Demonstration 23
Luminol - fractionating column.
This is our first demonstration of chemiluminescence in liquids. Whilst in a number of chemical reactions, especially in rapidly proceeding exothermic actions, the emission of radiation can be observed, the term chemiluminescence must be considered as having application only to a restricted class of such actions [1]. Although the term cold light has been applied to chemiluminescence, in some cases chemiluminescence is only to be observed in reactions proceeding at relatively high temperatures. In the case of fluorescence and phosphorescence, we are dealing with the emission of light either line or banded spectra resulting from the reversion to the normal state of excited or metastable atoms, or molecules, or molecular aggregates, originally excited by the absorption of radiation. Chemiluminescent reactions include not only those in which a similar reversion of excited or metastable molecules produced by chemical agencies occurs, but, in addition, those exothermic reactions in which, as a result of chemical action, the energy is at least partly emitted not as degraded thermal energy, but selectively, being the emission spectrum of some component of the reacting system.
Whilst the earliest record of a phosphorescent material, 1602, refers to impure barium sulphide - the so-called lapis Bologniensis - the supposed analogy to chemiluminescence provides us with the name of phosphorus, the oxidation of which provided the earliest recorded cause of a chemiluminescence of a pure material, although it may be noted that the long observed light of fire-flies, decaying wood, and of certain bacteria and fishes, are all due to chemiluminescent reactions. Many other vapours exhibit chemiluminescence on oxidation; thus the vapours of ether, carbon disulphide (see Experiment 22) and substances of the type RO•CSSR’ and R2NCSOR can readily be caused to undergo cold luminous combustion.
There are numerous examples of chemiluminescent reactions in solution, and many of them are remarkably beautiful. Oxidation of luminol (3-aminophthalhydrazide) is, probably a best-known and easiest to perform. Although 3-aminophthalhydrazide has been known since 1902, it was not until 1913 that Curtuis and Semper discovered that the substance was chemiluminescent [2]. They found that when it was dissolved in sodium hydroxide solution and then oxidised with hydrogen peroxide or sodium hypochlorite, it emitted a strong greenish light, brighter than that of any previously known reaction.
Luminol is commercially available from several suppliers and relatively inexpensive, however if desired it can be synthesised without any great difficulties. Numerous recipes have been published over the years [3-6].
During the reaction the luminol is converted into disodium salt of 3-aminophthalic acid:
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For the discussion of the mechanism of luminol oxidation see, for example, Scheider [7].
There are several ways of demonstrating luminol chemiluminescence, described in numerous publications [3,4,6,8-12].
Preparation. Prepare two solutions. Solution A: 0.1 g luminol + 5 ml of 5% NaOH + 1000 ml water. Solution B: 10 ml 3% H2O2 + 0.25 g K3[Fe(CN)6] + 1000 ml water. The demonstration can be performed by simply mixing these two solutions, but we are doing it in a slightly more elaborate way. Position a glass spiral (or a fractionating column) above a large flask and attach a glass funnel to the top of the spiral. Put a little of fluorescein, potassium ferrocyanide and a few ml of 5% NaOH into the flask.
Demonstration. Darken room completely. Simultaneously pour solutions A and B through the funnel: the glowing blue solution will run down the spiral (or the column). As soon as the solution reaches the flask the colour of light given out by the solution suddenly changes into bright green (due to the fluorescein).
Safety. The toxicity of luminol is not known exactly. Sensibilisation due to the inhalation of luminol dust or to skin contact is possible. Potassium hexacyanoferrate(III) is toxic. Skin contact must be avoided. Contact with NaOH can cause severe skin damage.
References.
1. E.K. Rideal, “Chemiluminescence”, School Sci. Rev., 1928/29, 10, 201.
2. R.E.D Clark, “Chemiluminescence”, School Sci. Rev., 1937/38, 19, 489.
3. M. Wilson and T. Wood, “Chemiluminescence”, School Sci. Rev., 1972/73, 54, 524.
4. M.A. Ivanova and M.A. Kononova, Chemical Lecture Experiment, Moscow, Vyschaya Shkola, 1984, p. 147 (in Russian).
5. E.H. Huntress, L.N. Stanley and A.S. Parker, “The preparation of 3-aminophthalhydrazide for use in the demonstration of chemiluminescence”, J. Am. Chem. Soc., 1934, 56, 241.
6. E.H. Huntress, L.N. Stanley and A.S. Parker, “The oxidation of 3-aminophthalhydrazide (“luminol”) as a lecture demonstration of chemiluminescence”, J. Chem. Educ., 1934, 11, 142.
7. H.W. Schneider, “A new, long-lasting luminol chemiluminescent cold light”, J. Chem. Educ., 1970, 47, 519.
8. A. Adey and G.C. Britton, “Chemiluminescence - a double reaction”, School Sci. Rev., 1975/76, 57, 314.
9. B.Z. Shakhashiri, Chemical Demonstrations, A Handbook for Teachers of Chemistry, vol. 1, The University of Wisconsin Press, Madison, 3-rd printing, 1986, p. 146-179.
10.H.W. Roesky and K. Möckel, Chemical Curiosities, trans. T.N. Mitchel and W.E. Russey, New York, VCH Publishers, Inc., 1996, p. 188.
11.Tested Demonstrations in Chemistry, ed. L. Gilbert, et al., Denison University, Granville, OH, 1994, vol. 1, p. H-41.
12.B. Iddon, The Magic of Chemistry, Poole, BDH, 1985 p. 36.