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0. D. BONNER, H. B. FLORA, AND H. W, AITKEN
combination rate constants determined for other sulfonphthaleins listed in Table 111,and is consistent with the upper limiting value calculated from the Debye equation for diffusion controlled reaction between a proton and a spherical dianion in water a t 25' and zero ionic strength.8 Table 111: Rate Constants for Several Aqueous Sulfonphthalein Indicators klz, lOlQM-1 sec-1
Bromocresol 8 . 0 purple0 Phenol reda 7.2 Chlorophenol 2 . 3 redb Bromocresol 5 . 4 green
Conditions
kzi,
sec-1
2.4 X 4.9 1.9
x x
lo4
10% 10%
8.5 X lo5
15O 15" 15O 0.1 M KN03 25'
Method
DFE DFE
T-jump This work
G. Ilgenfritx, Doctoral Dissertation, Georg-August University] Gottingen, 1966. M. Eigen and G. G. Hammes, J . Amer. Chem. Soc., 82, 5951 (1960). a
Our recombination rate constant for acetic acid is significantly faster than that reported most recently using conductometric detection3 and slightly faster than that determined by Eigen and Schoen.2 Our apparatus has been shown to be sufficiently fast for this system.
The fastest relaxation time observed for bromocresol green alone was 0.058 psec, more than three times as fast as the shortest relaxation observed for the coupled system. Our electrodes were fairly large and over 4 mm apart so we have minimized polarization effects that might have led to errors in the conductometric result. Our k13 = 7.2 X 1OO ' M-' sec-' agrees well with the acetic acid ion recombination rate constant estimated by the Debye-Eigen-Smoluchowski relation in aqueous solution a t 25018when this theoretical result is corrected to low4M ionic strength and the hydrodynamic interaction between reacting solute species described by Friedmang are taken into account: k13 (theor) = 7.6 X 1OO ' M-l sec-'. A further correction for the nonsphericity of the acetate ion woQld result in a still smaller theoretical k13, but whether this factor decreases the rate constant by as much as '/2'is open to question.
Acknowledgment. The design and construction of the high-voltage pulse generator and control circuitry by Steven L. Olsen, Ronald L. Silver, and Lloyd P. Holmes is gratefully acknowledged. This work was sponsored by AFOSR(SRC)-OAR, USAF, Grant No. 69- 1717-D. (8) H. Eyring and E. M. Eyring, "Modern Chemical Kinetics," Reinhold, New York, N. Y., 1963. (9) H. L. Friedman, J . Phys. Chem., 70, 3931 (1966).
The Ionization of Trichloroacetic Acid in Aqueous Solutions by 0. D. Bonner,* H. B. Flora, and H. W. Aitken Department of Chemistry, Universitg of South Carolina, Columbia, South Carolia 29808
(Received January 28, 1971)
Publicatwn costs borne completely by The Journal of Physical Chemktry
The degree of ionization of trichloroacetic acid in aqueous solutions has been calculated at several concentrations from Raman measurements in which the intensity of the carboxylate band is compared with that of the C-C1 vibrational band. These values have been compared with literature values determined by Raman and nmr measurements and found to be in satisfactory agreement in dilute solutions. A larger degree of ionization was found in more concentrated solutions. Activity and osmotic coefficients are reported for sodium trichloroacetate and these are used to calculate an ionization constant of 3.2 f 0.1 for trichloroacetic acid.
Introduction There have been at least four studies of the ionization of trichloroacetic acid in aqueous solutions, the most recent of which is that of Covington and coworkers,' They summarize the previous work in which ionization have been found constants ranging from 0*232 to and report a new value of 2 to 5 based upon Raman and The Journal of Physical Chemistry, Vol. 76,No. 16, 1971
mmr measurements. The large uncertainty in the value of the constants is caused primarily by the uncertainty in the values of the activity coefficient Corrections a t various concentrations. One Of the Present authors (1) A . K. Covington, J. G . Freeman, and T. H. Lilley, J . Phy8. Chem., 74,3773 (1970).
2493
IONIZATION OF TRICHLOROACETIC ACIDIN AQUEOUS SOLUTIONS had overcome this difficulty2 in determining the ionization constant of iodic acid and it was the initial intent of this work to merely make the necessary activity coefficient corrections and perform the extrapolation to infinite dilution in order to evaluate the thermodynamic constant. It became necessary, however, as will be indicated in the subsequent discussion to redetermine some of the values of a, the degree of ionization, and new Raman data are also reported.
Experimental Section
Osmotic and Activity Coeflcients. The osmotic and activity coefficients of the sodium salt of trichloroacetic acid were determined by the usual isopiestic technique with sodium chloride solutions being used as references. Sodium trichloroacetate was prepared by the careful neutralization of vacuum distilled trichloroacetic acid. It was then dried in a vacuum oven a t 65'. Raman Measurements. The Raman spectra were recorded using a Spex Ramalog spectrophotometer with a Spectra Physics Model 125 helium-neon gas laser. The frequencies recorded for all sharp lines are expected to be accurate to A 3 cm-l. Band intensities were reproducible to 1-1.5s.
Results The Raman spectrum of a 1.01 M solution of sodium trichloroacetate is shown in Figures 1 and 2. The intensities of the V I band (C-0 stretch) a t 1344 cm-l and the band a t 434 cm-1 (C-C1 stretch) are used for the calculations in this work. It may be noted that a relatively flat base line was obtained even when considerable amplification was used. The spectrum of a solution of sodium perchlorate over the range 1200-1500 cm-l, in which no band occurs, has the same near zero slope as the base upon which the carboxylate band is resting. This is in sharp contrast to the statement by Covington that it is situated on a sloping base line and that intensities could be estimated to only 4-59?.,. The differences in the observed spectra are undoubtedly caused in part by a difference in exciting sources. Solutions of organic compounds fluoresce much more when a blue source (Toronto arc) is used than when a red source is used. The degree of ionization of the acid in the various solutions was calculated from a comparison of the ratio of the intensities of the 1344- and 434-cm-1 bands with the ratio in a solution of the sodium salt for which the ionization is complete. This ratio was confirmed to be independent of solution concentration for the sodium salt and no difference was found between ratios of band maxima and ratios of integrated areas. Values of a for acid solutions a t six concentrations are given in Table I. Osmotic and activity coefficients of sodium trichloroacetate as determined from isopiestic comparison with sodium chloride standards are presented in Table 11. These coefficients are found to be larger than those of
do
L-L--+l
'
'
'
400 I
WAVENUMBER CMal
'
'
'
Figure 1. Raman spectrum of 1.01 M sodium trichloroacetate: A; rise time, 0.3 sec; slit full scale sensitivity 0.1 X width 12 cm-1; slit height 5 min; dynode voltage, 1800 V.
1300
800
WAVENUMBER CM-1 Figure 2. Raman spectrum of 1.01 M sodium trichloroacetate: (A) full scale sensitivity 0.03 X 10-6 A; rise time, 1 sec; slit width 12 cm-1; slit height 5 min; dynode voltage, 1800 V; (B) base line.
Table I : Degree of Ionization of Trichloroacetic Acid as Determined from Raman Measurements Concentration, mol/l.
Degree of ionization
0.31 0.63 0.94 1.25 1.57 2.27 3.35
0.93 0.88 0.82 0.76 0.66 0.57 0.42
(2) J. R. Durig, 0. D. Bonner, and W. H. Breazeale, J . Phys. Chem., 69, 3886 (1965).
The Journal of Physical Chemistry, VoL 7.5, No. 16, 1971
2494
0. D. BONNER, H. B. FLORA, AND H.W. AITKEN
sodium chloride and slightly larger than those of sodium acetate.
Table 11: Osmotic and Activity Coefficients of Sodium Trichloroacetate a t 25” M
d
Y
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 1.8 2.0 2.5 2.7
0.944 0.947 0.955 0.962 0.972 0.983 0.994 1.005 1.015 1.025 1.045 1.058 1.074 1.085 1.096 1.123 1.130
0.797 0.768 0.758 0.757 0.759 0.765 0,770 0.778 0.788 0.797 0.818 0.839 0.857 0.877 0.894 0.941 0.958
Discussion I t has been adequately ernphasized1s2 that ionic rather than stoichiometric activity coefficients are necessary for the correction of the quotient
k=- CY2C
1-CY
in order to determine the thermodynamic ionization constant. The former may not be determined directly for a weak acid and the latter ore also not possible by isopiestic methods for trichloroacetic acid because of its volatility. Emf measurements would be difficult because of the lack of a reversible trichloroacetate electrode. Ionic activity coefficients may be estimated, however, for many acids by a technique which was presented previously. Activity coefficient dataa are available for five strong acids4 and their sodium salts. It may be noted that for acids and salts containing anions as large or larger than iodide there is a constant ratio of acid to salt activity coefficient values a t any concentration up to 1.4 M . The ratio varies slightly for the bromides and somewhat more for the chlorides. With this knowledge and the values of the sodium trichloroacetate activity coefficients one may now estimate the ionic activity coefficients of the trichloroacetic acid. The product K p may now be calculated from the relationship
where frtis the ionic activity coefficient of the acid and p is the activity coefficient of the un-ionized acid. The Journal of Physical Chemistry, Vol. 76, No. 16, 1971
An extrapolation of K p to infinite dilution yields the thermodynamic ionization constant. A plot of the aegree of ionization, CY, as a function of concentration (Figure 3) reveals that there is substantial agreement among the three sets of data up to concentrations of about 2 M . Values of CY taken from the smoothed curve (Figure 3) of the three sets of data a t even concentrations were used to calculate the values of K p (Table 111). The value of K = 3.2 If: 0.1 which is obtained (Figure 3) falls nearly in the center of the range 5 > K > 2 reported by Covington.’ This is true, however, in spite of the fact that the activity coefficient corrections are larger than those which were used for his estimate. The behavior of Kp is quite reasonable in that one would expect. the activity coefficient of the un-ionized acid to remain relatively close to unity. The approximate values of p are 0.97 a t 1 M and 0.93 a t 1.5 M . Table 111: Calculation of the Ionization Constant of Trichloroacetic Acid
0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50
0.901 0.855 0.805 0.756 0.708 0.660 0.612 0.569 0.532
0.450 0.641 0.805 0.945 1.062 1.155 1.224 1.280 1.330
4.09 3.78 3.32 2.93 2.58 2.24 1.93 1.69 1.51
0.758 0.767 0.778 0.792 0.803 0.813 0.820 0.826 0.832
0.869 0.923 0,976 1.028 1.071 1.109 1.137 1.163 1.191
3.09 3.22 3.16 3.10 2.96 2.76 2.50 2.29 2.14
It should be noted that the values of CY obtained in this work for solutions more concentrated than 2.0 M are substantially larger than those reported by Covington. It is felt that the present data are more accurate for three reasons. (1) The use of the C-C1 band as an internal reference is inherently more accurate since it eliminates errors in sample positioning and there is less error due to instrumental drift as comparisons are made in a short time interval. (2) The use of the heliumneon laser as an exciting source reduces appreciably the fluorescence which is present in concentrated solutions of organic compounds and this requires less zero suppression and yields a flatter base line. (3) A calculation of the concentration of ionized acid, CYC,shows that a maximum value is reached in about 2.3 M solutions using the nrnr data and about 3.0 M solutions using the Raman data of ref 1. It seems unreasonable that the concentration of ionized acid should decrease with increasing stoichiometric concentration in soh(3) R. A. Robinson and R. H. Stokes, “Electrolytic Solutions,” Butterwortha, London, 1959. (4) Nitric acid is essentially completely ionized in the concentration range of interest.
IONIZATION OF TRICHLOROACETIC ACIDIN AQUEOUS SOLUTIONS
2495
3.5
0.8
0.h
2
n
-
Iu &
‘3 0
I
I
I
tions of 0.95 mol fraction solvent. The redetermined values indicate that LYCslowly approaches a limiting value of about 1.4 M a t a stoichiometric concentration of 3.5 to 4 M.
I
I
I
I
I
Acknowledgment. The authors wish to thank the Savannah River Laboratory for the use of the Raman instrument and Dr. A. L. Marston for his assistance in the measurements.
The Journal of Phyeical Chemistry, Vol. 76,No. 16,1071
