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THE PHOTOCHEMICAL REDUCTION OF THIONINE A Lecture Demonstration Based on its Reversibility LAWRENCE 1. HEIDT Massachusetts Institute of Technology, Cambridge, Massachusetts
TAEreversibility of a chemical reaction can be. demonstrated to a large or small audience by the following reaction in water solution: Fe++ less)
The structural formula of thionine, Cl2HSNaS,in acid as the monovalent purple cation, thio+, is
+ thionine (purple) = Fe++++ reduced thionine (color-
The reaction proceeds from left to right following the absorption of light by the thionine; the reverse reaction takes place in the dark. Both reactions are rapid and sufficiently complete, so that the solution in a welllit room can be made to change from purple to colorless and vice versa by turning on and off a photoflood lamp placed near the solution; this can be repeated as often as desired for the entertainment of the audience. The reaction has been studied in detail by E. Rabin~witch.'.~He states that the reversible bleaching process is similar to that in the ferrous-iodine system but the latter is about one-hundred times slower. "A similar reversible bleaching can be observed with other thiazine dyestuffs, e. g., methylene blue, calcozine (methylene green), toluidine blue, and thiocarmine. However, none of these dyes equals thionine in sensitivity. The sensitivity can be increased by addition of substances which form complexes with ferric ions" such as sulfuric and phosphoric acids, and by lowering the pH below unity. The rate of the back (dark) reaction is decreased by the formation of these complexes but is increased by the addition of ferric ions to the solution. "Under favorable conditions, [the] s stem required only a few seconds to reach both its staT? ionary state in light and its stable state in the dark. In the absence of oxygen, the bleaching is completely reversible: in [the] presence of oxygen, the reduced dye is partly reoxidized by oxygen with a resulting accumulation of ferric ions and a gradual weakening of the effect." The bleaching process can be made to give rise to a photogalvanic effect amounting to several tenths of a volt between bright platinum electrodes placed in the illuminated and dark portions of the solution. However, "the strongest photogalvanic effects are obtained in solutions which show hardly any bleaching at all" because in strong light the photogalvanic effect reaches a maximum between pH 2 and 3, depending upon the light intensity, whereas "at pH = 2.5 and above, the bleaching to the eye." - is no longer - recognizable ' RABINOWI~HE, E.,J . Chem. Phys., 8,551, 560 (1940). RABINOWIT~H. E.. AND L. F. EPSTEIN. J . Am. Chem.Soc.. 63.
The ion is in equilibrium with several equivalent structures with which it resonates, also the blue di- and the green tri-valent cations obtained by adding one and two protons, respectively, to the two other nitrogen atoms of the purple ion,' and dimers of the purple ion.z The transfer of one electron to the thionine produces semithionine which is believed to be colorless in dilute acid in the form of Thio H+. The transfer of two electrons to thionine produces leucothionine which is present in dilute acid mainly in the form of Thio H and Thio H h . The experiment was first designed and used as a lecture demonstration in the fall of 1942. Since then several of my colleagues and others have employed the experiment for the same purpose. In particular, Professor Arthur R. Davis demonstrated it to the New England Association of Chemistry Teachers a t their annual conference in the summer of.1945. The details of the demonstration, however, have not been published, so they are given here with the hope that this will serve to answer the requests of those interested in obtaining the information. The experiment also illustrates the dynamic property of opposing reactions, the conversion of light into chemical energy and the fading of certain dyes in light. Reagents. C. p. FeSOa.7H20 or Mohr's Salt ferrous ammonium sulfate and C. p. sulfuric acid. Thionine hydrochloride or thionine such as that distributed by the Eastman Kodak Company, Rochester, New York. The laboratory supply of distilled water or tap water free from strong oxidizing agents such as chlorine. Stock Solutions. 6 N HzSOn. A water solution containing 0.001 mol or 0.23 g. of thionine per liter of solution. Thionine solutions are most stable a t pH = 3.5 when kept in glass-stoppered pyrex bottle^.^ Preparation of Solution for the Demonstration. Thoroughly mix together 10 ml. of the 0.001 M thionine
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JOURNAL OF CHEMICAL EDUCATION
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THE B COMPLEX1
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discovery that human and animal food must contain certain vital elements in addition to a minimum of calories and prot, i w n.rs of i l l r n ~importnl.cr ~ in lhr conquest ofi!i, n r y 1111m~ .s. 'TI,? wmprom? ,i I,&L~ri wt w dc~vr~l~cvl ~n L'him ~ , U 2tilNl s.c. bur it wu. nor uwil lbX2 rll,,t Tnktki cured ir iu tit.. Japanese navy by feeding more vegetables, fish, and meat to the sailors. He was totally ignorant of the curative principle of these foods, and even in 1911 when Fuhk originated the term "vitamine" for such things, there was no krywledge of their exact chemical nature. Today, we not only have an expanding alphabet of vitamins but we know the chemical constitution of many of them and are able to produce them synthetically. From 1920-25 the so-called vitamin B was extracted with water or alcohol from rice polishings, wheat germ, yeast, and tho leaves of several plants. I t was the vital factor necessary for curing heri-beri snd pellagra. However, when heated to ahout 118°C. i t was no longer effective again& beri-beri, but was still active for pellagra. Evidently i t was a complex and not a simple material. Further study revealed that its thiamine content is the factor for beri-beri and its nicotine acid for pellagra. But its complexity did not stop there for now we know that the B Complex contains riboflavin, pyridoxine, pantothenic acid, choline, inositol, biotin, and folio acid. And the end is not yet in sight for the most recent discoveries are the B,, factor and the "animal protein factor." These discoveries are the culmination of effort in several fiolds of research. One aimed t o find the factor in chicken feed which produced the most rapid growth in chicks, and the most hetchwldc eggs from hens. Another xvaslooliing for the factor in snimal Reprinted from For Instance, No. 45, 1949.
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solution, 10 ml. of the G IV I&SOr, sufficient n.at,er to bring the total volume to 500 ml.. and finally about 2 g. of the hydrated ferrous sulfate or the equivalent in ferrous ion of Mohr's Salt. I
