evidence is presented to substantiate the olefin and aromatic calibration data on the basis that wide changes in coniposition do not cause appreciable differences between results from tn o different sources. Comparisons of saturate percentages are also made in Table VI. There is a significant discrepancy in the four higher boiling fractions. This may be caused by the influence of dicycloparaffins on the percentage of naphthenes by refractivity intercept. Table VI1 shows aromatic and olefin data for a variety of commercial motor fuels. The close agreement indicates t h a t a good approximation of the right compound distribution \\as achieved. The composition of the commercial fuels was largely unknonm but was assumed to be mostly catalytically cracked material. Since ASTM Method D1019 was used to relate total saturated and unsaturated hydrocarbons as percentages of the stabilized original, the precision of the distribution between these two types is dependent on the precision of D1019. Although this is 2% or better (S) the 11s does provide compari-
son data from the peak height ratios. It is therefore felt that total saturated and total unsaturated hydrocarbon percentages by this method agree somewhat within 27,. Unusual blends which would cause trouble in the analysis were not observed among the fuels tested. This method may readily be used for straight run or thermal gasoline provided calibrations specific for these streams are used. Other mass spectrometers which yield n-heptane patterns within the limits given and which have inlet systems a t 100” C. should be appropriate for this method of analysis. ACKNOWLEDGMENT
The authors are indebted to S. S. Kurtz, Jr., for his advice on normalizing the results to acid absorption values. They also thank C. C. Cerato who developed much of the reference data and Evelyn Remorenko who obtained the mass spectra.
(2) Am. SOC. Testing Materials, Committee D-2, RD IV, Section M, “Proposed Tentative Method. Hydrocarbon Types in Olefinic Gasoline by Mass Spectrometry,” 1959. (3) Am. Soc. Testing Materials, Standard8 on Petroleum Products and Lubricants (1960). (4)Zbid., Tentative Method D1658-59T, “Carbon Number Distribution of Aromatic Compounds in Naphthas by Mass Spectrometry.” (5) Brova, R. A., ANAL.CHEM.23, 430 (1951). (6) Ferguson, R. C., Howard, H. E., Zbid., 30, 314 (1958). (7) Frisque, A. J., Grubb, H. M., Ehrhardt. C. H.. Vander Haar. R. W.. Zbid., 33, 389 (1961). (8) Kurte, S. S., Jr., Mills, I. W., Martin, C. C., Harvey, W.T., Lipkin, M. R., Zbid., 19, 175 (1947). (9) LumDkin. H. E., Thomas. B. W., ‘ Elliott; Annelle, Ibid., 24, 1398 (1952). (10) bIcAdams, D. R., “Isotope Correction Factors for Mass Spectra of Petroleum Fractions,” Esso Research Laboratories, Baton Rouge, La. 1957. (11) Podbielniak, W. J., Zbid., 13, 639 (1941).
LITERATURE CITED
(1) Allen, J. G., Rood, J. C. S., Am. Soc. Testing hfuteriuls Bull. 238 (1959).
RECEIVEDfor review June 29, 1961. *AcceptedJanuary 16, 1962.
Spectrophotometric Titration of Primary Aliphatic Amines YU-LIN G. LIU and CHARLES A. REYNOLDS Departmenf of Chemistry, University of Kansas, Lawrence, Kan.
bA spectrophotometric titration method for the determination of primary aliphatic amines in the presence of secondary and tertiary amines and other basic substances has been developed. The amine is titrated in dioxane solvent with glacial acetic acid added as catalyst and a standard solution of 2-ethylhexanal as titrant.
P
the most versatile method for resolving mixtures of primary and secondary amines, especially in the presence of other bases such as ammonia, is that of Critchfield and Johnson (1). I n this procedure, the amine mixture is treated with excess of carbon disulfide, and the total amine content is determined by the titration of the resulting dithiocarbamic acids with standard base. An identical sample is assayed for secondary amine content by allowing the primary amine to react vrith excess %ethyl hexaldehyde to form the corresponding ROBABLY
542
0
ANALYTICAL CHEMISTRY
imine; the quantity of secondary amine is then determined by the same method as the total primary plus secondary amine, and the amount of primary amine is obtained by difference. The same workers (2) later developed a procedure in which the primary amine content is determined directly instead of by difference, but this procedure can not tolerate the presence of other bases such as ammonia. I n this work, a procedure for the direct titration of primary aliphatic amines with 5-ethylhesanal has been developed that will allow the determination of primary amine in the presence of secondary and tertiary amine and other basic substances. The amine sample is dissolved in a mixture of acetic acid and dioxane and titrated directly with a standard solution of 2-ethylhexanal in dioxane. The end point is detected by the appearance of the absorbance of the excess aldehyde a t 305 mp. Secondary
and tertiary amines and other basic substances, such as ammonia, do not interfere. EXPERIMENTAL
Reagents and Apparatus. An approximately O.1M solution of 2-ethylhexanal in dioxane was prepared by dissolving distilled 2-ethylhexanal which had been stored under nitrogen into dioxane which had been passed through activated alumina a n d refluxed over sodium. The solution was then standardized b y titrating weighed quantities of distilled n-butylamine by the procedure outlined below. The n-butylamine was analyzed previously by titrating weighed samples with standard hydrochloric acid in aqueous solution. The purity of the butylamine was 99%. The normality of the aldehyde solution was checked periodically and found to remain constant within =+=0.2% as long as it was protected from atmospheric oxygen by the use of a nitrogenfilled balloon over the top of the reagent bottle.
+
.cc 0
V o l u m e of
Figure
1.
3
2-Ethylhexanal
I
I
1
4
5
6
(mi.)
Titration of butylamine
All of the primary aliphatic amines used for analysis, with the exception of methyl amine, were distilled and stored under nitrogen. Other reagents used in this work, including methyl amine, were all of reagent grade. The spectrophotometric titration apparatus used has been described previously ( 5 ) . A quartz titration cell of 50-ml. capacity was used throughout this work. Procedure. Keighed samples of amine were dissolved i n 100 ml. of a dioxane solution which was 0.5M in acetic acid, a n d a 1-ml. aliquot of this solution was added t o exactly 35 ml. of t h e same solvent in t h e quartz titration cell. The amine was then titrated with t h e standard solution of 2-ethylhexanal; monochromatic, ultraviolet radiation of 305 mp wavelength R as employed, and absorbance readings were taken a t appropriate intervals as soon as the titration reaction had reached equilibrium. Dry nitrogen was passed through the cell compartment continuously during each titration. A more detailed photometric titration procedure has been described in a n earlier paper (6). RESULTS AND DISCUSSION
The results of the titration of a number of primary aliphatic amines are given in Table 1. The percentage purities were calculated from the results of the titrations of a t least three individual samples of each amine. The precision is given as relative standard deviation. ,4typical titration curve is shown in Figure 1. The concentration of the aniine to be titrated should be 10-23f or greater for maximum precision and accuracy. If smaller concentrations are used, the concentration of the titrant has to be reduced proportionally, as the molar absorptivity of 2-ethylhexanal is not
sufficiently large to produce a great enough slope of the rising leg of the photometric titration curve for accurate determination of the equivalence point. The photometric titration of an amine with 2-ethylhexanal could conceivably be carried out utilizing either the ultraviolet absorbance of the aldehyde or the imine. However, the imine absorbance does not take place appreciably a t m-avelengths greater than 270 mp, and the solvent dioxane absorbs radiation of this wavelength t o a significant extent. Also, a much sharper titration curve is obtained if radiation of wavelength 305 mp is employed, rather than radiation of 295 mp, where the absorbance peak of Zethylhexanal occurs, because no residual absorbance of the imine appears a t 305 mp. The rate of formation of the imine in dioxane alone is very slow, but the reaction is catalyzed by the presence of glacial acetic acid. Kresze and coworkers (3, 4 ) found t h a t acetic acidacetate buffers catalyzed the formation of Schiff bases in aqueous solution. -4lthough acetate salts are insoluble in
Table I.
dioxane, acetic acid alone seems to have a similar effect. Kinetic studies performed spectrophotometrically showed that the reaction between nbutylamine and 2-ethylhexanal was almost instantaneous when the added acid concentration was between IO+ and 2 X . R i t h amines that react more slon ly Kith 2-ethylhexannl, the maximum rate of reaction occurred when the acetic acid concentration was about 0.5Jf. The only exception to this was the reaction of ethylenediamine, whose acetate salt is insoluble in dioxane. However, ethylenediamine can be titrated effectively in anhydrous acetonitrile with 0.5M acetic acid as catalyst. As long as the acetic acid concentration is maintained a t 0 5-11, the titration reaction for all primary aliphatic anlines is rapid enough for a practical photometric titration. The rate of reaction seems to be deprndent also upon the strength of the acid used as a catalyst. For example, if anhydrous hydrogen chloride is used, or if a weaker acid than acetic is used, the rate of imine formation is too slow for a practical titration. Evidently the first step in the reaction sequence is the addition of a proton to the aldehyde, H R’-CH=O
+ H+
I
R’-C-OH I
followed by the addition of the aniine nitrogen to the positive center of the protonated aldehyde,
:
A
H
H H
This latter addition product then undergoes a dehydration reaction to form the amine,
H
R~-C=K-R
+ H ~ O+ H +
Results of the Photometric Titration of Primary Aliphatic Amines
Amine n-But ylamine n-Hexylamine Ethylenediamine p-Phenylethylamine Benzylamine 3-Methoxypropylamine !t%et.hylamine Cyclohexylamine Ethanolamine Allylamine
Millimoles of AmineTaken 0.3167 0.3346 0.1565 0.3542 0.3881 0.3216 0.3381 0.2676 0.4788
Relative Standard Deviation, (,C
Percentage Purity
0.3 0.5 0.9 0.6 0.6 0.4 0.4 0.6 1.2 0.7
99.0 99.9 98.2 99.0 99.3 99.4 23.4 97.4 100.8 98.0
VOL 34, NO. 4, APRIL 1962
0
543
If the acidity of the solution is too high, the amine is also protonated to a great extent and cannot add to the protonated aldehyde. If the acid concentration is too low, not enough of the aldehyde is protonated t o cause a significant rate of addition of the amine. Samples of n-butylamine mixed with equimolar quantities of di-n-butylamine, tri-n-butylamine, and ammonia were titrated according to this procedure, and no interference due to the presence of these added bases was noticed. The results of the titration of the above mixtures were all within two standard deviations of the mean of the results of n-butylamine alone. Repeated attempts were made to titrate primary aliphatic amines in the presence of primary aromatic amines
without success. Although the aromatic amines do not react appreciably with %ethylhexanal under the reaction conditions employed for the titration, the intrinsic ultraviolet absorbance of the aromatic amines a t 305 mp causes the absorbance of the solution being titrated to be so great that the relative change in absorbance during titration is very small. A number of aromatic aldehydes were tried as titrants, but none of these reacted rapidly enough with the aliphatic amines to make a practical titration possible. Crotonaldehyde, Lvhose carbonyl absorption peak is a t 325 mp instead of 295 mM, was also tried as a titrant, but unfortunately crotonaldehyde reacts to a significant extent with many aromatic amines while reacting only slowly with most
aliphatic amines. Thus, the procedure reported can tolerate the presence of only a small amount of an aromatic amine or any other substance whose intrinsic absorbance a t 305 mp is high. LITERATURE CITED
( 1 ) Critchfield, F. E., Johnson, J. B., ANAL.CHEM. 28,430 (1956). (2) Ibid., 29, 957 (1957). (3) Kresze, G.,Becker, H., 2. Nuturjorsch. IZB, 45 (1957). (4) Kresze, G., Manthey, H., 2. Elektrochem. 58,118 (1954). (5) McKinney, R. W.,Reynolds, C. A,, Talantu 1,46 ( 1958). RECEIVED for review December 7, 1961. Accepted February 9, 1962. Work sup-
ported in part by the Directorate of Chemical Sciences, Air Force Office of Scientific Research.
Evaluation of an Automatic Nitrogen Analyzer for Tractable and Refractory Compounds PAUL D. STERNGLANZ and HEINZ KOLLIG Southern Research Institute, Birmingham, Ala.
b Techniques are described which ensure precise and accurate analyses of tractable organic compounds with the Coleman automatic nitrogen analyzer. Precision and accuracy of the method are determined. A modification for refractory compounds is introduced. Refractory compounds are oxidized b y fusion with vanadium pentoxide in a platinum sleeve. Vanadium pentoxide serves as a combustion aid. The platinum sleeve protects the combustion tube from attack and acts as a catalyst. The temperature in the combustion zone is 900" C.
(pyrimidines, purines, pteridines) generally considered as refractory, behave like tractable compounds. Some heterocyclic compounds, however, require a more vigorous attack. They can be analyzed successfully by introducing a modification in the combustion procedure. The refractory sample is oxidized by fusion with vanadium pentoxide (16) in a platinum sleeve. Vanadium pentoxide serves as an oxidizing agent. The platinum sleeve functions as a catalyst and protects the quartz combustion tube a t the same time. PREPARATION O F EQUIPMENT
T
Coleman automatic nitrogen analyzer has been available commercially for only 2 years. Its reception by microchemists has been mixed, varying from rejection, to scepticism, to high praise. Therefore, a thorough study was undertaken in this laboratory t o evaluate its capabilities. The Coleman automatic nitrogen analyzer corresponds very closely to the prototypr designed and described by Gustin (6). Details can be found in the operating manual of the manufacturer (3). The precision and accuracy of analysis of all compounds is improved by simple changes of the procedure recommended in the operating manual ( 3 ) . Under these conditions, many heterocyclics HE
544
ANALYTICAL CHEMISTRY
Platinum Cylinder Sleeve. Platinum cylinder, of heavy gage platinum sheet, Item 8310-M, -4.H. Thomas Co., Philadelphia, P a . The platinum cylinder is almost closed a t the bottom by folding it 3 to 4 mm. from the bottom towards the center, thus forming a container which will hold solids b u t permit gases t o flow through. Combustion Tubes. Coleman Instrument Co., Maywood, Ill. The tubes are used for the unmodified procedure. Clear fused quartz tubing, Type 204, bore 9 mm., 0.d. 11 mm., 1313/leinches in length, General Electric Co., Willoughby, Ohio, is used with the Dlatinum cvlinder sleeve for better fit. Neoprene Gasket. Neoprene washer, Item V-232, Coleman Instrument Co., Maywood, Ill. The washer
is cut down t o an outside diameter of 10 nim. REAGENTS
Oxide (11). Cuprox reagent, Coleman Instrument Co., Maywood, Ill. Any other copper oxide, wire form, can be used when ground to pass 20 mesh b u t not 60 mesh. The copper oxide is heated a t 675' to 725' C. in a stream of oxygen for 30 minutes and then cooled to room temperature in an oxygen atmosphere. Vanadium Pentoxide. Vanadium pentoside, C.P. grade, Vanadium Corp. of America, Kew York 17, 9 .Y. Activated
Copper
PROCEDURE FOR TRACTABLE COMPOUNDS
The actual step-by-step operation of the instrument is omitted here because of the detailed instructions given in the operating manual of the manufacturer ( 3 ) . The recommended changes present no particular problem. These changes should be applied to the analysis of all compounds-tractable and refractory. At the beginning of the operation the temperature of the post heater is adjusted to about 525' C., and the temperature of the combustion furnace to 875' to 900' C. This ensures a temperature between 850' and 925' C. in the combustion zone during the combustion cycle. Under these conditions, the operator does not have to perform any temperature adjustment during the whole day.
