipparatus
("\
L
828
N O V E M B E R 1947 opeiate a t reduced pressures. This type of construction is advantageous, since the sample to be counted may be spread in a thin layer to minimize self-absorption. Such tubes can be made to have good performance characteristics. However, if such an insti ument is used, one encounters difficulties in the satisfactory deposition of barium carbonate on plates for the determination of the radioactivity. By the use of the technique described belon-, even and homogeneous deposits of barium carbonate can be prepared, and there can be vide variation of thc samplc weight and specific activity n-ithout lois of accuracv. CARBON.4TIOS O F GRIGX.ARD REAGEKT
The apparat,useniplol-ed is illustrated in Figure 1. The essential features are a manifold, L , attached to a high-vacuum system and carrying outlets for the attachment of the reaction flask, K , the carbon dioxide generator, F and G, a manometer, C, and ail inlet, B , for nit,rogen. K is fitted with an induction st,irrer,I and J , and is pear-shaped t o minimize the danger of cracking when the Grignard solution is frozen with liquid nitrogen. The carbon dioxide generator consists of an Erlenmeyer flask, G, for the barium carbonate, and a pressure-equalizing funnel, F , for the sulfuric acid. If the charge of barium carbonate is of such a quantity that a 50-ce. Erlenmeyer flask is filled to more t h a r one fifth its total volume, a round-bot,tomed flask of larger size IS substituted, since large Erlenmeyer flasks sometinies implode on evacuation. -4drying tube, E , is placed bet\vecn the generator and'the manifold. 1Ianomctrr C is of the simple open mercury type. Thc follon-ing procedurcl illustratw tho niaiiipulatioris performed vlien a run Tras madr v,-ith this apparatus. The Grignard reagent n-as prepared in an all-glass apparatus under an atmosphere of nitrogen. An aliquot of the et'hereal Grignard solut'ion was removed and titrated t o determine the concentration of t'he reagent (3,7 ) . The concent.ration should be low enough so that t.he Grignard reagent, will remain in solution a t -2.5' C.; in general, this concentration will be less than 0.25 ;If. From the titer of the Grignard solution, the volume of the reagent equivalent to the quantity of barium carbonate to be employed FTas calculated. I 10% excess of the Grignard reagent, was used. While the Grignard reagent was being prepared, the requisitc: amount oi barium carbosatr was n-eighed into flask G, and funnel F was charged n-ith concentrated sulfuric acid. (A volume of 5 t o 8 cc. of acid was added for each gram of barium carbonate.) The generator wa? put together and attached to the manifold by means of the drying tube, E . The carbonation flask n-as assembled and also attached to the manifold. The entire system was evacuated to a pressure of 0.1 micron, stopcock -4 closed, and the system tested for leaks. Stopcock B then was opened and nitrogen Tyas admitted through this opening to bring the manifold sJ-stem to atmospheric pressure. (If a Ushaped manometer is employed, an additional outlet must be added to the manifold for this purpose.) The single-arm manometer serves as an outlet for excess nitrogen. Plug M n-as removed and the Grignard reagent quickly pipetted into flask K , using a pipet previously flushed with nitrogen and operated by a syringe. Jl then n-as quickly replaced, R was closed, the contents of the flask n-ere frozen with liquid nitrogen, and the system rras evacuated to a pressure of 0.1 micron. A mas then c l o d and the liquid nitrogen bath was replaced by an acetone-ice bath at -20". The nit,rogen previously entrapped, vhen the reaction mixture was quickly frozen, escapes a t this temperature. The Grignard solution was again frozen, the system evacuated, and t'he temperature raised to -20". (When this procedure \vas followed, the manometer reading was the same before and after the generation of the carbon dioxide. If the entrapped nitrogen JTas not allowed to escape, a greater pressure n-as noted after the generation than before, making the determination of complete carbon dioxide absorption difficult,.) Khen the contents of flask K had melted and had come to thermal equilibrium with the bath, the stirrer Tvas started and carbon dioxide generated by dropping sulfuric acid onto the barium carbonate. The first port,ion of acid was added cautiously, so that the initial surge of gas did not carry out particles of barium carbonate, then as rapidly as possible without allowing the pressure to rise above 50 em. The last traces of carbon dioxide were expelled from the sulfuric acid by rarming flask G with a small flame. The carbon dioxide vas absorbed rapidly and usually 5 t o 15 minutes vias sufficient time for the generation. Absorption is complete when the manometer reading is constant. The Grignard solution n'as frozen again n-ith liquid nitrogen to draw any remaining carbon dioxide in the reaction system into
829 flask K . The stopcock, H , was closed, and thc reaction mixture warmed to -20' and stirred for 15 minutes to ensure complete absor tion of the carbon dioxide. (When carbon 14 of high specilc activity is used, it is advisable t o attach an additional trap equipped with a three-way stopcock a t B while stopcock H is closed, evacuate the entire system, close stopcock A to shut off the manifold from the high-vacuum system, and cool the new trap in liquid nitrogen. H is then opened and K warmed to -20'. After any remaining traces of carbon dioxide have condensed in the trap, it is shut off from the system.) The manifold was then opened t o the atmosphere and the reaction mixture worked up in the usual manner. 4 s an example, benzoic acid was prepared by carbonating 0.031 mole of phenylmagnesium bromide in 37.6 cc. of ether with carbon dioxide generated from 5.885 grams (0.029 mole) of barium carbonate. Yield: 3.103 grams (85.4c/c),m.p. 122-123' C. INDUCTIO.l STIRRER
The induction stirrer illustrated in Figure 2 consists of a rotor of iron, fitted with an outer sleeve of copper tubing. This unit turns with the magnetic field set up by the field coil from a Selsyn m o l or.
IEi5"" Ir.d"r.d
Figure 2.
UllL ICO,.,
Induction Stirrer
The st irwi, liuusiiig, in which the bearings and rotor are mounted, consists of an upper body of Pyrex tubing, the lomw end of which is attached to a 29/26 outer ground joint liy fusing the upper end of the joint to the lower end of t'he body. This can be done without softening the lev-er half-inch of ground surface, which Ihen serves to center the lower bearing accurately. The stirrer is att,ached to the reaction vessel by a ground joint at the bottom. A lid a t t,he upper end of the body scrvcs to seal the stirrer and also carries the upper bearing. Two types of closures are shown in Figure 2. The flush type, shown a t the top, is sealed by de Khotingky or similar cement and is preferable t'o the flange type in applications where there is no possibility of chemical attack on the cement. The flange type is used in reactions where a refluxing solvent, or ot,her agent Tvould attack the wax. For such applications, a gasket of silicone rubber is used instead of \Tax, the pressure necessary to effect a tight seal being applied by a clamp (not s h o r n ) which pulls the flange and top together. \Tax can, of course, be used instead of a gasket if the environment permits. Since the flange Fill not slip through the Selsyn coil, the glass body must be longer for the flange type than for the flush type, in order that the rotor can be aligned with the pole pieces of the motor coil. The core of the rot,or is of soft iron, to maximize the magnetic flux, although mild steel is acceptable. It' is machined to slip into a copper sleeve with a clearance of 0.127 mm. (0.005 inch). Six longitudinal holes, located symnict~rically,are bored through t,he core to lighten it,. The entire rotor is coated with DC 801 unpigmented silicone resin which is baked a t 250" for 16 hours and serves t o prot,ect the metal from corrosive chemicals, as well as to bond the copper sleeve to and insulate it from the iron core. The rotor, the glass bodv, and tho field coil should fit closely,
V O L U M E 19,
830 since the power falls off sharply as the gap between the rotor and pole pieces increases. The rotor is coupled by three setscrews t o a 0.5-inch shaft passing through the center. The hole which accepts the shaft is machined 0.51 mm. (0.020 inch) oversize t o allow for the space taken up by the resin coating. A longitudinal hole bored part way into the shaft accepts the agitator rod, which is held in by a setscrew. The lower bearing is constructed by cutting a 1.59mm. (0.0625-inch) shoulder into the rotor shaft which rides on a bearing ring, which in turn rides on a bearing support. This is cut with a one-in-ten taper, which fits the standard-taper glass segment. This bearing support has three symmetrically placed longitudinal holes bored through it to facilitate evacuation of the rotor chamber in vacuum line applications. If desired, the lower bearing design may be modified by eliminating the standard-taper segment in the glass housing and bringing in the wall t o form a flat bottom into which is sealed the ground joint connecting the stirrer t o the reaction flask. The bearing support then lies on this flat bottom and is cut without a taper. Centering is achieved by machining it t o fit snugly against the wall of the housing. Care is taken t o make the bottpm flat, so that the bearing support will not rock during operation. All the internal parts except the metal rotor are made of hard Bakelite, which has good bearing qualities and is resistant to most chemicals. If conditions which erode Bakelite are encountered, parts may be made of Teflon. Teflon has satisfactory bearing qualities although a gradual flaking occurs a t the lower bearing. The Selsyn motor coil used is the General Electric Model 2JD55V1, which was selected because its internal diameter is such that 37-mm. Pyrex tubing fits it closely, and because its weight and over-all dimensions are convenient. The coil is operated on three-phase current if this is available; otherwise ordinary twophase current is used and a condenser is placed across two of the leads as shown in Figure 2, t o create a phase lag. The stirrer runs on 10 to 40 volts, At 40 volts the speed is around 2000 r.p.m. and the torque somewhat greater than that of the small Cenco laboratory stirrer. At voltages above 20, heating is great enough to require cooling the coil. This may be accomplished by an air blast or by building a water jacket around the coil. The over-all length of the flange type shown in the sketch is 15 cm., when a 24/40 ground joint is used; the flush type is 1.2 cm. shorter.
I
The absorber consists of an outer body made from a 100-cc. Pyrex graduated cylinder cut off a t the 70-cc. mark and sealed to an inner 24/40 ground joint; the inner member is made from a fritted-glass disperser to effect complete absorption. (The gas disperser used was Corning KO.395334 with fritted cylinder. This porosity gives a small enough pressure drop across the fritted disk so that the suction of a Mariotte bottle plus the pressure of the carrier oxygen will drive the gas through a t the recommended rate of 5 cc. per minute, and yet produce bubbles small enough so that complete absorption occurs.) The reservoir, B, holds water for rinsing the inner tube and has a capacity of about 10 cc., or enough to fill the inner tube with water up t o the dropper tip, E. Tube C is of such a diameter that a rubber pressure bulb can be slipped over it to force the rinse water through the sintered disk. LLCarbonate-free"sodium hydroxide was prepared by dissolving carefully cleaned sodium metal in ethanol and diluting the solution with carbon dioxide-free water to a concentration of about L N . (The alcohol present in the solution does not interfere.) The solution was stored in a siphon bottle with all-glass conneciions, and the air inlet was protected by a soda-lime tube. So stored, this alkali will remain at least 3 months without showing a blank greater than 0.2 mg. of barium carbonate per 50 cc. of solution. The combustions were carried out in the following manner: A little alkali was run out of the delivery tip of the siphon and discarded, then a quantity equal to five times the expected requirement was run into the outer cylinder and diluted with carbon dioxide-free water to a volume of 45 cc. The inner joint was greased lightly, the inner tube put in place, and the assembly attached at A t o the outlet of the combustion tube. When the combustion was over, the absorber was disconnected, the inner tube raised until it was above the level of the liquid in the cylinder, and the outer wall washed off with a fine stream of carbon dioxide-free water from a wash bottle fitted with a pressure bulb. The washings were allowed to run into the cylinder. Reservoir B was filled, and the stopcock opened to fill the inner
11
l-c
TO
BOTTLE
Figure 3.
COMBUSTION OF ORGASIC COMPOUND
The combustions were executed in the manner described by Pregl and Grant (IO)and the carbon dioxide formed in the combustion was absorbed in the absorber shown in Figure 3.
NO.
Carbon Dioxide Absorber and Precipi. tation Flask
Table I. Benzoic Acid Weight of Sample 3.204 10.350 10.540
n'eight
of BaCOI 36.4 117.2 118.7
Theoretical Weight of BaCOi
Theory
36.6 117.0 119.3
99.45 100.17 99 :49
%
01
tube with water. The water was then forced through the disk into the cylinder by a pressure bulb at C. The grease was wiped off the joint and the contents of the cylinder were rinsed into the precipitation flask shown in Figure 3. The contents of the flask were mixed well and an amount of ammonium chloride equivalent to the alkali used was added; then a twofold excess of barium chloride was added, the joint greased lightly, and the cap put on. The flask was swirled to mix the contents and most of the air removed by evacuation. (The air was removed because it was found that in a small combustion room with a Bunsen burner going, the air in the free space above the solution contained sufficient carbon dioxide t o give a blank of about 1mg.) After standing 15 minutes the flask was opened, the grease wiped off the joint and the liquid filtered through a weighed 15cc. sintered-glass alter crucible of medium porosity. After the barium carbonate had been washed three times with carbon dioxide-free water, once with alcohol, and once with ether, the solid was dried a t 120" and weighed. When all operations were done carefully, it was possible to fill the absorber with 50 cc. of the undiluted stock alkali, pass 150 cc. of oxygen through the solution, and obtain a blank of less thaD 0.2 mg. of barium carbonate. This procedure made it possible t o obtain the carbon analysis of the substance a t the same time that it was being burned for counting. The results of three determinations on benzoic acid are given in Table I. For samples of sufficiently high specific activity, an amount as small as can be accurately weighed on a microbalance can be used. In these cases, a known amount of inactive sodium carbonate was
NOVEMBER 1947
831
added t o the absorber to act &s a carrier and to bring the total weight of barium carbonate obtained to the amount necessary for radioactivity determinations. An alternative procedure was to insert a secona boat containing a known amount of beueoic acid into the combustion tube and, after the sample of the desired compound was burned, to burn the sample of the carrier benzoic mid. This method sweeps out the combustion tube after ignition of the active sample.
g Method
Self-
Weight
'dm'
35.7
3.10
132.6
7.12 11.51
3000 ' 2 1 4373 * 2 4
8J1.6
Z8.78
457" '84
42.1 81.9
. 195.4 . COUNTING METHOD
Thiol Mg./q.
,orptian Teetion Activity Specific 'sotor Calculated Activity c./m. c./m./mu.
3.65
2613 - 1 7
2829 *I5
1.429
1.531 2.183
;Ita** :3
?E.$ I"1.J
3.174
8515 18880
104.0 104.7 104.0
7.576
849'1
1u4.1
The mean apeeifio activity of 16 samples was 104.4 o./m./mz.
The samples far counting consisted of thin layers of barium oarhonste deposited on disks cut from hard aluminum sheet 0.153 mm. (0.006 inch) thick. The disks were 45 mm. in dim e t e r and the covered none was 39 mm. in diameter. The Samples were mounted by a technique similar to that used for barium sulfate by Hendricks and co-workers (6). A tared disk, C , was placed in a brass cup (Figure 4) with straight sides and an accurately machined flat bottom, D, in which were three small holes. I n this CUD was placed a thick.
---- ---- degree, the oonce'ntration of t66 cayhonate p a r h l e s in'the edgeof the meniscus. When the. disk wm completely dry, i t was removed by placing the bottom of the cup on a brass plate fitted with three prongs. These slim prongs lift. the disk over the edge of the cup, 80 that it can be removed easily. (A semiskilled operator oan prepare six to eight samples per hour with no dit& rulty.) ~
~
A
Figure 5.
Counter Shield
when the background counting rate is determined.
I
D Figure 4.
Sample Mounting Cup
This assern-
square inches and a horiaontal sectional aria of 3.1 square inches has a background of about 40 counts per minute. (The sample carrier was developed on suggestions of T. H. Norris.) The construction of the counter tubes is shown in Figure 0. All end-window Geiger-Miiller tubes embody the same general
5.1 cm. (2.0 inches) thick and of a n aluminum inner lining 0.95 om. (0.375inch) t.hick, is of more or less conventional design. At the brtek of the cylinder is a brass guide, A , to which is attached a
features of design. The windows of these tubes are approximately 5.5 em. in diameter and it is necessary to support the thin mica, of which the window is made, with a grid if the tube is t o operate a t other than atmospheric pressure. Several investigators have constructed tubes of this type, including the supporting grid, with windows as thin as 4.5 mg. per square centimeter. By polishing tat? high brilliance that surface of thesupport which touches the mica, counter tubes have been prepared with windows only 1.6 mg. per square centimeter in thickness. ( I t is possible to mount nindows as thin as 0.6 mg. per square centimeter and eliminate the supporting grid if the tube is filled with 74 em. of helium and 1 em. of ethanol.)
the t h i i window of the tube. This slide his tnio positiios, one for a sample, H , and the other for a clean disk, G , t o he used
The counter used in this research had a window 1.6 mg. per sq. cm. in thickness and was filled t o a total pressure of
~~~~~~~~~
.~~~~~~~
of;the specific activity' d a given sample of'harium carbonate agreed within 1.5%. Typical data are shorvn in Table 11.. ~
The shield and counter tube employed are shown in Figure 5. T h e body of the shield, coasisting of an outer envelope of lead
Tubes of this design have excellent electrical characteristics.
V O L U M E 19, NO. 1 1
832 10 cm. of mercury with 5% ethanol-95yo argon mixture. The threshold was a t 950 volts and the plateau was flat, * 1%, for 350 volts when the tube was used with a modified Keher-Pickering type of quenching circuit. The cathode of the tube was cleaned in concentrated nitric acid, thoroughly washed in distilled water, dried, and oxidized in a large flame before assembly.
The supporting grid is insulated from the cathode. There is a possibility that this arrangement might lead to an erratic charging of the grid with accompanying lack of reproducibility in counting. In practice the authors have varied this grid cathode separation from 0.5 to 15.0 mm. and have observed no effect attributable t o the presence of the T5indow support. These experiments were conducted both with the cathode grounded (Nehcr-Pickering type of quenching circuit) and ungrounded (Seher-Harper type of quenching circuit). If the cathode can be grounded the grid may actually he soldered to it.
15MM BOREX15 CAP1 ILARY T U
-2MM. .ANT
I6 G A . (050 D I A ) COPPLR WIRE
Y-1-
BORC
M
SPOT WLLD
LIMITS OF COUNTABILITY
The maximum activity countable with a tube of this design is 25,000 to 50,000 counts per minute. The minimum activity countable depends on the time spent and accuracy desired.
62 M M 0
0
-
PYREX
For example, with a background of 40 counts per minute, an error of 5y0is involved in determining the activity of a 27 counts per minute sample if background and sample are each counted for one hour. If the sample mount described above is used, a 50mg. sample is 4.34 mg. per sq. cm. thick. The self-absorption correction factor for this thickness is 1.77. If this sample liad an observed activity of 27 counts per minute, its actual activity would be 27 X 1.77 = 47.8 counts per minute and its specific artivity would br 47.8/50 = 0 956 counts per minutc p e ~mg. LITERATURE CITED
Buchanan, Hastings, and Kesbitt, J . B i d . Chcm., 150, 413 11943). Dauben, Reid, Yankwich, a n d Calvin, J . -4m. Chem. Soc., 68, 2117 (1946). Gilman, Wilkinson, Fishel, and Meyers, Ibid., 45, 150 (1923). Good, Kip. and Brown, Rev. Sei. Imtrziments, 17, 262 (1946) Hcndricks, Bryner, Thornas, and Ivie, J . Phys. Chem.; 47, 469 (1943). Hcnriquea, Kistiakowaky, LIargiietti, and Schneidw, IKD. ENG. CHEhI., A K A L . E D . , 18, 349 (1946). Houben, Boedler, a n d Fisher, Ber., 69, 1766 (1936). Kuster, Z . anorg. Chem., 13, 134 (1897). Livingston and Bethe, Rev. M o d e r n P h y s . , 9, 245 (1937). Pregl and Grant, "Quantitative Organic ;hislysis," English cd., p. 34, Philadelphia, Blakiston Co., 1946. Ruben, dllen, and Nahinsky, J . Am. Chem. Soc., 64, 3050 (1042). Sakarni, Evans, and Gurin, I b i d . , 69, 1110 (1947). .
I
Figure 6.
Counter Tube
(13) Beaborg, C'hem. Rets., 27, 199 (1940). (14) Treadxell-Hall, "hnalytical Chemistrl-," Vol. 11, p . 490, 9th English ed., John Wiles. Br Sons, New York, 1940. (15) Weinhouse, Medes, and Floyd, J . Bid. Chem., 155, 143 (1944). (16) Wood, Werkman, et al., Proc. SOC.EzptZ. B i d . X e d . , 46, 313 (1941). (17) Yankwich, Norris, and Huston, ASAL.CHEY.,19, 439 (1947). (18) Yankn-ich, Rollefson, and Sorris, J . Chem. Phys.. 14, 131-40 (1946).
RECEIVED May 26, 194T. Based on work performed under Contract W7405-Eng-48 with the Aromic Energy Commission in connection with t h e Radiation Laboratory and the Department of Chemistry. University of California.
Instrument for Continuous Measurement and Recording of Low Concentrations of Oxygen in Gases GUNTIIER COHN, Chemical Research Laboratories, Baker 6;: Co., Znc., ,Yewark, .V. J .
AKY industrial, processes require gaseous atmospheres which are virtually free from oxygen. For instance, in order to ensure satisfactory results, the nitrogen-hydrogen mixture for ammonia synthesis, the nitrogen or the rare gases used in the lamp and radio industry, and t,he nit,rogen with which high-power electric cables are filled are allowed to contain only traces of oxygen. In recent times the production of inert or reducing atmospheres has become increasingly important in the processing of metals such as sintering in porrder metallurgy, brazing, and heat treatment of stainless steels or silicon steels. In some applicaticns thc critical limit for thc pcrniissihle oxygen
concentration is in the neighborhood of 0.001c7, by volume, while in ammonia synthesis, for instance, oxygen concentrations as high as a few hundredths of 1% might be permissible. An instrument has been developed in this laboratory for the rapid detection and assay of the oxygen and the control of such low oxygen concentrations in a comparatively simple way. SELECTION OF METHOD
Continuous industrial gas analysis i- frequently carried out conveniently by means of thermal conductivity methods (6) which, hoivever, ale not sensitive enough for the determination of
