Determination of Microgram Quantities of Sulfur by Reduction with Raney Nickel LAWRENCE GRANATELLI American Oil Co.(Texas), Texas City, Tex. A, rapid, sensitive analytical method for the determination of microgram quantities o f sulfur is described. Organically bound sulfur present in nonolefinic hydrocarbon solutions is reduced to nickel sulfide b y Raney nickel. The addition o f acid liberates hydrogen sulfide, which is absorbed in a causticacetone solution. Absorbed hydrogen sulfide is titrated with mercuric acetate using dithizone as indicator. Standard solutions containing elemental, thiol, sulfide, disulfide, and thiophenic sulfur were prepared and analyzed. Avera g e recovery achieved on 24 standards covering the concentration range of about 1 to 375 p.p.m. of sulfur was 99 =t 1.6%. Olefins present in the sample, even a t 2% concentration, introduce appreciable error. The method, employing simple apparatus, requires less than 1.5 hours for a completed analysis and is reliable for the determination of as little as 5 y of sulfur. For a maximum recommended sample size of 50 grams, the method is applicable to samples containing as little as 0.1 p.p.m. of sulfur.
T
petroleum industry expends considerable time and money on the determination of sulfur in its ran materials and products. Any method, therefore. nhich s h o w promise of effecting economics, yielding sensitive and accurate results, is highly regarded. This investigation was prompted by results reported by Trifonoff, Ivanoff, and Pavloff (a), who employed Raney nickel as a reducing agent in the determination of sulfur in organic sulfur compounds using samples of about 2 mg. With the use of calibration curves for each compound and polarographic estimation of hydrogen sulfide recovered, they showed excellent analysrs of compounds containing thiophenic, sulfide, and disulfide groups, as well as with elemental sulfur, sulfanilic acid, and thiosemicarbazide. Their method was extended to the determination of about 2% elemental sulfur in benzene and compared favorably with the lamp method of sulfur analysis. A recent review on desulfurization with Raney nickel is presented b) Lieber and Norritz (6). The method of Trifonoff, Ivanoff, HE
434
ANALYTICAL CHEMISTRY
and Padoff (8) required about 30 minutes for a determination. This short analysis time, together with the apparent sensitivity of the method, prompted the investigation reported here. Experimentation \vas directed to the detrrmination of sulfur in the parts per niillion range ITith a high degree of accuracy and determination of the types of organic sulfur compounds which could be quantitatively reduced. ThP method developed employs simple apparatus and requires less than 1.5 hours for a complete determination. A nonolefinic hydrocarbon sample is heated TT ith Raney nickel, converting organically bound sulfur to nickel sulfide. The addition of acid liberates hydrogen sulfide which is purged into a caustic-acetone solution. Absorbed sulfide is determined titrimetrically m-ith mercuric acetate, using dithizone as indicator ( 2 ) . Consistent recoveries of about 95% are obtained for a variety of types of sulfur compounds. For this reason, the method is placed on a n empirical basis by standardizing niercuric acetate solution against hydrogen sulfide recoxered from a standard solution of elemental sulfur in 2,2,4-trimethylpentane (iso-octane). REAGENTS
RAXEYXICKEL. Raney nickel was prepared by adding 0.3 to 0.6 gram of nickel-aluminum alloy (Raney catalyst powder, Raney Catalyst Co., Chattanooga, Tenn.) to a 100-ml. distillation flask (d,Figure 1). Ten milliliters of 2 . 5 S sodium hydroxide \?as added to the alloy and the flask was swirled gently in a fume hood until vigorous evolution of hydrogen ceased. Alloy adhering to the side of the flask was washed down with a minimum amount of mater and the preparation \vas alloffed to activate overnight. After activation, sodium aluminate, which was formed, and excess sodiuni hydroxide were decanted from the Raney nickel. The active nickel \?as washed three times with 5-ml. portions of water and preserved under 10 ml. of 2-propanol until ready for use. Decantation and washing were accomplished with a minimum loss of Ranev nickel. ~IERCUR ACETATE, IC 30 y of sulfur per ml., prepared by dissolving 0.4045 gram of oven-dried reagent grade mercuric oxide in 50 ml. of water containing
l-l
F
h-a 5
G G O
!
?
INCHES
?
Figure 1. Apparatus for reduction, evolution, and absorption of sulfur
2 nil. of acetic acid, and diluting to 2 liters. This solution mas used to determine the sulfur content of the nickelaluminum alloy. ~ I E R C UACETATE, RIC enipirical sulfur titer of about 100 y per ml., prepared by dissolving 1.3 grams of mercuric oxide as described above. The empirical sulfur titer of this solution is determined by titration of hydrogen sulfide recovered from 100 to 1000 y of sulfur dissolved in sulfur-free iso-octane and treated as described in Procedure. SODIUMHYDROXIDE, 2.51: and LY. HYDROCHLORIC ACID, 1.5 to 1 solution. DITHIZONE(diphenylthiocarbazone), prepared by dissolving 0.1 gram in 100 ml. of acetone. SULFURCOMPOUNDS. Monoclinic sulfur was prepared by recrystallizing commercial sulfur. 1-Butanethiol, cyclohexanethiol, 4thiaheptane, thiacyclohexane, 3,4-dithiahexane, thiophene, and 2,3-benzothiophene were standard samples obtained from American Petroleum Institute. APPARATUS
Assembly of the apparatus used for the reduction, evolution, and absorption of sulfur is shown schematically in Figure 1. Raney nickel was prepared, and the sample was desulfurized in a 100-ml. distillation flask, A. Delivery tube B ,
terminating in a 1-nun. hole, was used to purge the system with inert gas. T o the connecting head, C, \\-as attached an acid reservoir, D , and delivery tube E. The absorber, F , was provided with a long delivery tube terminating in a spiral, 9 to 10 inches in length, and had a n opening of about 1 mm. at G. A hemispherical, Glas-Col heating mantle was used to heat flask A . L4 small fluorescent lamp was used to illuminat~~ the absorbent.
Table I.
Solvent 2,2,4-Trimeth~-lpentane Benzene Saphtha n-Heptane Xylene Table 11.
Sulfur Analyses of Solvents
s o . of
Detns.
sample Size, Grams
4 5 4 5 4
18-51 17-42 8-33 10-34 12-17
Sulfur Found, P.P.M. Mean Std. dev. 0 28 0 017 0 0 2 5
32 61 34 38
0 0 0 0
030 053 035 133
Analyses of Standard Sulfur. Solutions
Sulfur, P.P.M. Found -
PROCEDURE
Sample Size, Grams
~
Decant 2-propanol from a flask containing Raney nickel. Introduce a weighed amount of sample and wash down the side of the flask with 5 nil. of 2-propanol. Set the flask in the heating mantle; lubricate the connecting head with Cello-Grease and place it n the flask; allow nitrogen (or hydrogen) to flow through the sample at a rate of one bubble per 1 or 2 seconds, and turn the heater on. Select a temperature which will cause bubbles to emerge from the Raney nickel surface n 10 to 15 minutes. Introduce 10 nil. of I S sodium hydroxide and 10 ml. of acetone into the absorber, F . Purge this solution n i t h nitrogen during the time the sample is being desulfurized. Allou- desulfurization to proceed for a n arbitrarily chosen period of 30 minutes. Shake flask and its contents occasionally during this period. Aftcr the desulfurization period, place 10 nil. of 1.5 to 1 hydrochloric acid solution in the acid reservoir, D. Disconnect the nitrogen from absorber F and attach it to the acid reservoir through a glass tube in a cork stopper. Attach tlie nitrogen delivery tube, E , to which a short length of Tygon tubing is connected, to the drlirery tube of absorber F . Add 2 drops of dithizone solution to the absorbent. Slonly add acid dropwise to the contents of flask A while maintaining nitrogen flon a t B. Introduce a few drops of niercuric acetate into the absorbent; the color of the solution will turn from vellow t o pink. When all of the acid has entered the flask, raise the temperature setting t o the minimum \\-hich nil1 bring the contents of the flask to reflux. The first few niicrograms of hydrogen sulfide absorbed will change the color of the absorbent from pink back to yellow. Further absorbed hydrogen sulfide is titrated with mercuric acetate until the color of the absorbent remains definitely pink. After 15 to 20 minutes of hydrogen sulfide evolution, discontinue the nitrogen flow momentarily, and cool the flask by placing the hand around it. This cooling allows absorbent to travel u p the delivery tube in the absorber and recovers any hydrogen sulfide which may be adsorbed on its surface. Repeat this process several times, taking caution t o prevent absorbent from wetting the horizontal portion of the delivery tube. E X P E R l M E N l A1
Initial experiments to reduce sulfur
Compound Sulfur
Solvent Benzene
Calcd.
Mean
Cyclohexanethiol
Saphtha
183 19 5 111
184 le 5 112 38 1 9 12 41 0 2 83 59 6 20 2 6 90 2 39 I 03 7 2 -3 24 0 8 43 3 31 190 19 9 373 125 43 4 29 6 7 68
3i 9 41 2 60 20 6
Thiacyclohexane
Kaphtha
Diethyldisulfide
Xylene
4 11 8 93 3 3 94 2 50 1 02 i2 4 24 5 8 58 3 27 190
Thiophene
n-Hept an e
378
1-Butanethiol 4-Thiaheptane
2,3-Benzothiophene
P,P,-l-Trimethylpentane 2,2,4-Trimethylpentane
Benzene
19 7 126 44 1 30 3 7 81 4 07
4 14
s o . of Detns.
Std. dev.
5 0 2 0 0 0 0 0 0 0 0 0
2 65 4 99
4 9
7 8
269
37 052 99 21 194 126 023 1 03 0 40 0 162 0 131 1 5 0 70 2 4 1 0 0 43 0 54 0 0i3 0 075
-I
7
6 8 0
7 8 6 12 11 8 9
6 6 4 4
5 6 5 6
1 -5-5
1-14 1 i-6 1 1-14 7-17 *3-7 9-35 1-6 5 7-14
4-24
0-20 10-36 0 2-6 4-12 4-18 9 5-23 1 4-3 2 2-12 0 15-3 5 1 5-6 .3 5-14 1 5-9 10-18 13-19
Equivalent results obtained also with Eastman Organic Chemicals: 1-heptanethiol, benzenethiol, 2-naphthalenethiol, n-propyl sulfide, n-butyl methyl sulfide, isobutyl sulfide, phenyl sulfide, n-butyl disulfide, benzyl disulfide, and phenyl disulfide. in standard solutions with c.ommrrcially prepared Raney nickel and n i t h Raney nickel prepared according to directions of Pavlic and Adkins (7j indicated that sulfur v a s introduccd from an eatraneous source. This source of sulfur contamination TI as traced to sulfur present in the nickel-aluminum alloy. The presence of sulfur in the alloy necessitated the application of a blank sulfur correction to the analyses. This blank sulfur correction n a s established by treating portions of alloy, ranging in size from 0.2 to 1.0 gram, as dcscrihed in the Procedure, except that no sample was added. Recovered sulfide was titrated with mercuric acetate solution having a calculated sulfur titer of 30 y per ml. A sulfur blank of 33.1 y per q a n i of alloy was established. The plot of sulfur recovercd 2's. grams of alloy extrapolated to the origin of the graph. This indicates that sulfur originated entirely in tlie alloy and none was contributed b y the reagents used to prepare the Raney nickel. Standard sulfur solutions \yere prepared b y dissolving sulfur compounds in various solvents of lo^ sulfur content. By serial dilution, solutions were pre-
Table Ill.
Sample Analyses
Sulfur, P.P.M.
Samplea 1
2
3 4 5 6 i 8 9
E F G
Raney nickel reduetion
