I therefore investigated derivatives of benzene-sulphonic acids and evaluated the measured acidity constants with Hammett's equation for m- and p-substituted benzenes. The accompanying table shows the results of the potentiometric determination of the pK-v in water at an ionic strength JJL = 0-10 and the a-values calculated with these constants. ACIDITY CONSTANTS AND 7-VALTTES(ionic strength fx = = 0-10 : 25 0?C.) PKX ffm-SOa" op-SOfAniline 4-71 Metanilic acid 3-650-39 Sulphanilic acid 3-120-58 Phenol 9-86m-Phenolsulphonic acid 9-070-39 2>-Phenolsulphonic acid 8-700-58 Benzoic acid 4-09m-Sulphobenzoic acid 3-780-31 -Sulphobenzoic acid 3-720-37 These a-values of the m- and p-sulphonic group may be compared with those of the other substituents mentioned in the literature. From the table it is apparent that the effect of the p-SO3~ group is not accurately described by a single a-value. The constant for _p-sulphobenzoic acid is 0-21 smaller than the values of the constants for sulphanilic acid and p-phenolsulphonic acid. Comparable differences have been observed for p-NO2 (0-49)3, p-CH3CO (0-44)4, p-CN (0-34)5 and?>-CH3SO2 (0-42 and 0-26, respectively)46. By analogy with the interpretation for these substituents, we conclude that there is a conjugative effect in the sulphonic ions, too, and that the sulphur atom in sulphonic acids is capable of extending its octet to a decet (or dodecet) structure. There is a peculiarity of the a-constants for m-and p-sulphonie groups. So far as I know, this is the first negatively charged substituent for which a-values for several equilibria have been calculated. All other substituents have a net charge of zero : only one of the groups investigated has a positive charge, namely, the trimethylammonium ion studied by Roberts and co-workers7. The pi?2-values shown in the table relate to the ionization of a second proton of the molecule (the first, more acidic proton refers to the ionization of the SO3H group). Therefore the acidity constants of these benzene sulphonic acid derivatives show a different kind of dependence on changes in ionic strength from the unsubstituted compounds phenol, anilinium ion and benzoic acid. This explains why the a-values for m- and?>-SO3~ (but not for electrically neutral groups) depend in a high degree on the ionic strength. The a-values calculated from acidity constants at ionic strength [x = 0-01 and 0-001 are smaller than those shown in the table. The observed differences of a-values for the?)-SO3~ group are important evidence of the mesomeric effect. The acidity constants at low ionic strengths were measured spectrophotometrically in the range 220-255 mjx. A description of the experimental procedure as well as a detailed discussion of the results will be published later in Helvetica Chimica Acta.