Free Sulfur in Petroleum Distillates Effect of Peroxides upon the Copper-Strip Method. Quantitative Method for the Determination of Free Sulfur S. COMAY, 356 West 34th St., New York,
N. Y.
sulfur compounds. Free sulfur may also appear as an oxidation product of hydrogen sulfide either by air-oxidation or by the action of sulfur dioxide. The presence of free sulfur in gasoline finished by the so-called sweetening process is too well known in the petroleum industry to require elaboration. Free sulfur in any motor spirit presents a serious problem; it is very corrosive to many parts of the fuel system, as has been shown by Hoffert and Claxton (11) in their exhaustive investigation of the corrosive effect of elementary sulfur upon copper. Various methods have been proposed for the quantitative determination of free sulfur. The Ormandy and Craven method ( 1 4 , modified by the Institution of Petroleum Technologists and adopted as a British Standard Method (b), APPARATUS FOR DETERMlNATION OF yields accurate and reproducible results and is perhaps the FKLE SULFUR IN GASOUNE best so far proposed, although slow and tedious. The method is based on the conversion of free sulfur to mercuric sulfide by shaking with mercury. Recently Garner (10) proposed as a quantitative method for the determination of free sulfur FIQURE1. APPARATUS FOR DETERMINATION OF FREESULFUR refluxing the gasoline samples in the presence of copper bronze IN GASOLINE and then oxidizing the copper sulfide to sulfate by bromine. A . Inert gas supply I. Variable-speed motor The sulfate is determined as barium sulfate in the usual B. Reducing valve J . Trap (100-cc. Erlenmeyer) C. 3-Neck liter flask K. Three-way stopcock manner. D . Dropping funnel, 50 cc. L. Gas-washing bottle for absorbThe American petroleum industry has no generally used E. Mercury seal in,g hydrogen sulfide filled F . Condenser with ammoniacal cadmium method for the quantitative determination of free sulfur but G. Stirrer chloride solution H . Pulley M . G a s inlet tube uses the qualitative method of the American Society for Testing Materials ( I ) , known as the copper-strip method. That the copper-strip method is not reliable has been demonstrated in several cases by Hoffert and Claxton (11). pr'o corrosive REE or elementary sulfur is present in nearly all cracked effect upon the copper was observed with some samples of and straight-run gasolines, but its presence in crude oils benzene even in the presence of over 5 mg. of free sulfur per has not been established generally. 100 cc. These investigators attempt to explain the failure of the free sulfur to corrode under the conditions of the test A Pennsylvania crude oil, which had the usual sulfur content, by the presence of impurities, such as sulfur dioxide, dialkyl was analyzed and found to contain no hydrogen sulfide, free sulesters of sulfuric acid, sulfonic acids, etc., that act as inhibitors. fur, or mercaptans. When about 50 per cent of the crude oil had been taken over by fire-distillation, the overhead product That none of these compounds has any inhibitory effect has contained hydrogen sulfide, free sulfur, and a mercaptan or merbeen proved in actual tests. An untreated cracked gasoline captans in small amounts. Birch and Norris (..$) report that when that gave a positive corrosion test by the copper-strip method a distillate from maidan-i-naftun was heated to 120" C., free failed to do so after several weeks of standing. A considerasulfur appeared in the distillate. This finding, if true, is an extraordinary case and an exception t o the rule, unless the free tion of this phenomenon led to the conclusion that oxidation sulfur was formed by oxidation of hydrogen sulfide in the rewas the principal change that had occurred in the sample and ceiver. If free sulfur was present originally in the crude oil or, that its products were involved in the failure of the test. as these authors state, was a product of decomposition in the It was later found that no indication of corrosion was shown liquid phase in the still, it would not distill at such a low temperature and concentration. A gasoline to which 0.10 per cent of on the copper after several hours a t 100" by some oxidized sulfur had been added gave no free sulfur in the distillate when gasolines to which 100 mg. of free sulfur per 100 cc. of gasoline the vapor temperature had reached 150' C. It has been found; were added. But when the gasolines were treated with sulfur however, that sulfur does appear at a vapor temperature of 200 dioxide for 5 minutes, mashed with caustic soda, and tested if the distillation is conducted rapidly. The partial pressure of sulfur is relatively high at this temperature and the rapidity of with a copper strip, heavy corrosion and scaling occurred the distillation does not permit the sulfur to react completely after a few minutes a t room temperature. Brooks (6) to with the hydrocarbons. These distillations were conducted the author's knowledge was first to employ sulfur dioxide in through a Hem el column. detecting the presence of peroxides in gasoline. The same Hoffert and 8laxton (11) claim that free sulfur in motor benzene distills over when open steam is used at 100' C. Many results were obtained when the peroxides were reduced with steam distillations made by this author contest this assertion a water-alcohol solution of ferrous sulfate. A series of experistrongly. A gasoline t o which 100 mg. of free sulfur per 100 CC. ments was made, the more striking results of which are given were added gave no elementary sulfur in the overhead product in Table I. when it was steam-distilled at 150" t o 160' C. After this work was almost completed, a paper by Kiemstedt (12) reported that peroxides are inhibitory agents in the The occurrence of free sulfur in the products of distillation corrosive action of free sulfur on copper. Although peroxides of a sulfur-bearing charging stock is easily explained by the were found to be the inhibiting agents that affect the corresults obtained by Faragher, Morrell, and Comay (9) in rosive action of free sulfur upon copper, the mechanism their investigation of the thermal decomposition of organic
F
460
NOVEMBER 15, 1936
ANALYTICAL EDITION
of this phenomenon is not easily understood, especially in the light of the results obtained by Hoffert and Claxton (11) concerning the composition of the black deposit formed. They found that it consists almost entirely of cuprous and cupric sulfides. They report that in the absence of impurities that act as inhibiting agents the deposit from pure benzene and sulfur approximates cupric sulfide in composition, while those formed by the action of crude benzenes on copper contain a much larger proportion of the cuprous compound. Hoffert and Claxton say that this difference is “probably explained by the later discovery that the presence of other impurities in the benzene tended to inhibit the action of free sulfur on copper.” It is rather beyond possibility that peroxides, which are the true inhibitory impurities, should act as reducing agents in the reaction between copper and sulfur. It must be emphasized that the accumulations of peroxides will cawe the A. S. T. M. method, known as the copper-strip test, to fail to show the presence of free sulfur. On the other hand, the mercury test alone cannot be used as a qualitative test for free sulfur, since it was shown by Kingzett ( I S ) and Antropoff (3)that peroxides give a black deposit with mercury. It follows, therefore, that in order to show qualitatively the presence of free sulfur, the peroxides in the sample of gasoline must be reduced. The reduction is readily accomplished either by sulfur dioxide or an aqueous solution of ferrous sulfate to which alcohol is added. The sample may then be tested either with mercury or by the copper strip. It follows that the method proposed by Garner for the quantitative determination of free sulfur will fail in case of aged or oxidized gasolines, and if this method is used the gasoline must first be reduced to eliminate the peroxides. TSBLE 1. KO.of Expt.
INHIBITORY OF
EFFECTOF PEROXIDES SULFUR ON COPPER
Solution Sulfur Peroxide
Mg. M g . / l O O
Effect on Copper Strip Discoloration Discoloration
50
50
75
25
No discoloration 212O, F. No discoloration
4b
75
25
5a
87.5
12.5
1
100 50
3 4a
7
90
10
ACTION
Time
cc
None None
2
UPOK
212’ F.
at
Instantaneous Almost instantaneous After 2 hours
at
After 15 minutes
Slight spotty disooloration a t 2 ! 2 O F. No discoloration at
Discoloration
After 2 hours After 5 minutes
Instantanc
For reducing the peroxides, sulfur dioxide or a water-alcohol solution of ferrous sulfate was mentioned. The question arose of‘ using nascent hydrogen-subjecting the oxidized gasoline to the action of zinc and hydrochloric acid. The answer was soon obtained by an actual test with the oxidized sample of gasoline to which 100 mg. of free sulfur per 100 cc. were added and which failed to give the corrosive test in the copper-strip test. Kot only were the peroxides reduced but the free sulfur also was reduced to hydrogen sulfide. Therefore, this ready reduction of free sulfur by nascent hydrogen can be easily utilized for a quantitative method of free sulfur in gasoline or any other motor fuel.
Quantitative Method for Determination of Free Sulfur The principle underlying the newly proposed method for the quantitative determination of free sulfur is in reality based upon the discovery by Cloez (7) in 1858 that hydrochloric acid and zinc, aluminum, or iron convert free sulfur suspended in water into hydrogen sulfide. Cossa (8) obtained the same conversion of free sulfur, suspended in water, into hydrogen sulfide by electrolysis of the water.
461
It has been found that the action of nascent hydrogen in converting free sulfur dissolved in gasoline is very rapid and complete. Therefore, it was not difficult to devise an apparatus in which the hydrogen sulfide could be formed and driven out readily from the gasoline into an ammoniacal solution of cadmium chloride. Titration with iodine solution by the usual analytical procedure was used to determine the hydrosulfide. When zinc dust and hydrochloric acid were used, although all the sulfur present was converted into hydrogen sulfide and all the sulfide was driven out from the apparatus, low results were obtained in most of the tests made, especially when a large amount of zinc dust was used. This was found to be due to zinc sulfide suspended between the gasoline and acid layers. For this reason, iron powder, reduced by hydrogen, was used. The results are given in Table 11. TABLE11.
QU.4NTITY OF SULFUR
Sulfur Used
Sulfur Found
Jkfg.
Me.
10
10 10
9.92 9,83 9.92
5 5
4.98 4.96 4.95
5
DETERMINED BY REDUCTION Sulfur Used MU. 3 3 3
Sulfur Found Mg. 3.01 3.04 3.01
2 2
1.98 2.03
An experiment with a solution of 2 per cent di-isoamyl disulfide gave no hydrogen sulfide, but mercaptans mere formed as a result of the reduction.
Description of Apparatus The apparatus consists of a cylinder supplying hydrogen or nitrogen through A , connected by rubber tubing to a glass inlet tube, M , that nearly reaches the bottom of the flask, C. The stopper holds also dropping funnel D, leading almost to the bottom of three-neck round-bottomed flask C. Through the central neck of the flask the stirrer, G, supplied with mercury seal E is inserted; the remaining side neck of C is used for condenser F , connected by glass tubing to trap J (serving for trapping any gasoline that may be swept out from C), which is connected by glass and rubber tubing to three-way stopcock K and then to gas-washing bottle L.
Procedure The synthetic solutions of free sulfur were made in a naphtha that had an initial boiling point of about 135” C. Five grams of iron reduced by hydrogen (containing0.03 per cent of sulfur) were placed in three-neck flask C and the sample of naphtha was added. The apparatus was then closed, the stirrer, G, started, and hydrochloric acid (1 to 4) added in several portions through the dropping funnel, D. Usually, 40 cc. of the hydrochloric acid are more than enough. As soon as the acid covers the reduced iron, gas is evolved. The addition of the acid is stopped until the evolution of the gas ceases, then acid is added and the procedure repeated. After 10 or 15 cc. of acid have been added, the precipitate of cadmium sulfide in the gas-washing bottle, L, appears. After all the acid has been added the iron will have disappeared. Inert gas from A is allowed to bubble through the liquids in flask C, which are heated to about 80’ C. In order to determine whether or not the hydrogen sulfide has been driven out, gas is released through the cock, K , and is tested with cadmium chloride solution. The complete test requires less than 20 minutes. After the test is completed, the gasoline in C must be tested for free sulfur by shaking with mercury and for hydrogen sulfide with lead acetate paper. Obviously, running blank tests for the sulfur content in the reduced iron powder is a precaution not to be avoided. Cleaner’s naphtha free from elementary sulfur may be readily used in the above procedure for the determination of sulfur in the reduced iron. Since the percentages of free sulfur in petroleum distillates are small, it would be advisable to use the purest obtainable iron powder, reduced by hydrogen.
462
INDUSTRIAL AND ENGINEERING CHEMISTRY
I n order to utilize this method for determining elementary sulfur present in gasoline, the following procedure is suggested: Two hundred cubic centimeters of the gasoline to be tested are vacuum-distilled till about 50 per cent comes over. The sulfur determination is then made on the residue and the result obtained is calculated as milligrams of sulfur per 100 cc. of gasoline. Birch and Norris (3) stated that a copper-corroding substance is obtained when a petroleum distillate containing a mercaptan is treated with sulfuric acid. They found that this substance, dialkyltrisulfide, on reduction with zinc and hydrochloric acid, yields a mercaptan and hydrogen sulfide. Therefore, in order to analyze an acid-treated distillate for free sulfur, the quantity of hydrogen sulfide which will be formed by reduction of such substances must be deducted from the total amount of hydrogen sulfide. This can readily be accomplished by treating a sample of the distillate to be analyzed for free sulfur with mercury for the purpose of removing the free sulfur. The sample thus treated is then analyzed by the above described method for the corrosive trisulfide, and the amount of hydrogen sulfide obtained is then subtracted from the total hydrogen sulfide.
VOL. 8 , NO. 6
All the experimental data, with the exception of the results contained in Tables I and 11,were obtained in the laboratories of the Houdry Process Corporation a t Paulsboro, N. J. The writer expresses his appreciation for the permission to publish these results, and, especially, feels indebted to W. F. Faragher for his suggestion that peroxides must be the inhibitors which affect the corrosive action of sulfur on copper.
Literature Cited (1) Am. SOC.Testing Materials, D 130-30, Part 11, p. 488, 1930. (2) Antropoff, J . prakt. Chem., 77, 273 (1908). 21, 1087 (1929). (3) Birch and Norris, IND.ENQ.CHEM., (4) Birch and Norris, J. Chem. SOC.,127, 898 (1925). (5) British Standards, Standard Methods, 18 (1929). (6) Brooks, B. T., IND.ENQ.CHEM.,18, 128 (1926). (7) Cloez, Ann. Chena. Phys., 47, 819 (1858). (8) Cossa, Ber., 1, 117 (1868). (9) Faragher, Morrell, and Comay, IND.ENQ. CHEM.,20, 527 (1928). (10) Garner, J . Inst. Petroleum Tech., 17, 451 (1931). (11) Hoffert and Claxton, Second Report of National Benzole
Association.
(12) Kiemstedt, Petroleum Z.,28, No. 28, 1 (1932). (13) Kingzett, J. Chem. SOC.,12, 511 (1874). (14) Ormandy and Craven, J. Imt. Petroleum Tech.., 9,133 (1923). RECIPIYED
December 13, 1935
Metal Extractor for Laboratory Use J. M. LEMON, F. P. GRIFFITHS, AND M. E. STANSBY Technological Laboratory, U. S. Bureau of Fisheries, College Park, Md.
I
N T H E course of chemical and biochemical studies and analyses, it is frequently necessary to extract various quantities of such materials as wheat germ, casein, or protein meals. The usual glass apparatus found in laboratories is rather difficult to set up so as to form a continuous extractor. Soxhlet and other extractors are not ordinarily of sufficient size to permit use of large samples. McCay (2) and Bryant (1) have described apparatus which work well for relatively large amounts of material (20 to 50 pounds). As no simple and inexpensive apparatus for the continuous extraction of from 1to 5 pounds of material was found listed in various scientific catalogs, the extractor illustrated in Figure 1 was designed for use in this laboratory. Two are
now in use and have proved to be efficient and to require very little attention. A is a 20-quart cream-setting can, 9 inches in diameter and 20 inches high, supplied with a cover which fits closely over the top and 2 inches down the side. The condenser, B, consists of a 15-foot spiral of soft-copper tubing 0.25 inch in outside diameter and is soldered inside this cover, a proximately even with the bottom edge of the lid. In bending tiis ty e of tubing, it is necessary to fill it with some material which wilp revent its collapsing. in this instance, the tubing may first be slEd with fine dry Band and the ends tightly stopped. After the bending is completed, the sand is easily removed by lightly tapping the tubing and revolving the coil at the same time. The container for the material, C, is made by removing the bottom of a £ ether can. A co per siphon tube, D, is soldered through the cap of the can anzbent so that the top of the siphon loop is about two-thirds up the side of the container. The end of the siphon extends about 2 inches below the side of the can. The extractor is supported by an ordinary iron Iaboratory tripod, the legs of which have h e n cut to a length of 7 inches. The tripod is kept in place by three U-strips of tin soldered to the can.
A wire gauze is placed in the bottom of container C and over this is laid a layer of cotton; C is then filled with the material to be extracted and placed inside the large can, A . About one and one-half tunes as much solvent is added as is necessary for the operation of the siphon. After setting the Iid in pIace and starting a good flow of water through the coil, the can is set on a three-heat, 600-watt electric heater. When using alcohol or acetone the heater is turned on high, but with ether i t is advisable to use medium or low heat. The cost of materials for constructing this apparatus, not including the heater, is less than three dollars. FIGURE 1. EXTRACTION APPARATUS A. Cream-setting can B. Cover with oopper coil C . Inside container for material D. Siphon tube The apparatus assembled for use is shown on the right.
Literature Cited (1) Bryant, L. R., IND.ENQ.CHEM.,And. Ed., 1, 139 (1929). (2) MoCay, C. M., Ibid., 5, 219 (1933). RECEIVED
June 3, 1938.
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