ABSORPTIOIY COEFFICIEXTS BY HENRY G . DE LASZLO*
A great divergence of views' as to the exact mechanism of absorption in photo-chemical reactions st,ill exists. We shall understand this mechanism better if research be undertaken on the absorption spectra of each of the several components of the reacting system, be it in solution, or in gaseous phase. These spectra should be measured quantitatively, Le., the exact' positions of the absorption bands in the spectrum and their absorption coefficients should be determined accurately before commencing a study of the reaction. This might give a clue as to why for a given reaction, light of one wave-length is photo-chemically active, and inactive if of another wavelength. Until very recently, absorption coefficients have only been determined for solutions or pure liquids. These will therefore be discussed first. The methods may be divded into two classes, h and B. (A) In general, photographic methods are the best and nearly all of them depend on the folowing principles: h common light-source, which is usually an iron spark, is optically divided into beams of equal intensity I. One (a) passes straight into the spectrograph and the other (b) reaches it after having traversed the solution. S o w if by some means (a) can be varied equally t'hroughout the spectrum with one of the methods cited below, or if the time of exposure f be changed for any given concentration one would be able to find some position (A) common to the two spectra where the lines are of equal intensity. This is plotted against the corresponding absorption coefficient S. The molecular absorption coefficient Z is calculated from the relations I = I,. IO-^^^ where c is the molecular concentration and d the thickness of the solution or gaseous layer, I being the intensity of the light before entering the solution and I, the light after traversing same. Since I is proportional to f, the time of exposure for a given photographic effect, then Z = -xc * log -.t d to z is the constant for the photographic emulsion used, and lies between .8 and .9. Thus we see that either I or t may be varied. There are a variety of ways of changing I and they may be roughly classed as follows: I. 11. 111. IV.
Varying distance of the light source, Diaphragm in path of rays. Absorbing medium. Xicol prisms.
Research Associate, Massachusetts Institute of Technology. Trans. Faraday Society, 21, 438 (1926).
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HENRY G. DE LASZLO
I. This is usually the most accurate and theoretically correct method, but it is difficult to carry out in practice. Schaefer has, however, develooed it and the apparatus can be obtained from Zeiss- Jena.2
11. Sectors An equipment for visual spectrophotometry has recently been adopted by Guilde3 This method uses a rotating sector with variable aperture and depends upon the validity of Talbot's Law as shown by H ~ d e Slany .~ experiments have shown that the latter holds to within the accuracy of the best photometric measurements down to quite small angular apertures. The original sector photometer was introduced by Hilger in 1913, and has been mostly used in conjunction with quartz or glass spectrograph^.^ An improved form of sector photometer was originated by Judd LewisJ6 but the apparatus is both expensive and difficult to keep in adjustment, though very rapid in use. When using the photographic method, it is best to use only two sets of exposures, and one aperture of the sector, a t the same time varying the thickness of the solution layer. This sector method can be used throughout the visible and ultra-violet regions and gives very accurate curves. W z r e Screens Winther' describes the method of reducing I by means of blackened wire screens placed in the path of one of the beams. These screens are perfectly achromatic and any reduction of I can be obtained by varying the spacing and thickness of the wires. h set of screens can thus be made t o cover any range of 2. 111. Merton* uses a sheet of quartz upon which a platinum wedge has been cathodically deposited. This is found to be entirely non-selective in its absorption and can be calibrated accordingly, and then placed in the path of one of the light beams. IV, hlost of the methods for investigating the visible spectrum are of this type. The Konig-Martens spectrophotometer enables Z to be determined for each A. It has been both universally applied and frequently des~ribcd.~ h spectrophotometer has recently been constructed by Bellingham and Stanley.10 They use nicol prisms of special construction which are transparent down to the limit of calcite 2 5 0 0 A. It is attached straight onto the slit of a spectrograph and as in I1 employs the juxtaposition of two spectra. It is perhaps more accurate than the sector photometer owing to the difficulty of cutting sector openings accurately. Its shape, however, does not allow of having solution layers thicker than 5 cms. Z. angew. Chem., 33, 2 j (1920). Trans. Opt. Soc., London, 26, 74 (19zj). Bull. Bur. Standards, 2, I (1906). Cat. of Adam-Hilger, H-3 (1924). J. Chem. Soc., 115, 312 (1919). 7 Z. wiss. Phot. 22, i z j (1923). 8Proc. Royal Soc., 97, 181 (1920). Ann. Physik, (4) 12 984 (1903). lo Address: Hornsey Rise, London. 2
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ABSORPTION COEFFf CIENTS
A method whereby the light intensity is kept constant, and the time of exposure and concentration being varied, has been used for many years by Victor Hemi.“ He2ri’s photographic method is applicable over the whole range of 8000-2000 A using a glass spectrograph for the visible, and a Hilger quartz E-z instrument for the ultraviolet. It is simple and its accuracy has been rigorously tested.’* The author has used i t for many years, and the following refinements may prove useful. A light source of unvarying intensity is a prime necessity to insure accuracy. We use a z mm. spark between iron electrodes (8000-2500 A) and copper for the range Z ~ O O - I ~ O O quenched by a synchronous rotating gap to prevent arcing and assure constancy. Solutions of known molecular concentration are made up in optically pure hexane,13 alcohol or ether. A Baly absorption tube constructed of fused quartz with the inner tube ground into the outer like a hypodermic syringe is then filled with the solution. A similar tube is filled with the pure solvent. The spark spectrum after having traversed a thickness d of the solution for a time t is photographed with the spectrograph using a slit height of 1-2 mm. After moving the plate carrier down a distance equal to the spectrum width a second exposure for time to (5-10seconds) is made through the same thickness d of solvent. A metol-hydroquinine developer is used. A Leitz-Metzl4 binocular microscope with a magnification of 25 X may be employed without eye-strain for identifying the lines of equal intensity for each set of exposures in which d is constant. BJ varying t , d , and c, the 2 corresponding to any X can be determined to &/A and is plotted against I/X. To supplement the above methods a continuous source of light should be passed through the solution in order to locate intensity maxima of the smaller absorqtion bands. A gas-filled tungstenl5? lamp is satisfactory down to 3000 A, or a high frequency spark17v under distilled water between duralymin or copper electrodes, using the iron spark as a standard, covers the whole spectrum down to 2000’A. (B) Photoelectric Cell and Microphotometer Methods. Direct reading methods consist in using a thermopile or photoelectric cell instead of the eye in conjunction with some form of constant deviation spectroscope to act as a monochromator. Thus a direct measurement of light absorbed for each wavelength may be made b y noting the galvanometer or electrometer deflexion before and after traversing- the absorbing medium. The most accurate work
a,
“Etudes de Photochirnie.” Henrich: “Theorien der-organischen Chemie,” 348 (1924); Ley and Volbert; Z. wiea. Phot., 23, 41 (1924). l 3 Castille m d Henri: “Bull. SOC.Chim. biol., 6,299 (1924). L3de Laszlo: Proc. Roy. SOC.,111, 363 (1926). I 3 de Laszlo: Ind. Eng. Chem., 19, 1367 (1927). 14U. S. P. I , 501,059 Leita (Wetzlar). l5 Loebe and Ledig: Z. tech. Physik, 6, 325 (1925). ladeLaszlo: J. Am.Chem. SOC.,50, 899 (1928). “Howe: Phys. Rev., (2)8, 676 (1916). Strachan: Konen-St,ahler, Vol. 11, p. 519 (1919). There are a number of other methods of Spectrophotometry in the visible re ion which and in Bur. Bur. Btandards, are well described in the J. Opt. SOC.America, IO, 170 (19251, 18, No. 40(1922);Weigert: Ber., 49, 1496, 1530 (1916);Jones: J. Opt. Soc. America, 10, 561 (1925). 11
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HENRY G. DE LASZLO
has been that of Halban and his pupils,lg who extended their observations throughout the visible and ultra violet portions of the spectrum. Gibson?”has developed a null method which seems promising, but until now has only been used for the visible region. Photographic”’ ?‘ . Here a constant beam of continuous light is passed through the solution onto a spectrograph for a knoivn time. The same exposure is repeated without the solution, the spectrum being in juxtaposition to the former. This is followed by an iron comparison spark. The absorption will be clearly seen on the continuous background and is measured in some form of direct reading microphotometerz3,24 using either nicol prisms of neutral tint glass wedges to obtain a matching of photographic density. Automatic registering2js26 photometers are the most convenient. In all cases the instruments may be directly calibrated in S beforehand by means of some known substance. The Determination of the Absorption Coeflcients of Vapors and Gases has been much neglected until very recently. Considerable work has, however, been done by Hcnri and his pupils in measuring the exact position of absorption bands and their fine structure. For substances with high vapor?’ pressure a t 70°C we use tubes of fused quartz into which, after evacuation, a small amount of the vapor is allowed to enter. By varying the pressure and passing continuous ultra-violet light through the tube for the same length of time t , we get a complete curve showing all fine structures with comparative data as to their absorption coefficients. The latter may be more accurately determined by methods similar to solution spectra using an iron spark. For substances with a very low vapor pressure a t 2oCCwe use quartz tubes of a special design electrically heated throughout a wide range of temperatures. The exact values for E may be found by inserting weighed quantities of substance into an apparatus consisting of a number of quartz absorption tubes of known total volume and off varying lengths connected to a common reservoir. We then will know the number of gram-mols per litre and hence from the time of exposure and thickness of layer can calculate Z for each X. The same system may be adapted for bodies with low V.P. by making the apparatus smaller and inserting all but the reservoir in a furnace which is heated to the highest temperature t o be used in the experiment. The reservoir is heated in a separate furnace whose temperature can be accurately measured and may be raised in steps, photographs being taken a t each step through each of the several tubes. Such a series of photographs will again give us exact data as to both 2 and the position of the bands. 19Halban and Siedentopf: Z. physik. Chem., 103, 71 (1922); Halban and Eisenbrand: Proc. Roy. SOC.,116, 153 (1927). 2o Bull. Bur. Standards, No. 349, 327 (1919). 21 Riboux: Ann. Phys., 11, 107 (1919). 22Henri:J. Ph s., 3, 181 (1922). 2a Buisson and gabry: J. Phys., I, 25 (1920). *‘Yvon: Rev. d’Optique, 1, 499 (1922). 26 Koch and Goos: Ann. Physik, (4) 39, 705 (1912). 28 Moll: Proc. Phys. SOC. London, 33, 207 (1920). 2’deLmzlo: 2. physik. Chem., 118, 369 (1925);J. Am. Chem. SOC.50, 899 (1928).
