Crystal Violet - a pH Indicator

Demonstration Experiment on Video

Crystal Violet - a pH Indicator

Objective: Protonation of Crystal Violet

Peter Keusch

German version

Chemicals:

5 × 10 ^{- 5}M aqueous solution of crystal violet ( 21 mg / L H₂O)

1 N HCl

conc. HCl

2 N NaOH

Apparatus and glass wares:

hot plate

thermometer

4 conical measures, graduated, 500 mL

3 glass stirring rods

beaker 2000 mL

beaker 50 mL

beaker 100 mL

snap-cap vial 10 mL

volumetric pipette 3 mL

1 pipette bulb

2 measuring cylinders 100 mL

Hazards and 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.

Sodium hydroxide solution is corrosive! Contact with skin can cause irritation or severe burns and scarring with greater exposures. Swallowing may cause severe burns of mouth, throat, and stomach.

Safety goggles and protective gloves must be worn. The experiment should be performed under a portable fume cupboard giving all-round visibility!

Experimental procedure:

Four conical measures are each filled with 75 mL of aqueous crystal violet solution.The measures are made up to the 400 mL mark with dist H₂O heated to 40°C. Hydrochloric acid is added to two of the solutions. Another solution is mixed with aqueous NaOH.

conical measure 1	400 mL of crystal violet solution	-
conical measure 2	400 mL of crystal violet solution	3 mL of 0.5 M HCL
conical measure 3	400 mL of crystal violet solution	35 mL of conc. HCl
conical measure 4	400 mL of crystal violet solution	50 mL of N NaOH

Results:

A color change can be observed in the glasses 2 and 3. The violet color turns blue and yellow, respectively. The violet solution in conical measure 4 fades gradually and becomes colorless.

Video clip (Download RealPlayer .rm file)
The fading of crystal violet in alkaline solution (conical measure 4) is shown in fast motion.

Discussion:

· The crystal violet cation present in the aqueous solution (pH = 7) of conical measure 1 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 the positive charge on the central carbon atom. The blue solution in measure 2 contains the dication. The color of the dication resembles that of malachite green. Two aromatic rings (equation (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 blocks finally all three amino groups. The color of the trication (conical measure 3) formed is similar to the color of the triphenylmethyl cation.

· In the alkaline medium (measure 4) the colorless carbinol base is formed (equation (2)).

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

References:

Absorption Maxima of Triphenylmethane Dyes

Light Absorbance of Triphenylmethane Dyes - Determination of the Absorbance Maxima - Triphenylmethyl Cation, Crystal Violet, Malachite Green, Fuchsin, Protonated Crystal Violet and N-substituted Fuchsin

Light Absorbance of Triphenylmethylium Salts

Kinetics: Fading of Triphenylmethane Dyes - Pseudo First Order Reaction

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