V O L U M E 2 4 , NO. 5, M A Y 1 9 5 2
785
INTERPRETATION OF DATA
ACCURACY OF METHOD
The capacitance of a cylindrical, concentric two-plate capacitor such as the micrometer described above is directly proportional t o the length of the inner cylinder. In the circuit described, thcx caparitance of the dielectric constant cell is likewise proportional to the dielectric constant of the liquid within it. It follows, tlieii. that changes in dielectric constant of the solution may be read directly on the micrometer. For the micrometer readings to follo~vthe progress of the reaction directly would require that the dielectric constant of a solution be an additive property of the constituents. Heston et ai. have shown that for a dilute solution (less than 0.1 M )of a polar solute in a nonpolar solvent, the dielectric constant of the solution varies linearly with the mole fractiou of the solute ( 4 ) . In a reacting medium where the number of products is equal to the number of reactants, concentration will vary di~ectlywith mole fraction, so that if Heston’s relationship is valid for a niulticomponent r;yetem,
Results obtained using this method for studying the kinetics of the reaction of acetic anhydride with ethyl alcohol in carbon tetrachloride ( 2 ) demonstrate the high precision of which the method is capable. I n a single study the second-order velocity constant was determined ten times with an average deviation from the mean of 0.12%. Even greater precision might have been possible if a less crude reaction cell had been used. The dielectric constant is not the only property which may be studied with essentially this same equipment. For example, i n gas reactions, a mercury manometer with a metal film on the outside of the glass tubing might be substituted as the variable capacitor in place of the dielectric constant cell.
A R = kA[.4]
ACKNOWLEDGMENT
The author wishes t o express his deep appreciation to Donald H. .%ndrews,under whose guidance this instrument was completed. John H. Lehman constructed the reaction vessel.
ill
LITERATURE CITED
Anderson, K., Bettis, E. S.,and Rei-inson. D., .~s.LL. CHEM..2 2 , 743 (1950). ( 2 ) .Ixtmann, R. C., .I. .4m. C‘irixin. Soc., 7 3 , 5367 (1951). (3) Burrell, C. >I., h1agur.y. T. G., and Melville. H. W., Proc. ROU. Soc. ( L o n d o n ) , 205, 309-22 (1952), also pp. 323-35. (4) Heston, IT. M., Jr.. Franklin, .4.D., Hennelly, E. J., and Still-th. C. P., J . Ani. Chem. SOC.,72, 3443 (1950). (5) Jensen, F. W., and Parrack, .I.I,.. IXD. EIFG.C H m r . , A 4 x a ~ ED.. . 18. 595 (1946). ( G I IT.&, P.If., Burkhaltei, T. S . , and Broussard, L., -IN.&L. CHEli., 22, 469 (1950). ( 7 ; Zeluff,V.. and Markus, .J.. “Electronics Manual for Radio Engineers,” p. 227, S e w Yoik, McGraw-Hill Book Co., 1949. R E C E I V Efor D review October 4. 19.51, Accepted Rlarch 5 , 1952. Taken (1)
ir.lwrr A f i is the change in micrometer reading, k is a constant, and 1-4 ] ir the concentration of ally component. Heston’s relationship does appear to apply in at least some multicomponent systems (P). In the case of a second-order reaction with t,he two reactaiitinitially present i n the same concentration, a, the integrated expression for the velocity constant, becomes in terms of thc micrometer readings, R, where the subscripts refer to readings taken at zero, infinite, and 1 time:
k = J
Rr-Ro - Rt)
t a(R,
from t h e doctoral dissertation of R. C. Axtmann at Johns Hopkins Uni\.ersity, 1950.
Polarographic Studies of Oxygen-Containing Organic Compounds Functional Groups of Autoxidation Products C. 0. WILLITS, CONSTANTINE RICCIUTI, €I. B. KNIGHT, AND DANIEL SWERN Eastern Regional Research Laboratory, Philadelphia 18, Pa.
C
I’IIREXT intercet in autoxidation of fats, oils, paints, s l y rene polymers, rubbers, and gasoline has result,ed in nunierous papers on this subject. Lack of direct means for the qualitative and quantitative determination of the autoxidation products has been a handicap in such studies. Lewis and coworkers (8, 9) obtained well-defined polarograpliic waves for autoxidized methyl oleate, methyl linolenate, and l a d and found that the wave heights were proportional to the concentrations. Bovey and Kolthoff ( I ) also found this to be true of autoxidized styrene polymers. These results suggest the possible usefulness of the polarographic method for the study of autoxidation processes. The use of many different electrolytic solutions in the past has, however, resulted in an accumulation of polarographic data on various organic oxygen-containing groups which c.annot he correlated. Inasmuch as the lithium chloride methanol-benzene electrolytic solution of Lewis and Quackenbush proved satisfactory for the study of water-insoluble compounds, it was believed that this solution would provide the means for obtaining comparable data on water-insoluble as m-ell as water-soluble oxy-
yen-containing organic compounds. A wide variety of pure (,ompounds was selected tvhose oxygen groups have been either detected in autoxidation reaction substances or are suspected of occurring in them. These compounds contained the oxirane, hydroperoxide, peroxide, hydroxy, dihydroxy, keto, diketo, and aldehyde groups attached to various types of organic structures. This study using nonaqueous electrolytic solutions of several classes of oxygen-containing compounds of diverse structural formulas, but having similar groups, has further substantiated the observation of other workers that the polarographic reduction is generally a characteristic of the functional group and its immediate structural environment and not of the compound as a whole. Because the oxygen group is usually the determining factor, it should be possible to identify it in an autoxidized substance from its polarographic waves. The polarographic data also provide a means for measuring the purity of water-insoluble oxygen-containing compounds which are reducible a t the dropping mercury electrode. EXPERIMENTAL
Apparatus and Reagents. A Sargent Model XX polarograph
ANALYTICAL CHEMISTRY
786
Lack of direct means for the qualitative and quantitative determination of autoxidation products has been a handicap in studies of autoxidation. The use of a nonaqueous electrolytic solution in the present investigation has made i t possible to stud!and compare the polarographic behavior of the functional groups of autoxidation products. The polarographic characteristics of 41 oxygen-containing compounds whose functional groups have either been detected or suspected of occurring in autoxidation reaction mixtures are described. Polarographic data obtained for reducible compounds such as
was used to obtain the current-voltage curves. The current scale was calibrated. Capillary I had m and t values of 4.63 mg. per second and 1.47 seconds, respectively, which gave a capillary constant, mz41'6, of 2.96 mg.2/*set. -1 '2. Capillary I1 had m and t values of 3.09 and 1.54, respectively, which gave a capillary constant of 2.28. The m and t values were for an open circuit, a t 25" C. and with the capillary in the electrolytic solution.
0 I
I '
2
3
4
.
SCALE
INCHES
Figure 1. Modified Lingane H-Cell The electrolytic cell (Figure 1) was a modified Lingane H-type with a saturated calomel reference electrode (IO). The two halves of the cell, the sample-containing arm, A , and the reference saturated calomel electrode, B , were connected by spherical joints, C. The top of the sample arm was provided with the socket of the spherical joint, D. The cap, E, was made from the ball of the spherical joint, and fused into this cap was the dropping mercury capillary and an inlet tube, F , which extended to the bottom of the sample arm. I n addition, the cap had two vent tubes, G and H , with stopcocks for venting the nitrogen. The airtight cap eliminated the need for continuous sweeping of nitrogen over the test solution, because once the solution and head space had been purged of oxygen with nitrogen, no oxygen waves were formed, even after long periods. The fixed position of the capillary maintained a constant capillary height. The spherical joint connection made for easy replacement of the agar bridge and facilitated cleaning of the cell.
peroxides, hydroperoxides, aldehydes, ketones conjugated with a double bond, and diketones have further substantiated the observation of other workers that the polarographic behavior is dependent upon the oxygen group and its immediate structural environment and not upon the compound as a wThole. Compounds containing these reducible oxygen groups could be determined qualitatiyely and quantitatively by the polarographic method. A n equation has been developed for calculation of the percentage pnrity of organic hydroperoxides.
The sample arm, A , of the cell was glass-jacketed, and the temperature of the cell was maintained within f 0 . 1 ' C. The electrolyt'ic solution described by Lewis and uacbenbush, which consisted of 0.3 M lithium chloride in e q u l volumes of absolute methanol and benzene, was used in these studies. The cell wit,h the solution had an open circuit resistance of 1175 ohms. The half-wave potentials were corrected for ZR drop. Preparation of Organic Reference Compounds. Preparation and purification of tert-butyl hydroperoxide, Tetralin hydroperoxide, cumene hydroperoxide, cycIohexene hydroperoxidcL, di-tert-butyl peroxide, acetophenone, 12-ketostearic acid, cyclohexanone, methyl isobutyl ketone, 9,10-epoxyoctadecanoI, melting point 53", 9,lO-epoxystearic acid, melting point 59". methyl g:lO-epoxystearate, 1,2-epoxydecane, methyl undecylenate, and methyloleate have been described (5,6,14-16). 4-ICetostearic acid, melting point 96-97', was prepared by the chromic acid oxidation ( 6 ) of 4-hydroxystea:ic acid ( 2 ) ; 9,lO-diketostearic acid, melting point 80.8-81.6 , was similarly prepared from 9,lO-dihydroxystearic acid, melting point 95" (-00.
P i n a n e hydroperoxide j
s.
-0.76
__ .,:e, "OX
a-Pinene hydroperoxide Alethyi oleate hydroperoxide
I ,
,
?OH,
ic-c-c.
,
