Medium effects on the dissociation of weak acids in methanol-water solvents

Abstract

A spectrophotometric method has been used to determ in e the di ssociation constants of o-chloroanilinium ion, m-n itroanili ni um ion , and 4-chloro-2,6-din itrophenol in methanolwater solvents at 25°C. The ranges of solvent composition s (in weigh t percent metha nol) were as foll ows: o-ehloroan ilinium ion , 0 to 99.9; ?n-nitroanilinium ion, 0 to 93.7; a nd 4-ehloro-2,6-dinitrophenol, 0 to 33.4. The 1)K of t h e first two ac ids f a lls with addition of methanol and passes through a minimum when the mole fraction of methanol in the solve nt mixture is abo ut 0.7 . The pI( of the substituted phenol, however, rises as the di electric constant of t he solvent is decreased by addition of m ethanol. It is suggested that the total m edium effect for both t y pes of acid can be explain ed by the superposit ion of an electrostatic e ffect and a non electrostatic effect. The latter is a constant for each particu lar s olvent co mposition and probably characterize's the acid-base proper t.v of the m ed ium itself.

The dissociation constant of m-nitroanilinium ion in water-tetrahydrofuran solvents at 25 C

“…The figure also contains a plot of the dissociation constant of m-nitroanilinium ion as a function of E-1 , in water-methanol solvents, using 1 data published previously [5]. This figure emphasizes once again, what has been said several times before in earlier papers from this laboratory, that the Born effect does not give a complete description of solvent <: effects on acidity, particularly if the macroscopic dielectric constant is used.…”

“…The dissociation constant of m-nitroanilinium ion in a number of water-tetrahydrofuran solvents was determined by the spectrophotmetric method described in several previous publications [5][6][7][8][9][10]; the notation used in this paper is that used before. The validity of Beer's Law for this system was demonstrated by measurements of optical density of alkaline solutions in 62.5 wt percent tetrahydrofuran with the con-' centration of m-nitroaniline varying from 2 X 10-4 to 8 X lO-4m.…”

“…The medium effect on the dissociation constant of m-nitroanilinium ion has been determined [5] in watermethanol mixtures as solvent; this paper is concerned · Present address: Oe parlme nt of C he mi s try, S lal e Uni vers it y of New York at Binghamton , Binghamton , New York 13901. 1 Fi gures in brac ke ts indica te th e lit era ture re fe re nce at th e end of thi s paper.…”

Study of the Dissociation Constants of Some Weak Indicator Bases in Ethyl Cellosolve - Water Solvent Mixtures

“…Thus instead of having the structure (H 2 O) 4 4 H + is less than that in the compact structure. The latter will be holding the proton more firmly [20][21][22][23][24] . Figure 5 shows the effect of the changing composition of some organic solvents on the acidity function, for a definite acid concentration in case of pnitroaniline.…”

Acid-base behavior in 50-percent aqueous methanol: Thermodynamics of the dissociation of protonated tris(hydroxymethyl)aminomethane and nature of the solvent effect

“…These "observed " values of p(sK) can be re presented within a bout 0.001 unit by the fo llowing eq uation , which is of the form proposed by Harned and Robinso n [8]: where T is the temperature in oK. The p(sK) calculated by eq (2) is given in the next to the last column of table 2_ It is evident that the protonated form of tris(hydroxymethyl)aminomethane is a stronger acid in 50 wt percent methanol than it is in pure water, where the pK is 8.072 at 25°C [3,5] _ This increase of strength is in accord with earlier measurements of the solvent effect of methanol on acids of this charge type [9, 10,-11] _ In general, the p(sK) goes through a minimum in the range of solvent compositions between 70 and 90 wt percent methanol and then rises sharply as the concentration of solvent water becomes low_ It has been shown that this behavior cannot be explained by the electrostatic effects of the changing dielectric constant alone_ Nevertheless, qualitative consistency among the solvent effects on pK for acids of different charge types can be achieved if a second parameter characterizing the alteration in the basicity of the medium is introduced [12] …”