Determination of Carbon in Low-Carbon Iron and Steel Low-Pressure Combustion Method L. A. WOOTEN AND W. G. GULDNER Bell Telephone Laboratories, 463 West St., New York, N. Y.
An improved apparatus and method for the determination of carbon in low-carbon iron and iron alloys are described. The method consists of combustion of the sample in an all-glass apparatus, collection of the carbon dioxide by condensation in a liquid nitrogen trap, and, after pumping
I
N COKNECTION with the analysis of hydrogen-treated iron some years ago (8) i t became necessary to devise a method of high sensitivity for the determination of carbon. This was an application of techniques described b y Cioffi ( 2 ) for heat-treating metals in vacuum in conjunction with the low-pressure method of gas analysis (8). It consisted of combustion of the sample in a n all-glass apparatus, collection of the carbon dioxide by condensation in a lowtemperature trap (liquid air or nitrogen), and, after pumping out the excess oxygen, determination of the carbon dioxide by a pressure measurement in a calibrated volume. The sample was heated by means of a high-frequency induction furnace. More recently the method has been improved and applied mit'h satisfactory results to iron-silicon and iron-nickel alloys of low carbon content. This paper describes the improved method and some of its applications. Somewhat similar methods for the determination of carbon in lorn-carbon iron have been described previously (9, IO, I I ) , but these methods do not employ high-frequency heating; hence they cannot utilize the advantages of an all-glass system. The induction furnace has been applied (7) to the determination of carbon in tungsten and chromium steels and other alloys difficult to burn by the conventionalmethod,but in this case the carbon dioxide was determined by the usual gravimetric method.
Apparatus The apparatus developed for the determination of carbon is shown in Figure 1. It consists essentially of a purification system, a combustion chamber, and an analysis system, approximately 200 cc. in volume, connected to a mercury diffusion pump backed by a mechanical oil pump (Cenco Megavac) PURIFICATIOX OF OXYGEN. The oxygen, admitted from a tank connected a t A through a flexible copper tubing, first passes through a liquid nitrogen trap, then over a palladium catalyst at 800" to 900" C. (Evidence of the presence of hydrocarbon impurity, probably methane, in some tanks of commercial oxygen made this step necessary. A temperature of 400" C. has been found satisfactory for oxygen now available), and finally through two more traps at liquid nitrogen temperature before being admitted to the combustion chamber. The palladium catalyst is contained in a quartz tube connected to the glass by graded seals. The oxygen is purified by condensing approximately 10 ml. in trap 2'1 and then evaporating a portion of this and recondensing it in traps 2'2 and 2'8. These traps collect any oxidation products formed by passing the oxygen over the palladium catalyst and also hold the liquid oxygen until ready to be admitted to the system. At liquid nitrogen temperature the vapor pressure of oxygen is approximately 16 cm., so that it is possible, with the cutoff at I
.
out the excess oxygen, determination of the carbon dioxide by a pressure measurement in a standard volume. The sample is heated by a high-frequency induction furnace. The precision of the method is within * 10 per cent on samples containing 0.003 per cent carbon.
closed, to hold a supply of ure oxygen in trap !fa while evacuating and outgassing the anag-sis and combustion systems. The mercury cutoff at I is employed as a manometer. The capillary by-pass (1-mm. bore) in I is a device for admitting oxygen a t a relatively high pressure of about 16 cm. to the evacuated system. The capillary a t K serves a similar purpose in exhausting the excess oxygen subsequent to the combustion of the sample. COMBUSTIOX SYSTEM. The combustion system is sealed on at D and is shown in the upper left of Figure 1. It consists of a Pyrex tubular chamber in which is suspended coaxially a platinum crucible which contains the refractory oxide crucible. The sample, in the form of millings or thin turnin s, is introduced into the side tube, &, which is then sealed off. $he sample is held in the horizontal portion of the loading tube, &, at point a, until the system has been exhausted and a final blank run on the crucible, after which the sample is introduced into the combustion crucible by means of a magnet. With additional horizontal branches on the loading tube, provision can be made for admitting several samples a t one time, thus reducing by a considerable factor the average time required for a determination. This technique also permits a blank to be run on the crucible immediately before introduction of the sample, thus eliminating any gas adsorbed on the crucible. The platinum crucible, which serves as the heatin element of the high-frequency induction furnace, is of standar! form with reinforced rim and attached bail, of approximately 15-ml. capacity, and was obtained from Baker & Co. The refractory oxide crucible which fits snugly in the platinum crucible is made of alumina or magnesia and was supplied by Norton and Company (For sug estin the use of magnesia the authors are inAsh ey, General Electric Company, Pittsfield, debtid to S. E. Mass., in whose laboratory multiple loading was first employed.) The blank on these crucibles which were specially prepared for the analysis of low-carbon iron is readily reduced to a low and constant value by prefiring in oxygen. Both alumina and magnesia crucibles, however, will adsorb a small quantity of carbon dioxide when exposed to air at room temperature and this will introduce an error unless removed by heating in vacuum or in oxygen before introducing the sample. For the combustion of nickel or iron-nickel alloys which require a higher temperature, the sample was placed directly in a small platinum crucible. The rapid formation of an oxide film on the sample prevents alloying with the crucible. The high-frequency unit used in this work was a vacuum tube oscillator designed and constructed by the Western Electric Company. A coil of 15 turns with an inside diameter of 6.25 cm. (2.5 inch) was used. A current of about 35 amperes at a frequency of approximately 500 kc. was required to give a temperature of 1100' C. under the conditions specified in the method. The coil was air-cooled by means of a small blower. Temperature was measured by means of an optical pyrometer. Stopcocks SI and S, are lubricated with Apiezon L, which is a very low vapor pressure grease formerly supplied by Technical Products, Ltd. Traps 2'1 and T6, which are held at liquid nitrogen (or liquid air) temperature, serve to protect the system from condensable gases originating in the stopcock grease as a result of oxidation.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
ANALYSISSYSTEM. The analysis system consists simply of a RIcLeod gage and the double trap, 2 ' 6 , separated from the other sections of the apparatus by the mercury cutoffs, J and K . [Contrary to a prevalent opinion, the McLeod gage can be satisfactorily employed to measure carbon dioxide and other condensable gases. The accuracy and precision of the age applied to carbon dioxide are well within the requirements ofthe present method-for example, the maximum error due to nonideality under the working conditions IS less than 0.3 per cent ( 5 ) . ] The McLeod gage, M , has a sensitivity of 5 X 10-5 mm. and a range up to 1 mm. of mercury. The gage was calibrated by the usual procedure (4),and the volume was calibrated by expanding measured quantities of gas from the RIcLeod gage.
Procedure The first step in the procedure is to exhaust and degas the entire system, by heating the glassware with a Bunsen flame with all cutoffs lowered and the pumps in operation. Holding the glassware at a temperature of approximately 425" C. for a period of 30 minutes is sufficient to condition the system. During this period furnace F is heated to 900" C., and the crucible is heated a t 1150" C. by means of high-frequency induced current for a 10-minUte period. During this and subsequent heatin of the crucible the combustion chamber is cooled by a stream ofair from a smali
Vol. 14, No. 10
blower. A final degassing of the analysis and combustion sections of the apparatus is made with cutoff I closed. After the initial degassing of the apparatus it is unnecessary to give it more than a few minutes' flaming in subsequent runs, provided the system is kept under vacuum. For routine analysis of samples of higher carbon content, the outgassing procedure can be shortened considerably. When the system has been satisfactorily conditioned (as indicated by ability to maintain a vacuum of approximately 5 x 10-8 mm. of mercury for at least 1 hour with cutoffs Z and K closed), traps Ti, Tq, T3,and 6'2 are surrounded by liquid nitrogen, oxygen is admitted, and purified, and blank determinations are run under the conditions used in the combustion of a sample. An analysis is run as follows: The sample (0.5 gram) is introduced into &, the tube is resealed, and the system is immediately exhausted. Oxygen is introduced and purified as described above. It is important first to collect liquid oxygen in Ti and then t o redistill it to 2'2-7'3, where an excess of the liquid oxygen (over that required for the combustion of the sample) is collected and held until ready to be introduced into the combustion chamber. This procedure of filling the combustion system by evaporation of oxygen from T$-T3 prevents condensable impurities from being swept through the traps mechanically. Having a supply of liquid oxygen in Ta and T3,K is closed, and the RlcLeod gage is filled with mercury by admitting air through
\
I
n
'I T
A N
0 CM
0
D
C
e
E 54
1
U
FOR DETERMINATION OF CARBON IX IRON BY FIGURE 1. APPARATUS
8. Flexible copper tube with coppei-to-glass seal B , C. Outlet t o rough vacuum (Cenco Hyvac) D. Connection t o analysis system E . Outlet t o oil p u m p (Cenco Megavac) F . Electric furnace G . Palladium catalyst on asbestos H . Clear quartz tube I, J , K . Mercury cutoffs
THE
LOW-PRESSCRE COMBUSTIOS METHOD
M. l\lcLeod gage S. Platinum crucible with reinforced rim supplied by Baker & Co. 0. Magnesia crucible supplied b y Korton and Co. Q . Side tube f o r admitting sample St-87. StODcocks T; Heater' T I - T ~ . Traps
a37
ANALYTICAL EDITION
October 15, 1942
stopcocks S6and Ss,respectively. Assuming the pressure in the system has been tested and found satisfactory, oxygen is admitted to the combustion chamber by lotyering the mercury in I (by applying suction to stopcock S3)to a point n-here the oxygen will just pass through the capillary by-pass and bubble up through the mercury column in the right arm of Z. When the pressure differential between the purification and combustion systems is somewhat reduced, Z is opened completely to allow the pressure in the two sections to equalize more quickly. Z is then closed to a point just above the capillary by-pass and held there during the heating of the crucible. Having admitted the oxygen as described above, a liquid nitrogen (or liquid air) bath is placed on T 6and a solid carbon dioxide- Cellosolve acetate bath on T4. The platinum crucible is now heated by high-frequency induced current to a temperature of 11.50' C. Heating is continued for 10 minutes. A t the end of this period the excess oxygen is evacuated in the following manner: The main stopcock, S7, is closed and then K is lowered very slowly to a point where the oxygen can escape slo\r-ly through the capillary at K . When the pressure has equalized in the sections separated by K , S,is opened just enough to allow the oxygen to be slowly pumped from the system through the capillary a t K . When a pressure in the system of less than 1 mm. is reached, K is opened completely and the system is pumped down to a vacuum of about 5 X 10-5 mm. The elapsed time from the escape of the first bubble of oxygen until K is opened completely is about 10 minutes. An additional 10 minutes is usually required to reduce the pressure to a value of approximately 5 X 10- 5 mm. As soon as the evacuation of the system is completed, K and J are closed and the carbon dioxide condensed in 2'5 is released in the calibrated volume and allowed to attain thermal equilibrium at room temperature. If the blank is too high-i. e., greater than 10 per cent of the expected value on the sample-the blank run is repeated until satisfactory. When a satisfactory blank has been established a sample is transferred from the loading tube to the crucible by means of a magnet, while the system is maintained under vacuum. Oxygen is admitted and the sample is burned according to the procedure described above. As the combustion proceeds and the oxygen pressure decreases in the combustion chamber, I acts as a valve, allowing more oxygen to enter the combustion system, thus tending to keep the pressure constant at slightly below 16 cm. of mercury while riot allowing any of the gaseous combustion products to escape. The same crucible may be employed for the combustion of several samples one after the other. A high blank value is never obtained after one sample has been burned in a crucible. Knowing the volume of the system and the pressure, the carbon content of the sample may be computed. One cubic centimeter of carbon dioxide at 1 mm. of mercury and 25" C. (1 cc. mm.) is equivalent to 0.6454 microgram of carbon. Therefore Pressure ( m m . ) X v o l u m e (cc.) X 0.6454 X 0.5000
10-6
x 100 = per cent carbon
SENSITIVITY OF THE METHOD. The sensitivity of the method as described is of the order of 0.02 cc. mm. of carbon dioxide. This is equivalent to less than 0.02 microgram of carbon. Practically, however, it is not possible to realize the full sensitivity of the method because of the difficulty of reducing the blank on the apparatus and oxygen used as reagent. With care, however, it is possible to hold the blank to the equivalent of 1 microgram or less of carbon. Experimental I n the method described above it is assumed that all the carbon in the sample is converted quantitatively t o carbon dioxide and as such is condensed a t low temperature in trap T6;and that the gas so collected after passing through trap T4a t -80' C. is substantially pure carbon dioxide. It is of particular interest to determine the influence of sulfur on the purity of the carbon dioxide obtained, since sulfur is usually present in small quantities in the iron samples to be analyzed. In the following paragraphs data are presented on the completeness of combustion of the sample, the composition of condensable gas obtained, and the influence of sulfur upon the results obtained b y the method as applied to some typical samples.
0 10
0 09
II
I
I
I
0 TEMPERATURE
IN
DEGREES
K
FIGURE 2. A4N.41,YsIsO F GASES
COMBUSTION OF THE SAMPLE.In order to determine whether the carbon is completely removed by this method, the fused iron oxide obtained from the combustion of thirteen 0.5-gram samples was collected, pulverized, and reanalyzed for carbon by firing in oxygen for a 30-minute period at' 1200" C. The value obtained was equivalent to 0.0001 per cent carbon on material which originally contained approximately 0.01 per cent carbon. The presence of the platinum crucible, heated to 1100" C., and of the platinum oxide which forms and evaporates to the walls of the combustion chamber, catalyzes the oxidation to carbon dioxide of any carbon monoxide formed in the combustion reaction, thus making possible the elimination of the copper oxide reagent previously employed (9, 11). CONDENSATIOX OF CARBOX DIOXIDE. By inserting an additional cutoff and trap between 2'5 and K it was established that the condensation of carbon dioxide in 2'5 under the conditions employed is substantially complete. No carbon dioxide was detected in the added trap after making a run in the usual way. COMPOSITION OF CONDEKSABLE Gas. The gas collected in T6 on combustion of typical samples was identified as carbon dioxide by the following tests: The gas obtained from a typical sample was completely absorbed when exposed to soda-lime. Portions of the gas obtained from the combustion of 'two different samples of iron, containing approximately 0.02 per cent carbon and 0.02 per cent sulfur, rr-ere analyzed by the low-temperature distillation method (f, 8). The vapor pressure curves obtained were characteristic of carbon dioxide. S o evidence of sulfur dioxide was found in either case. A plot of the vapor pressure curves obtained on these two samples is shown in Figure 2 in comparison with a curve on a synthetic mixture of carbon dioxide and sulfur dioxide. It is clear that the presence of sulfur dioxide can be readily detected by this method. In an attempt to determine the effect of sulfur, if present in larger quantities, 5 mg. of ferrous sulfide were added to a 0.5gram sample of Bureau of Standards Sample S o . 55,4 and the mixture was analyzed by the procedure described. The carbon dioxide obtained was equivalent to 0.011 per cent carbon, the same value as obtained without the addition of ferrous sulfide. Sulfur dioxide was not detected in the condensable gas. The apparatus was then opened and the walls of the combustion chamber and Ta (the dry ice trap) were extracted with a solution of hydrochloric and nitric acids. These solutions, analyzed separately for sulfur by a microgravimetric procedure, contained 1.1 and 0.1 mg. of sulfur for the combustion chamber and trap, respectively. A portion of the magnesia crucible used was also analyzed and was found to have increased in sulfur content. The total sulfur recovered from all sources was roughly equivalent to that added. However, when two consecutive runs were made on samples to which ferrous sulfide had been added, without changing the crucible or cleaning the combustion chamber be-
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tween runs, a high result was obtained on the second sample burned and sulfur dioxide was found in the carbon dioxide trap, indicating incomplete oxidation of the sulfur to sulfur trioxide. EFFECTOF PREHEATING SAMPLE IN VACUUM. In the early work the sample was preheated in vacuum for a short period at 500” C. to remove adsorbed carbon dioxide (9, 11). I t was found, however, that such a preheating treatment is unnecessary if the crucible is preheated and the sample transferred to it in vacuum. Furthermore, it appears to be unsound practice to preheat the sample in vacuum because of the probable loss of carbon as carbon monoxide. Some carbon dioxide is no doubt adsorbed on the magnesia crucible when exposed to air and is later evolved when the crucible is heated, but this can be eliminated by heating the crucible in vacuum before the sample is transferred.
the combustion chamber, is oxidized to sulfur trioxide and is adsorbed on the ceramic crucible and the glass walls of the apparatus or condensed in trap T4 a t -80” C. The presence of sulfur on the walls of the combustion tube and in T4 following the combustion of a sample containing larger quantities of sulfur supports this explanation. The evidence indicates, however, that the method is not suitable for the analysis of samples containing greater than 0.1 per cent sulfur, unless modified to ensure complete oxidation of sulfur to sulfur trioxide. Most samples of low carbon content, however, are also low in sulfur, and on these samples no significant error is introduced. The method deserves wide application not only to low-carbon iron and iron alloys but to higher carOF CARBON BY THE LOW-PRESSURE bon alloys where only very small samples are available-for TABLEI. DETERMINATION example, the authors have successfully applied it to samples METHOD of vacuum tube filament core made of nickel and nickel Carbon Blank Sample (Corrected for Blank) Deviation Correction alloys, weighing only a few milligrams. The method is gener% % % ally applicable to the determination of microquantities of 0.0231 $0.0008 Armco iron carbon. For application to samples of higher carbon content 0,0229 $0.0006 0.0216 0.0007 the procedure may be simplified and the time for a deter0.0210 -0.0013 0.0220 -0.0003 mination correspondingly reduced. -
-
Av. Armco iron HZtreated
0.0228
10.0007
0.0031 0.0031 0.0030 0.0030
+0.0001 +0.0001 0.0000 0.0000
Av. Carbonyl iron
0.0030
==0.0001
0,0087 0.0078 0.0090
+0.0002 - 0.0007 +0.0005
0.0085
*0.0005
0.0060 0,0059
0.0000 -0.0001
0,0060
==0.0001
Av. Iron-silicon alloy
Av.
0.0003
TABLE11. DATAON BUREAU OF STANDARDS SAMPLE 55A D a t e of Analysis
0.0002
Carbon
11/26/41 12/4/41 12/4/41 12/30/41 12/30/41 1/2/42 1/5/42 2/3/42 2/3/42
0.0003
0.0002
%
0.0106 0,0109 0.0108 0,0108 0.0105 0.0109 0,0109 0,0109 0.0107
-0.0002 $0 ,0001 0.0000 0.0000 0.0003 +0.0001 +o. 0001 +o ,0001 -0.0001
0.0108
~0.0001
Av.
Deviation
%
-
Precision of t h e Method The precision of the method applied to typical samples is shown in Table I. I n order further to test the method Bureau of Standards Sample No. 55A, which has a certificated value of 0.014 per cent carbon, was analyzed. Results are listed in Table 11.
Discussion The data presented in Table I show that the precision of the method is well within the limits of * 10 per cent of the quantity of carbop determined in the range of 0.003 to 0.02 per cent carbon. The precision obtained on hydrogen-treated samples of very low carbon content is generally higher than that obtained on samples of higher carbon content. This indicates a more uniform distribution of the residual carbon in the hydrogen-treated samples of low carbon content. The data in Table I1 show that the method gives a value of 0.011 per cent carbon as compared with the certificated value of 0.014 per cent. Further work showed that the low-pressure method, when vacuum loading is employed, consistently gives slightly lower results than are obtained with the standard carbon combustion method (6). This difference may be due to the elimination of adsorbed carbon dioxide from the crucible by heating in vacuum or in oxygen immediately before the sample is introduced, a procedure not possible with the standard high-pressure method. It was found, for example, that the crucible employed in this work 1%-ouldadsorb carbon dioxide equivalent to 0.002 to 0.003 per cent (0.5-gram sample basis) on exposure to air for 30 minutes. The absence of sulfur dioxide in the condensable gas obtained on combustion of samples containing small quantities of sulfur suggests that this gas, in contact with the crucible and the platinum oxide observed to deposit on the walls of
Summary An improved apparatus for the determination of carbon in iron by the low-pressure combustion method is described and applied to samples of very low carbon content. The method is inherently of very high sensitivity. To realize the advantages offered by the high sensitivity of the method, however, it is necessary to exercise care in the purification of the oxygen and in the selection and conditioning of the combustion crucible. Methods for the purification of the oxygen and crucible and for minimizing other sources of error are described and precision data on the method are presented.
Acknowledgment The authors are indebted to James Morrison for the data in Table I1 and for performing the experiments on the influence of sulfur.
Literature Cited f1)
iij
(3) (4) (5) (6) (7)
(8) (9) (10) (11)
Camnbell. N. R.. Proc. Phvs. SOC..33. 287 11921). Cioffi, P. P,, J. Franklin I&., 212, 601-12 t1931j. Cioffi, P. P., Phys. Revs., 39, 363-7 (1932); 45, 742 (1934). Farkas, A., and Melville, H. W., “Experimental Methods in Gas Reactions”, pp. 73-5, London, Macmillan Co., 1939. Francis, M., T r a m . Faraday SOC.,31, 1325 (1935). Lundell, G. E. F., Hoffman, J. I., and Bright, H. A,, “Chemical Analysis of Iron and Steel”, p. 157, New York, John X7iley & Sons, 1931. Smith, G. F., and Hockenyos, G. L., IND.ESG. CHEM.,ANAL. ED.,2, 36 (1930). Wooten, L. A., A . S. 2’. M. Bull., p. 39 (Jan. 1941). Yensen, T. D., Trans. Am. Electrochem. Soc., 37, 227 (1920). Zhuravleva, G., and Chufarov, G. I., Zavodskaya Lab., 9, 498 (1940). Ziegler, N. A., Trans. Am. Electrochem. Soc., 56, 231 (1929)
