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Demonstration of Microscale Projection Experiments - Chemistry en miniature

Crystal Violet - a pH Indicator

Objective: Protonation of Crystal Violet

Peter Keusch





 German version




Chemicals:
10 - 4 M aqueous solution of crystal violet
1 N HCl
conc. HCl
1 N NaOH


Safety precautions:

Crystal violet may cause cancer. Severe eye irritant. Harmful by inhalation, ingestion and through skin contact.
Conc. hydrochloric acid is extremely corrosive. Inhalation of vapour can cause serious injury. Ingestion may be fatal. Liquid can cause severe damage to skin and eyes.

Safety goggles, protective gloves and effective ventilation required.


Experimental procedure:

1 mL of aqueous crystal violet is pipetted into each of three tubes. Afterwards hydrochloric acid is added to two solutions.

R1
1 mL of crystal violet solution
R2
1 mL of crystal violet solution    2 drops of 1 N HCl
R3
  1 mL of crystal violet solution    4 drops of conc. HCl  


In a second experiment the reaction of crystal violet with sodium hydroxide solution is demonstrated.

Test tube 3
1 mL of crystal violet solution
Test tube 4
  1 mL of crystal violet solution    2 drops of 1 N NaOH  



Results:

Color
Photo1
Color
Photo2
Test tube 1 violet Test tube 4 violet
Test tube 2 blue Test tube 5 Decolorization
  after approx. 2 minutes  
Test tube 3 yellow





Foto1
Photo 1

Foto2
Photo 2
























Discussion:

· The crystal violet cation present in the aqueous solution (pH = 7) of T1 is stable. The high stability of the propeller shaped crystal violet cation may be attributed primarily to the strong electron donating dimethyl amino groups that delocalize (stabilize) the positive charge on the central carbon atom. The green-blue solution in T2 contains the dication. The color of the dication resembles that of malachite green. Two aromatic rings  (1),  highlighted in grey) are aligned on the same plane (degree of conjugation between the coplanar rings is at a maximum, absorbance maximum is shifted to longer wavelengths in the visible region of the spectrum). The third ring is twisted out of the plane.


An excess of acid finally blocks all three amino groups. The color of the trication corresponds to the color of the triphenylmethyl cation.

· After approx. 5 minutes (water in the cuvette is 40 °C) the green-blue color in R2 turns green, while the yellow solution in test tube 3 is completely decolorized. The trication is unstable. The addition of H2O disrupts the system of conjugated double bonds  (2).


· In the alkaline medium (test tube 5) the colorless carbinol base is formed (Photo 2V)   (3).



Summary:

The pH-dependent color changes of basic triphenylmethylium salts are based on the following structural modifications:

· Blocking or regeneration of auxochromic groups by acid-base reactions

· Disruption or regeneration of the chromophoric system


Reference:
  Demonstration Experiment on Video Video:   Crystal Violet, a pH Indicator


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