0
A N EFFICIENT CHEMILUMINESCENT SYSTEM AND A CHEMILUMINESCENT CLOCK-REACTION EMIL H. WHITE The Johns Hopkins University, Baltimore, Maryland
ENmm in the form of visible light is. emitted by many chemical reactions a t room temperature;' however, only two reactions have been found that are markedly chemiluminescent. These are oxidation reactions of the phthalic hydraaides and of the biacridinium salts.z The various phthalic hydraaides differ considerably in their ability to yield light, the &amino derivative, luminol, being the most efficienL3 The chemiluminescence of this compound has been studied rxte~rsivelyin the past, and in all of the systems that 0
uyNH @' &N
1,uminol:
NH
I
0 5-Amino-2,3dihydro-l,4-phthalazinedione I
'Footnote 2, and also AUDUBERT,R.,Z'ran8. Faraday Soe., 35, 1 0 i (1939). Background material and general references can be found in the following review articles. ANDERSON,R. S., Ann. N . Y . Acad. Sei., 49,337 (19481;SIMPSON, J. C. E., :(The chemistry of Heterocyclic Compounds-Condensed Pyridazine and Pyanine Rings," Interscience Publishers, Ine., New York, 1953, p. 153; see footnote 4 also. a I ~ R E WH. , D. K., A N D F. H. PEARMAN, J Chern. SOL.,1037, 586.
VOLUME 34, NO. 6, JUNE, 1957
have been devised, the oxidation of an alkaline solutiorl of luminol is involved. Three types of oxidants have been used: (I) Hydrogen peroxide-metal ion mixtures (e.g., traces of iron or copper in the form of their chelates). The light observed in "metal-free" systems is due in large part to the trace amounts of iron, etc., in the hydroxides used and on the surfaces of the reaction flasks. (2) Hydrogen peroxide-oxidizing agent mixtures (e.g., NaOCl or K3Fe(CN)6). (3) Potassium ferricyanide-oxygen mixture^.^ The nature of these systems strongly suggests that free radicals are involved, and indeed, in the presence of radical reactants such as hydroquinone, no light is emitted. Little more than this is known about the chemical processes involved. The following two systems were developed during a study of the mechanism of chemiluminescence. The brilliance possible with the first system and the "clock" nature of the second recommend their use as demonstrations. AN EFFICIENT CHEMILUMINESCENT SYSTEM
I n order to study the chemiluminescent reactions at low temperatures, modifications were made of the above systems. It was found, for example, that alkyl WILHELMREN, P. C., R. LUMRY, A N D H. EYRINO, "The Luminescence of Biologicsl Systems,".4rneriran Asaocistion for the Advancement of Science, Washington, I). C., 1955,p. 77.
hydroperoxides could be used in place of hydrogen peroxide if iron-polyamine6 catalysts were used. Copper is not a catalyst for this system. Furthermore, acetone or methanol could be used as the solvent in most of these cases. The most striking change in the reaction occurred, however, when dimethyl sulfoxide ((CH,)2SO) was used as the s o l v e n t . V n this case, oxygen alone serves as the oxidizing agent, and the light evolution is far superior to that of the aqueous systems. For demonstration purposes, 70 g. of KOH, 60 ml. of dimethyl sulfoxide which has been dried with sodium sulfate, and 0.1 g. of luminol are placed in a 1000-ml. flask. Tetramethylammonium hydroxide can also be used; whereas, NaOH and NaNH2 are less satisfactory and amines are ineffective. The flask is stoppered and shaken. At first, the surface of the KOH pellets glows brightly. Eventually the solution becomes brighter than the pellets, reaching a maximum brightness in 1-2 minutes. A brighter light can be obtained if the flask is first filled with oxygen instead of air. If larger amounts of luminol are used, the solution becomes dark when the shaking is stopped and only the surface in contact with oxygen remains bright. The solution becomes bright again on renewed shaking. If oxygen or air is admitted occasionally, such systems can be used intermittently for days. The color of the light emitted can be changed by addition of dyes to the reaction mixture. A CHEMILUIvlINESCENT CLOCK REACTION
This reaction is based on the aqueous chemiluminescent system containing luminol, hydrogen peroxide, ammonia, and copper. The latter can he added as copper(II), copper(I), or copper metal; in each case, the solution becomes deep blue in color due to the presence of Cu(NH&++ If copper wire is lowered into a solution of the other reagents, only the surface of the wire glows a t first; later, enough copper has dissolved so that light is produced throughout the solution. Cyanides stop both the light emission and the decomposition of the hydrogen peroxide, but only temporarily. The role of the cyanide ion is more apparent when potassium copper (I) cyanide'is used as the source of copper. An aqueous solution of luminol, hydrogen peroxide, ammonia, and the complex cyanide remains invisible in the dark and colorless in visible light for a predictable time interval; then, suddenly,
light is emitted throughout the solution. At that instant, oxygen gas is evolved and the solution becomer blue when examined in visible light. For demonstrations, 5 ml. of a 0.1 A' solution of K2Cu(CN)3is added to 100 ml. of a 0.2 M solution of luminol in concentrated ammonia in a 500-ml. flask. Dilute ammonia can be used, but longer induction periods will be observed. Forty ml. of 3% hydrogen peroxide is then added and the resulting solution is stirred in the dark. If the solution is not stirred, local blue chemiluminescent zones may develop; these disappear when the solution is stirred. After 3 minutes a t 21°C., a flash of light signals the start of chemiluminescence. The induction times are approximately doubled by halving the hydrogen peroxide concentration; whereas halving the concentration of the cyanide complex leads to a decrease in the induction time. Systems with large amounts of the cyanide complex have a long induction period terminated by a bright flash of light, the subsequent reaction being a dark one. 0% is still evolved, however. Conversely, systems with small amounts of the cyanide complex have a short induction period and the bright period lasts for many minutes. Systems with large amounts of hydrogen peroxide also emit light for longer periods of time. If 30% hydrogen peroxide is used, large volumes of oxygen are released, and interesting chemiluminescent fountain experiments can be carried out. No doubt, similar results could be obtained by replacement of the cyanide complex with pre-mixed solutions of sodium cyanide and copper salts. The luminol, in effect, is acting as an indicator in the reaction and the chemilumillescence is incidental to the clock reaction proper. The latter can be studied in the absence of luminol by timing the appearance of the blue tetraammine copper(I1) ion in visible light. Although the chemistry of these reactions has not been investigated, the following plausible reactions are consistent with the observations. Reactions 1,2, and 3 account for the "clock" nature of the system.
'For the Use of similar systems in polymeri~ation,see EMBREE, W. H., R. SPOLSKY, AND H. L. WILLIAMS, Ind. Eny. Chem., 43,
2553 (1051). The usefulness of dimethyl sulfoxide in chemiluminescence was first noted by Dr. Joyce Geidusehek in a systematic program of evaluating organic solvents for this reaction. 'Available irom Amend Drug and Chemical Co.,New Yark, N. Y.
fast
+ H.02 S H , Cu(iXHa)*+++ H203+ luminol-
(3) Cppper(1) complex (4)
Co(XH,),++ light
+ OHe
+ products
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