ADVANTAGES
The main advantages of the absorptiometric method over conventional electrolytic and gravimetric procedures are its improved simplicity, speed, sensitivity, and accuracy. Lead may be determined down to about 1 p.p.m., the lower limit depending on the amount of background interference. ACKNOWLEDGMENT
Thanks are due to Jack Z. Falcon and Borji Pavlovich, who did much of the
analytical work in helping to prove the method. Permission of the Firestone Tire and Rubber Co. to publish this paper is gratefully acknowledged. LITERATURE CITED
(1) Am. Soc. Testing Materials, Philadel-
phia, Pa., “ASTM Standards. Chemical Analysis of Rubber Products”, Method D 297-43T, 1952. (2) Brooks, L. A , , Rowley, A. C., Vunderbill News 22, No. 1, p. 10 (1956). (3) Davis, C. C., Blake, J. T., “Chemistry and Technology of Rubber,” Reinhold, Kern York, 1937.
(4)India Rubber World, “Compounding Ingredients for Rubber,” Bill Brothers, New York, 1947. ( 5 ) Kress, K. E., ANAL.CHEM.27, 1618 (1955). (6) Kress, K. E., Mees, F. G. S., Zbid., 27, 528 (1955). (7) Memmler, D. K., “The Science of Rubber,” American ed. by R. F. Dunbrook, V. N. Morris, Reinhold, New York, 1934. (8) Merritt, C., Hershenson, H. M., Rogers, L. B., ANAL. CHEM.2 5 , 572 (1953).
RECEIVED for March 21, 1956. Accepted December 15, 1956.
Potentiometric Method for Determination of Carbonyl Sulfide in Petroleum Gases DOUGLAS B. BRUSS, GARRARD E. A. WYLD, and EDWARD D. PETERS Shell Development
Co., Emeryville, Calif.
b Carbonyl sulfide in petroleum gases can b e determined potentiometrically after absorption in alcoholic rnonoethanolamine solution b y titrating it with standard silver nitrate solution. Mercaptans and hydrogen sulfide are first removed from the gas stream b y absorption in alcoholic caustic solution. Concentration as low as 2 mg. per cubic meter of gas may b e conveniently determined.
D
of sulfur compounds in refinery gas streams is of considerable importance in the petroleum and petrochemical industries. Sulfur constituents such as hydrogen sulfide, mercaptans, carbonyl sulfide, and free sulfur may act, in certain processes, as catalyst poisoning agents and corrosive agents. A potentiometric method has been developed for the determination of carbonyl sulfide in gases in concentrations as low as 2 mg. per cubic meter. Soloveichik (IO) determined carbonyl sulfide in soil gases colorimetrically by absorbing the gas in alcoholic potassium hydroxide and converting the carbonyl sulfide to hydrogen sulfide. dvdeeva ( I ) described a method of determining carbonyl sulfide and carbon disulfide by absorption in selective reagents, oxidation, and measurement as the barium sulfate precipitate. A method of determining carbon disulfide and carbonyl sulfide in gases by adsorption in piperidine-chlorobenzene or alcoholic potassium hydroxide and resolution of the ETERMISATION
proportion colorimetrically or iodometrically was reported by Riesz and Wohlberg (9). Field and Oldach (3,4) used a catalytic method to convert organic sulfur to hydrogen sulfide which is absorbed in caustic, converted to a bismuth sulfide suspension, and read photometrically. They also developed a procedure for the determination and identification of sulfur compounds in gas mixtures based on differences in solubility in an inert solvent ( 7 ) . Haken-ill and Rueck ( 5 ) absorbed carbonyl sulfide in alcoholic potassium hydroxide and determined the amount iodometrically. The use of selective absorbents for analyzing manufactured gas for sulfur compounds is described by MacHattie and McNiven (6). Thiophene, carbonyl sulfide, and carbon disulfide in producer gas have been determined ( 2 ) by treating a gas sample with a piperidine-ethyl alcohol reagent and measuring in the ultraviolet region. Rapoport (8) used a series of absorbents and a combustion technique for the determination of carbon disulfide, carbonyl sulfide, and thiophene. The method reported here consists of the passage of the sample gas through two scrubbers, the first of which removes hydrogen sulfide and mercaptans by absorption in 30% caustic and the second of which absorbs carbonyl sulfide in alcoholic monoethanolamine. The contents of the monoethanolamine scrubber are titrated with silver nitrate solution in acidic alcoholic titration solvent, using the potential difference between a glass reference elec-
trode and a silver sulfide electrode t o indicate the end point. The precipitates from several titrations were filtered from the solutions, washed, and dried in a desiccator. They were then weighed, dissolved in nitric acid, and analyzed for silver content by potentiometric titration with potassium iodide. The results showed a silver content of approximately S6%, which corresponds closely to that of silver sulfide. The titration of the contents of the monoethanolamine scrubber requires 2 moles of silver nitrate per mole of carbonyl sulfide. The following equations are postulated as a possible explanation of the stoichiometry. Equation 4 describes the over-all reaction. COS
+ HOCHZCH~KH~-P HOCHzCH2NH
1
(1)
‘SH HOCHZCHZNH
+ +
HzS
8 + \
SH HOCHzCH2NHZ+ (HOCHzCH2NH)zC=O
(2)
+ +
H2S 2AgNOa+AgzS 2HNOs (3) COS 2HOCHzCHsNHs 2AgNOa+ AgzS (HOCHzCHzNH)zC=O 2”Os (4)
+
+
+
APPARATUS AND REAGENTS
Absorbers.
Gas
washing
VOL. 29, NO. 5, M A Y 1957
bottles,
807
125-m1., with coarse-porosity, sinteredglass disks. Gas meter, \vet type, having a measuring capacity of about 3 liters per revolution and a cumulative register up to about 300 liters. The meter was equipped n ith a thermometer and water manometer for measuring the gas temperature and pressure, respectively. Flow control valve. A stainless steel, l/s-inch needle valve, for samples taken from the liquid state. An arrangement was supplied for heating the valve. Titrorneter. Precision Dual A.C. Titrometer equipped with a glass reference electrode, a silver sulfide indicating electrode, and a 10-ml. buret graduated in 0.05 ml. Acidic titration solvent. Sodium acetate trihydrate, 2.7 grams, Jyas dissolved in 20 nil. of oxygen-free water and 97.5 nil. of anhydrous ethyl alcohol, and 4.6 nil. of glacial acetic acid was added. Refined isopropyl alcohol, which haq been passed through a small column of activated alumina, or Formula 2B denatured alcohol (a mixture of absolute ethyl alcohol and 5% by volume of benzene) may be substituted for the anhydrous ethyl alcohol. The solution \vas purged with a rapid stream of nitrogen for 10 to 1.5 minutes each day prior to use to remove dissolved oxygen. Ammonium hydroxide, concentrated. Carbonyl sulfide, Matheson Co., 97% as determined by mass spectrometry. Monoethanolamine, Eastman Kodak White Label, 5% solution in ethyl alcohol. Sitrogen, Linde, H. P. dry, containing less than 0.1% oxygen. Silver nitrate solution. Silver nitrate, 17 grams, \vas dissolved in 100 ml. of distilled water and diluted to 1 liter with 92% isopropyl alcohol (prepared from refined, peroxide-free alcohol and distilled rTater). A 0.01X solution was prepared by exact dilution of the alcoholic 0.1S solution with refined 92% isopropyl alcohol. The silver nitrate solutions were standardized against standard 0.1.Y potassium iodide. Sodium hydroxide solution, 30% aqueous. This was purged with nitrogen for 10 to 15 minutes each day prior to use to remove dissolved o x y g ~ n .
Pass sufficient gaseous sample through the absorbers to afford a titration of 2 to 10 ml. of 0.01,Y silver nitrate (usually 8 to 12 liters). The gas sample should pass through the absorbers a t a rate of 150 to 180 ml. per minute. Record the meter reading, the temperature, and the barometric pressure. Flush the absorbers with 5 to 10 liters of nitrogen and disconnect them. Quantitatively transfer the monoethanolamine solution from the second absorber into a 250-n11., tall-form titration beaker. Dilute to 125 ml. with acidic titration solvent and titrate potentiometrically, using the glasssilver sulfide electrode system. Select the end point a t the bottom of the straight line portion of the break in the titration curve (Figure 2). The end point potential should be approximately +SO mv. Unpublished work ( 2 1 ) on the titration of pure halides and mercaptans with silver ion has shown empirically that the stoichiometric point is a t the end of the steepest portion of the titration curve. I n the present application, the end points have been taken in this manner, although the difference from the conventional inflection point is within experimental error. If the titration falls appreciably outside the 2- to 10-ml. range, use a larger sample or an aliquot of the scrubber solution. Calculate the gas volume a t standard conditions by means of the following equation:
where Ty0
=
observed liters
808
ANALYTICAL CHEMISTRY
of
gas,
mercury vapor pressure of water at temperature T Calculate the carbonyl sulfide content of the sample using the following equation: p
=
carbonyl sulfide, mg. per cubic meter = A*%-30,000
v
where A N
V
= = =
volume of silver nitrate, ml. normality of silver nitrate volume of sample, standard conditions EXPERIMENTAL
A number of synthetic and refinery gas samples were tested for carbonyl sulfide using this method. Synthetic samples were prepared by injecting known amounts of carbonyl sulfide into a stream of propane at a point before the caustic prescrubber. Table I compares the results obtained on several samples with those obtained using the mass spectrometer or a lamp combustion method. Carbon disulfide, mercaptans, and hydrogen sulfide would interfere with the titration, as they form titratable complexes with monoethanolamine. However, hydrogen sulfide, mercaptans. sulfur dioxide, and carbon dioxide are absorbed in the 30% aqueous caustic solution. Carbon disulfide is usually present in such small amounts that it does not constitute a major interference. Hydrolysis of carbonyl sulfide in the 307' caustic solution may become n
N,
4
Scrubber
Line
Figure 1.
PROCEDURE
Connect the gas absorbers in series as shown in Figure 1. Connect the outlet tube of the second absorber to the wet test meter. Connect the inlet tube of the first absorber to the flow control valve on the sample line and to the rate control valve from the nitrogen line so that either the sample or nitrogen may be passed through the absorbers. Make all connections as short as possible with glass tubing joined either by groundglass joints or short lengths of Tygon tubing. Place 20 ml. of 30% aqueous sodium hydroxide in the first scrubber and 20 ml. of 5% alcoholic monoethanolamine in the second scrubber, which should be completely masked \%-ith black tape. Flush the assembled apparatus with 25 liters of nitrogen.
volume
P = barometric pressure, mm. of
Table I.
Apparatus for absorption of carbonyl sulfide
Comparison of Methods for Determination of Carbonyl Sulfide" Carbonyl Sulfide, Calcd. as Sulfur, Wt.
Lamp method
for total
Sample Depropanizer tops Untreated propane Treated propane Untreated propane Untreated propane Untreated propane Blend of COS in c, Blend of COS in C? (1
>ile:er
sulfur
0.280 0,055 0.066 0,057
Comparison data from Taylor ( 1 2 ) .
Potentiometric titration method 0.0059 0.0045, 0.0050 0,0005. 0.0005 0.0068 0.0059 0.0051 0,255 0.068 0,066 0.058
Mass spectrometer 0.0053, 0.0055
0,0040, 0.0055 0,0005, 0,0005
0.0073 0.0062 0.0046
Table 11.
cos -\Ig. -
Analysis of Synthetic Samples
J
Experiment A
Added
Found
lT.9
17 0 0.5
Recovery,
Remarks. MEA* scrubber Caustic scrubber B 22.2 20.6 M E A scrubber 0.7 3 Caustic scrubber c 17.9 16.7 93 S o caustic scrubber D 17.8 16.4 92 Scrubber at 0" C. E: 17.8 16.7 93 Scrubber at 35" C. F 22.2 20.5 92 Isopropyl alcohol as titration solvent G 22.2 21.5 9T Solution titrated after 2 hours in dark H 22.2 12.6 5i Solution titrated after 2 hours in light I 22.2 1.5 7 Solution titrated after 8 hours in light J 22.2 18.0 81 Scrubber exposed to sunlight during run Ii 193 1TO 88 1st ME-1 scrubber 2 2nd MEA scrubber a Conditions of determination are same as given under procedure unless otherwise specified. * Monoethanolamine. c
95 3 93
what higher rcaults when the acetic acid was omitted and lower results when the sodium acetate as omitted. Variation of the temperature of the absorber or tit'ration solution did not affect t'he results measurably over a range from 0" to 35" C. There was some indication of incomplete alisorption of the carbonyl sulfide, amounting to about 2'7& with larger sample sizes (Table 11, K ) . This was determined by placing two monoethanolaniiiie scrubbers in series. A x-ariety of other anlines (5% solutions in alcohol) and a 5% alcoholic potassium hydroxide solution were tried in place of nionoethanolamiric in the carbonyl sulfide scrublxr. The other amines studied were triethanolamine, diethanolamine, n-butyltliethanolaniine, ethylaniine, and di-n-amylaniine. dlcoholic potassium hydroxide gave a poor t,itration curve and low recoveries. The other amines were unsatisfactory from the standpoint of reaction rate, stability, or reproducibility. These amines may give more suit'able results by the proper adjustment of the acidity of the titration medium. Di-n-amylamine proved satisfacbory when the apparent p H was controlled a t 10.6 in the beginning of the titration. However, the results obtained were one half t'hose obtained using monoethanolamine. The following equation was postulated for the reaction of carbonyl sulfide with di-n-amylamine and silver nitrat'e :
-30 +
+
(CsH1i)zSH COS AgSOa(CjHii)aSCOSAg HSOJ (5)
+
ACKNOWLEDGMENT
The authors are indebted to L. W. Tayler, Shell Oil Co., for data comparing the present method with other methods for carbonyl sulfide.
100.
zoo1 0
I 2
Figure 2.
I 1 I 6 8 V o l u m e of T i t r a n t , ml.
4
I 10
12
Typical titration curve
appreciable if certain precautions are not taken. Satisfactory results w r e obtained using a flow rate between 150 and 180 ml. per minute and a coarseporosity. sintcred-glass disk immersed about 0.5 inch in the caustic solution. Titrations of the caustic scrubber from determinations of synthetic samples of carbonyl sulfide in propane indicate that betnecn 1 to 3% of the carbonyl sulfide is hydrolyzed in the caustic scrubber solution. (Table 11,A and B ) . Light appears to be the most serious factor affecting recovery of the carbonyl sulfide. Successive determinations were niade allowing the monoethanolamine scrubber solution to stand for different periods of time before titrating (Table 11, G, H ,and I ) . Recoveries of u p to 977, were obtained on solutions which were placed in the dark for several hours
before titration. Upon standing in normal room light for 2 hours, only 5iYO of the normal titration was obtained, and on standing 8 hours in the light, recovery dropped to 7%. This effect was verified by allowing sunlight to fall on the monoethanolamine scrubber during a run and titrating immediately after completion of the run. Recovery of the carbonyl sulfide dropped to 81%. For this reason the monoethanolamine scrubber should be completely masked with black tape and the solution should be titrated immediately on completion of the run. The acidity of the titration solvent was varied by omitting the acetic acid, the sodium acetate, or both. The composition of the titration solvent made the recovery of carbonyl sulfide vary by several per cent, giving some-
LITERATURE CITED
Avdeeva, A . V . j Zanodstaya Lab. 8 , 279 (1938). Brady, L. J., AXAL. CHEM.20, 512 (1948). Field, E., Oldach, C. S., IXD.EAG. CHEII.,A ~ A LED. . 18, 665 (1946). Ihtd., p. 668. Hakewi!l, H.. Rueck. E. 11..-4 it!. Gas .issoc. Pro;. 28, 529 (1946). MacHrtttie, I. J. K . , McSiven, S . L., Can. Cheni. Process Inds. 30, S o . 7 , 87 (1946). Oldach, C. S., Field, E., AXAL. CHEJI.,18, 669 (1948). Rapoport, F. )I., Zavodskaya Lab. 16, 560 (1950). Iliesz, C. H., Wohlberg, C., A m . Gas Sssoc. Proc. 25,259 (1943). Soloveichik, S. I., Zacodskaya Lab. 6 ,
1151 (1937), Tamele, 31. 11 ., Ryland, L. B., Shell Ilevelopment Co., Emeryville, Calif., unpublished work. Taylor, L. W.,Shell Oil Co., Wood River, Ill., private communication.
RECEIVEDfor review July 19, 1956, Accepted December 15, 1956. VOL. 29, NO. 5, MAY 1957
809
