ANALYTICAL CHEMISTRY
812
c
.-
8 o Thermally C r o c k e d Goso I in e
80 C o t o l y t i c o l l y
80 Thormolly Reformed Gasoline
Crocked Gorolino
0
-2E 6 0 (I)
c
ALPHA
0
I
9.5
10.0
I
10.5
I
I1.O
1
11.5
20
2ot OLS.5
Id.0
10:s
11.;
11;
-
0
-
INTERNAL
9.5
10.0
10.5
11.0
11.1
Wavelength in Microns Figure 14. Infrared Spectra of Plant Stream Olefin Concentrates from Silica Gel Adsorption
Table
V.
The infrared portion of the presented application required less than 0.5 ml. of sample and approximately 30 minut,es of analytical time.
Comparison of Olefin Types i n RefineryProductsa
Plant Stream Catalytically cracked gasoline Thermally reformed gasoline Thermally eracked gasoline Thermally cracked gasoline after hydrotreatment
Alpha 9 13 43 10
Internal Double Bond
ACKNOWLEDGMENT
14 13 32
The authors wish to thank J. 11. Martin, Jr., mho obtained many of the experimental data and prepared the figures.
50
a To enable comparison of relative concentration in stream, ratios are corrected t o total olefin basis of stream.
LITERATURE CITED
Inst. Hydrocarbon Research Project, Ohio State University Research Foundation, Fifth Annual Report, Table 11,p. 40, Xug. 31, 1943. (2) A m . Petroleum Inst. Project 44, National Bureau of Standards, , Washington, D. C., “Infrared Absorption Spectrograms.” (3) A w r y , iT. H., J . Optical Soc. Am., 31, 633 (1941). (4) Brattain, R. It., Rasmussen, R. S.,and Cravath, A . >I., J . A p p l i e d P h y s . , 14,418(1943). ( 5 ) Fry, D. L., Nusbaum, R. E., and Randall, H . M., Ihid.,17, 150 (1946). (6) Kurts, S. S.,Jr., Mills, I. IT., Martin, C. C., Harvey, IT. T., and Lipkin, &I. R., I N D . ESG. CHEM., .kSiaL. ED.,19, 175 (1947). (7) Mair, B. J., J . Research S a t l . Bur. Standards, 34, 435 (1945). (8) Rasmussen, R. S., and Brattain, R. R., J . Chem. Phys., 15, 120 (1947). (9) Ibid.,p. 135. (1)
mally cracked gasoline blend, and (4) a hydrotreated product of (3). The spectrum of the olefin concentrate of the hydrotreated product was shown in Figure 13 a t a liquid hourly space velocity of 10, and the spectra of the olefin concentrates of the other three gasolines were determined for the region 9.5 to 12.5 microns, and are shown in Figure 14. Using representative olefin type calibration data as shown in Figure 10, the olefin distributions listed in Table V were obtained. I n these analyses the olefin type concentrations were corrected to the total olefin concentration of the original sample as determined by bromine number. The sums of the uncorrected concentrations of the olefin types were from 3 to 12’33 below the value found by bromine number.
-hi.Petroleum
RECEIVED October 7, 1947. Presented before the Southwest Regional Meeting of the ;IYERICAN CHEMICAL SOCIETY, Houston, Tex., December 12 and 13, 1947.
Infrared Analysis of Organic Mixtures Using C-H
Band Structure Resolved by a Lithium Fluoride Prism ELEANOR L. SAIER AND KORRIAN D. COGGESHALL Gulf Research & Development Company, Pittsburgh, Pa.
T
HE present-day availability of lithium fluoride prisms for the various commercially available infrared spectrometers extends the usefulness of infrared spectroscopy _ _ very greatlv. It is well known ( 1 , 9) that lithium fluoride provides a large i5000 increase in dimersion over sodium chloride in the 2 to 5 . 5 ~ to 1820 cm.-l) region. This brings out structure characteristics, particularly in the spectra due to the stretching vibrations of the C-H valence bonds. FOXand Martin ( 6 ) and Rose (8) have shown that different types of substitutions in hydrocarbons “
1
I
produce different, recognizable C-H absorption bands. and Martin considered the following groups:
I
=CH2, =C-H,
\
\
/ /
/ /
-C-H,
Fox
CH?, and --CHI
The bands due to these groups all lie near 3 . 4 and ~ are so closely spaced that they are not satisfactorily resolved in the ordinary instrument that employs a sodium chloride prism.
V O L U M E 20, N O . 9, S E P T E M B E R 1 9 4 s
813
W t h a prism of sodium chloride the usual infrared spectrometer does not resolve the structure of the absorption bands due to C-H groups. A lithium fluoride prism which possesses high dispersion in this spectral region resolves these bands 5 ery satisfactorily and makes them available for analytical determinations. Such a use of a lithium fluoride prism has been explored and a number of specific analyses have been developed. This method of analysis allows the examination of compounds and mixtures in dilute solutions of carbon tetrachloride as well as the analysis of compounds subject to complex formation such as hydrogen bonding and the analysis of mixtures possessing large differences of intensity of absorption in the sodium chloride region. It makes possible a new approach to systems of very similar isomers; and it extends the usefulness of instruments equipped with such a prism for molecular structure determination work.
region and the limited number of bands available, the number of components per sample cannot be as large as can be handled when a sodium chloride prism is used in the longer wave-length region. However, the advantages of the present method lie in the handling of special types of mixtures which often have only a small number of components. One advantage in the use of the C-H absorption band structure lies in the availability of an excellent solvent for this region. Carbon tetrachloride is very transparent throughout thii -CH, SCH ;CH, region and many compounds and systems of compounds are soluble in it. It is so transparent that cells up to several centimeters in length can 7I be used. This makes it possible to examine coni2300 2050 cm-1 2950 3000 3050 3090 pounds and mixtures in dilute solutions. Viider 3.448 35 0 9 ~ 3.389 3.333 3.279 3.236 these conditions the inherent difficulties due Figure 1. Absorption Frequencies Observable for Different Classes to molecular association such as hydrogen bonding of C-H Groups in Hydrocarbons When Sufficient Resolution Is may be avoided. Also it makes possible the Achieved w i t h Lithium Fluoride Prism simultaneous analysis in the same mixture of Data from Fox and Martin (6) conipounds that may have large differences of intmsitv of absorption in the longer wave-length regions such as paraffins and some of the polar or unsatuThe use of a lithium fluoride prism, however, makes possible rated materials. In some cases, mixtures of homologous conithe utilization of the C-H structural bands for analytical appounds which do not have appreciably different spectra in the plications. The present paper reports some of the results oblong wave-length region can be satisfactorily analyzed by this tained in these applications. method. A further advantage of the method is a spectrometric Figure 1 presents an absorption band diagram synthesized one. Because the spectral region used is near the maximum for from the empirical data of Fox and Martin (6) for hydrocarbons. the radiation curve, the scattered light is very weak. In fact, This shows the positions and multiplicity of the absorption it is so weak that the authors have detected none a t all. This bands due to the various types of carbon-hydrogen groupings. eliminates a correction procedure that must be applied a t the The relative heights of the lines are some indication of the inlonger wave lengths for best results. A further advantage lies tensities of the observed absorption bands. However, neither in the extension of usage to analytical problems of an instruthese nor the wave-length positions may be regarded as exact, as ment equipped with a lithium fluoride prism and otherwise variations occur in going from compound to compound. The primarily used for molecular structure determination m-ork. dashed lines below the base line represcnt absorption bands obThe use of the samples in solution also gives greater accuracy, served only in some of the cases studied. The dashed lines above in that it allow a true correction for the absorption, reflection, the base line are for the cases where a single band splits into a and scattering of the radiation beam by the cell itself. This is doublet. Despite these limitations it is evident from the figure done by comparing the transmittance of the sample in solution that with the satisfactory resolution of these bands, which is with the transmittance of the solvent alone. possible using a lithium fluoride prism, the scope of applications The accuracy attainable for a specific mixture depends upon of infrared spectroscopy may be greatly enlarged. -As more the differences in intensity of absorption that can be utilized. data become available in the future it will be possible to determine In some cases where unique absorption bands can be found, the. the specific limitations and generalities of such data as those average errors are of the order of 0.1 to 0.2% of total sample. presented in Figure 1. Where the wave length and intensity discrimination are not very It is to be observed in Figure 1 that the absorption bands for good, the average errors may be of the order of 1.0% of total the saturated carbon-hydrogen groupings occur at longer wave sample. High accuracy was not an all-important aim of the lengths than do those for the unsaturated groupings. This present work. Rather, a compromise in reasonable acimmediately suggests the use of the C-H bands for the quantitacuracy, rapidity of analyses, and ease of analyses by nontechtive analysis of mixtures of olefins and paraffins. These systems nically trained personnel was desired. All calibration data niay be handled, as well as ones containing aromatics and oxyand synthetic samples were processed under routine conditions. genated compounds. Because of the relatively narrow spectral
1
I
I
ANALYTICAL CHEMISTRY
814
y w s for the quantitative analysis oi multicomponent mixtures oi hydrocarbons, using a sodium chloride prism and the longer wave-length rogion. Tests for the scattered radiation intensity were made by the total absorber method (2). For some of these &n absorption cell of 0.036-inch thickness was used. Almost any material containing C-H groups is suitable a8 solute for such tests in the 3.4" reeion. Drovided a strong enouih ioncentration is used
Figure 2.
Liquid Absorption Cells and M o u n t i n g t r r a n p e m e n t s Used in knalytical Work
.An m a y of the accuracy of the method under routine conditions iusy he made from the results reported for the synthetic samples bPl"\V.
EXPERIMENTAL METHODS AND TECHNIQUES
In the present work d l dilutions were made volumctcioally in graduated pipets and burets. In certain cases care must he taken to follow a definite procedure in making the calibration blends, diluting the samples, and obtaining the data, in order to avoid trouble caused by differences in evaporation rates. It is well known (7) that the high boiling points oi sim,,l,le alcohols, in
parison with other compounds of similar molecular uTeights, are due to the hydrogen bonding forces between the molecules. These hydrogen bonding forces, due to the hydroxyl groups, are responsible for molecular association between the molecules and the attondant reduction in vapor pressure. However, in dilute solutions in a solvent such &s carhon tetrachloride the relative degree of hydrogen bonding is reduced because of the greater average distance between hydroxylated molecules. Consequently, the alcohol molecules in such 8. solution will escape much inure rapidly, on a relative basis, than from a eonoentrated soh; tion. This effect is so strong that the same procedure as regards t,ime intervals must be used on the samples t o he analyzed and 011 t,he odihration blends. Otherwise errors will mise due to loss of some of the material by evaporation. In Figure 3 may he seen the opt,ic+l density plotted as a function oi the number of times analyzed for a 2.5% by volume solution of ethyl alcohol in carbon tetrachloride. This sample was kept in a glass-stoppered battle and analyzed an successive days. During these tests the concentration of othyl alcohol chauged radically. The
The instrument on which the present tiork was done was is Perkin-Elmer Model 12B infrared spectrometer. Sodium chloride and lithium fluoride prisms are used in it interchangeably. A cell-in-cell-out arrangement is used wherein the ahsorption cells are ~eeurelyclamped to a movable carriage. I n Figure 2 may be seen one of the cells clamped in place and anothcr onc resting on top of the instrument. The type of cell used has boen described (8). The carriage arrangement is the same BS usod in routine gas aoalysis (4).The liquid cells are equipped with 8 mounting frame which allows them to be held in the movablt: carriage by means of thumbscrews. The cell employs t a o needle valves, one fabricated to act as a filling piston Tq-hen loading the cell. These cells have int,erchangeableperts and are easily fabricated and repaired. The thiekncss i i s d most. corni:mnly in the present work w&s 0.006 inoh. For quantitatively measuring the radiation intonsiLies at Lhc various wave lengths, the same system of null measurements (4) ~wisused. A high sensitivity ralvanometcr is used merelv as an indicator and The thcrmocouple signals i r e balanceiI out by I raiatol 3s due to tho high sensit.ivity"grtlvanomser are thereb? eliiminatcd. The instrument is equipped n i t h an Amphenol connector ior the thermocouple outlet. With ETHYL ALCOHOL dhis arrangement either the galvanometer and a. 3 . 4 7 ~ null system equipment or the automatic rccorder may be conneoted a t will. It has been found very convenient to use the automatic r c c o r d i ~ ~ obtain a spectrum of the^ sample throughout aave-length region used. This may t,hen hv exmined t o determinle what compounds arc present or. if the samr,le is B calibration st,snd&d, to determine t h e m ost desirable wave lengths tu use in the analytica1 procedure. Har~ever, 0 i i 4 b 6 f 8 9 i o i l l i for auantitative measu rements of transmittancc DISTURBANCES a t specific wave lengt,hs consistently more atFigur,3 3 Optical Density of Dilute Solution of Ethyl-Alcohol i n curate data are ohtailled manually, using the, Carbon Tetrachloride Solution as a Funotion of Number of null method. The same equipment and general T i m e s Analyzed (Disturbances) methods have been in use here far scvrii~l
L
.
~ $
V O L U M E 2 0 , NO. 9, S E P T E M B E R 1 9 4 8
815
authors have found it most advisable to obtain the data for such a solution on the same day as it was prepared. .is Beer's law of absorption is obeyed by the compounds undcr investigation the calculatiorrs and handling of data are straightiorward. The absorption, reflection, and scattering effects duc, to the cell may eliniinatcd tiy comparing the tranamittanc*c~ or' the sample in solution with the transmittance of the xolutiori ~ i lH ~suital>lcT ) l i ~ ~suhtiwt ~ ~ ~ ~i ol n slone. .Ictually this i v : i ~ ~ ~ ~ i ) 1)y cii optical den-itiv~.
approximations. In the method of successive approximations concentrations C.2 and Ca are assumed to he zero in the first equation and it is solved directly for C1. This value is used in the weond equation with the assumption that. C3 is zero and a value f o b CZis determined. r e s t the values of C! and Czobtained in the first two steps are used in the third equation and it is solved for C,. The cycle is repeated with the values of Cz and C3 from the latter two steps substituted in the first equation and a new value of CI is determined. The second equation is then solved for C? using the most recent values of C1 and Ca, etc. Several cycles can he quickly run with a semi-automatic: nr automatic calculating machine in a few minutes. -1nave-length calibration 01' tlic C-€1 region was prepared i'wm thc data of Fox arid JIartin (5,6). S o attempt was made to makc. a highly precise calibrat,ion. In practice the wave lengths for Ypec-ificanalyses are specified in terms of the instrument vernier rcadings. For that reason specific wave lengths are not given t)i:low. For Dome of the examples the wavelength values may 1 ~ appraised s from the figures. I t is believed that these may tie rc~licdon to within about 0 . 0 0 5 ~or better. APPLICATION TO SPECIFIC ANALYSES
From the information given in Figure 1 a prediction may often made as to whether or not a specific mixture can be analyzed. For example, a binary mixture containing two compounds with differences in types of unsaturated groups may generally be analyzed with ease. kJV
\. 2,4,4
---
- TR I METH YL-I-
--- 299 -TR
P ENT E NE 1 ME THY L-2-PE NTEN E
p /.\
Figure 4. Infrared Absorption Spectra in C-I1 Region of Isomeric Trimethplpentenes
To see this let us consider the transmittance of a solution 01' We have for the solution:
I
pure compound at wave length A,.
D, = log (Io/I) = diC
+K,
OCTENE- 2 N- OC TAN E OCTENE- I
ill
where Di is the optical density, 10 is the incident radiation i l l tensity, I is the transmitted radiation intensity, A i is the calibration coefficient for the pure compound a t wave length X i . C' is the concent,ration of the solute, and K ; is an attenuation factor due to the absorption, reflection, and scattering by thv cell and absorption by the solvent. Since the concentrations 01 solute used are small, between 1 and 6% for the present work, the Ki in Equation 1 may be evaluated by obtaining the optiral density for the cell fillcd n.ith solvent only. A subtraction of this value from the D, of Equation 1 then gives the component of optical density due to the compound. With this method of obtaining the optical density, Beer's law xas tested for a numlwi of the compounds in the examples belox and found to hold i i i tw-h ws?.
3.20 3130 3hO 3.60
3:609
Figure 3. C-H Structure -4bsorption Bands f o r a-Octene, n-Octane, and 1-Octene
L\'ith the above method oi obtaining true optical densitit*:tnd ivith the additivity of optical densities that occurs ivheii
Hew's law is obeyed, siniple equations of the typcx below ai'c, recorded spectra, the transmitted energy is plotted as a function of r a v e length. Here, as indicated by the arrow, may be seen the absorption band due to the terminal =CHz group in one of the compounds. Actually, the difference in absorption is great cnough here t o permit taking data a t this point only. Then instead of having two equations of the type of Equations 2 ive have but one plus the equation:
-
c, + C? = c \\ tit'
Iicxrc C is the concentration of sample in solution.
(3,
The resulth arialyws of two synthetic samples may be seen in Table I.
A N A L Y T I C A L CHEMISTRY
816 Table I. Analyses of Synthetic Mixtures of Binary Mixture of Trimethylpentenes Synthetic,
Calculated,
70
Compound 2,4,4-Trimethyl-l-pentene 2,4,4-Trimethyl-2-pentene 2,4,4-Trimethyl-l-pentene 2,4,4-Trimethy1-2-pentene
%
Differenre,
R
50.4 49.6 30.6 69.4
50.0 50.0 30.0 70.0
Table I1 also gives the reaults for two synthetic samples of the binary system of n-heptane and benzene, for which considerably better accuracy is possible.
0.4 -0.4 0.G -0.6
---.-.
OCTENE- 2 N - OCTANE CA RBITO L
Table 11.
Analyses of Synthetic Mixture of Hydrocarbons Containing Paraffins and Unsaturates Compound Synthetic, % Calculated, % Difference, R
n-Octane 2-Octene 1-Octene n-Heptane Benzene n-Heptane Benzene
'.
.'
18.9 31 .O 50.1 49.9 50.1 75.1 24.9
20.0 30.0
50.0 50.0 50.0 75.0 25.0
\
-------
\
-1.1
1.o 0.1 -0.1 0.1 0.1 -0 1
B E N Z Y L ALCOHOL N- OCTANOL M E T H Y L ETHYL KETONE
,-\--.
t
!Y
.
I
I
3.20 3.30 3.40
I
I
I
I
I
3.20 3.30 3.40 3.50 3 . 6 0 ~ Figure 7. C-H Structure of Absorption Bands for a-octene, n-Octane, and Carbitol
In the preceding type of examples predictions could be made in advance concerning the success of the method. I n other cases, such as the examples discussed below, an idea of the feasibility cannot be reliably formulated until the spectra of t,he pure coinpounds.are obtained. A number of ternary mixtures of particular interest in the study of azeotropes may bc easily analyzed. In general, these systems contain a paraffin, an aromatic'or olefin, and an oxygenated compound. Other systems containing all oxygenated conipounds may in some cases bc analyzed with good accuracy. In Figure 6 may be seen the spectra in the 3 . 4 region ~ of bcnzyl alcohol, n-octanol, and methyl ethyl ketone. Here it is observed that three distinct and unique wave lengths, indicated by arrows, may be chosen for analysis. Table I11 gives the results on synthetic samples made up to test the accuracy on such a systcni.
t 1
r--.
/-
I
3.50 3.60~
Figure 6. Infrared Absorption Structure in Region for Benzyl Alcohol, n-Octanol, and Methyl Ethyl Ketone
C-H
Continuing with hydrocarbon systems we see in Figure 5 that satisfactory wave-length discrimination is obtainable for the ternary system of n-octane, 2-octene, and 1-octene. The wave lengths used for analysis are indicated by the arrows. 4 t about 3.26~ may be seen the band characteristic of the terminal =CH2 group of I-octene, a t about 3.3211 the band characteristic of the
=CH group of 2-octene, and a t about 3.50~the band character-
\
istic of the CH2 groups, which is stronger for n-octane than for
/
the others. The 2-octene shows some of the bands characteristic of the other compounds, because the sample used for recording was not pure. Although definite differences in spectra are found, the differences in intensity of absorption are not great (Figure 5 ) . Despite this, fair accuracy is obtainable, as may be seen in Table 11.
Table 111. Analj-ses of Synthetic Mixtures of the System L n-Octanol-llethyl Ethyl Ketone-Benzyl Alcohol Compound n-Octanol Methyl ethyl ketone Benzyl alcohol n-Ootanol 3Iethyl ethyl ketonr Benzyl alcohol
Synthetic, "0 Calculated, % 30.0 30.3 30.0 29.4 40.0 40.3 z5.0 .IO.0
25.0
24.5 50,s
25.0
Difference,
