A SIMPLE REVERSIBLE PHOTOCHEMICAL EXPERIMENT I. W. GROTE
AND
J. H. BARNETT
University of Chattanooga, Chattanooga, Tennessee
A mixture of sodium nitrofirusside, sodium bicar- ber of times (until the active ingredients are decomposed). bonate, and thiourea i n aqueous solution will develop The reaction goes at a much slower rate in the absence a n intense blue coloration when exposed to sunlight for of the bicarbonate. .a few minutes. The blue solution changes to a deep By preparing a series of tubes, i n each of which a difcrimson when p h e d i n darkness for a few hours. I t ferent one of the ingredients is missing, a n experiment again becomes blue when returned to the sunlight. This i s offered illustrating both a reerersible photochemical alternate color change will take place a n indefinite num- reaction and the action of a catalyst.
+ + + + + +
A
LTHOUGH many reversible photochemical reactions are known, not many are suitable for use as elementary chemical experiments. Most of them go in one direction or the other at a very slow rate, or the change is not easily apparent to the unskilled observer. Thus the reaction light I S + HsO-2HIf darkness
0
although involving a color change, goes very slowly from left to right under normal conditions. The following simple experiment is suitable either for a lecture demonstration or as a laboratory experiment in general chemistry. The solutions required are: 20 cc. of freshly prepared 0.5y0 sodium nitroprusside (to he kept in an opaque bottle) ; 20 cc. of saturated sodium bicarbonate; and 20 cc. of 0.5% thiourea.
PROCEDURE
Four test tubes are numbered and placed in a rack so that equal exposure to light may be secured. I n tube 1 is placed 5 cc. each of the three solutions, nitroprusside, bicarbonate, and thiourea. I n tube 2 is placed 5 cc. each of the nitroprusside and bicarbonate solutions only. In tube 3 is placed 5 cc. each of the thiourea and bicarbonate solutions only. In the last tube 5 cc. of nitroprusside and 5 cc. of thiourea solution are mixed. The four tubes are then placed in the sunlight for from four to six minutes. A blue color will form quickly in 1and slowly in 4, 2 will darken slightly, and 3 will remain unchanged. The reaction will proceed in ordinary light, hut at a much slower rate. The tubes are removed from the sunlight as soon as 1 and 4 are distinctly blue, and 5 cc. of thiourea solution is added to 2. A blue coloration will take place quickly without the aid of further exposure to light. Five cc. of nitroprusside solution is then added to 3. The blue color will not be formed in this case, showing that the sunlight does not cause any change with thiourea in bicarbonate solution. The tubes are now placed uncorked in total darkness for a period of three to six hours or more. Upon removal it will be noted that tubes 1and 2 have changed to a deep crimson, 4 will be blue and practically unchanged, while tube 3 will still be colorless, although all the necessary reagents for the reactiou have been present for several hours. If 5 cc. of bicarbonate solution is added to 4, the blue color immediately takes on a purplish tinge, and if kept in the dark the color will be decidedly red after about one-half hour, showing that the tube "stored darkness" even though it was not apparent, as in 1 and 2, until the bicarbonate addition. All the tubes are then re-exposed to sunlight and the blue coloration will immediately form in each, since all the reactants are now present in each tube. Another period of darkness will bring the red, etc., until finally the d i v e ingredients are decomposed, due to side reactions. If too long exposures to sunlight are avoided after the blue color is produced, the reaction may be reversed 12 to 20 times. If the solution is placed in a completely filled, sealed container, the change blue to red will fail to take place after several times, showing that the presence of air is necessary, probably for the spontaneous re-oxidation of the iron previously reduced by the action of the sunlight. DISCUSSION
This reaction was first described by one of us in a recent paper and full details may be found therein.' The exact chemical reaction involved is still unknown, but the alternate change blue to red is most likely caused by alternate valence changes in the iron atom, probably from the divalent state in the light to a trivalent state in the dark.
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GROTE,"A mew Color Reaction for Soluble Organic Compounds," 1.Bid. Chem.. 93,25-30 (Sept.. 1931).
Sodium nitropmsside (sodium nitroferricyanide) solution, after several days' exposure to indirect light, or a few hours to direct sunlight, changes from a cherry red color to yellow, then green, and finally Prussian blue coagulates out and the solution may be filtered to yield a yellow-brown filtrate. During the reaction in the light a strong odor of cyanide is present and bubbles of nitrogen are given off. This final solution gives the intense blue color with compounds of the C=S type very rapidly even in the dark. If placed in the dark without filtration for a few weeks, the light-treated solution will go back to the original cherry-red color, and no longer react with C = S , but will again do so after re-exposure to light. A solution giving this blue coloration with thiourea instantly, without action of light, may be prepared by dissolving 0.5 g. of sodium nitroprusside in 10 cc. water and adding 0.5 g. of hydroxylamine hydrochloride, followed by 1 g. of sodium bicarbonate. After evolution of gas has ceased, 2 drops of bromine are added. Excess bromine is then removed by aeration, the solution is filtered and the dark greenish or blackish brown filtrate made up to 25 cc. with water. This solution behaves like that prepared by exposure of nitroprusside to sunlight, and is stable for about two weeks. The blue coloration is formed not only with compounds containing the C S group, but also with compounds containing N==S (as sodium thiocyanate), S=S, (as sodium thiosulfate), and probably in general by the type E-4,where E is any single non-metallic element. We have not been able to obtain the color change, blue to red, when left in darkness, except with thiourea derivatives The reaction is extremely delicate in the case of thiourea, a distinct blue coloration being obtained in a dilution of 1:100,000 or more. For freshman classes, a simple explanation of the chemistry involved in the reaction can be made by stressing the reducing action of sunlight upon ferric compounds, as illustrated by blueprint paper, in which a mixture of potassium or sodium ferricyanide and ferric ammonium citrate are involved. Since sodium nitropmsside is more correctly called sodium nitroferricyanide, the analogy is very close. The change from blue to red tak'mg place in the dark (in the presence of air) can be assumed to be simply a re-. versal to a more stable oxidized form of iron. The importance of a catalyst in speeding up the rate of a reaction is well illustrated by the rapid rate of coloration of tube 1 as compared to tube 4 when exposed to light. The failure of tube 4 to pass over to the red color in the dark, and the rapid change taking place, even in diffused light, when sodium hicarbonate is added, also emphasizes this catalytic r61e. Numerous modiications of the experiment easily suggest themselves, and the very lack of information as to the actual chemical reactions involved in such striking color changes will doubtless make it even more interesting to the advanced or saentifirally curious student.
