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Computer-Interfaced Experiments - Absorbance Measurement Light Absorption of Triphenylmethane Dyes Fuchsin, Protonated Crystal Violet and N-substituted Fuchsin Peter Keusch |
German version
Hazards and safety precautions:
Safety goggles and gloves must be worn. The preparation of the corresponding solutions is carried out in a fume hood!
| Experiment 1: Absorption maxima of triphenylmethyl cations, crystal violet and malachite green Experimental procedure: In order to record the absorption spectra the following solutions are used: Solution 1: solution of triphenylcarbinol in conc. H2SO4 Solution 2: 5 · 10 -5 M aqueous solution of crystal violet, in the ratio 1:2 diluted with water Solution 3: 5 · 10 -5 M aqueous solution of malachite green Using solution 1 the wavelength range from 360 nm to 600 nm is considered. Using solution 2 and 3 the series of measurements begins at 400 nm and ends at 640 nm. The recorder output of the photometer is connected to the input B of the INTERFACE. Matching of the program 'Multimeter': - In the program 'Measuring and Evaluating' the subprogram 'Multimeter' is activated and via the menu item <F3> 'Select measur. quantities'®'Reselect channel B' the quantity 'Voltage DC' is selected. - After the program has been switched to <F4>'Automatic/Param./Select formula'® Enter parameter wavelength is entered. Also l and nm are entered. l stands for 'Physical symbol' and nm for 'Physical unit'. - Under the menu item 'Enter formula' A is entered. A stands for 'Physical symbol'. The 'No. decimal places ' is set to 4. According to A = 2 - logU·100 (CASSY registers a voltage of approx. 1 V at 100% transmittance) the beginning of the formula A (n, l, U) = is completed by entering - lnU / ln10 (Fig. 1). 
Fig. 1: Matching of the program Measurement: Using the wavelength knob a wavelength of 360 nm is selected. After zero calibration is completed, a cuvette filled with water is placed into the sample compartment. With the sample cover closed, using the light control knob the meter needle is adjusted to "0" on the absorbance scale (100 % T). A voltage of approx. 1 V is displayed on the measuring screen. Now the cuvette (filled with water) is replaced by a cuvette, those to 2/3 with the dye solution filled is. After striking the function key <F1> the appropriate wavelength is to be entered. By pressing ¿ the measured value is confirmed. Afterwards the wavelength is changed by increasing the wavelength by 5 nm. The cuvette filled with water again is placed into the light-tight sample holder. The needle is adjusted to "0" on the absorbance scale. Now the cuvette containing the dye solution is inserted and the absorbance is monitored at the appropriate wavelength. Before the data are stored, by means of <F7>'Select representation'® 'Display' l is selected for the x-axis and A for the y1-axis. Under 'Select graph options' one ensures that the data points are displayed as crosses. Graphical analysis: A direct comparison of the measurements is permitted in an overlay mode, which can be activated by switching to <F8> 'Disc operations'®'Multigraph on'. The desired series of measurements are selected individually, in order to represent them together in the main menu under <F6>'Evaluate in graph'. By pressing <F3> and by entering the number of the graph considered is drawn a best fit curve through the measuring points (Fig. 2). 
Fig. 2: Triphenylmethyl cation: lmax = 445 nm (1) crystal violet: lmax = 590 nm(2) malachite green: lmax = 600 / 425 nm (3) Discussion: · In conc. sulfuric acid from the triphenylcarbinol a colored species, the triphenylmethyl cation, is reversibly formed. It can be isolated using weakly nucleophiles (e.g. BF 4 -, SbCl 6 -) and can be determined X-ray-structure-analytically.  The carbocation center is in conjugation with three benzene rings, whereby the positive charge is strongly delocalized. Thus the triphenylmethyl cation possesses ten resonance structures, in which the positivecharge is distributed on six ortho and three para positions. · The extent of the bathochromic shift increases in the order triphenylmethyl cation (1) < crystal violet (2) < malachite green (3). The position of the absorption maxima of the mentioned triphenylmethane dyes depends on the geometry of the chromophore and on the character of the (para-) substituents on the phenyl rings.  Fig. 3: Triphenylmethyl cation (1) crystal violet (2) malachite green (3) 
Experiment 2: Absorption maxima of the protonated crystal violet Experimental Procedure: Immediately before the recording of the individual absorption spectra the following solutions are prepared: Solution 1: 10 mL 5 · 10 -5 M aqueous solution of crystal violet + 20 mL water Solution 2: 100 mL 5 · 10 -5 M aqueous solution of crystal violet + 100 mL water + 5 mL 1 N HCl Solution 3: 20 mL 10 -4 M aqueous solution of crystal violet + 2 mL conc. HCl The absorbance is measured in the intervall from 400 nm to 640 nm (solution 1 and 2) and in the intervall from 360 nm to 570 nm (solution 3). 
Fig. 5: Crystal violet cation (1) dication (2) trication (3) 
Fig. 6: Crystal violet cation: lmax = 590 nm (1) dication: lmax = 600 / 425 nm (2) trication; lmax = 410 nm (3) Discussion: The color changes of crystal violet (Fig. 5) are based on the blocking of the auxochromes by protons (Fig. 7). 
Fig. 7: Protonation of crystal violet - crystal violet cation (1) crystal violet dication (2) crystal violet trication (3) · The blue solution contains the dication. The spectrum of the dication is very similar to that of malachite green (Fig. 8). Two aromatic rings are in the same plane (Figure 7, rings highlighted in white). The p - electron conjugation between coplanar rings is at a maximum. The absorbance maximum is shifted to longer wavelengths in the visible region of the spectrum). The third (unsubstituted) ring does not participate in the resonance of the two coplanar rings. It is twisted out of the plane .  Fig. 8: Crystal violet dication: lmax= 600 nm / 425 nm (1) malachite green: lmax= 600 / 425 nm (2) · A large excess of acid finally blocks all three amino groups (Fig. 7). The color of the crystal violet trication formed thereby corresponds to that of the triphenylmethyl cation (Fig. 9).  Fig. 9: Crystal violet trication lmax= 410 nm (1) triphenylmethyl cation lmax= 445 nm (2) The crystal violet trication is unstable. The addition of H2O destroys the congugation between the aromatic rings and results in the formation of a colorless carbinol base (Fig. 10). Therefore, the recording of the absorption spectrum of the triphenylcarbenium ion is to be carried out speedily. 
Fig. 10: Decolorization of crystal violet trication Experiment 3: Absorption maxima of fuchsin and N-substituted fuchsin SCHIFF'S REAGENT (fuchsin-sulfite reagent) is mixed with some drops of formaline. · Sulfurous acid decolorizes fuchsin. The hydrogensulfite ion is added to the central C-atom of the triphenylmethyl compound and thus the branched conjugated system is disrupted (Fig. 11). 
Fig. 11: Decolorization of fuchsin with sulfurous acid SCHIFF'S REAGENT reacts with aldehydes regenerating the chromophore system. Via a carbinolamine a diimine is formed, which reacts with sulfurous acid to give a resonance stabilized cation (Fig. 12). The reaction is kinetic controlled. 
Fig. 12: Regeneration of the chromophoric system The addition of bisulfite to aldehyde is a competing reaction. The reaction is thermodynamic controlled.   Fig. 13: Substituted 'fuchsin'lmax= 575 nm (1) fuchsin lmax= 540 nm (2) The pH-dependent color changes of basic triphenylmethylium salts are based on the following structural modifications: · Blocking or regeneration of auxochromes by acid-base reactions · Disruption or regeneration of the chromophoric system. Reference: Computer-Interfaced Experiments Absorption Maxima of Triphenylmethane Dyes Computer-Interfaced Experiments Kinetics: Fading of Triphenylmethane Dyes - Pseudo First Order Reaction Computer-Interfaced Experiments Kinetics: Fading of Phenolphthalein in Alkaline Solution 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
Index of CASSY Experiments 
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