the photochemical reduction of thionine

2
a THE PHOTOCHEMICAL REDUCTION OF THIONINE A Lecture Demonstration Based on its Reversibility LAWRENCE 1. HEIDT Massachusetts Institute of Technology, Cambridge, Massachusetts TAE reversibility of a chemical reaction can be. demonstrated to a large or small audience by the follow- ing reaction in water solution: Fe++ + thionine (purple) = Fe+++ + reduced thionine (color- less) The reaction proceeds from left to right following the absorption of light by the thionine; the reverse reac- tion takes place in the dark. Both reactions are rapid and sufficiently complete, so that the solution in a well- lit 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. Rabi- n~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 sensi- tivity. 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 sta T? 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. How- ever, "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 is no longer recognizable to the eye." - - - ' RABINOWI~HE, E., J. Chem. Phys., 8, 551, 560 (1940). RABINOWIT~H. E.. AND L. F. EPSTEIN. J. Am. Chem. Soc.. 63. The structural formula of thionine, Cl2HSNaS, in acid as the monovalent purple cation, thio+, is 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 elec- trons to thionine produces leucothionine which is pres- ent in dilute acid mainly in the form of Thio H and Thio Hh. The experiment was first designed and used as a lec- ture demonstration in the fall of 1942. Since then several of my colleagues and others have employed the experiment for the same purpose. In particular, Pro- fessor Arthur R. Davis demonstrated it to the New England Association of Chemistry Teachers at their annual conference in the summer of.1945. The details of the demonstration, however, have not been pub- lished, 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 illus- trates 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 con- taining 0.001 mol or 0.23 g. of thionine per liter of solu- tion. Thionine solutions are most stable at pH = 3.5 when kept in glass-stoppered pyrex bottle^.^ Preparation of Solution for the Demonstration. Thor- oughly mix together 10 ml. of the 0.001 M thionine

Upload: lawrence-j

Post on 21-Dec-2016

212 views

Category:

Documents


0 download

TRANSCRIPT

a THE PHOTOCHEMICAL REDUCTION OF THIONINE

A Lecture Demonstration Based on its Reversibility LAWRENCE 1. HEIDT Massachusetts Institute of Technology, Cambridge, Massachusetts

TAE reversibility of a chemical reaction can be. demonstrated to a large or small audience by the follow- ing reaction in water solution: Fe++ + thionine (purple) = Fe+++ + reduced thionine (color- less)

The reaction proceeds from left to right following the absorption of light by the thionine; the reverse reac- tion takes place in the dark. Both reactions are rapid and sufficiently complete, so that the solution in a well- lit 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. Rabi- n~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 sensi- tivity. 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 sta T? 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. How- ever, "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 is no longer recognizable to the eye." - - -

' RABINOWI~HE, E., J . Chem. Phys., 8, 551, 560 (1940). RABINOWIT~H. E.. AND L. F. EPSTEIN. J . Am. Chem. Soc.. 63.

The structural formula of thionine, Cl2HSNaS, in acid as the monovalent purple cation, thio+, is

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 elec- trons to thionine produces leucothionine which is pres- ent in dilute acid mainly in the form of Thio H and Thio Hh.

The experiment was first designed and used as a lec- ture demonstration in the fall of 1942. Since then several of my colleagues and others have employed the experiment for the same purpose. In particular, Pro- fessor 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 pub- lished, 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 illus- trates 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 con- taining 0.001 mol or 0.23 g. of thionine per liter of solu- tion. Thionine solutions are most stable a t pH = 3.5 when kept in glass-stoppered pyrex bottle^.^

Preparation of Solution for the Demonstration. Thor- oughly mix together 10 ml. of the 0.001 M thionine

526 JOURNAL OF CHEMICAL EDUCATION

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<. Itsbinowitch (Ref. 1, p. 554) states t,hat a 1000-watt lamp will completely bleach in a few seconds a snlut,ion 10-4 ,?if in thionine, 10-I M in Fe++ and 10-' M in HzSOa, or a solution 1.5 X iCf in thinnine, 4 X lo-? .%I' in Fe++, 0.2 M in H3PO4, and lo-' M in Fe+++. The color will return to any of these solutions a few seronds aft,er the lamp has been turned off.

Apparatus and I'rocedwe. A rolorless glass reaction vessel having a capacity near 2 litcrr, sochasa tall-form glass beaker or lecture jar, is used to hold the 500 ml. of solution. A light shield such as a piece of cardboard is fitted around the upper inside or ontside of the reaction vessel in such a way as to complet~ely screen t,he lamp from the audience. A convenient source of intense light is a 250 watt photoflood bulh, which is lowered into the reaction vessel until it is ahout a cm. from the surface of the solution. The lamp is connected to the 110-v. alternating or direct-current line through a switch which is used t,o turn the lamp on and off. Thr ext,ent of the bleaching car? he limited by reducing the intensity of the linht fallinn on the solut,ion. c. o.. hvdimminnt ,he - ~~, ~ ~~ " , .~,, ..-~~.~..-~- --.-

~ ~ ~ ~ ~ t ~ ~ t i ~ ~ . ~ f ~ ~ ~ i ~ ~ . ~ t lamp, or by incrcasing thc distalrce het,ween thc lamp U P P ~ ~ : Light 05 Lower: Light on and solution.

THE B COMPLEX1

Tm 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 1111-

m~ .s. 'TI,? wmprom? ,i I,&L~ri wt w dc~vr~l~cvl ~n L'him ~ , U I

2tilNl s.c. bur it wu. nor uwil lbX2 rll,,t Tnktki cured i r 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 Com- plex 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 to find the factor in chicken feed which produced the most rapid growth in chicks, and the most hetch- wldc eggs from hens. Another xvaslooliing for the factor in snimal

Reprinted from For Instance, No. 45, 1949.

feed which produced thc best rcprvduction and lactation in rats. And another wantcd to know thc cract f a c h in liver rhich re- stored pernicious anomia victims to health.

Since 1927 i t was known that satisinrtory rhieken iced should contain some animal protein, but in l!W it was found Lhat this cruld he replaccd with the product from a icrmenta1,ion process. This product apparently containcd thc mima1 protein factor. The bacillus for tho icrmenl was ubtaincd from chicken or cow mmure and such organisms as Slrcplofy!,ees aureofaciens. After concentrating nurl rdiuing the product ft.om the fermentation process it was announcai, in July, 191X,,that it was also thera- peutically aetivc for pcrnieious ancmia. %

In tho Spring of l O Z R mscarcll in Engla~iil and U. S. A. isolated a red, crystdlinc compound from liver TI-llirll aasaetive for perni- cious enemia and i t was c:~llcd vitamin UI? in this country. \Vhen added to an unsntisfsetory dict for rals, it improved lactation and reproduction. I t i~nprovcd certain dcfieient dicts for chicks hut i t was not so complcte in this respcrt as t.he animal protein factor obtained from the fermentation process. Therefaro, it appears that the animal protein factbr consists of B,: plus somc fltetor as yet unident,ificd.

Subskquent research indicstcs that thc important animal pro- toin factor may bc produced from many materisls by the corrccr formcntatian process. This will onsurc an adequate and uniform supply. Feed manufacturers then will produce special foods s o that the farmer may convert chicks and porkers into maximum quantities of eggs and meat in the shot.tcst possible time. This ~vill help to prevent mankinds's pantry from cvcr looking likc .Mother Hubbard's cupboard, and it will provide mothel. factor to combat dietary disease.

D~spi te recurrent lorcc%sts that thc rapidly increasing popnla- tion of the world is dostined for starvation because of insufficient food supplies, there n:cms no nccd for fear if science is kept active. For indeed, i t is tho province of scientists not simply to maintain life, but to maintain it molt abundantly.