Experimental Technique for Quantitative Study of Photochemical

EXPERIMENTAL TECHNIQUE FOR QUANTITATIVE STUDY O F PHOTOCHEMICAL REACTIONS* BY GEORGE SHANNON FORBES

Introductory. Experimental photochemistry is such a many-sided subject that the scope of this paper must be severely restricted. It deals with chemical reactions evoked by light from external sources, and from the red to the beginning of the Schumann region. Heterogeneous systems are not discussed. Photochemical literature is burdened with an unduly large proportion of articles which are useless or misleading because part only, if any, of the essential variables were measured. Photochemical generalization, for insufficiency of sound experimental data, is still uncertain regarding many vital matters. Only consistently quantitative methods can banish the dubious hypotheses which the pseudo-quantitative type of paper engenders. I n what follows, therefore, the emphasis is placed upon monochromatic procedures, in which both energy absorbed and substance transformed are adequately measured. It will be evident that a considerable degree of accuracy is compatible with reasonable simplicity and economy. The treatment is in no sense historical or encyclopaedic, but rather selective and critical; the same is true of the bibliography, which includes less than a third of the papers which received more than passing consideration. Sources of Information. Most details are given by reference; also whole topics when connected and satisfactory expositions are accessible. A dagger indicates a reference list of importance. Grateful acknowledgment is made of assistance rendered by Mi-. R. M. Fuoss in connection with the library work. It will be recalled that the Research Information Service of the National Research Council, Washington, D. C., “places at the immediate command of individuals, institutions and firms, information concerning scientific instruments, apparatus, lantern slides, laboratory construction and equipment, bibliographies and reference lists both published and unpublished, research problems, projects, methods, funds, personnel and other subjects of interest to investigators.” It will also give the names of dealers, domestic and foreign, which can supply specified instruments and apparatus. Many dealers, in turn, are glad to send reprints, folders or booklets on the theory and use of photochemical instruments. * Contribution from the Chemical Laboratory of Harvard University. General References “Ueber den Energieumsatz bei photoohemischen Vorgangen.” E. Warburg: Sitzungsber. preuss. Akad. Wiss., 11911, 746; *1912,216;31913,644;‘1914,872; 61915,230; ‘1916,314’ 133 (1921).The ’1918,300;81918,1288;9 1919,960;1°Z. Elektroohem., 26,54 (1920);’~~27, foundation of quantitative photochemistry.

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“Optische Methoden der Chemie,” F. Weigert. Akad. Verlagsges., Leipzig (1927). The best summary of the field. 11 “Arbeitamethoden der Photochemie,” G. Jung. “Handbuch der Arbeitsmethoden in der anorganischen Chemie,” 2 11, 1477-1560. Edited by Tiede and Richter. Walter de Gruvter and Co., Berlin and Leipzig (1925). Modern and scholarly, includes quantitative details. 12 “Hand- und Hilfsbuch physiko-chemischer Messungen,” 682-739. Edited by Drucker. ([Akad’.Verlagsges., Leipsig (1925). Elementary, but deserving of careful study. 13 Photochemische Versuchstechnik, J. Plotnikow. Akad. Verlagsges., Leipzig (1912). Still of value in qualitative work. 14 “Leh,rbuch der Photochemie,” J. Plotnikow. Walter de Gruyter and Company, Berlin and Leipzig (1920). Brief on the experimental side. 15 “The Chemical Action of Ultraviolet Rays.” Ellis and W e b . Chemical Catalog Co.. Inc., New York. An extended survey of the qualitative side of the subject. 16 “Ultraviolet Radiation.” M. Luckiesh. D. Van Nostrand Co., (1922). Less comprehensive but more critical and quantitative than 15. 17 “Die Lehre von der strahlenden Energie,” G. Schmidt, in Chwolson’s Lehrbuch der Physik, 2 11. Vieweg and Son, Brunswick (1922). Helpful in radiometric work. “Die Lehre von der strahlenden Energie” (0 tik), 0. Lummer, in Muller-Pouillet’s Lehrbuch der Physik, I I th Edition. Vieweg and Son, grunswick (1926). Authoritative and up-to-date in optics. Light production and radiometry are not included. l o “Experimental Optics,” G. F. C. Searle. Cambridge University Prew (1925). Clear, practical and complete enough for photochemista. 2o “Dictionary of Applied Physics,” 4, R. Glazebrook. Macmillan (1923). Not written primarily for the photochemist. *l Photochemistry, 1g14-1g25. A. J. Allmand. Annual Reports on the Progress of Chemistry, London (1926). The best summary of the period. Does not discuss experimental methods. A Review of Photochemistry, W. A. Noyes, Jr., and L. S.Kaasel. Chem. Reviews, 3 , 199 (1926). Less comprehensive than 21. 2t “Photochemie und Photographie,” 1, K. Schaum. J. A. Barth, Leipzig (1908). 2‘ “Photochemistry.” S. E. Sheppard. Longmans, Green and Co., New York (19x4). Y “Lehrbuch der Chemie,” 3, 865-88 M. Trautz. Walter de Gruyter and Co., Berlin and Leipzig (1924). Not detailed r e g a r k g experimental methods. “Spectroscopy,” 2, E. C. C. Baly. Longmans Green and Co., Sew York (1927). lob

Light Sources. (General). The ideal photochemical light source is extremely intense and concentrated, and should be efficient, convenient and economical. Much of its total energy is radiated through a relatively small number of lines 26 evenly spaced in the actinic region. The continuous background should be relatively weak. It should be shaped like a monochromator slit, and better still have the same dimensions. Rapid variations in intensity about a constant mean value can usually be tolerated, and evenly progressive changes invite interpolation, but violent fluctuations are incompatible with quantitative work. A concave mirror behind the sourcez7or a projection apparatus2*increases the intensity of illumination. Mirrors can reflect radiation upon the sides of the vesselzgand a mirror placed behind the vessel multiplies absorption by a factor between I and 2 1 2 4 ,* 284. A weak or unsuitable source takes such heavy toll in time spent and in quality of results, that economy is the worst form of extravagance. The permissible minimum of light intensity is discussed below under “Analytical Methods.” ReferenceS1-2 , 11, 1 2 , 13, 15s 16, 23, 24, 1 2 0 8 “Handbuch der Spectroscopie,” 7, Kayser and Konen. S. Hirsel, Leiprig (1924). 27, 359 (1921). Kiss: Reo. Trav. chim., 42, 665 (1922). ‘9Plotnikow: Z. wisa. Phot., 21, 103 (1921).

*’Noddack: Z. Elektrochem.,

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GEORGE SHANNON FORBES

Sources with approximately Continuous Spectra SunlightK6,pp.1J-34 costs nothing, and is extremely intense in the visible, especially at high altitudesr6’pp. 17-” or in the Its spectrum, which practically terminates a t 3oomp16vp. z4 is for photochemical purposes best imitated by the blue gas-filled tungsten lamp, 16, p . ‘ I 7 or by the snowwhite flame Combinations of the mercury vapor lamp with various thermal radiators are its equivalent for color-matching only.” The mercury vapor lamp with a crown-glass filter is said to behave like sunlight in fading tests.34 Lenses or concave mirrors concentrate sunlight further, and water filters16’p. 49 mitigate the thermal effect. The extreme variability of sunlight,35 the need of a heliostatK8’ pp. 114-117, and the fact that its spectrum is continuous, except for the dark lines, all handicap quantitative monochromatic work. Such drawbacks, however, must not discourage further investigation of its immense possibilities. Dhar and others: J. Chem. Soc. 123, 1856 (1923). J. Phys. Chem., 29, 926 (1925). 3* hlott: Trans. Am. Electrochem. Soo., 28,371 (1915). 33 Ives: Bur. Standards Sci. Paper, 271 ( 1 ~ 0 s ) . 34 Flynn: American Dyest& Reporter, 12, 293, 837 (1923). 3 Coblenz and Kahler: Bur. Standards Sci. Paper, 378 (1920). 3O

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SoEd fidiat0rs~61PP. 107-164 . Incandescent filaments, straight or coiled, should, if used with a monochromator, approximate the dimensions of the slit. The thin glass bulb transmits much ultraviolet, and a quartz windowa6can be added. Overloading greatly increases the ultraviolet though shortening the life of the lamp. The Nernst glower3’ is still made by the GlascoLampen Gesellschaft, Berlin; a laboratory recipe has been p u b l i ~ h e d ; ~its~ spectrum is very faint beyond zjomp16*p. log. “Glowbar” is useful for infra’. Io’ could be fed uniformly into oxygen over a red work only. If magnesiumz4~ considerable period, i t would be of greater value. Flames of burning gases are infrequently used in photochemical work. Considerable ultraviolet is emitted by acetylene properly burned in oxygen 14r p. IO1 and by the carbon disulfide-oxygen lamp39,4 0 . High Tension Discharges under Water1Stpp. 244 between electrodes of aluminum, brass, tungsten, etc., from a step-up transformer coupled or not with a Tesla coil produce spectra continuous to 2 romp. Such sources are more valuable for photographic work41,(2 on absorption spectra than for photochemical reactions. Gelhoff: Z. tech. Physik., 1, 224 (1920). Coblenta: Bur. Standards Bull., 9, 103, 109 (1913). 38 Griffiths: Phil. Mag., 50, 263 ( 1 9 ~ s ) . 39 Wulf: Ann. Physik, 9, 946 (rgoz). 40 Tassily and Gambier: Compt. rend., 151, 342 (1910). 41 Tyndall: Bur. Standards Tech. Paper, 148 (1920), Appendix. ‘* Gibson: Bur. Standards Sci. Paper, 440 (1922). 86

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Sources with Discontinuous Spectra A ~ ~ ~PP.I2 7s- 2 9, ; 15,PP 107-164 , excepting flaming arcs, are not well shaped for use with slits. Feeding mechanisms allow considerable fluctuations. The ordinary carbon arc’s spectrum is weak beyond 2 5ompr6’p . Io’. The spectral energy distributions in crater and flame are unlike, and the lines are disadvantageously spaced. B a ~ s e t has t ~ ~studied carbon arcs carrying several hundred amperes. Carbon electrodes can be cored or impregnatedr6* I Z 4 The flaming arc has been carefully studied, especially by Mott4I4’. Of many metals, copper, cadmium, iron and tungsten are most frequently employed as electrodes. King46347 has operated arcs between metallic rods with IOOO2 0 0 0 amperes, producing normal and enhanced spectra in great intensity. The copper arc of Meyer and Wood4* gives radiations of very short wavelength. W. Taylor4” found the “Pointolite” lamp very intense in the visible.

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Bassett: Trans. Am. Electrochem. Soc., 44,153 (1923). Mott: Trans. Am. Electrochem. Soc., 31, 365 (1917). 43 Trans. Am. Electrochem. Soc., 37,665 (1920). 4( King: Astrophys. J., 62,238 (1925). Science, 65, January 7, supplement I O (1927). ( 8 hleyer and Wood: Phil. hlag., 30,4.49 (1915). 15’. Taylor: Phil. hlag., 49, 1166(1925).

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Sparks between metallic electrodes are almost indispensable for work below zsomp. Warburg’s zinc sparksIoa< 13’ gave a t z 54 mp a thousand-fold the intensity of his mercury vapor lamp. The elements of the second and third groups give advantageous spacing of lines.26 Many papers on the subl6 r40-144b ut more data on absolute spectral inject have accumulated49~ tensitics under specified conditions would be welcomed by photochemists. Warburg” *, I o , Lenard and Ramsauer’” ’, I s Z 1 , also otherss0’r633 ‘03 h ave described adequate installations. Many such outfit^'^, pp. 22-4 are undcrpowered. h transformer consuming several K.W. and yielding a t least 20,ooo volts should be used. Special wiring may have to be run through the building. Mica condensers are better than Leyden jars. Warburg‘ housed his spark gap in a chambe,r with a quartz window and an air-blast. He threw its image on a screen, and kept it constant by a feed worked by a glass rod,* but intensity varied 3-67c.6 Harrison and Hesthalsl combatted unsteadiness by rotating the lower electrode, and by regulating the gap and the primary impedance. K o w a l ~ k ?by ~ ~adjusting the capacity, brought the greatest intensities into the middle ultraviolet.

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Pfliiger: Ann. Physik, 13, 904 (1904). Henri: “Etudes de Photochimie,” 9. Gauthier-Villars, Paris (1919). 51 Harrison and Hesthal: J. Opt. SOC.America, 8, 472 (1924). 618 Kowalski: Compt. rend., 158, 1337 (1924). 49

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Mercury Vapor Lamps of fused quartz approximate the ideal light source in so many respects that most photochemists use them. Numerous modifications are de~cribed”~ pp. 30-s0. The cathode is always of mercury, the anode of mercury or of tungsten. I n the latter case reversal of polarity must be avoided. The commercial lamps have sealed-in electrodes, and not all of them

486


can be operated in both a horizontal and a vertical position. For alternating currents a rectifier is ordinarily used instead of a three-electrode lamp, but a two-electrode lamp for high frequency has been describedP2 The manufacturers supply detailed directions for operation. A lamp with sealed-in electrodes is slow to attain a steady state. Light intensity varied with line voltage and ventilation. They will not stand heavy overloading. First cost is high, and freshening, when the ultraviolet transmission has deteriorated 539 5 4 8 546, is expensive in time and money. The unsealed constant-pressure lamps recently described avoid the above difficulties55*56. 57, 5 8 . The disadvantages of heavy overloads are relatively small. Dr. P. A. Leighton of this laboratory has constructed an open lamp water-cooled and with adjustable pressures which can maintain for hours galvanometer deflections constant within a fiaction of a percent. When extreme concentration of light is not required, the bore of the constricted arc should be four rather than two millimeter^.^^ The ends of the luminous column should not be opposite the slit. The constriction can be bent to match a collimating slit curved to neutralize distortion of the beam at the exit slit. The constriction can be omitted and the lamp immersed in a tank so that the water level stands about a centimeter below the slit. The capillaries used to restrain oscillations@ can be made quite large if blocked with steel wires. A powerful air-blast can replace water cooling.59 Fuses afford protection against short circuits caused by collapse of the luminous column. The recently invented mercury vapor induction lamp devised by Foulkeao has great possibilities for photochemistry. It has no electrodes, but the vapor is excited by a rapidly changing electromagnetic field. Intensity is greatly increased by small amounts of inert gas, as is the case with lamps having electrodes.6* A lamp of I O cm. radius run on 2 . 5 K.W. gave a horizontal candle power of 20,ooo. Unfortunately the cost of installation is considerable. I* George: Rev. d’optique, 4, 82 iI925). 68 Coblents, Long and Kahler: Bur. Standards Sci. Paper, 330 (1918). =Reeve: J. Phys. Chem., 29, 39 (1925). 55 Harrison and Forbes: J. Opt. Soc. America, 10, I (1925). 56 Forbes and Harrison: J. Opt. Soc. America, 11, 99 (1925) 5’ Forbes and Harrison: J. Am. Chem. SOC.,47, 2449 (1925). 58 Forbes and Leighton: J. Phys. Chem., 30, 1628 (1926). 6*Villar~:J. Am. Chem. Soc.,49, 326 (1927). “JFoulke: New York Regional Meeting of the A.I.E.E., Nov. I I (1926).

Stark and KuchB1tried out Vapor Lamps with Other melted cadmium, zinc, lead, bismuth, antimony, tellurium, selenium. Lowry and Abramse2used solid cadmium. To avoid breakage Sand“ added zirconia, and BatesB4gallium. Sodium-potassium alloy has been used.B5 Amalgams contain such ingredientsI3’P. 14; 16, P. 150: 66 as cadmium, zinc, thallium and caesium to enrich the spectrum; the added metal collects a t one pole or on the walls, and inconstancy results.67 The Bates lamp would seem to be the most intense, steady, and practical of this group. Dr. George K. Burgess of the Bureau of Standards, in a private letter to me stated that all the cadmium lines including those in the region 250 mp to 185 m p can probably be obtained with greater intensity from this lamp than from any other source.


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Precautions. Although the dangers of ultraviolet light have been minimized,68 all will agree that high-grade welding goggles69'7 0 , I s pp. 72-76 with sides must be worn near a powerful source. It will pay to buy a light and a dark pair. Avoiding lateral exposure, ordinary spectacles afford some protection. Welding masks, gauntlets, and aprons follow goggles as exposure becomes more severe. Ellis and Wells discussrs'p. 299 susceptibility, with emphasis upon diabetes. A ventilated lamp-house with heavy red glass windows protects the experimenter and keeps ozone out of the room. Rubber is shellacked or wound with electric tape. A paint of zinc oxide and lampblack absorbs stray radiations.?' Stark and Kiich: Physik. Z., 6, 438 (1905). Lowry and Abrams: Trans. Faraday SOC.,10, 103 (1915). 83 Sand: Proc. Phys. Soc. London, 28, 94 (1915). Y Bates: Bur. Standards Sci. Paper, 371, 45 (1920). Xewman: Phil. Mag., 44, 944 (1922). Callier: Ann. Physik., 33, 1061 (1910). 6' von Halban and Siedentopf: 2. physik. Chem., 103, 73 (1922). 6 s Verhoef and Bell: Science, 40, 453 (1914). 89 Gibson and McNicholas: Bur. Standards Tech. Paper, 119 (1919). lowith Tyndall: 148 (1920). Andrews: Gen. Elec. Rev., 24, 866 (1921). 61

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Spatial Energy Distribution. If the distance exceeds ten-fold the greatest dimension of source or reaction vessel, the inverse square law holds within one percent'" p. 42' . Deviations for other conditions have been worked out20. "Distance" is neither to the front face nor to the centre of the mixture'5, p. 65 but rather to that imaginary surface at which half the total absorption has occurred. Unsymmetrical distribution, as with carbon arcs or incandescent filaments, can be mapped'of PP. 4'9-433; 2 3 , PP. 123-129. Integrals for vafious di? ~ a formula mensions of source and cell arz given17,pp. 435-9. T r i i m ~ l e r gives for mercury vapor lamps at short distances. The conduction of heat away from the ends of such lamps makes the energy E, radiated perpendicular to their axis, a t the ten-fold distance d , nearly equal to E/47rd2 per square centimeter of receiving surface, where E is total energy consumed by the lamp. Trumpler: 2 . physik Chem., 90, 395 (1915).

Quantitative Variation of Light Intensity a t constant distance without changing spectral energy distributionr6,p . I S o 8 p . I O 6 . ( I ) Multiplication of sources20.p . 445. ( 2 ) Variation in effective area of source (area is not always proportional t o energy transmitted). (3) Punched plates or calibrated wire l o b , P . 136 , in irregular motion.51 (4) Variation in electrical conditionsS5'5 6 . ( j ) Sector wheel.74 Certain objections should be noted,75, P 342: 100 P 458 . Rapid rotation is often important76 7 7 . (6) Double neutral wedge.78 (7) Polarizer and analyzer.79 (8) Oblique plates of glass or quartz (hard to f i y r e quantitatively). 23s

Davis: Bur. Standards Sci. Paper, 511 (1925). '5Coblentz: Bur. Standards Bull., 4, 458 (1908). ' B D a v i ~Bur. : Standards Sci. Paper, 528 (1926). 7' Briers, Chapman and Walters; J. Chem. Sac.. 1926, 562. 7 s Butschowitz: Chem. Ztg., 47, 382 (1923). 79Rosenberg: Z. Physik, 7, 18 (1921). li

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The Reciprocity Law predicts that, with other variables constant, total photochemical effect is proportional to the product of intensity and time. Unless the fraction decomposed is small, it is better to find whether the effect is constant for constant products of intensity and time. The limitations of the “law” have been systematically studied72 for the photographic plate only. Allmand”’ .’ 342 gives a non-critical summary of verifications and violation^'^ reported, 19 14-25 . He also discusses threshold intensities. Wood,8o also Pringsheima1 report that enormous intensities evoke reactions non-existent with moderate ones. If the “law” does not hold in a given case, light intensity must be defined in stating quantum yields. Also the usual mathematical treatment of the illuminated system would have to be corrected for variation in photochemical efficiency with varying depth in the reaction layer. The need for consideration of dark reactions, induction periods and after effects in studies of the reciprocity law is obvious. 80W00d: Phil. Mag., 43, 757 (1922). 81 Pringsheim: 2. Physik., 10, 176 (1922). 72 Jones and Huse: J. Optical SOC.America, 7, 1079 (1923);11, 319 (1925).

Quantitative Measurement of Radiation Intensity”’ pp- 731-4. Intensimeters8* with Sensitivity Curves.83 Photo-electric cells are now made in America which are practically free from time-lag,88 fatigue and deterioration, and with intensity-current curves that are almost linear. These are points of great superiority over older types.” Photo-electric cells are highly selective, but different alkali metals or hydrides have sensitivity peaks at different wave-lengths.*j They are most useful as null instruments86’ in comparing two intensities a t the same wave-length. F l u ~ r o m e t e r s ~are ~ ~ simple and inexpensive though not very precise. Sensitivity curves, and relations between incident and emitted light are given by Winther.89 Photographic plates have been studied by Harrison’” ’I, who mapped their “characteristic surfaces” generalizing the three coordinates intensity, time and density for each of a number of wavelengths. The ultraviolet range is increased by bathing with fluorescent Such data make it possible, by a simple photographic photometer,93 to measure the spectral energy distribution of false light (see below), and to measure absorptions (see Dr. de Laszlo’s report). Report on Spectroradiometry, J. Opt. SOC.America, 7,439 (1923). Coblentz: Bur. Standards Bull., 14, 507 (1918). 84 Kriiger and Moeller: Physik Z., 13, 729 (1912). 85 Rougier: Rev. d’Optique, 2. I33 (1923). von Halban and Siedentopf: Z. physik. Chem., 100,2 1 2 (1923). 87 Gibson: Bur. Standards Sci. Paper, 349 (1919); also J.Opt. SOC.,Am. 7,693(192.3). 8 8 General Electric Company, catalogue, 1927. See also Goos and Koch: Physik Z., 27,41 (1926). Winther: Z. Elektrochem., 19, 390 (1913). go Gyemant: J. Opt. Soc. America, 12, 65 (1926). 9’HarI.ison: J. Opt. SOC.America, 11, 341;DI I I J ; 93 157 (1925). g1 Biichi: Z.physik. Chem., 111, 269 (1924). 82

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Chemical Actinometers should employ only photochemical reactions which are reproducible, which involve only one phase, and whose photochemical efficiency is known for each of a number of wave-lengths. When speed of reaction falls off with decreasing concentrations, fresh reactants must be substituted. Silver halide paper actinometers are extremely crude devices. The Bunsen'4~P , 373 is not reproducible, the Eder14*p . 46x forms a precipitate. The uranyl oxalate p h o t o l y s i ~ in , ~ ~spite of complications, has many advantages. It is unfortunate that none of the reactions so carefully investigated by Warburgl-10 are convenient for general use. Self-integrating actinometry for specific purposes, valid even in polychromatic light, has been exemplified by Porcas and Forbes.g5 Photogalvanic Cells involve too many obscure phenomena to promise well as actinometers. None the less some use of them has been made.g6 Seleniumg7and Thalofideg8cells have sensitivity peaks a t the longer wavelengths, and other drawbacks for photochemical work. Dorcas and Forbes: J. Am. Chem. SOC.,49, 3081 (1927). geAthanasiu:Ann. Phys., 4, 319 (1925). g1 Pfund: Phya. Rev., 34, 370 (1912). gs Coblentz: Bur. Standards Sci. Paper, 380 (1920).

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Intensimeters practically non-selective"5v 99;' I , pp. 1534-44 produce thermal or electrical effects. In the thermal group the radiometers with vanesl"oslol~ 102 , torsion radiation bal~nce*0~, thermophotometer'M and radiocalorimet,er 105. 106 and in the electrical group the microradiometer107 and radiomicromcterIo7all leave something to be desired in range, or convenience or simplicity IO8, IO9, ' I x , 'Iz and the bolometer I O o . 'OB; of interpretation. The thermopile 109, p. 1 7 1 : 111, I13 , have preempted the photochemical field, and both can be made in the linear or the surface form4,p . The building and operation of the bolometer requires more technical information. The linear thermopile with a very thin quartz window, not exhausted, is robust, convenient and gives a relation almost linear between intensity and galvanometer deflectionsIo8, j 9 . For accurate work this relation should be plotted, and the zero kept a t a fixed point on the scale. By taking readings in a number of positions and integrating graphically the linear thermopile serves the purposes of the surface form. Following the detailed directions of Coblentz1"07I O 8 , IO9 thermopiles can be constructed and mounted by anybody having good eyes, a steady hand and considerable patience. Full directions for installation and use are given also. Several constructed in this laboratory, 30 to 40 X I mm. in area and having resistances between I 5 and 20 ohms have given excellent service. They have about two thirds the c.m.f. of the newest Hilger thermopiles and the deflections of the two types stand in a very constant ratio throughout the spectrum. At high intensities, shunt resistance, s, and series resistance, ( I , are provided. A serviceable relation is s = p (p - a)/a, where p is the resistance of the pile. Moll""* ' I 4 , 'Is and Burger have described vacuum thermopiles with exceedingly thin junctions. While too sensitive for ordinary photochemical purposes, they should prove useful in determining the spectral energy distribution of false light delivered by a monochromator.

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GalVanOmeferlOOsP. 415; 108. With thermopiles described above, a d’Arsonval galvanometer of resistance I 5-20 ohms and sensitivity 15 mm/mv116 will be satisfactory. The new loop galvanometers116”are very sensitive. Owing to drift, it is difficult t o measure the e. m. f. of thermopiles by potentiometry using the galvanometer as a zero instrument. If a Julius suspension is necessary to eliminate vibration, a heavy slab can be made of reinforced concrete. The scale support is cut from a thick board with the proper curvature, the mirror being a t the centre of curvature. Millimeter scales, with numbers written as mirror images, are tacked on. A row of small electric bulbs with a shield of bright tin plate is placed in front. 9 9 Fabry: J. Opt. Soc. America, 10, 543 (192 5). Io0Coblentz: Bur. Standards Bull., 4, 391 (1908); 101 12,503 (1916). I M Marsh, Compton and Loeb: J. Opt. Soc. America, 11, 257 (1925). I03Tear: J. Opt. Soc. America, 11, 135 (1925). ’MPlotnikow: Z.tech. Physik, 5, 113 (1924). IO6 Bowen, Hartley and Scott: J. Chem. Soc., 125, 1218(1924). ‘O6Bowen: J. Chem. Soc.,123,2328 (1923). 101 Johansen: Ann. Physik, (4)33, 517 (1910). lo* Coblentz: Bur. Standards Bull., 9, 7 (1913); 1°9 11, 131 (1914). Moll: Physica, 6,233 (1926). lllCoblentz: J. Opt. Soc. America, 5, 259 (1921). ll*Voege: Physik. Z.,22, 119 (1921). ”3Abbot: Astrophys. J., 18, I (1903). 11‘ Moll and Burger: Z. Physik, 32, 575 (1925). J. W. T.Walsh, Constable and Co., London (1926). 116 “Photometry,” Chapter 1 1 . 116 “Notes on Moving Coil Galvanometers.” Leeds and Xorthrup Co., Philadelphia (1925). 116* Mechau: Physik. Z., 24,242 (1923).

Radiation Standards and their Use. Of the various standard flames de171 P. 442 the Hefner 1ampl17, 118, 1191 1 2 0 is most often used as a total radiation standard. I n defining such standards the assumed value of the p. 88 should be mentioned. More convenient are the black body standard carbon larnps,’l7 with certificate and directions for standardizing thermopiles, ordered from the Bureau of Standards. Two should be used successively in crucial comparisons. Only standard models of ammeters can be used. I n particular, overloading must be avoided. A separate calibration curve may be established for each shunt-series resistance, and then all can be combined into one by proper reduction factors. The conditions of standardization and use must agree closely. To standardize the thermopile for intensities higher than those corresponding to the highest rated amperage of the lamp at the standard distance of two meters the distance is cut down and another set of readings taken for the same amperages. The deflections at the lower current strengths and the lesser distance are interpolated upon the standard curve, and the corresponding energies incident upon the thermopile are found. The ratios of these to the energies at standard distance with equal currents are averaged. The average ratio times the energy for any other current a t the standard distance will give the energy incident upon the thermopile at the shorter distance, and the


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energy-deflection curves are extended accordingly. No assumption is made, in the process, concerning the relation between intensity and distance. 11'Coblentz: Bur. Standards Bull., 11. 87 (1914). 118 Boltaemann and Basch: Sitaungsber, Akad. Wiss. Wien, 131 IIa, 57 (1922). 119 Gerlach: Physik. Z., 21, 299 (1920). 120 Kussmann: Z. Physik, 25, 58 (1924).

Spectral Energy Distribution of Light SourcedZoamay be roughly worked out by surface integration of a spectral energy curve taken with a spectroradiometer or monochromator having a narrow slit; correction for slit width mustbe made120b: I Z I ; 1 1 5 , P. 287 . Corrections for aberrations, reflection, absorption, and scattering in the optical train would be desirable, especially in the ultraviolet. Absurd errors result from uncorrected surface integration of such diagrams made with a wide slit. Fabry and Buisson122measured total energy I, transmitted by n different filters, of thickness d (or n different thicknesses of f . . , . . .. the same filter). They solved n equations of the form L = . . . A , the n monochromatic int'ensities of the original source. Of course ml . . . . . . .m, are determined separately for each filter. It is hazardous t o assume that two sources of the same type, especially mercury vapor lamps'23, have the same spectral energy distributions even if dis4. The photomensions, electrical conditions and the like are comparable533 chemist seldom needs to know the spectral energy distribution of the source itself, as he is primarily interested in monochromatic intensities at the exit slit of his monochromator or behind filters. Indeed he could interpret the total effect of polychromatic light only as a sum of monochromatic effectslZ4. The difficulties of such comparisons will be inferred from a study of the literaturezI, P . 342; 1 2 5 , 1 2 6 Coblentz, Dorcas and Hughes: Bur. Standards Sci. Paper, 539 (1926). Coblentz: Bur. Standards Bull., 10,41 (1914); 17, 36 (1920). 121 Franklin, Madison and Reeve: J. Phys. Chern., 29, 713 (1925). 122 Fabry and Buisson: Compt. rend., 152, 1839 (1911). 123Ladenburg:Physik. Z., 5, 527 (1904). 1Z4 Wegscheider: Z. physik Chern., 103, 295 (1923). 125 Padoa (andothers): Trans. Faraday Soc., 21, 573 (1925);~*'Gaea., 56, 164,375(1926). lzOB

~ 0 b

Polychromatic Light acting directly upon a sensitive system produces a total effect which is a highly complicated integraP4 involving spectral energy distribution of the source, absorption coefficients of reactants, products and perhaps of solvent or other substances, concentrations, thickness of layer, quantum yields, time and temperature. Its chief value is to detect photochemical reactions, sensitizers, catalysts, inhibitors, to study concentration effects involving non-absorbing (dark) reactants, and in some (though perhaps not all) cases to investigate temperature coefficients and the reciprocity law. Pome quantitative information may be gained regarding the above by instituting comparisons, throughout which the source, the thickness of the absorbing layer and the concentrations of all absorbing substances are practically constant. Obviously these conditions forbid the transformation, during any experiment, of any considerable fraction of any absorbing substance iinless all absorption coefficients are measured, or unless the sensitive reactant

492


absorbs all the light a t all stages of the reaction. On the other hand those who delight in “unimolecular constants” have only to start with a mixture which absorbs but a small fraction of the actinic light.

Quantum Yields in Polychromatic Light may be very crudely estimated as follows: The total radiation incident upon the cell face is calculated from the total wattage of the source and the inverse square law, or from thermopile measurements. Concentration and thickness of layer should be sufficient to insure complete absorption of actinic light. Only one absorbing substance should be present. Some arbitrary fraction of the total radiation can be assumed to be actinic. Better, the actinic region is located by trial with filters of given transmissions, and laid off upon a spectral energy diagram of the source, whichmust include all the infra-red. The ratio of the twoareastimes totalenergy isactinic energy absorbed. Average quantum yield follows from the latter, average frequency, and number of molecules transformed (with correction for dark reaction if any). It must of course be proved that no two spectral regions used cause the reaction to proceed in opposite d i r e c t i ~ n ‘ ~”*.” Light Filters”, x529-34:1 2 s p .694-6. The sum total of data on light adsorption is very great, but relatively few of the systems would make good lightfilters. The substance must be definite, reproducible and fast to light even if warmed up. If colloidal, its absorption coefficient mcst also be unaffected by dilution, by lapse of time or by any substance added. Many dyestuffs 1 2 9 1 130 fail to meet these specifications. If oxidizable, as ferrous s o l ~ t i o n s ~ 3 ~ , it must be covered by a suitable liquid. Transmissions, including the container if any, must be known or determined by the investigator for each wavelength of significance. When data on thin sheets are given, small variations in thickness are serious. The methods and precautions of spectrophotometry are given in Dr. de Laszlo’s report. If non-selective intensimeters are to be used behind filters, infra-red transmission must not be forgotten. Keyes13* interposes a vacuum cell as a protection against the heat of the lamp. A quartz water cell I cm. thick transmits 58YG at 945 mp and 1 4 7 ~a t 1 1 9 0 mp133. Coblentz has suggested filters to absorb infra-red completely’34,13’ Spectrograms of light sources taken through filters give little quantitative idea of the transmissions of the latter because it is impossible to make due allowance (except by the method of Luckiesh’“ *. ) for spectral energy distribution of the source136,sensitivity curve of emulsion, characteristic curves of the plate a t different wave-lengths, and the effects of reproduction processes. As filters have a t best rather broad transmssion bands, they should be used with discontinuous sources having strong emission lines nearly coincident with their respective transmission maxima. Some filters have a narrow absorption band which blots out one line of a particular source. The didymium filter, glass137or solution, absorbs the yellow line of the mercury vapor lamp, transmitting the green. Unfortunately such coincidences are rarc. Gaseous 36-45. While nitrogen is transparent even at 1 2 5 mw, 1 6 , p . 36 oxygen absorbs from 186 mp16sp.36, a fact which prevents the shorter wave-lengths from exerting their striking photochemical effects except at


493

very short distances from the source'38813g. Calcite, also, has been used to intercept these.138 If the ozone formed by mercury vapour lamps is not removed by a gentle current of air, radiations between 230 mg and 280 mp will suffer to some e~tent.13~Dr. A. L. Marshnll has suggested to the author that mercury in pumps or manometers may introduce enough vapors to cause trouble in working With resonance radiations ( I ) by opacity ( 2 ) through excited atoms. The most useful of the gaseous filters contain bromine and chlorine140f 141 or chlorine142in quartz and transmit Xz j q . VillarsI43finds the Peskov filter more nearly monochromatic. Mathews: Ind. Eng. Chem., 15, 885 (1923). Baly: Ind. Eng. Chem., 16, 1018 (1924). 129 Kruss: Z. physik. Chem., 51, 257 (1905). 13OUhler and Wood: Atlas, Carnegie Inst. Wash. Pub. (1907). 131 Byk: 2. physik. Chem., 49, 659 (1924). '32Keyes: U. 9. P. I , 237, 652 (1917). 133 Xichols: Phys. Rev., 1, 1 3 (1893). *34 Coblents: Bur. Standards Bull., 7, 655 (1911); I u 9 , I I O (1913). 136 Bielecki and Henri: Ber., 46, 3640 (1913). 137 Bur. Standards Sei. Paper, 148,9 (1920). Baly and others: Sature, 112, 323 (1923). 139 Ind. Eng. Chem., 16, 1016 (1924). Peskov: J. Phys. Chem., 21, 382 (1917).

1*7

128

Liquid and Solid Filters 2 6 t "O1. , Lack of space forbids any attempt to review critically the data as a whole. The variety of ultraviolet filters is inferior to that for visible, also the information about them. Certain glass filters and dyes were carefully measured up by the Bureau of Standards r44, IJ5 including monochromats for mercury vapor lamps. Commercial filters include Agfa, Corning, Cramer, Fuess, Ilford, Jena, Xaison Calmels, Wratten. P ~ t a p e n k o 'gives ~ ~ complete directions for making gelatine-dye filters. Wood's nitro~odimethylaniline1~7filter transmits the long ultraviolet only. Hood14Sdescribes glasses with rather extraordinary transmissions. Lists of filters are given in a number of books 1 1 , P P . 1529-34: 1 2 . P P . 694-6: 1 5 . P P . 78-84; 1 6 , PP. 4692' pp. 7 2 - 7 5 . Nickel oxide glass (Corning Glass Works) transmits 366 mp. 141 2. Physik, 18, 235 (1919). LP201denberg:2. Physik, 29, 328 (1924). 48, 1874 (1926). 143Villars: J. Am. Chem. SOC., Gibson and others: Bur. Standards Tech. Paper, 148 (1920); 1 4 5 ~Gibson: J. Opt. SOC. America, 13, 267 (1926). I45b Gray: J. Phys. Chem., 31, 1732 (1927). Potapenko: 2. wiss. Phot., 18, 238 (1919). 147 Wood: Phil. Mag., 5 , 257 (1903). 148 Hood: Science, 64,281 (1926).

Sci. Paper 440 (1922).

Procedures with Light Filters, if qualitative, require no extended comment here. Outfits including lamp, case, also holders for sheet filters and quartz or glass cells are advertised. In the absence of a monochromator, or with cells having large surfaces, a thermopile can be read, in several positions, between filter and cell and behind the cell. If radiometer is non-selective, the infrared radiation absorbed by the sc!utions must be considered. NoddackZ7de-

494


scribes a typical apparatus. Correction for reflection', p. can be made as described below. Reflected infra-red might erroneously be reckoned as a part of the actinic light absorbed. Such a method would work well using for instance a mercury vapor lamp, with filter 86 of Gibson, Tyndall and McNicho l a ~ ' ~ The ~ . line a t 366 mp146bis very strong and well isolated and would be transmitted almost free from contamination. On the other hand, the weak line a t 302mpcould scarcely be separated effectivelyfrom the strong line a t 313mp. General Methods for Monochromatic Light. Low-voltage arcs deliver monochromatic resonance radiation of low intensity149. That evoked in cold mercury vapor by transverse illumination from a mercuryvapor lampxsO~ Is' is about one fortieth as intense in A 2 5 4 mp as the source15o,and has possibilities for practical work. Ruled gratings are l i g h t - ~ e a k but , ~ ~ would ~ be the only obvious expedient for monochromatic photochemistry in the Lyman region. A monochromator based upon Priest and Gibson's rotary dispersion is imaginable. Focal isolation, so successful in infra-red work164 was used by AndrewsIs' p. to separate visible from invisible and by TereninIs5 for different ultraviolet regions. Professor F. A. Saunders kindly tried out the method with the author, but spectrograms of the ultraviolet showed much false light over a range of IOO mp. Cf. Foote and hleggers: Bur. Standards Bull., 16,317 (1920). Wood: Physik. Z., 13, 353 (1912). 151Cario and Franck: Z. Physik, 17, 205 (1923). 152E. C. C. Baly: "Spectroscopy," 1, 157 (1924). 153 Priest and Gibson: Bur. Standards Sci. Paper, 443 (1922). 154RubensandWood:Phil. hlag.,21,249 (1911). Terenin: Z. Physik, 31, 33 (1925). 150

Monochromators employing refraction dispersion are a t present relied upon for the best practicable compromises between intensity and monochromatic purity. Both improve with the size and optical refinements of the outfit. The simplest type is a slit-shaped source a t some distance from a prism, then a lens and a movable slit. I n practice a collimating lens is added, as well as other optical parts. The beam is usually horizontal, but may be made If a ready-made instrument is to be purchased, objectives less than 5 cm. in diameter are of limited usefulness in the case of weaker lines. The suggestion of Fabry157 that any spectroscope plus an exit slit is a monochromator was hardly intended for photochemists. A great variety of suitable prism monochromators has been described. 156B~ll: Ann. Phys., 2, 5 (1914). I57 Fabry: Rev. d'optique, 1,413,445 (1922). 158 Athanasiu: Rev. d'optique, 4,65 (1925). 159 van Cittert: Rev. d'optique, 2, 57 (1923); 5 , 393 (1926). Beatty: J. Sci. Inetr., 1, 33 (1923). 161 Feuas: Berlin-Steiglita, catalogue. 161 Hilger: London, catalogue. 163 Henri and Wurmser: Compt. rend., 157, 126 (1913). 184 v. Halban and Siedentopf: Z. physik Chem., 100,212 (1922).

EXPERIMENTAL TECHNIQUE FOR PHOTOCHEMICAL REACTIONS Houstoun: “Studies in Light Production,” chapter and Publishing Co., London. Kurta: J. Opt. Soc. America, 13,495, 1927 (1926). 167 Leias: Ann. Physik, 31, 488 (1925). le8 Pfund: Phys. Rev., 7, 291 (1916). In9 Rudberg: 2. Physik, 24, 257 (1924). 1 7 0 Schoof: Z. Instrumentenkunde, 42, 82 (1922). 171Wadsworth:Phil. hlag., (5) 38, 346 (1894).

12,

495

The Electrician Printing

“Home-made” Monochromators are relatively inexpensive, and if carefully constructed will prove very satisfactory. Dr. Villars of this laboratory has briefly described having a fused quartz prism ~ o Xj 105 X ~ o Xj 80 mm. Certain of the following suggestions were made by him or by Dr. P. A. Leighton of this laboratory. The above literature will suggest many variations. The necessary rigidity may be had by using a slab of soapstone drilled for bolts or reinforced concrete poured around the various supports. Several front slits are provided, with increasing widths, and curvatures sufficient to neutralize distortion of the emerging beam a t various wa~e-lengths~~l’; the flat side of the bevel is toward the source. A constricted mercury vapor lamp can be bent so that its luminous column will have the average curvature of the slits. I n this way vertical intensity gradients in the emergent beam are avoided. With most other light sources a quartz condensing lens of the type used in removing picture projectors can be used. Lenses figured to remove spherical conspherical aberration152are preferable. h’ickel or cave mirrors, though changing the spectral energy distribution of the IO3 . The source, have no chromatic aberration and can replace the rays should not be too far from perpendicular incidence. A fused quartz prism, 60°, and one of extra heavy flint 1 J d = 1.69 make an excellent outfit. Cob l e n t ~ l ’has ~ reported on the optical constants of lens and prism material. Fig. I , kindly prepared by Mr. W.G. Leighton of this laboratory shows the distribution of wave lengths on the focal plane of a monochromator for four different materials. The high dispersion of water174from 2 5 0 m p has suggested a hollow quartz water prism.175 The lenses will usually be made of fused quartz. Striae can be seen between crossed nicols or by other methods 17‘j. Combinations with fluorite or even water removes chromatic aberration. A prism table with lens taken from a small discarded spectroscope can be made over for larger units. A special table must carry the exit slit (of variable width) along the focal plane; it holds an exchangeable mounting for thermopile behind it, and another mounting for thermopile’ogvp . I S 4 still farther back. This last mounting can be moved across the light beam by a thread with millimeter pitch. Various automatic devices are suggested to keep the above system perpendicular to the light beam’” 16’. The parts are set up by trial, using a screen soaked in alcoholic anthracene“’ p. IZs6 . Collimation and minimum deviation a t X366 m p will usually suffice for other wave-lengths. To set the exit slit squarely on a given line the position of maximum galvanometer deflection is found, or spectrograms of the emerging radiation 4. P . 8 7 4 are made with an auxiliary spectrograph.

496


The emergent light may be made parallel or less divergent" p. **I by an additional lens. This greatly diminishes the light incident upon unit area of cell face, and cuts down percentage accuracy of thermopile readings. Also, a greater mass of reactant is needed to absorb a given amount of energy, which

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magnifies analytical errors. To meet this difficulty Villars used a cell of trapezoidal cross-section placed close to the exit slit. The glass sides diverged a t the same angle as the emergent beam of 366 mp. The front quartz window was only slightly wider than the exit slit used. Instead of integrating the light with a surface bolometer'' 749 or thermopile, VillarsS9read his linear thermopile in about ten positions behind his cell, and made a graphical integral of intensity and area. He has shown mathem a t i ~ a l l y the l ~ ~propriety of measuring absorption coefficients in an analogous fashion. It is possible a t least to halve the minimum error of 10% given by v. H a l b a ~ ~ ' "' ~ ~for . absorption measurements by thermopile except for a


497

weak line near a much stronger one, The thermopile should cover the whole beam in a vertical direction, or else an integration vertically should be attempted. Complications are introduced by fluorescence86 p , ' 1 7 and by turbidity 17'. The method of P l o t n i k ~ w ' p. ~ 'I s 4 for continuous removal of precipitates is not intended for quantitative work. Changes in absorption may occur when gasess3p. '39 or s01utes~~*' p . 3 2 1 t 3s9; r 7 8 a are mixed. Lambert's law should be proved to hold before assuming that the absorption coefficient varies with concentration. If two or more substances absorb light, this must be properly apportionedl'o between them, whether reactants or products, a t each stage of the reaction. Water must be taken into account a t the shorter wave-lengths, especially if the solute has small absorption; Kreusler's figures1**are not based gives the proper corrections for reflected upon a unit layer. Warburg'. p . light; v. Halban and EbertI7*describe a more refined correction for light absorbed and reflected by the vessel full of solvent. Absorption by good quartz is negligible to 250 m p a t least,'** but a small trace of iron tells a different story. The thermopile deflection a t z 54 m p should not be decreased by more than 8 per cent by interposing a I cm. water cell. At 2 0 7 mp absorption by quartz becomes perceptible4'4 7 . Boll: "Recherche8 sur I'Evolution photochimique," 80 (1914). Hulburt: Astrophys. J., 42, 205 (1915). Coblents: J. Optical Soc. America, 4, 432 (1920). 174 Duclaux and Jeantet: J. Phys. Radium, 5 , 92 (1924:. 175 Duclaux: Rev. d'optique, 2, 384 (1922). 176 Smith, Bennett and Merritt: Bur. Standards Sci. Paper, 373 (1920). 1I7 Villars: J. Optical SOC. America, 14, 29 (1927). 176v. Halban and Ebert: 2. physik. Chem., 112, 335 (1924). liba v. Halban: 2. physik. Chem., 120, 2 7 0 (1926). 179 0. WarburgandNegelein: Z. physik. Chem., 102, 241 (1922). noLutherand Weigert: Z. physik. Chem., 53,408 (1905). I 8 l Kreusler: Ann. Physik, 6,421 (1901). Joos: Physik. Z., 25, 376 (1924). 17'*

False ~ i ~ h f 8 18, 4 ;PP. '03-7 . The purity of monochromatic light, A i A A , may attain IO^ in extreme cases,16ibut no such refinement is attempted in photochemical work. The degree of monochromatism which is necessary to attain a given degree of accuracy in a quantum yield will vary greatly with the system under consideration. If the photochemical efficiency $I reckoned in molecules per erg is practically constant over a given spectral region, the false light in the same region is equivalent to that of the principal wave-length. The error is worst for a weak line, near a strong line, in a region where d@'dX is large. KuhnlsSrequired z to 2 . j quanta per molecule of ammonia over the interval 202.5 mp to z 14 mp, but when a second prism was added and only the lines 206.3 mp and 2 I O m p were transmitted, no reaction at all could be directly observed. One may not agree with Kuhn's contention that the ammonia molecule must first absorb hv, and then hv' not coinciding with any frequency absorbed by the normal ammonia molecule. But his findings should give pause to all those who work with imperfectly resolved light, especially when gases are involved.

498

GEORGE SHAXXOX FORBES

Thermal radiators, especially, emit an amount of infra-red large in proportion to the light a t shorter wave-lengths. Coblentz*86has emphasized the importance of taking occasional thermopile readings behind a shutter of electric smoke or Corning red glass, and subtracting the indicated correction. False light in other spectral regions can be cut down by using a narrower monochromator slit, or a double r n o n o ~ h r o m a t o r16’ ~ ~which ~ ’ squares (roughly) the fraction of the false light. If Lambert’s law holds over a considerable range of thickness of layer of any given absorbent, the degree of monochromatism is probably g 0 0 d . ” ~ ~331 Filters used with monochromators cause a loss in intensity which magnifies the effect of analytical errors without necessarily making the monochromatism much better. Warburg63P. 3 2 2 also Vilset up an auxiliary spectrograph with a narrow slit behind theirexitslits, and examined the spectrogram for false light without attempting a quantitative correction. To make this, such a spectrogram could be taken after each photochemical experiment, measured up on a densitometer and relative intensities estimated at suitable intervals after some correction for change in photographic density with ~ a v e - l e n g t h . ~The ~ relative intensities could be more directly found by an auxiliary spectroradiometer. These would next be plotted for each position of the exit slit, on the same wave-length scale as that of the auxiliary spectrograph. The “pure light” would be arbitrarily marked off from the “false light,” and the ratio between the two areas determined by graphical integration. The thermopile reading for the corresponding experiment would be accordingly corrected. Xext the ordinates on the above curve would be multiplied by the uncorrected photochemical efficiencies found for the corresponding wave-lengths. Finally one would mark off the areas under the new curve corresponding to “pure light’’ and “false light,” and deduct the latter from the total photocheniical reaction, after correction for any dark reaction. A cruder method of correcting for false light, when using light filters, has been described by Luther and Sutting: Bur. Standards Bull., 2, 439 (1906). lS5Ruhn:J. Chim. phys., 23, 521 (1926). Coblentz: Bur. Standards Bull., 14. 229, j34 (1918). I a 7 Luther and Forbes: J. Am. Chem. Soc., 31, 776 (1909).

Polarized Light. Ghosh states that circularly polarized light has a slightly greater photochemical action upon solutions than unpolarized light of equal ~ t 192 I 9finds 0 . that polarized light produces changes intensity188. 189. ~ v ~ i ~ ~ 191, in light absorption by certain solids. Sofids12s P. 7 3 8 : 2 2 , P. 2 2 4 , Methods for photochemical study of solids may be found from the references given. P l o t n i k ~ w ’*~r44 , formulates the progress of the reaction zone in gelatinized media where a colored reactant is decolorized by light without c o n ~ e c t i o n ’p~. 144; , 188 Ghosh and Pukayestha: Chem. Abs., 20, 1953 (1926); I89 Ghosh and Kappanna: 20, 3646 (1926). 1gOWeigert: 2. Elektrochem., 24, 2 2 2 (1918); 1g1Z. Physik, 5 , 410 (1921); I g 2 Z . physik. Chem., 101, 414 (1922). IQ3 Padoa: Atti hccad. Lincei, 28 ( 2 ) 372 (1919). lo( Soyes: Compt. rend., 176, 1468 (1923).

EXPERIMENTAL TECHNIQUE FOR PHOTOCHEMICAL REACTIONS 135 1g6

499

Bowen and others: J. Chem. SOC.,125, 1218 (1924). Benrath and Schaffganz: Z. physik Chem., 103, 139 (1922).

found that the speed of photolysis of potassium Stirring. Karburg" p. nitrate was the same whether the solution was stirred or not. In this case the absorption was not high, and the reaction was slow. On the other hand, stirring was indispensable when a mixture of maleic and fumaric acids', 960 was irradiated because absorption was high and the speed of isomerization considerable. These principles are generally applicable. It would be superflous to recount here all the types of stirrers for liquids, including gas bubbles. Warburg', p . 300 shows how to keep a gas-stream well mixed while passing through a light-beam. If the stirrer is opaque to the radiation there must be corrections for its volume and for the shadow behind it. If transparent, one corrects only for the "hole" made in the reaction mixture, unless absorption is complete anyhow. Temperature Regulation is nowz1,*346-8 believed to be less important than formerly,'48pp. 6 z - ' 2 unless there is a considerable "dark reaction." The temperature coefficients of photochemical reactions proper seem to be small. Much ingenuity has, however, been expended on "light-thermostats," few of which would be suitable for monochromatic work. Running water,lQ7watercooled flasks,1gsliquid thermostat with glass walls,199stirrer of coiled tube through which chilled brine circulates,18i airbath and fan, transparent vapor or liquid baths,200envelope of refluxed naphthalene,201electric ovens '02, '03 are typical devices. Plotnikow'3: pp. 68-106; Plotnikow": pp. 122-130; Jung": pp. 1504-5, Igi Bolin: Z. physik. Chem., 93, 721 (1919). Allmand: J. Chem. Soc., 127, 822 (192j). 199 Berthoud: J. Chim. phys., 21, 308 (1924). * O 0 Weigert: Ann. Physik, 24, 55 (1905). 2D1 Bodenstein and Lutkemeyer: 2. physik. Chem., 114, 208 (1924) 202 Coehn and Becker: Z. physik. Chem., 70, 93 (1910). ?03 Kuhn: Compt. rend., 178, 708 (1924).

Dark Reaction Velocity. This is determined inside or outside of the reaction vessel by the usual methods. If the dark reaction is important all work should be carried out a t the lowest possible temperature, and the reaction layer should not' absorb more than half of the incident light. W'egs~heider~~* points out that the photochemical reaction is not the difference between total, and dark reaction determined separately. The error in such an assumption would be negligible, however, if total reaction is small in comparison with amount of reactant initially present. If the temperature is not constant during exposure to light, the temperature coefficient,of the dark reaction may have to be measured, and the total dark reaction found by summation for a number of short time intervals. Calculation of Results. Wegscheidern4 gives various equations for reaction velocities, and integrals, developed exclusively from the standpoint of dark reactions. He compares his deductions with those of Plotnikow,'* taking issue with him upon certain points183. Warburg'-9 was the first to make adequate calculations of quantum yieldsjg; he included all the important cor-

500

GEORGE SHANNOK FORBES

rections except thac for false light. Allmand’s reportz1summarizes recent conclusions based on observations of reaction velocities, while the Oxford symp o ~ i u m ‘is~concerned ~ primarily with quantum yields. These two methods of forniulating photochemical reactions are supplementary,’17 p . 3449 and are interconvertible when complete quantitative data are available for monochromatic intensities, absorptions, concentrations and quantities transformed during given intervals. The less complete such data are, the more cautious must be any interpretation, and the more commodious the holes provided by the author for his own escape. Ia3

Wegscheider: Rec. Trav. chim., 44, 1118 (1925).

Analytical Methods are chosen with reference to their probable errors and with reference to the quantity of product which can be formed with the given light source in the time available. Time may be limited by a rapid dark reaction or by laboratory conditions. To form a millimol of product, for instance, with unit quantum yield a t 366 mp, 3.3 X 109ergs must be absorbed. If the intensity is L ergsjsec and n the number of millimols needed, the time will be n X 9 X 105jLhours (assuming complete absorption). A brand-new commercial mercury vapor lamp of standard type, run on 2 . 2 amperes and I O volts/cm will deliver a t an exit slit 40 X I nim, of a monochromator with 50 mm. objectives and an aperture ratio 4.5, 8 X 1 0 3 ergs/sec a t 366 mp, and 4 X rozergs/sec a t 280 mp where the quantum is nearly a third greater.57 Therefore the exposure times per millimol would be I I O and 3000 hours respectively.) The other important wave-lengths of the lamp have intensities between the two extremes. The time necessary to get n millimols would be increased by unfavorable quantum yields or by low absorptions. These considerations show why BolPS6despaired of doing monochromatic work with a mercury vapor lamp if he had to determine the product by chemical analysis. Bodenstein and T,iitkemeyerZo1used the full radiation of the tungsten arc to study hydrobromic acid formation “as dispersed or filtered light would have taken too long.” Instances of this sort could be multiplied. The constricted mercury vapor lamp without its maximum overloads7will give ten times the slit illumination of the commercial lamp. Sparks from a 3 K W transformerxoa, 13’ give and z x 1 0 4 ergs/sec a t 2 o j mp. I t is difficult to increase monochromator intensities much beyond such values. Chemical methods in such special cases as iodimetry’, p. *” orelectrometric titration16ggive with a small fraction of a millimol an accuracy of a few percent, which is the probable error of determining intensities. If a cylinder or bulb is fused to the top of the light reaction vessel the titration can be completeted without transfer of the contents. Pregl’s microchemical methodszos are suggestive to the photochemist. Gas analysis using chemical absorbentszu4 may suffice. W ’ a r b ~ r p. g ~764~ has stressed the importance of rapid removal and determination of unstable products, making correction for the fraction lost. Certain physical methods”, 1492-6 of analysis require small amounts of substance. They are indispensable when chemical methods change the concentration of the substance to be determined or when a number of measurements must be made without heavy inroads upon the reaction mixture.

EXPERIMENThL TECHNIQUE FOR PHOTOCHEMICAL REACTIONS

501

For gas reactions where the number of molecules changes one can use the quartz manometer with flattened tube,206the quartz thread m a n ~ m e t e r , ~ or~ ' the various manometers containing mercury'' 746 or other liquids. Hydrogen has been determined manometrically after condensing chlorine and hydrogen chloride in a side tube by adequate refrigeration.208 Such a method is particularly useful when the number of molecules does not change. Light abif feasible, are advantageous. For liquids also sorption meas~rements,~09 absorption measurements have been used.21° Conductivity9' Is6< l'* and potentiometry210 are very sensitive. The fluorescence change in ultraviolet lightZ12has been suggested as a means of following concentration. Photo'I4. electric effects have been Bredig and Goldberger: Z. physik. Chem., 110, 536 (1924). Pregl-Fvleman: "Quantitative Organic Microanalysis." P. Blakiston's Son and Company, Philadelphia (1924). 206 Bodenstein and Kistiakowski: Z. physik. Chem., 116, 371 (192.j). ? O i Haber and Kerschbaum: Z. Elektrochem., 20,296 (1914). * O s Bodenstein and Dux: 2. physik. Chem., 85, 300 (1913). *09 Pusch: Z. Elektrochem., 24, 336 (1918). Rideal and Fiorrish: Proc. Roy. Soc., 10JA,357 (1923). *11 Schwartz: Ber., 5 8 , 746 (1925). 212 Mellet and Bischoff: Compt. rend., 182, 1616 (1926). "3Volmer: Ann. Physik, 40, 775 (1913). ? I 4 Volmer and Riggert: Z. physik. Chem., 100, 502 (1922). 204

206

Reaction Vessels. For quantitative work the faces should be plane, and perpendicular to the light beam. W a r b ~ r g ' -developed ~ in a high degree the cylindrical cell with plane parallel ends for liquids, and for gases in rest or in flow a t pressures from I to 400 atmospheres. He used a lens behind the opening in the focal plane to make the rays parallel.29p.I'' It was necessary to fill the cell with radiation when an unstable product such as ozone was flowing through it. The trapezoidal vessel for use in diverging light has been described above. Crystal quartz must be cemented or clamped, but fused quartz cells cylindrical, rectangular or trapezoidal can now be made either in the oxyhydrogen flame or by applying in powdered form a flux slightly more fusible than the quartz and fusing the joints together all a t the same time in an electric furnace. A new glass has recently been described by Hood148which would be serviceable down to zoo mpj and which can be had in sheets several inches square. More information concerning its resistance t o chemicals would be welcome. Other promising glasses are said to be in process of dcvelopment. Cements'" P. 6g3: 235 should be avoided if possible, but they will give good service if intelligently selected. Warburg" '"ground his surfaces accurately and found that the thinnest film of paraffin, with a clamp, was sufficient. Fused silver chloride, used as described by HulburtZ1j makes good joints between quartz and glass, and graded seals are possible. Other cements that have been found useful are fish glue, gelatine and concentrated acetic acid, wax and colophonium, "picein" (a German product), de Khotinsky, lead oxide and glycerine,235lead borate and various dental cements.

502

GEORGE SHANNOX FORBES

Spherical vessels should in general be avoided, and cylindrical vessels illuminated perpendicular to their axis are unsuitable for quantitative work. Formulas for absorption in the latterI4> I S 5 are not corrected for reflected light. Concentric annular vessels, one of which may be a mercury vapor lampzl6and another a light f i 1 t e F or a water cooler*17constitute very compact and efficient systems, especially for qualitative work, and make it possible218to use the shorter ultraviolet, which is absorbed by air. Among recent specimens of apparatus for investigation of gases include one of Coehn and ,Jung219,the whole of which could be heated to the softening point of glass to remove adsorbed gases, also one of Marshallz1g"which could be baked in a furnace a t 300' to 400' for the same purpose while passing electrolytic chlorine. Xorrish and Kidea12z0devised a train for mixing hydrogen, chloride and oxygen in any proportions, dried with pentoxide, and measured manometrically. Porter, Bardwell and LindZ21realized a valid comparison of hydrogen-chlorine mixtures of equal sensitivity subjected to light and to radiochemical action. 2LJHulburt:Science, 56, 147 (1922). ?IeFischer: Physik. Z.,6 , 575 (1905). 217 Marshdl: J. Phys. Chern., 30, 34 (1926). 21* Coehn and Stuckhardt: 2. physik. Chem., 91,722 (1916). 219 Coehn and Jung: Z. physik. Chem., 110,714 (1924). Marshall: J. Phys. Chem., 30,757 (1926). 220SorrishandRideal: J. Chem. SOC.,127,787 (1925). 221 Porter, Bardwell and Lind: J. Am. Chem. SOC.,48, 2603 (1926).

Special Procedure with Gases. Intensive drying has been discussed at lengthinthe literature21, PP.349,368; 11, PP. 1557-60; 2 2 2 , 2 2 3 . . TheBuddeeffect has been unraveled by Lewis and RideaLZz4 Weigert and KellerrnannZzbhave devised a most ingenious apparatus for taking many photographs of the Draper effect have followed during the initial 1/40 second of exposure. Coehn and HeymerZz6 the hydrogen-chlorine reaction in a jet from which the gases emerged into an atmosphere of hydrogen, to eliminate effects of contact with walls. Of extreme importance in photochemical theory and practice are the reactions initiated by rayless collisions with excited atoms or molecules"' p* 3b1367; 226s. Experimentation with excited mercury vapor has developed some special types of apparatusZz7. 222 Tramm: Z. physik. Chem., 105, 356 (1923). m3Piorrish: Trans. Faraday SOC.,21, 575 (1925). 224 Lewis and Rideal: J. Chem. SOC.,1926, 583, 596. 221, Weigert and Kellermann: 2. physik. Chem., 107, I , (1923). 116 Coehn and Heymer: Ber., 59B, 1794 (1926). 2*8a Franck and Jordan: "Anregung von Quantenspriingen durch Stosse." Berlin, Springer (1926). n7Franck and Cario; Z. Physik, 11, 161 (1922); 228 17, 202 (1923). 2 z 9 Duffendack and Compton: Phys. Rev., (2) 23, 583 (1924). 230Xoyes: J. Am. Chem. SOC.,47, 1003 (1925). 231 Hirst: Proc. Cambridge Phil. SOC.,23, 162 (1926). 232 Taylor, Marshall and Bates: Nature, 117,267 (1926). 233 Rideal and Hirst: Nature, 117, 449 (1926). 234 Dickinson and Sherill: Proc. Nat. Acad. Sci., 12, 175 (1926). 2s Neville: J. Phys. Chem., 30, 1181 (1926).

Experimental Technique for Quantitative Study of Photochemical

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