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Demonstration of Microscale Projection Experiments - Chemistry en miniature

Negative Solvatochromism

Objectives: Hypsochromic Shift, Solvent Polarity

Peter Keusch




German version




Chemicals:
N-(4'-hydroxy-3',5'-diphenyl)-2,4,6-triphenyl-pyridinium-betaine - Reichardt's Dye    (I)
4-(4'-hydroxystyryl)-N-methyl-pyridinium-iodide   (II)
2-(4'-hydroxystyryl)-N-methyl-quinolinium-betaine   (III)<
acetone
isopropanol
96 % ethanol
n-butanol
methanol
chloroform
piperidine


Hazards and safety precautions:

Acetone, Ethanol and Isopropanol are highly flammable.
Methanol may be a reproductive hazard. Ingestion may be fatal. Risk of very serious, irreversible damage if swallowed. Exposure may cause eye, kidney, heart and liver damage. Chronic or substantial acute exposure may cause serious eye damage, including blindness.
Chloroform causes cancer in laboratory animals, and is IARC listed as a probable human carcinogen. Inhalation and ingestion are harmful and may be fatal. May cause reproductive damage. Irritant. Exposure to alcohol may increase toxic effects. Prolonged or repeated skin contact may cause dermatitis.
Piperidine is a poison. May be fatal if inhaled or swallowed. Severe irritant. Skin contact may cause severe irritation or burns. Contact with the eyes may lead to permanent damage.

Safety glasses and gloves must be worn. The experiments should be performed under a portable fume hood giving all-round visibility!


Experimental procedure:

1. Some crystals of phenol betaine (dye I) are dissolved in 1 mL of acetone (T1), isopropanol (T2) and ethanol (T3), respectively. A drop of piperidine is added to each of the solutions. A Pasteur pipettes containing some crystals of fuchson is placed in each of the three test tubes. The dye is introduced into the solvents by gentle squeezing of the pipette bulb.

2. Some crystals of pyridiniumiodide (dye II) are dissolved in 1 mL of acetone. The solution is mixed dropwise with water.

3. Dye II is dissolved in 1 mL of n-butanol (R1), methanol (R2) and water (R3), respectively.

4. The quinolinium betaine (dye III) is dissolved in 1 mL of water. The solution is mixed with chloroform.


Results:

Experiment 1:

Solvent   Color of solution  
Test tube 1 acetone blue-green
Test tube 2   isopropanol   blue
Test tube 3 ethanol purple


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Experiment 2:

The procedure lesds to solution colors ranging from blue to yellow.

Experiment 3

  Solvent     Color of solution  
Test tube 1   n-butanol   violet
Test tube 2 methanol orange
Test tube 3 water yellow


Experiment 4:

Methyl quinolinium betaine is blood-red in water and ink-blue in chloroform.

Discussion:

Solvatochromism is the ability of a chemical substance to change color due to a change in solvent polarity. The dyes used exhibit negative solvatochromism. As the solvents become more polar, the light absorbed by the dye shifts from the low energy, long wavelength to the high energy, short wavelength end of the spectrum.




·   The phenomenon of solvatochromism arises from a change in the electronic structure and distribution of charge of the excited state as compared with the ground state. If the ground state is more polar than the excited state  (1),  it will be better stabilized by polar solvation and its energy lowered  (2)  so that the transition will occur at a shorter wavelength i.e. there will be a hypsochromic shift ("blue" shift) with increasing solvent polarity.





Summary:

The sign of the solvatochromism depends on the polarity of the dye molecules in the ground state. Two fields of application result from the phenomenon of the solvatochromism:

·   Determination of the polarity of organic dyes in the ground state.

·   Determination of solvent polarities.


References:
Microscale Projection Experiments   Positive Solvatochromism
  Demonstration Experiment on Video   Negative Solvatochromism
  Demonstration Experiment on Video   Positive Solvatochromism
Computer-interfaced Experiments   Dyes - Negative Solvatochromism
Computer-interfaced Experiments   Dyes - Positive Solvatochromism


General experimental instructions and index of experiments






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