Microdetermination of Carbon and Hydrogen by a Rapid Combustion Procedure GEORGE I. ROBERTSON, LOUISE M. JETT, and LOUIS DORFMAN Research Deparfmenf, Ciba Pharmaceufical Producfs, Inc., Summif, N. J.
b The rapid carbon-hydrogen determination has been made semiautomatic with respect to the operation of the movable burner. One analysis now requires a total elapsed time of only 20 minutes-1 5 minutes for the combustion and 5 for the weighings.
pound pressure. The flow rate is controlled with a fine Hoke needle valve placed in the line after the conventional gas-reducing valve, and after the setting is made, the on-off flow of oxygen is controlled with a Hoke toggle valve.
the standard Pregl absorption tube the diffusion rate must be kept a t a minimum, as the tubes are customarily weighed after 10 to 15 minutes. In designing a new tube the only permissible variable is enlargement of the capillary in the side arm of the absorption tube. In this manner, the rate of diffusion can be increased and the resistance to the fast flow of oxygen decreased. It is therefore possible to balance the gain in weight against the loss in weight. The length of the capillary arm must be kept short, as the oxygen-introducing arm of the water absorption tube must be kept warm (50" C.) to prevent condensation of water. When the capillary is 2 to 3 inches long, the temperature equilibrating period is prolonged.
APPARATUS
N
uivmous publications (1-3, 6, 8, 9, 11, 13-16) in recent years
have presented rapid combustion procedures for the microdetermination of carbon and hydrogen. The trend has been toward a simpler apparatus and the use of an empty combustion tube. -4partial filling is used only to remove interfering gases which might be absorbed in the water and carbon dioxide absorption tubes and not to aid oxidation. In addition, external absorbants for the oxides of nitrogen have to be employed. The accelerated techniques yield results comparable to, if not better than, the modified Pregl procedures. In principle there are but two methods; in one the sample is approached by the burner and vaporized toward the stationary furnace (2) and the combustion tube is modified, so that a longer oxidizing zone exists, and in the other the sample is pyrolyzed in a capsule as originally suggested by Friedrich (6). In the latter method, the capsule is inserted into the combustion tube with the open end facing the movable burner, which in its initial position is next to the long furnace. The burner is moved back over the sample and then forward again toward the long furnace. This technique (la, 13) has given excellent control of the oxidation of organic compounds in an oxygen stream. It was chosen because the standard Pregl combustion equipment could be used with minor changes. The physical setup is similar to that described by others (8, 14, 15), but the Nariotte bottle and the liquid pressure regulator are unnecessary. When the oxygen flow is maintained a t 50 ml. per minute, small variations in the flow appear to have no influence on the accuracy of the determination (14, 16). The oxygen, obtained from liquid air, is fed directly into the system from the cylinder at approximately 3-
.
132
ANALYTICAL CHEMISTRY
Absorption Tubes. S o carbon and hydrogen determination can be truly rapid unless the absorption tubes can be weighed immediately after removal from the combustion train. As a result of years of experience with various types of absorption tubes it was concluded that only the Pregl type could be so modified. Standard Pregl tubes must be conditioned for 10 to 15 minutes in the balance room in order to attain a constant weight, because the temperature in the area of the furnaces is usually higher than in the balance room. The same conditioning period exists for the redesigned tubes, as is shown by Figure 1, A . During this period the tubes gain weight. As the tubes are weighed filled with oxygen, diffusion occurs, accompanied by a loss in weight. The rate of diffusion is directly related to the size and length of the capillary openings in the arms of the absorption tubes. In
The body of the absorption tube, was left a t 155 mm. long; however, the perforated inner partition was eliminated (7). .An open standard-taper 10/10 joint is used instead of one constricted to a fine capillary opening. A single capillary constriction of 0.6 to 0.62 mm., 4 to 5 mm. long, is made in the side arms near the body of the tube. The length of the arm is 3 cm. To test their stability, the absorption tubes are filled in the usual manner and placed on the combustion train, and after 15 minutes they are removed and weighed. If the weight is
after remvoi f r m wrbuilion tmin
.
\,
8-Absorption tuber weighed immedialely after removal from combustion traln C-Abrorpllon lube6 eondittmed I" balance room for 15 minutes 4fler o x j w IS parsed ,,,through for 2minutes lube is reightd.
,.
'.
4 t
I
I. 0
Figure 1.
Conditioning of absorption tubes
constant within the desired weighing period, the tubes are satisfactory. The data presented in Figure 1 establish the possibility of attaining a stable absorption tube. The tubes are weighed each minute for 20 minutes; the points on the curve are the average of many readings taken over a period of 3 months. Any one point is accurate to * 2 y. As the water tube is weighed a t approximately the third minute, its readings were averaged a t this time interval. Jf7hen the absorption tubes are completely sealed, A , there is a constant gain of approximately 10 y per minute for the first 5 minutes; the gain then tapers off to a constant weight a t the end of approximately 16 minutes. As this was verified a number of times, it was possible to construct a tube in Jyhich the gain in weight could offset the loss in weight in this 5-minute period. C represents the loss in weight of unsealed tubes that were conditioned in the balance room for 15 minutes. Oxygen was then passed through the tubes for 2 minutes. Care was taken not to touch the tubes. At the end of this time the tubes were weighed; the curve is almost flat for 20 minutes. B represents the weight stability of the absorption tubes when weighed after each combustion. The over-all change in ryeight in an 8minute period was never more than 5 y . The data were taken on a Nettler microbalance (AI-5). In normal practice, the carbon dioxide tube is weighed after the first minute, during which time the zero point of the balance is adjusted. An initial change of 4 to 5 y is always observed when the tubes are placed on the microbalance. This reading is ignored. The one variable not taken into account is the gain in weight of the tubes due to the absorption of water and carbon dioxide from the atmosphere. It is believed that this is offset by the blank correction. Further work is contemplated on constructing absorption tubes with different capillary dimensions. The absorption tubes are operable under the follom-ing conditions. A rapid weighing microbalance is essential, as the weighings must be completed within 8, preferably 5 minutes. Both the balance room and the laboratory should be air-conditioned. Any up and down fluctuation in either temperature or humidity seriously interferes with the weighings. A gradual positive change is without effect. The temperature in the area of the absorption tubes when placed on the combustion train must be higher than in the balance room. Under reverse conditions the tubes are inoperable. The temperature differential was 1.5" to 2" C. in the above experiments. On the whole, by the very nature of the physical arrange-
ment, these conditions exist in most air-conditioned laboratories. However, each laboratory may have to vary the capillary dimensions of the absorption tubes according to physical setup. The water tube is filled with Anhydrone and the carbon dioxide tube is filled first with a 2-em. lager of Anhydrone and then with Ascarite. Quartz wool is used to separate and contain the fillings, as it forms a more compact mass than the glass wool usually employed. I n the rapid combustion procedure the absorbents are exhausted rapidly. Low carbon values occur after the tube containing Ascarite has absorbed 400 to 450 mg. of carbon dioxide. These results have been attributed ( 2 ) to loss of water, as the small amount of Anhydrone used in the absorption tube is soon spent. This point was confirmed. Consequently, both tubes are removed a t the end of the third day of use and occasionally sooner, if many samples contain high percentages of carbon. In Figure 2, a copper heating coil, 8, on the absorption tube, D,prevents condensation of moisture in the side arm. If condensation occurs, oxides of nitrogen dissolve in the water and high hydrogen values are obtained. A Prater absorption tube is placed between the water and carbon dioxide tubes. It is filled with manganese dioxide granules (10 to 20 mesh), a reagent which effectively removes oxides of nitrogen (11). At the exit side a 1-em. layer of Anhydrone retains any traces of water which might otherwise pass from the manganese dioxide to the Ascarite tube. Before use, the manganese dioxide is dried for 2 hours, in vacuo, a t 100' C. The filling lasts for 100 to 120 nitrogen-containing samples and its exhaustion is indicated by high carbon Values. The tube is changed every fifth day. The effectiveness of the manganese dioxide will vary from lot to lot. Newly filled absorption tubes are conditioned by burning an unweighed sample. All absorption tube connections are made with silicone rubber tubing. This absorbs neither water nor carbon dioxide and needs no lubricant. Combustion Tube. Because the furnaces are fixed in place on the unit, it was necessary to lengthen the standard quartz tube to 60 em. This bears a No. 12/2 spherical ball joint on its side arm and a N o . 14/35 standard-taper outer joint a t its mouth. These two ground joints are not necessary, as the same blank is obtained with silicone rubber connections and a stopper; however, they are convenient. Because of the difficulty in routinely fabricating quartz capsules of the desired size, the outside diameter of the combustion tube was increased to 12.5 mni. -45-em. coil of 1-mm. silver wire, six turns, is placed a t the exit end of the combustion tube, follorred by a 13-cm. layer of silver wool. Electrolytic silver wool (16) is used because it has a high absorptive capacity for halogens and sulfur per unit of weight. A n indentation is made in the tube a t the center of
the long furnace to retain a 2-em. layer of quartz wool which will trap carbon particles swept down the tube during the combustion of the sample. A second indentation is made 1 cm. in front of the movable furnace to prevent the capsule from traveling down the tube. KO conditioning of the combustion tube is necessary. The tube is replaced after 850 to 900 analyses, principally because in the author's laboratory many samples contain halogen. -4 tube that was retained for 1300 analyses yielded erratic results because the silver filling no longer completely retained halogen. When the silver filling was omitted, no chlorine or bromine combustion products were retained in the mater absorption tube (IO) and all the chlorine was absorbed by the manganese dioxide. However, bromine \vas incompletely absorbed and passed into the carbon dioxide absorption tube. Iodine condenses in the water tube. Sulfur oxidation products are completely retained in the manganese dioxide tube. Consequently, the combustion tube is replaced after 2 months of continual use, long before the silver is saturated with halogens. The quartz area, above the sample, becomes opaque after one month's service and must be replaced, if one desires to watch the sample during combustion.
Capsule. The capsule is constructed from thin-walled quartz tubing, 7 t o 8 mm. in outside diameter and 5 cm. long. The internal diameter should be sufficient t o accommodate a standard platinum microcombustion boat. The closed end of the capsule has a loop of quartz, so that it can be easily removed from the combustion tube. A thin platinum sheet. 5 X 1 cm., is fitted into the capsule. If the platinum sheet is omitted, many explosions occur, irrespective of how carefully the sample is approached by the movable burner, and erratic values are obtained. This unevenness in rate of burning is due to the poor heat conductivity along the walls of the quartz capsule. The incidence of explosions is greatly reduced by the use of the platinum sheet; if one does occur, it is mild and does not affect the results. Under these conditions of combustion less nitrogen oxides are formed ( I d ) , as the oxidation is accomplished with a deficiency of oxygen. The burning characteristics of organic samples under the above conditions can be roughly classified into three types. In one, the sample merely chars and the carbon is gradually burned off. Most samples burn in this manner. In the second, vapor of the sample ignites as it forms, and a reddish flame gradually travels forward in the capsule, accompanied by the streaming of fine carbon particles. In the third, VOL. 30, NO. 1, JANUARY 1958
w
133
4
igurt
F, Silver coil G. Silver wool € Short I . burner I . Long burner J . Movable burner
the sample explodes as the burner approaches. In the last two situations it was customary in the past to bring the burner back and slowly approach the sample again, so that the greatei part of the sample would not flow pas1 the furnace unoxidiaed. This proce. dure has now been found unnecessary. Thus, the rapid technique appears tc be readily adaptable to a completelj automatic combustion apparatus.
Furnaces. A modified Hssli combustion apparatus is used. The heat. ing mortar is replaced by a 7.5-em furnace set a t 500" C., which heats the largest part of the silver filling. The long stationary and movable burners are 20 and 6.5 em. long and are set a1 800' to 850" and 950' C., respectively An oxygen-purifying furnace is neces. sary, as variable blanks are obtained from one oxygen cylinder to another. The usual IO-em. furnace is incapab!e of completely purifying the oxygen in its rapid flow and consequently it is replaced by one that is 20 em. long and set a t 750" C . To absorb the water and carbon dioxide a large 8-inch U-tube, filled with Ascarite and Anhydrone, is used, the latter reagent being on the exit side. No cement or grease is used on the ground-glass joints, including those on the combustion tube. The joints are held in place by springs and clamps. As the system is always under pressure, 134
ANALYTICAL CHEMISTRY
Figure 3.
Micro rarbon and hydrogen assembly w
sugnr IeaKS DeIore me ComDuSClUIl zuue are of no importance. To test for a leak in the combustion and absorption tubes, the oxygen inlet arm of the combustion tube is disconnected and closed with a n b b e r stopper. The flowmeter is replaced by a Mariotte bottle a8 with this the smallest leak can be easily observed. The flowmeter will not register a flow of less than 1 ml. per minute and, therefore, small leaks cannot be detected. The setuo is given in Fieures 2 and 3. ~
PR, uL"."..uu ... The ahsorpt101. the usual manner a t the beginning of each day. With the tubes in position y"vIu
system is flushed with oxygen for 10 to 20 minutes a t 50 ml. per minute, during which time a number of samples can he weighed. The rapid flow of oxygen is continued all day, so that no atmospheric contamination can enter the combustion tube while the capsule is being removed or reinserted. After the flush-out period, the ahsorption tubes are removed from the combustion train with the aid of chamois gloves and are not handled again in this manner until they are replaced on the train. The arms of the tubes need not be cleaned, as silicone n h h e r tubing leaves no deposit. Under these conditions excellent weighing stability is ob-
tained. The carbon dioxide tube is removed first, in order to maintain a positive flow of oxygen through the absorption tubes. The Prater tube is then closed. The tubes are weighed immediately on a Mettler microbalance (M-5) in the order of removal. The tubes are then replaced on the combustion train. The capsule is now removed from the combustion tube, the sample is inserted toward the closed end, and the capsule is reinserted into the combustion tube, open end first. Solid samples are weighed in platinum boats and liquids in capillaries. The movable burner, which is adjacent to the long burner, is slowly moved manually toward and over the capsule in 3 minutes. The burner is kept over the capsule for 3 minutes more, after which it is automatically set in motion toward the long burner a t such a rate that the total time of combustion is 10 minutes. The combustion tube is now flushed for an additional 5 minutes, during which time the next sample is weighed. After exactly 15 minutes the absorption tubes are removed and weighed immediately. Total elapsed time for one analysis in a series is 20 minutes. With the exception of compounds listed by name, all the others were analyzed as received from the research staff. To date, no trouble has been encountered in analyzing compounds that have been said to be difficult to burn. Good results have been obtained on samples weighing as low as 2 mg.; below this value extreme care should be taken. A blank should be run before the sample is analyzed, the zero reading for the balance taken after each weighing, and the weight of the tubes thus corrected.
Table I.
Substance Cystine Acetanilide
Sulfanilamide p-Bromoacetanilide Anisic acid Benzoic acid Steroids Cholesterylacetate C~i”iBr05
Determination of Carbon and Hydrogen
Weight, hlg.
Found 70
6.065 1.170 4.843 3.057 2,115 3.589 5.370 4.132 4.520 4.S91 3,917
29.99
5.03
71.04
6.70
41.86
4.68
44.90 63.15 68.84
3.77 5.30 4.95
4.353 3 565 4.180 4.014 4 127 4 656 4.713
81.25 59.10
11.29 6.69
74.01 71.74
9.00 9.15
72. SO
4.876 3.955 4.106 3.660 3 690 4 744 4 173 3 950 3.389
3.227 4.386 4,160 2.863 3.146 2 998 3.123 3.488 4.481 4.177
BLANK
A constant blank of 30 + 10 y for the carbon dioxide tube and 50 =t10 y for the water tube is always obtained, but in any one day the blank does not vary more than 10 y. It is difficult to attribute the blank to any particular phase of the determination. The diffusion of oxygen or the absorption of carbon dioxide and water from the atmosphere cannot contribute much to the blank, as the tubes do not lose or gain appreciable weight on the microbalance. It has been suggested (12) that when carbon is deposited on quartz a t elevated temperatures silicon carbide is formed, which will yield high carbon values in subsequent analyses. Though the apparatus is operated under conditions favorable for silicon carbide formation, this is not the cause, as the blank is the same even when a new tube is used, before any samples are oxidized. The possibility that a film of moisture is always present on the weighing equipment (4)has not been definitely ruled out. However, as the
Required C% H
C%
H%
30.03 29.95 71.10 7i.ii 70.82 41.87 41.98 44.86 63.33 69.03 68.98
5.06 5.31 6.80 6.65 6.70 4.91 4.97 3.79 5.31 5.02 4.98
8.73
81.38 59.22 59.21 74.19 71.38 71.47 72.91
11.33 6.80 6.80 9.17 9.21 9.27 8.85
46.66 65.11 67.52 82.57 83 17 81.38 73.59 68 97 67.i2
4.48 6.62 6.61 7.29 6 61 8 63 8 03 6 67 7.58
46.70 65.05 67.35 82.39 83 36 81 16 73 62 69 16 67.81
4.66 6.86 6.66 7.36 6 62 8 76 8 25 6 59 7.53
34.26 49.10 41.10 69.82 50.38 60.23 61.67 55.70 62.28 68.92 50.30 79.96 58.19 66.96 78.68 61.00 60.57 61.54 62.33
6.71 9.27 5 52 5.86 7.69 7.16 3 48 7.19 .?I23 . 8.31 3.70 8 20 5.35 6.84 3.85
34. OF, 49 04 41.07 69.97 50.42 60.06 61.59 55.43 62.17 68.90 50.18 79 90 58.22 66.94 78.78 61.01 .~ 60.36 61.40 62.25
6.81 9.58 5.82 6.12 7.75 7.17 3.73 7.15 5.18 8.39 3.94 8.30 5.39 6.95 4.02 6.25 7.53 4.11 7.22
atmospheric conditions of the room are constant, differential Iveighing should cancel out this error. Further work is contemplated on determining the source of the blank. LITERATURE CITED
(1) Belcher, R., Fuel 2 0 , 130 (1941). (2) Belcher, R., Ingram, G., Anal. Chim. Acta 4, 118 (1950). (3) Belcher, R., Spooner, C. E., J . Chem. SOC.1943,313. (4) Charlton, F. E., Analyst 81, 582 (1956). (5) Colson, A. F., Zbid., 73, 541 (1948). (6) Friedrich, A., Angew. Chem. 45, 476 (1932). (7) Hozumi, K., Imaeda, IC., J . Pharm. SOC.Japan 74, 574 (1954).
5.. 9. .7
7.82 4.06 7.47
~~
(8) Ingram, G., Analyst 73, 548 (1948). (9) Ingram, G., Mikrochemie ver. Mikrochim. Acta 36/37,690 (1951). Ingram, G,, Il.likrochim. Acta 1953, 71. Zbid., 1956,877. Kirsten, W., Mikrochemie ver. Mikrochim. Acta 35, 217 (1950). Klimova, V. A., Korshun, bl. O., Bereznitskoya, E. G., Doklady Akad. Nouk. S.S.S.R. 96, 287 (19.54). ---, Korshun, M. O., Shiveleva, N. S., Zhur. Anal. Khim. 2, 274 (1947): Ruck, J. A., Altieri, P. L., Mikrocham. Acta 1956, 1550. llitsui, T., Sato, H., Zbid., 1956, 1608. \
RECEIVED for review March 13, 1957. Bccepted July 26, 1957. VOL. 30, NO. 1 , JANUARY 1958
135
