Determination of Sulfur Truces in Naphthas by Lamp Combustion and Spectrophotometry R. W. KLIPP and J. E. BARNEY II Research Deparfmenf, Sfandard Oil Co. (Indiana), Whiting,
b A
w.m. SULFIIII
sensitive method for determining
1 to 400 p.p.m. of sulfur in naphthas by lamp combustion and spectrophotometry has been developed. Ten grams of naphtha is burned in the conventional ASTM lamp combustion apparatus, and the sulfuric acid formed is determined spectrophotometrically with barium chloranilate. A buffer of sodium acetate and acetic acid simplifies the spectrophotometric determination by eliminating the need to adjust pH. The two absorption peaks of chloranilic acid solutions a t 330 and 530 mp give satisfactory sensitivity for the entire concentration range without the use of aliquots. Compared to the widely used combination of lamp combustion and turbidimetry, the new method is more precise, a t least as accurate, and four times as sensitive.
T
most widely used method for determining sulfur in naphthas ( I ) burns the naphtha in a specially designed lamp and absorbs the vapors in 1.5y0 hydrogen peroxide. The resulting sulfuric acid is determined acidimetrically by titration with standard sodium hydroxide solution, or gravimetrically by precipitation as barium sulfate. These methods lack sensitivity below 100 p.p.m. Trace quantities of sulfur usually are determined by a turbidimetric method, but skill is required to obtain good precision. Cryst,al size of the barium chloride used to obtain the suspensions must be carefully controlled, and the suspensions themselves are inherently unstable. A more sensitive, simpler, and more reliable method would be desirable. A spectrophotometric method for sulfate (a, 3) appeared attractive as a means of determining trace amounts of sulfate ion in absorber solutions. In this method, solid barium chloranilate reacts with sulfate ion in buffered 5Oy0 ethyl alcohol solution to release the strongly absorbing acid chloranilate ion. Metallic ions, which would interfere in the spectrophotometric method, are not present. Two absorption peaks of chloranilic acid solutions a t 530 and 330 mp provide satisfactory sensitivity; intensity of the peak a t 330 mp is approximately 20 times that a t 530 mp, HE
596
ANALYTICAL CHEMISTRY
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Figure 1.
buffering capacities
Intensity at bct,h wave lengths is proportional to sulfate ion concentration, provided pH iri carefully controlled. I n the spectrophotometric method for determining silfate ion, a 0.005M potassium acid phthalate solution provides relatively low buffering capacity (2). Experiments indicated this buffer was unsatisfac m y because sulfate is present in the absorber solutions as sulfuric acid, :tnd the adjustment of p H mas time consuming. A buffer with much gret,ter capacity was sought to eliminate ad, ustment of pH. Of buffers xsted subsequently in aqueous solutioils, the most satisfactory was a solul#ion0.02M in both acetic acid and sodium acetate. Figure 1 shows the effec.t of sulfuric acid upon the p H of 0.0211%sodium acetate-acetic acid, 0.OliM pc tassium acid phthalate, and water. V'ith sulfuric acid solutions containing; up to 0.08 mg. of sulfur per ml., the acct,ate buffer controls the p H a t 4.6 A 0.1, whereas the phthalate buffer allows the p H to change from 4.1 to 3.0. Thus, the acetate buffer has over ten times the capacity of the phthalate buffei-. Because of the higher p H of the acemte buffer, a Soy0 loss in sensitivity VILS expected (4). However, absorban :es were actually measured in Soy0 et8hyl alcohol, and when the aqueous ,;elutions were diluted with ethyl alcohol the apparent p H was 5.65 for tho acetate buffer and 5.53 for the phthahte buffer. Sensitivity mas thus reduced only 10% by use of the acetate bwi'er.
The sodium acetate-acetic acid buffer has been incorporated in a new method for determining sulfur in naphthas containing 1 to 400 p.p.m. Ten grams of naphtha is burned in the conventional ASTM lamp combustion apparatus, and the combustion products are absorbed in 1.5% hydrogen peroxide. After addition of the buffer, ethyl alcohol, and barium chloranilate, sulfur is determined as sulfuric acid by measuring the absorption of the resulting chloranilic acid solution a t 530 or 330 mp. The acetate buffer has enough capacity for sulfuric acid to be determined directly without adjustment of pH. CALIBRATION
Two calibration curves are used: One a t 530 mp covers the 40- to 400p.p.m. range; the other a t 330 mp covers the 0- to 40-p.p.m. range. To prepare either curve, add 10 ml. of a buffei solution, 0.lil.f in both sodium acetate and acetic acid, to several 100ml. volumetric flasks containing known amounts of sulfuric acid. Add 50 ml. of ethyl alcohol to each flask and fill to the mark with distilled water freed of sulfate by passing through Amberlite LIB-3 ion exchange resin. Add 0.3 gram of barium chloranilate (3) to each flask and shake it intermittmtly for 15 minutes. Filter through Whatman No. 42 filter paper into 150-ml. beakers. Within 2 hours measure the absorbance of each solution against)a reagent blank a t 530 nip in a Beckman Model B spectrophotometer (or equivalent) in 5-cm. cells, or a t 330 mp in a Beckman &/Iode1 DU spectrophotometer (or equivalent) in 1-cm. cells. Plot absorbance against milligrams of sulfur per 100 ml. PROCEDURE
Burn about 10 grams of naphtha in an ASTM lanip combustion apparatus ( I ) , but do not add indicator or preneutralize the hydrogen peroxide solution. Run a piocedure blank simultaneously with each sample or group of samples. Transfer the absorber solutions for the unknown and for the procedure blank to 150-ml. beakers, using about 20 ml. of water to rinse esch sprag trap, chimney, and absorber. Evaporate each solution to about 25 ml., and transfer it to a 100-ml. volumetric flask. Add 10 ml. of the buffer and 50 nil. of ethyl alcohol,
mix, and fill to the mark with distilled water. Add 0.3 gram of barium chloranilate to each flask and shake it intermittently for 15 minutes. Filter and measure the absorbance of the filtrates within 2 hours a t 530 mp in a 5-cm. cell against a reagent blank. If absorbance of the unknown is below about 0.1, measure again in a 1-em. cell at 330 mp. Use the appropriate calibration curve to convert absorbance to sulfur concentration. Correct for the sulfur content of the procedure blank. Absorbances of a typical reagent blank against distilled water are 0.05 a t 530 mp and 1.2 a t 330 mp. They should be checked periodically against distilled water to ensure absence of contamination, DISCUSSION
Eight naphthas containing from 1 to 373 p.p.ni. of sulfur were used to test the precision and accuracy of the method. An eleven-unit lamp combustion apparatus (1) was used to burn the naphthas. A single run consisted of three procedure blanks and a single determination of each of the eight naphtha samples. For comparison with the spectrophotometricmethod, sulfuric acid in the absorber solutions from alternate runs was determined by a niodification of a turbidimetric procedure used in these laboratories for the past eight years. In the tu1bidimetric procedure, which was also used in a recent ASThI cooperative study, the absorber solution is transferred to a 50-nil. volumetric flask containing 3 ml. of 1N hydrochloric acid, and the solution is made up to volume vith distilled water. This solution is added to 10 ml. of 2 to 1 ethyl alcohol-glycerol, 0.3 gram of 20- to 30-mesh barium chloride crystals is added, and the mixture is stirred for 3 minutes. Turbidity is measured in a 5-cm. cell in a Beckman Model B spectrophotometer a t 425 m p exactly 4 minutes after the end of the stirring period. Results shown in Table I represent averages of five determinations. The cyclohexane samples were used in the ASTRl study. Excellent agreement between methods and good sulfur recovery were obtained, except for the last sample. I n this high concentration range, the turbidimetric method is less accurate than the spectrophotometric method because aliquots must be taken to keep within the range of the calibration curve, Figure 2 compares the precision of the two methods. Only in the narrow
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Figure 2. Precision of methods
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concentration range between 30 and 40 p.p.m. of sulfur is the precision of the turbidimetric method as good as that of the spectrophotometric method; above this level, precision of the spectrophotometric method is better. At the 375-p.p.m. sulfur level, the standard deviation of the turbidimetric method was 9.3 p.p.m., compared with 5.5 p.p.m. for the spectrophotometric method. The spectrophotometric method a t 330 mp is four times as sensitive as the turbidimetric method. Limits of detection, provided that a difference of 0.005 absorbance can be distinguished, are 1 y of sulfur a t 330 mp agd 18 a t 530 mp for the spectrophotometric method compared to 4 y for the turbidimetric method. For a 10-gram naphtha sample, these limits correspond to 0.1 and 1.8 p.p.m. for the spectrophotometric method and 0.4 p.p.m. for the turbidimetric method. The lower sensitivity of the spectrophotometric method a t 530 mp is no handicap above 40 p.p.m. of sulfur and serves to increase the range of sulfur determination.
4
CONCLUSION
The spectrophotometric procedure for determination of sulfur in lamp absorber solutions offers four advantages over the turbidimetric procedure: Greater stability of color Absence of crystal-size effects of solid reagent Greater sulfur range Greater sensitivity When naphthas of unknown sulfur content are being analyzed, the third
100
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IN NAPHTHA
pp,m. SULFbR
Table 1.
Comparison of Methods Parts per Million Sulfur SpectroTurbidiphotometric metric Cyclohexane 3.4 3.2 +5 p.p.m. S 8.1 8.3 +15p.p.m.S 17.9 17.9 Desulfurized naphtha 1.4 1.5 +37p.p.m. S 37.7 39.4 +74p.p.m S 75.3 73.9 + 148 P.P.nl.
s
+372 p.p.ni. S
146
375
148
399
analysis mus; be repeated. Time for a single analysis is about the same for both methodE. The improved spectrophotometric procedure can be applied to the determination of sulfate in nearly any aqueous solution. Interfering cations can be removed by ion exchange (9). A higher buffer concentration could be used to increase buffering capacity still further, The high sensitivity a t 330 mp preserts a challenge for even better precision than that obtained in these studies. LITERATURE CITED
(1) Am. SOC.Testing Materials, Philadelphia, Pa., “ASTM Standards on Petro-
leum Products and Lubricants,” Method
D 1266-57T, 1J57. ( 2 ) Bertolacini, 1%. J., Barney, J. E., 11, ANAL.CHEIW. ;!9, 281 (1957). (3) Ibid., 30, 202 (1958). (4) Schwartzenbr.ch, G., Suter, H., Helu. Chim. Acta 24, 617 (1941).
VOL. 31, NO. 4, APRIL 1959
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