Decomposition of Crystal Violet

Crystal violet (aka gentian violet and numerous alternative trade names) reacts with alkali to form a colourless compound as shown below.

    crystal violet

The reaction can be followed using a colorimeter to measure the disappearance of the violet colour.


eye-protection Solutions of crystal violet must be prepared with care since the material is harmful if swallowed, a severe eye irritant and there is limited evidence that it may cause cancer.

The dilute solutions of crystal violet used for the investigations here do not constitute a hazard (final concentration in the cuvette typically 4μg per cm3).

Solutions of sodium hydroxide between 0.05M and 0.5M should be labelled ‘Irritant’. Concentration greater than 0.5M should not be needed and it is not difficult to achieve good results with concentrations lower than 0.05M.

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Transmittance vs Time

    The results shown in below were obtained using cuvettes containing 4cm3

    2cm3 of NaOH at twice the final concentration shown was placed in the cuvette and the reaction started by adding 2cm3 of crystal violet solution containing 16μg of crystal violet (8mg per litre prepared by dilution).

    transmittance vs time

Effect of NaOH concentration

    NaOH concentration
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