Determination of Trace Amounts of Arsenic in Petroleum Distillates N. C. MARANOWSKI, ROBERT E. SNYDER, and RALPH 0.CLARK Co.,Pittsburgh 30, Pa.
Gulf Research & Development
Arsenic in petroleum fractions can b e determined by a reflectometric method in concentrations as low as 1 p.p.b. In principle, the method depends upon wet oxidation, followed b y isolation of the arsenic on paper as the colored reaction product between arsine and mercuric bromide. The intensity of the resulting colored complex is evaluated quantitatively with a spectrophotometer fitted with a diffuse reflectance assembly. Recoveries of 90% or better were obtained in most cases on samples containing from 0.1 to 0.5 y of arsenic. The standard deviation is within the range of 0.020 to 0.025 y of arsenic. Interference by most of the common metals, except antimony above 100 y, is not a problem.
W
catalytic reforming processes, it has become necessary to devise and refine analytiral t,echniques foi, the determination of certain trace elements that may affect the activity of the catalyst. Important aniong the contaminating constituents when platinum catalysts are used is arsenic! which may be introduced during the crude oil production operations and carried through in the distillate fractions prepared as charge stocks for reforming. Depending upon the arsenic content of the charge, the catalyst may become deactivated to an extent where it' is no longer feasible or econoniical t o maintain operations. necessitating replacement of the catalyst inventory a t considerable cost. For this reason, an accurate and reliable method for the determination of arsenic in concent1,ations of the order of a feiv parts per billion is needed as a rontrol for tlie quality of reforming charge stocks. Xumerous methods have been described for the determination of low concentrat,ions of arswic in various materials. The preferred colorinietric procedure appears t o be one based on the formation of molybdenum blue (3, 5, 8-20, 12). Although the method is simple and reproducible. it' necessitates tlie use of large sample sizes when applied in the part per billion range. A more ITH THE U E T E L O P M E S T Of
sensitive test for arsenic is the familiar Gutzeit reaction which has been used by a number of investigators (4. 7 , 11-15). This method depends upon the evolution of arsine and its reaction nith mercuric bromide to form a colored complex. Satterlee and Blodgett (11) described a n ultramicro procedure whereby the reaction is concentrated by vacuum filtration through a gaged area of the sensitized medium. The method uses photographic reference scales adaptable to either visual or densitometric evaluation of the spot reactions. .Jay and Dickson (6) simplified the Sattcdee and Blodgett procedure and applied it successfully to petroleum prodw t s . Although the latter method has I m n used with limited success a t this lahoratory, it retains the undesiiable feature of a visual comparison of test disks \\-ith standards. The task of extending the application of the Gutzeit reaction to trace amounts of arsenic using impregnated strips, stiings. or disks together with visual comparison embraces many factors. These include such difficulties as the appraisal of stains due to the unrqual distribution of the arsenicmercuric bromide complex and the occasional intensification of color in localized areas. The necessity of analyzing standards with each set of samples to compensate for operational variables unnecessarily complicates the analysis. The method described here is a modification of that of Jay and Dickson (6). Use of their technique led to incomplete recovery of arsenic with the relatively large sample sizes involved. An improved digestion procedure is described n hich allows for more intimate contact between sample and acids and yields quantitative recovery of arsenic. d Beckman Model DU spectrophotometer equipped n i t h a diffuse reflectance attachment is used to evaluate the intensity of the colored spot. APPARATUS
The digestion apparatus consists of a three-necked, round-bottomed borosilicate flask of 500-ml. capacity, fitted with a precision bore bearing stirring unit (Ace Glass Co., Inc.) and ti\-o reflux condensers.
The evolution apparatus consists of a 250-ml. borosilicate Erlenmeyer flask connected to a hydrogen sulfide trap by means of a ground-glass joint. The trap is fabricated from a 75-mm. section of 11-mm. glass tubing having a 24/40 T inner ground-glass joint a t one end and a 14/35 T joint a t the other end. Filter clamp assembly (6) fabricated from Plexiglas and fitted n-ith two Teflon gaskets. A 550-watt electric heater fitted with a Transite top having a hole 3 inches in diameter and connected to a variable transformer. Open end manometer fitted with a bleeder valve. Vacuum pump. X a t e r bath maintained a t 50" =t1" C., fitted with a suitable wire mesh rack for supporting the evolution flasks. Beckman DU spectrophotometei equipped n ith the diffuse reflectance attachment. The accessory is fitted with masking plates fabricated from 1/32-inch stainless steel having a centered 1/4-incli hole reamed to &0.000. The disks are cut to fit the carriage receptacles t c =tO.OOl inch and are sprayed with optical flat black paint. REAGENTS AND SOLUTIONS
Isopropyl alcohol, 99%. Hydrogen peroxide, 30%. Glass wool. Distilled Jyater. Ammonium oxalate, saturated ayueous solution. Lead acetate solution prepared by dissolving 17.5 grams of lead acetate trihydrate in nater containing 5 ml. of glacial acetic acid and diluting to 100 ml. Xercuric bromide solution prepared by dissolving 2.5 grams of mercuric bromide in 95% ethyl alcohol and diluting t o 25 ml. The solution is stored in a borosilicate. glass-stoppered, amber bottle. Kitric acid, specific gravity, 1.42, arsenic content less than O.OOOOOOl%. Sulfuric acid, specific gravity, 1.84, arsenic content less than 0.0000001%. Potassium iodide solution, 15%. Filter disks prepared by cutting Schleicher and Schuell S o . 575 filtei paper into disks approximately 5i/8 inch in diameter. Sensitized filter disks. Prepare a supply of disks, sufficient for 1 week, as follows. Place the filter disks in a small amber jar and corer with 25 ml. of VOL. 2 9 , NO. 3, MARCH 1957
353
mercuric bromide solution. Evacuate for 1 hour a t approximately 0.5 atm. The disks may be safely kept for 1 week if protected from bright light and the atmosphere. Standard arsenic solution, prepared by dissolving 1.320 grams of primary standard arsenous oxide in 25 ml. of sodium hydroxide solution (250 grams per liter) and diluting to 1 liter with water. For calibration purposes, the stock solution is diluted 1 t o 10,000 with water. Stannous chloride solution, 40% in hydrochloric acid (specific gravity, 1.19). Sulfuric acid, 1 to 1. Zinc metal. Screen 20-mesh zinc, containing less than O . O O O O l ~ o arsenic, through a 20- and then a 30-mesh sieve. Transfer the zinc remaining on the 30mesh screen to a beaker and cover with water. Activate it by placing the beaker on a hot plate, adding 50 ml. of concentrated hydrochloric acid, and stirring until the zinc becomes bright in appearance. Decant the acid solution and wash the pickled zinc with hot water until the washings react neutral to litmus paper. Store the activated zinc under distilled water. Just prior to use transfer the zinc t o a small Buchner funnel, rinse with several small portions of acetone, and dry on the funnel with vacuum. EXPERIMENTAL
Seasoning
of Glassware. Kew di-
gestion equipment is usually contaminated with small amounts of arsenic and must be seasoned with hot acids as follows. Place 50 ml. of concentrated sulfuric acid and 10 ml. of nitric acid in each digestion flask and attach the water condensers and stirrer units. Place the flasks on heaters and reflux for 4 hours. Remove the condensers and allow the seasoning t o proceed for an additional 3 hours. After the flasks are cool, discard the acid and thoroughly rinse the digestion equipment with water. Calibration Curve. Pipet 1, 3, and 5 ml. of the diluted arsenic standard into separate evolution flasks; add 5 ml. of distilled water to a fourth flask as a blank. To each flask add 35 ml. of distilled water, 25 ml. of 1 to 1 sulfuric acid, and 20 ml. of isopropyl alcohol. Place the flasks in the constant temperature bath and allow to equilibrate. Add 5 ml. of potassium iodide to each flask and allow to equilibrate again, Finally, pipet 1 ml. of stannous chloride solution into each flask. Insert a loose plug of glass wool into the lom-er half of each absorption tube and moisten with lead acetate solution. Insert a loose plug of cotton in the upper half of the tube and in the tapered portion of the Plexiglas assembly. Blot a sensitized disk between filter paper, air dry, place betneen the two Teflon gaskets, and assemble the apparatus. Introduce 3 grams of zinc into the flasks, then immediately attach the hydrogen sulfide absorption tubes and clamp assemblies. Adjust the vacuum to 190 i 5 mm. of mercury and 354
ANALYTICAL CHEMISTRY
allow the reaction to proceed for 30 minutes. F i t h a 2-mm. slit width, measure the reflectance of the test spot against a clean, unsensitized disk a t 400 m,u. Plot the reflectance readings, expressed as absorbance, us. total micrograms of arsenic. Procedure. Introduce a weighed amount of sample, equivalent to 0.1 t o 0.5 y of arsenic, into a seasoned digestion flask. Add 25 ml. of concentrated sulfuric acid and assemble the apparatus. Add approximately 5 ml. of nitric acid and stir the contents vigorously for 30 minutes. Then apply heat, gradually increasing the input while allowing the digestion to proceed for 2.5 hours. Add I-ml. portions of nitric acid whenever the mixture begins to darken. Remove one condenser from the flask and continue the addition of small increments of nitric acid as darkening occurs, until the lighter nonoxidized fraction is completely volatilized. At this point add 2 ml. of 3070 hydrogen peroxide to each flask, followed by nitric acid if darkening occurs. Repeat until the acid remains colorless or light amber. After the flasks have cooled to room temperature, add 7 5 ml. of lvater and 10 ml. of ammonium oxalate solution. Heat the flasks until dense white fumes of sulfur trioxide are evolved. Then cool the digestion mixtures to room temperature and transfer into separate evolution flasks. If the concentration of arsenic in the sample is suspected of being less than 2 p.p.b., pipet 1 ml. of the diluted standard arsenic solution into the evolution flasks containing the sample. Prepare a spot reaction as described for calibration, beginning with the addition of 20 ml. of isopropyl alcohol. Convert the reflectance measurements to micrograms of arsenic by means of the calibration curve, correcting for added arsenic if necessary. Determine the magnitude of the method blank, which consists of arsenic inherently present in the reagents as well as in the glassware, in the same way. If the value exceeds 0.025 y of arsenic it is indicative of contamination of the reagents, improper seasoning of glassware, or both.
with nitric acid and hydrogen peroxide. The evolved arsine was reacted on mercuric halide-impregnated paper strips and evaluated visually. The procedure, although simple and rapid, was unsatisfactory from the standpoint of recovery of arsenic. The next digestion study embraced a wet oxidation method (6). Specifically, the technique consisted of oxidizing the sample with sulfuric and nitric acids and hydrogen peroxide in a Kjeldahl flask fitted with a water condenser. The digestion was simple to perform but, when applied to standards prepared from triphenylarsine and iso-octane, was erratic and gave low results (Table I). However, the data indicated that increasing amounts of arsenic were recovered as the sample size mis diminished. This suggested that more intimate contact between sample and acid was necessary to determine arsenic concentrations of the order of a few parts per billion. To obtain this intimate contact. the digestion mixtures
Table I. Arsenic Recovery with Nonstirred Mixtures
Sample, Grams 20 15
71
25 25 25 30 3,5 40
5
163
86 100
41
Table II.
DISCUSSION
A sample size should be chosen so that the total arsenic content falls between 0.1 and 0.5 y. The maximum size sample which can be handled conveniently is approximately 40 grams. Based on the recommended maximum sample size, the lower limit of the method 1%-ithoutthe addition of the standard arsenic solution is approximately 2 p.p.b. Digestion Studies. I n the search for a method sensitive to about 1 p.p,b. of arsenic, several digestion procedures were tried. One method involved the extraction of arsenic from petroleum distillates with sodium hypochlorite and 72% sulfuric acid, followed by oxidation of the extract
-krsenic, P.P.B. Added Found
Av *
Recovery, c /C
61 46
57
Effect of Glassware Seasoning
(Triphenylarsine-iso-octane blends) Arsenic Found, y Arsenic Added Before Aft 91' P.p.b. y seasonlng seasoning"
13
0.227
15
0 261
0 242 0 310 0.232 0.266 0.363 0.346 0 267
Std. dev., 0.021 -J.
0.222 0 197 0 193 0 213 0 249 0.240 0 257
Table 111.
Arsenic Recovery with Stirred Digestion Mixtures
Sample Percolated naphtha
Brsenic Added
Arsenic Found,
P.p.b.
Y
Y
1.1
0.048
0.027 0.034 0.033 0.024 0.026
3.1
0.115
0.034 0.089
0.116 0.109 0.107
0.082
5.3
0.196
0.098 0.190 0.156 0.146 0.198
Base naphtha (contained 20 p.p.b. of arsenic; sample taken was equivalent t o 0.180 y arsenic)
0.216 0.189 6.2
0.229 (0.409, total)
0.402 0.438 0.368 0.432
Std. dev., 0.025 y .
were stiired in a 500-nil. flask fitted with a precision bore stirrer unit to minimize possible loss of volatile arsenic compounds. When this apparatus was first tested, high results for arsenic were obtained. Further study showed that the glassware had to be well seasoned before use (Table 11). A number of determinations were carried out on a naphtha that was previously percolated through silica gel t o remove arsenic conipounds. This percolate was then blended with known amounts of triphenylarsine in the range from 1 to 5 p p.b. of arsenic. The arsenic content was also established for the base naphtha and triphenylarsine was then added. Table I11 shows the results obtained on the base and percolated naphthas. As the presence of coke or carbon in the digestion mixture enhances the formation of volatile, reduced valence arsenic compounds, the oxidation must not be allowed to proceed beyond incipient darkening before nitric acid is added. Experimental m-ork demonstrated that, after a preliminary treatment with the sulfuric and nitric acids, the lighter fractions of the sample may be alloTyed to escape without loss of arsenic. When analyzing samples with an arsenic content less than 10 p.p.b., it is imperative that the niet,hod blank be checked periodically. Color Reaction Area. T o evaluate a spot reaction reflectometrically the light beam must be incident upon a fixed area of the colored product. If the spot area is constant, it need not be known; if the area varies, its value must be determined. Therefore, t o
standardize the exposed reactant surface and to make certain t h a t the measurements were due entirely t o the intensity of the spot reaction, masking plates were fabricated with a precisely drilled circular hole in the center, slightly smaller than the diameter of t h e test spot. Type of Paper. The make of filter paper does not appear t o be unduly critical as far as reflectance measurements are concerned. Each type of paper offers certain advantages and disadvantages in producing uniform spots. Schleicher and Schuell No. 575 filter paper, a thin, hardened, smooth textured paper, permits a spot reaction with minimum diffusion. The color reaction produced with this paper appears to be more intense than with Whatman S o . 50 or similar grade papers. Coarse grained or soft papers should be avoided for obvious reasons. Unsensitized filter disks may occasionally vary in reflectance, presumably because of variations in surface texture or accidental discoloration of the paper stock. For this reason, the reference disk used to adjust the spectrophotometer for 100% reflectance must be a representative one. To determine if the disk meets this criterion, several clean, unsensitized paper disks, selected a t random, are compared for reflectance. If the difference in reflectance is not greater than that equivalent to 0.01 absorbance unit, the disks are considered suitable for use. If the differential reflectance is greater than this value, the disks should be discarded and a different lot of the paper employed.
Spectral Curve. An absorption spectrum of the arsenic-mercuric bromide complex was determined between 350 and 600 mp using a spot reaction equivalent to 0.57 of arsenic. Although no sharp peaks were found, a slight plateau occurred a t 400 111,~. All subsequent measurements were made a t this wave length using a 2-mm. slit vidth arid normal sensitivity. Evolution of Arsine. The variables governing the intensity of the arsenic spot reaction must be critically controlled if accurate results are to be obtained. These variables include the concentration of sulfuric acid, weight and mesh size of the zinc. regulation of x-aciium, temperature of the evolution mixture, and impregnation of the test disks. Several of these factors have been thoroughly investigated by other workers and their conclusions were directly employed in this method. These include concentration of sulfuric acid (4), rreight of zinc (1, 4 ) , and impregnation of the test disks ( 2 , 4). A study was made to determine the effect of several brands of zinc and mesh size on aisenic recovery. T K O makes of granular zinc labeled as 10and 20-mesh and a zinc produced electrolytically were investigated. For uniformity of particle size, which is related to surface area. the granular zinc was screened through a No. 20 and EO.30 sieve. The zinc collected on the KO. 20 screen and on the No. 30 screen were reserved for subsequent testing. As indicated in Figure 1, the 20-mesh granular zinc appears t o be the most active, and the electrolytic the least. This fact has been corroborated by Cassi1 ( 2 ) . The evolution studies were made using a bath maintained a t 20" C. and pressures approaching 0.5 atm. Occasional erratic staining and lo^ recovery of arsenic suggested that not only \vas incomplete arsine evolution occurring but that perhaps other factors were affecting reproducibility. Because the method was an adaptation of a vacuum spot-filtration technique pressure might be a deterniinative factor. Table IV shows data obtained by careful regulation of pressure in the evolution system on a series of arsenic standards. The pressure
Table IV. Effect of Pressure on Evolution of Arsine a t 20" C.
Pressure, Mm. 3 90
i9o
190
380 380
380
Arsenic, y
Added 0.10 0 30 0 0 0 0
50
10
30
50
Recovered 0.108.0.059 0 245,O 2 i O 0 463,O 418 0 079,O 076 0 248,O 251 0 413,O 415
VOL. 29, NO. 3, MARCH 1957
355
studies were conducted with 20-mesh zinc and a bath at 20" C. This experiment demonstrated that consistent recovery of arsine was not attained. During the course of this n-ork it was found that the test spot area was unifornily stained when the pressure in the reaction flask ivas evenly controlled.
0.400 A=ECECTROCYTIG ZINC, 5 GRAMS E = BAKER'S ZINC, 2 0 MESH, 3 G R A M S G:MALLINCKRODT ZING, IO MESH, 3 G R A M S
D 0 300
C
--IC
s
(1
Table V. Reproducibility of Calibration Data at 50" C. and 190-Mm. Pressure
Arsenic Added. 3
0.10 0.30 0 50
y 0200 2
week 0 082 0 237 0 38;
2nd \wek 0 080 0 2.38 0 389
A
W u
U
Absorbance 1st
B
w
E W
4th week 0.100
0 078
0.23Y 0.391
01
Because of the incomplete evolution for marginate diffusion of the spot reaction in the prwence of evcessive m t e i vapor a t high bath tenipeiatures ( 1 1 ) . a median temperatuie of 50" C. was selected. The water lapoi produced at this temperatuie was effectively removed as condensate in the lead acetate absorption tube, tlic final traces being trapped by cotton placed in tht. filtiation assembly. Furtheiinore, this temperature seemed to provide coniplete evolution of the arsine at a pressure of 190 nim. Analytical data (Table V) shorv that consistent reflcctawe values were obtained over a peiiod of 1 month on three arsenic standards. Calibration Curve. The fact that the aisenic-mrrcuiic Ix oniide ieaction gives a complex \%hich follons 3 straight line function is advantageous in t h a t fewer points are required t o define the standard calibration cuivc. The linearity of the curve a p ~ ~ l ionly r s to amounts of arsenic less than about 0.5 y. A lack of reagent is apparently not responsible for the c u r ~ a tule. as thp mercuric bromide is present in large excess (approuimately 1000 y per disk). It seems 11101e likely that t1w proportion of aisenic combined a t or near the surface of the disk approaches a maximum intensification rmulting in a leveling off of tlie curx-e for larger amounts of aisenic. Loss of arsenic thiough the disk docs not occur within the recomInended concentration range, as shon-n by a lark of color on n second disk in the filtration assembly. Other Variables. The spot reactions are stable for 1 t o 1.5 hours without significant change, although a gradual change in the shade of the color may be noted if the disk is not protected from laboratory fumes. For precise work. the disk should be 356
ANALYTICAL CHEMISTRY
02
03
04
05
ARSENIC, MICROGRAMS
at 20" C, and the ieported tendency
Figure 1. Effect of mesh size and type of zinc on arsenic recovery
Table VI.
Reproducibility of Method
Arsenic Found, P.P.B. Gasoline Xaphtha 2 2,2 0 19,21 4 1,4 4 17,iT 1 i, 1 4 19,Ii i 9, 7 3 10, lt5 1 0,l 0 23,21 6 0,6 3 21,22 0 9.0 8 21.22 i 717 5 22:22 1.2,1 0 19,20 G.9,6 5 24,25 Std. dev., p.p.b. 0 . 2 0 8
measured as sooii as possible following the evolution step. Direct sunlight oi strong artificial light have a bleaching effect on the iraction product. Of the possiljle intei feiing constituents. antimony should be corisidcicLd 111 the analysis of samplcs Iia\ ing a petroleu~iiorigin. When varying amounts of antimony were mixed \\ ith knon-n concentrations of ni senic, the results indicated that about 100 y of antimony can be toleiated when 0.50 y of aisciiicis present. The removal of antinion! by the hydiobromic acid technique (11) was iiot investigated, because thtl antimony content of naphtha cliargc stocks is not expected to exceed the 100-7 limit. Hydrogen sulfide interference is eliminated by the use of lead acetate. Sonir nietallic salts tend to increase the ratc of ai*sincevolution; this effect is usuall) not serious. lion e w . Rrsidiial hydro-
carbons tend to inhibit t'he rate of evolution and should be absent. The ACS specification for i,eagent grade sulfuric and nitric acids is not more than 0.000003~,arsenic. There~ fore. the limit' of 0 . 0 0 0 0 0 0 ~nrsenica specified for these acids in this prowdure might appear to be unrealisbir. However, t s t s perfoi,nied on Baker & Adanison sulfuric and nitric acids foi, over 1 year have showed that they never exceeded the specified lon-er limit of arsenic content. Strict adherence to the details set forth in this inethod is absolutely iiccessary for accurate results. On the othei, hand, the procedure is relatively siniple and iiot part>icularly tedious t o carry out. Six determinations arc asily completcd by one optmtor in s 11ours. ACCURACY A N D PRECISION
A statist.ical st,udy of bhe recoinmended procedure T T ~ Smade by applying it to a srrics of synthetic samplw ranging from 1 to 26 1i.p.b. of ai,eenic.. The standard deviations obtained from this study ai'c shown in Tables I1 and 111. The culculntioris indicatr the precision to be within the range of 0.020 to 0.025 y of arsrnic. A w ~ r a c y of 90% or better is obtained on samples cont,ainiiig from 0.1 to 0.5 y of aiwnic,. Listed in Table T'I are some i w i donily selected results obtaincd foi. 10 gnsolines and napht1i:is snl>mittcsd for tlic detemiiiiation of arsenic,. .\ltliough the dat:i cannot be relial~l nith tlie othcr indicatcd I
I)elieved tile accuracy is of the saillc: oydcr of magnit,ude a s tha,t of the synthetic samples.
(4) Ho\v,A. IC., [bid., 10, 226 (19i38). ( 5 ) H11t)t)ard, D. 11.1 [bid., 13, 915 (1941). (6) Jay, R. It., Dickson, I,. It., Petrolemi Processing 9, 374 (1952).
LITERATURE CITED
(1) hssoc. Offic. Agr. Chemists, "Official ?*lethods of Analysis," 7th ed., 1950. (2) C a d , C. C., J . dssoc. O j i c. Agr. Chemists 20, 171 (1937). (3) Chaney, -4.L., Magnuson, H. J., IND.ENG.CHEII., ANAL. ED. 12,
691 (1940).
(T) Lachele, C. E., I s u . KXG. CIIEM.,
. ~ s A I , . ED.6, 256 (1934). (8) Niteching, E. H., Flinn, F. 13., J . Lab. Clin. .\led. 15, ii9 (1930). (I))Morris, H. ,J., Calvcry, H. O., ISD. ENG.CHEAT., .4x.i1,. ED. 9 , 447 (1937). (10) Itodgew, D., IIcron, .1. E., .IriaZyst
(11)
71, 414 (1946). Satterlee, €1. J., Hlodgett, G . , ISD.
ESG. CHEAT.,. 4 s a ~ .ED. 16, 400 (1944).
(12) Scott,, W. IT., "Standard Methods of Chemical Analysis," 5t.h ed., pp. 99, 115, Van Nostrand, Yew I'ork, 1939. (13) Smith, D. G., British Petroleum Co., London, England; private conirniiiiication (1952). RECEIVEDfor review June 11, 1956. Accepted December 26, 1956. Group Session on Analytical Research, Division of Refining, American Petroleum Institute, Montreal, Canada, Ma>- 1956.
Hydrocarbons in the 116" to 126" C. Fraction of Petroleum AUGUSTUS R. GLASGOW, Jr.,l RAYMOND J. GORDON, CHARLES B. WILLINGHAMI2 BEVERIDGE J. MAIR, and FREDERICK D. ROSSlNl Petroleum Research laboratory, Carnegie Institute of Technology, Pittsburgh I 3, Pa.
Twenty-two hydrocarbons were found in the fraction of petroleum normally boiling between 116" and 126" C. This fraction of petroleum constitutes approximately 5.37% by volume of the representative petroleum which has been under investigation for many years by the American Petroleum Institute Research Project 6. These compounds were concentrated by extended use of the fractionating processes of regular and azeotropic distillation. Identification of the compounds was made by mass and infrared spectrometric examination, together with measurements of the simple physical properties. The names and estimated relative amounts by volume of the twenty-two compounds, given in order of decreasing amount in the petroleum, are as follows: n-octane, 35.5; 2-methylheptane, 1 6.7; 1,cis-3-dimethylcyclohexane, 1 1.8; 1 ,trans-2-dimethylcyclohexone, 5.8; 3-methylheptaneI 5.6; 1 ,trans-4-dimethylcyclohexane, 4.5; 4methylheptane, 3.7; 1 ,trans-2-ethylcyclopentane, 2.5; 3,4-dimethylhexane, 2.4; 1 -methyl-trans-3-ethyIcyclopentane plus 1 -methyl-cis-3-ethylcyclopentane, 2.3; 3-ethylhexaneI 1.7; 1 ,cis - 4 - dimethylcyclohexane, 1.6; 1 ,frans-3-dimethylcyclohexane, 1.3; 1,cis - 2,trans - 3 trimethylcyclopentane, 1,l -dimethylcyclohexane, 1.2; 1.3; 1 ,1,cis 2,trans- 4 tetramethylcyclopentane, 0.8; 1 -methyl- 1 -ethylcyclopentane, 0.6; 3-methyl-3-ethylpentaneI 0.3; 1 ,cis-2,frans-4-trimethylcyclopentane, 0.2; cycloheptane, 0.1 5; 2,2,5trimethylhexane, 0.03. In addition,
-
-
-
trace amounts of an unidentified bicycloparaffin occur in the distillate fraction boiling near 124.5" C.
A
p . 4 ~of~ the continuing work of the American Petroleum Institute Research Project 6 on the composition of its representative petroleum (7), analysis has bccn eomplcted of the hydrocarbons in that fraction of this petroleum normally boiling betn cen 116' and 126' C., nhich constitutes approximately 5.37% by voluinc of the original petroleum. An early invrstigation ( 4 ) revealed the presence of four hydrocarbons in this fraction: 2-niethylheptanc at 117.65" C.; l,cis-3-dimetliylcyclohexane at 120.09' C.; l,trans-2-dimetliylcgclolie\ane :it 124.45' C.; and noctane a t 125.66' C. The present in\ estigation of this portion, using distillation equipment of much greater separating efficirncy than \\as earlier available, in conjunction \\ ith the modern spectroscopic mcthodq of analysis has shov n that it contains 22 hydrocarbons. The dctnils of the :inalysii of thc adjacent Ion er lioiling portion arc given in (3), and tliow of the :icljncciit highrr boiling portion in ( 2 ) .
s
PROCEDURE
=ill of the paraffin-cycloparaffiIi material remaining from the earlicr in\-rstigatioii (4) was combined and distilled a t 725 nini. of mercury. Further processing of this material by distillation (regular, azeotropic. and at reduced pressure) was designed to concentrate the indi-
vidual 11) dioc:irhiib SO that they could be identified 11 ith reasonable certainty. The apparatus and procedurrs used in the distillations are drscribed in ( 8 ) . These distillation oprrations required the repeated blending of portions of distillate to product charges for redistillation. d total of 55 charge< n:ts prepared :ind cliatillcd. The amounts of the indi\.idiial conipounrls \\-rr(' coniputed from thc r t w l t s of ipwtrographic analyses (principally mass spectra, with ~ n i infrared c spectr:i) made for L I on ~ a total of j i intc~rmediatc~ and final frnctions by the Rcsearch 1,aI)oratoi-y of the Humble Oil :lad R h i i n g Co. In addition t o the -pcctrobcopic :in:ily.eq, the cr:d coinpounds \ r a t determincti from nic~isuremtnt~of the freezing point. Thc dttails of t h c v operations givtw in a rc>port of thv ilnierican Petrolvuni Iiiititutcx Ilriearch Project 6. c n t i t l d "Hydrocarbon. in the 116" to 126' C.Fraction of PctroIruni". dated AInrch 31, 1952, 11 hich is a v d a h l r from the Petrolrum Rpsearcli Laboratory, Carnegit, Inatitiitc. of Technology, Pittsburgh, Pa. The results of the distillation of the starting material are given in Figlire 1. In this figure, the solid lines refer to the distillation a t 725 mni. of mercury. and the dashed lines refer to the redistillation of a portion of the material at 30.5 mm. of mercury. It will be noted that the distillation at 30.5 nim. of inercury \vas very effectiw in separating the re1 Piesent iddress, Satioiiitl Rineau of Standards, Kashington 25, D. C 2 rlddrees, llellon Institute 01 Industrial Research, Pittsliurgh 13, Pa.
VOL. 29,
NO. 3,
M A R C H 1957
357
