The Phenomena of Chemiluminescence' WILLIAM GEORGE LEEDY Rose Polytechnic Institute, Terre Haute, Indiana
I. REACTIONS INVOLVING CAEMILUMINESCENCE A . Between Alkali Vapors and Halogens MONG the best known of chemiluminescent reacbons are those between alkali vapors and halogens or halides. Haber and Zisch found that sodium vapor and chlorine a t low partial pressures yielded the D-sodium line. Lialikov and Terenin studied the chemiluminescence given off by the reactions of sodium, potassium, rubidium, and other metals with iodine vapors. Most of these reactions gave spectral line emissions corresponding to the lines of the alkali. The reactions between sodium or potassium and mercuric halides offered an exception to this. In this case, as noted by Haber and Zisch, a band spectrum was emitted extending into the ultraviolet region. They believed that the molecule HgCl was the light emitter.
A .
B. Oxidation Processes 1. Of Silicon Compounds and Phosphorus. The
especially of the &amino compound, is probably the most brilliant example of this phenomenon. Carl Zellner and Gregg Dougherty made some extensive tests on the phthalhydrazide derivatives in order to determine the nature of chemiluminescence and the reactions which produced it. By means of a Weston photronic cell and a very sensitive galvanometer they measured the light given off by a number of phthalhydrazide derivatives. Reactions were carried on a t two different concentrations. specifically 0.001 and 0.004 molar concentrations, of phthalhydrazides in a 0.25 per cent sodium hydroxide solution, to which 2 cc. of 0.2 N sodium hypochlorite were run in. The results appear in the following table. In all cases except that of 3-aminophthalhydrazide (which is commonly known as "luminol" and will be called such hereafter) no resistance was used. With "luminol" i t was necessary to use 700,000 ohms resistance in series with the galvanometer.
N CSOME B PBTBALHYDRAZIDB DBRW*TIV&S oxidation of yellow phosphorus is probably the best RBLATIVB C H ~ X I L D V I N ~ S C I OF Cm. D i m cm. Dire. known of the chemiluminescent reactions involving Devhriuc 0.001 M 0.004 M oxidation. The oxidation, which occurs a t reduced pressure and between - 10' and 40°C., is believed to he a chain reaction. Its light is a whitish green luminescence, giving many lines in the ultraviolet region. Kautsky and his co-workers investigated the chemiluminescence of various unsaturated silicon compounds as they underwent oxidation. Kautsky used a mixture of siloxen and oxy-siloxen which he called silikone. When silikone was oxidized by air or oxygen, and more a. Effect of substituted groups. From the above rapidly by hydrogen peroxide or chromic acid, a feeble table i t was deduced that the intensity of luminescence green light was given off. upon the nature of the group substituted 2. ~ ~ , cmpounds. j ~ ~ ~h~ ~ ~~ dr i ~ ~ is dependent ~ d 1 x 1 the benzene ring and the position of the group. pounds are characteristic of a number of organic In running ortho-, meta-, and para-benzhydrazides, compounds which emitlight much greaterin intensity than any other chemiluminescent compounds. When Zellner and Doughert~found that the ortho derivative chloropicrin reacts with phenylmagnesium bromide, gave an easily visible light, the meta hardly visible, a greenish blue light is given off. Grignard com- and the Para no visible light a t all. b. Effect of oxidation rate. By measuring the amount pounds will react with other nitro compounds and proof nitrogen evolved during the reactions i t was posdnce a similar light. sible to determine the rate of oxidation of each of the that in this type of ~ ~ f andf E~~~~ ~ ~ claimed d above compounds. The rate of oxidation was found reaction a magnesium atom coupled to an unsaturated by Zellner and DoWherty to be independent of the carbon atom was necessary for light emission to substitutin~ proun in the benzene rina, although occur. 3. phthlhydrae& ~ ~ , j ~ ~h~ ~ t che,,,ii ~ ~ chemiluminescence ~ . was very dependent on it. " ~ i r luminescence of the phthalhydrazide compounds, and example, the nitrophtha~h~drazideswhich @ve no light give the same rate curve as the aminophthal-
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L Presented before the Division of Chemical Education, of the American Chemical Society, lOlst meeting, St. LOUIS,Mxssouri. April 10, 1941
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142
"The rate was found to be accelerated markedly
with increasing hydroxyl ion concentration, which is interesting in connection with the fact the light decreases with increasing hydroxyl ion." 11.
THE CAEMILUMINESCENCE OF ~-AMINOPHTHALHYDRAZIDE
The luminescence given off by the oxidation of "luminol" is many times more brilliant than any other chemiluminescent reaction, and in this respect is treated separately from the other light-giving compounds.
A. The Reaction Involved 1. Reagents Used. In order to produce chemiluminescence with "luminol" it is necessary to use four reagents and a solvent. Omission of any one of the five constituents will result in either no light a t all or a very faint emission of light. The first substance needed is "luminol" itself as the chemiluminescent substance. Second, an oxidizing agent is needed. Three per cent hydrogen peroxide is usually used for this although the same function can be performed by calcium or sodium hypochlorite, the persulfates, and other compounds. Third is needed another oxidizing agent, usually potassium femcyanide. Some workers in this field think that this second oxidizing agent performs the process of decomposing the hydrogen peroxide, while others think that it is the main oxidizing agent. Potassium femcyanide may be substituted by ammoniacal solutions of copper salts, by hemin crystals and oxygen, or by alkaline hypochlorites. The fourth constituent is an alkali which is used to aid the rate of oxidation of the "luminol." The solvent is water. The reaction is performed by mixing two solutions. The first contains 0.1 g. of "luminol" and 10 cc. of 5 per cent sodium hydroxide solution per liter of aqueous solution. The second solution contains 0.25 g. of potassium ferricyanide crystals and 10 cc. of 3 per cent hydrogen peroxide per liter of aqueous solution. 2. Nature of the Reaction. The oxidation is very rapid and profound. It is thought that a threestage oxidation is reached long after the light has ceased. When additional femcyanide is added, however, light is again given off and the reaction probably proceeds to a fourth stage. In any case 3-aminophthalic acid is formed as an end product.
B . Variables and Effect of Concentrations 1. Oxidetion and Alkalinity. The data of Zellner and Dougherty in their experiments with phthalhydrazides show best the effect of increasing alkalinity upon the oxidation of "luminol": Norrnoliry of NaOE 0.007 0.015 0.03 0.06 0.125 0.25 0.5
Gob. Dir9. in Cm. 42 66 (Max.)
60 50 30 18 8
2. Concentration of Reagents. Here the concentration of "luminol" is referred to. It has been found that the maximum light intensity is obtained when the concentration of "luminol" is 0.02 molar. A 0.5 molar solution will cause only half the galvanometer deflection of the 0.02 molar.
C. Results of the Reaction I . Energy Emitted.--a. Method of measuring. Harris and Parker carried on tests on the reaction of the sodium salt of "luminol" hy arranging a condensing lens in front of a reaction tube of quartz glass of 3.8cm. diameter. In front of this was a thermopile and a sensitive galvanometer. They considered the thermopile to be more accurate than a photronic cell. Readings were then taken using various "luminol" concentrations. b. Results of reuctwns. Harris and Parker found that the quantum efticiency varied with the rate of flow and inversely with the concentration of "luminol." A maximum was determined of 0.005 quantum per mole which, although low for energy emissions of light-giving reactions, is more than 10,000 times more efficient than other chemiluminescent reactions. 2. Light Emitted.--a. Method of measuring. Harris and Parker used a spectroscope to determine the extent of the wave-length band of the light obtained in their experiments. By nse of optical filters and photographic plates they were able to tell where the larger quantity of light occurred. b. Results of experiments. The band emitted extended from the violet deep into the red. The range of the largest portion of the light was from about 3800 to 5000 A.U. and about 2 per cent of the light extends beyond 5780 A.U. Thus i t is seen that the light from the chemiluminescence of "luminol" covers a wide band. c. Effat onfluorescent dyes. The light from "luminol" has been found to excite various fluorescent dyes. For example, the addition of fluorescein to the reaction vessel will change the pale blue light of "luminol" to a greenish yellow. Rhodamine B causes an opal rose color, and sodium naphthionate fluoresces deep blue in alkaline solution and green if an acidifier is added while the reaction is going on. 111.
THE INTERPRETATION OF CHEMILUMINESCENCB
A . Definition and Distinction front nescences
Other Lumi-
Chemiluminescence is in effect "cold light"; in other words, light produced by means of a chemical reaction in which the light emitted is entirely independent of any heat involved. One of the best examples of natural chemiluminescence, and one which has been the basis of investigation of Harvey in this field, is the cold light given off from a firefly's tail when chemiluminescent substances in his body are oxidized. The phenomena may be interpreted as the reverse of the photochemical process where:
A
+ B + light
AB = A
= AB (photochemical process)
+ B + light (chemiluminescent process)
Chemiluminescence is different from fluorescence in that fluorescence or even phosphorescence involves the utilization of energy from external sources such as rays, heat waves, or light waves, while chemiluminescence involves the energy which comes from the reacting substances themselves.
cules to yield energy-rich hydrazide molecules, which emit light." 2. Bv Transfer , of Enerev. This is best illustrated bv the reaction between sodium and mercuric chloride. Here it seems that the energy of the reaction is transferred to the chloride molecules which are excited and emit a broad band of light. ~
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