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Table I Electrolyte’ (in propylene carbonate)
0.257 m LiC10412
(mA/am*) at time indiaated18ec 10 aec 1 min 10 min
10.2 7 . 8
>10 >5 -3
8
3.1 -1
5.4 2.8
90 1 hr
1.6
80
1.5 0.30 0.026 -0.1
.. .
.. .
time. Values previously r e p ~ r t e dare ~ ~consistent ~~~ with our measurements after 10 min to 1 hr. Our values a t t = 0 were obtained by extrapolation on a tl’l scale. I n the dry (. h
70
60
98 5 0 5 40 6 E 30 20
e 35‘C A 60’C
10 I
O(
0.1
I
I
I
I
I
I
I
I
I
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 HEAD SPACE (mi)
Figure 1. Change in the molar absorptivity for the OH and C H stretch of methanol in carbon tetrachloride at 35 and 60’ as a function of the head space over the solution. The solution concentration was 0.0048 M at 35”.
ment incorrect. Swenson,’ in an attempt to determine the cause of this temperature dependence for methanol in carbon tetrachloride solution, has ascribed the decrease of the molar absorptivity with increasing temJAMES N. BUTLER TYCO LABORATORIES, INC. perature to reduced concentration of the methanol in WALTHAM, MASSACHUSETTS 02154 DAVIDR. COGLEY solution due to evaporation of the methanol into the JOHN C. SYNNOTT head space above the solution. Thus Swenson conRECEIVED AUGUST18, 1969 cludes that a true temperature dependence of the molar absorptivity does not exist. Because of the relative importance of the temperature dependence of the molar absorptivity to the interpretation of spectral data, it Temperature Dependence of the Molar seemed that some of our preliminary results in this Absorptivity of the OH Stretching Vibration area should be reported. The change in the molar absorptivity as a function Sir: Several have reported the temperaof increasing head space over a solution of methanol in ture dependence of the molar absorptivity for the carbon tetrachloride has been studied. The molar fundamental OH stretching vibration of various monoabsorptivity for the CH and OH stretching modes of meric molecules in dilute solution; corrections for this methanol was monitored with some of the results given temperature dependence have been made by Hamin Figure 1. The plots of the molar absorptivity maker, et al.,4 for solutions of methanol, t-butyl alcohol, and di-t-butyl carbinol in carbon tetrachloride. (1) I. Motoyama and C. H. Jarboe, J . Phys. Chem., 70, 3226 (1966). Finch and Lippincott6 have suggested that a true tem(2) U. Liddel and E. D. Becker, J . Chem. Phys., 25, 173 (1966). perature dependence should exist for hydrogen-bonded (3) R. H. Hughes, R. J. Martin, and N. D. Coggeshall, ibid., 24, 489 (1966). complexes. Thus, if the solute should hydrogen bond (4) R. M. Hammaker, R. M. Clagg, L. K. Patterson, P. E. Rider, to the solvent, a temperature dependence would be and 9. L. Rock, J . Phys. Chem., 72, 1837 (1968). expected. However, Fletcher and Heller6 argue that (6) J. N. Finchand E. R. Lippincott, ibid., 61,894 (1957). the molar absorptivity should not change with tempera(6) A. N. Fletcher and C. A. Heller, {bid., 72, 1839 (1968). ture, and therefore consider any temperature adjust(7) C. A. Swenson, ibid., 71, 3108 (1967). VoZume 73, Number 11 November 1060
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Table I : Change in Molar Absorptivity with Temperature for the Fundamental OH and CH Stretch of Methanol in Carbon Tetrachloride"
A B 1\/20'C
04 61.0 f 0.8 55.8 rfr 1.1 52.8 rfr 0.6 50.5 It: 0.8
40.7 f 0.6 39.3 0.7 39.9 f 0.7 39.8 f 0.7
30 40 50 60
*..
...
8.5 13.4 17.2
3.7 2.0 2.2
All spectra were recorded on the Beckman DK-PA spectrophotometer equipped with the Beckman temperature regulated cell holder. One-cm Teflon stoppered cells with CClr serving as the reference were used. The cells were considered adequately sealed since recycling of the temperature gave identical results. The solutions were prepared in a drybox. The molar absorptivities have been corrected for changes in concentration with temperature. * The molar absorptivities were obtained by extrapolation of a plot of e us. concentration to infinite dilution. The errors reported are the errors in the intercept as calculated by leastsquares analysis. The concentration range was 0.005 to 0.02 M. a
O
A€ =
(€30"
- €T)/€30°.
8 0.3
I 0
v)
m
0.2
0.1
against head space for the OH stretch a t 35 and 60" do not converge a t zero head space as they should if evaporation of the methanol into the head space is the WAVELENGTH (m+) cause of the reduction of the molar absorptivity. ConFigure 2. Temperature dependence of the spectra of phenol versely, the molar absorptivity for the CH stretch is and catechol in carbon tetrachloride solution. A, phenol in only slightly different a t the two temperatures. The CC14(0.00212 M a t 25'); B, catechol in CClr (0.00182 M slope of the OH and CH absorptivity curves for 60" a t 25'). The spectra have been corrected for changes in indicates that some evaporation of methanol does occur. concentration with temperature. We find for head spaces in excess of 1 ml and temperatures around 60" sufficient evaporation of methIf evaporation of the methanol were the cause of the anol from solution could occur t o reduce the solution decrease of the molar absorptivity with temperature, Concentration substantially. At a head space of 0.5 any temperature dependence of the molar absorptivity ml or less, evaporation of methanol would cause only for other nonassociated species in carbon tetrachloride a 4-5010 reduction in the molar absorptivity from the solution should also be liable to the same interpretation. value a t zero head space at 60". At temperatures Since phenols are relatively nonvolatile, the temperature lower than 35", evaporation is negligible even for head dependence of the molar absorptivity of these comspaces considerably greater than 1 ml. pounds, previously reported, a cannot easily be ascribed We have also estimated the Henry's law constants to evaporation. We investigated the temperature for methanol in carbon tetrachloride solution a t 35" dependence of the molar absorptivity of phenol and and 55" from the data of ref 8. Calculations using catechol in carbon tetrachloride with the results given these constants also indicate that for head spaces of in Table I1 and Figure 2. For catechol, the molar the order used in this work (0.5 ml) evaporation of the absorptivity for the OH involved in intramolecular methanol would account for only about 2% of the hydrogen bonding is seen to decrease 2.501, while that observed decrease in the molar absorptivity in changfor the OH not involved directly in the intramolecular ing the temperature from 35 to 55". bonding decreases 11.3% with increasing temperature. The temperature dependence of the molar absorpThe free OH and the intramolecular hydrogen-bonded tivity for the CH stretch of methanol in carbon tetraOH peaks for catechol would be expected to behave chloride has also been determined with the results given similarly if evaporation solely were responsible for the ,~~~ in Table I. I n agreement with earlier w o r k e r ~we decrease of the peak height. find slight variation of the molar absorptivity for the The results of our studies indicate that a temperature CH stretch while the molar absorptivity for the OH dependence of the molar absorptivity for the free OH stretch changes significantly. The relative constancy stretch of nonassociated species in carbon tetrachloride of the CH molar absorptivity indicates that any evaporation is not sufficient to account for the observed de(81 G . Scatchard, S. E. Wood, and J. M. Mochel, J . Amer. Chem. crease in the OH molar absorptivity. Soc., 68,1960 (1946). The Journal of Physical Chmhtry
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Table 11: Change in Molar Absorptivity with Temperature for the Fundamental OH Stretch of Phenol and Catechol in Carbon Tetrachloride Wavelength, m p
Phenol Catechol
@
Ae =
(e200
2777 2772 2808
- E ~ O ~ ) / ~ ~XO O100.
to 60" in all cases.
&%a
1l.Ob 11.3b 2.5b
The temperature range was 20
* Average of three samples.
solution does indeed exist. Evaporation of the solute in question may occur but not to such an extent to account for the observed change in the molar absorptivity provided small head spaces are used. We explain this temperature dependence in the manner of Finch and L i p p i n c ~ t t ;i.e., ~ the solute-free OH forms a weak hydrogen bond with the carbon tetrachloride. Some support for this can be derived from the work of Fumio and Katsuhiko, 9 who have isolated complexes of certain diols and carbon tetrachloride, and from Fletcher, lo who has presented evidence for the existence of a 1 : l complex between carbon tetrachloride and l-octanol.
Acknowledgment. The authors wish to acknowledge Lebanon Valley College for partial financial support for this research. (9) T.Fumio and A. Katauhiko, TetrahedronLett., 33,3695 (1968). (10) A. N.Fletcher, J . Phys. Chem., 73,2217 (1969). (11) To whom correspondence should be addressed.
DEPARTMENT OF CHEMISTRY LEBANON VALLEY COLLEGE ANNVILLE,PENNSYLVANIA 17003
G. P. HOOVER E. A. ROBINSON R. S. MCQUATE H. D. SCHREIBER J. N. SPENCER^^
RECEIVED AUGUST20, 1969
The Homoconjugation Constant of Picric
Acid in Acetonitrile
Sir: I n a recent paper, D'Aprano and Fuossl reported that the homoconjugation constant, K'~pi,- = [HPi2-]/[HPi] [Pi-], of picric acid in acetonitrile (AN) is of the order 2 X lo2, a value some one hundred times greater than that reported previously by US.^ I n the where [SI denotes notation of FUOSS, = Kf~pi*-[S], the molarity of pure AN, i.e., 18.9 M . The method used by these authors is subject to unusually large errors. In brief, they added successive portions of solid picric acid to very dilute solutions of tetramethylammonium or tetrabutylammonium picrate. From the difference in electrical conductance of the mixtures after correction for basic impurities in the solvent and
x
that of the picrate solution without acid, they arrived at the large value of KfHpia- and the mobility of the HPi2ion ranging from 40 to 63 taking Xopi- = 77.a The correction to be applied for basic impurities was rather large as compared to the difference between the conductivity of the picrate solution with and without picric acid. From our work on the effect of picric acid on the solubility of potassium picrate in AN2 we arrived at the value of KfHpil- of the order of 2. Also, paH measurements in dilute mixtures of picric acid and tetrabutylammonium picrate indicated that KfHpig- must be very small. In order to substantiate our previous value we have measured the paH in mixtures of tetrabutylammonium picrate containing a large excess of picric acid. The equation used in the calculation of K f H p i n - from the experimental paH data had been derived previously.2 Also, we have estimated ioHPia by determining the effect of a large excess of picric acid on the conductance of dilute tetraethylammonium picrate solutions in AN. Acetonitrile was purified and dispensed as described previ~usly.~The water content as found from Karl Fischer titration was 1-2 X M . Picric acid2 and tetrabutylammonium picrate2 were products used previously. Tetraethylammonium picrate was prepared in this laboratory by H. Smagowski. Conductometric measurements and potentiometric paH techniques with the glass electrode have been described elsewhere. The following paH values were found in mixtures M in tetrabutylammonium picrate and 3.53 X 0.226, 0.439, and 0.835 M in picric acid: 8.92, 8.51, and 8.10, respectively, from which an average value of K f ~ p i a= - 2.4 f 0.5 was obtained. This value is in good agreement with that of 2.0 reported previously from solubility data of potassium picrate in presence of picric acid.2 Conductivity data of mixtures of picric acid and tetraethylammonium picrate are entered in Table I. Even after correction for viscosity, an appreciable decrease in conductivity was observed with increasing picric acid concentration a t a given salt concentration as observed by Fuoss.' The specific conductivity of 0.91 M picric acid alone was 1.5 X ohm-' cm-l, corresponding to C B H + = 1.1 X M . Since this value is less than 1% of the concentration of salt taken in the mixtures, C B H + can be neglected as compared to CEtrN t. Values of [Pis-], the subscript referring t o the simple ion in the solvent, and [HPiz-] in columns 5 and 6 in Table I were calculated using K f ~ p i e=- 2.0 and i4
(1) D. D'Aprano and R. M. Fuoss, J . Fhys. Chem., 73,223 (1969). (2) I. M.Kolthoff and M. K. Chantooni, Jr., J.Amer. Chem. Sac., 87, 4428 (1965). (3) J. F.Coetzee and C.P. Cunningham, ibid., 87,2529 (1965). (4) I. M. Kolthoff, S. Bruckenstein, and M. K. Chantooni, Jr., tbtd., 83,3927 (1961). Volume 73, Number 11 November 1969
