B. “BASIC”TITR.4TION SOLVENT.Dissolve 13.7 grams of sodium acetate trihydrate in 500 ml. of absolute methanol and dilute to 1 liter with benzene. C. AMMONI.4CAL TITRATIOS SOLVENT. Dissolve 13.7 grams of sodium acetate trihydrate in 500 ml. of absolute methanol, add 20 ml. of concentrated ammonium hydroxide, and dilute to 1 liter with benzene. D. BASIC POLAROGRAPHIC SOLVENT. Mix 12.5 nil. of 1.1.1 tetra-n-butyl ammonium hydroxide nith 400 ml. of 99% isopropyl alcohol and dilute to 500 ml. with distilled water. E. PYRIDINE POLAROGRAPHIC SOLVENT. Add 47.5 ml. of pyridine and 2.5 ml. of concentrated hydrochloric acid to 450 ml. of methanol. F CAUSTIC TITRATION SOLVENT.Dissolve 100 grams of sodium hvdroxide and 20 ml. of- concentrated ammonium hydroxide in distilled water and make to 1 liter. G. ACIDIFIEDCADMIUM SULFATE SOLUTION. Dissolve 150 grams of C.P. cadmium sulfate (3 CdSO, 8Hz0) in distilled water; add 10 nil. of 1 to 1 sulfuric arid and dilute to 1 liter. H. SILVER NITRATE, 0.lN. Dissolve 17 grams of C.P. silver nitrate crystals in distilled water and dilute to 1 liter. Standardize by titrating- against potassium chloride. I. ALCOHOLIC SILVER NITRbTE. 0.01-v. Dilute 50 ml. of aqueous 0.1N silver nitrate solution to 500 ml. with 99% isopropyl alcohol free of contaminants which react with silver nitrate. J. ACIDIFIEDSTANDARD IODINE SOLUTIOS, 0.LV. Dissolve 20 to 25 grams of potassium iodide in a minimum amount of water and add 12.7 grams of resublimed iodine. Dilute with water to 1000 ml. after all the iodine has been dissolved.
Standardize with known thiosulfate solution or arsenous acid. Prepare acidified solution by adding 5 ml. of hydrochloric acid to 25 ml. of standard iodine solution immediately before use. K. MAXIMUM SUPPRESSOR. Black Nigrosine, oil-soluble, Central Scientific Co. No. 6604. Dissolve 0.20 gram of black nigrosine, soluble in C.P. benzene, and dilute to 1 liter with benzene. Use 1 ml. (0.2 mg.) per sample. L. IODINE-ISO-OCTANE SOLUTION.Dissolve 1 gram of iodine crystals in spectroscopically pure iso-octane and adjust volume to 1 liter.
LITERATURE CITED
Ball, J. S., “Determination of Types of Sulfur Compounds in Petroleum Distillates,” U. S. Bur. Mines, Rept. Invest. 3591 (1941). Brown, R. A., ANAL.CHEM.23, 430 (1951).
Earle, T. E., Zbid., 25, 769 (1953). Elliott, Annelle, “Electrochemical Analysis of Phenyl Sulfides, Benzothiophenes, and Thiophenes,” Southwide Chemical Conference, Memphis, Tenn., December 1956. Hale, C. C., Quiram, E. R., McDaniel, J. E., Stringer, R. F., ANAL.CHEM.29, 383 (1957). Hall, M. E., Zbid., 22, 1137 (1950).
ELECTRODES
A. For Potentiometric Techniques. SILVER-SILVERSULFIDE,Beckman 1261 billet style. Polish the silver surface to a bright finish with crocus or other fine emery cloth. Immerse the tip of the electrode in 100 ml. of titration solvent containing 10 ml. of a 1% sodium sulfide solution. With stirring, add 10 ml. of 0.1A7 silver nitrate dropwise over a 10- to 15-minute period. After thorough washing, soak the electrode for a few minutes in 0.01N silver nitrate to remove excess sulfide, rewash, and buff lightly with crocus cloth. EXTERNALCALOMEL,Beckman No. 4970-71, sleeve type with sleeve removed. Connect to titration beaker by an agarpotassium nitrate salt bridge as shown in Figure 5. B. For Polarographic Analysis. CALOMEL, Beckman No. 4970-71, sleeve type with sleeve removed. Place in H-Type polarographic cell as shown in Figure 4. DROPPINGMERCURY CAPILLARY, 2- to 5-second drop time, Sargent No. 5-29419. Place in H-Type polarographic cell as shown in Figure 4.
Zbid., 25,556 (1953). Hastings, S. H., Zbid., 25, 420 (1953). Zbid., 27,564 (1955).
Hastings, S. H., “Percolation-Mass Spectrometric Method for Determining Thiophene,” 10th SouthTTest Regional ACS Meeting, Fort Worth, Tex., December, 1954. (.1 1 ), Karchmer. J. H.. ANAL.CHEM.29.
425 (1957). ’ (12) Karchmer, J. H., Walker, hf. T., Zbid., 26, 271 (1954). (13) Zbid., 30,85 (1958). (14) Sicholson. M. M.. J. Am. Chem. SOC. 76.2539 (1954’1: (15) Purdy,-K. ‘M., Harris, R. J., ANAL. CHEM.22,1337 (1950). (16) Tamele, M. W., Ryland, L. B., IND. ENG.CHEM..ANAL.ED. 8. 16 (1936). (17) Tamele, M. W., Ryland, L. B., Irvine, V. C., Zbid., 13, 618 (1941). .
I
RECEIVEDfor review May 6, 1957. -4ccepted August 26, 1957.
Determining Disulfides in Petroleum Naphtha Modification of the Acetic Acid-Zinc
Reflux Method
J. H. KARCHMER and MARJORIE T. WALKER Humble Oil
13 Refining Co., Bayfown, lex,
,An investigation of the low results sometimes obtained by the acetic acid-zinc reflux reduction method of determining organic disulfides in petroleum naphthas, showed that they were due to the volatility and water solubility of certain mercaptans of low molecular weight formed by reduction of the disulfides. These errors can be avoided by use of an absorption trap, with nitrogen purging, to catch the lower mercaptans and by titration of the reflux bottoms without separation of the acetic acid. Errors associated with the reduction of some disulfides of high molecular weight were also investigated. Seventeen primary, secondary, tertiary, and aromatic disulfides ranging from dimethyl to di-
fert-dodecyl disulfide have been studied and a modified procedure for obtaining accurate results is reported.
A
procedures for determining organic disulfides in petroleum fractions have certain limitations, The polarographic method (6), perhaps the most rapid, the cold reduction method using powdered zinc and alcoholic potassium hydroxide ( I S ) , and the sodium borohydride method (15) do not determine secondary or tertiary disulfides. The “acidicstirring”method of Hubbard, Haines, and Ball (7) is satisfactory for rapidly and accurately determining normal and secondary disulfides, but is not applicable t o tertiary disulfides. The VAILABLE
acetic acid-reflux method described by Faragher, Morrell, and Monroe (5) and later modified by Malisoff and Anding ( I I ) , Bell and Agruss (d), andBall ( I ) is more widely used, because all secondary disulfides and a large portion of the tertiary disulfides can be determined. However, it may not be effective with some of the disulfides likely t o be present in petroleum fractions. Ball (1) describes optimum conditions for the acetic acid-reflux method and recovery of benzyl, phenyl, n-butyl, and p-tolyl disulfides, and s h o w that these conditions do not break down organic sulfides and thiophenes and cause them to be mistaken for disulfides. While the Ball modification has given reasonable results on a large number of refinery VOL. 30, NO. 1, JANUARY 1958
0
85
samples, work with synthetic samples has indicated that several improvements could extend its usefulness. The Ball modification of the acetic acid-reflux method for determining disulfides is carried out by refluxing 50 ml. of a naphtha sample for 3 hours in a 250-ml. flask fitted with a watercooled condenser, with 50 ml. of glacial acetic acid and 10 grams of 20-mesh zinc. As the glacial acetic acid is miscible with the hydrocarbon, only one liquid phase is present. The zinc in presence in the acetic acid reduces the disulfide to a mercaptan (thiol): Zn
+ RSSR + 2H+ -,2 RSH + Z n + + (1)
At the end of the reflux period the contents of the reflux vessel are cooled and placed in a separatory funnel. Addition of water causes separation of the hydrocarbon phase from the aqueous acid. The hydrocarbon phase contains the mercaptan, which is subsequently determined by an indicatorargentometric (3, 11) titration. Mercaptans present in the original sample must be either removed or determined initially, so that proper correction can be made. Since Ball’s work in 1941, the more accurate potentiometric (16) and amperometric (9) methods have been developed. Although the amperometric method is extremely sensitive, the potentiometric method can differentiate between mercaptans and hydrogen sulfide, which is sometimes produced. Another innovation is the use of acrylonitrile (4) to convert the mercaptans to sulfides prior to reduction. I n this laboratory 50 to 75% recoveries of tertiary disulfides were obtained by the Ball method. Although these recoveries were low, they were accepted as a compromise because more drastic reduction conditions could result in formation of hydrogen sulfide from other types of disulfides (1.9) or sulfides. Hydrogen sulfide would cause the indicator (3) or the amperometric (9) results to be too high. Investigators (10) using the amperometric method have reported some results over 100% under more severe reduction conditions. Synthetic samples like diethyl disulfide consistently yielded low results. This led to an investigation and subsequent modification of the Ball acetic acidzinc reflux method. This paper reports this modification and the experience of this laboratory with synthetic samples, describes precautionary steps that will lead to increased recoveries, and discusses the significance of pertinent factors. DEVELOPMENT
OF
uted to the loss of the reduction product, ethyl mercaptan, during the reflux period, as its boiling point is 36” C. Consequently, a dry icealcohol knock-back trap was placed on the reflux condenser. Although this trap effected some improvement, the over-all recovery was only 67%, indicating that there was still an additional source of error. Even though the dry ice knock-back trap prevented escape of the mercaptan, the mercaptan dropped back into the reflux vessel and revolatilized in the vapor space above the liquid. As significant amounts of ethyl mercaptan could be held up in the vapor phase of the reflux vessel, even a t room temperature, the cold trap was replaced with a 20% sodium hydroxide scrubber (immersed in ice water), and the system purged with nitrogen. This caustic solution was then titrated with silver nitrate to determine mercaptans and hydrogen sulfide. With such a setup the diethyl disulfide was completely recovered as ethyl mercaptan in the overhead caustic scrubber (Table I). Bell and Agruss ( 2 ) recommended an overhead scrubber of a silver nitrate solution to recover volatile mercaptans. The proposed way of handling the volatile mercaptans offers the following advantages over the Bell and Agruss system: The scrubber solution of sodium hydroxide can be titrated directly to determine both sulfide and mercaptide ion, and the nitrogen purging ensures complete removal of the mercaptan from the vapor space in the reflux system. Although the nitrogen purging tends to volatilize greater amounts of the mercaptans, this is of no consequence, as the mercaptan content is computed by totaling the
c ;or* --Dirlii!ed
contents of the scrubber and the reflux bottoms. Elimination of Water-Washing Step. With diamyl disulfide essentially all of the resultant mercaptan was found in the bottoms of the reflux vessel, with negligibly small amounts in the overhead caustic scrubber. With intermediate disulfides, such as di-n-butyl, di-isobutyl, and di-n-propyl, part of the mercaptan was found in the bottoms and part in the overhead. However, total recoveries of these intermediate disulfides were again low. These poor recoveries were traced to the partial solubility of these mercaptans in water during the step in which the hydrocarbon is separated from the acetic acid and water-washed. It is not necessary to separate the acetic acid from the hydrocarbon prior to titration. The contents of the reflux vessel, or an aliquot thereof, may be mixed with a special solvent, and titrated directly with alcoholic silver nitrate solution, using the Precision Recordomatic titrator with a silver sulfide indicator electrode and a glass reference electrode. The “break” of the titration curve can be improved by incorporation of sodium acetate in the special solvent. While the titration break in the presence of the large amount of acetic acid was not as sharp as titrations without the acid, the over-all accuracy was better, as mercaptan losses were prevented by elimination of the acid separation step. To study this titration step, a number of mercaptans were placed in a solution of iso-octane and glacial acetic acid to simulate the reflux bottoms; excellent recoveries were obtained. I n some cases, a double wave appeared in the end point
stosper
klctei
Calomel Electrode W i t h Sleeve Removed
3% & O f Sol. KNO
MODIFIED METHOD
Recovery of Light Gases.
The low results obtained on synthetic solutions of diethyl disulfide were attrib-
86
ANALYTICAL CHEMISTRY
Figure 1. Salt bridge for use with Precision Recordomatic titrator
GLASS REFERENCE ELECTRODE
r-
-0.6
CALOMEL
REFERENCE ELECTRODE
i-
-0.4 -0’5 -0.3
;-0.2 0 *
li‘
-0.1
I’ 0.0
w
10.1 t
I
0.2
to.41
I
I
I
I
I
I
L O L U M E OF S I L V E R N I T R A T E
Figure 2. titration
Comparison of reference electrodes used in potentiometric mercaptan
Titrants. Aqueous and alcoholic silver nitrate Indicator electrode. Silver sulfide Acetic acid-solvent mixture made to simulate solution of reflux bottoms with titration solvent (HOAc: iCs: MeOH: 82:2:2:1:1)
Figure 3.
Reduction apparatus
vicinity. The inflection point of the first wave of the doublet yielded the correct results. Choice of Reference Electrodes. The glass electrode used as a reference in the initial 11-ork was ultimately replaced with a n external calomel electrode, electrically connected to the titration vessel with a n agar-potassium nitrate bridge t o avoid possible chloride contamination (Figure 1). The glass electrode was replaced by the calomel cell, in order to be consistent with electrodes used in titration of the 20% sodium hydroxide solution, as even the high pH glass electrodes had a short, life. I n the latter
titration an external calomel is not needed, because ammonium hydroxide which represses possible chloride ion contamination from leakage of the calomel cell is added to the sodium hydroxide solution. The calomel cells are equal in performance to the glass cells, although the initial voltages and the magnitude of the voltage breaks may differ slightly. The glass-silver sulfide and the calomel-silver sulfide electrode systems in 207& sodium hydroxide solution and in the “special solvent” are compared in Figure 2. I n the caustic solutions the calomelsilver sulfide system produces with mercaptans curves of higher negative
values; in the acetic acid-hydrocarbon solvent the mercaptan voltages are more positive and the breaks perhaps a little sharper than corresponding curves produced using the glasssilver sulfide system. I n the acetic acid-hydrocarbon solvent very little variation in the initial voltages was observed for any mercaptans. The glass electrode operates more satisfactorily when connected to the shielded terminal. Therefore, the calomel when used to replace the glass as the reference electrode was also connected to this plug. The polarity signs are correct as shown in Figure 3 only 1%-henthe electrodes are connected in this manner. Effect of Increased Temperature. The temperature of the reflux is understandably a factor in the completeness of the reduction of the disulfides. Hon ever, the higher temperature not only hastens reduction of the disulfide, but can also promote formation of hydrogen sulfide instead of mercaptans. The recoveries of above 100% which some investigators hare reported are presumably due to the formation of hydrogen sulfide and its subsequent titration with silver nitrate along mith the mercaptan. As the sulfide ion reacts mith two silver ions and the mercaptide ion with only one silver ion, high results will be obtained unless the two are differentiated. The indicator or the amperometric determination does not make this differentiation; the potentiometric method does. The modified method, with provision for an overhead trap to catch all the volatile sulfur gases, and potentiometric determination of both reflux bottoms and the trap prevents errors of this sort. Therefore, certain disulfides, like benzyl disulfide (la),which are known to produce some hydrogen sulfide upon reduction, cause little concern. The temperature of the reflux is important, however. The reflux temperature is influenced by the boiling point of the naphtha, unless its boiling point is higher than that of glacial acetic acid (llS.1’ C.), in which case the temperature will be that of the glacial acetic acid (except in azeotropic formation). Therefore, with stocks boiling loffer than gasoline the refluxing temperature of the mixture should be kept sufficiently high to permit reduction of the disulfides. I n many cases, the polarographic (6) or alkaline reduction ( I S ) methods can be used satisfactorily, as there is less chance that secondary and tertiary mercaptans will be present. Amount of Zinc Used. Initially 10 grams of 20-mesh zinc were used t o effect the reduction, but in a n effort to increase the yield of tertiary mercaptans other amounts and sizes of zinc mere used. 153th powdered zinc VOL. 30, N O . 1, JANUARY 1958
87
the reduction conditions were too severe for certain disulfides, and apparently other decomposition products were obtained t h a t were not recoverable by silver nitrate titration. In general, larger amounts of the 20mesh zinc gave better mercaptan yields from the tertiary disulfides. However, it was felt that 25 grams was the largest practical amount that could be employed in this procedure. PROCEDURE
The modified procedure developed during this study differs from the Ball method in that: Twenty-five grams of 20-mesh zinc are used, if necessary, instead of 10 grams. An absorption bottle containing 20y0 sodium hydroxide is connected on the outlet side of the reflux condenser, and nitrogen is used to sweep out the more volatile mercaptans and hydrogen sulfide. The period of reflux is 3 hours; during the latter 1.5 hours the sample is blown with nitrogen a t the rate of 10 liters per hour. The sample is cooled before the nitrogen flow is discontinued. The reflux bottoms are diluted with solvent and titrated directly without phase separation. Apparatus and Reagents. The reduction apparatus (Figure 3) consists of a Tri-Flat Flowrator tube No. 08-F-1/16-12-4 with 316 stainless steel float, a 250-ml. flask fitted with a side arm for nitrogen entry and a B 24/40 outer joint, a Graham glass condenser with a-ater-cooled B 24/40 inner joint, a connecting tube, and a gas-washing bottle containing 50 ml. of 20y0 sodium hydroxide solution. A Precision-Dow Recordomatic Titrator is used. Two sets of electrodes are needed, one for titrating the caustic scrubber, and the other for analyzing the hydrocarbonacetic acid mixture. For the caustic, a sleeve-type calomel reference electrode, such as Beckman KO. 1170, and a silver indicating electrode with large area, such as Beckman No. 1261 billet style, are used. For the hydrocarbon titration, a sleeve-type calomel electrode with sleeve removed and electrically connected to the titration beaker by an agar-potassium nitrate salt bridge, as shown in Figure 1, and a silver indicating electrode are used. The silver electrodes are prepared for use as follows: Immerse the tip of the electrode in 100 ml. of 20% sodium hydroxide containing 1 ml. of ammonium hydroxide and 10 ml. of a 1% silver sulfide solution. K i t h stirring, add 10 ml. of 0.1N silver nitrate dropwise over a 10- to 15-minute period. After thorough washing, soak the electrode for a few minutes in 0.0lN silver nitrate to remove any excess sulfide, then rewash and buff lightly Kith crocus cloth. Titration solvent, 0.1N sodium acetate in a 1 to 1 mixture of methanol and benzene. (This is miscible with the
88
ANALYTICAL CHEMISTRY
glacial acetic acid and hydrocarbon in the reflux bottoms.) Aaueous silver nitrate standard solution,.O.Ol N . Alcoholic silver nitrate standard solution, 0.01N, prepared by diluting 10 ml. of standard aqueous 1.000N silver nitrate to 1 liter with 99% isopropyl alcohol. Outline of Method. Determine the mercaptan content of a portion of the sample by dissolving in titration solvent and titrating with alcoholic silver nitrate solution. If this value is large in comparison with the anticipated disulfide content, remove or complex the mercaptan by methods described by Ball (1) or Earle (4). Otherwise, subtract this value from the mercaptan titration obtained after reduction of the disulfides. LMeasure 50 ml. of 20% sodium hydroxide into the gas-washing bottle, immerse the bottle in an ice bath, and connect to the apparatus as shown in Figure 3. Pipet a portion of sample containing from 0.5 to 20 mg. of d i s u ~ d esulfur (1 to 8 mg. is optimum amount) into the 250-ml. reflux flask, and add enough iso-octane to bring the total hydrocarbon volume to approximately 50 ml. Then add 25 grams (10 grams is sufficient if tertiary disulfides are known to be absent) of 20-mesh zinc metal and 50 ml. of glacial acetic acid. Connect the flask to the condenser and reflux. At the end of 1.5 hours, purge with nitrogen through the side arm a t a rate of approximately 10 liters per hour. After 3 hours of reflux, remove the hot plate and cool the reflux flask in ice water for 15 minutes, continuing the nitrogen flow. Disconnect the nitrogen line and remove the caustic scrubber. Pour the caustic into a 250-ml. tall-form beaker, rinse the scrubber several times with small portions of 20% sodium hydroxide, and add the washings to the beaker. Add approximately 1 ml. of concentrated ammonium hydroxide and titrate potentiometrically with 0.01N aqueous silver nitrate. Disconnect the overhead line, and rinse down the condenser with approximately 10 ml. of the titration solvent. Remove the reflux flask and pour its contents into a 250-ml. tall-form beaker. Rinse the flask twice with 20-ml. portions of titration solvent and add the washings to the beaker. Titrate potentiometrically with 0.01N alcoholic silver nitrate. Calculate the milligrams of disulfide sulfur per 100 ml. of sample by the following: Volume A
X normality A ~ N OX ~3206.6 volume of sample
~ N O ~
DISCUSSION OF RESULTS
Recoveries on Synthetic Samples. The modified procedure was evaluated on a series of synthetic alkyl and aromatic disulfides. Table I presents the results obtained by the modified procedure and by using zinc of particle size and amount other than those recommended. The three secondary disulfides were obtained from the Synthesis Purification and Property
Section (at Laramie, Wyoming) of
-4PI Project 48A, “Synthesis, Properties, and Identification of Sulfur Compounds in Petroleum,” and were of 99+ mole % purity. The purity of disulfides indicated as “pure” as presumed by definition to be 95+%; the purity of those indicated as “practical” grade was not known. The base stock in which the disulfides were dissolved was purified iso-octane, except in two cases when xylene was used. Table I indicates that good recoveries (above 97%) were obtained for all except tertiary disulfides by the modified method. Recoveries of 87.5, 95.7, 96.0, 85.0, and 47.6% were obtained for di-tertbutyl, di-tert-amyl, di-tert-hexyl, diieri-octyl, and di-terf-dodecyl disulfides, respectively. This suggests that reasonably good recoveries may be expected through the di - tert - octyl disulfides. There is some evidence that recoveries of the tertiary disulfides are more d f i c u l t with increasing molecular weight; when the di-tert-dodecyl disulfide was reduced under increasingly severe conditions the recoveries increased from 23.1, to 47.6, to ‘79.1%. However, in 12 experimental runs on the di-tert-butyl disulfide, 91 3% was the highest value obtained under the most severe reduction conditions. -4s 96% yields were obtained on the diterb-amyl and di-tert-hexyl disulfides, the di-tert-butyl disulfide may have been impure. The secondary disulfides offer no problem, as essentially complete recoveries were obtained. Most of the methyl, ethyl, propyl, and tert-butyl mercaptans were found in the overhead caustic scrubber (98.2, 99.4, 82.6, and 64.8%, respectively), while the n-butyl mercaptan and those having higher boiling points were found in the reflux bottoms. This finding, which is essentially consistent with the boiling points of the mercaptans, may be used for partially characterizing the disulfides in the sample. The recovery of 98% of the diphenyl disulfide should be noted, as 85% was the maximum recovery by the regular procedure. Variation of Reduction Conditions. The severity of the reduction was altered by raising the boiling point of the reflux mixture (substitution of xylene for iso-octane in the synthetic blends), by using larger quantities of 20-mesh zinc, or by using powdered zinc. The increased reflux temperature increased the recovery of the di-tert-butyl disulfide from 69.5 to 90.37,. Large quantities of 20-mesh zinc, in general, produced greater yields (di-tert-amyl, di-tert-butyl, and di-tert-dodecyl disulfides.) The use of powdered zinc increased the yields.
amount of zinc was increased. The powdered zinc actually produced lower results for the dibenzyl, the dimethyl, and di-sec-butyl disulfides. This perhaps can be interpreted to mean that other sulfur-containing decomposition products are being formed either by too drastic reduction conditions or by some reaction in which powdered zinc acts as a catalyst. Production of Hydrogen Sulfide. Several possible reactions could account for t h e formation of hydrogen sulfide. I n some early experiments it was observed that whenever hydrogen sulfide
However. when large amounts of 20mesh metal or the powdered metal were used, hydrogen sulfide was produced. Although production of hydrogen sulfide will not prevent complete recoveries, its presence indicates too drastic a reduction of the disulfide, and perhaps other decomposition products are being formed, not measured by the silver nitrate titration. For this reason powdered zinc should not be used and no more than 25 grams of the 20-mesh zinc should be employed. The dibenzyl disulfide produced hydrogen sulfide even with 10 grams of 20-mesh zinc: the quantity increased as the
Table 1.
was produced, low results were also obtained. This was initially explained by the following equation:
RSSR
+ 2 H + + ZnO
+
H2S
+
RSR Zn++ (2)
+
where the reduction of one mole of disulfide produces one mole of a n organic sulfide and one mole of hydrogen sulfide. As the organic sulfide is not titrated with the silver nitrate, it would appear that this sulfur is "lost." This equation implies that sulfur converted to a n organic sulfide could be accounted for by simply multiplying the hydrogen sulfide content by 2, as equal moles of hydrogen
Determination of Disulfide Sulfur by Modified Acetic Acid-Zinc Reflux Method
(Synthetic samples in iso-octane)
Organic Dimethyl
Source and Grade Eastman, pure
Diethyl Di-sec-propyl Di-n-propyl Di-tert-butyl
Eastman, pure API, 99%+ Eastman, pure Phillips, prac.
Group
Di-sec-butyl
API, 99%+
Diisobutyl Di-n-butyl Di-ferf-amyl
Eastman, pure Eastman, pure Phillips, prac.
Dicyclopentyl Diisoamyl Di-w-amyl Di-tert-butyl Di-tert-octyl Di-tert-dodecyl
API, 99 7% Eastman, pure Eastman, pure Phillips, prac. Phillips, prac. Phillips, prac.
Diphenyl Dibenz yl
+
Eastman, pure Eastman, pure
Disulfide Sulfur Present, Mg. 5.70 5.54 12.45 6.14 8.85 7.12 7.12 7.12 7.12 7.12 7.12 7.12 7.12 7.12 7.12 7.12 7 .8ja 7.14 7.14 7.35 12.65 5.53 5.53 5.53 6.01 7.32 1.85 6.00 4.20 8.45 5.07 5.07 3.00 1.40 1.40 13.35 5.68 6.84 5.68
Disulfide Found In Overhead Scrubber -
P: size 20 mesh Powdered 20 mesh 20 mesh 20 mesh 20 mesh 20 mesh 20 mesh 20 mesh 20 mesh 20 mesh Powdered Powdered Powdered Powdered Powdered 20 mesh 20 mesh Powdered 20 mesh 20 mesh 20 mesh 20 mesh Powdered 20 mesh 20 mesh 20 mesh 20 mesh 20 mesh 20 mesh 20 mesh Powdered 20 mesh 20 mesh 20 mesh 20 mesh 20 mesh Poq-dered Powdered
1.I
Total 10 25 10 10 10 10 25 50 75 100 200 10 25 50 75 100 10 10 25 10 10 10 25 25 10 10 10 25 25 10 25 25 10
10 10 10 25 25 25
0.00 0.00 0.00 0.4@ 1.28 2.42c 2.4W 2.2gC 3.34c 2.8OC 2.62O 2.5OC 2.52c 2.3lC 2.13c 1.45c 2.13c 5.74" 4.9@ 5.40c 11.44 3.53 4.72= 4.32c 5.94 7.1? 11.54 5.13" 3.1P 1.73 2.27 3.68 12.74 10.24" 10.2P 12.3AC 3.OOc
0.00 0.00 0.00 7.5 14.5 34.0 34.8 32.2 46.9 39.3 36.8 35.1 35.4 32.4 29.9 20.4 27.1 80.4 69.5 73,s 90.4 64.2 85,4 78 1 98.8 97.7 97.4 !5.5 15.5 20.5 44,8
72.6 98.1 89.8 89.8 92.4 52 8 0.8P 12.9 0.80" 1 4 . 1
Mixture Di-ethyl 22.49 .95 Di-n-propyl Diisobutyl Di-n-butyl 2.53 20 mesh 10 11.21 62.9 Diisoamyl Di-n-am 1 Diphenyy 2.60 17.83) Tot a1 a By recommended modified procedure. Slight darkening of caustic observed. Double break observed; inflection point t o first break used for end point, Xylene used as solvent instead of iso-octane.
0.00
0.00
5.60 98.2
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.0
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
5.68 7.31 2.53 3.75 4.07 3.05 3.72 3.80 3.70 3.91 3.93 4.26 5.06 4.96 1.47 1.74 1.76 i.12 0.42 0.57 0.81 0.10 0.00 0.13 0.63 0.16 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.17 0.26 0.16
Traceb Trace 4 . 9 5 8 9 . 4 0.00 0.00 12.36 9 9 . 3
Traceb Traceb Traceb Traceb Traceb
Trace Trace Trace Trace Trace
0.00 0.00 0.00 0.00 Traceb Trace 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Traceb Trace 0.00 0.00 0.00 0.00 0.00
0.00
024 022 014 0.33 0 00 1.22 i.01 1.20 2.48 1.06 1.02
5 7 2 6 2 8 6 5 0 00 1~. 0.7 8.9 9.0 43.7 15.5 18 0
0.00
0.00
Traceb Trace
92.5 82.6 35.5 52.7 57.2 42.8 52.2 53.4 52.0 54.9 55.2 59.8 71.1 63.2 20.6 24.4 23.9
8.9
7.6 10.3 14.6 1.7 0.00 1.1 10.5 3.8 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.0 3.8 2.8
5.60 98.2" 4.95 89.4 12.36 99.3" 6.14 100.0' 8.59 97.1a 4.95 69.5a 6 . 2 3 87.55 6.36 8 9 . 4 6.39 89.7 6.52 91.5 6.42 9 0 . 2 6.20 87.1 6.42 90.3 6 . 2 4 87.6 6.39 89.7 6 . 5 1 91.5 7.09 90.3" 7.21 101.05 6.70 93.9 7.16 97.4" 12.56 99.3= 3.97 71.P 5.29 9 5 . i a 5.13 92.7 6.04 100.5a 7 . 1 5 97.7" 11.67 98.5" 5.76 9 6 . W 3:57 85:o" 1 . 9 5 23.1a 2 . 4 1 47.@ 4 . 0 1 79.1 12.74 98.1"
1.98
34.9
i
6.55 36.7
17.76
VOL. 30, NO. 1, JANUARY 1958
99.@
89
Accuracy and Limitations. Some idea of the over-all accuracy of the method may be judged from t h e recovery of 99.6% on the mixture t h a t contained seven different disulfides of widely varying boiling points (Table I). Previous work (1) has indicated that alkyl sulfides, aromatic sulfides, and thiophenes do not interfere with the zinc-acetic acid method; no information was available on polysulfides or elemental sulfur. I n experiments using tri- and tetrasulfides and elemental sulfur it was found that these materials were reduced :
sulfide and organic sulfide were produced. However, as other types of disulfides were examined, and the data studied, it became apparent that Equation 2 did not represent the complete picture. For example, in an experiment using dibenzyl disulfide (Table I) when 5.68 mg. of disulfide sulfur were reduced with 25 grams of 20-mesh zinc, 3.17 mg. of sulfur were recovered as benzyl mercaptan and 2.48 mg. as hydrogen sulfide sulfur, a total of 5.65 mg. or 99.57, recovery. If the hydrogen sulfide sulfur content mere doubled, the recovery would be 143.1% instead of 99.5%. I n an experiment (Table I) using di-tert-dodecyl disulfide, when 5.07 mg. of disulfide sulfur were reduced with 25 grams of 20-mesh zinc, 2.27 mg. of mercaptan sulfur and 0.14 mg. of hydrogen sulfide sulfur were r& covered. Even doubling the hydrogen sulfide content would not yield a complete balance. When the reduction conditions were more severe with normal and secondary disulfides, small amounts of hydrogen sulfide were produced, but the losses could not be balanced by doubling the hydrogen sulfide found. Although no postulation can be made on the basis of these data regarding the relationship of the production of hydrogen sulfide with low results, some information is available regarding the hydrogen sulfide produced by reduction of dibenzyl and di-tert-dodecyl disulfide. As tertiary mercaptans can be decomposed by heat to an olefin and hydrogen sulfide, an experiment was conducted to determine whether the mercaptan formed by the reduction was decomposed under the prescribed conditions. Accordingly, 10.0 mg. of ditert-dodecyl mercaptan was refluxed for 3 hours with 25 grams of 20-mesh zinc, and 0.7 mg., of hydrogen sulfide and 9.1 mg. of the original tertiary dodecyl mercaptan were recovered. Similarly, when dibenzyl mercaptan mas refluxed, 42% of the sulfur was recovered as hydrogen sulfide sulfur and 58% of the original mercaptan was obtained. Reduction of normal and secondary mercaptans under the same moderate conditions did not yield hydrogen sulfide. Thus it appears in the cases of dibenzyl disulfide and the tertiary disulfide, one of the sources of hydrogen sulfide is the decomposition of the mercaptans. Olefins are formed from the tertiary disulfides and the hydrocarbon is formed from the dibenzyl disulfide. Equations for these reactions are: R-
AI
-S-S-C-R
So
I
Zn”
CLH&!H2-f!-S-CHzCsH6
+ 2H”
heat
H+
ANALYTICAL CHEMISTRY
H+
-P
4
+ zHzS
HZS
(5) (6)
While recoveries of hydrogen sulfide from polysulfide were quantitative, only 50 to 60% yields of hydrogen sulfide were obtained from elemental sulfur. The alkylthio group (RS) of the polysulfide is converted to a mercaptan, as is this same group in the disulfide, and the “polysulfide” sulfur atoms are converted to hydrogen sulfide. Thus, when hydrogen sulfide is obtained overhead, it may have been derived from (1) polysulfide sulfur or elemental sulfur, or (2) from certain decomposabledisulfides, such as dibenzyl disulfide, or the tertiary disulfides of high molecular weight. If it was known that the hydrogen sulfide was due solely to either of the two above categories, the titration curves could be properly interpreted, but if it came from both categories, interpretation would be difficult. While a knowledge of the boiling range of the sample may be helpful in deciding whether certain compounds are excluded, this is not reliable, as the samples may have been oxidized and both polysulfides and high molecular weight disulfides could form from the lower molecular weight mercaptans present. If polysulfides and/or elemental sulfur are absent, it can be assumed that the hydrogen sulfide obtained on reduction was due to disulfides. If polysulfides and/or elemental sulfur are present, the hydrogen sulfide may not be due solely t o these materials. Certain polysulfides are difficult to detect and even more difficult to remove quantitatively. The mercury blackening (17) and Sommer (14) tests are satisfactory for elemental sulfur and most polysulfides; when peroxides are
I
CH3
+ 2(1 + z)H”
2RSH
CHI
CH3
90
RSS,SR
I
CHs
CH1
R-4
I
Zn, H +
ACKNOWLEDGMENT
The authors wish to thank the Humble Oil & Refining Co. for permission to publish this paper. LITERATURE CITED
Ball, J. S., Bur. Pllines, Rept. Invest. RI 3591 (December 1941). Bell, R. T., Agruss, M. S., IND. ENO. CHEK, ANAL ED. 13, 297 (1941). Borgstrom, P., Reid, E. E., Ibid., 1 , 186-7 (1929).
Earle, T. E., ANAL. CHEW25, 769 (1953).
Faragher, W. F., blorrell, J. C., Monroe, G. S., I n d . Eng. Chem. 19,1281-4 (1927).
Hall, M. E., ANAL. CHEX. 25, 556 (1953).
’
Hubbard. R. L..Haines. W. E., Ball, J.’S., Ibid.; 29, 91-3’(1958). ‘ Karchmer, J. H., Walker, plf. T., Ibid., 26,271 (1954). Kolthoff, I. M., Harris, IT. F., IND. ENG. CHEM., ANAL. ED. 18,161 (1946).
Kolthoff, I. M., May, D. R., alorgan, P., Ibid., 18,442 (1946). Malisoff, W. M., Anding, C. E., Jr., Ibid., 7 , 8 6 4 (1935). Moses, C. G., Reid, E. E., J . A m . Chsm. SOC. 48,776-7 (1926).
Rosenwald, R. H., Pdrol. Processang 6 , 969 (1961).
Sommer, H., IND. ENQ. CHEM., ANAL.ED.12,368 (1940). Stahl, C. R., Siggia, Sidney, ANAL. CHEM.29, 154 (1937). Tamele, M. W., Ryland, L. B., IKD. ENQ. CHEM., ANAL. ED. 8, 16 (1936).
Uhrig, K., Levin, H.,
+ HzS
(3)
CH,
heat
C~H~CHZ-SH
present and for polysulfides like di-le+ butyl trisulfide, the more reliable polarographic techniques (8) should be used. The problem engendered by formation of hydrogen sulfide during reduction illustrates the difficulty of characterizing a class of organic compounds by some common grouping (such as the disulfide group) without regard for other substituent groups that may affect the chemical properties of that group. Therefore, many group classifications on samples containing a mixture of many different compounds of varying substituent groups must be a compromise. Often, however, the difference in reactivity may form the basis of a “practical” test. For example, the procedure described, without regard for the presence of polysulfides, could serve as an index of the “potential” hydrogen sulfide and mercaptans present in the sample.
CeHsCH3
+
H?S (4)
.%SAL.
CHEM.
22, 1137 (1950).
RECEIVEDfor review May 6, 1957. Accepted August 26, 1957. Division of ilnalytical Chemistry, 131st Meeting, .4CS, Miami, Fla., April 1957.
