V O L U M E 20, NO. 5, M A Y 1 9 4 8
471
-
For a temperature change of 5' C., p - CY = 10 X lO-'/"C. that is, the permissible difference is inversely proportional to the temperature change. For most room conditions a variation of 1 5 ' C. is sufficient, although variations of 110" C. are encountered in extreme cases. Many plastics are available to permit variations of a t least *loo C. in room temperature and maintain a precision of 0.1 mm. in 200 mm. (Table I). These data were abstracted from the Modern Plastics Encyclopedia (8). Vinylite scales, printed and laminated, were found to be suitable for a maximum change of *5" C. in ambient temperature and still maintain a precision of 0.1 mm. in 200 mm., or a maximum of * 10" C. if the tolerance in precision xere increased to 0.2 mm. in 200 mm.
tube on the scale. The amount of mercury held up in the trap has negligible effect on the zero setting of the instrument, as can be seen from the calculation below, in which the length, L, of the mercury column in the trap is taken as approximately 70 mni.:. ~h~ =
L
D a (2)
=
70
(&)2
=
0.08 mm.
COiYCLUSION
A simplified absolute and differential m2tnometer has been found both accurate and more convenient than any of the ordinary types used in the laboratory. Besides the smaller gage, a larger one reading about 800 mm. has been used with comparable accuracy and ease of manipulation, cleaning, and observation.
OPERATION
The use of the manometer described is almost self-evident from its construction. In Figure 2 is shown a possible construction of a finished instrument mounted on a stand and ready for use in the laboratory. After the glassware has been scrupulously cleaned and dried, which is a simple matter since the tube is open a t both ends, and the stopcock has been carefully greased with a low vapor pressure lubricant, clean dry mercury (preferably reagent grade) is poured into the reservoir, until the level in the manometer tube corresponds to a zero reading on the scale (with the use of a vernier, the mercury level should just reach the lower edge of the vernier when set to read zero). Differential pressure readings can now be made with the stopcock open and the higher pressure connected to the reservoir. To use this manometer as an absolute gage, mercury must be drawn up the manometer tube into the trap and the stopcock sealed off. This may be done with vacuum, pressure, or by simply tilting the stand. The recommended procedure is t o use vacuum connected to both outlets simultaneously with a threeway cut-off stopcock in the connection t o the reservoir. After the system is pumped out, the cut-off stopcock is carefully closed and turned to the air, permitting mercury to rise gradually in the manometer tube. When the mercury level reaches the stopcock above the trap, it is closed off. The gage is now ready for absolute measurements OR connecting the system to the reservoir outlet and reading the resulting mercury level in the manometer
ACKNOWLEDGMENT
Thanks are due R. E. Bader of the Emil Greiner Company for his kind assistance in making the literature survey, and to the Emil Greiner Company for permission to publish this information. LITERATURE CITED
(1) (2) (3) (4) (5) (6) (7) (8)
Burton, IND. ENG.CHEM., ANAL. ED.,9, 335 (1937). Cameron,ZbM., 5,419 (1933). Doja, J . Chem. Education, 10, 574 (1933).
Ferguson, IND.ENG.CHEM.,ANAL.ED., 14, 164 (1942). Handbpok of Chemistry and Physics, 29th ed., p. 1653, Cleveland, Ohio, Chemical Rubber Publishing Co., 1929. LeRoy, D. J., private communication. Malmberg and Nicholas, Rev. Sci. Instruments, 3, 440 (1932). Modern Plastics Encyclopedia, Chart 2, New York, Plastics Catalog Corp., 1946. Robertson, IND.ENG.CHEM.,ANAL.E D . , 17, 238 (1945). Weatherill, J . Am. Chem. Soc., 47, 1947 (1925). Werner, IND.ENG.CHEM.,ANAL.ED.,10, 645 (1938).
(9) (10) (11) (12) Zbid., 17,805 (1945). (13) Zimmerli, Ibid., 10,283 (1938).
RECEIVED May 24, 1947. Presented before the Division of Physical and Inorganic Chemistry at the 113th Meeting of the AXERICANCHEMICAL SOCIETY, Chicago, Ill.
Determination of Small Amounts of Carbon in Steel Evaluation of Low-Pressure Combustion Apparatus JOHN J. NAUGHTON' AND HERBERT H. UHLIGZ General Electric Co., Schenectady, N . Y . The discrepancy between carbon values for low-carbon steels as determined by the low-pressure combustion method and the standard combustion method has been investigated and possible causes of the differences have been checked. All results show the reliability of the low-pressure combustion method, especially on low-carbon steels, where accuracy to 0.001% carbon is sought.
T
HE low-pressure combustion method has been investigated by several laboratories and has proved satisfactory for the determination of carbon in lowcarbon iron, even for use on a routine basis. The limitations of lowcarbon determinations by standard methods using an absorption train were pointed out by Yensen (6), who proposed measuring the pressure of the carbon 1 Present address, Department of Chemistry, University of Hawaii, Honolulu, T. H. 2 Present address, Department of Metallurgy, Massachusetts' Institute of Technology, Cambridge, Mass.
dioxide resulting from carbon combustion instead of weighing it, a method which was extended by Ziegler ( 7 ) . Modification of their method culminated in the low-pressure method of carbon analysis of Wooten and Guldner (5),reported on by Gurry and Trigg ( I ) , and used with further modification by Murray and Ashley (2) for routine determinations. An extension of this method was reported by Stanley and Yensen (4). Following the form suggested by Murray and Niedrach (3),an apparatus has bgen set up for investigation in this laboratory.
478
ANALYTICAL CHEMISTRY
These investigators all reported low values on determinations by this method run on Bureau of Standards sample 55A. This was checked by the authors, and similar low results mere also obtained with Bureau of Standards samples 55B and 8G. This paper reports the investigation of two aspects of this method: lack of agreement on carbon content of stecls with values reported by the standard methods of carbon analysis (Bureau of Standards samples), and source of blank.
TO UMPS ---L
TO SAMPLE T R E E
APPARATUS
The apparatus is essentially that described by Murray and Kiedrach ( 3 ) . The sample is burned in an allglass apparatus, the carbon dioxide is collected by condensation in a low-temCOMBUSTION TUBES perature trap (liquid nitrogen-cooled), W and, after the excess oxygen has Figure 1. Types of Combustion Chambers and Measuring System been pumped out the carbon dioxide . . is determined by a pressure measurement in a calibrated volume. The weight of sample 0.01% carbon for a 0.5-gram sample was always found. The used depends on the carbon content; thus, for low carbon values source, however, was found to be not incompletely oxidized carin this apparatus, the weight used is 0.5 gram, and for higher bon, but carbon dioxide absorbed from the air by both the oxide carbon contents it is proportionally less. and the ceramic crucible. Indeed, any ceramic crucible or boat alone, ordinarily used for carbon determinations, was found The sample is burned in oxygen a t 15 to 20 cm. of mercury greatly to increase its blank on exposure to the atmosphere-for pressure in a beryllia or alumina crucible, which is contained in a example, absorbed carbon dioxide was equivalent to an average of platinum crucible heated by high frequency. The equipment is shown in Figure 1 (furnace A.and measuring system D).. After 0.003% carbon based on 0.5-gram sample weight. This extranecombustion is completed (5 minutes), the excess oxygen containous source of carbon on exposure of ceramic materials to the ing the gaseous products of combustion is pumped out through atmosphere could be the source of the hisher results in the standthe water trap, Tz, and the carbon dioxide trap, TI. When the pressure in the system has been pumped to a value of ard combustion method, as has been suggested by Rooten and 10-4 mm. or better, as indicated qualitatively by the thermoGuldner (5). [Stanley and Yensen ( 4 ) successfully used a nickel couple gage, P, stopcocks 3 and 5 are closed, the carbon dioxide boat previously decarburized in wet hydrogen a t 1200" for 50 trap is allowed t o R'arm UP, and the pressure of the carbon dioxide hours.] released in the known volume of the system is measured by means of the RlcLeod gage. -4simple calculation converts this to per As an alternative test, the burned residue from a sample was cent carbon in the original steel sample. Carbon dioxide colreduced by palladium-diffused hydrogen a t 700' C. in situ for 10 lected in the C-type, trap a: liquid nitrogen temperatures can be minutes. Examination of another specimen treated in like quickly released by immersion of the trap in water. Such traps manner showed it to be completely reduced. The sample, tohbve been shown by experiment to remove carbon dioxide completely from an oxygen stream under conditions of the test and gether with the hydrogen used in the reduction, was then burned their use greatly speeds up the determination. again. No additional carbon dioxide beyond the normal blank Water vapor is removed in trap TZby the use of a frozen acevalue iyas obtained from the second burning. tone bath (obtained by pouring liquid nitrogen into acetone-solid Samples have also been burned with and without tin as a flux, carbon dioxide) which cools the trap to -95' C. A t this tembut without any appreciable difference in the results. If carbon perature, water vapor is removed efficiently,as the corresponding vapor pressure of ice is only 2.7 X 10-5 nun. of mercury. is held in the iron oxide residue, one would expect some difference of available carbon dioxide in this case because of the different, CARBON LOSS oxide environment. The conclusion seems to be in order, therefore, that no carbon That the low-pressure combustion method gives uniformly is left in the residue which results from the burning of low-carbon lower results than standard combustion-weighing technique when steels in oxygen. Any carbon found probably results from applied to low-carbon samples, has been checked by many experiadsorption of carbon dioxide from the atmosphere on oxide and menters. In the authors' work an effort was made to trace sysceramic. tematically any nonevident source of error in the low-pressure Loss of Carbon Dioxide. The possibility of adsorption of the method. carbon dioxide resulting from the combustion was examined. The lower results might be due to the existence of some oxidized This could happen in the ceramic crucible and on the walls of the carbon as noncondensable carbon monoxide, incomplete oxidafurnace. tion of carbon in the sample, or inadvertent removal of some carCrucibles made of many different materials with many different bon dioxide en route to the measuring device. treatments and of many sizes and shapes were used. The blank Existence of Carbon Monoxide. Introduction of a glowing varied with crucible material but the determined values of the platinum filament or of copper oxide (400 C.) into the path of the carbon content for samples were all consistent. Increased precombustion gases obtained in the normal manner failed to show cision resulted from better crucibles. The best material was any evidence that carbon monoxide was present. fused aluhina or beryllia. No result indicated that the hot Incomplete Oxidation of 'Carbon. The oxide from burned crucibles adsorbed sufficient carbon dioxide to account for the difsamples was ground and carbon redetermined. As much as
479
V O L U M E 20, NO. 5, M A Y 1 9 4 8 ference in determined values between the low-pressure and the standard methods. After several combustions an appreciable film of platinum (eonfirmed by x-ray) collected on the glass walls of the combustion chamber. This film probably results from the formation and possible later decomposition of an oxide of platinum. The suspicion was entertained that this platinum film, acting as a sort of "getter," might adsorb some of the carbon dioxide present in the system during a run or during an initial blank run. It was also conceivable that the slow desorption of some of this carbon dioxide would account, for the blank.
-
@ SULFUR PRESENT
-
g
NO S U L F U R
004
0030.02
-
OOl-
:O
.:+..
E OO2L 001 0
-
I 04
0'8 12 16 2'0 2'4 2 6 PARTIAL PRESSURE OF COZ IN S Y S T E M ( m m H g )
the run and compared with the amount introduced. The results, shown graphically in Figure 2, essentially substantiate the conclusions reached from the experiments on desorption. The adsorption of carbon dioxide varies with the furnace wall area, as would be expected. While these experiments are significant in showing that some carbon can be lost under these conditions, the amount of carbon involved is insignificant in comparison m-ith the total amount of carbon being determined, even for high-carbon steels where the samples of small weight increase the percentage error. In the range of carbon contents included by the authors' work the maximum carbon per specimen released as carbon dioxide is about 0.4 mg. Maximum adsorption for the corresponding carbon dioxide pressure (0.3 mm.) is 0.005 mg. of carbon (0.02 mg. of carbon dioxide), or about 1% loss, which is within the deviation for such samples. For low-carbon samples, Tvhere disagreement exists with the Bureau of Standards' results, the adsorption is still less (total carbon per specimen released as carbon dioxide = 0.050 mg., corresponding to 0.03 mm. of carbon dioxide, at which pressure adsorption is negligible). To bring about, agreement with the Bureau of Standards' sample 55B, FURNACE WALL AREA=I15cmP for example, we would have to account for the large loss of 0.012 mg. of carbon (0.04 mg. of carbon dioxide). A series of experiments was also run using an oxygen flow method. The equipment is shown in Figure 1,C and D.
The suspicion of appreciable adsorption was confirmed by the Oxygen a t 1 atmosphere flowed over a beryllia crucible conobservation that if the walls of the combustion chamber were tained in a quartzcombustion tube heated by a platinum-wound strongly heated (400" to 500" C.) after such a platinum film had furnace. This eliminates the troubles resulting from the plati~ ~ ~ l num i - film in contact with the carbon dioxide released from the collected, considerable quantities of gas R-ere driven sample. The samples could be dropped into the furnace for comtatively, this desorption had the following characteristics: bustion without opening the system. The gases resulting from the combustion were pumped over a platinum catalyst to oxidize the sulfur dioxide and through a frozen acetone freeze-out trap to No appreciable gas was obtained by heating the glass walls remove water vapor. Finally, the carbon dioxide w s condensed without the platinum coating. in a liquid nitrogen trap and, after removal of the excess oxygen, The amount of gas driven off was roughly proportional to the was released into a known volume and its pressure was measured. number of runs made before heating the film. Thus, the quantity of carhon could be determined, as in the lowVapor pressure-temperature distillation curves run for the gas pressure method. obtained from a heated platinum film that accumulated during actual runs showed that it consisted of carbon dioxide and sulfur The are in agreement the l o w r e s s u r e method dioxide, and some noncondensable gas which was not carbon (Table I). A conventional horizontal furnace and boat arrangemonoxide (probably oxygen). The amount of carbon dioxide desorbed seemed to vary inment gave comparable results if provision was made for filling- the versely with the sulfur content of the sample. boat without exposure to the air. The procedure was clumsy and The amount of carbon dioxide adsorbed (and desorbed on the was heating the film) depended on the sulfur content of samples run 911 this leads to the conclusion that while the platinum film orior to thp samule in auestion. Greater desor^ptionof carbon dioxide was found on heating the collected on the furnace walls does adsorb some carbon dioxide, film in oxygen than in vacuum. Adsorption of carbon dioxide could be . # appreciable for high pressures of carbon dioxide (from high-carbon sampled and Table I. Carbon Values Obtained by Combustion at Low Pressure and at 1 in the absence of sulfur dioxide. Atmosphere Pressure These facts lead one to postulate that, carbon dioxide under these conditions is either physically adsorbed or ehemisorbed, and that sulfur dioxide is similarly adsorbed and in preference to t.he carbon dioxide. Better to evaluate the quantitative aspect of this process, a small known quantity of pure carbon dioxide was admitted to the system. Oxpgen was also admitted and a run was carried out in the *Orma' manner t'he heated and other factors the same as during an a c t d carbon determination. The carbon dioxide was measured a f k r
%
N o . of
Detns,
Carbon, Std. ~ ~ condition ~ hof Run ~
55-4
15
0.014
558
20
0.014
55A
11
0.014
55B
10
0.012
55B
4
0.012
8G
14
0.069
14C
13
0.791
Sucrose
2
Piano wire
7
42.1 0.88
Average Blank c, d Mg. c, % Lowpressurecorn- 1 . 5 X 10-8 0,0003, bustion 0.5-g. sample Low pressure corn- 0.53 x 10 - 8 0,00008, bustion cooled 0 . 5 - g . sample walls Atmospheric pres- 3.1 X 10-3 0.0006, sure-0%flow 0.5-g. sample Lowpressurecom- 1.8 X 10-8 0.0003, buation 0.5% sample Low pressure oom- 1.2 x 10-8 .,..... bustion-Sn flux Lowpressurecorn- 0.6 X 10-3 ....... bustion Lowpressurecorn- 0.7 X 10-8 0.0018, bustion 0.05-g. sample Low pressure com- 0 . 7 X 10-8 0.028, bustion 0.003-g.sample Low pressure corn- 1.6 X 10-3 0,003, bustion 0.05-g. sample
-
.4verage
%
Standard Deviation,
Carbon 0.0108
0.0003
0.0108
0.0003
%
0.0105
0.0003
0,0095
0.0003
0.0098
o.noo3
0.063j
0.0011
0.785
0.008
42.0 0.881
0.1
0.0007
ANALYTICAL CHEMISTRY
480
the amount is not sufficient t o affect the results of low carbon determinations. If all the sulfur dioxide was not removed by the platinum catalyst in the oxygen flow method outlined, higher values in agreement with those of the Bureau of Standards were obtained for 5L4. This removal of sulfur’dioxide was difficult, and depended on careful preparation of the catalyst and the presence of sufficient catalyst area. Gas was taken from a commercial combustion setup which is in everyday use for the determination of
This was true for both the commercial steel examined and the Bureau of Standards steels, and does not substantiate the claim that all high-carbon samples are too inhomogeneous t o give good precision under these circumstances. Another feature of the authors’ results that can be noted in Table I is the agreement between the results by the standard method and by the low-pressure combustion method for highcarbon samples. The writers believe that this agreement is the result of a corresponding loIver percentage error in the standard
g 0080 -
by the boat and liner material on exposure t o the air, or to sulfur dioxide that might be entrapped with the carbon dioxide. The errors thus introduced are small when compared with the large total amount of carbon dioxide resulting from the combustion of the relatively large sample used in the standard procedure. For example, 0.01 to 0.05 mg. of carbon (average 0.02 mg.) can be absorbed from the air by an alu-
a
e z O.OT0
-
0.060
-
0.050
-
e
*..
0 4
I-
E
0
p”
I
sponding error of 0.2%. However, a
Bureau of Standards steel 8G,0.069WC
0.01% carbon sample, factor weight 2.72 grams, would give 0.272 mg. of
The small amount of carbon dioxide adsorbed by the platinum film can probably account for the blank in the lon-pressure equipment. This is indicated by the following results. Thorough degassing of the walls by heating in the presence of oxygen reduces the blank 3- to 10-fold. d special furnace tube x a s built (Figure 1, B ) which was designed in such a way that all surfaces t,hat, xould collect platinum could be water-cooled. The operation of t,his equipment was identical with that of the apparatus previously used. The purpose of the water cooling was to reduce desorption, and thus reduce the blank if the latter was due to desorbed carbon dioxide. The blank was found to be lower by a factor of about 3 (see Table I). Consequently, to obtain low blank values and increase precision, it is best to keep the walls of the combustion chamber as free of plat,inum film as possible. 4 cleaning every ten runs will suffice. The use of fused alumina crucibles also leads to increased I . precision in the results.
carbon and involve an error of approximately 7%. The difficulty of the difference between carbon results by the two methods can be resolved easily by recognizing that the limitat,ions of the combustion-weighing method sets the apparent limit of accuracy of this method to not better than 0.003% carbon for the factor weight. For low-carbon determinations the discrepancy falls within this error (sample 55B, 0.012 us. 0:00950/,). On the other hand, were each method equally accurate and the error in each case of an indeterminate nature and purely random, .the mean of many determinations by each method should more nearly coincide. The observed discrepancy for low-carbon determinat’ionsdealt with here is based on such averages in each case. The magnitude of discrepancy indicates a fundamental deteFminate or method error. Examination of the low-pressure met,hod has been, in effect, a search for the source of this error which, however, has not been found. On t,he other hand, it has been shown that exposure of boats with liners to the air, and possible trapping of some sulfur dioxide in carbon dioxide traps might be, a t least partially, the source of error in the standard combustion-vxighing met,hod. It is the authors’ conclusion, therefore, that the low-pressure combustion method meets present requirements for analyses of low-carbon materials with respect t,o both high precision and accuracy.
SIZE OF SAMPLE AND CARBON CONTENT
LITERATURE CITED
Finally, another experiment having special significance was performed (Figure 3). Bureau of Standards steel 8G (0.069% carbon) was analyzed in samples of weight varying from 0.005 to 1.1 grams. This gives a variation in carbon from 0.003 to 0.70 mg. The average carbon content of the controversial samples 55A and 55B weighing 0.5 gram is 0.050 mg., well inside this range. All results are in agreement (0.06357,, standard deviation O . O O 1 l ~ o ) . This is additional proof that the equipment can handle all ranges of carbon content,s without appreciable loss or trend, depending on size of sample or carbon content.
(1) Gurry, R. W., a n d Trigg, H., IND.ENG.CHEM.,ANAL ED., 16, 248 (1944). (2) M u r r a y , W. M., Jr., a n d Ashley, S. E. Q., Ibid., 16, 242 (1944). (3) LMurray, W.M., Jr., a n d Kiedrach, L. W., Ibid., 16, 634 (1944). (4) Stanley, J. K., and Yensen, T. D., I b i d . , 17, 699 (1945). (5) \$-ooten, L. A, a n d Guldner, W. G., I b i d . , 14,835 (1942). ( 6 ) Yensen, T. D., Trans. A m . Electrochem. SOC., 37, 227 (1920). (7) Ziegler, N. A , ,I b i d . , 56, 231 (1929)
carbon in steels, a t the point where pure carbon dioxide is supposed t o exist. The gas was analyzed by running a vapor pressure-temperature curve, and 5% sulfur dioxide by volume was found (using sample 55A). BLANK
DISCUSSION
Results of a series of runs are shown in Table I. No difficulties or unusual deviations were experienced in making determinations on small samples (as low as 0.005 gram) of high-carbon steel.
RECEIVED August 26, 1947.
CORRECTIOS.I n the article on l‘Application of Corrections in Viscometry of High-Polymer Solutions” [ANAL.CHEM.,20, 155 (1948)], Equation 1 should have a multiplication sign, and not s minus sign, before the bracketed quantity. H. wAGNER Eastman Kodak Co. Rochester, N. Y.
