[Damiano0707]

Hi
I can't understand why indophdnol is blue in a basic medium and red in an acid one. Shouldn't it be the opposite? The structures in the to mediums are in the attachment. In a basic medium the conjugated system is bigger hence wave length would be bigger than indophenol in an acid medium.

[Corribus]

To truly make sense of it, you'd have to look at the actual absorption spectra, not inspection of visual color. The color you see is a consideration of (a) the color distribution of the incident light, (b) the relative reflection and absorption of these wavelengths by the sample, and (c) the relative sensitivity of your eyes to different light wavelengths. If there are multiple absorption bands, and god forbid any fluorescence, what your eyes see tell you very little concrete about the energetic states of the molecules in the sample. All thing being equal, when something looks red, it usually absorbs more to the blue, and vice-versa. But again, it's hard to say much from appearance alone - the spectrometer doesn't lie, though. It is a well calibrated instrument that compensates for the potential sources of variation described above.

Also, the "size" of the conjugated system doesn't always mean a whole lot, especially when comparing several species in which the spatial extent of delocalization is similar. In this case, charge transfer contributions and so forth can really influence the wavelength-dependent absorptivities of the compounds.