Determination of Small Quantities of Oxygen in Hydrocarbon Gases J. S. POWELL AND P. C. JOY Butadiene Production Department, Southern California Gas Company, LO3 Angeles, Calif.
A method is described for determining oxygen in gases in very low concentrations up to 2.5%. A known volume of gas is passed repeatedly over copper wire which is moistened with ammoniacal ammonium chloride solution. The amount of dissolved copper is determined by measuring, with a photelometer, the intensity of the blue color in the solution. Saturated or unsaturated hydrocarbon gases do not interfere, but hydrogen sulfide and mercaptans must be removed. The principle is well known; however, a portable apparatus and simple procedure have been developed which minimize the sources of error.
I
The commercially available Chore Girl scouring pads are sati8factory as packing in the absorption tube. The copper ribbon should be cleaned by heating it momentarily in a gas flame, the copper oxide coating formed being dissolved subsequently after the copper is packed in the tube and treated with the ammoniacal ammonium chloride solution. According to Uhrig et al. (S), cleaning the copper with nitric acid is not satisfactory, as the copper becomes inactive after being used a short time. Photelometer. The instrument must be capable of determining the transmission of light at approximately 640 millimicrons.
N THE production of butadiene by cracking of petroleum frac-
tions, oxygen in the cracking gas or finished butadiene is an undesirable contaminant, because it catalyzes the formation of gummy polymers and produces organic peroxides which in certain operations might present a hazard because of their unstable nature. A sensitive, simple, and rapid method was needed for the determination of oxygen, which would not be subject to interferences from olefinic hydrocarbon gases. An apparatus was desired which would be portable and could be taken to the gas source, thus eliminating the possibility of contamination that exists when a gas sample is taken in a container and transported to the laboratory. The manganous hydroxide method for determining small amounts of oxygen given by Phillips (8), which is a modification of the Winkler (4) method, and the methods for determining oxygen in gases by the reaction of the oxygen with reduced copper in the presence of ammonia-ammonium chloride as described by hlacHattie and Maconachie (1) and Uhrig, Roberts, and Levin (3)were investigated. The latter method offered the desired sensitivity and freedom from interferences but the apparatus required was complex and nonportable. The design of a simple, portable apparatus which would make possible the use of this method was successfully undertaken.
STORAGE 5U0C
REAGENTS
Ammonium Hydroxide-Ammonium Chloride Solution. Mix 300 grams of C.P. ammonium chloride, 1000 ml. of C.P. ammonium hydroxide, and 1000 ml. of freshly boiled distilled vater, and filter the solution to remove undissolved solids. Keep this solution in a bottle provided with a delivery tube under an atmosphere of essentially oxygen-free gas which can be introduced into the bottle under pressure to force the solution from the bottle when needed. Standard Copper Solution. Dissolve 3.928 grams of cupric sulfate pentahydrate in exactly 1000 ml. of the ammonium chloride-ammonium hydroxide reagent. One milliliter of solution contains 1 mg. of copper. Silver Nitrate Solution. Dissolve 20 grams of silver nitrate in 1 liter of water. This solution is used to scrub the gas if it contains hydrogen sulfide or mercaptans (thiols). APPARATUS
Apparatus. The apparatus (Figure 1) consists of a solution storage tube of about 115-ml. capacity and a tube packed tightly with copper ribbon. The storage tube is provided with 3-way, 120' stopcocks a t either end. The storage and absorption tubes are connected at their ends with heavy-walled rubber tubing, long enough so that one tube can be raised above the other.
. Figure 1. Apparatus for Determining Low Concentrations of Oxygen in Gas 296
V O L U M E 21, NO. 2, F E B R U A R Y 1 9 4 9 PROCEDURE
Calibration of Photelometer. Accurately measure 5 , 10, 15, 20, and 25 ml. of standard copper solution into separate 100-ml. volumetric flasks and make up to 100 ml. with ammonium hydroxide-ammonium chloride reagent. Mix well and determine the optical density a t approximately 640 millimicrons, compared to the ammonium hydroxide-ammonium chloride reagent. From these results, prepare a graph showing the relationship between optical density and the milligrams of copper contained in 100 ml. of solution. Operation of Apparatus. Determine the volume of the solution storage tube to a mark placed near its top. Determine the volume of solution that is retained by the copper packing after the absorption tube is filled with solution and allowed to drain for several minutes. Assemble the apparatus as shown in Figure 1 and purge i t with an inert gas. A small amount of oxygen in the gas is of no consequence. Connect the delivery tube of the reagent bottle to the left arm of stopcock A , and set the stopcock plug so that solution can be purged out of the right arm. Discharge enough solution to clear the tubing and stopcock of oxygen. Set stopcocks A and B so that solution can be introduced and retained in the solution storage tube. Fill this tube t o the mark by allowing the gas in the tube to be displaced through the absorption tube out of one of the arms of stopcock C. Close A and C and set B so that the solution in the storage tube can pass into the absorption tube. Raise the bottom of the solution storage tube above the top of the absorption tube, and allow the solution to fill the absorption tube, then invert the apparatus and allow the solution to pass back to the solution storage tube. Return the apparatus to its original position, and repeat the operation twice more. The solution must pass through the absorption tube and not merely in and out. These operations remove all the oxygen from the gas within the apparatus and leave an atmosphere that is absolutely oxygenfree. Store all the solution in the apparatus in the absorption tube and set stopcocks B and C so that all the solution can be discharged through one of the side arms of C. Introduce fresh solution into the solution storage tube, wash the copper packing with this fresh solution, and discard it in a like manner. Repeat the washings until the solution, when it is taken from the apparatus and air is bubbled through it, has no color or only a slight constant color due to the oxygen in the reagent, and retain i t as a blank. The apparatus is now ready for use. Fresh solution is in the solution storage tube and the absorption tube is empty. If the gas contains hydrogen sulfide or mercaptans, pass it through a scrubber containing silver nitrate solution. Then introduce the gas to be analyzed through the left side arm of stopcock C, and allow it to flow for a few moments out of the bottom side arm. Set C and A so that the gas can pass through the absorption tube and out of the right side arm of A . Allow a small amount of solution to run into the rubber tubing between B and the absorption tube. This solution saturates the gas bubbling through it and prevents the gas from drying the copper packing. Meter the gas with a meter placed downstream from the apparatus, and pass the gas through the apparatus a t a rate not to exceed 350 ml. per minute. The quantity of gas to pass is given in Table I.
Table I.
Gas Passed through Apparatus
Oxygen in Sample, % 0.5 to 2.5 0.08 t o O . 5 0.02to 0.08 0.004 to 0.02 0.0008 t o 0,004 0.0002 to 0.0008
Sample, M1. Fill storage tube 600
2,800 11,000 55,000 280,000
Where the gas contains from 0.5 to 2.5% oxygen fill only the storage tube with gas, by first passing all the solution in the apparatus into the absorption tube and setting stopcocks B and A so that the gas passes through the storage tube. Pass enough gas through the tube to collect a representative sample, and use this sample, equal to the volume of the storage tube, for the analysis. ilfter the prescribed volume of gas has been passed through the apparatus, close A and C and set 13 so that the solution can be rycled through the absorption tube. Cycle the solution through the absorntion tube a t least three times. store it in this tubc. and then discharge it from the apparatus while introducing fresh solution into the storage tube. Bubble air through the solution taken
297 from the apparatus until the color of the solution reaches its maximum density. Determine the optical density of this solution a t 640 millimicrons, setting the photelometer to 100% transmission on the blank solution. Calculations. The per cent oxygen in the gas analyzed is given by substituting in the following equation:
O =
0.099 c (S
v
+ h)
where 0 = volume per cent oxygen in gas C = milligrams of copper per 100 ml. of test solution obtained from optical density measurement and calibration graph S = milliliters of solution in solution storage tube h = milliliters of solution held up on copper packing T' = milliliters of gas sample Accuracy. The copper-oxygen conversion factor is defined as the milliliters of pure oxygen a t 0' C. and 760 mm. of mercury pressure equivalent to 1 mg. of copper and is theoretically equal t o 0.088 ml. corresponding to the formation of cuprous oxide. Apparently some cupric oxide is also formed by carrying out the described procedure, and by introducing known volumes of oxygen into the apparatus, an average factor of 0.099 was obtained. The results of several such tests are shown in Table 11. The maximum deviation from the average factor is 2.4%. MacHattie et al. (1) obtained an average factor of 0.0956, and Uhrig et al. (5)obtained results that indicated the reduction of any cupric oxide formed to cuprous oxide. However, the authors by limited tests were not able to duplicate the findings of Uhrig et al.
Table 11. Determination of Oxygen Equivalent per Milligram of Copper Oz a t 0' C. Optical Density Factor t o Convert
and 760 Mm.
of Solution
Copper Mg./100 ml.
0.336 0.330 0.135 0.140 0.205 0.205
24.6 23.9 10.1 10.4 15.4 15.4
1111.
2.44 2.43 1.01 1.01 1.51 1.51
AV.
M g . Cu t o Ml. Oz
0 .099
0.102 0.100 0,097 0.098
0.098 0.099
To fix the rate for the complete absorption of oxygen from a gas passed through the absorption tube, a second apparatus was constructed and nitrogen containing about 0.0270 oxygen was passed through the two apparatus in series a t various rates. At a rate of 360 ml. of gas per minute no oxygen was absorbed from the gas by the second apparatus in series. The results obtained from introducing known quantities of oxygen into the apparatus indicate that if the above rate is not exceeded the method is probably accurate to within a t least 2.5y0of the true value. ACKNOWLEDGMENT
The authors wish to acknowledge the able assistance of 1,. V. Iileiner, who did much of the experimental work and offered helpful suggestions. LITERATURE CITED
(1) MacHattie, I. J. W., and Maconaohie, J. E., IND.ENG.CHEM.. ANAL.ED.,9,364 (1937). (2) Phillips, F. C., Am. Chem. J . , 16, 340 (1894).
(3) Uhrig, K., Roberts, F. M.,and Levin, H., IND. ENG. CHEM., ANAL.ED.,17,31 (1945). (4) Winkler, L. W., Z . Nahr. Genussm., 47, 257 (1924). RECEIVED J U
I ~IO, 1948.
