Polar structure of some benzein indicators

Abstract

In the study of electronic effects associated with changes of color of indicators, the approximate electric moments have been measured for a-naphtholbenzein and thymolbenzein in benzene solution. The values obtained were 4.3 X 10-18 and 6.8 X 10-18 esu, respectively. These values are much lower than those predicted for the benzein type of molecule on the basis of t he static dipolar structure shown in figure 2. They are compatible, however, with the quinone-phenolate resonance theory and indicate that these molecules are predominantly in the structure shown as Air figure 1, with very little of the resonant dipolar form A 2•

Basic ionization constant of metacresolsulfonphthalein; pH values and salt effects

“…(6) The spectrophotometric data suggest that two separate reactions do not both take place simultaneously to any appreciable extent. Assuming then that the yellow form is a univalent negative ion, represented now as Y-, and, for the present, that the activity coefficient of the resonant ampholytic ion, represented now by R+-, is approximately unity at low concentrations, the Debye-Huckel theory yields the relation (7) in which z is unity; A and B have the values 0.5098 and 0.3298 f121, respectively, at 25° C; al is an adjustable parameter used to represent in angstroms the average distance of closest approach of ions; p. is defined as ~z~md2 as usual; and (:3 is an empirical correction factor. By arbitrarily assigning different reasonable values to aJ, a set of curves is obtained for pKo-log(jHjy-/jR+-) with small slopes which can be extrapolated accurately, as shown in figure 6.…”

“…Hence, as expressed linearly in eq 2, if a mixture of these two forms alone determines the observed absorp&If polar states (electronic tatitomers orelectromers) ofeacb ion, indicated by [+) and [-) signs in figUre 1, are present in instantaneous equilibrium witb tbe nonpolar forms. tbe absorption bands .may still be' used at any given temperature to calculate tbe over-all values of a and (1-a) for tbe red and yellow ions, including tbeir natural polar states or tbose "induced" by other ions [1,4,7,8 tion curve in their overlapping regions of spectral absorption, the isosbestic point must r emain unchanged with respect to both index and wave number for all stages of the color transformation. This may not be true when specific salt effects, association, or colored impurities affect this part of the spectrum, as shown in.…”

“…1) to the values of z and the activity coefficients. The yellow form will be assigned tentatively Z= 1 for the following reasons: (1) The work on the dipole moments of benzeins [7], whose quinone-phenol chromophores are identical with those of Y-, shows that the concentration of the resonant dipolar form is only a few percent of that of the ionic form If use is now made provisionally of the hypothesis of Lewis and Randall [18] that the activity coefficient of any univalent ion in dilute aqueous solution depertds only upon the ionic strength of the solution, a hypothesis that is also inherent in the Debye-Hiickel theory of dilute mixed electrolytes, the activity coefficients of the chloride and single-charged indicator ions would be approximately the same. Hence if we place JCI equal to jy-, we must get jR+-and .JjR+~ values appreciably less than unity, as shown in columns 4, 8, 12, 16, table 5.…”

Effects of hydrochloric acid and salts on the absorption of light by beta-naphthoquinonesulfonic acid

“…and the apparent general increase in absorption at all wave numbers in the ultraviolet 6 and visible with increased concentration of electrolyte, indicate that the effect of electrolytes on the absorption band (peak 26,500 wave number) is mainly a uniform increase in index at all wave numbers. There is, however, some flattening at the peak for high concentrations, and indications of a slight broadening of the band or a small shift in its position in the spectrum.…”

“…To minimize the effect of nonuniform changes in absorption in the more concentrated solutions, the average of the ratios of the molar indices, at 10 mJL intervals, for the solid portions of the curves (figs. 3 to 6) were used in the subsequent correIa tions.…”