Polarographic Determination of Elemental Sulfur a i Petroleum Fractions MAYNARD E . HALL Humble Oil and ReJining Company, Baytown, Tex.
A polarographic procedure is described for the determination of elemental sulfur in petroleum fractions. Evaluation with synthetic mixtures has shown the method to be accurate to *2% of the elemental sulfur content in the range of 1 to 100 p.p.m. The method is rapid (about u) minutes per sample), sensitive, and free from interference from organic sulfides, disulfides, and thiophene.
T
H E corrosiveness of elemental sulfur to automobile engine parts is well known, and a great deal of care is exercised by petroleum refiners to niinimke free sulfur in their products. In doctor sweetening careful control of free sulfur addition must be maintained to avoid production of a corrosive gasoline. A number of analytical methods have been proposed for determining the free sulfur content of gasoline and other hydrocarbons, but a survey of publications on the determination of elemental sulfur indicates that there is no recognized quantitative method for the direct determination of small quantities of elemental sulfur in hydrocarbons. The polarographic procedure described in this report was developed for the determination of free sulfur in the gasoline fractions of petroleum, although it is expected that it could be applied to the measurement of elementalsulfur in liquefied petroleum gases or heavy hydrocarbons. The method is rapid (about 20 minutes per sample), sensitive, and free of interference from organic sulfides, disulfides, and thiophene in gasolines. Evaluation with synthetic mixtures has shown the method to be accurate to *2y0 of the elemental sulfur content over the range from 1to 100 p.p.m. In surveying the literature for information pertaining to the qualitative and quantitative estimation of elemental sulfur, it was found that several A.S.T.M. methods ( I , $ , 4)make use of the copper strip corrosion test for detecting elemental sulfur. Another A.S.T.M. method ( 3 ) employs the mercury corrosion test to detect free sulfur. hlapstone ( 7 ) in a study of the Sommer test, the inverse doctor test, and the mercury corrosion test for the qualitative detection of free sulfur in gasoline, for plant control work, found that an approximate estimate of the free sulfur content of gasolines could be obtained by any of the three tests, but much more rapidly and easily by the Sommer test. Probably the most often used method for the quantitative determination of free sulfur in appreciable amounts is the butyl mercaptan (butanethiol) or inverse doctor test described by Wirth and Strong (10). In a study of the methods for determining elemental sulfur, Ball (6) found the butyl mercaptan test to be the best of the existing methods, but concluded that even this method was subject to inaccuracy when employed for low concentrations of elemental sulfur in hydrocarbons. In a recent paper Morris, Lacombe, and Lane (8)describe a method for the quantitative determination of elemental sulfur in aromatic hydrocarbons. The method is based upon the reaction
S
+ SaZS03+Xa2SzO?
but is satisfactory only for free sulfur in the range of 0.1 to 20%. Morris, Lacombe, and Lane studied bothrthe so&um sulfite and the butyl mercaptan methods for determining very small amounts of free sulfur and found both methods unsatisfactory for determining elemental sulfur in the range 0 to 100 p.p.m. They concluded that no reliable method appears to exist for the determination of elemental sulfur in this low concentration range.
Proske (9) published a polarographic procedure for the qvantitative determination of free sulfur that he used in connection with studies on the vulcanization of rubber. Proske extracted the free sulfur from the rubber with pyridine and ran a polarogram on the pyridine extract. The electrolyte solvent consisted of acetic acid, sodium acetate, and tylose. The procedure developed independently and discussed herein differs from Proske's method mainly in the nature of electrolyte-solvent employed. Inasmuch as free sulfur is not readily extracted quantitatively from petroleum fractions, an electrolyte-solvent consisting of methanol and pyridinium hydrochloride was chosen because of its miscibility with hydrocarbons. APPARATUS AND REAGENTS
A Sargent Model XXI recording polarograph was used in developing the method. An H-type electrolysis cell was employed, and a saturated calomel electrode was used as the reference electrode. The capillary constants for an open circuit were t = 4.0 seconds (drop time)
M
= 1.76 mg. per second (rate of flow of mercury) M z / 3 1 1 ' 6 = 1.64mg.z/3sec.1/6
The mercury column, h, was 67.0 cm. All experimental measurements were made in an air-conditioned room with the temperature held constant to 25" * 0.5"C. One hundred milliliter flasks were used to dilute tbe unknown sample to volume with the electrolyte-solvent. Baker's C.P. methanol, pyridine, and concentrated hydrochloric acid were used to prepare the solvent and none of the reagents required any further purification. Powdered monoclinic sulfur of better than 99.9% purity was used for the preparation of standard solutions. The electrolyte-solvent is prepared by mixing 90 ml. of methanol, 9.5 ml. of pyridine, and 0.5 ml. of concentrated hydrochloric acid. The pyridine and pyridinium hydrochloride form a highly buffered solution that has a pH of 6 as measured by a Beckman p H meter. A standard solution of known sulfur content is prepared by dissolving weighed amounts of the monoclinic sulfur in the solvent. The monoclinic form of sulfur is used because it is more soluble than other forms of sulfur, and also because the monoclinic form is used in doctor sweetening processes for gasolines. METHOD
Calibration. Because it is doubtful that the present chemical methods are reliable for accurate determinations of free sulfur in very low concentrations, synthetic solutions of known amounts of free sulfur were used as standards for plotting diffusion current against sulfur concentration as shown in Figure 1. (In refinery laboratories it is often desirable to express concentration of sulfur as milligrams per unit volume, and by expressing concentration of sulfur in such a way the density of the liquid in question does not have to be determined.)
1137
ANALYTICAL CHEMISTRY
1138
Analysis of Samples. The size of sample required for analysis dcpends on the free sulfur concentration; however, for petroleum fractions containing 5 to 10 p. .m. of free sulfur, 20 ml. of the sample to be analyzed are ad& to a 100-ml. volumetric flask and diluted to volume with the solvent. If the elemental sulfur content of the sample is approximately 1 or 2 p.p.m., the size of the sample should be increased to produce a diffusion current large enough to provide a determination that is *2% accurate. However, in the case of motor gasoline, the maximum amount of sample thht can be analyzed is usually about 40 ml. because this is the limit of miscibility of the gasoline and solvent. The solutions in the flask arc wcll mixed and then added to the electrolysis ccll. The solution is bubblcd for 5 minutes with an inert gas before the reduction wave is determined. The inert gaa should be bubbled through methanol prior to assing through the sample to prevent excessive evaporation of tKe sample solution. The amount of free sulfur present in the sample will determine what damping position and sensitivity setting should be used. The wave height multiplied by the sensitivity setting gives the diffusion current in microamperes. A calibration curve, such as Figure 1, is then used to obtain the concentration of free sulfur in the sample. DISCUSSION OF EXPERIMENTAL WORK
The principle of the polarographic method is based upon the measurement of the diffusion current that is produced when elemental sulfur is reduced a t the dropping mercury electrode. The number of electrons, N , involved in the electrode reaction was determined by the equation:
Ed. = E l / z -
log .-
ad
i
-Z
(6)
firient of elemental sulfur was calculated to be 2.2 x 10-5 cm.l sec.-l a t 25” C. in the methanol-pyridine solution. Because the method was to be applied mainly to petroleum samples, one of the major problems was to find a satisfactory solvent for the gasoline, sulfur, and a supporting electrolyte. A t first, mixtures of various ratios of pyridine, water, and concentrated hydrochloric acid were tried, but wwe found unsatisfartoiy mainly because not enough gasoline would blend with the electrolyte solvent to permit an accurate determination bf sulfur in the concentration range of 1 to 3 p.p.m. in the gasoline; a t least 30 ml. of the gasoline sample in a 100-ml. blend are necessary for most accurate work in this concentration range. Mixtures of various amounts of the low molecular weight alcohols with pyridine and concentrated hydrochloric acid were investigated as solvents, and the most suitable combination found for mixing with gasoline was the mixture of methanol, pyridine, and conrentrated hydrochloric acid already described. The methanol serves as a solvent for gasoline and the electrolyte; the pyridine serves as a solvent for free sulfur. The hydrochloric acid reacts with part of the pyridine to form pyridinium chloride which is the supporting electrolyte, and also helps form a highly buffered solution with a pH of 6. Using an electrolytesolvent of this composition, as much as 40 ml. of gasoline will blend with 60 ml. of the solvent.
Table I.
which is the fundamental equation of the polarographic wave. I t can be seen from the above equation that a plot of log i/(id i) versus Edeshould be a straight line with a slope of 0.0591/1V volt.
-
Free Sulfur in Synthetic Solutions Sulfur Taken, P.P.M.
Blend ?.io,
Sulfur Found, P.P.hl 1.02
.
1.00 5.00
yo Error +2.0 -1.4 -2.0
4.93 9.8
10.0
25.0
2:; -0.7
25.6
50.0
80.5 99.3
100,o
Average % error
*1.6
P m
,so
3
+-
A brief study of the effect of changes in pH revealed that the electrode reaction should be carried out below a p H of 7 . ’ Above pH 7 the reduction wave spread out over a range of 1 volt or more and was unsatisfactory to use for calculating the sulfur concentration. At a pH of 6 or lower, well defined waves were obtained with a half-wave potential of -0.50 volt versus the saturated calomel electrode.
24
20
I6
5
ID
3 U
Z
I
z
4
P Y
Table 11. Reproducibility of Polarographic Method for Determining Elemental Sulfur in Petroleum Fractions
n 0
0
SULFUR, P. P. M.
Figure 1. Diffusion Current us. Parts per Million Sulfur in Methanol-Pyridine Solution
Such a plot was made from the polarogram obtained with a Model XXI Sargent recording polarograph of a 10 p.p.m. sulfur solution; the points of the log plot formed a straight line with a slope of 0.031 volt which is in good agreement with the theoretical value, 0.0296 volt, for A’ = 2. On the basis of the determination that 2 electrons are involved in the electrode reaction, it is believed that the reduction of sulfur occurs according to the following equation:
S + 2 H + + 2e-
+H2S
From the magnitude of the diffusion current and by using the IlkoviE equation ( i d = 605 NDl/*Crn*/%1’6),the diffusion coef-
Detn. NO.
1
2 3
4
2-M~thylheptane Sulfur Found, P.P.M.
Kerosene Sulfur Found, P.P.M.
100
10.1
10.0 10.0
9.9 Av. 10.0
103
*
0.05
100 101 101 * 1
Because there appear to be no reliable chemical methods for the analysis of free sulfur in gasoline on which to base calibrations in very low concentration ranges, synthetic standard solutions were used for plotting diffusion current versus concentration of sulfur as shown in Figure 1. The diffusion current is directly proportional to the concentration of sulfur over a range of 0 to 100 p.p.m. Experimental data showing the accuracy and precis,ion of the method are given in Tables I and 11. Several organic sulfur compounds were added to synthetic. and plant gasoline samples to determine if such compounds interfered with the analysis of the.free sulfur. Butyl sulfide, propyl di-
V O L U M E 22, NO. 9, S E P T E M B E R 1950 sulfide, and thiophene offered no interference. When a mercaptan was added, the sulfur was consumed in forming a disulfide. The rate of consumption of the free sulfur by a mercaptan can' be followed with the polarograph. HydTogen sulfide, if present, will be removed upon bubbling.
1139 The materials of higher molecular weight appear to retain more sulfur. This is probably a kolubility effect rather than a variation in plant process. The method should be applicable to aqueous solutions as well as to hydrocarbons. LITERATURE CITED
Table 111. Elemental Sulfur Content of Gasoline Fractions and Kerosene after Doctor Sweetening (Determined by polarographic procedure) Sample s, P.P.M. 7 Light crude naphtha Heavy crude naphtha 10 Light cracked naphth;8 , high octane 4 101 High sulfur refined oil (kerosene)
APPLICATIONS
The polarographic method for elemental sulfur has been applied in connection with studies of different gasoline sweetening processes. Table 111 shows the concentration of sulfur found in various gasolines and kerosene after doctor sweetening.
(1 ) .\m. ?oc. Testing Materials, "Specification for Petroleum Spirits (Mineral Spirits)," D 235-39. (2) .Zm. SOC. Testing Materials, "Specifications for Stoddard Solvent," D 454-40. (3) Ani. 50c. Testing Materials, "Standard Method of Sampling and Testing Lacquer Solvents and Diluents," D 268-44. (4) Am. Soc. Testing Materials, "Standard Method of Test for Detection of Free Sulfur and Corrosive Sulfur Compounds in Gasoline." D 130-30. ( 5 ) Ball. ,J. Y., U. S.Bur. Mines, R e p t . Itmest. 3591 (1941). ( 6 ) Kolthoff. I. M., and Lingane, J. J., "Polarography," rev. reprint, p. 144, New York, Interscience Publishers, 1946. (7) Mapstone, G. E., IND. ENG.CHEM.,.-INAL. ED..18, 498-9 (1946). . (8) Morris, H. E., Lacombe, R. E., and Lane, Vi. H., A N . ~ LCHEM., 20, 1037-9 (1948). (9) Proske, G.. .4q7ew. Chem., A59, 121-2 (1947). (10) Wirth, C.. 111. and Strong, J. R., IND. ENG.CHEH.,ANAL.ED., 8, 344-6 (1936).
RECEIVED .4pril 20, 1950.
Copper Contamination and Ascorbic Acid loss in Waring Blendor MERTON P. LAXIDEN College of M e d i c i n e , University of V e r m o n t , B u r l i n g t o n , V t .
The Waring Blendor, which has extensive usage in food analysis, vitamin assays, and biochemical preparations, is regarded with disfavor by some workers. Loss in ascorbic acid and loss in enzyme activity during blending have been cited. Adverse results may arise from use of containers having worn chrome plating on their blending assemtiies. Copper dissolving from exposed brass parts may catalyze the loss of ascorbic acid in solutions' during blending or create false high results in the determination of the copper content of certain foodstuffs. Inhibition of the destructive effect of copper dissolved from the blending assembly on ascorbic acid is discussed.
T
HE Waring Blendor is an exceedingly popular and useful tool
for comminuting or homogenizing material in the laboratory, especially for food analysis, vitamin assays, and biochemical preparations (6). There are few reports concerning limitations on its use. The Blendor has been recommended for ascorbic acid determinations in tissues (1, 6, 8, 12), although Roe et al. (If) advise against use of any homogenizer that would introduce increased amounts of oxygen into the slurry. Stern and Bird ( I S ) found that treatment of wheat germ and mill stream suspensions in the Waring Blendor caused oxidation of sulfhydryl groups and inactivation of enzyme S.StemS.
Divergent results and conclusions in work involving use of the Raring Blendor in some instsnces may be traced to the indiscriminate use of containers. This study was initiated as a result of discrepant data on the ascorbic acid content of identical solutions blended in different containers. Inspection of a container in which significant losses of ascorbic acid occurred showed that
the chrome plate on parts of the metal blending assembly was worn, exposing the brass undersurface. Brass contains a high percentage of copper, which is' a known catalyst for the oxidation of ascorbic acid ( $ 3 , 7 ) . This paper shows that blending ascorbic acid solutions in different contajners resulted in widely varying losses in ascrobic acid, which can be attributed to the dissolution of copper during the blending procedure, EXPERIMENTAL PROCEDURE AND DISCUSSIO'Y
Two hundred milliliter portions of a solution of ascorbic acid (20 micrograms per ml.) in 5% metaphosphoric acid (HPOI) initially a t room temperature (23" C.), were blended for 3 minutes in triplicate in each container, The average t e p erature rise in the solutions a t the end of the blending was 8.7 with a low of 6.0" C. and a high of 12.0" C. The small differences in temperature rise were not reflected in the results. Ascorbic acid was determined by the 2,4-dinitrophenylhydrazine method ( 11 ) on aliquots of all blended solutions and nonblended controls. The copper content was determined by a dithizone method not subject to interference by other metals (9). The results from triplicate runs with a single Waring Blendor container were not in agreement where the loss in the first blending was high. Instead, these triplicate runs seemed to form a pattern in which there was progressive lowering of ascorbic acid loss with the two succeeding blendings. This phenomenon of lowered ascorbic acid loss with successive blendings was further investigated. Using only one container (No. 6 ) nine consecutive 3minute blendings of 200-ml. portions of ascorbic acid (20 micrograms per ml.) in 5% metaphosphoric acid were carried out. The ascorbic acid and copper conterlts were determined in each solution immediately after blending, as well as in an unblended control. This experiment was then repeated in the same container scrupulously cleaned, after an interval in which the container had general laboratory use in blending various materials.
8.
The values determined for ascorbic acid loss and copper concentration in all experiments are shown in Table I, and the correla-
