Quantitative Analysis of Isomeric Cresols and CresolPhenol Mixtures By Ultraviolet Absorption Spectra of Vapors W. W. ROBERTSON, NATHAN GINSBURG',
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The Llniv*sitv
sf Texas,
A procedure for the quantitative analysis of isomeric cresols and cresol-phenol mixtures is described. Spectrograms of the vapor in equilibrium with liquid mixtures are taken under fixed conditions for a number of synthetic samples. Working curves are prepared from the measured densities of select bands in the absorption spectra and the known concentrations in the sampler. The mean dovia-
AND
F. A. MATSEN
Austin 19, Texas
tion bohvoen the composition of the synthetic isomeric cresol sampler and the composition of these samples aa obtained from the working CUNOI is less than 9%. Phenol can be detected in cresol mixtures when present in amounts as low as 0.3% and can be analyzed for at this and higher concentrations with about the same precision as that mentioned above. CRESOL MIXTURES
T
H E ultraviolet absorption spectra of aromatic molecules in the vapor state have a number of features whioh suggest their use in analytical chemistry. All aromatic compounds absorb light in the region of 2500 to 3000 A,, owing t o a n electronic transition involving the excitation of the pi electrons in the ring, corresponding to the A r B m transition in benzene (4,6). The elect,ronic transition is very structure-sensitive and the ultraviolet absorption spectra should he effective in bringing out peculiarities in oompounds of similar structure. The ultraviolet absorption of ammatics in the liquid state has been widely used in analyses. The use of the liquid state has the important disadvantage that the fine structure so characteristic of the vapor spoctrum is largely wiped out and with i t much of the individuality of the spectra of the various aromatic compounds. Cole (8) has analyzed air for toluene and benzene, using u l t r a ~ o l e absorption. t I n 1944 Berton (1) discussed the analysis of aromatic compound mixtures by means of vapor ultraviolet spectrography. He reproduced spectrograms of mixtures of aromatic compoundv but gave no quantitative data except the results of analyses for toluene and benzene in air. I n this he had been completely anticipated by Cole who, three years earlier, published complete analytical data, including the working curves for both toluene and benzene. Berton probably did not hsvo access to the American journals during the war.
PREPARATION OF WORKINGCmvms. Spectrogrms were taken of the three pure cresols and of a number of synthetic mixtures. I n Figure 1, reproduced as negative prints, are the spectra of the pure components. I n Figure 2 me the microphotometer tracings of these plates, as well as of the plate obtained from sample 5 of Table I. The scale on the left of each microphotometer tracing is a density scale obtained by running anEastman stepwedge with each plate for the purpose of calibrating the microphotometer, This was necessary, since the sensitivity of the microphotometer was changed from plate t o plate in such a manner as to bring out as much detail as possible. The strongest bands of the spectra of 0-, m-, and p-cresol are designated, respectivelg, as Ao, A,, and A , with wave lengths 2744, 2779, and 2830 A,, respectively. I t will be evident from an examination of the spcctrograms of Figure 1 or the tracings of Figure 2 that the A . band and the A , band are relatively independent. Thus the density of the center of each band will sowe as an independent measure of the concentration of the corresponding cresol in the sample. I n Figures 3 and 4 are given the working curves for 0-cresol and p-cresol as they were obtained
EXPERIMENTAL
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with two outlets, one to a resesoir holding t6e sample, the other t o 8. Hyvac pump through a stopcock and trap. Ground-glass joints lubricated with Apiesan L were used. A 2.5-kv.-rtmp. hydrogen discharge tube provided the illumination. Synthetic samples were prepared and introduced into the reservoir and were maintained a t 25O C. The cell, cleaned by rinsing with acetone and ether and with the ground-glass joints completely relubricated after the preceding run, was exhausted for 10 minutes and a time interval of 10 minutes was allowed for attainine eouilibrium after the system was isolated from the pump. -Allthe exposures Were tiken in the first order of the spectrograph (slit width, 0.05 mm.) and were of 10 minutes' duration. Eastman 103-0 spectrographic plates were used and eivm of 8 minutes with D76e at 19" C. with ~ ~ . R . tmv ~ - dlevelooment . ~ ~ ~ I manual aatatioi, this development being estimated to give a gamma. of about 1 (a degree of contrast such that the slope of the curve of density versue the logarithm of the exposure would be about 1). The developer used h a d t o be mixed very carefully if consistent results were to be obtained from different batches of develooer. The d a t e s were scanned with a Lee& & Northrup microphotometer.. ~~~
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2600
Figure
1. Near Ultraviolet Absorption Spectrouramr of Cresol
50
2700
50
2800
Vapors '
746
I 50
50
Present Address. Physies Department, Syraouse University, Syrsouse.
N. Y.
I
a. o-Cnd.
b. m.Creml. e. P-CWOI.
ANALYTICAL EDITION
December, 1946
sity diffciencc between the background of the band and the crnter of the band. This roughly cancels the variations in the intensity of the light source and in development. The meta band, A,,,, has a relatively low extinction coefficient and is ovcarlapped by an ortho band of almost equal intensity. Therefore it was not possible to determine the meta concentration by the direct method which was used for the ortho and para isomers. The difference in denqity between bands from t n o different compounds in a single mixture is a more sensitive function of concentration than the density of either band alone. Thr density differcnce, called A', was taken between ban: A , and the second strongest ortho band, labeled BO(X = 2694 A.) on the spectrograms of Figure 1. Bo Tvas used instead of Ao to avoid the effect of strong absorption mmtioncd below. Since the para isomer also absorbs slightly in this region, a working curve of A' versus per cent meta was first prepared from meta-ortho mixtures. T h t ~ three-component synthetic mixtures were then analywd and a correction cuxve was determined (see Figure 5 ) that would eliminate the effect of the p-cresol on the two-compo-
nent density A'-% mcta curve mentioned previously. cedure is as follows:
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m
Each working curve that is drawn represents an average of the results in terms of the density A as a function of concentration. DISCCSSIOX OF RESULTS.The Beer-Lambert law is fairly well obcyed for low concentrations of the components but not for the higher concentrations, particularly in the case of the ortho isomer. This is because the absorptlon of these hands is SO strong that the exposure in the center of the bands falls below the linear portions of the density-exposure curve given in Figure 6. (This curve is the experimentally determined characteristic
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% ORTHO
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Figure 3. Working Curve for o-Cresol Based upon Band Ao of Wave Length 2744 A.
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W
A Is density difference between background and center of band
n Te P A R A
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Figure 5.
Correction Curve Used in Meta Analysis
Density corresponding to determined percenbge of para is to be added to density difference, A m - Bo
Table
a
Sample KO.
t I-
40
60
80
% PARA Figure 4. WorKing Curve for p-Cresol Based upon Band A, of Wave Length 2830 A.
Synthetic Sample Ortho 61.4 Meta 20.7 Para 17.8
Determined from Working Curve *
.. ..
20 22 21 61 18 59
Ortho Meta Para
20.6 20.0 59.4
3
Ortho AIeta Para
19.4 61.8 18.8
Ortho Meta Para
30.1 31.7 58.2 25.7 26.3 48.0
29 30 37 26 26 47
9.4 10.4 80.2 64.2 25.1 10.7 37.3
:3 1 8
n
20
Composition of Samples
2
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1.
6
Ortho 1Ieta Para
7
Ortho lleta Para
8
Ortho AIetn Para
9 10
Ortho Meta Para Ortho Xleta Para
27.4
36.3 63.1 26.2 10.6
17
.. ..
10 35 30 35
..
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9
ANALYTICAL EDITION
Deeember, 1946
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22. Figure 6.
Working Curve for m-Cresol
> 1.8.
m '*. I - .
intensity, its complete independence,of the cresol spectra, and its sharpness. I n these mixtures, then, o-cresol, p-cresol, and phenol can he determined directly, and rn-cresol is known if it is the only remaining component of the mixture. The apparently small differences in the intensities of the strong phenol bands shown in the tracings in Figure 9 are due t o the fact that the exposures in these bands fall on the rather flat, insensitive toe of the Characteristic curve of the photographic emulsion (see Figure 7).
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Figure 8.
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Figure 9.
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A
A?
I I 2000
b. 6 m d o 4 of phonolueml m i i t u ~ e ~
A,".
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PSI0
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Ultraviolet Absorption Spectrograms of Vapors
a. Pure Phenol.
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A0 APH I
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2aoo
Microphotometer Tracings of Spechograms of Figure 8
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750
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
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In Figure 10 is given thc working curve for phenol. The density A that is plotted against the phenol concentration is th(, difference in density between the center of the band P and thr region immediately outside the sharp edge of the band. froni RESULTSOF A ~ ~ The4 curve ~ in ~Figure ~ 10~departs ~ . linearity at concentrations of phenol greater than about 407,. This departure is due apparently to an unsyminetric developmenr of background absorption and not to the lack of linearity in the response of the photographic emulsion at the densities involved. Samples containing more than 40% of phenol can be analyzcvl hy following the suggestions given above. camparison is given in Table I1 between the percentage's of phenol in the synthetic samples and those read from the LT orking curvp. There is a mean deviation between the two of less than 3 7 . p-Cresol and o-cresol were determined in the first four sampleP taken in order to check the validity of the cresol working rurwb in a phenol-cresol system. The results given in Table 111 indirate that the phenol hnq not affected the 0- and p-cirsol hands nscd in the analyw~.
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%PHENOL
A N A L Y S I S OF S A M P L E S OF L O W P H E N O L C O N C E N T R A T I O N
h working curve for lon concentrations of phenol is given i r i Tigure 11. The strong band of phenol marked APH (A = 2750.3 A,) in Figure 9, b, was used for this curve. As a weak ortho band falls at the same wave length, a slight correction for the percen;age of o-cresol in the sample must be made. This corrertion v a s determined from separate samples containing no phenol, but was too small to justify a complete correction curve and can be taken as a density of 0.03 to be subtracted from the phenol drbnsity difference for concentrations of o-cresol up to 207,, 0.06 for concentrations between 20 and 4 0 7 , and 0.09 for concentrations between 40 and 60%. This curve indicates that concentrations ot phenol as low as 0.37, in phenol-cresol mixtures can bel drtrrmined with a reasonablr degree of accuracy.
Vol. 18, No. 12
Figure 11. Working Curve for Small Percentages of Phenol in Phenol-Cresol Mixtures Bared upon phenol band A ~ H of wave length P750 A.
Table II. Percentages of Phenol in Synthetic Phenol-Cresol M i x tures and Those Read from Working Curve of Figure 10 Sample
Synthesis
Analysis
%
%
4.0 12.4 16 4 23.1 21.5 26.5 41.5
4 13 19 20 22 31 37
2 3 4 5 6
7
8
Table 111.
Analyses of Four Arbitrarily Chosen Samples of PhenolCresol Mixtures Sample
Synthesis p-Cresol . 2 43 17 21 20
ACKNOWLEDGMENT
The cresols useti in this investigation were prepared by the Reilly Tar a i d Chemical Corporation and appeared pure under spectroscopic examination. These n-ere lent by R. J. Williams, to whom they \wre originally given. The authors wish to thank 1.D. Glover for the use of the sensitometer by means of which the characteristic curve x a s obtained and to acknowledge the continued support of the Researrh Institute of the TInivt\rsity of Texas.
0 10 203040 50 60
% PHENOL The apparatus for this research, with the exception of the hydrogen discharge tube and the quartz absorption cell, is available in all laboratories that perform emission spectrum analysis. The authors believe that the Beckman quartz spectrophotometer with compartment assembly No. 2510 and gas cells 2310-GS-100 can be used for this analysis. This instrument was not available to the authors a t the time this work was carried out.
LITERATURE CITED
(1) Berton, A , Ann. Chim., l l e serie, 19, 394 (1944). (2) Cole, P., J . Optical SOC.Am., 32, 304.(1942). (3) Ginsburg, N., and Matsen, F. A , J . Cliem. Phys., 13, 167 (1945). (4) Sklar, A. L., Ibid., 5 , 669 (1937). (5) Sponer, H., Nordheirn, G., Sklar, A. L., and Teller, E., I b i d . , 7, 207 (1939). P R E S E N T E in D part before the Division of Analytical and Micro Chemistry, Atlantic City. a t t h e 109th Meeting of the AMERICIN C H E M I C A LSOCIETY, N. J.
