Kinetics: Fading of Triphenylmethane Dyes

Computer-Interfaced Experiments - Absorbance Measurement

Kinetics: Fading of Triphenylmethane Dyes - Pseudo First Order Reaction
Objectives: Determination of Rate Constants and Activation Parameters

Peter Keusch

Datalogging and data analysis using the Program CHEMEX and the Analog-Digital-Converter CHEMBOX
IBK electronic + informatic

German version

Chemicals:
5 · 10 ^-5M aqueous fuchsin solution (17 mg fuchsin dissolved in 1000 mL H₂O)
5 · 10 ^-5M aqueous crystal violet solution (21 mg crystal violet dissolved in 1000 mL H₂O)
5 · 10 ^-5M aqueous malachite green solution (18 mg malachite green dissolved in 1000 mL H₂O)
0.1 M NaOH

Apparatus and glass wares:
magnetic stirrer hotplate
magnetic stirring bar
stirring bar remover
crystallizing dish 140 × 75 (for water bath)
contact thermometer
thermometer 0 - 50°C (resolution: 0.1°C)
4 volumetric pipette 2 mL
pipette bulbs
photometer fitted with recorder output
round, tube shaped cuvettes
disposal containe

Hazards and safety precautions:

Crystal violet may cause cancer. Severe eye irritant. Harmful by inhalation, ingestion and through skin contact.

Malachite green is harmful if swallowed. Contact with skin or eyes may cause irritation.

Safety goggles and protective gloves required. The preparation of the triphenylmethane dye solutions is carried out under a fume hood!

Theoretical background:

Triphenylmethane dyes are water-soluble organic compounds that contain a colored cation. The intense color of this ion is due to the extended conjugated system of alternate double and single bonds. The reaction of OH^- ion with triphenylmethane dyes results in a disruption of the conjugation and loss of color. The cation is converted into a (non-resonant) carbinol base.

equation

Fig. 1: Fading of crystal violet

Since the absorbance of the violet colored triphenylmethane cation is directly proportional to its concentration, the kinetics of the reaction can be determined by measuring the change in absorbance as a function of time.

The reaction is first order in dye and in hydroxide ion.

The following experiments are carried out in excess of hydroxide. Because one of the reactants in the rate equation is present in great excess over the other in the reaction mixture the resction is pseudo first order.

Kinetic equations (Download)

Calibration of the photometer and the matching of the program Chemex are carried out analogical to the procedure described under Bromination of reactive Aromatics.

The following absorption maxima are used: fuchsin 545 nm, crystal violet 590 nm and malachite green 620 nm (see Absorption Maxima of Triphenylmethane Dyes).

Experimental procedure:

Fig. 2: Experiment set-up

2 mL of the triphenylmethane dye solution are pipetted into a cuvette. 2 mL of 0.1 M NaOH are given into a further cuvette. The two cuvettes are placed in a water bath, in which a contact thermometer and a thermometer with a resolution of 0.1°C are immersed (Fig. 2). A reaction temperature below the room temperature is obtained and maintained by careful addition of ice or cold water to the water bath. After thermal equilibrium has been reached (15 minutes) the reaction temperature is read to the nearest 0.1°C.

The hydroxide solution is added rapidly to the dye solution and the cuvette is shaken. The outside of the cuvette is wiped to dry. Immediately the cuvette is placed into the sample compartment of the photometer and the sensing software is started. The measuring interval is 1 second.

The change in transmittance and in absorbance is displayed simultaneously on the measuring screen.

The following temperature ranges are recommended: for the fading of malachite green 17 - 25°C, of fuchsin 20 - 26°C, of crystal violet 39 - 49°C. In each case the reaction is studied at three temperatures to determine the activation parameters.

The in-situ determination of the reaction rate on the basis of a continuous logging of photometrical data is allowed in rapidly proceeding reactions (see temperature constancy).

Data analysis using Excel - determination of the rate constants and the activation parameters:

After creating a plot of A against t (Fig. 3), the absorbance values are converted. Thus a plot of -lnA versus t is allowed (Fig. 4).

Excel

Tab. 1: Malachite green measured values A (t) calculation of -lnA

absorbance

Fig. 3: Malachite green Temperature effect 1: 17°C 2: 21.1°C 3: 25°C

proportional constant

Fig. 4: First order kinetics plot determination of the pseudo-first order rate constant k'

The constant k' can be determined from the plot of -lnA against t according to the equation (10) Kinetic equations (Download).

Measurement	T [ °C ]	k' [ s^-1 ]	k [ L · mol^-1 · s^-1 ]
1	17	0.0448	0.896
2	21.1	0.0599	1.198
3	25	0.0811	1.622

Tab. 2: Calculation of the true rate constant k
(According to the reaction conditions: k = k' / 0.05)

If the reaction temperatures and the corresponding rate constants are entered into the table of the Excel file Activation parameters (Download), then all activation parameters
(Tab. 4) will be calculated and the plots according to the ARRHENIUS and EYRING relation will be generated (Fig. 5).

Tab. 3: Calculation of the activation parameters

ARRHENIUS and EYRING

Fig. 5: ARRHENIUS (1) and EYRING plot (2)

Under the described conditions also fuchsin and crystal violet are converted with sodium hydroxide solution and the activation parameters are determined.

Measurement	malachite green	fuchsin	crystal violet
E_a [ kJ · mol^-1 ]	53.3	53.7	57.6
lnA	21.99	21.86	21.39>
DH ^‡ [ kJ · mol^-1 ]	50.9	51.2	54.9
DS ^‡ [ J · mol^-1· K^-1 ]	- 70	- 71	- 76
DG ^‡ [ kJ · mol^-1 ] bei 298.15 K	71.8	72.4	77.5

Tab. 4: Comparison of the activation parameters

Discussion:

The branched chromophoric trityl system is disrupted by the reaction of the central C-atom with the nucleophile OH^- (Fig. 1). The three triphenylmethane dyes show a different reactivity (crystal violet << fuchsin < malachite green):

Fig. 6: Crystal violet (1) fuchsin (2) malachite green (3)

Fig. 4: Ring twisting
steric repulsion between ortho-hydrogen atoms (blue)

The geometrical structure of the triphenylmethyl (trityl) system seems to be primarily responsible for the rate by which reactions takes place at the central carbon atom.

· Unlike crystal violet, malachite green has only two out of three phenyl rings substituted with dimethylamino groups. The two substituted rings are nearly planar aligned. The third aromatic ring is turned out of the plane of the coplanar rings. The partial planarity of malachite green leaves the central carbon atom accessible for attack by the nucleophilic hydroxide ion.

· Crystal violet, on the other hand, reacts with hydroxide ion considerably slower. X-ray studies indicate that the structure of crystal violet (like fuchsin) resembles a three-bladed propeller. The planes of the phenyl rings are twisted out of the plane defined by the central carbon atom and its three bonds. The dihedral angle between the phenyl rings and the central coordination plane is 27.7°. The twisting can be understood as a compromise between the ortho-ortho steric repulsion involving aromatic hydrogens on adjacent rings and maximum resonance stabilization of the extended conjugated p-system which favours a planar conformation. Due to the propeller-shaped structure the approach of the nucleophile to the reaction center is sterically hindered.

References:
Computer-Interfaced Experiments Kinetics: Fading of Phenolphthalein in Alkaline Solution
Computer-Interfaced Experiments Absorption Maxima of Triphenylmethane Dyes
Computer-Interfaced Experiments Light Absorption of Triphenylmethane Dyes
Microscale Projection Experiments Light Absorbtion of Triphenylmethylium Salts
Microscale Projection Experiments Crystal Violet - a pH Indicator
Demonstration Experiment on Video: Crystal violet - a pH Indicator

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