INDUSTRIAL A N D ENGINEERING CHE.VISTRY
June, 1926
383
The Oxygen Bomb Method for Sulfur Determinations' By M. J. Bradley, R. M. Corbin, and T. W. Floyd UNIVERSITY OF
ILLINOIS,
T
HE quantitative determination of sulfur in petroleum and coal distillation products is difficult and tedious, and considerable care is necessary in order to obtain accurate results. The analytical procedures published may be roughly grouped into three classes: ( a ) dry fusion methods, (b) oxidation with liquid reagents, and ( c ) combustion methods. Any one method cannot be used successfully for all the various fractions of these hydrocarbons. However, in the combustion group for light oils the lamp method,2 and for the heavier fractions the oxygen bomb m e t h ~ d have ,~ been recommended. When checking the lamp method on a known organic sulfur compound, dissolved in light oil, the results were found to be low, even when such added refinements as washing the lamp with amyl acetate and finally burning the wick were employed. I n the usual oxygen bomb method the light fractions oxidize with explosive violence and the heavy tars and residues are difficult to get completely burned. The following research was undertaken in order to overcome some of these difficulties and to modify the methods, if possible, so that the light and heavy fractions might be safely analyzed by the oxygen bomb method. Theoretical
The oxidation of sulfur dioxide to trioxide] on account of its technical importance, has been studied by several investigators] and from the results of Bodenstein and Poh14 the writers have calculated the degree of dissociation of SO3 to SO2 '/go2 a t various temperatures and 1 atmosphere pressure. The graph of these results shows that dissociation commences around 450" C. and is complete a t 1000" C. From the equation
+
so, + '/* 02 e so*
the following expression for the dissociation constant,
may be obtained. Since this dissociation takes place with a n increase in volume, pressure will tend to inhibit it. Also a large excess of oxygen over that required for the reaction will tend to decrease the dissociation of sulfur trioxide a t a given temperature. For example, at 525' C. and 1 atmosphere pressure, sulfur trioxide was found by calculation to be 11.6 per cent dissociated. By increasing the partial pressure of oxygen to 20 atmospheres the degree of dissociation was decreased to around 4 per cent. However, the equilibrium is not affected by change in pressure due to the addition of inert gases, such as obtained as combustion products in calorimetric determinations. The equilibrium is affected] however, by the increase in temperature accompanying such a combustion and, as shown by the foregoing calculations, no sulfur trioxide would exist above 1000"C. I n calorimetric determinations on coal and oil the maximum temperature reached is well above 1000" C., so that no sulfur trioxide would exist until after the combustion was completed and the Received December 19, 1925. Bur. Mines, Tech. Paper 8 . 8 I b i d . , 298, p. 51. '2. Elekirochem., 11, 373 (1905). 1 2
URBANA,
ILL
bomb cooling down. It is possible t o cool the bomb so rapidly that even a t the optimum temperature there would not be sufficient time for all the sulfur dioxide to be oxidized to trioxide unless some activating catalystswere present. These assumptions were verified in the following experimental results. Experimental
The determinations were made in an Illium oxygen bomb,5 fitted with a double valve attachment which made it possible to release the gases from the bomb very slowly, after combustion, so that they could be bubbled through a wash train of solution. The procedure was similar to that recommended for calorimetric determinations, except in some instances a few cubic centimeters of 10 per cent sodium carbonate solution were placed in the bottom of the bomb to neutralize the acids formed in the combustion. The analysis of small amounts of sulfur dioxide and trioxide in the presence of each other entails considerable difficulty. The procedure finally adopted was to cool the bomb slowly after the combustion was completed and then release the gases slowly through a 10 per cent sodium carbonate solution in the wash train. The last traces of gas were obtained by applying suction to the wash train and a t the same time admitting a spray of distilled water into the bomb. The absorption solution plus the bomb and train washings were diluted to 500 cc. in a volumetric flask and aliquot portions used for analysis. I n some cases a smaller volumetric flask was employed. Sulfur dioxide was determined by titrating the alkaline solution with 0.002 N iodine solution, using starch paste as an' indicator. Sulfur trioxide was determined by making the alkaline solution acid with hydrochloric acid, boiling to expel any sulfur dioxide, and finally precipitating the SO4 as barium sulfate with a few cubic centimeters of hot barium chloride solution. The resulting precipitate was digested for a couple of hours, filtered, washed, ignited, and weighed as barium sulfate. Total sulfur was determined on the undiluted solution by treating with a few drops of bromine water to oxidize any sulfite to sulfate, taking to boiling to expel the excess bromine, and finally precipitating the barium sulfate as described. Pure Sulfur
Several combustions were made with pure sulfur, using 0.25-gram samples and 20 atmospheres initial oxygen pressure. No constant results were obtained, the amount of sulfur dioxide varying from 14.5 to 43 per cent. If the bomb was placed in hot water and allowed to stand more than one-half hour before releasing the pressure the percentage of sulfur dioxide was considerably lowered. Practically all the sulfur dioxide remaining was liberated when the pressure was released from the bomb. Increasing the initial oxygen pressure up to 40 atmospheres did not materially increase the amount of sulfur trioxide formed. The SO2 equaled the sulfur used, amount of sulfur in the SO3 indicating complete combustion. I n order to provide a gaseous atmosphere in the bomb, which might increase the rate a t which sulfur dioxide oxi-
+
6
Bradley, Rosecrans, and Corbin,
THISJOURNAL,
18, 307 (1926).
584
INDUSTRIAL A,VD E,VGINEERING CHEMISTRY
dizes to trioxide, 0.04 gram of ammonium nitrate was added to the 0.25 gram sulfur and combustions were run a t 20 atmospheres oxygen pressure. This would provide a nitrogen content approximately 5 per cent of the weight of sulfur, and under these conditions the resulting SO2 content varied from 12 to 20 per cent. Several determinations were made using 0.25 gram sulfur plus 0.04 gram ammonium nitrate plus 0.5 gram compressed benzoic acid a t 20 atmospheres initial oxygen pressure. KO sulfur dioxide could be detected in the resulting products of combustion. I n determinations on sulfur and benzoic acid without the addition of ammonium nitrate, mixtures of sulfur trioxide and dioxide resulted. Therefore, it seemed necessary to have nitrogen oxides and water vapor both formed in the combustions in order to get complete oxidation of sulfur dioxide to trioxide in these determinations. I n some determinations the ammonium nitrate was omitted and 5 atmospheres of commercial nitrogen were added with the 20 atmospheres of oxygen to the bomb. Complete oxidation of the sulfur resulted; however, when determinations were being made upon heavy oils the ammonium nitrate was preferable as complete combustion was more often obtained when ammonium nitrate was added. It was found that 0.011 gram of nitrogen was oxidized if calculated as nitric acid, when 0.5 gram of compressed benzoic acid was burned in the bomb with 20 atmospheres of oxygen and 5 atmospheres of nitrogen. Increasing the initial oxygen pressure by 5-atmosphere intervals up to 50 atmospheres did not materially increase the amount of nitrogen oxidized. Bituminous Coal
Several sulfur determinations were made on an Illinois coal of the following analysis: Per cent 44.60 43.95 11.45 4.88 74.8
Volatile matter Fixed carbon Ash Sulfur Total carbon B. t. u. Unit B. t.
12,433 14.370
U.
The addition of ammonium nitrate to the charge did not increase the amount of sulfur determined, and with this coal no sulfur dioxide could be detected in the gases when released, regardless of the rate a t which the bomb cooled down after combustion. Apparently, the nitrogen content of coal furnishes more than sufficient nitrogen oxides and the hydrogen compounds enough vapor to oxidize rapidly the sulfur dioxide to trioxide as the bomb cools down. When 1 gram of coal was oxidized under these conditions the maximum temperature developed was far in excess of that required completely to dissociate the sulfur trioxide. Allowing a heat loss of 50 per cent by the time the maximum temperature was attained, the maximum temperature was approximately 1800" C. These results are reported in another paper. Crude Petroleum
An Illinois crude petroleum of the following partial analysis: Specific gravity Unsaturated Sulfur B . t. u.
0 . 8 3 0 at 1.5 5' C 26 per cent 0 dU per cent 18,853
was used in determinations with and without the addition of ammonium nitrate. When ammonium nitrate was added the sulfur found was approximately 0.10 per cent greater than when it was not added. Moreover, better combustions were obtained when ammonium nitrate was added to the charge.
Vol. 18, S o . 6
A great many sulfur determinations were made on a heavy Mexican crude petroleum with a sulfur content of 4.94 per cent. I n the dry fusion methods from 25 to 50 per cent of the sulfur was lost. The sodium peroxide method in all instances gave slightly higher results than the oxygen bomb without the addition of ammonium nitrate. However, when ammonium nitrate was added to the charge in sufficient quantities to give a nitrogen content of 5 per cent of t h e sulfur present, the amount of sulfur determined was from 0.20 to 0.30 per cent higher than without ammonium nitrate and no sulfur dioxide was detected in the escaping gaseous products of combustion. Under these conditions the peroxide and oxygen bomb methods checked very closely on this high sulfur oil. It was also found that by the addition of ammonium nitrate or potassium chlorate to a charge of asphalt, low-temperature tar, or coal tar, complete combustion was more generally obtained. Light Oils
Crude benzene (light oil) in weights varying from 0.68 to 1.55 grams were sealed in thin-walled glass bulbs and these placed in an Illium crucible in such a way that the hot fuse wire caused the bulb to break and liberate the sample. T h e sulfur content checked a t 0.41 per cent when either a few drops of fuming nitric acid were placed in the bomb or some ammonium nitrate was added in the crucible. However, without these it was impossible to get check results, as much sulfur was liberated in the gases as sulfur dioxide, although theoretically there was more than sufficient nitrogen in the air in the bomb at the start to furnish the oxides deemed necessary to catalyze the oxidation to sulfur trioxide. Samples of commercial kerosene in weights up to 1.25 grams were oxidized by methods similar to the benzene experiments, but the results were so variable that larger samples were desirable. Accordingly, a thick-walled Pyrex bulb was filled with 2.25 grams of the sample and after being provided with a small ring of naphthalene (approximately 0.1 gram) around the capillary tube, i t was placed in the crucible in the bomb so that the fuse wire ignited the naphthalene, breaking the bulb. The sample oxidized with explosive violence, smashing the 0 to 200-atmosphere Bourdon gage which was attached. This part of the work is being continued and other methods of controlled oxidation for light oils give promise of making the oxygen bomb available for sulfur determinations on these lower boiling fractions. Summary of Results
1-Pure sulfur when burned in an oxygen bomb forms SOz and SOa in varying amounts. Increasing the initial oxygen pressure in the bomb up to 50 atmospheres does not materially affect the equilibrium. The bulk of the SO2 escapes when the pressure is released from the bomb. 2-The addition of compounds which form water vapor and nitrogen oxides during combustion to the sulfur charge provides an activating atmosphere in the bomb, under which the sulfur is completely oxidized to sulfur trioxide. 3-The addition of ammonium nitrate to a charge of heavy tars, pitches, and asphalts accelerates combustion and also promotes the sulfur trioxide formation. 4-Sulfur determinations upon coals, crude petroleums, crude benzene, and kerosene were satisfactorily carried out by the oxygen bomb method. Production of manufactured nonmetallic mineral products in Canada in 1925 reached a value of $115,587,316, an increase of nearly $4,500,000 over 1924 and the highest since 1920.
