COLOR DETERMINATIO?S O F F A I N T I,UMINESCENCE BY HARRY B. WEISER
I n connection with the study of the faint crystalloluminescence of certain salts the author was confronted with the problem of determining the color of the emitted light. The difficulty arises from the fact that the intensity and duration of the light is such that it does not admit of spectroscopic investigation. The same is true of many chemiluminescent reactions and particularly of those that take place when the reacting substances are brought together in solution. In these cases spectrographs can not be obtained even by longcontinued exposure. “Spectrographs of the strongest phenomena of crystalloluminescence and chemiluminescence could not be obtained even by repeated exposures for eight hours, using a large spectroscope with camera attachment.” Color photography of the luminescence2 is likewise manifestly impossible. I n consequence observers of faintly luminescent reactions have had to rely on the color sensitiveness of the eye to determine the color of the emitted light. For many reasons this is very unsatisfactory a t the best. I n the first place the glow of faint luminescence appears white, and it is obviously very difficult to determine whether this whitish glow has its maximum in the red,,yellow, green or blue. In speaking of the color of crystalloluminescence, Trautz says:3 “The color of the light is white, sometimes greenish; only in one case, that of sodium fluoride, is it yellowish. Usually the effect is very delusive. To all outward appearances the quality of the light is similar in all cases.” It is interesting to note that the single case of yellow crystalloluminescence referred to above was not observed by Trautz as he was unable t o detect light during the crystallization of Trautz: Zeit. phys. Chem., 53, I O I (1905). Lohr: Jour. Phys. Chem., 17, 675 (1913). Trautz: Zeit. Elektrochemie, IO, 593 (1904).
,
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H a w y B. Weiser
sodium fluoride. Berzeliusl and Rose2 each claim to have observed the luminescence but once. As might be expected, there is a difference of opinion as to the actual color of the light emitted by certain reactions. Thus Bandrowski3 describes the light emitted when a saturated solution of sodium chloride is precipitated with hydrochloric acid as bluish green. Parnau4 takes it to be bluish white. When the light is very faint the color seems to me to be bluish; but a t the point of maximum brightness which I have been able t o obtain by a careful adjustment of the conditions of precipitation, the color appears decidedly yellowish. This is likewise the consensus of opinion of a score or more of people to whom I have exhibited the phenomenon. A common source of error in the determination of the color of luminescence by the unaided eye is introduced by the “Purkinje Phenomenon.” “When spectral colors are examined, it is obvious that some of the colors are brighter than others, the extreme red and extreme violet for instance, possessing little luminosity as compared with the yellow. The relative brightness of the different spectral colors is found to vary with the amount of illumination With a brilliant spectrum the maximum brightness is in the yellow, but with a feeble illumination it shifts to the green. This accords with what is known as the Purkinje phenomenon, namely, the changing luminosity and color value of colors in dim lights. As the light becomes more feeble the colors toward the red end of the spectrum lose their quality, the blue colors being perceived last of all, just as in late twilight the sky remains distinctly blue after the colors of the landscape become indistinguishable.”5 On this account we should expect a very faint luminescence to appear blue to the eye even if the maximum color actually were in another portion of the spectrum. Jahresbericht, 1823,400. Pogg. Ann., 52, 443, 585 (1841). Zeit. phys. Chem., 15, 323 (1894). Eighth Internat. Congress Applied Chemistry, 2 0 , 133 (1912). Howell: “Textbook of Physiology,” 339 (1910).
Color Deternzinatioul of Faint Lumiulesceulce
441
The color of light by which luminescence is observed likewise has an effect. “It is striking,” says Trautz,I “that just those liquids which show the clearest luminescence appear to undergo a decided change in color toward the blue if they are viewed by red light (ruby glass light). As an example I should like to call attention especially to the above mentioned pyrogallol luminescence. I n the second stage of this reaction, after the first foaming has subsided, the liquid glows white in the dark and blue like petroleum in red light. The luminescence is too weak for spectroscopic investigation. . . . Glowing amarine behaves similarly. When treated with bromine water, the color of the light appears yellowgreen in the dark and more of a blue-green in red light.” In this connection I have noted that there is an apparent difference in the color of luminescence depending on whether I remained in total darkness while my eyes became sensitive enough t o make the observations or whether the dark room was lightedfaintly with a red light to facilitate the preparation for the experiments. Trautz2 claims that the addition of substances which change the color of the reacting mixture does not greatly change the color of the luminescence, but that such additions may diminish the intensity t o such an extent that it is impossible to judge the color. He has found, for example, that the addition of potassium dichromate, fluorescein, iodine in potassium iodide, or alizarine blue does not change the glowing reddish color of light emitted during the oxidation of pyrogallol, but that addition of the colored substances diminishes the intensity. Thus, strong coloration with alizarine blue renders the light almost invisible. It should be mentioned at this point that Trautz considers the pyrogallol oxidation with hydrogen peroxide to take place in two stages: the first stage glowing with a relatively bright reddish light, and the second (after the foaming subsides), glowing much fainter and white. As it seems to me, the apparent change in color 1 Zeit. 2
phys. Chem., 53, Ibid., p. 1 0 1 .
102
(1905).
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Harry B. Weiser
442
is brought about by two things: first, the diminished intensity which is largely the result of the diminished reaction velocity; and second, the effect in increasing amount of the presence of the very highly colored reaction product. For the color of the solution doubtless has an effect, particularly in view of recent work on the modifying influence on flame spectra of the presence of colored substances in the flameel Considering all t h e possible sources of error it is evident that the determination of the color of faint luminescence with the unaided eye is most unsatisfactory. A method has been devised, however, whereby the color may be determined with a considerable degree of accuracy. The method consists essentially of photographing the luminescence on a panchromatic plate, interposing a series of different colored color screens between the source of light and the plate. Obviously the plate will be fogged only behind those color screens that transmit the light and in direct proportion to the amount each transmits. By comparing the photograph so obtained with a photograph of white light, using the same color screens, the color of the luminescence is readily determined. The method is applicable in all cases where the glow is uniform and sufficiently strong to fog the most sensitive photographic plate brought very close to it. For the purpose of demonstrating the usefulness and applicability of the method, I will describe in detail the results obtained with reactions varying in intensity from the relatively bright oxidation of alkaline pyrogallol to the faint crystalloluminescence of sodium chloride. Experiment a1 The reactions were carried out in the mixing apparatus described at length in a previous communication.z This consists essentially of two concentric glass tubes: the outer one approximately 3 . 5 cm in diameter and the inner one 2 cm in diameter. With this apparatus practically instantaneous 1Bancroft and Weiser: Jour. Phys. Chem., 18, 281 (1914). Weiser: Ibid., 20, 314 (1916).
2
Color Determi?zatio?z of Fai?zt Luminescence
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and uniform mixing can be obtained and when necessary, the solution can be stirred by moving the inner tube up and down. The exposure of the plate was conveniently accomplished in an apparatus especially designed for the purpose: A vertical slit 7 cm long and 0 . 9 cm wide was cut in a board 28 cm by 28 cm and I cm thick. On one side of this board were fastened two parallel horizontal strips so grooved that they held a photographic plate holder flush against the board and yet allowed the holder to slide back and forth in front of the slit. By this provision a number of exposures could be made on one plate. On the opposite side of the board two vertical cleats 3 cm by 3 cm were fastened on either side of the slit at such a distance apart that ‘the mixer fit snugly between them. A groove was cut in the board to fit the curved surface of the mixer. Two buttons attached to one of the cleats could be so turned as to hold the mixer firmly in place during an exposure. To support the apparatus in a vertical position it was fastened to a strip of wood 4 cm by 9 cm by 28 cm. The apparatus was painted a dull black. I n this camera” the reacting vessel was held about I cm from the plate. The highly recommended1 panchromatic plates manufactured by Wratten and Wainwright were used when necessary to have a plate sensitive t o red light. When this was not necessary the faster Hammer ‘red label” plates were substituted. A collection of 35 gelatine color screens manufactured by Wratten and Wainwright were obtained from the Eastrnan Kodak Company. The color of light which each would allow through was determined by interposing each in turn between a source of white light and the slit of a spectroscope. Prom the assortment of calibrated screens a number were chosen varying in color from deep red to deep violet. Strips of suitable width were cut from each and pasted close together over the slit of the ‘(camera” so that they were next t o the plate during exposure. ( (
(
1
Baly: “Spectroscopy,” 372
(1912).
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The Oxidation of Pyrogallol .-Fifteen cubic centimeters each of 50 percent potassium carbonate solution, I O percent pyragallol solution and formalin were placed in the outer compartment of the mixer and 40 cc of 40 percent hydrogen peroxide in the inner c0mpartment.l This was set in the “camera” previously loaded with a panchromatic plate and the reaction started by mixing the contents of the two compartments. After several seconds’ exposure to the light of the reaction, another portion of the plate was moved over the slit and a photograph of white light taken by exposing to daylight for an instant. The development and fixing of the plates were carried out in absolute darkness. A description of the filters used is given in Table I. Statements of the color of TABLEI Color of filter
Light that passes through filter
5
Deep red Red Reddish orange Yellow Green
6
Green
7
Dark green
8
Blue (purplish.)
9
Purple
Red only Red to lithium beta line Red to the left of calcium beta line Red to the thallium line From lithium beta line to barium delta line. Small band in red From lithium beta line to barium alpha line. Small band in red From calcium beta line to barium beta band From barium beta band to midway between strontium delta line and rubidium alpha line. Little band in orange IFrom strontium delta line through the extreme violet. Red to lithium-alpha line From right of barium alpha line to right of indium alpha. Little band in orange Prom left of barium beta band to indium alpha line Blue only Mercury violet filter
Numbe: I 2
3 4
IO
Blue
I1
Blue
I2
Deep blue Violet
I3
This recipe is recommended by P. Schorigen: Zeit. phys. Chem., 53, 596 (1904). 4
Color Determi73atiovt of Faivlt LuYtzinesce.tlce
445
light that passes through the filters are of course only approximate as the absorption bands are not always sharply defined. The photographs which have been taken and which are referred to in the text have not been reproduced owing to the rather unsatisfactory effect of such half-tones. It is evident from the photographs obtained that the color of the light emitted during the oxidation of pyrogallol lies almost entirely in the red and yellow portions of the spectrum. There is practically no green or blue as but little light passes through the filters 5 to 1 3 , inclusive. The plate was fogged very slightly behind filters 5, 6, 9 and IO, but this hardly shows in the print. By reference t o the table it will be seen that these filters allow either a little red or orange light t o pass through. The plate was affected behind filters three and four almost equally, although when exposed to white light it is affected distinctly more behind Number 4. This indicates that the luminescence is made up of a band that extends from the red through the yellow with the maximum in the orange-red. Oxidation of Phosphorus.-When a solution of phosphorus in glacial acetic acid is treated with a solution of hydrogen peroxide a luminescence is produced that appears to be colored greenish yellow t o white, depending on the intensity of the light. This glow was photographed in the same way as the pyrogallol luminescence, using the same color screens. Examination of the photograph shows that there is practically no red, orange or yellow in the luminescence, since no light passed through filters I , 2 and 3 . A little passed through Number 4, which allows some green through in addition t o the red and yellow. It will be noted that the remainder of the plate was affected almost in the same way by the luminescence as by white light. Photographs taken with the series of filters described in Table I1 lead to the same conclusion, namely, that the luminescence is the same in quality as white light from the yellowish green through the violet. Oxidation of Anzarine with Chlorine avtd Bronz.ine.-It
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was discovered by Radziszewskil that the oxidation with air or oxygen of a solution of amarine in alcoholic potassium hydroxide is a luminescent reaction. Trautz2 found that the intensity of the luminescence is greatly increased by increasing the velocity of the reaction by the addition of 30 percent hydrogen peroxide or one of the halogens. With chlorine the color of the light is described by Trautz as blue-green, with bromine, yellow-green and with iodine, white. Amarine, which is an isomer of hydrobenzamide, was prepared by heating the latter for several hours a t a temperature of 125 degrees. I . j gram of the product were dissolved in 40 cc of an alcoholic solution of sodium hydroxide saturated at room temperature. This solution was heated to boiling and placed in the outer compartment of the mixing vessel and an equal amount of saturated chlorine water or bromine water in the inner compartment. The mixture was stirred continuously during the exposure of the plate. It was found that the luminescence with chlorine was fainter than with bromine but that it lasted longer and was more readily photographed. Better results were secured with bromine by adding a little of the liquid along with the water solution. Experiment showed that it was unnecessary to use a plate sensitive to the red as the emitted light contained no red. The filters used are described in Table 11. As before stated, there is no red in the amarine luminescence. Examination of the photographs shows that white light affects the plate most behind filter Number 7, whereas it is almost entirely unaffected a t this point by the luminescence. This eliminates all the color from the strontium blue line through the violet. Further examination reveals that the plate is fogged the most behind those screens which transmit the most green light. Filter Number 5 definitely fixes the maximum color of the luminescence in the green andNumber 6 shows it to be in the immediate region of the thallium green line. 1 Ber. deutsch. chem. Ges., IO, 70 (1877). 2Zeit. phys. Chem., 53, 86 (1905).
Color Determination of Fa&
Lumivlescence
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TABLE: I1 Number
1 I
~
Color of filter
I 2
Yellow Green
3
Green
4
Dark green
5
Light green
6
Dark green
7
Purple
8
Greenish blue
9
Blue
IO
Blue (purplish)
I1
Blue
I2
Deep blue
I
Light that passes through filter
Red t o the thallium line From lithium beta line to barium delta line. Small band in the red From lithium beta line to barium alpha line. Small band in the red From calcium beta line to barium beta line From lithium beta line to left of barium beta band. Red t o lithium alpha line From calcium beta line to right of barium beta band. Maximum a t thallium alpha line From strontium delta line through the extreme violet. Red to lithium alpha line From left of thallium line to indium alpha line From right of barium alpha line to right of indium alpha line. Little band in orange From barium beta band t o midway between strontium delta line and rubidium alpha line. Little band in orange From left of barium beta band to indium alpha line Blue only
Filter Number 1 2 shows that there is relatively little blue in the light as the fogging behind this screen is almost imperceptible. It is interesting to note that the photographs using chlorine and bromine as oxidizing agents indicate that the color of the luminescence is the same iii both cases even though they may not appear so to the unaided eye. The Crystallol.u.flzi.tzescel2ce of SodiwN Chloride.-When a saturated solution of sodium chloride is mixed with alcohol or hydrochloric acid, precipitation takes place with emission of light, the color of which is impossible to determine with any degree of certainty with the unaided eye. However,
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B. Weiser
it readily lends itself to determination by the method under consideration. Forty cubic centimeters of a sodium chloride solution saturated at room temperature were placed in the outer compartment of the mixing vessel and a like amount of hydrochloric acid, specific gravity I . 12, in the inner compartment. The precipitation was effected and the luminescence photographed as in the previous cases. To get a good negative it was necessary to expose the plate to the luminescence three times a t the same place. The filters listed in Table I were used. The photograph shows that there is no red, orange, yellow or green in the luminescence since the plate is unaffected behind filters I to 7, inclusive. Filters Numbers 8 and 9 show conclusively that practically all the color is beyond the strontium blue line. There is very little color in the blue to the left of the indium blue line since the plate is affected but slightly behind filters I O to 12, inclusive. The color of the luminescence is therefore bluish violet. The above series of experiments is sufficient to show that by the method described, it is possible to determine the color of luminescence with a degree of accuracy that is limited only by the number and choice of the ray filters employed. The results of this investigation may be summarized as follows: I . Spectroscopic investigation of faint luminescence is impossible. 2. The color of faint luminescence cannot be determined even with approximate accuracy with the unaided eye. 3. A method has been devised for determining the color of faint luminescence that is applicable whenever the glow is uniform and sufficiently strong to affect the most sensitive photographic plate brought very close to it. The method consists essentially of photographing the luminescence on a panchromatic plate interposing a series of color screens between the source of light and the plate.
Color Deternzivtation of Faivtt Luwainesce?zce
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4. An apparatus has been described for photographing the luminescence of reactions that take place in solution. 5. The applicability and usefulness of the method has been tested with a number of reactions, that vary widely in the quality and intensity of the luminescence. Department of Chemistry, The Rice Institute, Houston, Texas
