ANALYTICAL CHEMISTRY
914 Table IV. Comparison between Infrared and Freezing Point Determinations for Total Impurities
Purity by infrared, vol. % n-heptane Purity by infrared, mole n-heptane Purity by freezing point, mole % n-heptane Difference by infrared and freezing point, mole X n-heptane
1 97.7 97.4 97.62 0.2
Sample 2 98.5 98.1 97.84 0.3
Number 3 4 95.3 95,; 94.8 95.0 9 4 . 8 2 94.75 0.0 0.2 ~~
addition, the results expressed as the sum of the total high boiling impurities, the sum of the total low boiling impurities, and thc sum of the total impurities arr also accurate and usually within +0.37, on the total sample. A furt,her check on the accuracy of the method was ohtained for several samples by comparison of the infrared results for total impurities versus concentration of impurities as deterniined by the freezing point method. Results obtained on these samples are shown in Table 11.. Thew bedata indicate agreement t o within ==0.201,on thr :i~c~i':igcr tween infrared and freezing point determinations. The infrared method as described above has becw used for study of the composition of impurities in n-heptane concrntratev from eight crude sources. In a number of cases, calculated results showed a percentage of cis-1,2-dimethylcyclopentanerelative to n-heptane greater than 1%. In no case was the concentration of 2,2,4trimethylpentane relative to n-heptane greater than 0.3%. The calculated percentages of 2,2,4-trirnethylpentane which were obtained were not considered significant, inasmuch as they were almost within the reproducibility of the determination. Although this analysis requires a precision that is almost a t the practical limit of the instrument, it may be appliaable for fractional distillation column control for the plant production of high
purity n-heptane in the same manner as a similar analysis was used for fractional distillation column control for the plant production of reference fuel grade iso-octane ( 1 ) . In this case the analysis would be used to determine the relative amounts of high and lon. boiling impurities present in the n-heptane product. For this application, however, the results should always be esamined for an unespected or unusual distribution of the impurity components caused by the influx of another component no! allowed for in the calculations. Periodic cross checks of analyses by the freezing point method will also assure that all the signifirant components have been included in the analytical schemp ACKIVOW LEDG\IEST
The work described ill this publication p as carried out in the laboratory of the Humble Oil and Refining Compmy and the authors wish to express their thanks for permission to publish details of the analytical method employed. The contribut,ions of D. K. McDonald, who made arrangements for the segregation of n-heptane concentrates from various crude sources, and of G . A. Satterwhite, who performed nearly all calibration and calculation work involved in the development of the method, are herehv acknowledged. LITERATURE CITED
(1) Anderson, J. h.,Jr., ANAL.CHEM.,20, 801 ( 1 9 4 8 ) . (2) Gerbes, Otto, Hall, H. J., and Becker, -4. E., Petroleum f'rocesai n g , 2, 734 (1947). (3) Heigl, J. J., Bell, M . F.. and White, J. E., ANAL.CHEM., 19, 293 (1947). (4) Liston, M. D., Quinn, C. E., Sargeant, W. E., and Scott, C. G.. Rev. Sci. Instruments. 17, 194 (1946).
RECEIVED December 29, 1947. Presented before the Southwest Regional Meeting, AMERICAN CHEMICAL SOCIETY. Houston, Tex.. December 12 and 13, 1947.
Infrared Spectroscopic Determination of Ester Carbonyl ROBERT R. HAMPTON AND J. E. NEWELL, United Stcrtes Rubber Co., Passaic, N . J . Frequency and intensity of the carbonyl absorption have been measured for nineteen esters of low molecular weight and two polymers. Both the frequency and intensity of the ester carbonyl absorption vary considerably with the adjacent molecular structure, so that, for accurate analyses, it is necessary to know something about the molecular structure adjacent to the carbonyl group. Working curves have been prepared for the quantitative determination of polymethyl methacrylate and for ethyl adipate (selected as a representative ester).
I
T IS nominally possible, by means of infrared absorption spcc-
tra, to identify and determine various types of organic functional groups such as hydroxyl, nitrile, amine, and carbonyl. Qualitative analysis is based on the presence or absence of absorption bands a t frequencies characteristic of the groups to be identified ( 2 ) . Quantitative analyses are based on measurements of intensities of the absorption bands. Usually quantitative analyseq are made to determine a specific compound, and calibratiom are based on a pure sample of the compound to be determined. However, i t is frequently possible to determine (although less accurately) the amount of a specific functional group. This type of analysis can give useful information in many cases dealing with polymers or compounds whose exact structure is in doubt. For example, it should be possible to estimate the molecular weight of a compound when the number of functional groups per molecule
is known or can be inferred, or to determihe the composition of a tautomeric equilibrium mixture. The determination of ester carbonyl is useful in the analysis of copolymers containing esters (such as the acrylates), in studying hydrolysis, lactone formation, and other problems involving esters. Before analyses can be made with reasonable confidence i t is desirable to study the spectra of knoir-n compounds, to determine the effect of structure variation on the frequency and intensity of absorption, and to establish the quantitative relation between concentration and absorption intensity. Thompson and Torkington ( 4 ) have reported the absorption frequencies of a considerable number of esters, but their data were almost entirely on saturated aliphatic esters of monocarboxylic acids and did not give extinction coefficients. Anderson and Seyfried ( 1 ) have recently made an excellent and
915
V O L U M E 21, N O . 8, A U G U S T 1 9 4 9
c \?-a$deterniined for two different solutions of each sample, and the values (which in most caws agreed to within 2%) wrre a w r aged.
1.5
MATERI4LS STUDIEI)
c;
1.0
n -I
Q.
v c
8 0.5
0.0
I
0.001
0.002
PER SO. Figure 1
MILLIEOUIVALENTS
I
0.003 CM.
rwniprehensive study of the determination of specific functiond groups, including ester carbonyl, in samples such as hydrocarboil synthesis naphtha. They did not give data on individual esters, but gave an average value for the wave length and intensity of absorption for the esters studied (presumably of the simple aliphatic type). The authors have measured the frequency and intensity of the iwbonyl absorption for nineteen esters of low molecular weight anti two polymers. STorking curves have been prepared for the quantitative determination of ethyl adipate (selected as a reprew~it:itiveester), and polymethyl methacrylate. The model 12A Perkin-Elmer spectrometer used has been equipped with a blackened Thermistor bolometer, Western Electric amplifier Model KS-10281, and Speedomax Type A recorder. The recorder chart and Littrow mirror are driven through Selsyns to ensure accuracy of the wave-length calibration on the charts. Radiation is chopped a t 15 cycles per second by means of a synchronous rotating sector a t the globar. This recording system is stable, sensitive, and completelv free from zero drift. Except where otherwise speiified, a calcium fluoride prism \vas used, with a slit width of 0.155 mm., equivalent to a half intensity hand width of about 7 cm.-l. Each sample n'as measured twice, using two interferometrically calibrated ( 3 )sodium chloride cells 0.01045 and 0.01856 cm. thick, wspectively. The cells were vapor-tight, being made with amalgamated lead spacers and tapered lead plugs closing the hypodermic hub filling holes. Carbon tetrachloride was used as solvent for all samples except polymethyl methacrylate, which, bring insoluble in carbon tetrachloride, was measured in chloroform solution. I n every case the solution concentration was such as to give an optical density of about 0.15, after correction for solvent :ibsorption. The optical density was measured with the spectrometer set a t the frequency of maximum absorption by the carIroriyl bond (for the particular sample being studied). SVith the recorder running but the spectrometer drive stopped, I he cell containing the sample solution was alternately placed i n front of the slit, then removed, and the corresponding int,ensiti were read from the chart. The zero, determined using a gla shuttpr, was subtracted from each reading, thus correcting for the small amount of stray short wave-length light present. The readings were repeated using t'he same cell filled with pure solvent. If the chart showed any change in the cell-out reading, the readings were corrected for this change. The molar extinction coefficieiit, E, was calculated: d e = ~~
cl
where c 1 d d I
is the solution concentration in gram-moles per liter is the length in cm. of absorbing path (cell depth) is the optical density I = log 2 I is the intensity of radiation transmitted by the solution IO is the intensity of radiation transmitted hy the pure solvent
The two polymers were prepared in this laborator The pol) methylmethacrylate was prepared by heating pure mrth? lmetharrylate a t 60" C. in the absence of air, and had been stored lor 2 years before use, thus ensuring complete polymerization. It is believed to be pure. The pentamethylene pimelate was prepared by condensation of pentamethylene glycol and pimelic acid, and is probably considerably less pure. The low molecular weight esters were all purified by fractional distillation through a packed column a t reduced pressure. In each case a series of middle fractions of constant boiling point and refractive index were combined for use in this work. All unsaturated esters were distilled a t reduced pressure in an atmosphere of carbon dioxide and stored in the dark a t 3 " C. until needed. Polymerizable monomers were inhibited by the addition of tert-butyl catechol (50 p.p.m.). The infrared spectra showed all samples to be free from a p p r e ciable amounts of alcohol. Chemical analyses of many of the samples (Table I ) showed negligible free acid (
