January, 194.5
ANALYTICAL EDITION
real. Although no evidepce of the presence of insoluble manganese dioxide was found in caustic soda, it appeared desirable to add a small amount of sodium sulfite to prevent the oxidation of the oxine which, as pointed out above, is excessive urder some conditions and doubtleas occurs to some extent under any condition. I n addition, the phenolphthalein is oxidized rapidly if the sulfite is omitted. The accuracy of the method was determined by comparing the recovery of manganese added to caustic soda with a calibration curve which waa prepared without going through the extraction procedure. The maximum error observed (Table VII) is within the precision of the method as shown in Table VIII. It is concluded from this that the extraction procedure is free from constant errors and that the method is accurate. The limit of uncertainty of the extraction method under the
31
best conditions was found to be *0.03 part per million (Table VIII). The ratio of the limit of uncertainty (LUJ of *0.03 part per million by the extraction procedure to *0.18 part per million by the direct procedure is approximately what would be expected from the fivefold increase in sample size. This indicates that the added manipulations of the extraction do not significantly detract from the precision. LITERATURE CITED
Clark, N. A., IND. ENQ.CHEM.,ANAL.ED., 5, 241-3 (1933). (2) Moeller, T..Ibid.. 15,270-2, 346-9 (1943). (3) Moran, R. F.,Ibid., 361-4 (1943). (4) Willard, H.H.,and Diehl, H., “Advanced Quantitative Analysis”, p. 81, Ann Arbor, Mich.. Bloomfield and Bloomfield, 1939. (6) Willard, H. H.. and Greathouse, L. H., J . Am. CAem. soc., 39, (1)
2366-77 (1917).
Determining Oxygen in Hydrocarbon Gases KARL UHRIG, F. M. ROBERTS,
AND
HARRY LEVIN, The Texas Company, Beacon, N. Y.
A
method is described for determining oxygen in concentrations of O x y g e n is reacted with copper wetted with ammonia-ammonium chloride solution, the resulting mixed oxides are dissolved in the same solution and reduced to cuprous form, and copper is determined iodometrically as a measure of oxygen in the
0.001 to 5%.
A
STUDY of refinery operations for factors affecting catalyst life made it important to have a method for determining small quantities of oxygen in hydrocarbon gases. The usual methods of Orsat gas analysis employing alkaline pyrogallol, phosphorus, chromous chloride, etc., are obviously unsuitable for the small concentrations of oxygen with which this paper is concerned. Simmons and Kipp (Id) used sodium triphenylmethyl for determining traces of water and oxygen, presuming therefore the a b m c e of one when determining the other. An interesting test is described by Kautsky and Hirsch (4) who determined the time required for destruction of the phosphorescence of trypaflavine as a measure of very low concentrations of oxygen. A nephelometric method based on fume intemity in the reaction between oxygen and phosphorus was mentioned by Wagner (14). A colorimetric method, based on the red color produced by oxygen in a solution of pyrocatechol and ferrous sulfate, was described by Binder and Weinland ( 2 ) . Ambler (1) described a method based on the color imparted by oxygen to alkaline pyrogallol solution. Hofer and Wartenberg (3) based their method on the ready oxidation of sodium hydrosulfite and measured the consumption of the sulfite. Mugden and Sixt (7) determined small amounts of oxygen in gases by comparing the blue color produced in ammoniacal cuprous salt solutions with known cupric salt solutions. The use of manganous hydroxide for determining small amounts of oxygen in gases was suggested by Phillips (8) and h t e r elaborated by Schmid (9). This is a modification of Winkler’s (15) method which was originally proposed for determining oxygen dissolved in water. A method employing the same general principle, but ferrous instead of manganous salts, was described by Shaw (If), who completed the determirdon colorimetrically. A ,method claimed to be fast and practically independent of the composition of the test gas was c!escribed by MacHattie and Maconachie (6) who deposited the oxygen on reduced copper kept moist with ammonia-ammonium chloride solution, dissolved the resulting cop er oxide in the same reagent, and estimated ir, by titrating a glank with standard copper solution to the same depth of blue color. The last method mentioned appeared most promising of all covidered and was investigated extensively. When it was applied to knowns, following in detail the directions of its authors, low results were obtained. This was thought to be due to di5culties experienced in recognizing the end point of the colori-
sample. The method gives accurate results in rbturated r n d unsaturated hydrocarbon gases. Sulfur dioxide, hydrogen sulfide, and mercaptans must b e removed and means for doing so are provided, The method is based on well-known reactions but many modifications have been made in technique and equipment.
1.
Copper Determinations ns Measure of O x y g e n in Air (Air taken to be 20.9% oxygen by volume) Copper Calculated Oxygen Found5 Air Taken (Diluted to 100 from Air Taken Assuming Formation M1. with Pure Copper Assuming Formation of: of: CurO CuO CurO CUO Nitrogen) F&nd Me. MU. M1. 31g. R 70 10.8 5.4 19.0 38.0 4.53“ 9.8 21.5 10.75 17.0 34.0 9.06 17.5 11.32 24.1 26.9 13.45 18.7 37.4 17.8 35.6 13.61 27.5 32.3 16.15 22.75 46.1 54.0 17.8 35.6 27.0 Table
5
At 0’ C 760 mm.
b Calculaihd from copper determined
microelectrolyeis.
metric copper titration. This method of determining copper was therefore replaced by the more precise microelectrolytic procedure. Those authors (6) believed cuprous oxide was formed but determined from many experiments that 10.45 mg. of copper, instead cf theoretical 11.36,were equivalent to 1.0 ml. of oxygen (0’ C., 760 mm.) and adopted the former empirical value. The present authors have found this to be due to the fact that cuprous and cupric oxides are simultaneously formed. The data given in Table I indicate that mixed oxides are formed and that the ratio may not be constant, the prssent data yielding a copper value of 9.24to 10.33 mg. per cc. of oxygen. i f oxygen be calculated from the copper found in the cell washings, is necessary that it be present either as cupric or cuprous oxide, but not a Variable mixture. &me transformation of cuprous into cupric oxide involves adding oxygen, the cupric oxide must he reduced to cuprous and this is easily accomplished by shaking its ammdniacal solution with copper in the specially designed reaction ce!i described below. io this manner satisfactory results were obtained (Table 11). The rather involved microelectrolytic copper determination was then replaced by simple iodometric procedure without loss in accuracy or precision. To analyze a sample of very low oxygen content such a large portion (up t o 5 liters) must be taken that its passage through the
INDUSTRIAL AND ENGINEERING CHEMISTRY
32
coming nitrogen (rate approximately 10 liters per hour) is controlled by stopcock 2. The nitrogen and liquid meet a t 3. A mercuryfilled pressure regulator, 1, maintains the impure nitrogen supply a t approximately 100 + mm. Nitrogen and solution are proportioned by stopcocks 2 and 7, so a steady stream of !as-liquid mixture rises in the tube above 4. t is essential that the height of the liquid column in 6 and the tube leading immediately downward be large enough to produce a t 3 a pressure greater than that exerted by the nitrogen-liouid mixture in the riser tube above 4; dymensions shown in Figure 1 accomplish this. The slight downward slope of the tube between 3 and 4 is necessary for smooth operation of the lift-pump. The internal diameter of the riser tube above 4 should not exceed 6 mm.; otherwise an unnecessarily rapid current of nitrogen is required. The nitrogen-liquid mixture discharges above the copper ribbon in 5 and dissolves the copper oxide film as fast as it is formed, maintaining the copper in an active condition and completely freeing the nitrogen of the oxygen (13). Stopcock 8 is used to remove spent solution; fresh solution is introduced at the top of 5. The unit should be glass, except for the rubber stopper on 5 and the rubber connection near 2 , and must be absolutely tight. The solution in 6, which collects the copper oxide formed in 5, will be blue when a new charge is introduced but will become colorless soon after the lift pump is started and remains so for a long time if the system be tight. Occasionally, if the pump has not been in operation for days, a blue color may appear at.the top of the solution but this has no adverse effect on the punty of the effluent nitrogen. One charge of solution will supply oxygen-free nitroen for a t least a month even if it be used 8 hours every day. bperation of the pump is simple and, once started, does not require attention for the rest of the day. REACTION CELL. Only the cylindrical part of the reaction cell (Figure 2) is filled with copper ribbon, which must be cleaned before being placed in the cell. Copper which has been cleaned with nitric acid was found to be inactive after a few runs. Reduction with methyl alcohol yielded satisfactory copper but for present purposes the procedure is inconvenient. A simple satisfactory procedure consists of momentarily heating the copper in a gas flame to remove grease, etc., introducing the ribbon in the cell, and removing the resulting copper oxide by ammoniacal ammonium chloride solution contained in reservoir 12 (Figure 3) with which the cell comniunicates through stopcock 13. Operation of the cell is described below. Reservoir 12 has a capacity of 1 liter. Its constricted bottom connects with three-way stopcock 11, inserted in the exit line of the lift pump which carries oxygen-free nitrogen to the rest of the assembly. This stopcock is always kept open to all three legs, so that pure nitrogen is available for flushing the reaction cell via stopcocks 18 and 19 or if the nitrogen’is not used it I MM CAPILLARY CLASS TUBING TO STOFUZK will bubble through the liquid in 12 and keep it oxygen-free; the exhaust is strongly ammoniacal and should be disDosed of Droperly. Reservoir i2 should be kept full, since the height of -COPPER its liquid column RIBBON determines the D r e s a u r e of t h e purified nitrogen. Stopcocks 9 and 18 By arenotessential but are verv useful in washing out salt deTORESERVOIR posita which may 12 FIGURE 3 5MM. 1.0. form with sham A temperaturi changes. Stopcock 10 permits introduction of pure nitrogen into the receivers for diluting samples of high oxyFigure 2. Reaction Cell gen content.
Table II. O x y g e n Determinetions on Known, Using e N e w Reection Cell (Copper, equivalent to the oxygen, wan determined by microeleotrolysis) Run No. 1 2 3 4 a 5 6 7 8 9 Methane Air Isobutane Air Isobutene Air %oxygen,calculated 0 020 0.060 0.054 0.024 0.128 0.053 0.053 0.180 0.065 %oxygen,found 0 019 0.062 0.056 0.023 0.123 0.055 0.056 0.184 0.066 Run No. 10 11 12 13 14 15 16 Propane Air n-Butane Air oxygen,calculated 0 078 0.026 0.027 0.150 0.020 0.011 0.086 oxygen, found 0.080 0 . 0 2 5 0.030 0.146 0.018 0.010 0.088
9
+
+
+
+
STOPUXK FIG. 3
REAGENT AMMONIUM WDROXlOE AN0 PMr(ONIUM CHORlDE I r l
Figure 1.
@a
Lift Pump
cell dries the copper and renders it inactive. The reaction cell in its final form makes provision for keeping the copper moist. The knowns were prepared by introducing measured volumes of air (20.9% by volume of oxygen) from a buret into a 3-liter evacuated flask which was then charged with pure nitrogen or hydrocarbon gas to desired pressures. For low concentrations a 19.9-liter (bgallon) bottle was used. The following procedure was adopted by the present authors. Though described in considerable detail, the manipulations are actually simple. REAGENTS
Cop r ribbon (approximately 1 mm. wide and 0.1 mm. thick) k n i t t e r into a sponplike pad; the commercially available “Chore Girl” is made in this way and is arrtisfactory.. Ammoniacal ammonium chloride solution prepared by mxing equal volumes of concentrated C.P. ammonium hydroxide and saturated a ueow solution of C.P. ammonium chloride; C.P. potassium iolide, C.P. glacial acetic acid, 0.025 ‘N sodium thiosulfate, 1% starch solution, and commercial nitrogen. APPARATUS
The a p aratus consists of three principal sections: lift pump (Figure used in the production of oxygen-free nitrogen from the commercial product, reaction cell (Figure 2) where oxygen of a m p l e combines with copper, measuring ahparatus consishng of Shepherd buret, calibrated receivers, and three-leg mercury manometer. The entire assembly is shown.in Figure 3. LIFT PUMP. In the lift pump (13) incommg nitrogen e c t u a h a continuous circulation of ammoniacal arpmonium chloride solution t h r o u p copper (Figure 1)1the scrubbing serving to free the nitrogen o oxygen. Commercurl nitrogen enterin through stop cock 2 is forced through co per ribbon in 5, wettmfmth ammoniacal ammonium chloride soktion in 6 by the lift pump, 4, to which the flow of solution ia regulated by stopcock 7. The flow ofbin-
Vol. 17, No. 1
/
1
($ I
1
33
ANALYTICAL EDITION
January, 1945
-IFCOPPER
RIBBON
CHARGING LINE
6
20
21
24
3
MERCURY MANOMETER
n 3 LITER FLASKS
7
CELL
~
4
BY PASS
Figure 3.
w
Apparatus for Determining O x y g e n in Gases
MEASURING APPARATUS.The measuring apparatus consists of buret, manometer, and calibrated receivers connected through stopcocks 20 to 26 as illustrated in Figure 3, which is selfexplanatory. PROCEDURE
Before the apparatus is used the air in the system must be displaced by oxygen-free nitrogen and the oxide removed from the copper ribbon in the cell. Removal of oxide is necessary only after a new charge of copper ribbon has been introduced. The lift pump is started by careful manipulation of stopcocks 2 and 7. Stopcock 9 (Figure 3) is turned to close the system to the atmosphere but to permit discharge of purified nitrogen into the system. Stopcock 10 is closed, 11 is open to all three legs, and 18 is open into the line toward 19, which is closed. Nitrogen now bubbles through the ammonlacal ammonium chloride reagent in 12. The reaction cell is partially filled with the reagent from 12 via stopcocks 13 and 14 till the copper ribbon is immersed, displacing air through 15 and 17. Stopcocks 13, 14, 15, and 17 should be lubricated with petrolatum. Common stopcock greases are unsuitable in presence of the strongly ammoniacal reagent. Stopcock 17 is momentarily dosed while stopcocks 19, 16, 15, and 14 are turned to permit nitrogen to bubble u p ward through the reagent in the cell when 17 is opened. Stop cock 17 should be kept only partially open, merely enough to maintain a slight pressure of mtrogen within the cell to prevent entrance of air. The liquid in the cel! willenow be agitated by the bubbles of nitro en, the co per omde dissolved, and the cupnc ammomum com3ex r e d u A as indicated by the. dim pearance of the blue color. After about 15 minutes the am wip have.been displaced from the linea and the rea ent in the cell is mthdrawn. For this purpose 17 is closed and the nitrogen prewure be1o.w 19 permjtted to force the li uid of the cell out throu h 13 whrle 16 and 14 are turned 80 the%quid will not enter the iy-pass; stopcock 13 is closed to reservom 12; and the li uid drained from the absorption cell is discarded. Care must%e observed in turning 13, BO that air cannot back into the cell. This procedure of fillin the cell with reagent, agitating with nitrogen, etc., is r e p e a d are colorlesa when compared with fresh retill the cell washagent in 6Gm. N d e r tubes againat a white background. The
cell washings appear colorless immediately upon being withdrawn from the cell even if they do contain co per because it is present in the cuprous state. They must theregre be converted into the cupric form by shaking with air before comparison is made. The method is specific for oxygen and it would seem unnecessary to pretreat samples. It was found, however, that hydrogen sulfide, mercaptans, and sulfur dioxide must be removed; the fitst two because they render the copper in the cell inactive in a short time, the sulfur dioxide because it interferes with the iodometric'determination of copper. The sample should not be scrubbed directly with potassium hydroxide because mercaptans consume oxygen in presence of alkalies (6),as was confirmed in this study. A gas sample is best scrubbed by passing through aqueous silver nitrate solution for removal of hydrogen sulfide and mercaptans, and then through potassium hydroxide pellets to remove sulfur dioxide. Before introducing the sam le into a receiver, it must be purged of oxy en and this is best $one by evacuation, charging with purifief nitrogen, and reevacuation. Charging the evacu ted receiver with nitrogen from the lift-pump system must be lone carefully because of the high vapor preasure of the ammoniacal reagent. To avoid a boil-over of the reagent, stopcock 10 should be used as a rate cock by o ning it o@y slight!y with the other introducing the mtrogen, stopcook stopcocks wide o n 11is closed to all t k l e q to prevent the rea ent in 12from backing into the lines. There IS no ssibihty o f developing excessive pressure with regulator 1 in system. For ordinary p it is easier to eliminate oxygen by flushing the receivers m z sample and evacuating, repeating the cycle twice. Sample is charged into receivers to a preasure of lo00 p.of mercury. The bgallon (19-liter) bottle is large enough +J handle nsmples containing as little as 10 parts of oxy en per mllion, m which.case 5OOO cc. is a suitable sam le sine. f f the sample contains more than 1% and lesa than 5& oxygen, it is measured in the buret. If it containa more than 5 7 it mu+ be diluted with nitrogen aqd d c i e n t time allowed for mixlng of the geses.
Wh~g
te
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
34 Table Run No.
111.
O x y g e n Determination on Knowns and Unknowns by Proposed M e t h o d 1 2 3 4 5 6 7 8 9 10 Plant Plant Plant Methane .4ir Isobutane Air Gas 1 Gas2 G-3 0.0042 0 047 0.019 0 039 0 398 0.0009 0 . 1 5 6 Unknown Unknown Unknown 0.0040 0.049 0.017 0.038 0.390 0.0008 0 . 1 5 2 0.1750,00390,046 0.177 0.0085
+
+
oxygen calcd 2orygen:found Run No.
7
14 15 16 Butanes Air 0.056 0.009 0.087 0.054 0.0°9 O.Ogo Plant Gas 3
+
11 12 13. Plant Gas 3 0.024% 0% Iaobutene Air oxygen, calcd. 0,070 0.035 0.110 oxygen, found 0,067 0.038 0.112 Plant Gas 1 Plant Gae 2
+
% Methane Ethene Ethane Propene Propane Isopentane
29.6 54.1 0.5 13.0
2.2 0.6
55.9 2.7 3.7 13.4 5.8 9.5 4.9 4.1
% Propene Isobutane Isobutene 1-Butene n-Butane ciatrans-butene Pentane
IV.
Effect of Time of Standing before Analysis on O x y g e n Content of Isobutene-Air Blends Oxygen Found After After After Oxygen 24 hours 48 hours Calculated 1 hour % % 70 70 0.019 0,010 0.026 0.025 0,081 0.068 0.095 0.093
Table
V.
O x y g e n Determinations on Methane Containing Interfering Substances Oxygen, % Interfering Substance, % Present Found Hydrogen sulfide, 0.2 0.026 0.028-0 025 Sulfur dioxide, 0.2 0,032 0.033-0.033 Methyl mercaptan, 0.2 0.060 0 057-0.057
Table
For such samples corrections should be applied for deviations from perfect gas laws. Samples containing less than 1% oxygen are taken directly out of the receivers and measured by pressure difference. The system now being free of oxygen and the copper ribbon in the cell free of oxide, ammonia-ammonium chloride reagent is run from reservoir 12 into the lower bulb of the reaction cell through stopcocks 13 and 14 until it is one-third full, 14 being turned carefully so that no reagent enters the by-pass. This liquid serves to saturate the incoming sample with moisture, thus keeping the copper in the cell wet. This is very important, since dry copper will not absorb oxygen. The sample is now passed through the cell via stopcocks 19 and 16, the by-pass, 14, up through the cell, and out by 15 and 17. Stopcock 17 should be opened last and only partially, to maintain a slight positive pressure in the cell to prevent air entering it. I t should be noted that 16 must be put into the roper position before 19 is opened; otherwiqe, a small portion o!the sample is trapped in the line between 16 and 15. The sample should pass through the cell a t approximately 150 cc. per minute as indicated from the manometer. If the sample be in the .Fgrtllon receiver under a pressure of 1000 mm. of mercury, a drop of 5 rnm. per minute on the manometer would be the correct rate. After sufficient sample has passed through the cell, 17 is closed while 19 is opened to the nitrogen supply and 17 is partly opened again to permit nitrogen to flush through the cell such sample as remained in the lines. This requires about 2 minutes. Stopcock 17 is then closed again and reagent run into the cell from 12 by way of 13 and 14 a'gainst the nitrogen pressure above 15,stopcock 14 being set so no liquid can enter the by-pass. Sufficient reagent is run into the cell to subnierge the copper ribbon, then 13 is closed to all connecting lines. Nitrogen is bubbled for 5 minutes upward through the liquid in the cell through 19, 16 the by-pass, 14, 15,snd our through 17, which a ain is opened fast and partially. This agitation ensures the presence of only cuprous oxide in the solution by bringing the ammoniacal cupric complex in contact with copper. The cell washings are emptied into a beaker, and the washing is repeated with new portions of reagent until colorless by comparison with fresh reagent in Nessler tubes a t a depth of 12 cm.
minute, allowed to cool, about 1 gram of potassium iodide crjstala added, and the liberated iodine titrated with 0.025N thiosulfate using starch aa indicator. A microburet, divided in 0.01 cc. should be used for this titration.
+
% Methane Ethene Ethane Propene Propane Isobutane Butene Butane
Vol. 17, No. 1
+
4.13
30.3 10.0 22.7 25.1 6.4 0.9
CALCULATIONS 1 CC. of 0.025 N N&Oa . . - = 1.59 mg. of Cu 11.36 mg. of Cu = 1 cc. of O2 a t 760 mm. and 0" C. 1 CC. of 0.025 N NkS.0, 0.14 cc. of 02 at 760 mm. and 0" C.
Cc. of 0.025 N thiosulfate X 0.14 X 100 - = cc. of sample at 760mm., 0" C. % O2by volume
The precision and accuracy of the proposed method are evident from Table 111. Olefins and diolefins do not interfere. However, a sample containing unsaturates must be analyzed promptly, since oxygen appears to be consumed by the unsaturates (Table IV). A better indication of reaction between unsaturated gas and oxygen is had in the case of IJ-butadiene, to which 0.2% oxygen in the form of air was added. After standing overnight no oxygen was found but peroxides were present in the receiver though the original gas contained none. It is of interest that acetylene does not consume oxygen on standing under these conditions. Methane containing 5% acetylene and 0.010% oxygen still showed 0.009970 after 24 hours' standing. Hydrogen sulfide, mercaptans, and sulfur dioxide interfere but they are easily removed by successive scrubbing with aqueous silver nitrate and potassium hydroxide pellets, without affecting the oxygen content (Table V). A determination in duplicate requires about one hour. A raw plant gas consisting of. Cs and C, olefins and paraffins, containing 0.07% mercaptan by analysis, was found to contain 0.035y0oxygen when scrubbed as described above. No oxygen was found when the gas was scrubbed with potassium hydroxide pellets alone; in the latter case mercaptans had apparently consumed t,he oxygen. ACKNOWLEDGMENTS
The authors express their appreciation to B. R. Stanerson and R. VanVleck for assistance in the initial work. LITERATURE CITED
DETERMINATION OF COPPiR
(1) Ambler, H. R.,Analyst, 59, 14 (1934). (2) Binder, K.,and Weinland, R. F., Ber., 46, 255 (1913). (3) Hofer, G.,and Wartenberg, H. von, Angew. Chem., 38, 9 (1925). (4) Kautsky, H., and Hirsch, A., 2. anorg. a l l g e m . Chem., 222, 126 (1935). (5) Lahhman, A., IND.ENQ.CHBM., 23,354(1931). (6) MacHattie, I. J. W., and Maconachie, J. E., IND.ENG.CHEM.. ANAL.ED.,9, 364 (1937). (7) Mugden, M., and Sixt. J., Angew. Chem.,46,90 (1933). (8) Phillips, F.C.,Am. Chem. J.,16,340 (1894). (9) Schmid, A., Brennstof-Chem., 15,271 (1934). (IO) Scott, "Standard Methods of Chemical Analysis", 5th ed., Vol. I, p. 368,New York, D.Van Nostrand Co., 1939. (11) Shaw J. A., IND.ENO.CBBM.,ANAL.ED..14,891 (1942). (12) Simmons, J. H.,and Kipp, E. M., Ibid., 13, 328 (1941). (13) Van Brunt, C.,J. Am. Chem. Soe., 36, 1448 (1914). (14) Wagner, G.,Oester Chem.-Ztg., 42,270 (1939). (15) Winkler, L.W., 2.Nuhr. Genusm., 47,257(19%).
The combined cell extract and waahinga are boiled until the solution smells faintly of ammonia (IO). After cooling, it is acidified (litmus external indicator) with acetic acid, boiled anothQr
PRESENTED before the Division of Petroleum Chemistry a t the 1OSth Meeting of the AMEEICAN CHZYICAL SomErY, Detroit, Mioh.
