V O L U M E 20, NO, 1, J A N U A R Y 1 9 4 8 explained by the fact that considerably longer hours of digestion are required for these (28) and that loss of nitrogen does not commence appreciably until oxidation of organic to ammoniacal nitrogen is complcte. The catalytic effect of selenium is, as shown by Sreenivasan and Sadasivan (32j, most pronounced during the initial stages of decomposition of reducing organic matter and as, with ring compounds, oxidation is extremely slow from the commencement, the chances of better recovery of nitrogen on prolonged digestion are obviously more than with proteinaceous materials. I n view of the general ease with which conditions of digestion in semimicro and microprocedures can be controlled (fa),it is possible that, as recently reported (7, 11, fd, f7, 18, 2f j , these may yield more concordant results for nitrogen determinations by the Kjeldahl method and its modifications. It is desirable that further extended trials by the micromethod, using various types of compounds, be carried out with a view to verifying whether the period of afterboil following clearance that is necessary for completeness of conversion to ammoniacal nitrogen is not so rigid a factor in determining'the accuracy of the results obtained as in the macromethods. LITERtTURE CITED
(1) (2) (3) (4)
Acree, F., J . h s o c . Oficial .4gr. Chem., 3, 648 (1941). Ashton, F. L., J . A g r . Sci., 26, 239 (1936). Ashton, F. L., J . SOC.Chem. Ind., 56, lOlT (1937). Beet, A. E., Fuel. 11, 196, 406 (1932); 13, 343 (1934). Beet, A. E., Belcher, R.. and Godbert, A. L., IND.ENG.C m x . , ANAL.ED., 17, 746 (1945). Beet, A. E., and Furaey, D. G., J . SOC.Chem. Ind., 5 5 , 108T (1936).
Belcher, R., and Godbeit, A. L., Ibid., 60, 196 (1941). Bradstreet, R. B., Chem. Revs., 27, 331 (1940). Bradstreet, R. B.. IND. ENG.CHEM.,ANAL.ED., 12, 657 (1940). Chibnall, A . C.. Rees. S1. W., and Williams. E. F., Biochem. J., 37, 354 (1943).
65 (11) Clark, E. P., J . Assoc. Oficial A g r . Chern.. 24, 641 (1941). (12) Clark, E. P.. "Semimicro Quantitative Organic Analysis," Ken.
York. Academic Press, 1943. (13) Dalrymple, R. S.,and King. G. B., ISD.ESG. CHEM., ANAL.ED., 17, 403 (1945). (14) Davis, C. F., and Wise. M., Cereal Chem.. 10,488 (1933). (15) Gortner, R. A . , and Hoffman, R. F., J.'BioZ. Chem., 70, 457 (1926). and Ssolovjeva, li. ii., 2. anal. Chem., 100, (16) Illarionov, ITr. W., 328 (1935), (17) Jonnard, R., ISD.ENG.CHEM., ;Ix.%L. ED..17, 246, 746 (1945). (18) Kaye, I . A., and Weiner, N., Ibid.. 17, 397 (1945). (19) Korek. G. R., T r u d y Leningrad. Inst. Sovet. Torgovli, 1939, 30. 2 , 22-9; Khim. Referaf Zhur., 4, Xo. 1 , 89 (1941). (20) Lauro, M. F., IND.EX. CHEM.,ASAL. ED.,3, 401 (1931). (21) Miller, L., and Houghton, J. d.,J . B i d . Chem.. 159, 373 (1945). (22) Osborn, R. A , , and Krasnita, A., J . Assoc. Oficial Agr. Chem.. 17, 339 (1934). (23) Pepkowits, L. P., and Shive, J. W., ISD. Eso. CHEN.,ANAL.ED., 14, 856, 914 (1942). (24) Peters, J. P., and Van Slyke, D. D., "Quantitative Clinical Chemical Methods," p. 519, Baltimore, Williams & Wilkins Co., 1932. (25) Poe, C. F., and Nalder. M. E., ISD. ENG.CHEM.,- 4 i i . ~ ED., ~. 7, 189 (1935). Biochem. Z., 296, 434 (1938). (26) Sadasivan, V.,and Sreenivasan, -4.. (27) Sandstedt, R. M., Cereal Chem., 9, 156 (1932). (28) Shirley, R. L., and Becker, W.W., ISD.ENG.CHEM.,ANAL.ED., 17, 437 (1945). (29) Snider, S.R., and Coleman, D. A, Cereal Chem., 11, 414 (1934). (30) Sreenivasan, A , Ind. J . Agr. Sci., 4, 546 (1934). (31) Sreenivasan, A., and Sadasivan, V., ISD. ESG. CHEM.,ASAL. ED., 11, 314 (1939). (32) Sreenivasan, A, and Sadasivan, V., 2. anal. Chem., 116, 244 (1089\. , (33) Taylor, W. H., and Smith, G. F., ISD.ENG.CHEM..ASAL.ED., 14, 437 (1942). (34) Van Slyke, D. D., Hiller, -4,, and Dillon, R. T., J . Bid. Chem., 146, 137 (1942). (35) Wagner, E. C., ISD. ENG.CHEM.,-4x.4~.ED.,12, 771 (1940). (36) Wicks, L. E., and Firminger, H. I., Ibid., 14, 760 (1942). \ - - -
RECEIYED November 20, 1946
Determination of Gaseous Hydrocarbons Combustion over Precipitated Copper Oxide Containing 1% Iron Oxide ROBERT E. MURDOCK, FRANCIS R. BROOKS, AND VICTOR ZAHN Shell Decelopment Company, Emerycille, Calif. A method is described for the rapid and quantitative determination of gaseous paraffins by combustion at 700" C. over precipitated copper oxide containing 1% iron oxide. This method is an adaptation of that proposed by Briickner and Schick ( 1 ) and has great advantages over commonly employed methods that use copper oxide wire which requires a temperature of around 900" C., or combustion over a heated filament or catalyst which requires the addition of oxygen. Formation of explosive mixtures and the hazard created thereby are eliminated.
I
S THE analysis of gases containing only nitrogen and paraffinic hydrocarbons, the paraffin content is usually determined by combustion analysis. Methods most commonly used for the combustion (4,?) require admixture of a portion of sample with a suitable excess of oxygen and passage of this mixture over a heated catalyst or filament to promote combustion. Such methods necessitate a source of oxygen of accurately known purity and errors may be introduced by contamination or inaccurate measurement of the oxygen used. I n addition, it is difficult completely to avoid formation of explosive mixtures and the hazards which arise therefrom. These disadvantages can be eliminated by the use of a combustion agent having readily available combined oxygen. It is also desirable that
this agent display sufficiently high reactivity to permit combustion a t a moderate temperature, so that special apparatus and techniques are unnecessary. Copper oxide has often been used as such a combustion agent, but according t o Lunge ( 5 ) combustion of methane is not rapid even a t 950' C. and numerous passes over the oxide are required for complete combustion. At this temperature dissociation of copper oxide (2, 6) may introduce appreciable error if the oxygen formed is not removed prior to measurement of the residual gas volume. Campbell and Gray ( 3 ) have reported that copper oxide displays improved activity when impregnated with small amounts of cuprous chloride or other metallic oxides. Briickner and Schick ( 1 ) found that copper oxide prcpared by
, ANALYTICAL CHEMISTRY
65 precipitation, and containing 1% iron oxide, is capable of rapid and complete combustion of hydrogen at 220" C. and of methane a t 600" C. This combustion agent has been further tested by the authors and has proved t o be much superior to commercial copper oxide for the combustion of paraffins. APPARATUS AND MATERIALS
The precipitated copper oxide-iron oxide mixture (hereafter referred to simply as precipitated copper oxide), according to Bruckner and Schick, is prepared as follows: Dissolve in 3 liters of distilled water appropriate weights of cupric nitrate and ferric nitrate to produce 100 grams of 99 to 1 copper oxide-iron oxide, and add 3070 potassium hydroxide solution in slight excess. Boil the resulting mixture for 20 minutes to convert the precipitated copper hydroxide to copper oxide. Then cool, Jvash the precipitate several times by decantation v-ith water, and filter to the consistency of a heavy paste. Press pellets 1 mm. in diameter and 6 mm. long from the paste, dry in a stream of air a t 100" C., and remove the last traces of water by heating to 400" C. Reduce the dried pellets completely n i t h hydrogen and reoxidize with air a t 400 ' C. The authors found it more convenient t o prepare a hard filter cake of the precipitated oxides, crush it into small pieces, and dry it a t 400" C. The dry particles vere then reduced ryith hydrogen and reosidized in a stream of air for 3 to 4 hours a t 400 O C. During the reduction and subsequent reoxidation considerable shrinkage of the oxide particles took place. Following reduction and reoxidation the material was screened to obtain granules of the desired size and then packed into the combustion tubes. The combustion tubes used consisted of U-tubes about 120 mm. in length and 3.5 t o 4.0 mm. in inside diameter, and were fitted with spherical joints t o permit convenient attachment to the all-glass manifold. These combustion tubes held from 4 t o 7 grams of the precipitated copper oxide, depending to some extent on the mesh size of the material used. Since ordinary Pyrex softens excessively at 700" C., it is necessary to use combustion tubes of fused silica or of special heat-resistant glass, such as Corning S o . 172, a t that temperature. Both types of combustion tubes nere used and found satisfactory, but the heat-resistant glass is more easily worked and is less expensive. The copper oxide packing v a s retained in these tubes by plugs of loosely rolled sheet asbestos. EXPERIMENTAL
Combustion of Hydrogen. Preliminary tests confirmed the findings of Briickner and Schick that hydrogen can be burned rapidly and quantitatively by passage over the precipitated copper oxide a t 220" C. This high activity, however, proved to be of no special advantage for the selective combustion of hydrogen, for a t 220" C. hydrocarbons are oxidized by the precipitated copper oxide a t approximately the same rate as with commercial copper oside n-ire a t 270" C., the temperature used in this laboratory for the selective combustion of hydrogen. Combustion of Hydrocarbons. Samples of natural gas were passed over the precipitated copper oxide a t 600" and 700" C. Slightly loxer hydrocarbon values were found a t the lower temperature and the combustion rate was somewhat slower. It n a s disclosed by mass spectrometric analysis of the residues that small amounts of unburned methane remained from the combustions a t 600" C. No methane iyas found in the residues from combustions carried out a t 700' C. I n order to establish clearly the difference between the precipitated copper oxide and copper oxide wire, x i t h regard to activity in effecting combustion of hydrocarbons, a comparative test was made. The copper oxide wire used for this test was given the same preliminary treatment as was the precipitated oxideLe., it was reduced with hydrogen and reoxidized with air a t 400" C. before being packed into the combustion tube. Because
the true composition of the natural gas used above was not known, a sample of methane of high purity was prepared by careful fractionation of the natural gas. Methane prepared in this manner was found t o have a purity of 99.8+ % by mass spectrometric analysis. Samples of the methane were then passed over the two combustion agents and the rates a t which it burned were noted. As can be seen from Figure 1, a t least 18 complete cycle passes were required for complete combustion by the copper oxide wire while only 3 to 6 passes over the precipitated oxide were necessary.
-1 P Precipitated Copper Oxide Commercial
'I
Copper Oxide
I
I
6
Figure 1.
I
I
12 18 Number of Passes
2
Combustion of Methane over Copper Oxide at 700" C.
The pure methane was then analyzed by combustion over precipitated copper oxide a t 700' C. and by two other combustion methods: (1) by passage over an incandescent platinum filament in a slow combustion pipet, and (2) by passage over platinized silica gel catalyst a t 500" C. The results, shown in Table I, indicate that combustion over the precipitated copper oxide gives a t least as good accuracy and precision as are obtained by the other methods tested. The time required for an analysis was about the same for each method. T o determine the applicability of this method to the analysis of hydrocarbons other than methane, samples of purified propane, isobutane, and 7 ~ butane were also analyzed by combustion over the precipitated copper oxide. S o difficulty was encountered in obtaining rapid and complete combustion of these hydrocarbons and the results, s h o m in Table 11,indicate the method is reliable. I n the analysis of pure hydrocarbons by combustion it is necessary to add an accurately measured volume of a carrier gas t o permit the determination of the amount of carbon dioxide formed and the residual gas volume. Nitrogen v a s used for this purpose in the analyses summarized by Tables I and 11. Oxidation of Copper Oxide. Briickner and Schick reported that the precipitated copper oxide they prepared could be oxidized more completely than could the commercially available n-ire form. They found the combined oxygen content of the precipitated oxide'to be between 95 and 100% of the theoretical value when oxidized at 4.50 O to 550' C., n-hile under the optimum conditions only about 4Oc/c of the theoretical value could be obtained for the copper oxide nire. The authors have studied the effect of several variables on the rate a t which the reduced precipitated copper oxide is oxidized in an effort to arrive a t a rapid and convenient method for regenerating the oxide A sample of precipitated copper oxide was reduced and reoxidized a t 400" C. and screened t o retain three fractions, having
V O L U M E 20, NO. 1, J A N U A R Y 1 9 4 8 Table I. Combustion Method
67
Determination of RIethane bv \-arious Combustion Methods
Sa..AF.Volume
MZ.
_.-
sary to avoid too rapid oxidation, which may result in
Tntnl _ 1
Formed 31I .
Used Ml.
Contraction .Ill.
..
particle diameters of 0.5 t o 1.0 Platinized mm. appears to be superior silica gel 40.2 40.2 80.2 80.9 100.0 99.3 101.0 100.5 to the coaryer fractions tested, 40.2 40.0 80.5 80.7 99.5 99.3 40.3 40.1 80.9 80.8 99.5 98.7 100.3 inasmuch as it can be packed 81.0 80.7 100.5 99.8 101.1 39.9 40.1 40.7 40.5 80.6 80.3 99.5 99.6 98.4 more solidly into the com40.1 40.1 80.8 80.3 100,~99.6 99.8 99.1.3 99.4 100.2 bustion tubes,.~presents alarger Av. ... ... ... surface area, and reduces the P p t d . copper oxide 4 44 0 .. .. .. .. .. io1 100.0 . .. . . iikelihood of channeling of t'he 40 0 .. 3 9 0 .. 86 9 9 .n .8 100.0 40.6 40.4 , . 99.5 99.3 ... gas stream. I n the above 40.1 39.9 ... 99.5 100.0 ... 39.9 39.7 ... . . 99.5 100.0 . . tests it !vas noted that the 4... 0.3 4.0.. .0 . . 99.3 1'00.0 more finely divided oxide Av. .. . .. 99.8 99.9 ,. . .. generally n-as oxidized more Slow combustion pipet 40.7 40.8 82.4 82.1 100.2 99.8 100.5 rapidly than the coarser ma40.5 40.4 81.4 81.4 99.8 99.8 100.5 terial. -111 these factors tend 40.3 40.3 81.2 81.1 100.0 100.5 100.5 AV. ..: ... ... 100.0 100.0 100.R t o promote increased activity, % methane = c X 100.0 (for methane C qhould equal S). both in the combustion of 0 hydrocarbon gases and in t h e b % methane = ' 3 x 100.0. regenerative oxidation of the '?& methane = L'r(3TCS - O ) x 100.0combustion agent. Appreciably smaller particles impede where: C = volume of carbon dioxide formed on combustion S = initial sample volume the flow of gas through the tube S = volume of unburned residue T C = volume contraction on combustion and may be swept from the 0 = TTolume of oxygen conwmed combustion tube past the retaining plugs of sheet asTable 11. Determination of Gaseous Paraffins by Combestos. bustion over Precipitated Copper Oxide at 700" C. During the combustion of n-butane, it vias observed after Material Volume of Unburned Volume of Paraffins several portions had been burned vithout reoxidation of the ilnalyzed Sample Residue CO? Formed Founda copper oxide that the combustion rate gradually bemnie low and when the copper oxide became highly reduced, olefinic 40.3 Methane 40.9 hydrocarbons m r e found in the combustion residues. Appar40.6 ently this vias due t o cracking of the butane in the absence of 40.1 34 9 sufficient available oxygen for complete combustion. Since the 40.3 oxygen available in the copper oxide is fairly rapidly depleted, Propane 25.9 0.0 78.2 100.0 28.9 0.0 86.7 100.0 care must be exercised t o maintain the copper oxide in a mil28.5 0.05 S3.5 99.8 oxidized state; otherwise slow and incomplete combustions are n-Butane 33.3 0.05 95.3 99.8 0.0 88.0 100.0 21.3 likely to result. Reoxidation of the precipitated oxide after 0.0 78.6 100.0 19.1 combustion of about 50 ml. of butane or proportionately greater Isobutane 21.0 0.0 84.5 100.0 19.8 -0.03 80.4 100.2 volumes of the lo1Ter hydrocarbons Ivas found satisfactory.
...
24.2
a See footnoteb of Table
0.0
97.9
100.0
I for method of calculation.
SUMMARY AND COKCLUSIOKS
respective particle diameters of 0.5 to 1.0, 1.0 to 2.0, and 2.0 to 3.0 mm. Portions of each of these fractions were packed into combustion tubes and alternately reduced completely v i t h hydrogen and then reoxidized with air and/or oxygen a t various temperatures and for various lengths of time. Oxygen contents between 19 and 737, of theoretical were observed gravimetricallv. Oxidation temperatures above 700' C. were not investigated, since it was thought desirable to avoid the expense and inconvenience of combustion tubes made from fused silica. Oxidation periods in excess of 3 hours were not investigated, because such prolonged periods of oxidation would be impractical for routine use in most laboratories. The most favorable temperature range for reoxidation of the reduced oxide appeared t o be 400' to 500" C. However, by drawing air through the combustion tube while the furnace was being heated to operating temperature (700" C.), which required only 15 minutes, a n oxygen content of about 5 5 7 , of the theoretical was attained. Probably the shifting of particles which occurs while the oxide is being heated results in greater surface exposure to the oxidizing gas. At temperatures above 600" C., oxidation proceeds a t a diminished rate. Oxidation with oxygen proceeds somewhat more rapidly than with air but closer control of the flon rate is neces-
Precipitated copper oxide containing 176 iron oxide is a satisfactory combustion agent for the determination of gaseous paraffins and off'ers several important advantages over other existing methods. Its use not only eliniinates the need for supply of oxygen of accurately knon-n purity but also avoids errors arising from incorrect measurement of oxygen rolume or contaniination of the oxygen supply, and eliminates any explosion hazard from formation of an explosive mixture of hydrocarbon gases and oxygen. Results obtained by its use are equal or superior in accuracy and precision to those obtained by other combustion methods tested. LITERATURE CITED
(1) Brtickner, H., and Schick, R., Gas- u . Tasserjach, 82, 189 (1939). ( 2 ) Bunte, K., and Wunsch, W., Ibid., 66,481 (1923). (3) Campbell, J. R., and Gray, T., J . SOC.Chem. Ind., 49, 447, 450 (1930).
(4) Kobe. K . A , and McDonald, R. A , , IND. ESG.CHEV.,ANAL.ED., 13,467 (1941). ( 5 ) Lunge, G., "Technfcal Gas Analysis," rev., p . 147, New Tork, D. Van Yostrand Co., 1934. (6) Ott, E., J.Gasbeleucht., 62,89 (1919). (7) Winkler, C., 2. anal. Chem., 28, 289 (1889).
RECEIVED March 17,1947
