1806
ANALYTICAL CHEMISTRY
given on the certificate of analysis aa far as the precipitation of the iron a8 the sulfide. The washed sulfide was dissolved in 10 ml. of 1 to 1 sulfuric acid, and the mixture was heated until sulfur trioxide fumes were evolved. After the sample cooled, the sides of the beaker were washed down and heating to sulfur trioxide fumes was repeated. From this point, the analysis was concluded by the method given for iron(II1) nitrate. Because of the smaller amount of iron presentinthis sample, the titration of iron(11) was made using 0.01 hr potassium dichromate. The results obtained by this method are compared with the average analysis found by the National Bureau of Standards and its cooperating analysts. Iron, % 0.42, 0.44
Method of Analysis Aluminum NBS
0.45
DISCUSSION
The method of analysis for iron differs from those proposed earlier (1, 3, 4, 11, 18) in that a weighed portion of very fine aluminum powder is used as the reducing agent. This powder is allowed to dissolve completely. A determination of iron content of the powder provides a blank correction. The method is applicable to any type of sample for which the Jones reductor method is used. The action is rapid in solutions in which aluminum foil, chain, or sheet dissolve slowly. In spite of this rapid action, the effciency of reduction is good. The large surface area of the powder speeds the action of the metal on iron( 111) without favoring the evolution of hydrogen. ACKNOWLEDGMENT
The authors wish to thank the Aluminum Co. of ilmerica for
providing samples of foil, and the Reynolds Metals Co. for p r e viding samples of powder. LITERATURE CITED
Campbell, E. D., unpublished work. Carnegie, D. J., J . Chem. Soc., 53, 468 (1888). Carulla, F. J. R., J . SOC.Chem. Ind. (London),27, 1049 (1908). Fritchle, 0. P., Eng. Mining J., 70, 548 (1900). Furman, N. H., “Scott’s Standard lMethods of Chemical Analysis,” 5th ed., 1’01. I, p. 50, New York, D. Van Nostrand Co., 1939. Ibid., p. 469. IND.ENG.CHEY.,ANAL.ED., Henry, J. L., and Gelbach, K.W., 16, 49 (1944). Kohn-Abrest, E., Compt. rend., 147, 1293 (1908). Kolthoff, I. M , , and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” revised ed., p. 597, New York, Macmillan Co., 1943. Light, A. K., and Russell, L. E., IND.ENG.CHEM.,ANAL.ED., 19, 337 (1947). Schumann, C. L., J . Ind. Eag. Chenz., 7 , 431 (1915). Seamon, R. H., West. Chem. M e t . , 4, 105 (March 1908). Walden, G. H., J r . , Hammett. L. E’., and Edmonds, S. ll.,J . Am. Chem. Soc., 56, 350 (1934). Willard, H. H., and Diehl, H., “Advanced Quantitative Analysis,” p. 136, New York, D. Van Sostrand Co., Co., 1943. Willard, H. H., and Furman, K. H., “Elementary Quantitative Analysis,” 3rd ed., p. 203. S e a York, D. Van Nostrand Co., 1940. RECEIYED for review February 2 , 1952. Accepted July 14. 1952. Taken i n part from a thesis submitted by R. D. Schwarta to the Graduate School of the University of Buffalo in partial fulfillment of the requirements for the degree of doctor of philosophy.
Determination of Nitrogen in Petroleum and Shale Oil Report of the Subcommittee on Nitrogen Determination, Committee on Analytical Research, Division of Refining, American Petroleum Institute G . R . LAKE, Chairman, Union Oil Co. of California, Brea, Calif.
E. L. BALDESCHWIELER, Standard Oil Development Co., Linden, AT. J . J. S. BALL,U . S. Bureau of Mines, Laramie, W y . G. R. BOND,JR.,Houdry Process Corp., Marcus Hook, Pa. R. 0. CLARK, GulfResearch & Development Co., Pittsburgh, Pa. C. Ill. GAMBRILL, Ethyl Corp., Detroit, Mieh. RALPHGRIFFITH,Sinclair Research Laboratories, Inc., Harvey, Ill. B. J. HEINRICH,Phillips Petroleum Co., Bartbsville, Okla. C. K. HEWES,Richfield Oil Corp., Wilmington, Calif. W. H. JOKES,Esso Laboratories, Louisiana Division, Baton Rouge, La. S.S.KURTZ,JR.,S u n Oil Co., Marcus Hook, Pa.
T
HE occurrence of organic nitrogen compounds in crude petroleum and shale oils poses a problem of considerable importance to the petroleum industry. The deleterious effect of nitrogen compounds on cracking catalysts (14, 1 7 ) and the indication that they contribute to gum formation in gasolines (12, 13, 21, 22, I S ) are aspects of this problem. It is understandable, therefore, that the industry must have good, rapid analytical methods for determining the content of organic nitrogen in petroleum and shale oils and their products. The analysis of materials for nitrogen content is a fairly common laboratory operation. Despite this, there is a considerable difference of opinion regarding the relative reliability of the different analytical procedures that are being used. The unreliability of some of the commonly used procedures was strikingly demonstrated in an exchange program instituted and reported by the Bureau of Mines ( 4 ) where three shale oil fractions &-ere
I’. HARRY LEVI~V, The Texas Co., Beacon, LOUISLYKKEN, Shall Development Co., Emeryville, Calif. ROBERTMATTESON, California Research Corp., Richmond, Calif. LOUISR~ITTELMAN, Tide Water Associated 021 Co., Associated, Ca1i.f. J. M. POWERS, Humble Oil & Rejining Co., Baytown, Tex. J. €3. RATHER, JR.,Socony-Vacuum Oil Co., Inc., Brooklyn, 1Y.1’. A. R. RESCORLA, Cities Sercice Research and Development Co., Hillside, N . J . H. L. STAWE,Barrett Division, rlllied Chemical & Dye Corp., Edgewater, N . J . E. B. TUCKER, Standard Oil Co. (Indiana), Whiting, Ind. analyzed by seventeen laboratories. Less than 28% of the reported results were within 2% of the “most probable value.” Late in 1949, the Committee on ilnalytical Research, Division of Refining, American Petroleum Institute, considered the problem of sufficient interest to establish a subcommittee to study and evaluate the various procedures. Some fourteen laboratories, collaborating in the 2-year study reported in this paper, have demonstrated that nitrogen in shale and petroleum oils may be accurately determined by several methods if careful attention is given to the control of operational variables. Methods for the determination of nitrogen in organic mixtures may be classified into three basic groups: Kjeldahl, Dumas, and ter Meulen, which depend upon liquid phase oxidation, combustion, and hydrogenation, respectively. hlany modifications of the first two methods have been developed, each purporting to be of especial value for a particular stock or type of sample. On
V O L U M E 2 4 , NO. 11, N O V E M B E R 1 9 5 2
1807
Cooperative work of twenty laboratories has demonstrated that nitrogen in petroleum and shale oils may be accurately determined by the Kjeldahl, Dumas, or ter Meulen methods if careful attention is given to the pertinent operational variables. The digestion temperature must be maintained at about 700' F. for 1 hour in the Kjeldahl method. Mercury is the preferred catalyst. Twenty determinations may be made per man-day. Meticulous burning is required in the Dumas method to assure complete combustion. The gas-recycle, double furnace, and mass spectrometer correction modifications provide safety features to prevent or correct for improper comhustion. The ter Meulen catalytic hydrogenation method is applicable to lower concentrations of nitrogen than the the Dumas method. Fourteen determinations may be made per man-day.
the basis of their interest, members of the subcommittee formed four subgroups to investigate the macro- and micro-Kjeldahl, the Dumas, and the ter hleulen procedures. EXPERIMENTAL
Six samples were selected for the cooperative work: three synthetic blends, a gasoline and a gas oil fraction from petroleum, and a shale oil naphtha. Data on these samples are given in Table I. Pyridine, quinoline, and isoquinoline were used in KS-1 and SS-2, as it was believed that these compounds are as refractory as any likely to be encountered in shale and petroleum.
Table I. Sample NO. NS-1
Samples for Cooperative Analysis h'itrogen, % by Wt. 1.00
Composition 1257.9 grams of NS-3, 104.75 grains of blenda Synthetic 0,06!b 1000 ml. of NS-3,50 ml. of NS-I NS-2 Satural 0 01 East Texas light gas oil NS-3 Petroleum naphtha 124-770' F. Satural NS-4 0.1oc Katural 1.3b.C Shale oil naphtha 400-550' F. NS-5 Synthetic 858 grams of NS-3, 19.81 grams of NS-6 0.268 nitrobenzened a Blend of 33.0022 grains of pyridine, 38.2659 grams of quinoline, and 33.4797 grams of i,soquinoline, ai! of 99+ 70purity. b There was evidence of deterioration of these two samples in some laboratories, depending upon exposure to air and/or light. For example, Laboratory 14 obtained the following results: NS-2 0.063, 0.064, 0.064% by Kjeldahl in June 1950 when blends were made up. 0.060 0.058% by Kjeldahl in October 1950. KS-5 1.301: 1.355. 1.331% by Kjeldahlin June 1950. 1.206, 1.217% by Kjeldahl in July 1961. 1.207, 1.2037, by ter Meulen in July 1951. Other laboratories substantiated these trends. 0 Approximate values based on this work. d Carefully redistilled nitrobenzene. Type Synthetic
Macro-Kjeldahl Method. Based on prior work ( 8 ) indicating that the digestion temperature was the most important single variable, the following procedure, designed to give a digestion temperature of about 700" F., was selected for cooperative testing:
For 1 gram of sample, 35 ml. of concentrated sulfuric acid, 20 grams of potassium sulfate, and 1.3 grams of mercury were used; 5 ml. of concentrated sulfuric acid were added for each additional gram of sample. The sample was heated until clear and digested for 1 hour. The determination was completed in the usual manner. The data obtained, listed in Table 11, demonstrate that nitrogen in ring-type compounds may be accurately determined by a Kjeldahl procedure. In addition to the work reported, most laboratories compared the cooperative method with their current procedures on various types of stocks with entirely satisfactory correspondence. With adequate equipment, some 20 determinations per man per day may be made. The statistical summary of the data shows that the coefficient of variation (standard deviation as a percentage) of the method improves from about 4% a t the 0.05y0 nitrogen concentration level to less than 2% for samples with nitrogen content above 1 %.
A further phase of the program was to investigate the value of the macro-Kjeldahl method in determining nitro-type compounds. A modification (10)of the Association of Official Agriculture Chemists' method (1) wherein a mixture of salicylic acid and sodium thiosulfate is used was first tested, followed by tests of another modification using thiosalicylic acid (11). The results are given in Table 111. Here again, the data show that reproducibility and accuracy are satisfactory. A coefficient of variation of 1.3% was obtained on the sample containing nitrobenzene. It was the unanimous opinion of the seven cooperating laboratories that the macro-Kjeldahl method is applicable to the determination of nitrogen in petroleum and shale oils. The factors found to be essential in a satisfactory Kjeldahl method are the use of mercury as an oxidation catalyst and the maintenance of a proper digestion temperature during the oxidation. The method, when modified, may be used to analyze nitro-type compounds. The thiosalicylic modification was preferred because of simplicity, being essentially as rapid as the regular Kjeldahl method. However, it must be emphasized that the Kjeldahl method is not universal, hence should be used with care if certain types of nitrogen compounds are present. S p e cifically, hydrazines, pyrazoles, and others containing the N=N linkage are known to give low results.
Table 11. Analysis of Cooperative Samples by 3IacroKjeldahl Procedure Nitrogen, Weight % ' NS-1 0.993 1.001 1,003 0.982
NS-2 0.060 0.060 0.060 0.060
h-s-3 0,009 0,010 0.011 0.012
NS-4 0.097 0.102 0.103 0.099 0.099 0.103
NS-5 1.353 1,349 1.358 1.338
2
0.980 0.985 0.977 0.989
0.0554 0.0565 0,0632 0.0587
0.0077 0.0109 0,0061 0.0116
0.0929 0.0945 0,0908 0.0993
1.32 1.35
6
0.990 0.979 0.979
0.061 0.059 0.060
0.009 0,010 0.010
0.097 0.100 0.104
1.342 1.335 1.341
9
0.94 0.94 0.95
0 . 058O 0.049°tb 0,045'=1b
0.009 0.007 0.006
0.097 0.096 0.095
1.31 1.32 1.31
11
0.979 0.984 0.992
0.0622 0.0607 0.0624
0.015 0.011 0.014
0.0975 0,0990 0.0992
1.334 1.326 1.331
12
0.974 0,981 0.983
0.0591 0.0581 0.0582
0.0097 0,0099 0,0087
0.0957 0.0936 0.0948
1.320 1.342 1.335
14
0.999 0.977 0.992 23
0.061 0.062
0.011 0.012
0.097 0,096
1.301 1.355 1.331 21
Laboratory 1
Determinations
20
22
24
1.3333 0.01003 0.09760 Mean 0.9804 0.0593 Coefficient of vari1.2 3.9 22.0 3.4 ation 1.7 a Contained gummy deposit. b Not included in averages. Rejected statisticslly as being outside 99% confidence level.
ANALYTICAL CHEMISTRY
1808 Table 111. Analysis of Cooperative Samples by Modified Macro-Kjeldahl Procedures Nitrogen. Weight % Laboratory
Salicylic Acid-Sodium Thiosulfate Modification NS-2 Ius-3 NS-4 NS-5 ,..... ...... 0.097 ..... 0,100
NS-1
T h iosalicylic arid, NS-6
NS-6 0.286 0.252 0.254 0,244
1
.....
2
0,969 0.970
0.0494 0.0441
0,0079 0.0110
0,0958 0.0961
.33 .33
.....
6
0.968 0.990
0,055 0.067 0.058
0,009 0,009
0,090 0.093 0.0915
,315 ,310 ,306
9
.....
......
......
......
....
11
0.945 0.979 0,952
0.0589 0,0586 0,0574
0,0098 0.0097 0.0094
0.0968 0.0973 0.0972
1.301 1.305 1.310
0.246 0.243 0.250 0.251 0.261 0 249 0.257 0 255 0.251 0.257
0,979 0.979 0,973 0,999 0.984 1.005
0.0565b 0.055Ob 0,0548b 0,063 0.0134 0.064
0.0094
0,0929 0.0913 0,0910 0,100 0.106 0.097
0.257 0.262 0.259 0.263 0.258 0.261 0.264 0,243" 0.252 0,255 0 261 0 261 0,256 0 261 0 263 0 259 0 261 0.261 0.260 0.262 0.257 0.266 0.257
.....
salt would also be advantageous in the macroprocedure. Micro-Kjeldahl Method. Method A, the first procedure investigated, was based on one used by one of the cooperating laboratories. Sample size was 15 to 500 mg. based on nitrogen content of sample. Reagents were 4 ml. of concentrated sulfuric acid, 2.7 grams of potassium sulfate, 0.1 gram of mercuric oxide, and 0.05 gram of selenium. Digestion time was 1 to 1.5 hours after clearing. Standard completion was by distilling into boric acid solution and titration with hydrochloric acid. A pretreatment x ith hydriodic acid and phosphorus was prescribed for samples containing nitro compounds.
All laboratories reported low results using the optional pretreatment with 0,256 hydriodic acid and phosphorus probably 0.258 1,333 0,252 0.014 caused by the long vigorous heating 14 0.252 0.014 1.344 necessary to remove the hydriodic acid 0,254 0.250 after digestion. Determin10 16 13 21 22 14 ations 13 Pertinent results obtained by the 0.01022 0.09581 1.3161 0.2524 0.2604 0.05765 Mean 0,9763 regular procedure are shown in Table IV. Coefficient of 1.0 1.8 1.3 4.3 21. 9.1 variation 1.8 The data are erratic. Laboratories 5 , a Not included in averages. Rejected statistically as being outside 99% confidence level. 8, 9, and 10 indicate fair accuracy, b Contained gummy deposit. although reproducibility was rather poor; each had more or less trouble with samdes SS-4 and SS-5. LaboSubstitution of potassium pyrosulfate for potassium sulfate ratories 4 and 7 obtained consistently low values but reported considerably improved results by maintaining the digeswas not investigated. Later it was shown by the micro-Kjeldahl tion temperature between 630' and 660" F. Laboratory 9 resubgroup that potassium pyrosulfate caused less frothing and ported less frothing difficulties when potassium pyrosulfate was gave a smoother digestion. It is believed that the use of this 12
Table I\'.
Analysis of Cooperative Samples by Micro-Kjeldahl Procedure A
NS-1 Sample, Laboratory
mg.
Temp., O F .
%
336 349
750
0.90 0.62
b
384 814
660 660
0.96 0.97 0.96 0.94 0.814 0.626
7
... ...
....
. ..
NS-3
NS-2 Nitrogen,
4'
5
0,256
Sample, mg. 784 698 1115 658 708
... ...
Temp., O F .
750 660 660
....
Nitrogen,
%
0.015 0.0085 0.011 0.078 0.071 0.057 0,059
Sample, mg. 634 718 730 745
...
Temp., OF. 750 760 660 660
....
55-5
NS-4 Nitrogen,
%
0.029 0.026 0.028 0.024 0.015 0,015
Sample, mg. 728 761
795 800
0,028 0,0095
771 855
660 660
0.105 0.103 0.009 0.101 0,101
...
...
Temp.,
Nitrogen,
O F .
.. . . ... ,
%
IuitroSample,, T e m p . gen, O F . % mg. 960 0.019 399 800 0.021 370 355 360
... . ..
660 660
,...
....
0.582
630-60 630-60 630-60
....
9
...
....
0.621 0.642 0.720 0.514 0.712 0.946 0.952 0.973 0.93 1.00 0.97 0.95 0.991 0.996 1.020
...
630-60 630-60
...
...
,
...
....
..
0.061 0.062
,
0.053 0.052 0.066
...
630-60 630-60
... ....
0.059 0.063 0.060 0.063
0.013 0.013
...
630-60 630-60 630-60
0.013 0,009
...
.. . .
0.010
...
0.010
0.101 0.098 0.094 0.090 0.091 0,093
1.35 1.34 1.26 1.30 0.878 0.460 0.270 1.115
630-60 630-60
1.39 1.28
. ..
630-60
1.45
...
....
1.25 1.32 1.31 1.25 1.33
....
1.35 1.38 1.41 1.16 1.30
0.014
0.066
10
...
....
0.98 0.99 1.02 0.92 0.97
...
....
85 500 Pretreatment with H I and P. b Digestion temperature controlled as indicated. 8 Copper sulfate substituted for selenium. d Potassium pyrosulfate substituted for potassium sulfate. d
0
0.064 0.063 0.059 0.059 0.060 0.054 0.059
... 500
....
0.006 0.007 0.010 0.007 0.009
... 500
0.099
85
1809
V O L U M E 24, NO. 11, N O V E M B E R 1 9 5 2
, . 00
0 -
. .
00
. 0
. 2
0 0
s p
h
z
h
substituted for potassium sulfate. The substitution was made on the assumption that the following reaction occurs:
I(2SOI
z
+
HzSOa + PKHSO, + KzSpO, 4 H,O
d i d .
mmm 10 m L, lomu) I l l
coo
"N
(CCW
m 0 m -oc0mh
c-.o
aw(D w o w
006 o o
h P
L ? 0 d
0 0 0 .
l
m coo mm 00
o w hW
&A0
hhh
(Dww
0
(D (D
0
I.
I
x
.
hh0.
d (D
d.m
o o c ; '0
Q
0
w h
In addition to evolving water, which increases frothing, the potassium sulfate combines with an equivalent amount of sulfuric acid. This, together with the acid consumed by the sample, reduces the amount of liquid remaining so that the temperature may exceed the decomposition point of the ammonia salt, thus causing loss OS nitrogen during digestion. Several laboratories concluded that selenium was not a necessary or even desirable ingredient of the digestion mixture. Slthough all laboratoiies finally obtained reasonably satisfactory results, the cooperative method \vas revised in line with the findings of the first round of testing and all samples were retested by the revised Method B. Sample size was 15 t o 500 mg. based on nitrogen content of sample. Keagents \\ere 4 ml. of concentrated sulfuric acid, 3.0 grams of potassium pyrosulfate, and 0.1 gram of mercuric oxide; 0.5 ml. of concentrated sulfuric acid R as added for every 100 mg. over 100 mg. of sample to compensate for acid consumed by sample. Digestion time was from 1 to 1.5 hours. Temperature should not exceed 720" F. .i pretreatment using 0.3 gram of thiosalicylic acid s a q specified for SS-6. The data resulting from the second round of testing are given in Table V and, in general, are more consistent. The coefficient of variation is relatively high compared with that for the macro-Kjeldahl procedure, ranging from 10% at the 0.05y0 nitrogen level to about 5% for samples containing more than 1% nitrogen.
ANALYTICAL CHEMISTRY
1810 Laboratory 9 obtained low results on N S 1 until the digestion temperature was raised by increasing the salt-acid ratio, indicating a minimum safe digestion temperature, in line with earlier work (8, 19). The preferred digestion temperature appezrs to be in the range of 660' to 720' F. if the temperature data of Laboratory 8, which was measured with thermometers rather than thermocouples, are disregarded. The temperature range for macroKjeldahl is 680" to 770' F. with a minimum of 698" F. (370' C.) preferred (8) and there is no fundamental reason why the two methods should differ in this variable. This discrepancy in a p parent temperature readings may be due, a t least in part, t o the difficulties in accurately measuring the temperature of the small amount of liquid in the digestion flask. The digestion temperature in the microprocedure may be controlled by adjusting the sample-salt-acid ratio, but this is slightly more difficult than in the macroprocedure because of the small quantities involved. This may possibly account for the low results obtained on NS-1 and NS-6 by Laboratory 7 and on NS-1 by Laboratory 5 . The use of a constant temperature bath, block, or cabinet to enclose the digestion flask was considered to have more promise, but this has not been fully explored.
semimicro-Dumas procedure. All laboratories indicated that the method was not applicable to petroleum samples containing less than 0.5% nitrogen. It appears that erratic values may be the result of incomplete combustion (18); Laboratory 5 found methane in the combustion products from all samples, but using the gas recycle procedure (5) obtained excellent results. Laboratories 8 and 9 used a double furnace modification (9, 20) to obtain accurate data. It is generally agreed that the mass spectrometer correction method ( 2 , 2.4) would be equally valid for obtaining accurate answers. ~
~
Table VII. Laboratory 11 12 13 14
Table VI. Analysis of Cooperative Samples by Dumas Procedure Nitrogen, Weight 7% Laboratory
Modification
3
Semimicro
5
Micro Shell recycle'
8
Semimicro
Double furnace" 9
Double furnace"
11
Micra
13
Micro
XS-1
NS-4
1.07 1.15 1.17 1.30 1.30 1.30 0.99 1.00 0.95 1.27 1.06 1.33 1.14 1.01 0.97 1.02 1.03 0.97 1.26 1.08 0.99
..
.. 0.09 0.09 0.10 0.19 0.20 0.11
..
.. ..
NS-5 1.28 1.42 1.34 1.70 1.70 1.70 1.31 1.35 1.35 1.43 1.55 1.37
NS-6 0.58 0.71 0.49
....
..
.,..
..
....
1.37 1.37 1.60 1.62 1.66 1.37
.. ..
Determinations Meanb Coefficient of variation Not included in Statistical analysis, a8 data were obtained by modified method. b Moat probable value is 0.98 and 1.33%, respectively, indicating that technique used is not applicable to thls type of sample.
The consensus of the six cooperating laboratories was that the micro-Kjeldahl method is applicable to the determination of nitrogen in petroleum and shale oils. The most important variable in the procedure is the time-temperature relationship during digestion. This is of the order of 1 hour a t 660' to 720" F. Mercury is the preferred catalyst and there are definite advantages to using potassium pyrosulfate instead of potassium sulfate to attain the proper digestion temperature. Pretreatment with thiosalicylic acid permits the analysis of compounds containing nitrobenzene. The same limitations apply to the micro modification as to the macro-Kjeldahl. With proper equipment some 20 determinations per man per day may be made. Dumas Method. In the testing of this method, each laboratory used the modification currently in use in that laboratory. The analytical values are shown in Table VI. The results were extremely erratic as judged by a coefficient of variation of 11% compared with less than 2% for the macro-Kjeldahl. Only Laboratory 13 obtained accurate data by the regular micro- or
~~
Analysis of Cooperative Samples by ter Meulen Procedure h-S-1 0.982 0.977 0,968 0,987 0,987 0.985 0.945 0.958 0.953 0.963 0.929 0.916 0.931
Sitrogen, Weight % KS-3 SS-4" 0,013 0.0974b 0.015 0.0952 0.011 0.0981 , .... 0.08456 0.0793 0.0564 0.0738 0.0582 0.008 0.081d 0.055 0.009 0,074 0.055 0.078 0.051 0,007 0.057 0,009 0.094f 0.060 0 010 0.094
KS-2 0,0624 0.0393 0.0583 0.0563
0.064
0.068
NS-50 1.29b 1.31 1.30 1.226 1.21 1.21 1.21' 1.19 1.19 1.2070 1.203
NS-6 0,257 0,260 0.255 0.231 0.240 0.223 0.258 0.257 0.256 0.274 0.259 0,259 0.263
n -
2.s~ _._
Determinations 13 13 8 11 11 14 Mean 0.9624 0.0585 0.0101 0.0863h 1.231h 0,253 Coe5cient of variation 2.1 7.5 27 11 3.7 5.3 0 See footnoteb, Table I. b Run October 1950. 0 Run October 1951. d Run September 1951. Micro-Kjeldahl, same date, 1.17, l.l8YO. 6 Run September 1931. f Run July 1951. Macro-Kjeldahl, same date, 0.094, 0.094%. D Run July 1951. Macro-Kjeldahl, same date, 1.205, 1.217%. h Indicates marked deterioration in sample, as original values were 0.098 a n d 1.333%,respectively.
It was concluded by the six laboratories that little could be gained by additional cooperative work. If experienced operators are used, the standard micro or semimicroprocedure will give accurate results. The gas recycle, double furnace, and mass spectrometer correction modifications are all applicable. The regular Dumas method or the modified procedures are not attractive for control or routine operations because of the low output per man-day. They represent, however, the only methods not subject to limitations as to the type of nitrogen compounds to be determined and must be used unless the composition of the sample is sufficiently known to inspire confidence in the Kjeldahl or ter Meulen methods. ter Meulen Method. The ter Meulen catalytic hydrogenation method for the determination of nitrogen (6, 7 , 15, 16) has not been widely used in this country although it has gained acceptance in Germany. The success of the method depends upon the catalyst. An improved catalyst suggested by one of the cooperating laboratories (3) is prepared as follows: A slurry of 125 grams of magnesium oxide in 1.25 liters of distilled water a t 50" c. is prepared. and 400 grams of nickel nitrate hexahydrate are dissolved separately in 4 liters of distilled water a t 50" C. The nickel nitrate solution is added slowly to the magnesium oxide slurry with constant stirring. The precipitate is washed by decantation several times, transferred to a Buchner funnel, and washed free from nitrate, The solid is spread over filter paper, dried in the air and finally a t 100" C. in vacuum. I t is broken into 7- to 8-mm. pieces, heated in an electric oven to about 300" to 350' C., and reduced in a stream of hydrogen. After reduction the catalyst is broken up and screened through 4- and &mesh screens. The catalyst retained on the 8-mesh screen is used for filling the catalyst tuhe. The data obtained using this catalyst arc presented in Table VII. The statistical summary should be taken only as an indi-
V O L U M E 24, NO, 11, N O V E M B E R 1 9 5 2 cation of possible accuracy, as definite evidence is presented in Tables I and VI1 that samples KS-4 and NS-5 had changed in composition since they were run in 1950 by Laboratory lr. Lack of sufficient data and lack of experience with the method also make a rigorous statistical study unw-arranted a t this time. Despite this, the consensus of the four cooperating laboratories is that the ter hleulen method is a very useful procedure for the determination of nitrogen in petroleum and shale oils. Even though applicable to a greater variety of nitrogen-containing compounds than the Kjeldahl procedure, with the present catalyst and operating techniques, it cannot be considered a universal method. Some 12 to 14 determinations per man per day may be made if dual equipment is available. Further work is in progress on the ter Meulen procedure and several other laboratories have installed the equipment. Particular attention is being given to the determination of nitrogen in the low concentration region where the Dumas procedure is not applicable. I n addition, various types of nitrogen-containing compounds are being analyzed to establish the limitations of the method. CONCLUSIONS
In the considered opinion of the 14 laboratories participating in the coopwative work the folloviing conclusions are warranted. Kjeldahl Method. Either the macro or micro modification is applicable for the determination of nitrogen in heterocyclictype compounds such as pyridine and quinoline, and both are rapid, accurate methods for the analysis of normal petroleum and shale oils. The thiosalicylic acid pretreatment modification permits the determination of nitro-type compounds. Quantitative results are not possible if compounds containing the iY=N linkage are present. The digestion temperature is the most important variable, and to obtain accurate results should be maintained near 700” F. Low temperatures do not give complete conversion of nitrogen to ammonia, and high temperatures permit loss of ammonia by decomposition during the digestion. The necessary ingredients of the digestion mixture are mercury or mercuric oxide, potassium pyrosulfate or sulfate, and concentrated sulfuric acid. Other materials are superfluous or in somtx cases even harmful. For samples within its scope, the macroprocedure is best for routine operations because of its simplicity, rapidity, and accuracy. Dumas Method. The classical Dumas method is difficult to use with petroleum samples containing less than 0.5y0nitrogen.
1811 The Kjeldahl method is to be preferred for this type of sample. The Dumas, however, is the most nearly universal method investigated and must be used if the types of nitrogen-containing compounds present are unknown. The combustion step must be carried out by experienced operators, or added safeguards such as the gas recycle or the double furnace modifications must be employed to ensure complete combustion, or corrections for unburned hydrocarbons must be applied from mass spectrometer analysis. ter Meulen Method. This method is applicable to petroleum, shale oils, and also a greater variety of nitrogen-containing types of compounds than the Kjeldahl method. It cannot be considered a universal method a t the present stage of development. LITERATURE CITED
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sented before the Dirision of Refining, .imeriran Petroleuni Institute, 17th Midyear Meeting. San Franrisro, Calif.. May 12. 1052.
