terest, because there is a question whether the allrylat.ing agents exert their biological action through alkylation and cross linking, or through ionizing radicals as in x-irradiation. hlexander and Ross who studied the action of ionizing radiation in model systems. found t’hat the degradation of solid I)uly(niethacrylic acid) by x-irradiation could be prevent’ed b!. t’he addition of S-quinolinol ( 1 j . He suggested that 8-quinolinol may serve as a n dcceptor for liberated charges. d similar niech:inism is postulated for the protection of polymers in solutions 1., t’he reaction of cysteine or cysteamine with the ionizing radicals formed secondarily in the radiation treatment of such solutions. The reaction with cysteine followed the known pattern of near complete inhibition of the reactivitJ- of nit,rogen
mustard. It is suggested that the remaining portion of the nitrogen mustard was only loosely bound to cysteine via the carboxyl group, was more readily dissociable, and had reacted with the quinolinol reagent upon release. ACKNOWLEDGMENT
The author wishes to thank Gerald Sugarman for his competent technical assistance. LITERATURE CITED
(1) Alexander, P., Ross, h.C., Proc. R o y . Soc. ( L o n d o n ) A223, 302 (1954). ( 2 ) Boursnell, J. C., Cohen, J. A , , Dixon, M., Francis, G. E., Greville, G. D., Seedham, D. >I,, Wormall, A , , Bzochem. J . 40, 756 (1946). (3) Epstein, J., Rosenthal, R. \I-.>Ess, R. J., .tX..IL. CHEM. 27, 14.35
(195Merritt’, L. L., Jr., J . dm. Chem. SOC.73, 630 (1951j . (T) Phillips, J. P., Keolvn, R. W.,I b i d . , 73, 5483 (1951 1. (8) Skipper, H. E., Bennett, L. L., Langham, IT-, H., C a n c r t , 4 , 1025 (1951). (9) Trams, E. G., Kloppi C. T., Cancer Research 1 5 , 617 (1955). (10) Trams, E. G., Nadkarni, 11.V., unpublished data. RECEIVED for review April 10, 1957. Accept,ed October 8, 195i. Investigation supported by a contract with the United St,ates Atomic Energy Commission, XT(30-1)-llOT, and by reseuch grants from the Xational Cancer Institute. SiLtional Institutes of Health, arid Damon Runyon hlemorial Fund for C:inc.er Research (DRIR-42 C )
Dete rmina ti o n of PyrroIic Nitroge n in Petroleum Distillates M. A. MUHS and
F. T. WEISS
Shell Development Co., Emeryville, Calif.
b An improved colorimetric determination of pyrrolic compounds uses the reaction with p-dimethylaminobenzaldehyde. Optimum reaction conditions and a standard set of absorptivities have been established. Interference studies have shown the adverse effect of olefins and indicated the necessity of their removal, which has been satisfactorily accomplished by preliminary chromatographic separation. The method has broad general applicability; successful determinations have been made on gasoline, jet fuels, rubber extenders, and aromatic solvents. ~ E X V T I T E METHOD for the deterniination of pyrrole compounds is important because as little as 0.01%, cym,ised as nitrogen, can cause instability and promote gum and sediment formation in fuels (26’. SI). The presence of pyrrolic compounds in petroleum has been detected (19, 28, SI), but only from retorted shale oils and pyrolyzed asphaltites have a n y pure compounds been isolated and identified (p,yrrole, 2-methylpyrrole, 3-ethy1-2,4,5trimethylpyrrole, and 2,3,4,5 - tetraniethylpyrrole) (I?‘, SO, 3 5 ) . Reported analytical methods for the determination of pyrrolic nitrogen are far from satisfactory. Pyrroles. because of their loiv basicity, cannot
he dctermincd by available methods for basic nitrogen (5). Tn-o recent methods, one involving functional group analysis (16) and the other, chromatography and mass spectrometry (%), appear too lengthy for repeatcd application. Another, involving nieasurcnleiit of the infrared absorption of tlir nitrogen-hydrogen bond (26), does not have sufficmipt sensitivity to determine the small amounts of these compounds present in most petroleum distillatrs. Of the numerous known color reactions of pyrrolic compounds, only thc p dimethylaminobenzald(~hycio rcaction exhibits any generality. A procedure using this reaction has been tlevcloped by the Universal Oil Products Co. (%), but, although simple t o run, it is of liniitcd application and sometinies giws erratic results. This reaction has been further studied t o overcome t,hrse difficulties. p-Diniet hylaminobenzaldeh ?-de (I) condenses with pyrrolic conipounds in the presence of acid to produce a rrdviokt dye. The reaction may he depictrrt as follows ( 1 1 , 22).
I
L
I\
I n the presence of acid the condensation product exists as a resonance hybrid (I11 and IT’) and the rolor formation is due to t h r contribution of thc quinoidal form (IT7). The color, present in dilute acid, disappears in concmtrated acid, because of the formation of T’, n hich cannot exist in :I color-produciiig quinoidal form. This reaction n a s first reported b j Ehrlich in 1901 (8) and has been used nidely in biological analyses. I n 1940 Chernoff ( 4 ) rxtended the analysis to pyrrolic conipounds in solutions that n we insoluble in the reaction medium. by initially extracting with phosphoric acid. This procedure v a s later adapted to petroleum samples by the Universal Oil Products Co. (32). The p - dimeth) laminobenzaldehyde reaction is general for pyrrole and indole compounds. but gives negative results with carbazoles. It goeb readily n hen the alpha-position is unsubstituted, and in most cases nhen only a betaposition 1s open (la),but tetrasubstituted pyrroles react a t a slower rate (34, 35). Definition of Pyrrolic Nitrogen. As carbazoles do not produce a color in this method, “pyrrolic nitrogen” VOL. 30, NO. 2, FEBRUARY 1958
259
is defined as t h e nitrogen present in the five-membered heterocyclic rings of pyrrole, indole, and their homologs. EXPERIMENTAL
Apparatus. A chromatographic tube with a n inner diameter of 20 mm. and a length of 200 mm. is satisfactory. For best results vigorous shaking is necessary. A Burrell wrist-action shaker, Model BB, is satisfactory, if it is adjusted for maximum agitation. A Beckman Model B or D U spectrophotometer, equipped with 1-em. borosilicate glass cells is used. The wave length readings should be within 2 mp of the true value and the slit widths should not exceed 0.1 mm. for the Model D U and 0.3 mm. for the Model B. Reagents. %-Heptane, free from pyrrole and olefin. Extract 2 liters of Phillips’ pure grade n-heptane with five 100-ml. portions of concentrated sulfuric acid and four 100-ml. portions of distilled water. Dry the solvent over a mixture of anhydrous magnesium sulfate and potassium carbonate for 1 hour and filter. The solvent obtained should produce an absorbance of less than 0.015 a t 580 mp when subjected to the conditions of this method. Benzene, C.P. Florisil, 60 to 200 mesh. This is a synthetic magnesium silicate, available from the Floridin Co., Warren, Pa. Phosphoric acid, 85%, reagent grade.
p-Dimethylaminobenzaldehyde-phosphoric acid solution. Dissolve 1.0 gram of p-dimethylaminobenzaldehyde in 100 ml. of reagent grade 85% phosphoric acid. This reagent should be prepared from the purified aldehyde (14) and should remain colorless. Acetic acid, c.P.,glacial. Sampling. To obtain a readable absorbance sample sizes should be chosen as listed below : Pyrrolic Nitrogen, P.P.M. 0.5 t o 5 to 50 to 500 t o
5.0 50 500 5000
Sample Size, G. 10 I 0.1 0.01
Procedure. Add the accurately weighed sample directly t o a Florisil column 15 em. high, and elute with 50 ml. of n-heptane, followed by 75 ml. of benzene and finally with isopropyl alcohol to remove the benzene. Discard the heptane eluate (the border between the heptane and benzene zones is made apparent by a difference in shading of the zones) and collect the benzene eluate in three 25ml. portions. Using 50-ml. volumetric flasks, dilute each fraction to the mark with benzene, pipet out 25.0 ml. for the determination, and use the remainder for the blank. Make the determination on the first two benzene eluates and if the second has an absorbance greater
260
ANALYTICAL CHEMISTRY
than 25% of the first, run an additional analysis on the third eluate. If it is known that no interfering substances are present in the sample, omit the chromatographic separation and run the determination directly on a solution of the sample in 25 to 40 ml. of n-heptane, using an equal amount of sample for the blank. Place the solutions in a 125-ml. separatory funnel and add 5 ml. of the p dimethylaminobenzaldehyde-phosphoric acid solution (for the blank use 5 ml. of 85% phosphoric acid in place of the reagent solution). Shake the mixture for 10 minutes, using agitation equivalent to vigorous hand shaking (approximately equivalent to a setting of 7 on the Burrell shaker). Then add 50 ml. of glacial acetic acid and continue shaking for 5 minutes. If the solution is homogeneous a t this point, place in a 100-ml. volumetric flask; if there are two phases, put only the lower layer in the flask. Wash the stopper, walls, and tip of the funnel with glacial acetic acid and add the washings to the volumetric flask. Dilute the solution to the mark with glacial acetic acid and measure the absorbance of the solution us. distilled water a t 530, 540, 545, 550, 560, and 570 mp, using I-em. cells. Because some samples produce unstable colors, make absorbance readings within 10 minutes of the completion of the procedure. If the absorbance of the solution is too high (above l.O), dilute it and the corresponding blank suitably nith a 5% solution of phosphoric acid in glacial acetic acid and divide the final results by the dilution factor. Subtract the absorbance of the blank from that of the sample and add the corrected absorbances from each of the eluates that produced a violet color before making the calculation (generally only the first eluate produces any color due to pyrrolic nitrogen). Calculation. Calculate the parts per million of pyrrolic nitrogen in the sample by means of t h e following equation.
tained a t the six wave engths as the value for the determination. These absorptivity values are satisfactory, if the limitations on slit width given under “apparatus” are observed. Under these conditions the absorptivities are reasonably independent of the particular instrument used. DISCUSSION
-
Pyrrolic nitrogen, p.p.m.
=
1.4 X 106(A) (Tt-Xa)
where
A
=
W
=
a
=
total absorbance minus that of blank a t a particular wave length weight of sample used in determination, grams-Le., one half of that added to the chromatographic column molar absorptivity at a particular wave length. Lyse the values belov:
mr 530 540 .~. 545 550 560 570
Litere/ Mole-Cm. 49,500 254.000 000 57,100 50,000 38,000
Take the average of the values ob-
Development of Method. The basic procedure follow the outline of the UOP method, involving extraction of pyrroles from a hydrocarbon solution with a p-dimethylaniinobenzaldehyde-phosphoric acid solution and development of the color by dilution with acetic acid. A study of the reaction variables has indicated that, when optimum conditions are used, maximum sensitivity, conformity to Beer’s law, and good repeatability are realized.
EXTRACTION OF PYRROLIC COMPOUNDS. The
extraction consists of initial shaking of the hydrocarbon solution with the p-dimethylaminobenzaldehyde-phosphoric acid solution, addition of glacial acetic acid followed by further shaking, and separation of the lower phase for measurement of absorbance. For the initial step, one extraction n-ith phosphoric acid is equivalent to several using smaller volumes, and is: superior to the use of a phosphoric acid-acetic acid solution. The reaction of p-dimethylaminobenzaldehyde with pyrroles apparently goes best a t high acid concentration-i.e., undiluted phosphoric acid. Varying the shaking time from 5 to 60 minutes did not affect the efficiency of the first extraction. The volume of acetic acid added to the extraction mixture after the first shaking was critical, although the time of the second shaking period was not (2 to 10 minutes gave comparable results). Increasing the volume of acetic acid increased the intensity of the final color until a plateau (which corresponded to a 94% recovery with indole) was reached a t 50 ml. A uniform and maximum extraction of pyrrolic compounds (94% recovery with indole) was obtained from hydrocarbon solutions with volumes up to 40 ml. Above this there was a decrease in efficiency of extraction. COXCENTRATION OF REAGENTS.The marked effect of phosphoric acid concentration on the intensity of the color developed is illustrated in Figure 1. A broad niaximum is reached when the concentration of 85y0 phosphoric acid is 5y0, over a range of wave lengths about the absorbance maximum. Because the variation in absorptivity between 4 and 6% is slight, the volume of phosphoric acid used need not be exactly measured, if it falls within the above limits. However, to avoid gross
I' 20 S < % p H O S P H 0 8 I C .ZCID,
.
Figure 1 Reaction of 2-methylpyrrole with p-dimethylaminobenzaldehyde
Effect of phosphoric x i d concentration on absorptivity
W A V E L E N G T H . mu
Figure 2. Reaction of pyrroles with p-dimethylaminobenzaldehyde. Spectra with pyrrole, 1 -butylpyrrole, 2-methylindole, and 3-methylindole
C.
A . Pyrrole B. 1-Butylpyrrole changes in concentration, any dilution of the final solution (because of high absorbance) should lie made with a 5y0 solution of 85y0phosphoric acid in acetic acid. This dependence of absorptivity on phosphoric acid concentration is consistent with the proposed mechanism for color forniation discussed previously. Apparently the maximum concentration of the color-producing compound is present when the acid concentration is 5%. I n the UOP method (32),the concentration of 85% phosphoric acid in the final colored solution was 15%- and in a n y subsequent dilution with glacial acetic acid the phosphoric acid concentration was lowered. This lack of control over the acid concentration niay be a reason for t h e erratic results obtained by the COP method. Studiea on heptane solutions of indole indicated that concentration of pdimethylsminobenzaldehyde has no effect on the color intensity, if the p-dimethylaminobenzaldehyde indole molar ratio is between 2 and 2500. STABILITY. The color intensity with indole n-as essentially constant for 20 hours, but with gas oils absorbances were 98% of the original reading after 10 minutes and had dropped t o 90% in an hour. Therefore, with petroleum distillates the reading must be made
within 10 minutes t o keep the error due to fading below 2%.
COXFORMITT TO BEER'SLaw. Excellent conformity to Beer's law was obtained by using solutions of 2-~nethylpyrrole and indole, the maximum deviation of any point from the established line being less than 1%. K i t h both conipounds Beer's law mas obeyed at the ahsorption maximum and a t a
2-Methylindole
D. 3-Methylindole
point 25 mp from the peak. Results obtained using three gas oil samples indicate that Beer's law also applied (1.4%) under these conditions. PRECISION. The relative 95Oj, confidence limits for the determination of indole in heptane are =k2% of the mean, when the sample is of such a size as to give a n absorbance in the range of 0.1 to 1.0. For actual petroleum samples,
Table I. Results of Reaction of Pyrrolic Compounds with p-Dimethylaminobenzaldehyde
SDectraa ALP.,
amax,
Compound c. B.P., O C. Pyrrole . .. 125-127 2-1Iethylpyrrole , . . 80-82/80 mm. 92-95/90 mm. 1-Butvlrxrrole 2.4-Di~~thvl~i.rrole . . . 51-5217 mm. 2;5-Dimeth-l&rrole .60-61)13 mm. Indole 52153 ... ... 2-Methylindole 60-61 ... 3-Methylindole 95-96 Carbazole 240 .. 9-Methylcarbazole 87-88 .. 9-Ethylcarbazole 69-70 Bv method described in this paper. * Absorbance X cm.-l for 1 X 10-5X solution. For 1.40 X 10-SM. d For 4.54 X 10-6JI. e For 4.62 X 10-6M. O
n so
1.5090 1 5015 1.4741 1.49'70 1,5060
... ... ...
liter mp mole-cm. 545 55700 545 66500 555 75000 536 59200 522 91700 565 77300 540 81600 580 62100
A=,
ilreab 1265 1766 1198 1726 1684 1431 1609 1503
0
VOL. 30, NO. 2, FEBRUARY 1958
261
because of slight instability and interference problems, the confidence limits under the above conditions are =t4% of the mean. Results with Pure Pyrrolic Com-
pounds.
T h e results obtained with t h e method on a series of freshly purified pyrrolic compounds are given in Table I and t h e spectra of some typical compounds are shown in Figure 2. The efficiency of t h e extraction of these compounds from hydrocarbon solutions ranged from 94 t o 1 0 0 7 ~ . The absorption maxima of the curves fall betwren 522 and 580 mp with absorptivities varying from 55,000 to 92,000. The areas under the curves of absorbance us. wave number show a similar variation. There are insufficient data to show any definite correlation between structure and area, absorptivity, or position of the maxima, although indoles as a class probably absorb at higher wave lengths than pyrroles. These variations in area, absorptivity, and position of the maxima indicate that the p-dimethylaminobenzaldehyde method cannot be used for a completely quantitative determination of pyrrolic nitrogen, but by a judicious choice of absorptivity or area per mole a close estimation can be made. Results with Mixtures of Pure
Pyrrolic Compounds. As t h e pyrrolic
content of petroleum is made u p of a complex mixture of compounds of unkno71-n structure, t h e most practical choice for a reference standard would be one based on mixtures of a wide variety of pure compounds. Such a n approach was used in the present method, where t h e determination was run on a series of mixtures of varying composition. A standard was chosen such that the average of the errors from the determinations on the various synthetic mixtures was a minimum. The results obtained with this standard were superior to those calculated with the spectrum of 2-methylpyrrole as the standard, as was done in the UOP method (32). Their choice of a standard was based on the observation that the spectra obtained from three fuel oil samples were similar to that of 2-niethylpyrrole. However, the inadequacy of this standard mas indicated when determination of 34 fuel oils showed a wide variance in spectra (37). The mixtures studied for the derelopment of the standard \\ere made from available conipounds (Table 11). This series contained mixtures of only pyrroles, only indoles, large amounts of pyrroles in the presence of indoles, and vice versa, equal amounts of pyrroles and indoles, and in several mixtures
Table II. Synthetic Mixtures for Testing Pyrrole Method
hlixture 1
2 3 4 5 6
7 8 9 10 11
Pyrrole 30.2 ... ... 18.6 18.6 ~~
8 9
18.1 18.5 9.6 15.2 17.9
(Mole per cent composition) 2-Methyl- 1-Butyl2-Methyl- 3-Methylpyrrole pyrrole Indole indole indole Carbazole 40.2 29.6 ... ... ... ... ... ... 37.5 31.9 30.6 ... , . . ... 20.1 19.4 20.9 39.6 ... ... 42.1 20.6 18.7 31.1 18.8 10.7 10.4 10.4 ... 9 9 9 0 30 9 19.9 21.4 20. i 9.i 20.9 10.1 21.7 ... 20.5 9.3 21.3 10.3 10.3 9.8 10.7 ... 11.0 ... 10.7 58.0 41.7 14.8 ... 14.7 13.6 , . . 20.3 20.6 ... 21.4 19.8 ... . . I
rather large amounts of carbazole w r e present, in an effort to simulate actual compositions of petroleum distillates (28)* The spectra obtained from the reaction of these mixtures with p-dimethylaminobenzaldehyde all had similar shapes n i t h absorption maxima falling in the 540 to 550 mp range. METHODS OF CALCULATION
Two new niethods for the calculation of pyrrolic nitrogen have been evaluated and compared with the previously reported niethod of calculation using 2-methylpyrrole as a standard (32). I n one (absorptivity calculation), standard absorptivities were chosen a t six wave lengths about the peak-Le., 530, 540, 545, 550, 560, 570 nip-so that a t each wave length the average of the errors from the eleven determinations on the synthetic mixtures was a minimum. The standard absorptivities used and the results obtained at rach wave length as well as the average of these values are listed in Table 111. Because the area under a spectral curve (plotted as absorbance us. frequency) is directly proportional to the energy absorbed, the areas were studied, on the possibility that, although the shape and position of the spectra from pyrrolic compounds and actual samples varied, the energy absorbed was more nearly constant. I n the second method of calculation the curve areas (measured nith a planimeter) mere compared with a standard area nhich nas chosen, as with the first method, so that the average of the errors from the determinations on the synthetic mixtures was a minimum. This value was found to be 1523 absorbance units-cm.-l per 10-5 mole-and results of the calculations using this value are listed in Table IV. Both methods \\ere comparable, giving results with an average error of 4 t o 6% on the synthetic mixtures. However, the results obtained using the
Table 111.
Calculated Values of Pyrrolic Nitrogen Concentration in Mixtures by Assigned Absorptivities Method 5 i O mp Average from six 560 mp 550 mp 545 mp 530 mp 540 mp 38 000 Wave Lengths 50,000 57,100 54.000 57.000 49. 5OOa Calcd. 91, Calcd. % 70- Calcd. % Knoxn Calcd. % Calcd. % ' Calcd. % Calcd. , ~
hlixture C0ncn.b C0ncn.c error c0ncn.C error concn.c error concn.c error 0.2 0 . 1 1.015 1 1.017 1.007 1 . 0 1.018 6.8 2 0.961 1,017 5 . 8 1.067 11.0 1.026 4.4 8 . 3 0.665 3 0.637 0,655 2 . 8 0,690 2.4 5 . 4 0.588 4 0.574 0.586 2 . 1 0.605 0.3 3 . 1 0.990 5 0,987 1,000 1 . 3 1.018 0.5 4 . 2 1.035 6 1.030 1.031 0 . 1 1,073 5.2 1 . 8 0.963 7 1.016 0.966 4 . 9 0.998 4.0 3 . 6 0.863 0 . 1 0,867 8 0.899 0,877 2 . 4 0.898 8 . 2 0.355 11.9 5 . 2 0.370 9 0.403 0.370 8 . 2 0.382 8.0 6 . 5 0.537 3 . 6 0.546 10 0.584 0,536 8 . 2 0,563 7 . 8 0.737 10.2 0.726 11.6 11 0.821 0.734 10.6 0,757 ,.. 4.8 Av. error ... ... 4.3 ... 4.6 ... 4.4 a Absorptivities, liters per mole-em. b Based on pyrrole and indole compounds present. c hloles/liter X lo5.
262
ANALYTICAL CHEMISTRY
concn.c error concn.c error concn.c error 4.9 0.976 4 . 0 0,735 27.7 0 962 1,097 14.2 1.118 16.3 1.346 40.1 0.712 11.8 0.855 34.2 0.705 10.7 1.4 0.576 0 . 4 0.458 20.2 0.582 73 0.968 1.9 _ _ ~. . R 0.960 2 . 7 n- . 856 5.0 1.102 7 . 0 1.215 18.0 1.081 3.7 0 . 1 0.978 0.970 4 . 5 1.015 3.3 6 . 2 0.869 0.864 3 . 9 0,843 6.7 0.376. 0.8 0.380 5 . 7 0,400 5.3 0 . 5 0.553 0.550 5 . 8 0,587 8.7 2 . 0 0,750 0.740 9 . 9 0.805 ... 14.8 ... 6.0 ... 6.4
spectrum of 2-methylpyrrole as a standard consistently were high, with a n average errcr of 36%. The absorptivity calculation was used in the final procedure because it is less cumbersome than the area calculation. Results u-ith mixtures containing carbazoles indicated t h a t these compounds arc not determined nor do they interfere with the determination of pyrroles and indoles. I n each case the ralculated amount of pyrrolic nitrogen n-as very close to the nitrogen actually prescnt in the pyrrole and indole compounds. Interferences. p-Diniethylaminobenzaldehyde reacts n i t h a large variety of compounds (1-3, 6, 7 , 10, 13, 16, 18, 21. 23-26, 27, 29, 36) and such reactions can interfere with the pyrrolic Table IV.
Mixture 1
2 3 4
5 6 7 8 9 10 11
Concn. X lose, Theory
1.017 0.961 0,637 0 ,574 0,987 1 030 1 016 0.899 0.403
575
857 1232 ...
Concentration x 1 0 5 0 Area c a l m Bbs. calcn. 0,976 0,962 1.036 1.097 0.696 0.705 0.601 0,582 0.957 0.968 1.019 1.081 0.971 0 978 0.869 0.908 0,376 0.378 0.563 0.553 0 , 750 0.809 ... ...
-
0.821 .4v. error ... Moles per liter. Based on calculations using 2-methylpyrrole as standard. Table V.
x 10-6JI and that of the reagent was 3 x 10-3JI-i.e., the concentration of reagent normally used in the method. This represents a 430-fold e x e s \ of p-diniethylaminobenzaldehyde. The compounds studied n ere olefins, aromatics, and miscellaneous sulfur-, nitrogen- and OYJ gen-containing compounds. K i t h the exception of the olefins and aromatic 6ompounds. the amounts added ranged from 15 mg. (equal to about one half of the amount of reagent used) to 0 05 nig. K i t h the olefins and aromatic hj-drocarbonq the amounts studied were as high as 1 gram, which was felt t o reprrsent the amounts of these compounds that nould be actually found in a petroleum sample. The effect of definite neights vaq studied, a. it is the amount of these 7
Calculated Values of Pyrrolic Nitrogen Concentration in Mixtures iirea calculation method and comparison of methods
Area Abs. Units, Cm.-l 1486 1578 1060 915 1458 1552 1480 1382
0.584
nitrogen determination either by the production of a color or by the consumption of the reagent. Of the compounds that h a r e been reported to react p-dimethylaminobenzaldehyde, with only phenols and amines are likely to be found in petroleum products. Phenols have been isolated as one of the acidic constituents of petroleum (eo), and aniines are commonly used as inhibitors in fuels. I n the present work interferences h a r e been further studied, n i t h special emphasis on compounds that occur in petroleum. The study mas made by observing the eflect of the presence of various amounts of compounds on the reaction of indole with p-dimethylaniinobenzaldehyde in the presence of phosphoric acid. In all cases the concentration of indole n-as
b
.Area calcn.
0.969 1 .i75 1,128 0.601 1,128 1,604 1.340 1,110 0.528 0.775
3.6 7.8 9.3
co
Error, A ~ Pcalcn. .
4.4
3.0 1.1 4.4
1 .o 6.2 3.6 1.5 4.2
1.060 ..
b
4.9 14.2 10.7 1.4 1.9
4.7 84.7 77.1 5.4
5 0 :3 , 7
55.7
14 3
31.9 23 5
3.3 6.7 5 ,3 8.7 6.0
31 0 32 7 29 1
35.5
Effect of Unsaturated Compounds and Aromatic Hydrocarbons on Recovery of Indole
Recovery of Indole in Presence of Varying Amounts of Interfering Iraterial, Compounds Unsaturated hydrocarbons 1-Pentene 2-?\lethyl-l-butene 1-Octene Diisobutylenea 1-Hexadecene 1-Octadecene 2-RIethyl-2-butene 2-Heptene 2-Octene 3-Heptene Triisobu tyleneb Cyclohexene a-Pinene @-Methylstyrene Unsaturated acids and esters Cinnamic acid Ethyl cinnamate Ethyl oleate Methyl 10-undecanoate Methyl abietate Aromatic hydrocarbons Benzene Naphthalene Anthracene Phenanthrene
Source Phillips 9970 Phillips 9570 Phillips 997, Shell Chem. Humphrey Wilkenson Redist., b.p. 180°/ 15 mm. Phillips 9970 Phillips 997, Humphrey J17ilkenson Humphrey Wilkeonson Redist., b.p. 180 Eastman grade Hercules Eastman grade Eastman Eastman Eastman Eastman Eastman
grade grade practical grade grade
0.05 mg. 0 . 5 mg 5mg.
YC
15mg.
50mg. 100
96.6
98.0
98.0 100
99.4 92.9 100 97.0 97.2 100
100 100
98.8 98.6 74.6 91.0
88.9 62.9 90.7
100 94.8 99.5
98.6 97.4 46.2 80.2 74.6 22.6 76.0 99.3 99.6 96.6 73.2 87.5
0.lg.
0.25,.
97.5
92.5
95,6 100 83.8 87.2 14.5 g5.5
102 76.1 76.1 10.3 26.0 43.3 22 4
92.5 48.8 19.6 16.7 27.8
8.0
99.2 82.0
101
77.4 28.3 55.5
1.Og. 100
88.6 100 66.2 101 102 65.0 15.2 18.4 22.6 26.8 18.2 123 5.9 ...
08.3 27.8 49.1 20.2
Baker’s AR 102 Baker’s AR 100 Eastman practical 95% 102 ._ Eastman grade 96.7 807, 2,4,4-trimethyl-l-penteneand 20% 2,4,4-trimethyl-2-pentene (33). 57, 4,4-dimethyl-2-neopentyl-2-pentene, 36% 2,2,4,6,6-pentamethyl-3-heptene1 57, 2,3,4,6,6-pentamethyl-l-heptene and 5‘5; 2,4,4,-
6,6-pentamethyl-l-heptene (38).
VOL. 30, NO. 2, FEBRUARY 1958
263
Table VI.
Effect of Various Compounds on Recovery of Indole
Compound Gasoline additives Tetraethyllead Tritolyl phosphate Phenols Phenol Ionol Catechol Resorcinol Hydro uinone Pyrog&ol 1,3-Saphthalenediol 2,7-Naphthalenediol Other oxygen compounds 3-Methyl-1-butanol 4-Heptanol 3-Methyl-3-pent an
Scetone Furfural
Hydroquinone dimethyl ether Hydrocinnamic acid Ethyl stearate Sulfur compounds n-Hexylmercaptan n-Dodecylmercaptan Thiacyclopentane Phenyl sulfide Ethyl disulfide n-Amyl disulfide p-Tolyl disulfide Thiophene Benzothiophene Amines Cyclohexylamine Diiso ropylamine Triet lamine Piperidine 3,5-.Dimethylpiperazine
$
Aniline
. -.. . . . . ..
A--Methylaniline N,-V-Dimethylaniline p-LLminophedol p-Phenylenediamine UOP-ld UOP-5. UOP-copper deactivatorf Pyridines and quinolines Pyridine 3-Picoline 2,3-Lutidine 2,4,6-Collidine Quinoline 4-Methylquinoline Other nitrogen compounds Hydrazine Semicarbazide Urea Phenylacetamide Acetanilide Imidazole 3,5-Dimethylpyrazole 1-Phenyl-3-methylpyrazolone Benxtriazole Benzoxazole
Source
c/o Recovery of Indole in Presence of Varying mounts of Interfering Material 0.05 mg. 0.5 mg. 5 mg. 15 mg.
Ethyl Corp, Laboratory Redist., m.p. 42‘ Recryst., m.p. 70’ Eastman grade Baker’s AR Eastman grade Eastman grade Eastman practical Eastman grade
loa
96. 4b
94.4
9.1
90.8 61.0 34.6 99.2c 133c 180C
Baker’s AR Eastman grade Eastman practical Baker’s AR Redist., b.p. 86-87’/50 mm . Matheson, Coleman and Bell Eastman grade Eastman grade Eastman grade Eastman practical API Eastman grade Eastman grade Eastman grade Eastman grade Eastman grade hlatheson, Coleman and Bell
50.8
99.1
99.4 7.9 95.9
98.5 98.2 98.2 68.2 100 95.3 98.7 97.3 99.6
98.0 98.0
81.0 87.4
35.5 35.5
Eastman grade Eastman grade Sharples Eastman grade Eastman practical Baker’s AR Eastman grade Baker’s purified Eastman practical Eastman grade UOP COP UOP
98.0 97.8 95.7
14.1
24.1 98.5 96.2 99.8 98.0 100 97.3 98.4 98.8 96 1 97.4 97.8 98.4
79.7 80.3
100
23.0 7.4
46.5 4T.3 100
9.1 ppt 98.8 98.3 99.4
34.2 24.8 (8.7
51.6 PPt
Baker’s AR Eastman practical Eastman grade Eastman grade Eastman grade Eastman grade
31.3 36.4 71.8
99.6 97.4 98.2 97.4 98.4 98.0
Eastman grade Eastman grade Baker’s AR Monsanto Baker’s USP Eastman grade Eastman grade Eastman grade
67.5 97.7 101
Eastman grade Eastman grade
101
59.2 93.3 99.5
101
56.4 87.2 97.9
84.7
81.4 87.7 98.0 98.0 99.6 97.6 77.7 96.1 96.5
5
Value for 10 mg. of sample.
0
Unstable, after 1 hour values were: 0.05 mg., 101%; 0.5 mg., 93.8%; and 5 mg.,
* Value for 50 mg. of sample. 81.7%. a f
N-n-butyl-p-aminophenol and N,N’-di-n-butyl-p-phenylenediamine in alcohol. N,N-di-sec-butyl-p-phenylenediamine. N,N’-disalicylidine-1,2-diaminopropane in xylene.
264
ANALYTICAL CHEMISTRY
compounds in the size of sample taken, and not the concentration in the samde. which is of importance in relation t o interference. The results of these tests (as per cent recovery of indole) are given in Tables V and VI. Of the compounds listed in Table V, no interference occurred with normal unsaturated hydrocarbons with terminal double bonds or with aromatic hydrocarbons. However, other olefinic materials generally showed a marked tendency to reduce the recovery of indole. The values indicate that only 0.005 to 1.57, of such olefins could be tolerated in a 1-gram sample (normal samples are about this amount). The presence of these olefins not only reduces the intensity of the color but markedly changes the spectrum. Figure 3 depicts this effect 13-ith four olefins. These same olefins (in the absence of indole) produced absorption spectra in the visible range when they reacted with p-dimethylaminobenzaldehyde. The nature of the reaction among olefins, the reagent, and possibly indole is not understood, hut it is apparent that olefins interfere both by consumption of the reagent and by production of an interfering color. Two gasoline additives, tetraethyllead and tritolyl phosphate, showed no interference at amounts greater than would be found in normal gasoline samples. The interference of phenols u-as not as great as the previous studies indicated. Only resorcinol, pyrogallol, and 1,3-naphthalenediol affected indole recovery. The three phenols that caused interference have hydroxyl groups meta to each other. The other oxygen compounds tested (alcohols, carbonyl compounds, ethers, acids, and esters) showed no interference within the amounts of material studied, with the exception of 3-methyl-3-pentano1, a tertiary alcohol. The slight interference shown by this compound may have been due to partial dehydration to a n olefin under the acidic conditions used. Sulfides (cyclic and open chain), disulfides, and thiophenes did not interfere but mercaptans interfered strongly. The presence of 0.05 mg. (0.01 mg. of mercaptan sulfur) of the latter could be tolerated, but larger amounts caused the characteristic red-violet color to change to yelloworange. Of the basic organic nitrogen compounds tested, only the aromatic amines showed a marked influence on indole recovery, again by formation of a yellow-orange solution in place of the red-violet one. Polyfunctional compounds (p-phenylenediamine and p-aminophenol) produced the greatest interference, and substitution on the nitrogen atom caused a slight reduction in interference. Three commonly used additives-UOP4, UOP-5, and UOP copper deactivator -when tested a t concentrations greater
2 c
%[-
n
d & \ E L E Y G T H , mp
Figure 3. aldehyde
Curve A B
c
D E
Reaction of indole with p-dimethylaminobenz-
Effect of Olefins on Spectrum Olefin Added Compound Amount, G. None added 0 Cyclohexene 1 2-Heptene 1 3-Heptene 1 2-Octene 1
than would normally be employed, did not affect the recovery of indole. As had been reported, hydrazine showed a marked interference, n hile urea and the pyrazolone interfered slightly. Conflicting reports with respect to seniicarbazide were resolved with the observation that it also interfered. Increasing the amount of reagent tenfold (from 50 to 500 mg.) did not appreciably lessen the interferences caused by olefins and mercaptans. I n fact, with cyclohexene, recoveries were lowered when the amount of reagent was increased. Where removal of interfering compounds is necessary, acid or base extraction should remove the nonhydrocarbons (mercaptans, phenols, amines) ; a Chromatographic separation will allow the separation of olefins (and perhaps also the above-mentioned compounds) from pyrrolic compounds, Results with Petroleum Samples. To evaluate further the pyrrolic nitrogen determination, the method was run, without preliminary separation, on a series of samples. consisting of gasoline, jet fuels, gas oils, furnace oils, rubber extenders, aromatic solvents, and residues. From the qualitative standpoint the analyses mere satisfactory. Good spectral curves were obtained in almost every case and no particular difficulties were encountered when extraction of oil components caused appreciable coloration of the phosphoric acid layer. The pyrrolic nitrogen content of these samples was calculated by the three methods previously applied to synthetic
Indole Added, y. 90 90 90 90 90
MIAXE LEhGTH
--
Figure 4. Chromatographic separation of synthetic mixture containing cyclohexene and diisobutylene
A . Original sample (Solution A ) B. Benzene eluate from original sample
mixtures. The values obtained using the absorbance of 2-methylpyrrole a t 570 mp as a standard were consistently 50 to 100% higher than those of the other two methods. These results are not unexpected, as the absorbances obtained from the samples were much higher a t 570 mp than would have been obtained with 2-methylpyrrole. Consequently the calculated results were higher than those based on methods that more nearly approximated the mixtures of pyrrolic compounds found in samples. Area and absorptivity calculations gave similar results when the spectral curves obtained had no shoulders (straight-run and catalytically cracked samples), while area values were 25 to 50% higher, as would be expected, when a shoulder or second peak was present a t the higher wave lengths (thermally cracked samples and others high in olefin content).
PRELIMINARY SEPARATION OF PYR-
COMPOUNDS. The preliminary work on interference clearly indicated that olefine are the cause of shoulders or broadening of the spectral curves. This effect is strikingly shown by comparing the broadening of spectra with olefin content of several samples (Table 191). As these distortions of the spectral curves increased the error in the calculation of pyrrolic nitrogen, it became necessary to separate the pyrrolic compounds from the olefins. This has been done by a simple and fairly rapid chromatographic technique. The sample is placed on a Florisil column and eluted with heptane to remove the
olefins and then with benzene to recover the pyrrolic compounds in an essentially quantitative manner. X-Butylpyrrole behaved like pyrroles Kith an unsubstituted nitrogen, indicating that .I-substituted pyrroles would be collected with other pyrroles when the separation procedure was used. S o measures were taken to remove oxygen from t h e Florisil column or from the solutions. The good recoveries obtained indicate that a t the small concentrations used there is no appreciable reaction of the pyrrolic compounds with oxygen, as is the case with higher concentration. No difficulty was encountered in running the pyrrolic nitrogen determination directly on the benzene eluate; the use of this solvent gave results with pure
Table VII.
Effect of Olefin Content on Spectral Curve
ROLIC
Description of Sample Straight-run gasoline Aromatic solvent Straight-run gas oil Catalytically cracked gas oil Aromatic solvent Thermally cracked gas oil Aromatic solvent Aromatic solvent Thermally cracked gasoline Catalytically cracked gasoline
BroadOlefin ening Content, of Mg. Curve 0 0
