Demonstration 24
Luminol in flasks (Chemiluminescence of luminol in dimethylsulphoxide).
The alternative way to demonstrate chemiluminescent oxidation of luminol is to use dimethylsulphoxide (DMSO). When the DMSO, luminol, water, and caustic solution are shaken in the presence of oxygen from air, an oxidation reaction produces considerable bright blue-green light and as the reaction uses up the dissolved oxygen, light intensity decreases in about one minute to a low level resembling the light output of micro-organisms [1]. This low level light is continuously produced at the air-liquid interface for many days. Passing large amounts of air or pure oxygen into the fluid quickly quenches the light and when the excess oxygen has dissipated, light is again evolved. Even heating the fluid to 100oC does not prevent the normal reaction of light generation from taking place. The light, however, does go out faster because all the oxygen is used rapidly; because of the presence of caustic, this then becomes a way to produce nearly pure nitrogen. It was also noted that lowering the temperature to 13oC did not stop the chemiluminescence reaction, but that at 10oC no light was visible to the eye [1]. In order to increase the light brightness of the normal blue light it is necessary to add small amounts of fluorescein and then approximately 500 times the regular brightness was achieved. The colour was changed to yellow. An attempt to use Rhodamine B and many other organic colouring agents failed to produce other colours because none of these colouring substances are able to resist alkaline oxidation conditions [1].
When up to 5% water-DMSO solution was employed, excellent results for the chemiluminescent reaction were obtained, but as water concentration increased towards 20%, the chemiluminescent light became quenched [1]. Water concentrations above 20% also quench the light completely. Sodium or potassium hydroxide react with luminol in a regulated DMSO-water solution to form the dianion of luminol as shown in the figure. In the presence of air, the entire fluid bursts into light. Best results are achieved by using about five milligrams of luminol per ml of fluid.
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Preparation. In our demonstration we use a slightly simplified procedure employing pure DMSO [2,3]. Put approx. 10-15 g of potassium hydroxide pellets into two conical flasks (ca. 0.5-1 l) and wet KOH with dimethylsulphoxide (avoiding excess DMSO). A little of fluorescein is then added into one of the flasks.
Demonstration. To perform the demonstration add a little of solid luminol (3-aminophthalhydrazide) into each flask and stopper the flasks. Darken room completely. Shake the flasks vigorously to dissolve oxygen from the air. Brilliant blue light is emitted by the flask containing pure luminol and bright yellow light by the flask containing added fluorescein.
There are many variations of this demonstration, see, for example, [4]. The colour of the emitted light can be modified in several ways as described in [3].
Several practical applications have been proposed for the DMSO-luminol system [1]. These included emergency light sources or a light source in hazardous areas (e.g., in the presence of flammable fluids and vapours), night survival sea rescue, etc. A different system (based on oxalyl chloride esters) however is used in practice, see Experiments 25 and 26).
Safety. The toxicity of luminol is not known exactly. Sensibilisation due to inhalation of luminol dust or to skin contact is possible. Contact with solid KOH can cause severe skin damage and must be avoided.
References.
1. H.W. Schneider, “A new, long-lasting luminol chemiluminescent cold light”, J. Chem. Educ., 1970, 47, 519.
2. M.A. Ivanova and M.A. Kononova, Chemical Lecture Experiment, Moscow, Vyschaya Shkola, 1984, p. 149 (in Russian).
3. Tested Demonstrations in Chemistry, ed. L. Gilbert, et al., Denison University, Granville, OH, 1994, vol. 1, p. H-55.
4. M. Wilson and T. Wood, “Chemiluminescence”, School Sci. Rev., 1972/73, 54, 524.