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Interfaced Experiments - Absorbance Measurement Peter Keusch |
IBK electronic + informatic |
Hazards and safety precautions:
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. 
Fig. 1: Fading of crystal violet
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2 mL of triphenylmethane dye solution are pipetted into a cuvette. 2 mL of 0.1 M NaOH are
transferred 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. When 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 rapid reactions (small change in temperature during te reaction). |
malachite green bei 298.15 K |
 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. |
Demonstration Experiment on Video: Crystal violet - a pH Indicator
