Spot Reaction Experiments Part ZX: Catalyzed Reactions Brought About by Photolysis of Ferric Oxalate' FRITZ FEIGL Laboratorio Central da Produ$io Mineral, Ministerio da Agricultura, Rio d e Janeiro, Brazil
(Translated by Ralfih E. Oesfier, University of Cincinnati)
NE of the most familiar photochemical processes 1s the decomposition by light of Eder's solution: a mixture of mercuric chloride and ammonium oxalate. The resulting production of mercurous chloride and carbon dioxide,
0.
2HgC1,
-
+ (NH,)nC20~
HgrCln
+ 2NHXI + 2COs
is proportional to the intensity of the light. Winther2has shown that the decomposition in visible light is due to traces of iron in the commercial materials. In blue light, a concentration of ferric chloride of the order of 3.10-' mols per liter is sufficient to cause a marked decomposition of Eder's solution. It appeared likely that this activity of femc salts was a catalysis and that it involved the photolysis of ferric oxalate. Solutions containing ferric salts and oxalic acid, and consequently, complex ferric oxalate, keep almost indefinitely in the dark. On exposure to light, they decompose:
-
F ~ Z ( C ~ O , ) ~2FeCnO. *
+ 2CO2
The resulting ferrous oxalate can react with mercuric chloride in the presence of excess oxalic acid: Accordingly, ferric oxalate is regenerated and by exposure to light can again be reduced to ferrous oxalate, which, in turn, can react with mercuric chloride. Summation of these partial reactions gives: 2HgCIs
-
+ HECSO,
HgsCls
+ 2HCl + 2C0,
This net equation represents the Eder reaction, but the catalytically active ferric salt does not appear in it. If the foregoing explanation of the activity of femc salts on the Eder reaction is valid, the photolysis of ferric oxalate should accelerate catalytically other reducing actions of oxalic acid. If the photolysis in equations (1) and (2) below occurs in the presence of a compound M that is reducible by ferrous oxalate, then (in the presence of free oxalic acid) ferric oxalate will be regenerated (3). This again decomposes in light, fnr-
nishing ferrous oxalate which reduces M. Consequently, the series of reactions (1) to (3) constantly repeats. Summation of these partial reactions gives (4) which represents merely the reduction of M by oxalic acid on exposure to light, but without any visible signs of the participation of iron oxalate.
M
M
+ H2Cz04
COI
(2) secondary reaction
CO,
H.C204
hv
(1) primary reaction
Fer(GO&*
2FeCnOl
Fe2(C90&* 2FeGOd
Fe?(GO&
M (reduced)
+
M (reduced) (3) (rapid)
(4) = (1)
+ (2) + (3)
This series of reactions is the typical representation of an intermediate reaction catalysis; In its general form, it states that every slow reducing action of oxalic acid will be catalytically accelirated by the combined action of iron salts and light. The writer has found that the following redox reactions with oxalic acid are catalyzed by adding ferric chloride: H&Or
-- + ---
+ 2H~C204
Se
3Hz0
+ 4COn
+ H3C*04 2HI + 2COn PdClr + HnC204 Pd + 2HC1 + 2COn 2W08 + H2CnOl W20s + HqO + 2CO2 HsO + 2COn + leuco methylene Methylene blue + H G O , blue (colorless) It
The occurrence of all these reactions is easily perceived through the formation of a precipitate or a color change. They all proceed very slowly in direct sunlight. The addition of small quantities of an iron salt catalytically hastens them. 'The acceleration is due to the photolysis of ferric oxalate and to the participation of the resulting ferrous oxalate in the partial reaction (3) of the general reaction scheme. The rapid partial reaction (3), i. e., the actual reduction, can be formulated in these instances: H2SeOs
-
1 The qualitative experiments described in this paper illustrate hitherto unknown catalysisreactions effected by photolysis of ferric oxalate. The author makes no claim concerning the correctness of his interpretations of the course of these catalytic effects. Definitive explanations are not pmsible without ouantitative studies of the kinetics of these systems. This is particularly true because recent studies have shown that a large number of factors must he considered in the cage of the well-known Eder reaction. WINTHER,2.W ~ SPhotogr., S. 8,197,237 (1910)
- ++ + + - + +
F ~ S ( G O , ) ~ hv
+ 2FeCz04+ 2HzGO.+ In
PdCln 2WOs
+ 2FeC2011+ H&Od-
Fez(CsO&
--
+ 3Hn0 + 2C01+Se
FedCzOdr
+ 2HI
+ 2FeC~0, + HnC2O4 FedGOSa + 2HCI + Pd + 2FeCz0, + &GOl Fez(CzO& + H20 + 2C01 + W2Os
Methylme blue
+ 2Fe2C04+ HsCz04
-+
+
Fe.(C%O,). H.0 4leuco methylene blue
The catalytic acceleration of reducing actions of oxalic acid through the combined action of light and an
iron salt is a striking example of the fact that, in the strict sense, an intermediate reaction catalysis is accomplished not by a compound as such, but by a reaction. In the present cases the catalyzing reaction is the photolysis of ferric oxalate. Consequently, this catalysis can be inhibited by shutting off the light. This catalytic effect can be demonstrated simply in the form of spot reaction^.^ (48) Reduction of Selenious Acid by Oxalic Acid. A mixture is prepared from 15 ml. of 1 N oxalic acid and 5 ml. of 10 per cent selenious acid. Single drops are placed in three adjacent depressions of a white spot plate. A drop of 0.5 per cent ferric sulfate solution is added to two of the specimens. One of these is then covered with a metal shield. All three are then subjected to direct sunlight. Within a few minutes a red color develops in the insolated specimen containing iron. After about 15 minutes the cover is removed. It will be seen that free selenium was deposited only from the specimen that contained iron and was exposed to the sunlight. (49) Reduction of Iodine by Oxalic Acid. The test solution consists of 15 ml. of 1 N oxalic acid and 5 ml. of 0.02 N iodine solution. Three drops are placed in three adjacent depressions of a white spot plate and treated as in (48). After exposing to sunlight for about 10 minutes, all the specimens are treated with a drop of 1 per cent starch solution. It will be found that the exposed specimen containing iron gives no blue, whereas the others still contain free iodine. If the
-
Cj. the writer's "Laboratory Manual of Spot Tests." Academic Press, New York, 1943, and "Qualitative Analysis by Spot Tests." 2nd ed., Nordemann Publishing Company, New York, 1939.
starch is added a t the start, even the iron-bearing exposed specimen does not lose its color. Obviously, this is because the blue starch-iodine solution does not permit the passage of the active light. (50)Reduction of Palladium Chloride by Oxalic Acid. Three drops of 1 N oxalic acid are placed in each of three adjacent depressions of a white spot plate. One drop of 1 per cent palladium chloride in dilute hydrochloric acid is added to each. They are treated as in (48). In a short time, the irradiated specimen containing iron turns dark (metallic palladium). The cover is removed after about five minutes. It will be found that palladium has not been precipitated from the other two specimens. (51) Reduction of Tungstic Acid by Oxalic Acid. Solid WOa.aq. (tip of knife blade) is placed in three adjacent depressions of a white spot plate. Three drops of 1 N oxalic acid are added to each and stirred. The specimens are treated as in (48) and exposed to strong sunlight for about 15 minutes. The cover is removed and i t will be found that the light yellow WOa has changed to green or blue only in the specimen that contained iron and was irradiated. The same results will be observed if a 20 per cent aqueous solution of phosphomolybdic acid is used in place of solid WOa. (52) Reduction of Methylene Blue by Oxdic Acid. The test solution is a mixture of 15 ml. of 1 N oxalic acid and 5 ml. of 0.2 per cent methylene blue solution. Three drops are placed in three adjacent depressions of a white spot plate and treated as in (48). The blue color of the exposed solution containing iron disappears in about 30 seconds. Removal of the cover reveals that the unexposed solution containing iron has remained unchanged.
A Banker Study Chemistry? WHY should I study chemistry? I'm going to be a banker. M y dad is a "cotton man"; he didn't study chemistry. Chemistry is necessary for medicine, but for the insurance business, of what value are chemical formulas and reactions? These are typical remarks of those who plan t o enter a profession or vacation which an the surface seems far removed from chemistry. But let us stop t o examine the case of the banker more closely. As a banker, your chief problem will be that of loans and investments. Of the people who come t o borrow money, a large per cent will he either directly engaged in the manufacture of chemicals or the production of articles by chemical processes. A progressive company not only improves its products but is constantly finding new uses and looking for new products. Without a knowledge of chemistry, how will you distinguish between a good sound idea and a "fly-by-night scheme"? Must you take someone's advice, or would you rather decide yourself? Are you going to invest in corporations on the basis of past earnings or future possibilities? If you choose the latter as you should, how will you decide without a knowledgeof chemical processes? , As yon try to correlate the cotton buyer on Front Street with chemistrv. h a ~ o e nto remember that the National Cotton ,. vou , Council nmintaios several laboratories nnd is constantly cryerimcntmg. But how is chenktry rclatcd to the production of cotton? Since certain varieties of cotton thrive better in certain areas, the soil seems to be an important factor. Immediately we can see that experiments in plant breeding and improving the soil are necessary, both of which require a knowledge of chemistry.
..
AS t o new uses, how can cotton compete with rayon, nylon, and other svnthetic fibers! So you only buy and sell cotton' But the prductmn and uces of cotlon will detwmine its price on the world market Wouldn't a knowledge of the chemistry of production and of new uses help you t o anticipate t h a t market? The insurance business will serve as the third example. I believe we all agree that the success of a n insurance company depends upon how well its losses can be anticipated and t h e rates fixed accordingly. WiU a plant making cosmetics catch fire as easily as one making Boor sweep? How does the danger of explosion in a refinery compare with t h a t in a T N T plant? How do a glass factory and a plant manufacturing wood preservatives compare as to injury to their employees' health? Three examples of the value of chemistry t o the professions and vocations considered nonchemical have been discussed. Of the hundreds of vocations and professions, it is difficult t o find one in which chemistry has no value. Chemistry is certainly important t o the attorney with a murder case involving blood stains or poisons, t o the manufacturer who must find new uses for his products, to the farmer with problems of sail, feeds, a n d snraVF. .-~~ > -~
Many today are carrying on their work while depending on others for their knowledge of chemistry. With a knowledge of fundamentnl chemicnl facts. Rrcatcr happiness and succesq can be attained in the chemical world of tomorrow.-Raymod 71 Ihughn in The Memph-Ion (.llemphid S ~ r r i n n A . . C. S . )
