700
processes. This similarity may be fortuitous, however, as it has been shown that atomic and molecular hydrogen have different precur~ors,8~~ and that reactants like C O ( N H ~ ) ~affect + ~ the yield of molecular hydrogen without diminishing the yield of atomic hydrogen in neutral solution.1g It may be concluded, therefore, that the origin of the "molecular" hydrogen in radiolyzed water remains an open question.
Acknowledgments. The authors wish to thank Dr. F. S. Klein for helpful discussions. (19) iM.Anbar and D. Meyerstein, J. Phys. Chem., submitted.
Pure Acetic Acid and Acetic Anhydride and the Electrical Conductance and Dielectric Constant of This Systeml&ib
by R. Thomas Myers Department of Chemistry, Kent State University, K a t , Ohio 44240 (Received September 16, 196.4)
Research was contemplated using acetic acid as solvent. Acetic anhydride was used in the purification of the acid, as described below. Consequently, it seemed proper to study the effect of added acetic anhydride on the properties of the acid. The density, electrical conductance, and dielectric constant of the system acetic acid-acetic anhydride are reported below, along with novel-but very simple-methods of purifying these substances.
Experimental Puri$cation of the Acetic Acid. The first step was to determine the water content of the stock supply of reagent grade acetic acid by a calorimetric method similar to that of Greathouse, et aL2 In order to avoid the rise of temperature upon addition of the perchloric acid catalyst (due to reaction of the water therein with the acetic anhydride) a solution of perchloric acid in acetic acid was used as catalyst. The catalyst solution consisted of 6 ml. of 70% perchloric acid dissolved in 39 ml. of acetic acid to which was then added in small portions slightly more than the theoretical amount of acetic anhydride (14.0 ml.). A typical run was carried out as follows. To 200.0 ml. of acetic acid in the calorimeter (a common silvered vacuum bottle) was added 8.0 ml. of anhydride. The liquids were mixed by swirling the bottle and the temperature was estimated to the nearest 0.01" on a thermometer calibrated in 0.1" steps. The Journal of Physical Chemistry
NOTES
Then 2.0 ml. of the catalyst solution was added and a temperature-time curve drawn. The straight portion of this curve was extrapolated back to zero time and the temperature rise computed. This can be duplicated to within about 0.03" in separate runs. The heat c&pacity of the calorimeter was determined by adding to the stock acetic acid a known amount of water and repeating the experiment. This gives two equations in two unknowns (the amount of water in the stock acetic acid and the heat capacity of the calorimeter). Using the heat of reactiop and specific heat data given by Greathouse, the two equations can be solved simultaneously. Then, knowing the water content of the stock solution, the pure acetic acid is prepared by adding the theoretical amount (plus a 2% excess) of acetic anhydride necessary to react with the water in the stock solution. The reaction is catalyzed by the addition of 1g. of anhydrous 5-sulfosalicylic acid/l. This solution is kept overnight at about loo", then distilled at total takeoff through a 1-m. column, packed with glass helices and insulated with 2.5 cm. of glass fiber insulation. This new and simple process will result in acetic acid with a specific conductance of 0.6 X ohm-l em.-', provided it is distilled directly into the conductance cell. Purijkation of Acetic Anhydride. The acetic anhydride is purified by a novel azeotropic distillation process. Toluene is added to reagent grade acetic anhydride and the mixture is distilled through a 38-em. column with silvered evacuated jacket, packed with glass helices or Heli-Pak,3 at a reflux ratio of 30: 1. The first material to come off is the acetic acid-toluene azeotrope, boiling at 100.6" and containing 28% acetic acid.4 Next, toluene distils at 110.8", and finally acetic anhydride. (Within the limits of accuracy of the experiment the acetic anhydride-toluene system is nonazeotropic at 730 mm.). At this stage the reflux ratio is changed to 20:l. Acetic anhydride of a specific conductance of about 0.5 X 10" can be obtained if distilled directly into the conductivity cell. The distillate reaches this low conductance after about half of the charge has distilled. Conductance Measurements. The conductance cell was a Washburn type with a cell constant of 0.0161 (1) (a) Presented in part at the 138th National Meeting of the American Chemical Society, New York, N. Y., Sept. 1960 (Abstracts of Papers, p. 58s) : (b) partially supported by the Directorate of Chemical Sciences of the Air Force Office of Scientific Research, under Contract No. AF 49 (638)-631. (2) L. H. Greathouse, H. J. Jannsen, and C. H. Hagdill, A w l . Chem., 28, 357 (1956). (3) Obtained from Podbielniak, Inc., Chicago, Ill. (4) L. H. Horsley, "Aaeotropic Data," American Chemical Society, Washington, D. C., 1952, p. 48.
NOTES
cm.-1, The bridge utilized a Leeds and Northrup shielded ratio box with Wagner ground, a 1 to 111,111ohm General Radio Company decade resistor, with a 100kilohm resistor in parallel. The accuracy of resistance readings is considered to be about 0.1%. Dielectric Constant Measurements. The dielectric constant was measured at 1 Mc., using a sample holder similar in design and dimensions to the one described by Sniyth and co-w~rkers.~In view of the extremely corrosive nature of the liquids used in the investigation, the inner parts of the copper cell were gold-plated and given a dispersion coating of Teflon about 0.1 mm. thick. This latter, although offering very good resistance to corrosion, had the effect of making the calibration curve nonlinear, and the cell slightly variable in capacity. The cell was calibrated using Spectrograde liquids from Eastman: CCI,, CeH6, CHCla, CH2C12, (CH2C1)2,C5HSN,(CH3)&HOH, CH3COCH3,and CH3OH. The capacitance was measured by use of a TwinT impedance measuring circuit from General Radio co. Preparation of Mixtures. Mixing of solutions was accomplished by direct distillation into a small flask attached by ground joints to the distillation apparatus. The samples were weighed and then transferred through ground joints to the conductance cell. After conductance measurement the liquid was poured into the dielectric constant cell. Of course, some contact with air was unavoidable at this stage, but the effect of traces of absorbed moisture on the dielectric constant is much less than on the conductance. When density measurements were made, the liquid was transferred to the pycnometer from the conductance cell via ground joints. All measurements were carried out at 25 f 0.02", as judged by a thermometer calibrated by the U. S. Bureau of Standards.
Results The results for electrical conductance and dielectric constant are shown in Figure 1. The density and refractive index of mixtures are given in Table I. The data for density are in perfect qualitative agreement with the results of Drucker and KasseP at 15". Refractive index data apparently are not available. Discussion The fact that the procedure described for of water from acetic acid yields a material of such low conduct,ance certainly indicates that water is the chief volatile impurity in present-day reagent grade acid. The situation with respect to acetic anhydride is more complicated. After the acetic acid-toluene azeotrope
701
10
20
30 40 50 60 Mole % of acetic acid.
70
80
90
100
Figure 1. Specific conductance and dielectric constant of acetic acid-acetic anhydride mixtures: upper left curve, conductance data of Hoover; lower curve, conductance data of present study and of Brun (A); middle curve, dielectric constant. ~~
-
~
Table I : Density and Refractive Index of Acetic AcidAcetic Anhydride Mixtures Mole fraotion of AcOH
d 264
0 0.0619 0.1203 0.1215 0.1513 0.3400 0.4208 0.5933 0.8533 0.9212 1.0000
1.0747 1.0738 1.0724 1.0733 1.0717 1.0679 1.0656 1 .0607 1.0498 1.0478 1,0435
n%
1.3878 1.3874
....
1.3871 I.3862 1.3842 1.3826 I . 3799 I.3738 1.3724 1.3704
and the excess toluene have come over, the anhydride which distils has a high electrical conductance, around 5 X 10-8 ohm-I cm.-l. As this product which first distils over stands in the conductance cell there is a gradual increase in conductance, with a constant value reached in about 24 hr. This is true also of the mixtures of this first distillate with acetic acid. Neither this high conductance nor the increase in conductance can be due to dissolving adsorbed water from the cell, because the same result is obtained when the cell has stood for a long time with pure acetic anhydride. However, as the distillation proceeds, the conductance of the distillate becomes less and less, reaching an ( 5 ) W. P. Conner, R. P. Clarke, and C. P. Smyth, J . Am. Chem. Sac.,
64, 1379 (1942).
(6) J. Timmermans, "Physic*Chemical Constants of Binary System," Vol. 2, Interscience Publishers, Inc., New York, N. Y., 1959, p. 625.
Volume 69, Number I
February 1966
NOTES
702
almost constant value after about half of the charge has distilled. A product has been obtained with a conductance as low as 0.3 X lo+ ohm-l cm.-l. This product of low conductance does not exhibit the phenomenon of increasing conductance on standing in the conductance cell which is shown by the product in the early stageof distillation, either by itself or mixed with the acetic acid. The. previous facts indicate that the conducting impurity is volatile, with a boiling point just slightly lower than that of the anhydride. Qualitative experiments show that his impurity is not water, acetic acid, 2,4-pentanedione1 or ethylidene diacetate. The increasing conductance on standing indicates that a chemical reaction involving the impurity is occurring. Except for the very slightly probable simultaneous distillation of two impurities, the only probable single substance is one involving a keto-enol isomerization. At this tinze diketene, or some reaction product of d i k e tene, is suspected. This problem is being investigated further. In any event, the conductance found for pure acetic anhydride is about one order of magnitude less than most values previously reported.’ Likewise, the conductances of mixtures of the acid and anhydride are about one order of magnitude less than previous values.’ An unusual phenomenon was the relatively enormous increase in conductance when the first fractions of acetic anhydride (of about 5 x lo-* ohm-l cm.-1) were mixed with pure acetic acid. Conductances of these mixtures were as high as 50 X and were in essential agreement with the results of Hoover. The conductance of these mixtures increases slowly for about 24 hr. The indication is that a chemical reaction is occurring between acetic acid and an impurity in the anhydride. The presence of an ionic solute in supposedly “pure” acetic anhydride (and the enormous increase in conductance caused by this impurity on addition of acetic acid) is reason to question a great deal of the data reported in the literature using acetic anhydride as solvent. The conductance data reported here essentially verify the results of Brunls with the exception that the conducting impurity in acetic anhydride appears to be volatile, whereas Brun’s method of distillation indicates that the impurity is nonvolatile. Acknowledqment. A. large amount of the data was obtained by h r , Donald &app and Miss Viola M. L. Sun. (7) For a good summary, see T. B. Hoover and A. W. Hutchinson, J. Am. Chem. Soc., 8 3 , 3400 (1961).
(8) T.S.Brun, CJn:/niv.i Bergen Arbok, Naturuitenskap. Rekke, No.12,l (1952).
The Journal of Physical Chemistry
On the Polarizability of Rare Gas Atoms by Ralph L. h e y Douglas Aircraft Company, Inc., Missile and Space Systems Monica, CahfOTnb (Received Octobsr 6 , 1964) Division,
Calculations of several authors1,2indicate that the effective polarizability, a,of an atom may decrease with increasing density, contrary to the predictions of the dielectric theory derived for very low densities.S These predictions have been experimentally vedied in the case of liquefied rare gases and methane.415 ten Seldam and de Groot, employing an isolated, compressed atom model, unaffected by fluctuations in a,as well as by any attractive forces between atoms, demonstrated a decrease in a at high pressures. Jansen and Mazur showed that a remains a molecular constant for a harmonic oscillator but that, when a more realistic model is considered, the effective polarizability becomes a function of temperature and density. In the latter calculation, their employmentof large intermolecular distances, corresponding to lower densities, is equivalent to the use of a potential softer than that of the harmonic oscillator. In this note are presented calculations based on two rather simple potential energy models. The first is that of a harmonic oscillator, bounded at the points X = *tx0 by an infinite potential barrier
V(x> = =
@ J
‘/2
x > XO] kX2 [-xo 6 x 6 X,] [X< -Xo,
The second is that of a particle in the potential field
V ( X ) = V0tan2 In the first case the boundary condition changes the normalization constant, N,, of the harmonic oscillator through the orthogonality calculations so that it now appears as
N , = [An
+ 2”n!
SIa
e - @ d[]-”’
where (1) C. A. ten Seldam and S. R. de Groot, PhlJsica, 18, 905 (1952). (2) L.Jansen and P. Mazur, ibid., 21, 193 (1955). (3) J. G. Kirkwood, J. Chem.Phys., 4 , 592 (1936). (4) R. L.Amey and R. H. Cole, ibid., 40, 146 (1964). (5) G.0.Jonea and B. L. Smith, Phil. Mag., 5 , 355 (1960); C. M. Knobler, C. P. Abbiss, and C. J. Pings, J . Chem. Phys., 41, 2200 (1964).
