Direct Determination of Oxygen in Petroleum Products Adaptation of Schuetae- Unteraaucher Method I{. A. DINERSTEIN AND R. W. KLlPP Stundurd Oil Company (Indiana), Whiting, Znd. Adaptation of the Schuetze-Unterzaucher method to the direct determination of oxygen in petroleum products is described. Modifications of the apparatus include enlargement of the combustion tube to permit the use of larger samples, elimination of the contentional side arm on the combustion tube, and use of an inexpensive commercially ai ailable furnace in place of special equipment previously described in the literature. Results are given for known and unknown compounds containing from 0.15 to 2670 oxjgen.
0
XYGES in organic compounds has usually been deter-
I n 1'330 dchuetze ( 5 )proposed a new seiiiiniicroiiiethod involving passage of the decomposition vapors of a pyrolyzed sample over carbon a t 1000" C. and oxidation of the resultant carbon monoxide to the dioxide with iodine pentoxide. Either the carbon dioxide or the iodine could be determined. I n 1940 Unterzaucher (6) adapted the method to the microchemical scale b y making various improvenients in the apparatus. .1recent publication by -2luise et al. ( 1 ) describes an adaptation of the Unterzaucher niethod t o Anierican equipment and reagents. There are also references to a similar method in the recent Russian literature (3,4 ) as well as in work by the Sational Bureau of Standards on the analysis of rubber polymers ( 7 ) . The present paper describes the aditptation of the SchuetzeUnterzaucher method to the direct determination of oxygen in petrolcum products. The apparatus has been modified to permit the use of larger samples and thus minimize titration errors in cases n-here the oxygen content is i o n . The conventional side arm of the combustion tube has been eliminated by the use of a special adapter embly which permits more efficient forward and backward nitrogen sweeping. I n addition, an inexpensive comniercially availahle furnace has been satisfactorily substituted for the special furnace referred to in previous publications.
mined by difference in conjunction with carbon and hydrogen deterniinat,ions. This method has attributed to the oxygen value the sum of all the errors of carbon and hydrogen measurement, and, when constituents other than carhon, hydrogen, and oxygen are present, has necessit,ated additional analyses for such components. The methods for direct determination of oxygen in use before 1939 were based on either complete oxidation of the compound with measurement of the oxygen consumed, or catalytic hydrogenation to form water. The coniplete oxitlation method requires very accurate determination of hydrogen, carbon, and consumed oxygen, and if sulfur or nitrogen is prrsent in the compouricl under inwstipation, an additional determination niust bc made. The catalytic hydrogenation method determines oxygen directly, but other elements interfere and sulfur or halogrn compounds poison the cat alyst ( 2 ) .
FIGURE,
OXYGEN DETERMINATION APWRATUS
APPARATUS
A diagram of the apparatus is shown in Figure 1. IDENTIFICATION KEY I h l T R O G E N CYLINDER
2 REOUCINC VALVE 3 STOPCOCK
4 GASOMETER 5 STOPCOCKlnYDROGENENTRYi 6 COPPER DEOXYGENATING FURN4CE 7 STOPCOCK
,
1
~
B DR'IUG TUBE 9 BUBBLE C W N T E R IO U-TUBE I1 STOPCOCK 12 REVERSE FLUSHING TUBE I3 STOPCOCK I4 4DAPTEP Ls OUARTZ iOMBUSTION TUBE
16 I7 18 !9 20
CCMBLSTOW 'dRN4CE THERMOCOUPLE TRENCH STOPCOCK ASCbRITE TUBE
2'23;
TUBE
HARlOTTE BOTTLE 2 4 GSADllbTE
REVERSE FLOW T U B E
ASEESTO7 QUARTZ
18/7 ADAPTER
-r
-1e17-- STOPCOCK
TRENCH-
r19.5-3+
10.5tIl.O--C
1 5 . 0 4 11.04ZO
7 7 CM.
I
Figure 2.
Combustion Tube Assembly
545
The nitrogen tank, I, is equipped with a reducing valve, 2, connected to a 3-way stopcock, 3, by Tygon tubing. One arm of this stopcock leads t o the gasometer, 4, in which nitrogen is stored for use in maintaining an inert atmosphere in the apparatus when it is idle for prolonged periods. The other arm of the stopcock leads through a 3-15-a~stopcock, 5, to nitrogen-purification tube, 6, which is a vertical 22-cm. length of heavy-walled 18-mni. Pyrex tubing filled PTith 40- t o 60-mesh copper grits and wound with 300 cm. (10 feet) of Nichrome ribbon (1.206 ohms per foot) under asbestos tape. The extra arms of stopcocks 5 and 7 permit int'roduction of hydrogen for reduction of the copper in place. The effluent end of the nitrogen-purification tube is joined through a 2-mni. stopcock, 7 , and an 18/7 ball-and-socket joint t o drying tube 8, which is filled with porous barium oxide. This tube is connected by meaiis of a second 18/7 balland-socket joint to bubble counter 9 and U-tube 10. The bubble counter is filled with concentrated sulfuric acid, the inlet arm of the U-tube
546
ANALYTICAL CHEMISTRY
is filled with porous bariuiu oxide, and the outlet arm with phosphorus pentoxide and glass wool. A 2-mm. three-way stopcock, 11, permits the purified and dried nitrogen to pass to the combustion tube, 15, either through the reverse-flushing tube, 12, or through stopcock 13 and adapter 14. T.he combustion tube, detailed in Figure 2, is made of clear quartz (13 mm. in inside diameter) with Pyrex-to-quartz graded seals a t either end leading to ball-and-socket joints. A trench, 18, a t the bottom of the tube holds the sample and is designed to prevent liquid or melted samples from flowing to the unheated adapter end of the tube. The tube is filled with 15 cm. of carbon held in place on either side by quartz chips and asbestos fibers. The combustion tube is placed in an electric furnace, 16, Hoskins Type 303-4, regulated by a rheostat. (This furnace has been found adequate for maintenance of 1120" C. in the central IO- to 18-em. portion which is sufficient for the lj-cni. carbon filling. The present furnace has been in operation for 11 months with no significant change in operating characteristics.) A chromel-alumel thermocouple, 17, is used with a portable potentiometer for temperature measurements. The furnace entrance and exit are covered with Transite drilled to admit the combustion tube and thermocouple. The tube has a 10-cm. Xichrome-gauze roll covering t,he portion preceding the furnace where the sample is heated by a blast Meker burner. The forward portion of the system is shielded from the heat by the asbestos sheeting indicated to the left of the burner. Three-way stopcock 19 and drying tube 20-filled with Ascarite to remove nitrogen] sulfur, or halogen compounds-t ransfer the exit gases from the combustion tube into the oxidation tube, 21. This tube (8 mm. in inside diameter) is filled with granular iodine pentoxide and is surrounded by a jacket containing boiling glacial acetic acid to niaint,ain a temperature of 118' C. The jacket is heated by Xichrome ribbon (1.206 ohms per foot) and is equipped with a water-cooled reflux condenser. Connected to the oxidation tube by a 19/38 standard-taper joint within the acetic acid jacket is the indented Vigreux absorption tube, 22, the walls of which are moistened with 20% sodium hydroxide for collection of iodine vapors. The absorption tube is joined to a Mariotte bottle, 23, which is used to measure the volume of gas passing through the system.
This assembly includes several changes from the Uriterzaucher apparatus. The chief improvement lies in the substitution of the straight combustion tube for Unterzaucher's quartz tube with side arm, and the elimination thereby of a pocket not completely swept out by the passage of nitrogen into the combustion tube. hIore efficient nitrogen flushing and less danger of sample loss behind the flame result. I n addition, leakage has been minimized by eliminating all rubber connections. Except for the Tygon tubing joining the nitrogen cylinder t,o the apparatus, all connections are of glass-either ball-and-socket joints, where flexibility is required, or standardtaper joints. Silicone lubricants are employed for the glass joints. REAGEKTS
Nitrogen, Linde, high purity, dry (oxygen content 0.001 %). Barium oxide, porous grade, Barium & Chemicals Company, Inc. Carbon, Wyex Compact Black, pelleted amorphous, ,J. h1. Huber, Inc. Iodine pentoxide, Ba.ker's C.P. Formic acid, 98 to loo%, Eastman Kodak Company. Phosphorus pentoxide, Baker's C.P. Ascahte. Sodium hydroxide] 20% solution of Baker's C.P. in distilled water. Starch, 0.5% solution in water. Bromine , hlerck's c .P. Sodium thiosulfate, 0.02 N solution of Baker's C.P. Potassium iodide, 6y0 solution of Baker's C.P. Potassium acetate, 10% solution of Baker's C.P. in glacial acetic acid. Sodium acetate, 20% solution of Baker's C.P. in distilled water. PROCEDURE
The sample (weighed on the anal tical balance) will vary in size from 200 to 400 mg. for materia$ containing less than 0.5% oxygen to 15 t o 25 mg. for samples with about 2570 oxygen. For crystalline materials, a platinum or porcelain boat has been found a satisfactory sample vessel. For liquid samples, short lengths of quartz tubing 4 t o 5 mm. in inside diameter are sealed a t one end and drawn to a constriction a t the other end with the opening
fiared. The sample is introduced through the constriction with a hypodermic needle attached to a 2-ml. syringe. If the sample is volatile the flared end of the tube is filled with paraffin wax immediately after weighing to prevent loss of the sample. When this sample tube is inserted in the combustion tube, the wax-filled end is placed facing the furnace. Newly assembled apparatus is swept out with nitrogen for 24 hours with the various heating units (6, 16,. 21) operating. To ensure maximum activity of the deoxygenation tube, the copper is reduced a t 400" C. with hydrogen a t the start of each day's work. The entire apparatus is swept out with nitrogen for about an hour ryhile the furnace is allowed to come to 1120" C. The nitrogen rate js set a t 10 to 15 nil. per minute. Stopcock 11 is set to pass nitrogen through tube 12 and theii back into the combustion tube in reverse flow via stopcock 19, stopcock 13 being set to permit the gas to leave the system. The adapter, 14, is opened, and the sample is introduced against the nitrogen stream and placed in the trench, 18, by means of a glass rod with a platinum-nire hook. The adapter, 14, is replaced and the combustion tube and adapter are swept out in reverse flow for an additional 30 minutes. The u;alls of the absorption tube are thoroughly moistened with 20% sodium hydroxide, and the tube is placed in position. At the end of the sneeping-out period the nitrogen flow is changed to the forward direction, through the combustion tube to the Mariotte bottle, by closing stopcock 13 to the air and shutting off side arm 12 by stopcocks 11 and 19. The RIariotte bottle side arm is lowered to a horizontal position above a 1000-ml. graduated cylinder. Pyrolysis of the sample is then begun. I n the case of a liquid sample, the wax a t the end of the sample tube is first melted by the application of a small flame. The Meker burner, adjusted to provide a low flame, is then placed about 3 cm. to the left (Figure 1) of the sample, and the initial evaporation of the sample is begun slowly, so that the back pressure does not exceed the forward nitrogen pressure. After the sample starts to volatilize, the burner is moved in increments of about 1 em. toward the furnace at such a rate a' to reach the furnace in about 20 minutes. The burner is then returned t o a position about 10 cm. to the left of the sample and the flame adjusted to bring the combustion tube to white heat, The burner is again moved toward the furnace over a period of about 10 minutes to sweep any remaining portions of the sample into the furnace. Following the second sweeping, nitrogen is papsed through the system for 10 minutes more. The absorption tube, 22, is removed and its contents are rinsed viith about 125 nil. of distilled water into a 250-ml. Erlenmeyer flask containing 10 drops of bromine (to osidize all iodine to iodate) and 10 ml. of a 10% solution of potassium acetate in glacial acetic acid. The contents of the flask are stirred, 10 ml. of a 20% solution of sodium acetate are added, and formic acid iq added dropwise until the excess bromine is destroyed. After 4 or 5 minutes, 5 ml. of 6% potassium iodide solution and 5 ml. of 10% sulfuric acid are introduced, and the liberated iodine i? titrated with 0.02 X thiosulfate using a starch indicator. A blank run is made by introducing an empty sample vessel into the combustion tube and carrying out the complete procedure. The blank usually requires about 0.5 to 1.0 ml. of 0.02 S thiosulfate except that, when paraffin wax is used to seal the quartz tubes containing volatile samples, about 1 ml. additional is required. The per cent oxygen is calculated as follows:
(Y
- b) X
normality of Ka?SaOjX 6.667 X 100 = %oxygen milligrams of sample
where Y = ml. of sodium thiosulfate required for sample, b = ml. of sodium thiosulfate required for blank, and 6.667 = milligrams of oxygen equivalent to 1 mi. of 1 S sodium thiosulfate. The total time required for a determination is about 70 minutes. I n practice, either five analyses and one blank or four analyses and two blanks may be run per day, depending on the stability of the blank value for the particular apparatus and the accuracy desired. Carbon deposition on the combustion tube occurs in the vicinity of the sample. It has been found convenient to remove this carbon, which obscures the view of the sample, by introducing a fine stream of air a t low pressure into the tube against a reverse flow of nitrogen while a hot flame is applied to the tube. Care must be taken to avoid the entrance of air into the filled portion of the tube; hence, the reverse nitrogen flow 1s continued after the stream of air is withdrawn.
547
V O L U M E 21, NO. 5, M A Y 1 9 4 9 Tahle I.
Anal>ses of Pure Compounds Benziiir Arid
Oxygen calculated, % Sample size, mg. 0.02 .\-thiosulfate used, ml. Oxygen found, %
26.20
Average, % Average deviation from known, %
Tahle 11.
Benzoic Acid in Xylene
1.20 290-395 34-36 1.17. 1.23 1.19 1.20 0.03
18-28 35-55 26.10,26.52,26.34 26.00,26.32,26.07 26,23 0.17
Analyses of Sulfur and Nitrogen (:ompounds
Oxygen calculated, $7, Sample size mg. 0.02 A' t h i o h f a t e uspd, ml. Oxgpen found, % Avrrnge, "/c Average deviation from knoivn,
:4
Thio-tertbutoxyy propionic Acid Methyl Ester
n-Heptadecanoamide
18.16 15-30 20-40 17.65 17.96 18.11 17.91 0.24
5.94 45-70 20-30 5.91 5.90 6.01 5.94 0.05
suliur tu demonstrate that these elements do not interfere with the method. Table I11 shows results of typical analyses of hydrocarbon samples from Fischer-Tropsch synthesis. The accuracy of the method with the known samples is about 1% of the oxygen content. The precision in the determination of unknown samples containing over 1% oxygen averaged about 2% of the amount present; samples with lower oxygen content show poorer prerision. The precision of the method is dependent on the reproducibility of the blank titer, which varies from day to day by about 0.3 ml. of 0.02 S thiosulfate. This variation is equivalent to 0.4% oxygen for a 10-mg. sample or 0.027$ oxygen for a 200-mg. sample.. By mass spectrometer analysis, the nitrogen has been found to c-ontain about 0.03% oxygen by the time it reaches the copper deoxygenating tube and, because the copper does not remove all this oxygen, a positive blank value is obtained. Air adsorbed on the sample vessel or the walls of the combustion tube must be removed; the efficient sweeping technique described above minimizeq errors from this source. CONCLUSIONS
'I'ahle 111.
Typical Analyses of Fischer-Tropsch Product
Sample size, iny. 0.02 5 thiosulfate used, ml. Oxygen found, % Average,
A
B
c
D
230-310 45-76 1 75 2.68 2 75 2.73
110-13.5 15-18 1.78 1.81 1.77 1.79
218-260
210-235 2.5-2.6 0.14 0.16
11-13 0.69 0.62 0.73 0.68
o:i5
The Schuetze-Unterzaucher method for the direct determination of oxygen has been adapted to the semimicroanalysis of petroleum products. The modifications include a redesigned combustion tube which permits more efficient sweeping of gases to reduce the blank value and the use of an inexpensive, commerrially available furnace. ACKNOWLEDGMENT
arbon also is deposited on the quartz chips preceding the carbon filling of the combustion tube. This deposition cannot be removed by heating in the presence of air because of the proximity of the carbon filling. It has been found advisable to refill the combustion tube after about 80 analyses of saniples containing 1 to 10% oxygen. If allowed to accumulate further, the carbon deposition ail1 interfere with the passage of gas through the tube and become increasingly difficult to remove. It is good practice to check the fillings of the cornbustion and oxidation tubes to avoid losses due to channeling and also the drying agents to ensure complete dehydration of nitrogen. (
RESULTS AND DISCUSSION
The analyses of benzoic acid and of a dilute xylene solution of benzoic acid are shown in Table I. Table I1 gives the results of oxygen determination of pure compounds containing nitrogen or
The authors wish to express their appreciation to Miss L. 11. Kotarski for assistance in carrying out a portion of the analytical work. LITERATURE CITED
hluise, 1'. A., Hall, R. T., Staats, F. C., and Becker, W. W., AXAL.CHEW,19,347 (1947).
Elving, P. J., and Ligett, W.B., Chem. Rev., 34, 129 (1944). Korshun, M. O., Zacodskaya Lab., 10, 241 (1941). Korshun, M. O., and Gellnian, N. E., Ibid., 12, 500 (1946). Schuetee, M., Natur~issenschafte7z,27, 822 (1939). Gnterzaucher, J., B e r . , 73B, 391 (1940). T a l t o n , W. W.,McCulloch, F. W., and Smith, W. H., J. Research S a t l . Bur. Standards, 40, 443 (194s). IthCLIvED August 7, 1948. Presented before Petroleurn Division a t One-Day Technical Conference, Chicago Section, A M E R I C A N CHEMICAL SOCIETY, Dwprnher 26, 1947.
Analysis of Gaseous Hydrocarbons Combining Infrared and Mass Spectra DANIEL MILSOM, W. R . JACOBY, AND A. R. RESCORLA Cities Service Refining Corporation, Lake Charles, La.
ITH the advent of cracking petroleum stocks, the complexity of samples received for light hydrocarbon analysis has necessitated a search for faster methods of evaluation without dacrificirig the accuracy of the results. The chemical similarity and the difficulty of separating compounds of isomeric composition by distillation or chemical methods have led to the use of other physical properties for their detection and estimation. Two instruments, the mass spectrometer and the infrared spectrometer, have been accepted for analyzing hydrocarbon samples. These instruments are faster and a t least as accurate as low temperature fractionation and other accepted methods
of gas analysis. Each instrument, however, has definite limitations. The mass spectrometer is accurate for compounds that have no isomers, but as the number of isomers increases the degree of accuracy rapidly decreases, owing t o the similarity of ionization patterns for isomeric compounds. The infrared spectrometer is faster and more accurate for isomeric compounds than the mass spectrometer. However, its use is limited to the number of spectral positions a t which the absorptions of different compounds do not interfere sufficiently to affect the accuracy. I n analyzing samples of complex mixtures of hydrocarbons, it is customary to determine the composition of distillation frac-
