PHOTOCHElIICAL OXIDATIOS WITH P O T . i S S I C ~ I DICHROhlhTE * Downloaded by UNIV OF NEBRASKA-LINCOLN on August 28, 2015 | http://pubs.acs.org Publication Date: January 1, 1928 | doi: 10.1021/j150302a002
BY D. S. MORTON
The reaction between quinine and potassium dichromate in a solution containing sulphuric acid has been studied as a photochemical oxidation by Luther and Forbes,‘ who reached the conclusion that only the light absorbed by the quinine was active, the chromic acid functioning merely aq an inner light filter. “ K e chose the reaction between quinine and chromic acid, already studied by Goldberg,* whose results seemed to show that the velocity was dependent on the quantity of light absorbed by the chromic acid. Experiments with monochromatic light soon convinced us that quinine was the substance sensitive to light, and that the chromic acid acted only as an indifferent light filter, absorbing a part of the light and turning it into heat. . . . “Since the yellow ray is very slightly absorbed by the reaction mixture, it cannot cause appreciable cheniical change; hence experiment ( I ) indicates that the dark correction is adequate, and the consistency of the following results points to the same conclusion. The blue ray is virtually inactive; in the table, correction is made for 0 . 0 2 [406] which filter B transmits. The violet and the ultraviolet rays are active, and the constancy of the results in the last column show that the photochemical action is proportional to the incident light in each case. . . . “The figures thus far exhibited prove ( I ) that the amount of chemical change in each unit of time is proportional to the fraction of a given ray absorbed by quinine in the same period. ( 2 ) the amount of light absorbed by the chromic acid has no effect on the result. From ( I ) and ( 2 ) the apparent inertness of the blue ray is no longer surprising. (3) KO sensible error is involved in finding the light reaction by difference. . . . “From these figures it is inferred that the photochemical reaction takes place in two stages: first, the formation of sensitized quinine with a velocity proportional only to light absorption; then a reaction in the ordinary sense between this product and chromic acid and of sulphuric acid, as in the dark reaction. At very small concentrations3 the second stage is so slow that it determines the speed of the reaction as a whole; but when C ~ > 0 . 0 0 1 3the speed of the second stage is very great and the speed with which sensitized quinine is produced regulates the progress of the reaction; this sensitizing *This paper is part of the programme now being carried o u t a t Cornell University under a grant to Professor Bancroft from the Heckscher Foundation for the Advancement of Research established a t Cornell University by August Heckscher. J. Am. Chem. SOC.,31, 770 (1909). * Leipzig dissertation (1906). [C, is the concentration of chromic acid in gram equivalent6 per liter (Cr,Oa/3 = 33.4 grams:].
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D. 6 . JIORTOS
is probably a reversible reaction, otherwise the product would accumulate rapidly \ h e n CSbecame small and prevent any approach to a reaction of the first order. Finally, the smaller the value of c 1 2 , the more in proportion should the hydrogen ion accelerate the photochemical reaction as a whole. . , , “The percentages of each ray passing through the light filters, and the percentages absorbed by quinine and by chromic acid, respectively, in each experiment, are calculated. The speed of the light’ reaction is proport,ional to the quantity of light of given wave-length absorbed by the quinine alone unless the concentration of chromic acid is very small; this is consistent with the hypothesis that the light reaction runs in two stages.” There is no hedging about these conclusions and there is also no recognition of the fact that the results call for explanation. The general accuracy of Luther and Forbes’ experiment is, of course, beyond question: but their conclusions cannot be the whole truth, because there are technical processes1 based on the photosensitivity of potassium dichromate. “When mixed with organic substances, potassium dichromate is reduced on exposure to light; gelatine under such circunietances is rendered insoluble. This reaction is taken advantage of in the ‘carbon’ process of photography. The gelatine is mixed with a pigment of any colour and the paper carrying this film is sensitised by floating on a solution of potassium dichromate. On exposure under a negative, the gelatine beconies insoluble in those portions exposed to t>helight, and retains the pigment, xhile the portions protected by the darker portions of the negative are almost unacted upon, and may be dissolved in warm water. I n this manner photographs of great beauty and permanence may be produced. . . . The insoluble gelatine does not absorb water, but will take oil, which may be mixed with any desired pigment and thus becomes the basis of the oil and bromoil processes. Gum, mixed with dichromate, is also made insoluble by exposure to light, and this is made use of in photography in the gum dichromate process.” The conclusions of Luther and Forbes have been questioned by Plotnikow,2 though perhaps not with the definiteness that one would like. He showed that light, which was absorbed by’ an ammonium bichromate solution, causes this solution to oxidize alcohol. He also offered a tentative explanation for the failure of Luther and Forbes to obtain normal results. “It is to be assumed that both components will be photochemically active when quinine is oxidized by chromic anhydride in acid solutions; the reaction also takes place in the dark. X a n y of the active rays, such as the violet and the ultra-violet, for instance, are absorbed by both components, in consequence of which the chemically active light of these wave-lengths is divided between the two components. It is not impossible that the two bands of photochemical absorption, which belong to the quinine and the chromium are masked to some extent by thermal absorption. [It is questionable whether this means anything a t all.] From what has been said it, is evident how com1
Thorpe: “Dictionary of Applied Chemistry,” 2 , 241 (1921). “Lehrbuch der Photochemie,” 198, 2 1 5 i1920:.
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PHOTOCHEXICAL O X I D l T I O l ’ T I T H POTASSIU.\l DICHROMATE
I I3 7
plicated this reaction niust be. Since the light absorption is middling strong, it will hardly be possible to formulate for this reaction a differential equation which can be integrated. It is therefore not surprising that t,he investigators [Luther and Forbes] who have studied this reaction without considering all tht, factors have not obtained any agreement betiyeen experiment and theory.” Apparently as an after-thought, Plotnikow says, four short paragraphs later, that “it would be rather interest,ing to study the oxidation of quinine by chromates, I\r:Cr?O7 or K?CrOI,in neutral solutions. One mould probably get simpler relations and the pure photochemical properties of the b o ’ coniponents ~ o u l dbe recognized more easily.” Unfortunately Plotnikow never tested this suggestion of his and it evidently seemed of no importance to Luther and Forbes, because, in a later paper, Forbes, JToodhouee and Dean’ discuss Plotnikom’s niatheniatical relations. xhich are of no special importance, and ignore entirely the fundanirntal question n h y the chromic acid should apparently be photochemically insensitive. Both constituents in the reaction niust be photochemically actire, as Plotniliory points out. If one of them is not, it must be because the experimental conditions are so abnormal that the photosensitiveness of one constituent apparently disappears. As a matter of fact, t,he conditions under which Luther and Forbes worked were most extraordinary. hi1 their reacting solutions were 5.4 X in sulphuric acid, while the dichromate concentration was a l m y s less than 0.01s. It has long been known that the oxidizing power of a dichromate solution increases with increasing acidity. Only recently it was shovn by T-incrnt,2in experiments on the electrolysis of potassium dichromate d u t i o n s that this salt does not act as a depolarizer in neutral solution with platinum electrodes, there being one hundred percent evolution of hydrogen at the cathode. The depolarizing action appears and increases as sulphuric acid is added, and is nearly complete for a sat,urated solution of potassium dichromate in normal sulphuric acid. It seemed probable that, by using j . 4 S sulphuric acid, Luther and Forbes had pushed the oxidizing power of the dichromate to the limit. so that the absorption of light could produce no appreciable additional activation. I n order to test this assumption experiments were niade with potassium dichromate and ethyl alcohol. If the concentrations are chosen rightly, this makes a very satisfactory photochemical experiment. An ordinary hand-regulated D.C. carbon arc spotlight A, operated at j o amperes served as the source of light, Fig. I . The rays were made to converge, by means of glass lenses L,, L? and mirror 31 on the bot,toni of the glass absorption cell C. (6 X 6 X 6 cnij, which contained the reacting mixture. The outer cell F (6 X I O X I O cui), supported in the position shown, contained the liquid employed as light-filter. The contents of F were kept cool by circulating ~~
~
J. d m . Chem. POC.,45, 1891 (1923). J. Phys. Chem., 29, S j j (rgzj!.
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D. S. XORTON
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cold water through a spiral copper tube T immersed in the liquid. Cell C was provided with a glass cover D, silvered on the top surface. For the dark reaction a similar combination of cells was used. They were protected from light and were maintained at approximately the same temperature as the cells in which the light-reaction was studied.
Q.0 2
CONC.OFHzSO!
FIG.j
Curve 4.
I
5 alcohol, 0.025 S I i d h O l
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PHOTOCHEMICAL OXIDATIOS WITH POTASSIUM DICHROMATE
I139
Kahlbaum’s pure potassium dichromate was used and 9jq ethyl alcohol. Keither was purified further. The volume of the reacting solution was 100cc. in all cases. The course of the reaction was followed by withdrawing samples a t intervals (usually every fifteen minutes) and determining the concentration of dichromate iodimetrically. Experiments with water and with alcohol as light-filters showed no appreciable difference, so any activation of the alcohol by light of the visible spectrum is negligible. A potassium bichromate filter brings the light-reaction to a stand-still, thus showing that the dichromate is the photosensitive substance. The photochemical reaction is not prevented by a copper sulphate filter, showing that the light which is absorbed by the dichromate is the photochemically active light. The observed amounts of reaction in the light and in the dark, with varying initial concentrations of sulphuric acid a;.e given in Table I and 11, and shown graphically in Figs. 2 - 5 in which the decrease in concentration of dichromate is plotted as ordinates against concentration of sulphuric acid as abscissas.
TABLE I 5% ethyl alcohol; 0.04 Time minutes
30
45 60 Time minutes
N K2Cr207
Decrease in conc. of Ii2Cr20;in equivalents per liter Seutral 0.1 S H2S01 Dark Light Dark Light
0.0004
0.0022
0.0032
0.0128
0.0044
0.0060
0,0223
Decrease in conc. of KICrlO, in equivalents per liter S HzS04 0.75 S HzdO1 Dark Light Dark Light 0.j
30
0.02j6
0.0272
45 60
0.035 2
0.0388
0 0240
0.02j2
0.0322
0.0324
0.03jo
0.0376 0.0388
0,0384
TABLE I1 1%
Time minutes
I
Dark
30 45 60
ethyl alcohol; 0.025 S K2Cr207 Decrease in Conc. of KzCr20,in equivalents per liter S HzSO1 2 S H2SOa Light Dark Light
0.008;
0.0127
o ,0180
0.0189
0.02jo
0.0196
0.0222
0.0223
K i t h neutral 0.04 S K2Cr207 and 5 % alcohol, the light reaction is approximately ten times that of the dark-reaction at the end of an hour. K i t h 0.1S H2S04 the light-reaction is more than twice as fast as the other one and the total change in the light is five times as much as in the corre-
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D. 6. MORTOX
sponding neut’ral solution. TVith 0.7 j S HzSOl the light-reaction and the dark-reaction are practically identical, which means t,hat, under these conditions, the dichromate appears to be photochemically inactive, which is what Luther and Forbes found. K i t h more dilute alcohol and more dilute bichromate, there is a distinct difference betrreen the light-reaction and the dark-reaction with normal sulphuric acid ; but this difference is negligible with double-normal sulphuric acid. The results of Luther and Forbes are absolutely accurate; but they are misleading as stated. When studying the rate of inversioii of a dilute cane sugar solution by acids, we are justified in saying that, under these conditions, we may treat the concentration of water as practically constant; but it would be inaccurate to say that water does not enter into the reaction. Luther and Forbes were quite justified in saying that, under their conditions, chromic acid could be treated as practically insensitive photochemically; but it was unfortunate to imply, as they do, that chromic acid is photochemically insensitive in the reaction between quinine and chromic acid. The general relation seems to have escaped them entirely, even after it’ had been suggest’ed to them by Plotnikow. I t has seemed desirable to straighten out this matter because even Iiistiakowsky’, who should be the last word in photochemistry, has failed to recognize the limitations in the xork of Luther and Forbes and accepts it as right without any reference to sulphuric acid changing the general rela t’ions. “.In interesting, but not completely explained, photochemical reaction is the oxidation of quinine by chromic acid in presence of sulphuric acid. This reaction was studied by Goldberg,? mho noticed a very small tcmperrtture coefficient ( 1 . 0 2 ) . It was later the object of an extensive investigation by Luther and for be^,^ which revealed interesting relations. “ S o t only quinine, but also chromic acid, absorbs the light which i q causing the photochemical reaction. . . . Luther and Forbes measured the absorption coefficient (a) uf both substances and demonstrated, by applying equation I , that the light absorbed by chromic acid is ineffective; the lattcr is therefore acting only as an inner screen. The authors demonstrated, further, that the rate of reaction was proportional to the amount of light energy absorbed by quinine and was independent of the chromic acid concentration, a t least when this lat,ter was varied from 0.012 to o . o o r S . .It still lower concentrations of chromic acid, a decrease in the rate of reaction was observed. . , . “Recently Forbes and his eo-workers4 have extended the earlier experiments of Luther and Forbes and studied in particular the influence of sulphuric acid. They found that the rate of reaction is practically independent “Photochemical Processes,” 39 (1928)
* 2. miss. Phot., 4 , 56 (1906).
J. Am. Chem. Soc., 31, 7 7 0 (19091. J. Am. Chem. Soc., 45, 1891 (1923).
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PHOTOCHEMICAL OSIDATIOS WITH POTASSICM DICHROMATE
II41
of the concentration of sulphuric acid, only, however, if this is above 0.j normal; in more dilute solutions, a decrease in the rate of oxidation approsirnately proportional to the concentration of the acid was observed.” The reason why Tiistiakowsky did not detect the iTeak point in the csperinients of Luther and Forbes is apparently because he is not intereFtcd at a11 in the conditions for a photochemical reaction to take place, but only in the kinetics of a reaction which does take place. Practically all he says on this point is that “only in the nineteenth century was the first and fundamental law of photochemistry deduced. Grotthuss published in 1817 a paper which contained a clear outline of this law. d s formulated by him, the law states that only light which is absorbed can produce chemical change. Grotthuss’ paper attracted little attention on the part of his contemporariej, and in I839 Draper derived independently the same law and supported it by experiments on the hydrogen-chlorine reaction.” There is not a word said about theexperiments of Grotthuss on depolarizers or about the later formulation that all light which is absorbed by a substance tends to eliminate that substance and that what actually happens is a question of chemiet’ry. Instead Kistiakowsky jumps to the q u e h o n of light quanta. “h starting point for the rapid development of photochemistry along new lines was undoubtedly the introduction of quantum theory. General attention turned from rather unsuccessful thermodynamic speculations to a quantitative study of photochemical kinetics, resulting in a deeper understanding of the rBle of radiant energy in producing chemical changes and in many valuable contributions t’o the general theory of chemical kinetics.” JYithout questioning any of this, the fact remains that Iiistiakowsky’s view-point is defective to the extent that the contradiction between the results of Luther and Forbes and the behavior of bichromated gelatine either never occurred to him or did not seem to him of any importance. The general results of this paper are: I , Under suitable conditions dichromate solutions are photochemically acltive for light xhich is absorbed by the dichromate. With increasing acidity the oxidizing power of the dichromate solution 2. increases and the apparent action of the light consequently decreases. 3 . With ethyl alcohol, potassium dichromate, and varying amounts of sulphuric acid, the light-reaction can be made practically the whole thing or practically zero as one wishes. 4. Since Luther and Forbes studied the photochemical oxidation of quinine by chromic acid in j.4X sulphuric acid, they were working under conditions which should and did make the chromic acid practically photochemically insensitive. 5 . The conclusions of Luther and Forbes in regard to the photochemical activity of chromic acid are right for the extreme conditions under which they worked; but they are not generally true and are consequently misleading. Cornell Uniuersity.
