ilIEASUREMENTS O F T H E IONIZATION CONSTAXT OF BEXZOIC ACID USING SILVER CHLORIDE ELECTRODES1 H. T. BRISCOE AND JOHX S . PEAKE Department o j Chemistry, Indiana University, Bloomington, Indiana Received February 19, 1938
A method has been described by Harned and his coworkers for determining the ionization constants of weak acids and weak bases. In the present investigation this method has been applied to the determination of. the ionization constant of benzoic acid. Briefly, the method consists in measuring the electromotive forces of cells containing benzoic acid, sodium benzoate, and sodium chloride. These cells may be conventionally represented by
Hz 1 HB(ml), NaB(mz),NaCl(md) I AgCl 1 d g The ionization constant, K , is calculated by means of the equation:
At zero ionic strength
(p
= 0), In
ymgbecomes equal t o zero, so that YB
K’ then equals K . Since E , Eo, and the molalities are measurable, K’ can be determined for a number of solutions and its value a t zero ionic strength be determined by extrapolation. The values of the electromotive forces of our cells and the corresponding values of the terms of the above equation are assembled in tables 1 and 2. EXPERIMENTAL PROCEDURE
The solutions were prepared from recrystallized benzoic acid, sodium benzoate, and sodium chloride by weighing out the thoroughly dried solid reagents and adding them to weighed quantities of water. Crystals of electrolytically deposited silver and metathetically precipitated silver chloride were used in the preparation of the silver-silver chloride electrodes. The hydrogen electrodes were prepared by the This paper is abstracted from the thesis presented by John S. Peake to the Faculty of the Graduate School of Indiana University in partial fulfillment of the requirements f o r the degree of Doctor of Philosophy. 637
638
H. T. BRISCOE AND JOHN 8. PEAKE
BOLUTION
I.. . . , . , , , , , . . . . . . ,
11. . . . . , . . , . , . , . . . 111.., , . , . , , , , . , . . . IV . . . , . , , , . . . , V... . . , ,.,,. . .. V I . .. . , . , . , . . . . . ,
,
,
,
,
,
I
0.01636 mHB 0,01385 0,02338 0.01858 0.01734 0.01879
mNaCl
"NaB
P
E
0.01197 0.02637 0.01009 0.02877 0.02668 0.06823
0.01240 0.00670 0.00480 0.00162 0.00516 0.00900
0.02445 0.03320 0.03381 0.03363 0.03204 0.07736
0.6785 0.5472 0.5514 0.5294 0.5354 0.5229
TABLE 2 Calculation of ionization constant SOLUTION
I.,, . . . , , . , , . . . . . . I1 . . . , , . , . . . . . 111. . . , . , . . . . . IV. , . , , , , . . . . V . . .. , . . ..., . . . VI.. , . . . , , , , , , . . . ,
,
,
, ,
,
___
,
,
E
- Eo
"H
0.05918
0,00008 0.00013 0,00029 0.00024 0.00020 0.00013
5.0243 5.4953 5.5662 5.1944 5.2958
mHbmCl
mB -___ -1.8065 -1,2760 -1.3394 -0.9643 -1.0690
LOG
K'
-4.2178 -4.2193 -4.2268 -4.2301 -4.2268 -4.2289
K' X 10s
6,056 6.035 5.932 5.387 5.932 5.903
I _
Extrapolated value of K' = K = 6.05 X
FIG.1. The cell
The cells used in this investigation were contained in the ghss vessel shown in the figure. The silver-silver chloride electrodes mere placed in
IONIZATION CONSTANT OF BENZOIC ACID
639
the bottom of the vessel as shown, and the vessel was filled with enough solution so that the platinum foil of the hydrogen electrode was covered. The electrodes were washed repeatedly by decantation with fresh solution. This solution was first saturated with hydrogen by bubbling it through the solution before it was used for washing the electrodes or filling the cell. During measurements the cells were kept at 25°C. f 0.02' by means of a thermostat, and readings were taken at 15-minute intervals until equilibrium was reached. Readings were made by means of a Leeds and Northrup Type K potentiometer and Type R reflecting galvanometer. EXPERIMENTAL RESULTS
The values obtained for the electromotive force of cells using six solutions of benzoic acid, sodium benzoate, and sodium chloride have been listed in tables 1 and 2. Each E.M.F. value listed was obtained by comparing the values from six or more different cells containing the same solution. The molalities of the constituents of the solutions and the values of all terms of the equation for each solution are also listed in the tables. The value of Eo is taken as 0.2221 volt. Because of the extreme difficulty encountered in obtaining reproducible results with aqueous benzoic acid solutions in these cells, the accuracy is not claimed to be as high as that claimed by Brockman and Kilpatrick and by Saxton and Meier, who used the conductivity method. The electromotive forces were not reproducible to more than 0.1 millivolt. The difficulty in reproducibility was due chiefly to the fact that the range of acid concentrations that can be used is necessarily very small, and the activity of the hydrogen ion is limited to very low values in aqueous solutions owing to the low solubility of the acid. This method for determining ionization constants has been found by Harned and his coworkers to yield very accurate results with acids of comparatively high solubility. The results of this investigation show that the method will also yield reasonably accurate results with the slightly soluble benzoic acid. The value of 6.05 X for K is slightly higher than the value of 5.9 X obtained by Larsson from solubility measurements and lower than the values of 6.312 X obtained by Brockman and Kilpatrick and of 6.295 X low5obtained by Saxton and Meier, both from conductivity measurements. It is much lower than the value of 6.7 X which was obtained by Kilpatrick and Chase from measurements with a quinhydrone electrode. SUMMARY
Measurements of cells containhg benzoic acid, sodium benzoate, and sodium rhloride using silver-silver chloridp electrodes gave a value of
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H. T. BRISCOE AND JOHN S. PEAKE
6.05 X lop5 for the ionization constant of benzoic acid in aqueous solutions. REFERESCES (1) BROCKXAX A N D KILPATRICK: J. Am. Chem. Soc. 66, 1483 (1931). (2) HAMNETT A X D LORCH:J. Am. Chem. SOC. 66, 70 (1933). (3) HARSEDA N D EHLERS:J. Am. Chem. SOC. 64, 1350 (1932). (4) HARKEDASD OWKE:J. Am. Chem. SOC.62, 5079 (1930). A K D CHASE:J. Am. Chem. SOC.63, 1732 (1931). (5) KILPATRICK (6) LARSSOK: Z. physik. Chem. A148, 148 (1930). (7) SAXTONAND MEIER: J. Am. Chem. SOC.66, 1918 (1934).
