Detection of Elemental Sulfur in Gasoline by the Sommer Test GEO. E. MAPSTONE, National Oil Pty., Ltd., G l e n Davis, N e w South Wales, Australia
9
tense as that produced by the addition of the pyridine to the gasoline-caustic soda mixture. Pyridine-Gasoline Ratio. Various volumes of a 0.001 yo (wt./w) solution of free sulfur in gasoline were added to 5 cc. of pyridine and 1 cc. of 2 N caustic soda solution was added and shaken. The results were:
The sensitivity of various tests for the detection of elemental sulfur in gasoline has been determined; and a modification of the Sommer test is described which i s about five times as sensitive as the customary inverted doctor test using butyl mercaptan.
Test positive Test strongly positive Test maximum color
S
OMMER ( 1 ) describes a color test for free sulfur in pyridine
solution, in which the addition of a small amount of an alkaline solution precipitates the sulfur as a blue colloidal solution. A positive reaction was obtained with as little as 2 parts per million of free sulfur; but hydrocarbons were observed t o decrease the sensitivity of the test. I n the presence of pyridine, doctor solution gave a visible reaction with 10 micrograms of free sulfur per cubic centimeter (10 p.p.m. on w/v basis). Sommer's test was applied to the detection of free sulfur in gasoline by mixing the sample with pyridine and adding caustic soda solntion. I n the test, three phases were'formed: an upper gasoline phase, a middle pyridine phase, and a lower aqueous phase. The color appeared in the middle pyridine phase, occasionally in the lower aqueous phase, but never in the upper gasoline phase. A preliminary survey showed that this test was a t least as sensitive as the inverse doctor test, using butyl mercaptan solution. This is customarily used in plant testing but has, a t times, appeared t o be insufficiently sensitive. The alternative mercury test is generally considered too sensitive for plant control. The conditions for the maximum sensitivity of the Sommer test were determined, and the sensitivity of the various tests was compared.
2 to 20 cc. of gasoline 3 to 15 cc. of gasoline 7 to 9 cc. of gasoline
Pyridine-Caustic Sods Ratio. Different volumes of 2 N caustic soda solution were added to a mixture of 3 cc. of pyridine and 5 cc. of O.OOl7, (w/w) solution of sulfur in gasoline. The t,est was definitely positive with up to 1.6 cc. and questionable with 2.0 cc. of caustic soda solut'ion. The maximum color was obtained with 0.3 cc., though there was very little difference between t'he colors produced with 0.3 to 0.6 cc. The maximum was obtained with the same pyridine-caustic soda ratio (10 to 1) as uwd by Sommer. S a t u r e of Gasoline. The condit'ions of the test (5 cc. of gasoline, 3 cc. of pyridine, and 0.5 cc. of 2 N caustic soda solution) and t'he sensit,ivity were unchanged with 100% cracked gasoline (from the thermal cracking of shale oil). KOhighly aromatic gasoline was available, so mixtures of motor hcnzene and commercial iso-octane were examined. It was found that thc proportion of t,he middle pyridine phase decreased as the benzene content of the blend increased but the sensitivity of the tcst appeared to be unchanged. Sixty per cent of benzene in the blend the maximum that could be tested without changing the conditions of the test. Increasing the amounts of pyridine axid caustic soda and decreasing the caustic soda concentration onablcd the tcet to viork satisfactorily Tvith higher benzene blends:
EXPERIMENTAL
cc.
cc.
Caustic Soda Solution
5 5 5
4
1.5 2 N 2 2N 2 N
Volume of Blend
70 60
70 80
REAGENTS USED.Gasoline. For the purposes of this investigation, except as noted below, commercial iso-octane of specific gravity 0.691, freed of possible traces of sulfur and filtered before use, was used as gasoline a n d is referred to as such. The commercialiso-octane, asreceived, was shakenwith mercury t o remove possible traces of free sulfur and filtered before use. Only a negligible amount of mercuric sulfide was formed. Standard Sulfur Solution, prepared by dissolving flowers of sulfur in 1 cc. of hot pyridine and allowing the solvent to evaporate almost to dryness before adding sufficient gasoline to give a solution containing O . O l ~ ,of sulfur by weight'. Other concentrations were obtained by diluting the original standard with gasoline. Pyridine. A water-white, commercial grade of pyridine containing approximately 5 % of alpha-picoline was used untreated as i t was not discolored on shaking with mercury. Butyl h.lercaptan Solution, 1 cc. of n-butyl mercaptan dissolved in 1400 cc. of gasoline, thoroughly shaken with mercury, and filtered before use. This treatment satisfactorily removed any free sulfur present, and did not affect the mercaptan content of the solution. Doctor Solution, 10% caustic soda solution shaken with excess litharge for a quarter of a n hour, allowed t o settle, and the clear supernatant liquor decanted for use. Caustic Soda Solution, 2 N . Sodium Bicarbonate Solution, cold saturated. SOMYER TEST.The necessary conditions for the test to give m a n m u m sensitivity were obtained as follows: Alkaline Solution. Sommer used 2 N caustic soda and saturated sodium bicarbonate solutions and obtained greater sensitivity with the latter. However, in the presence of gasoline, the caustic soda solution was found t o give several times the sensitivity of the sodium bicarbonate solut'ion, which also required boiling t o develop the color. The caustic soda solution was therefore used. Order of Mixing. The addition of the caustic sodasolution to the pyridine-gasoline mixture gave a color several times as in-
Volume of Pyridine
Benzene in Blend
.
5 5
cc.
Blends containing much more than 80% benzme did not give three phases in the test', though a blue color was developed in the pyridine-gasoline phase. The sensitivity of the test, was, however much reduced. \;C?iththe benzene blends, the blue color was less stable to air and was destroyed by shaking for a minute or so. TESTSCOMPARED FOR SENSITIVITY. I n order t o compare the relative sensitivity of the different tests available, the following tests were carried out: Inverse Doctor Test. Five cubic centimeters of the sulfurcontaining gasoline were shaken with 5 cc. of butyl mercaptan solution and 5 cc. of doctor solution. I n the absence of free sulfur, the lead butyl mercaptide formed colored the gasoline phase yellow and some yellow mercaptide collected a t the interface. In the presence of free sulfur, the color deepened through orange to dark brown or even black, depending on the concentration. Owing to the color of the mercaptide, it was necessary to use a blank i est for comparison with borderline tests. Modified Inverse Doctor Test'. Five cubic centimeters of the sulfur-containing gasoline were mixed with 5 cc. of pyridine and 5 cc. of butyl mercaptan solution before addition of 5 cc. of doctor solution. The addition of the pyridine prevented the color formation due to the lead mercaptide. Three liquid phases were formed and, in the presence of sufficient free sulfur, the lead sulfide formed collected mainly a t the liquid phase interfaces, making this test easier to observe than the unmodified test. Sommer Test. Five cubic centimeters of the sulfur-containing gasoline were mixed with 3 cc. of pyridine and then shaken with 0.5 cc. of 2 N caustic soda solution. I n the presence of free sulfur, a sky-blue coloration was formed in the middle phase. On long shaking in the presence of air, the color disappeared, this being due to the oxidation of the colloidal sulfur t o thiosulfate. Mercury Test. Five cubic centimeters of the sulfur-containing gasoline were shaken with a little mercury. I n the presence of free sulfur, black mercury sulfide was formed and the suspension colored the gasoline gray. 498
499
ANALYTICAL EDITION
August, 1946
OTHERAPPLICATIUK OF TESTS. ilttempts to use purified pyridine homologs (boiling range 120” to 250” C.), isolated from the cracked pressure dist,illate, in place of pyridine in the Sommer test failed. This was due to the higher miscibility of these bases with the gasoline, and the failure of the mixture to separate into three phases on the addition of water. This is probably the desensitizing action of hydrocarbons on the test that was observed by Sommer. An attempt was made t o apply these different tests to detect free sulfur in crude shale oil, but alkali-soluble coloring matter in the oil masked any color reactions. ’
RESULTS
The results are expressed in sulfur concentration in parts per million by weight required t o give a definite, questionable, or negative test. With the standard solutions used, 1p.p.m. (TV/W) is equal to 0.7 microgram per cubic cent’imeter. Test Inverse doctor test Inverse, modified b y presence ofpyridine Sommer test Mercury test
Definite P.p.m. 20 15
4
Questionable P.p.m. 15
Negative P.p.m. 10
10 8 3 2 Positive even below 0.1 p.p.m.
The mcrciiry test is by far the most sensitive, but it is too sensitive for plant control. The inverted doctor test is posit’ive
to 15 p.p.m. of free sulfur, but the color of the lead mercaptide tends to make detection difficult near the limiting concentration; in the presence of pyridine, the modified test is ? a s h to observe and a shade more sensitive. The Sommer test is sensitive to 3 p.p.m. of free sulfur in the gasoline, being therefore about five times as sensitive as the inverted doctor test, and yet not too sensitive for plant control. With experience, a plant operator is able t o obtain an approximate estimate of the free sulfur content of the gasoline from any of the first three tests, but much more readily from the Sommcr test. The Sommer test has been found both quicker to operate and easier to observe than the inverse doctor test previously ured. If the gasoline being tested is highly aromatic, i t would br advisable to determine the optimum proportions of the reagents for the test before putting it into routine operation. ACKNOWLEDGMENTS
The author wishes to acknowledge with thanks the permission granted by the management of National Oil Pty., Ltd., to publish this paper and the assistance of -4.L. Kremer in carrying out the work. LITERATURE CITED
(1) Sommer, H., IND. ENG.CHEM.,AXAL.ED.,12,368-9 (1940).
Qualitative Test for Carbohydrate Material ROMAN DREYWOOD’, Paper Service Department, Eastman Kodak Company, Rochester, N. Y
A solution of anthrone in concentrated sulfuric acid gives a permanent green coloration ‘with carbohydrate material. The reaction is o f value as a qualitative test and for the preliminary classification of synthetic resins into a cellulose or noncellulose group.
A
LTHOUGH there are a large number of specific tests for certain types of saccharides (1, 2 ) , there are very few general tests for carbohydrate materials. The Molisch test, using 0naphthol, is well known and generally applicable to soluble carbohydrates. Anthrone, which is used for the determination of glycerol ( d ) , was found to give a green color with cellulose. Further experiments indicated that a positive test was obtained with all of a group of eighteen carbohydrate materials examined, including several cellulose derivatives. Furfural is the only noncarbohydrate material, thus far encountered, which gives a green color with anthrone. The test as given by furfural is, however, different from that given by carbohydrate materials. The green color given by a furfural test is rapidly obscured by a brqwn precipitate, and when the sample isdiluted with 50% sulfuric acid or glacial acetic acid, a heavy brown precipitate forms. Carbohydrate samples, on the other hand, may be diluted to any extent with these reagents, and the green color persists even a t extreme dilutions. Positive Test with Anthrone Cellulose Starch Dextrin Dextrose 1-Arabinose G u m arabic G u m tragacanth Agar Pectin Algin
Ethvlcellulose (ether) l I e t h \ lcellulose (ether) Cellulose acetate Cellulose acetate phthalate Cellulose acetate butyrate Cellulose acetate stearate Cellulose propionate Cellulose nitrate Furfural (noncarbohydrate)
A negative test was obtained by all of a large group of noncarbohydrate materials examined, which included a variety of noncellulose synthetic resins, organic acids, aldehydes, phenols, fats, terpenes, alkaloids, and proteins. PROCEDURE
One milliliter of water is placed in a small test tube containing approximately 1 mg. of the material to be tested, and 2 ml. of a 1
Present address, Tower Drug & Chemical Company, Rochester, N. Y.
0.2% solution of anthrone in concentrated sulfuric acid are then added. The final sulfuric acid concentration in the test solution should always be greater than 50%; otherwise the anthrone %rill come out of solution and produce a milky suspension. The heat produced by the dilution of the sulfuric acid is a necessary part of the test. I n the presence of carbohydrate material a clear green color iTill appear and rapidly increase in intensity until a dark blue-green solution result’s. The test solution can be diluted for comparison with glacial acetic or 50y0 sulfuric acid, I n the absence of carbohydrate material, but in the presence of other organic compounds, a brown color is often produced by t,he action of Dhe concentrated sulfuric acid. The anthrone, which is not a readily available chemical, can be prepared according to the directions of Schutz (4), or following the procedure given in Organic Syntheses (3). Care should be taken during the preparation to avoid contamination of the anthrone by carbohydrate material, especially filter paper pulp, which would cause a green color to develop in a blank test with the reagent. DISCUSSION AND APPLICATIONS
S o study ivas made of the mechanism of the reaction, but t lie appearance and deepening of the color seemed to be as rapid with a polysaccharide as with an equal weight of a monosaccharide. This would suggest t,hat hydrolysis may not be a neccssary stvp in the test. -1useful application of this reaction is the identification of synthetic resins. Even t,he most insoluble cellulose resins TTill give a poaitive test with anthrone, thus affording a preliminary classification into a cellulose or noncellulose group. However, many tic molding compositions contain n-ood flour and would giw a tive test for that reason. .-In attempt’ is now being made to apply this reaction to the qiiantitative colorimetric determination of small quantities of ccllulose in solution, particularly to the analysis of p- anti ycellulose. The anthrone test is extremely sensitive. I n tests with starch it proved t o be 10 to 40 times as sensitive as iodine for the detection of this carbohydrate. Approximately 1 part of starch in 900,000 parts of viater can be detected. LITERATURE CITED
(1) Browne, C. A., and Zerban, F. W., “Sugar Analysis”, p. 641, Sew York, John Wiley & Sons, 1941. (2) Degering, E. F., “An Outline of the Chemistry of Carbohydrates”, Chapter XVII, John S.Swift Co., 1943. (3) Gilman, H., and Blatt, A. H., editors, “Organic Syntheses”, Collective Vol. I, p. 60, New York, John Wiley & Sons, 1941. (4) Schutz, F..Papier-Fabr. (Tech. TI.,. 36,55 (1938).
