493
V O L U M E 2 3 , NO. 3, M A R C H 1 9 5 1 Table
V.
Hygroscopicity of Several Standards
(10-day tests) Gain in Weight, % Relative Potassium Humidity, Tris(hydroxymethy1)hydrogen Benzoic % aminomethanea phthalate6 acid 0.19 0.06 0.00 31 0.00 0.17 0.07 51 0.00 0.19 0.17 71.2 0.01 0.27 0.20 91 .4verage of 3 determinations. b Average of 2 determinations.
Potassium chloride
0.08 0.13 0.14 0 47
stoppered and weighed, then laced in the proper desiccator, and placed in the thermostat. TR ese bottles were weighed daily for 10 days, using the technique described for taring the weighing bottles. Representative data arc shown in Table V. The hygroscopicity tests were carried out in May, during which time the air temperature and humidity varied widely. The adsorbed moisture on the weighing bottles is the cause of the major error in the low adsorption range. Errors as high as 0.5 my. on unfilled weighing bottles were not uncommon when these bottles were weighed on days of widely varying humidity. The hygroscopicity of tris(hydroxymethyl)aminomethane compares favorably with that of the common primary standards Lvhich were tested over all humidity ranges that might be encountered in 1:bboratory work. CONCLUSION
Tris(hydroxymethyl)aminomethane fulfills many of the requirements of a good standard. I t is commercially available a t a moderate price; it can be prepared in a high state of purity with
constant composition; and it has a favorable equivalent weight, 121.136. The amine is nonhygroscopic a t usual laboratory humidities and compares favorably in this respect with potassium hydrogen phthalate. Tris(hydrosymethy1)aminomethane and solutions of this salt do not adsorb carbon dioxide from the air. It can be used directly as a primary standard for strong acid solutions by a simple and rapid procedure. Solutions of the amine are stable under all investigated conditions of standardization. The equivalence point can be easily detected either potentiometrically or by use of the proper indicator. Tris(hydroxymethy1)aminomethane has the disadvantage that it cannot be heated above 100” C. indiscriminately. It is a weak base and has the inherent disadvantage of this class of compounds as primary standards. Its dissociation constant has been reported recently by Bates and Pinching ( 1 ) as 1.202 X lop6 at 25” C. This value is comparable to that of potassium hydrogen phthalate. which iq 3.9 X at 18” C. LITERATURE CITED
( 1 ) Bates, R. G., and Pinching, G. D., J . Research Natl. Bur. Stand-
ards, 43,519 (1949).
F.,and Lundell, G. E. F., “Applied Inorganic Analysis,” 9th printing. pp. 137-8, 141, Kew York, John tT‘iley & Sons, 1946. (3) International Crltical Tables, Vol. IV, New York, McGraw-Hill Book Co., 1928. (4) Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis.” revised ed., p. 548,New York, Macmillan CO.,i945. (5) Waldbauer, L.. McCann, D. C., and Tuleen, L. F., IND.ENO. CHEM.,ASAL. ED.,6, 33B (1934).
( 2 ) Hillebrand, W.
RECEIVED M a y 4, 1950. Presented before the Division of Analytical Chemistry a t the 117th 11eeting of the . I v E H I c . * s CHEVICAL SOCIETY, Houston, Tex
Determination of n-Paraffins in Gasoline, Oils, and Paraffin Waxes WOLFGANG LEITIIE’, Carl Borschweg 1, Linz, .4ustriu Chemical processes with hydrocarbons of 5 arious boiling ranges (gasoline, oils, waxes) frequently require knowledge of their contents of n-paraffins and isopamffins, which has required tedious or expensive cxperiinents; an easily workable chemical method for providing this information fills a definite need in hydrocarbon analysis. The method described in this paper is based on a simple chemical reaction with antimony pentachloride, followed by an indirert determination of the n-paraffins which have not been affected by this reagent, while the isoparaffins are converted into insoluble tarry matter. A n accicrac) of almut 3 to 6’3, of total sample may be
T
HE commercial value of hydrocarbon mixtures sometimes
depends largely on their contents of straight carbon chains. In some cases, such as the preparation of detergents, unbranched hydrocarbons in the source material are preferred; on the other hand, n-para&is in motor gasolines are h:irmful because of their low octane rat>ing. In spite of the considerable importarice of this special arialytical question, no simple chemical method of gmeral applicability for the determination of straight-chain piratfiris in hydro1 Present address, Oesterreichische Stiokstoffwerke Aktiengesellschaft, Lins. Austria.
reached. The method applies also to other substances such as olefins, alcohols, fatty acids, etc., after they have been converted to hydrocarbons b y usual methods. There are many possibilities of applying this simple and cheap analytical method in scientific and commercial hydrocarbon processes, as in most cases the contents of branched or normal hydrocarbon chains in the sourcc material, in the intermediates, and in the final products reflects largely on the efficacy of the process and on the quality of the products. I t is hoped, therefore, that petrochemistry will draw considerable advantages from this new method.
carbon mixtures has been known, as there is but little difference in the chemical properties of n-paraffins and isoparaffins. This analytical field is chiefly covered by physical methods, such as t,he applicat’ionof infrared absorption spectroscopy ( I ). Some years ago Schaarschmidt and Marder ( 3 ) found a difference in the reaction rate of n-paraffins and isoparaffins with antimony pentachloride, and they based a rough and qualitative method for distinguishing n-paraffins from isoparaffins on this fact. Under the conditions outlined by these authors a quantitative distinction and separation of n-paraffins in hydrocarbon mixtures of various molecular weight are not possible. This.
ANALYTICAL CHEMISTRY
494 Table I.
Calculated Values of n-Paraffins Boiling Point,
n-Para5n
760
15
mm. H g
mm. Hg
... ...
... ...
...
... ... ...
... ... 80
93 113 128 142 155 168 180 190
... ...
...
C. 0.5 droOC. (in mm. Hg CClr Solution)
203
235 252 294
... 40 55 70 84 96 108 118 128 140 170 190 225
0.650 0.675 0.695 0.709 0.720 0.730 0.740 0.748 0.755 0.760 0.764 0.767 0.770 0.773 0.775 0.781 0.785 0.795
whereas, n-paraffins of lower molecular weight may be affected by too great an excess of this reagent. If nothing is known about the approximate n-paraffin content of the sample, 1.5 to 2 grams are weighed out and the determination is repeated with the proper amount of the sample according to the result of the fist determination. A convenient and sufficiently accurate form of density bottle is a 25-ml. volumetric flask sealed with a glass stopper, and about 4 mm. in diameter a t the neck. These flasks are easily filled and emptied by introducing a glass capillary tube in order to discharge the air. CALCULATION
The n-paraffin content of the sample is calculated by the rule of mixtures from the density of the carbon tetrachloride solution obtained by the procedure described above d (mixture) =
objective could be reached by modifying the reaction conditions and by applying a physical method for the determination of the n-paraffins which remain unchanged during this chemical reaction. The reactivity of antimony pentachloride with hydrocarbons was decreased by applying carbon tetrachloride as a diluent, choosing concentration, reaction temperature, and time in such a way that isoparafis react completely in the carbon tetrachloride solution, but n-paraffins remain practically unchanged. As the separation of the unaffected n-paraffins from the carbon tetrachIoride by distillation is impracticable in samples of lower boiling range, a physical method for the analysis of binary mixtures of n-paraffins and carbon tetrachloride based on the widely differing densities of both substances was outlined. The mixture is placed in a density bottle, and the n-paraffin content may be calculated from the density of the mixture by the mixture rule, using an adaptation of a pycnometric fat determination method ( 2 ) .
grams of CCl4 vol. of CC14
+ g r a m of paraffin + vol. of paraffin
(1)
(2) dpsraff.equals the density of the n-paraffin of the sample in carbon tetrachloride solution. This value has been shown to be about 1% lower than the density of the pure hydrocarbons without a solvent. The values of the n-paraffins calculated from the density of their solutions in carbon tetrachloride are listed in Table I. The value to be used for the calculation of the nparaffin content by Formula 2 as dpararr.must correspond with the mean boiling point of the sample. The volume of carbon tetrachloride has been fixed as 30 ml.
Table 11. Tests APPARATUS
A 50-ml. bulb flask is attached to a Liebig condenser by a ground joint. A small calcium chloride tube is placed a t the top of the condenser. PROCEDURE
Weigh out such an amount of sample that about 0.7 to 1.0 gram of isoparaffins or naphthenes is present. Add exactly 5 ml. of pure carbon tetrachloride with a pipet, dissolve completely, and add immediately 8 ml. of pure antimony pentachloride from a small graduated cylinder. Attach the condenser and cool by placing the flask in ice water for a short time, in case a vigorous reaction starts. Place the flask in a water bath at 40" C. and maintain a t this tem erature for 3 hours. If after 1 hour no reaction, indicated by a !ark color and deposit of tarry matter, has taken place, add a small drop of vaseline oil, which contains no n-paraffins but may act as a starter, and heat 2 more hours a t 40"C. After 3 hours take the flask from the oondenser, add exactly 25 ml. of carbon tetrachloride by a pipet, shake gently, and pour the carbon tetrachloride layer as completely as possible into a small separatory funnel, but avoid inclusion of the tarry matter. Shake the carbon tetrachloride solution n-ith 100 ml. of a mixture of 2 volumes of concentrated hydrochloric acid and 1 volume of water vigorously for about 1 to 2 minutes. Place a small pledget of cotton in the dry stem of the separatory funnel and run the lower layer, which now contains only carbon tetrachloride and the unchanged hydrocarbons, into a 28-ml. density kottle. Place the density bottle in a water bath a t exactly 20 * 0.2" C for half an hour, adjust to the mark, and weigh. NOTES
It is necessary to keep these conditions of concentration, time, and temperature, which have been proved by many tests, because some isoparaffins need an excess of antimony pentachloride,
% n-ParaffinsSubstance SamDle. G. Pure n-paraffins 1 n-Heptane n-Heptane 2.5 4 n-Heptane 2 n-Nonane 1 n-Hexadecane n-Hexadecane 3 2 n-Eicosane 3 Cvclohexane Pure isoparaffins, naphthenes, and olefins Hexadecene 0.7 Decalin 0.8 3-Methylheptane 0.8 Vaseline oil 1 Deoalin 3-methylheptane iso-octane 0.6 Mixtures n-Heptane 3-methylheptane
+
+
+
+ Deoalin + Decalin + iso-octane n-Heptane + Deoalin 4-3-methylheptane + iso-octane
n-Heptane n-Heptane
Calcd.
Found
100 100 100
100 100 99 100 102 100
loo
100 100 100 100
+ 3-methylheptane
n-Hexadecane
+ Decalin
+ hexadecene + vaseline oil
n-Hexadecane n-Eicosane
n-Eicosane
+ synthetic i s o p a r a 5
0
0 -1 0
+:
100
0 0
0
2 2 3 2
+2 +2 +3 +2
0
3
+3
0 0
0
100
26 11
22 10
--1 4
50 85 65
43 80 66
-7 -5 +1
49 65 25 25
53 66 25 19 22 81
+ 4I 0 -6
+ 20
56 89 58 14 12
92 59 17 58 90 57 18 12
95 28 52 59 93 96
94 28 56 63 97 98
26
n-Hexadecane
Diff.
86 90 59 11
;;
-4 --J
+6
+2
+1 -1 +4 0
5; -1 0 4-4 +4 +4
+2
V O L U M E 23, NO. 3, M A R C H 1 9 5 1 Pipets adjusted with water have to be adjusted to the actual delivery of 25 and 5 ml. of carbon tetrachloride. DISCUSSION
This procedure is applicable to gasolines (mixtures of paraffins and naphthenes) from about six carbon atoms in the molecule, to oils and paraffin waxes if they are soluble in carbon tetrachloride a t 20" C. The folloiving hydrocarbons are stable against antimony pentachloride: all saturated hydrocarbons containing only
C CHB--, -CH2-,
I
and C-7-C
c groups, such as n-paraffins, cyclohexane, and compounds with quaternary carbon atoms such as 2,2-dimethylbutane. Thus, the true n-paraffi content of a hydrocarbon mixture is found by this method only if the other hydrocarbons listed in this group are absent; otherwise, their percentage is included in the n-paraffin content. The following hydrocarbons react with antimony pentachloride t o form products which are insoluble in carbon tetrachloride: compounds with a tertiary carbon atom such as isoparafhs, substituted cycloparaffins (naphthenes), and olefins. Aromatic compounds and oxygenated compounds react with antimony pentachloride to form products which are partially eolublc in carbon tetrachloride; thus they interfere with this
495 method if present in considerable amounts, and must be removed before antimony pentachloride is applied, by treatment with sulfuric acid according to well-known procedures. This method is not restricted to mixtures of paraffins and naphthenes, but applies also to olefins, alcohols, ketones, aldehydes, fatty acids, etc., after these compounds have been transformed into paraffins by adequate hydrogenation. The accuracy of this method varies with the molecular weight of the sample; in oils and paraffin waxes the average absolute error is about 2 to 4% of the total sample and in gasolines the error is sometimes greater (3 to 6%), as may be learned from the data in Table 11. I t is probable that by a further study of the individual reaction rates of hydrocarbons R ith antimony pentachloride and by slight modifications of the procedure this range of errors may be narrowed. ACKNOWLEDGMENT
The experiments described in this paper were carried out by the author in 1940 a t the Ammoniaklaboratorium Oppau of the former I. G. Farbenindustrie Aktiengesellschaft, Ludwigshafen/ Rhein, Germany. Appreciation is expressed to the Badische Anilin- und Sodafabrik, Ludnigshafen /Rhein, for permission to publish this work. LITERATURE CITED
(1) Hibbard and Cleaves, A N ~ LCHEM.. . 21,486 (1949). (2) Leithe, W., 2.Unlersuch. Lebensm., 67,441(1934). (3) Schaarsohmidt and Marder, Z . angew. Chem., 1933, 151. RECEIYED April IO, 1950.
Infrared Analysis of Five C,, Aromatics JOHN A. PERRY' Monsanto Chemical Co., Texas C i t y , Tex.
The need for a means of analvzing mixtures of five Clo aromatics led to the method presented, which was designed to be rapid and accurate, and to have maximum simplicitj of execution. No dilution of standards or of samples was required in either calibration or analysis. Measurements of cell length and absorbancy were eliminated by the condition of normalization and the use of reference wave lengths, respectively. The acciiracy is show-n
I
S T E R E S T in the composition of isomeric butyl and diethylbenzene mixtures boiling between 16.5' and 190" C. led to the development of a method for determining the composition of such mixtures. The analysis was set up using the ult'raviolet region, but was found inadequate for complete determination of the individual isomers (?). A mass spectrometric determination ( 5 ) of traces of the Ci0 isomers in ethylbenzene has been reported, but determination of concentrations of the individual isomers was not indicated. S o analysis of these mixtures by Raman spectrometry has been reported, although it has been indicated as a possible approach ( 4 ) . Fractionation is not a sufficiently powerful tool to permit easy separation of the isomers, and no rapid and accurate chemical methods have been found in the literature. Resort was therefore made to infrared spectrophotometry: about 45 minutes are required to obtain results having an average absolute error of 0.5%. 1 Present address, College of Chemistry a n d Physics, Louisiana State University, Baton Rouge, La.
to compare favorably with similar reported work, the error being 0.570 absolute over the whole percentage range. An infrared normalized multicomponent analysis of hydrocarbon liquids can be set up under the following restrictions, each of which materially simplifies performance of the analysis: assumption of validity of Beer's law, no dilution of standards or samples, and no knowledge of the length or absorbancy of the rock salt cell.
APPARATUS
A Perkin-Elmer Model 12C spectrometer equipped with a Model 51 breaker-amplifier and Brown recorder was used. HYDROCARBONS
Sources and given purities of the hydrocarbons used were an follows: Compound
Purity, Mole 7c
Source
o-Diethylbenzene m-Diethylbenzene p-Diethylbenzene Isobutylbenzene sec-Butylbenzene
99.95 0.03 99.93 f 0.04 99.93 * 0 . 0 2 99.87 * 0 . 0 9 99 minimum
Sational Bureau of Standards National Bureau of Standards National Bureau of Standards National Bureau of Standard8 Phillips Petroleum Co.
f
DEFINITIONS
The following terms and meanings are used in this paper:
A = l o g 3 = Kcd I
