Dee., 1962
ACID-BASEEQUILIBRIA
IN
COXCENTRATED SALTSOLUTIONS
2687
ACID-BASE, EQUILIBRIA IN CONCEKTRATED SALT SOLUTIONS. I. POTENTIOMETRIC MEASUREMENTS, INDICATOR MEASUREMENTS, AND TJNCHARGED BASES IN DILUTE ACID SOLUTIOXS BYDONALD ROSENTHAL~ AND JAMESS. DWYER Department of Chemistry, The University of Chicago, Chicago, Illinois Received June 13. 1969
An acidity function, Ho, was established in 4 and 8 M LiCl solutions containing small concentrations of strong acids. pMH HO(where pMH = -log total strong acid concentration) was shown to be a constant for a given salt solution. ~ H G . E . pMH (where ~ H G . Eis. the pH measured using a cell with a glass indicator electrode and a saturated calomel reference electrode) also was found to be a constant for 4 and 8 M LiC1,6 A1 NaC104, 6 df NaNOa, and 4 M CaCL solutions which contain dilute strong acid. Identical results were obtained in 4 and 8 M LiCl solutions where the hydrogen electrode was substituted for the glass electrode. The significance of p H 0 . ~ .and Ho measurements is discussed. In dilute acid solutions of weak uncharged bases KBE+ = ([B] [total strong acid] /[BH+])QBH+ where KBH+is the thermodynamic dissociation constant of the acid BH+, and QBH+ 18 a constant for a particular base B and a particular salt solution. The values of ~ H G . Eand . Ho calculated using this equation are in good agreement with the experimental values obtained for aniline and 2-aminopyrimidine solutions containing dilute strong acids and concentrated salts.
-
Introduction Some studies of acid-base equilibrium in concentrated salt solutions have previously been reported.3-@ Potenti~metric~.~J and indicator5t6 measurements have been made on these solutions. The present studies were initiated in an attempt to explain the quantitative aspects of the potentiometric and indicator results, and of acid-base equilibria in these solutions. This work extends the measurements to more concentrated solutions than were previously I n this paper potentiometric and HO5measurements and acid--base equilibria in dilute acid solutions are discussed. Subsequent papers will consider charged bases in dilute acid solution, the situation in basic and concentrated acid solutions, reaction kinetics, and the practical aspects of acidbase titrations in these solutions. The results obtained in this study are consistent with the hypothesis that the situation in acidified salt solution is formally similar to that in water, e.g., the reaction BH+ e B H f can be written for an uncharged base, B, and its conjugate acid, BH+. Further, the quantitative aspects of equilibrium in these solutions can be accounted for using the equation
+
where K B H +is the thermodynamic molar acid dissociation constant and Q’BH+ = fBfH+/fBH+ depends upon the nature and concentration of the salt and the nature of the base, B. Since undis(1) Taken in part from the Ph.D. research of James S. Dwyer. This work was supported by two grants from Research Corporation, principally in the form of Research Corporation fellowships for J. S. D. A portion of this work was presented at the 138th National Meeting of the American Chemical Society, New York, N. Y.,September 12, 1960. (2) Department of Chemistry, Clarkson College of Technology, Potsdam, N. Y. (3) (a) H. S. Harned and B. B. Owen, “The Physical Chemistry of Electrolyte Solutions,” Reinhold Publ. Corp., New York, N. Y.. 1958, pp. 675-881; (b) M.Rilpatriok. et al., J . A m . Ckm. Soc., 76, 584,586. 588 (1953). (4) A. Ellila, Ann. Acad. Sci. Fennicae, Ser. A, 11, No. 51 (1953); Acto Ckm. Scand., 8, 1257 (1954). (5) M.A. Paul and F. A. Long, Chem. Reo., 67, 1 (1957). (6)F. E. Critchfield and J. B. Johnson, Anal. Chem., 80, 1247 (1958);81, 570 (1959).
sociated strong acid may be present (e.g., ”01 in 6 M NaNO3),’ it is desirable to use a somewhat more general form of eq. 1 KBH+= ([B][total strong acid concentration]/ [BH+l>&BHt (2) The following additional symbols are used in this and subsequent papers pMH = -logarithm of the total molar concn. of dissociated and undissociated strong acid ~ H G . Eis. the pH as determined with a pH meter using a glass electrode and a saturated calomel reference electrode ~ H isH the pH as determined using a hydrogen electrode and a saturated calomel reference electrode PuH = -log aHt = -log ~ H + [ H + ] + is the activity of hydrogen ion, and f H + is the molar activity coefficient No = ~ K B I I-+ log [BH+]/[B] = p1lH log & B H + (3) where B is one of a series of indicator bases (usually substituted anilines), thc Ho indicators5 a, is the activity of water Experimental Reagents .-Bakrr Analyzed Reagent grade LiCl, XIallinckrodt Analytical Reagent grade NaNOa and anhydrous CnCls, and G. F. Smith anhydrous NaClOa were used. The maximum limits of impurities listed on the labels were small, except for the presence of magnesium and alkali salts in CaClz. Fisher rcagent grade aniline w m purified by dietillation. Mathetion, Coleman and Bell 2-a~ninopyriniidine ww recrystallized from benzene, then from absolute ethanol. The indicators were purified as described elsewhere.* Solutions.-All solutions used in this study were prepared (7) (a) T. F. Young, L. F. Moranville. and H. M. Smith in “The Structure of Electrolyte Solutions.” W. J. Hamer, Ed., John Wiley and Sons, Inc., New York, N. Y., 1959,pp. 38-48; (b) A. A. Krawetz, Thesis, University of Chicago, 1955. ( 8 ) D. Rosenthal and J. S. Dwyer, “The Acidity Function in Aqueous Concentrated Acid and Salt Solutions,’’ Con. J . Chem., in press (1963).
2688
DONALD ROSENTHAL AND
JAMES S. DWYER
Vol. 66
TABLE I MEASUREMENTS O F
DILUTEHCl
HCI concn., moles/l.
x 7.97 x 1.00 x
4.98
10-3 10-3 10-2 2.99 X 1W2
7.97
x
10-2
Indicator
Ho
pKA" pN-4 pKA pNA ON.4
1.237 1.044 0.970 0.497 0,007
SOLUTIONS I N
4 M Licl AT 25'
PHG.E.
PHH
glass electrode
hydrogen electrode
1.36 1.13
1.35 1.14 1.07
1.0G
pMH
- Ho
=
PMH
log QBH+
1.066 1.055 1.030 1.027 1.092
0.55 0.15
-
-
Av. = 1,054 S.D, = 0.012 90yolevel = f 0 . 0 2 6 a
- PHGE. 0.94 .96 .94 .97 .94
0.95 0,0063 f0.013
py.4 is p-nitroaniline; 0 3 ~ 4 is o-nitroaniline.
TABLE I1 MEASUREMENTS OF DILUTEHCl SOLUTIOXS IX 8 AI LiCl PHG.E. PHH IICl concn., moles/l.
2.000 x 1.000 x 1.996 x 3.997 x 4.98 x 7.98 x 1.000 x 2.99 x 4.00 x 4.98 x 7.98 x
10-4b 10-3 10-3 10-3 10-3 10-3 10-2 10-2 10-2 10-2
Indiaator"
oXA oNA oNA 4C12NA 4C12NA 4C12NA 4C123A
I10
-0.194 -0.406 -0.480 -0.967 -1.075 -1.177 -1.389
glass electrode
2.92 0.79 .44 .I1 .02
