Crystal Violet - Dye Colour

Dye Colour

When dissolved in water the dye has a blue-violet colour with an absorbance maximum at 590 nm and an extinction coefficient of 87,000 M−1cm−1. The colour of the dye depends on the acidity of the solution. At a pH of 1.0 the dye is green with absorption maxima at 420 nm and 620 nm while in a strongly acidic solution (pH of -1), the dye is yellow with an absorption maximum at 420 nm.

The different colours are a result of the different charged states of the dye molecule. In the yellow form all three nitrogen atoms carry a positive charge, of which two are protonated, while the green colour corresponds to a form of the dye with two of the nitrogen atoms positively charged. At neutral pH both extra protons are lost to the solution leaving only one of the nitrogen atoms positive charged. The pK_a’s for the loss of the two protons are approximately 1.15 and 1.8.

In alkaline solutions, nucleophilic hydroxyl ions attack the electrophilic central carbon to produce the colourless triphenylmethanol or carbinol form of the dye. Some triphenylmethanol is also formed under very acid condition when the positive charges on the nitrogen atoms lead to an enhancement of the electrophilic character of the central carbon which allows the nucleophilic attack by water molecules. This effect produces a slight fading of the yellow colour.