Colorless solvents turn green, blue, violet and red when added to the same substance.
name: Reichardt’s dye change solution color due to changes of the solvent polarity.
Wear goggles and disposable gloves. The methanol is very toxic, be extremely careful in handling it.
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A number of substances are able to change color due to changes of the solvent polarity. This phenomenon is called solvatochromism. Reichardt’s dye is one of such compounds and solvatochromism is the reason it turns different colors in various solvents. In Reichardt’s dye molecule charges are situated in different parts of molecule and it could be considered a dipole in its ground state. When the molecule adsorbs light it moves to the exited state:
C41H29NO (ground state) + hv → (C41H29NO)* (exited state)
Charge transfer transition of Reichardt’s dye molecule lies in a visible region and is highly medium dependent. Polar solvents stabilize the polar ground state, nonpolar solvents – the nonpolar exited state. The color change of substances that demonstarate solvatochromism is frequently used for estimating solvent polarity. The Reichardt’s dye is characterized by bathochromic or red shift when going from nonpolar to polar solvents. This means that it is easier to excite the molecule in a nonpolar solvent. To move from relatively nonpolar acetone to polar methanol one needs more energy to excite the molecule. The light with longer wave length has the lowest energy (red light in our case). The light with shorter wave length has the highest energy (green light in our case). So in nonpolar solvents when low energy light is enough to excite the Reichardt’s dye molecule the red light is adsorbed and the solution turns green. Oppositely in polar solvents green light is adsorbed and the solution turns red.