V O L U M E 2 5 , NO. 9, S E P T E M B E R 1953.
1417
(Table 111) in which three working curves were obtained, the first of which was obtained in the presence of sodium chloride, the second in the presence of sodium sulfate, and the third in the presence of sodium nitrate, the salts being present in equimolar concentrations (400 mg. of sodium per liter). A statistical analysis of this data showed no significant difference between the curves obtained with the nitrate or chloride, but the slope of the sulfate curve differed significantly from the other two. In addition to the interferences already mentioned, it has been found that the approximate maximum concentration of ferric ion and of mercuric ion that may be tolerated is 1mg. per liter. This same concentration of hafnium or of zirconium strongly interferes. STATISTICAL ANALYSIS
The statistical method was used throughout this investigation, wherever applicable, with a significance level of 5%. The study of the effects of the certain cations was limited to the linear portion of the working curve, zero to 1.2 mg. per liter, in order to facilitate the analyses of covariance. ilnalysis of covariance is particularly useful in this instance, since it allows one to detect sig-
nificant differences among the slopes of working curves prepared under different conditions. I n the case of iron, mercury, hafnium, and zirconium, only a brief survey of possible interferences was desired, and the experimental design utilized was analysis of variance with multiple classification. The equation 2 = bA,, where 6 is the estimated calcium concentration corresponding to an absorbancy A,, and b is the estimate of the slope of the working curve, was fitted to the data h y the method of least squares. LITERATURE CITED
(1) Ostertag, H., and Rinck, E., Chim. anal., 34,108-9 (1952). (2) Ostertag, H., and Rinck, E., Compt. rend., 231, 1304-5 (1950). (3)Ibid., 232, 629-30 (1951). (4) Sohwarzenbach, G., and Gysling, H., Hela. Chim. Acta, 32, 131425 (1949). RECEIVED for review November 11, 1952. Accepted May 8, 1953. Preaented before the Section of Analytical Chemistry a t the Pacific Northweat Regional Meeting of the AMERICAN CHEMICAL SOCIETY a t Corvallis, Ore., June 20-21, 1952. Approved for publication by the Oregon State College Monograph Committee. Research Paper 229, Department of Chemistry, School of Science.
Ultraviolet Analysis of Isomeric Cresol Mixtures GERTRUDE E. CARNEY AND JANET K. SANFORD Research Laboratory, Barrett Division, Allied Chemical & Dye Corp., Glenolden, Pa.
techniques have been developed for analyzing mixtures Cryoscopic, colorimetric, infrared, and ultraviolet vapor (3) methods have beendescribed. Many of these methods are applicable only to binary mixtures. Ultraviolet spectrophotometry has been used in this work to analyze mixtures of any two or all three of the cresols simultaneously and directly in isooctane solution. The procedure is comparatively simple and rapid and can be used as a routine method. EVERAL
S of
0-,
m-, and pcresols.
Table I.
Analyses of Synthetic Mixtures of 0-, rn-,and p-Cresols
o-Cresol Known Found Error 90.1 9.0 0.0 10.2 1.9
89.1 9.0 -0.1 11.5 2.0
-1.0 0.0 -0.1 11.3 +0.1
W t . 7%) m-Cresol Known Found Error 9 . 9 1 1 . 3 +1.4 91.0 90.5 89.8 89.1 69.2 67.6 17.5 17.2
-0.5 -0.7 -1.6 -0.3
p-Cresol Known Found Error 0.0 0.0 10.2 20.6 80.6
-0.1 0.1 10.4 21.1 80.9
-0.1 +0.1 +0.2 +0.5
+0.3
Substances, other than cresols, which absorb between 2715 and 2860 A. will interfere with the analysis. Those most likely to be present and to cause interference are phenol and alkylphenols other than the cresols. The ultraviolet spectra of the simpler alkylphenols are so similar that it is impossible to prove by its ultraviolet spectrum alone that an unknown sample contains cresols only. Hence, care should be used to remove or demonstrate the absence of phenol, xylenols, and higher homologs. Since the three cresols, like most other phenols, have zero absorbances a t 3000 A., contaminants which absorb a t this wave length can easily be detected. To find the best set of wave lengths for the analysis, seven combinations of measurements made a t three wave lengths from the group 2719,2728,2774,2790,2798, and 2858 A. were used to calculate the compositions of five synthetic mixtures. At each of these wave lengths the absorptivity of a t least one isomer was strong, and the variation in absorptivity of every isomer with slight changes in wave length was small. Each set of three wave lengths was chosen so that the equations expressing the absorb-
ance-concentration relations had a high degree of independence and would yield relatively accurate analyses. Best results for the series of samples tested (Table I) were obtained a t 2728 (meta), 2774 (ortho), and 2858 A. (para). These are shown with the spectra of the individual cresols in Figure 1. From Beer's law, a t a given wave length the allsorbance, A , of a mixture of cresols is
A
=
(GC~
+
~
m
+
~ a p cnp ) b
(1)
where a is the absorptivity of a constituent in liters per gram centimeter; c, the concentration of a constituent in grams per liter of solution: and b, the optical path length in centimeters. The subscripts designate the individual isomers. For the general case where all three isomers may be present, measurements a t three wave lengths and three equations are required. In the special case where it is known that only two specific isomers are present, measurements need be made a t only the two wave lengths characteristic of these isomers, and only two equations need be solved. Deviations from Beer's law, resulting from association or the interaction of any of the cresols in isooctane solution, were not observed with concentrations having absorbances between 0.5 and 0.9 a t the characteristic wave length of the isomer. Spectrophotometric deviations, which reduce the precision of the method, were avoided by choosing slit widths to provide spectral band widths of 5 A. (2). EQUIPMENT
A Beckman quartz spectrophotometer, Model DUV, was used with a liquid-cooled lamp house thermostated a t 30" C. The wave length scale of the spectrophotometer was calibrated with a mercury arc lamp. To permit the use of narrow slits, a 10,000megohm phototube load resistor was substituted for the 2000megohm resistor. Solutions were measured in matched 1-em. quartz cells with covers. If the room temperature is variable, a thermostated cell compartment is desirable. ISOMERS
o-Cresol was purified to a constant melting point by four or five recrystallizations from a low boiling (b.p. 80" to 120' C.) petro-
ANALYTICAL CHEMISTRY
1418
2400
2700
2600
2500
2900
2800
3000
WAVELENGTH, A. Figure 1. Ultraviolet Spectra of Isomeric Cresols and Wave Lengths Used in .4nalysis Concentration. 0.1 gram/liter i n isooctane; cell length, 1 c m .
leum solvent and one simple distillation; the f.p. was 30.937" C . : analysis, 99.99 mole % by the method of Glasgow, Streiff, and Rossini ( I ) (which gave a freezing point of 30.944" C. for pure ocresol). rn-Crepol was prepared by distilling material with B m.p. of 12" C. in a Podbielniak high temperature distillation analyzer and collecting a heart cut; a f.p. of 11.6i" C. was obtained from the cooling curve which was determined with a Bureau of Standards calibrated thermometer graduated to 0.05" C.; analysis, 99.3 mole % ' based on afreezing point of 12.20" C. (4)for pure ni-cresol. p-Cresol was purified to constant melting point by four or five recrystallizations from a low boiling (b.p. 80' to 120" C.) petroleum solvent; the f.p. was 34.i34" C.; analysis, 99.99 mole yo b y the method of Glasgoa, Streiff, and Roseini (which gave a Ereezing point of 34.739" C. for pure p-cresol). PROCEDURE
Freehlg distilled samples were stored in ampoules; samples removed for analysis were kept in a desiccator over anhydrous calcium chloride. Spectrographic isooctane was used for making dilutions. For each cresol, t,hree solutions containing approximately 0.03. 0.04, and 0.05 gram per liter were prepared. The absorbances of these solutions (approximately 0.5, 0.7, and 0.9 a t the characteristic wave length of the isomer) were measured accurately a t 2728, 2774, 2858, and 3000 A. against isooctane a t spectral hand widths of 5 A. Solutions having an absorbance greater than 0.003 a t 3000 A4. were discarded as contaminated. iibsorptivities of the t.hree cresols were calculated from Equation 1, a t each wave length, and the values corresponding to the three concentrations were averaged. Except a t a wave length where the absorptivity of an isomer was Ion- (less than l.O), the mean deviation of the three averaged values was less than 1%. A vieighed amount (0.5 gram) of the sample for analysis was dissolved in isooctane and successively diluted to a concentration of approximately 0.05 gram per liter of solution. The absorhances of this solution were measured as above. The concentration of each isomer was calculated by solving the the simultaneous equations: = 16.41 co An;a = 15.89 C" A?w = 0.716 co
A2726
+ 15.70 e,n + 14.68 en + 10.28 + 15.33
+ 0.905 c , ~+ 19.30 e p c
~
L
or by substitution in the inverted equations:
~p
(2)
co
=
-0.1222
+ 0.1918 +
A2ij~
A2774
- 0.05945 A28;~
+
c , ~= f0.1958 - 0.2027 A2774 0.01219 A m c p = -0.00465 A?;?, 0.00239 Amr 0.05345 Azsja
+
(3)
The method of checking foi wave length shift described by Tunnicliff, Brattain, and Zumwalt ( 5 ) was followed to avoid :t I edetermination of absorptivities during a series of analyses. DISCUSSION
Five synthetic samples containing different proportions of isomers were prepared and analyzed. The results are shown in Table I. The largest error in the results was 1.6% (absolute). Standard deviations calculated from the analyses for 0-, m-, anti p-cresols were 0.736, 1.03, and 0.283%, respectively. The data were analyzed statistically by the straight-line method described by Youden (6). No constant errors were detected, and the standard deviations of the slopes and intercepts of the lines gave t values n-ithin Student's critical limits at the 95% probability lrwl ACK\OWLEDGMEYT
The authors are indebted to H. L. Stasse and J. F. FVeiler for the purification of the cresols, to Miss J. H. BIcNeil and JV. C. Cameron for the analrses by the Glasgow, Streiff, and Rossini method, to 11.B. hlueller and A. C. JT'erner for their assistance in the statistical treatment of the data, and to C. F. Glick for his many valuable suggestions. LITERATURE CITED
Glasgow, A. R., J r . , SGreiff, A . J., a n d Rossini, F. D.. J . Resenrch S n t l . Bur. Standurds, 35,355-73 (1945). Haendler, H. M., J . Opticnl Soc. Anier., 38, 417-19 (1948). Robertson, W.W., Ginshurg, S . , and Matsen. F. A , , IND. Em-. CHEM.,ANAL.ED.,18,746-50 (1946). (4) Ytasse, H. L., unpublished d a t a . (5) Tunnicliff, D. D., B r a t t a i n , R. R . , and Zumwalt, L. R . , .\X.
