The protonation of weak bases. A study of the protonation of some nitroaniline indicators and the determination of the H 0 values of aqueous sulphuric acid

Abstract

The protonation of some nitroaniline indicators has been studied in aqueous sulphuric acid over the temperature range 15-55".The log / values of these indicators decrease with increasing temperature and the pKBH+ values vary linearly with temperature. These log / and pK, , + values have also been used to calculate the Hammett acidity function (H,) of the acid media over the temperature range 15-55". MANYweak organic bases are protonated only in concentrated acid solutions to give their conjugate acids. The measurement of the acidity of such solutions poses a difficult problem which Hammett attempted to overcome by introducing the H , acidity function concept, whereby the acidity of a solution is determined relative to standard basic indicators. For the correct formulation of the H , scale, as expressed by equation (l), the conditions that must be met by the indicators are (i) that the pK values of the indicators are accurately established, and (ii) that the indicators are valid H , indicators such that the activity coefficient ratio of the free base to the protonated base for the compounds B and C are equal. The relationship between p K o ~+ and pKBH+ (which throughout the rest of this paper is written simply as pK) as expressed in equation (2) can

Extrapolation from concentrated to dilute aqueous acids

“…For example, the Long and Paul ( I I) -Jorgenson and Hartter (12) H , scale for H2S0, as compiled by Boyd (1 b) is only 0.10 more positive than ours, and the Bascombe and Bell (13) H , scale for H2S04 is also more positive but by a smaller amount: 0.02 to 0.05. More recent determinations of H , in H2S04 were made by Tickle, Briggs, and Wilson (14), whose scale is 0.07 to 0.14 more positive than ours, and also by Johnson, Katritzky, and Shapiro (15), whose scale is 0.01 and 0.07 more positive than ours.…”

Thermodynamics of protonation of weak bases in H2SO4-H2O media, determined using the excess acidity method

“…Results obtained by the different groups agree quite well, on the whole (e.g. at 25"C), although the measurements by Marcus' group at 60°C (17) are somewhat divergent, as are the lowest points at the higher temperatures by Tickle et al (8). In general divergent points tend to cancel out, as can be seen in Fig.…”

“…As an example chosen at random, the experimental data for 2,5-dichloro-4-nitroaniline (7,8,17,26) are illustrated in Fig. 1.…”

“…The purpose of this paper is to see if this practice is justified in general, and to calculate enthalpies and entropies of protonation in the aqueous standard ' Revision received January 18, 1984. state, by extrapolation, from existing ionization ratio data obtained as a function of temperature. Many of these data are provided by major studies of the variation of the Ho (7,8), HR (9, lo), and HA (I I) acidity functions with temperature. The excess acidity X is easily corrected to different temperatures by taking advantage of its definition as a combination of transfer activity coefficients as discussed previously (4), by multiplying by the absolute temperature ratio: XT = 298.15X/T.…”

The acid-catalyzed hydrolysis of phosphinates. III. The mechanism of hydrolysis of methyl and benzyl dialkylphosphinates

“…In subsequent correlations the Hammett acidity functions Do and Ho are also used. The Do values are corrected for temperature (9,10); however, the H, values are for 25°C (11). Rate-pH profiles are given in Figs.…”