order to study of acid–base indicator properties of prepared compounds, dyes 3d with electron-withdrawing group (R_CN), and 3e including electron-donating group (R_OEt) were selected. The instru-mental pH values of the studied compounds were also determined spec-
trophotometrically in 80:20 (v/v) methanol/water mixtures (aqueous
hydrochloric acid and sodium hydroxide) at 25 ± 2 °C. A digital pH
meter Genway model 3505 was employed for determination of pH.
The instrument was accurate to ±0.01pH unit. It was calibrated using
two standard Genway buffer solutions at pH 4.01 and 7.00. The instru-
mental pH values were corrected for non-aqueous solutions according
to the literature [34]. The absorption maxima and spectra recorded for
dyes3d and 3e are given in Table 2 and Figs. 3–4. As shown in Table 2
and Fig. 3, the shape, intensity and wavelengths of recorded spectra