Determination of Trace Kjeldahl Nitrogen in Petroleum Stocks E. D. NOBLE California Research Corp., Richmond, Calif.
Phenol solution, made by dissolving 8 grains of C.P. phenol in 100 ml. of freshly distilled water. If the solution ,becomes cloudy. iyarm it slightly prior to use. Standard nitrogen solution for calibration, made by dissolving 0.382 grams of dry C.P. ammonium chloride in 1 liter of lresli distilled water. Pipet 100 ml. of this solution into 3 1-liter volumetric flask and make up to mark. This standard contains 0.01 mg. of nitrogen per ml.
The only previous11 reported method for determining very low levels of total nitrogen in petroleum stocks is unsuited to routine analysis. In this work, a reproducible low blank has been achieved by purification of the Kjeldahl reagents, permitting detection of as little as 1 p.p.m. total nitrogen by Kjeldahl digestion of a 5pram sample. A systematic study was made of the phenol-sodium hypochlorite colorimetric method of ammonia assay, resulting in improved reliability of this hitherto unpopular method. The procedure lends itself well to multiple analyses.
COLORIMETRIC PROCEDURE
The Kjeldahl distillate, in which the ammonia is fixed with tv,w drops of 0.5N sulfuric acid, is concentrated by evaporation in t,he presence of boiling beads to 20-ml. volume in the receiving flask. Two standard samples are prepared in similar flasks by adding 0.0 and 25.0 ml. of the nitrogen standard t o 175 ml. of fresh distilled water containing two drops of 0.5N sulfuric acid and theri concentrating by boiling to 20-ml. volume. T h e flask contents :ire transferred to 50-ml. borosilicate mixing cylinders, and t h volume is increased to 40 ml. with washings from the flask. A t this point it is convenient to carry six t o eight cylinders through the remaining steps together. Pipet 5.0 ml. of the 8% phenol solution into each of the series of cylinders. Mix the contents by upending several times. Pipet 5.0 ml. of the sodium hypochlorite solution into each cylinder and mix again. Plunge the cylinders into boiling water to a depth sufficient to cover liquid rontents. Loosen the ground-glass stoppers. After 6 but before 8-minute exposure to the boiling water bath, remove the cylinders and cool them quickly in tap-water bath. Measure the absorhance of the solution in 1-cm. cells using a 610-w filter, and estimate the milligrams of nitrogen in the samples from the linear r w v e established by the two reference samples. If samples contain under 0.1 mg. of nitrogen, it is advantageous to use 5-cm. absorption cells and a standard rurve based on 0 and 1 0 ml. of standard nitrogen solution.
T
H E nitrogen content
of petroleum stocks is of interest bccause even small qurtntit’ies of nitrogen poison the catalyst in reformer units and promote the formation of gum in stored products. Traces of basic nitrogen may be determined readilj. down to 1 p.p.m. by perchloric :wid titration ( 5 ) ,but basic nitrogen may- be only a minor fraction of the total nitrogen to be found in a petroleum stock. -4recent publication (11)reviews the limitations of the Kjeldahl ( 3 ) and ter hluelen ( 2 ) procedures and aolves the problem by catalytic hydrogenation of a very largc. (1000-nil.) sample. Themethod is not suited t o routine analysis. This investigation covers the necessary modifications of th(, basic Kjeldahl procedure to permit its use for the det,ection of a,little as 1 p.p.m. total nitrogen with the digestion of only :I 5-gram sample. The investigation consisted of t h r w part*: the establishment of a colorimetric procedure for routine drterniination of low concentrations of ammonia, the purification of thc. Kjeldahl reagents t o give a reproducible low blank value, and a study of the ability of the Kjeldahl procedure t o convert. n-ithout loss, trace amounts of nitrogen in organic media to animonia. T h e trace nitrogen so determined is termed Kjeldalil nitrogen because it is consistent with macro Kjeldahl nitrogrii on the crude oils on which the method has been tested. So claim is made for it being an absolute total nitrogen method.
.30-
a
75
I 0
z .ro-
v
Y
u z 4
.15-
m w
COLORIMETRIC DETERRIINATION OF AM3IONIA
4
The ammonia detection procedure had to be selected and it? reliability established before reagent blanks could be determined. The phenol and sodium hypochlorit,e reagent, which gives a clear blue color wit,h ammonia, was chosen because the bluc color would be more suitable for spectrophotometric evaluation than the colloidal Sessler color. It was also thought the reaction would be lese sensitive to reagent formulation, pH, and temperature than the Nehsler reaction. The phenol-sodium hypochlorite method is included in laboratory manuals (8) and ha:: been studied by many invest’igators (1, 7 , 9, 10). I t is reconimended for its high sensitivity but warnings usually appear concerning poor reproducibility and color instability. A systematic. study was made of the variables in this reaction, and a sensitivt. :md reproducible procedure was developed as follows:
m 4
10-
I
O O
I
2
3
4
5
e
7
a
8
!O
M I N HEATED I N H20 BATH
Figure 1. Time cs. color intensity 0.25 mg. N nb NHI +
Table I.
Effect of Sequence of Addition of Reagents on Color Intensity
Absorbance with 810-1np Filter .Idd i>henol,add NaOCI, stand 5 minutea. heat 0.276 .Idd phenol, add NaOC1. heat 0 278 .Idd phenol stand 5 minutes add h-aOC1 heat 0.282 Add NaOCi. add uhenol. s t a i d 5 minutes: heat 0.187 .Add WaOC1, add phenol, heat 0.200 Add NaOC1, stand 5 minutes, add phenol. heat 0,032 ‘ c Each treatment was performed on 0 . 2 5 mg. N as “4Cl and one drou 0 . 5 S &SO4 made u p t o 40 ml. with distilled water. Additions were made as listed under treatment. Each sample was inverted twice after each addition of reagent t o niix and heated 7 minutes in boiling water bath.
APPARATUS
Cenco Photelometer with 1- and 5-cm. cells and 6lO-mM hlter. \Tater bath maintained a t 100’ & 0.5’ C. ITsual laboratory glasswarr. REAGENTS
Freshly distilled water, ammonia-free. Sodium hypochlorite solution, commercial Clorox. labeled as 5.25% by weight KaOC1.
1413
1414
ANALYTICAL CHEMISTRY
Table 11. Effect of Volume to Which Reagents Are Added on Ultimate Color Intensity Absorbance with 610-mp Filter Add phenol add NaOCl, add 20 ml. water heat 0.274 Add phenol: add NaOC1, add 20 ml. water' heat 0.268 Add phenol, add 20 ml. water, add NaOC1: heat 0.218 Add phenol add 20 ml. water a d d NaOCl heat 0.212 Addpheno1,'add 10 ml. water, Add NaOCl a'dd 10 ml. water heat 0 . 2 3 8 Add phenol, add 10 ml. water, a d d NsOC1: add 10 ml. water: heat 0.235 Treatmento
Each treatment performed on 0.20 mg. N as NHaCl in 20 ml. of distilled water.
04 0
6
2
a
6
8
I2
(0
YL. PHENOL SOLUTION (8%) IN
14
I8
I8
SO ML.
Figure 3. Effect of phenol concentration on color intensity 0.10 mg. N as NHa +
EXPERIMENTAL
T h e effect of one to five drops excess of 0.5N sulfuric acid was studied over the range of 0 to 0.25 mg. of nitrogen and found to have no effect, The effect of heating time was studied over the range of 3 to 9 minutes, and 7 minutes in boiling water was found to be optimum. This is shown in Figure 1. The stability of the color formed was observed for several hours and found t o be stable even in the presence of air.
4
Oo
7
1
i
z
3
ML.
4
s
6
7
B
NaOCl SOLUTION IN 50 ML.
Figure 4. Effect of sodium hypochlorite concentration on color intensity
Table 111. Nitrogen Found in Kjeldahl Reagents
2 U.
'
10
04 0
5
IO
I5
20
25
30
35
40
45
50
MILLILITERS VOLUME IN WHICH COLOR DEVELOPED ( A L L DILUTED' TO 5 0 M L . FOR COLOR MEASUREMENT)
Reagent HzSOa, C.P. HgSOa, distillation heart c u t K*SOa, C.P. KzSOa, recrystallized Hg. C.P. triple distilled HgO. C.P. CuSOa, C.P.
Quantity Tested 50 ml. 50 ml. 10 g . 10 g . 1 . O g. 0.7 1 . 0 g.
Nitrogen Content P.P.11. nitrogen In quantity blank i f 5 - ~ . tested, mg. sample digested 0.03-0.12 7-25 0.01 2 0.03-0 06 0.01 0.00 0.01
6-12 2 0 2
Figure 2. Effect of volume in which color developed TS. color intensity DISCUSSIOY
The sequence of the addition of the phenol and sodium hgpochlorite solution was found to be important. The phenol solution must be added first and mixed in well. The time betryeen steps is of little significance, provided this is done. The data are shown in Table I. The volume of solution in which the color is developed affects the color intensity. If the color is formed in a volume of less than 50 ml. and then diluted to 50 ml. when cool, a more intense color results. This effect is shown in Figure 2. 4180 if the volume of solution is less than 40 ml. when the 5 ml. of phenol and 5 ml. of sodium hypochlorite are added, and then the volume is increased to a total of 50 ml. before heating, a more intense color results. The smaller the initial volume, the greater the color intensity as shown in Table 11. If the volume is 45 ml. before adding the sodium hypochlorite, however, it appears to make no difference a t what dilution the phenol is added. T h e effect of phenol concentration was studied and is shown in Figure 3. The optimum amount is near 5 ml. but is not critical. The effect of sodium hypochlorite concentration is shown in Figure 4. Here, too, the optimum is a t about 5 ml. and is not too critical. I n Figure 5, the spectral absorption curve of the blue solution between 540- and 800-mp wave length shows a peak near 610 mp, which is the reason for choice of the designated filter. I n Figure 6, typical concentration curves versus absorbance show the linear relationship with either the 1- or 5-cm. cell.
It is evident from Figure 2 and Table I1 that sensitivity beyond the normal effect of concentration can be obtained by developing and measuring the color in a minimum volume. It was not necessary to take advantage of this greater sensitivity for this use of the method. Since this work was done, Riley (6) has reported a Fystematic study of development of the indophenol color. His procedure differs considerably from the one developed here in that it uses more complex reagents and a longer color development period. REDUCTION OF KJELDAHL BLANK
Chemically pure sulfuric acid gave a high blank value which differed widely from lot to lot. A simple distillation in all glass equipment yielded a heart cut (10 to 80%) of uniformly low nitrogen content. Chemically pure potassium sulfate also gave a high and variable blank value. A simple recrystallization reduced the blank to a uniform low value. Catalytic materials were also tested for their nitrogen content. The nitrogen content of various reagents is summarized in Table 111. If the Kjeldahl procedure is run on a &gram sample with 50 ml. of distilled sulfuric acid, 10 grams of recrystallized potassium sulfate, and mercury metal as a catalyst, the blank corresponds to 4 p,p.m. of nitrogen in the result, 2 p.p.m. coming from the acid, and 2 p.p.m. coming from the sulfate. I n this range a difference of 1% transmittance, which is the accuracy of the detecting instrument, is equivalent to just under
V O L U M E 2 7 , NO. 9, S E P T E M B E R 1 9 5 5 1 p.p.m. of nitrogen using 1-em. cells aad about 0.3 p,p.m. using the 5-em. cells. Consequently, since the blanks vary less than 1%, they are considered t o be sufficiently reproducible t o determine & 2 p.p.m. of Kjeldahl nitrogen in B sample up t o 50 p.p.m. and to detect as little as 1 p.p.m. with certainty. FURTHER MODIFICATIONS AND R E S U L T S
Reagents.
POTASSICM SGLFATE. Recrystallize C.P. grade potassium sulfate by saturating 2000 ml. of boiling distilled water and then allowing it t o cool overnight. Filter off the crystals, rinse lightly with distilled water, and store wet for future use. COPPERSULFATE,C.P. anhydrous powder. MERCURIC OXIDE,C.P. powder. MERCURY, C.P. triple distilled. SULFURICACID, 10 to 80% heart cut from C.P. concentrated acid of 1.84 gravity prepared in an all-glass still. Distill the acid batchwise a t atmospheric pressure from a 2-liter round-bottomed flask n i t h a ground-glass standard-tapered neck, connected to an air condenser by a 75" connector with standard taperedground glass fittings a t both ends. It is not necessary to lubricate the joints, to record the temperatures, or to protect the overhead from air. Approximately 4 hours are required t o obtain the 10 to 80% cut from a 1700-ml. charge. FIFTYPER CENT SoDIuhi HYDROXIDE SOLUTIOS. Dissolve 2 kg. of C.P. sodium hydroxide pellets in 2 liters of water. Let the solution stand 24 hours to cool and permit the carbonate to settle out. Filter through glass wool into a gallon bottle. Kjeldahl Digestion Procedure. \Veigh into a clean, dry 300ml. Kjeldahl flask 10 grams of purified potassium sulfate, 1 gram of copper sulfate, and 0.7 gram of mercuric oxide. (One gram of mercury may be used instead of the copper sulfate and mercuric oxide.) Add three or four glass beads and 50 ml. of the distilled sulfuric acid. Weigh into the flask 5 grams of sample to the nearest 0.01 gram. If more than 50 p.p m of nitrogen are anticipated, take a smaller weight of sample, or complete analysis by distilling into boric acid and titrating with hydrochloric acid. Several blanks should be run t o establish the amount of nitrogen contributed by the sulfuric acid and catalysts. I n the place of the sample. use 5 ml. of a pure hydrocarbon such as isooctane. Once the blank value is known, it need not be redetermined until different reagents are used.
1415 apparatus has been used for other work, the inside of the still should be cleaned by drawing distilled water through it by vacuum, or by distilling off a flask full of water.) Add 20 ml. of distilled water and two drops of 0.5A- sulfuric acid t o a 250-ml. Erlenmeyer flask and mount on the distillation apparatus t o receive the distillate. Swirl the Kjeldahl flask t o mix the contents thoroughly, and then distill until serious bumping commences. The Kjeldahl portion of the procedure required relatively little investigation or modification. Steps in the procedure, which differed from the conventional Kjeldahl procedure, were checked to make certain that no ammonia or nitrogen was lost.
o
/
1'
0
5 CM
002
004
006
008
010
CELL
012
014
016
018
020
M G NITROGEN IN 50 ML
Figure 6.
Standard curves
Cenco photelometer with 610-mp filter
When a 5- to 6-gram sample was digested in 50 ml. of concentrated sulfuric acid plus the catalyst, a large portion of the lower boiling paraffinic and naphthenic hydrocarbons distilled off and was lost. However, digesting a t a lower temperature for longer periods, so that no sample distilled off, did not increase the apparent nitrogen recovery. Because of the rapid distillation of hydrocarbon, a large quantity of sulfuric acid remained after the sample had cleared. To obtain a temperature high enough t o ensure complete reduction of the nitrogen (e), the amount of sulfuric acid for the 10 grams of potassium sulfate present must be reduced. Raising the temperature of the burner after the solution had cleared to distill off the excess sulfuric acid resulted in no discernible loss of nitrogen from standard samples.
0
io0
540
560
Si0
660
700
7k0
700
820
IN MILLIMICRONS Spectral absorption curve
WAVE LENGTH
Figtire 5 ,
Sodium phenate reagent 10-cm. cell
Swirl the flasks vigorously until the sample and acid are well mixed. (If the material being analyzed, or any fraction thereof, has a boiling point below about 100" C., digest for 1 hour on a steam plate with periodic vigorous swirling.) Place the flasks on the Kjeldahl digestion rack and proceed with the digestion, using as much heat as necessary. Excessive foaming can be largely controlled by periodically removing the flasks and swirling them for a few seconds, or small additions of sulfuric acid m a y b e used. However. the additional sulfuric acid is not desirable and consequently should be avoided if possible. After the acid has become clear and light-colored, and the neck of the flask has been cleaned down by increasing the heat and letting the sulfuric acid reflux, reduce the total volume about 25 ml. with a little additional heat. Reduce the flame and boil another 3 hours t o ensure complete reduction of the nitrogen. Cool the contents of the flasks, sloivly add 175 ml. of distilled water, and cool again. Incline the Kjeldahl flask a t 45" and add 55 ml. of 50% caustic solution s l o ~ l y ,so that it settles t o the bottom without mixing. .4dd a pinch of 20-mesh metallic zinc. Place the flask in position on the distillation apparatus. (If the
Table IV.
Results of iinal>-sisof Diluted Crude Oils
Crude Oil Diluted K~~~~ Xitropen Total N by content, Dilution, Name % P.P.11. Midway 0.50 5 Midway 0.50 10 Santa Maria 0.81 5 Santa Maria 0.81 10 a Catalyst 2 1,, CuSOa Hg. a n d HgO.
~~~i~ Nitrogen Found,
Total Nitrogen Follnd. P.P.hf. Catalyst Cataiyst P.P.M. 1: 2 1.0 6 5 2.5 10 8 0.8 5 4 2.2 11 12 .
Two California crudes, one Santa Maria and the other Midway, which were high in sulfur and nonbasic nitrogen, were analyzed and then diluted to 5 and 10 p.p.m. of total nitrogen with nitrogenfree kerosine. The analyses of these stocks are shown in Table IV. This shows good recovery of the total Kjeldahl nitrogm a t very low levels and is the basis for extending the method t o naphtha analysis. The procedure has been in routine use for about a year and a half and has been of value in distinguishing basic from total nitrogen a t low levels. The method has been
ANALYTICAL CHEMISTRY
1416 cross checked with the ter Muelen and basic nitrogen procedure3 where applicable, and the analyses have been thoroughly consistent.
16)
LITERATURE CITED
(8)
(5) Moore, 11. T., lIoCutchan, P., and Young, D. A , , Ibid., 23, 1639 (7)
(1) Foxwell, G. E.. Gus W o r l d (Coking Section), 64, 10 (1916~.
(2) Holowchak, *J., Wear, G. E. C., and Baldeschwieler, E. L., AI., CHEM.,24, 1754 (1952). (3) Lake, G . R., I b i d . , 24, 1806 (1952). ( 4 ) Lake, G. R.. 1lcCutchan. P.. Van Meter, R., and Keel, J . C., Ibid., 23, 16.74 (1951).
(9) (10)
(11)
(1951). Riley, J. P., Anal. Chiin. Acta, 9, 575 (1953). Russell, J. A., J . Biol. Chern., 156, 457 (1944). Snell. F. D., and Snell, C. T., "Colorimetric 1Iethods of -Inslysis," 2nd ed., vol. I, p. 658, April 1943, and 3rd ed., vol. 11, p. 818, 1949, Van Xostrand, Yew York. Thomas, P., Bull. soc. chim.,11, 706 (1912). Van Slyke, D. D., and Hiller, A., J . H i d . Chem., 102, 499 (1933). Wankat, C.. and Gatsis, J. G., . ~ . I L . CHEx., 25, 1631 (1953).
RECEIVED for review February 14. 1935. Accepted RIay 31, 1955
Precipitation of Barium Carbonate HARRY TEICHER' M o u n d Laboratory, M o n r a n t o Chemical Co., Miamisburg, O h i o
Critical studies of the several factors affecting the solubility of barium carbonate in analytical procedures have not been reported, I t was found that a dense, easily filtered, and easily washed product was obtained by hubbling carbon dioxide into an ammoniacal solution containing barium. Using radiochemical tracer techniques, the solubility loss of barium carbonate was determined with respect to final pH, alcohol concentration of the wash solution, addition of alcohol to the mixture after precipitation of the barium, and the presence of excess ammonium salts. The recommended procedure results in a solubility loss of 0.00015 gram of Imrium in 375 ml. of solution.
medium-porosity glass frit. Filtration was performed by connecting the exit tube to a bell jar which could be evacuated by gentle suction. .2beaker placed in the bell jar served to collect the filtrate and washings. Reagents. Barium nitrate. General Chemical Co. reagent grade barium nitrate was recrystallized twice from water and dried overnight at 110' C. I
,
T
HE quantitative precipitation of barium carbonate is recommended in various texts (8, 1 4 ) of qualitative analysis. .I quantitative barium carbonate precipitation has been recommended by Fresenius and Hintz (S), although the presence of ammonium salts, present in the procedure, have been reported as preventing complete precipitation ( 9 ) . .i survey of the literature ( 1 2 ) indicated that no critical study of the factors affecting the solubility of barium carbonate undei analytical conditions has been reported. Towley, Whitney. and Felsing ( I S ) have reported the solubility of barium carbonatr. in pure wat,er and in the presence of alkali chlorides. The soluhility in water of barium carbonate a t various pressures of carbon dioxide was investigated by Haehnel ( 5 ) . Semiquantitativt. estimations of the solubility of barium carbonate in the presencr of varying concentrations of ammonia, ammonium carbonate,: and ethyl alcohol have been published by Bray ( 2 ) . Sidyn.ic!i ( 2 6 ) has accepted the soluhility in r a t e r a t 18" C. as heing 8.ti mg. per liter. . This paper describes tests made to determine the solubilit>. loss (solubilit,y in mother liquor plus wash eolution) of barium carbonate formed by the int,roduction of carbon dioxide into an nmmoniacal solution containing barium followed by the addition of alcohol. The precipitate, is then washed with aqueous alcohol. The solubilit,y loss was investigated with respect to the final pH! the addition of alcohol to the reaction mixture after precipitation. the presence of excess ammonium salts, and the composition of the wash solution.
&SEL
Figure 1.
Reaction vessel
Barium-140. This material. carrier-free, vas obtained from Oak Ridge Sational Laboratory in dilute nitric acid and Kith a radiochemical purity greater than 9Syo. Two solutions were prepared from this material: One solution contained approximately 0.1 mc. per 0.3 ml. and the second solution, which was used in the preparation of counting standards, contained approximately 25,000 counts per 0.1 ml. ~~~
.
~
-
.~
~~
Table I.
Effect of Precipitation Time on Total Solubility Loss Alcohol Alcohol - Barium Lost, $7-
APPARATUS . \ Y U MATERIALS
Added, 1\11. 0 10 20
30 Present address, Inorganic Clieliiicalr Division, Monsanto Chemical Co.. P:Yrrrtt Station, Boston 49, .\lass.
II w
Apparatus. The reaction v e s ~ e lis illustrated in Figure 1. Iluring precipitation carbon dioxide was introduced through the !
m/ -
I
40
Concentration,
5%
0.0 4.4
8.5 12.2 15.6
Time, 30 min. 0.037 0.025 0.051
Time, 40 min. '
0,030 0.08R
0.036
0.051 0.040
0,028
0,050
