8624
from high-precision density measurements by means of a magnetic float densitometer as described by Millero. The apparent molar volumes of the un-ionized forms of the acids were corrected for ionization by the method Sir : of King" and extrapolated to infinite dilution. The Hepler5m6has reported that while the acidities of mapparent molar volumes of the sodium salts were deterand p-nitrophenol differ by more than one pK, unit, the mined in excess sodium hydroxide to prevent hydrolyenthalpies of ionization are almost identical (4.90 and sis, the apparent molar volume data being corrected 4.65 kcal/mol, respectively). The entropies of ionizawith the known NaOH values.'Z In dilute solution tion for the meta and para isomers, however, are -21.8 the apparent molar volumes varied with I1/?according and - 17.1 cal/(deg mol), respectively. Following the to the Debye-Huckel limiting law for 1 : I electrolyte.13 reasoning suggested by other workers in the a ~ - e a , ~ - ~ Examination of Table I reveals that A p i a is more Hepler attributes the difference in acidity to solvation negative for m-nitrophenol and m-formylphenol as comsuggesting that the entropy of ionization (AS;")qualipared to their corresponding para isomers. This aptatively reflects the relative degree of solvent orientation pears to be consistent with the qualitative explanation around the solute species involved in the equilibrium. of the entropy data cited above, in that the more negaIt is argued that m-nitrophenoxide is more effective in tive the AS;" the more electrostriction is expected in the orienting solvent molecules since the negative charge is ionization process. Indeed, this type of behavior is more localized on the oxygen than in the p-nitrophenqualitatively what might be predicted in view of the oxide case where the negative charge may be delocalized general correlation between AS" and A V O reported by over the entire molecule (electromeric interaction). Hepler for the ionization in water of a wide variety of The entropies of ionization of m- and p-cyanophenol structurally different acids. l 4 Quite surprisingly, the (-21.8 and -20.0 cal/(deg mol), respectively) and mpartial molar volumes of the isomeric sodium salts were and p-formylpheno14r6(- 23.4 and - 20.2 cal/(deg mol), found to be almost identical (within experimental error) respectively) are also consistent with the above interindicating that the origin o f the A v ; " difference lies expretation based upon the assumption that solvation clusively with the un-ionized forms. This appears to be differences are more important for the ionized forms of in direct contradiction to the interpretation given above the acids than for the un-ionized.6 We wish to report for the origin of the difference in entropy changes. data which bear on this explanation. Based upon the data presented in this communication, The volume changes on ionization (AVi") of m- and it appears that the solvation contribution to differences p-nitrophenol and m- and p-formylphenol have been in acidity of isomeric phenols may have their primary determined in water at 25". Table I contains the inorigin in solvation differences in the un-ionized forms. Acknowledgment. The work was supported by the Table I. Partial Molar Volume and Volumes of Ionization. Department of the Interior, Office of Water Resources in Water a t 25cb,c Research, as authorized under the Water Resources Acid BoH* Boxa+A-dB o H t A - e AB' Research Act of 1964, and by the National Science Foundation (Grant No. G P 14437). m-Nitrophenol 99.71 85.67 86.87 -12.84 Influence of Solvation Factors on Acidity. Volumes of Ionization (A vi of the Meta and Para Isomers of Nitrophenol and Formylphenol in Water at 25
">
p-Nitrophenol m-Formylphenol p-Formylphenol
98.23 97.87 96.94
85.71 83,32 83.53
86.91 84.52 84.73
-11.32
- 13.35 -12.21
+
Ionization process: H A H" A-. All volumes are in units of millilitersimole. c As a n experimental check of the method and procedures, the A V , ' of phenol was determined (-18.01 0.20). This compares favorably with the literature value ( - 17.3 I 1.0): S. D. Harmann and S. C. Lim. Aust. J . Chem.. 7, 329 (1954). The difference between the lowest observed apparent molar volume for each salt and the 9"was less than 0.1 ml/mol in !a cases. '-Found from the values in the previous column by adding V'H - -
*
(10) F. J. Millero, Rec. Sci. Itistrum., 38, 1441 (1967). (11) E. J.I
