T H E ELECTROCHEMISTRY O F LIGHT, X' BX- TVILDER D . B.\NCROFT
In 1818, Grotthuss' formulated two laws of photochemistry: I. Only those rays of light ivhich are absorbed can produce chemical action. 11. The action of a ray of light is analogous to that of a voltaic cell. The first of these lan-s is ~ i s ~ i a l lattributed y to Draper Tvho rediscovered it. In the preceding papers of this series, I have shown the usefulness of the second laiv as a working hypothesis. The time has non- come ivhen both of these l a m can be n-orded more broadly. The general understanding of the first law is that only those rays of light, which are absorbed, can produce chemical action but that all of the absorbed rays are not necessarily active. For instance. Ryk'j points out that Fehling's solution is decomposed by light having a Ivave-length somewhat less than +oo;r;l n-hile it is not sensitive to light corresponding to the absorption band in the red and yellon.. There seems to be no sound, theoretical reason for distinguishing t\vo kinds of absorption bands, one of Lvhich corresponds to a conversion of light into heat only, ivhile, Tvith the other, we have a partial conversion of light into chemical energy. It is much more rational to assume that, in some cases. the action of the light is not sufficient to produce measurable chemical change under the conditions of the experiment. This is in harmony with ivhat n e already know, namely, that many substances are sensitive to light only when suitable depolarizers' are present. I therefore propose changing the first law to make it read that all radiations tend to decompose the substances n-hich absorb them.' Khether any measurable 1 Hased on a paper read before the Eighth International Congress of Applied Chemistry in Ne\v York. September, I ~ I Z . Cf. Jour. Phys. Chem.. 12, 2 1 2 ( 1 c ~ o i 3 ) . :I Zcit. phys. Chem.. 49, Gjg i I < ) O ~ ) . ' 13ancroft: Jour, Phys. Chem.. 12, 2 , j o (1908). I3ancroft: Jotir. .\m. Chcm. Soc.. 33, '12 i I C ) I I ) .
Tlic Elccti ochetii istqi
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Light
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change takes place depends upon other conditions. \i,'ith some sih-er salts or ivith Eder's solution of mercuric oxalate. we get visible decomposition by light. J\-ith chromium salts we get no measurable change unless some reducing agent is present but i t can be a very ]Teak reducing agent. lT'-ith some substances, the action of light causes fluorescence or phosphorescence. thus indicatirig the occurrence of chemical changes el-en though there may be no appreciable decomposition. IVith a copper sulphate solution there is n o apparent effect due to light and yet it is certain that the difference in the light sensitii-eness of a copper sulphate and a silver sulphate solution is merely a difference in the strength of the depolarizer needed. This point of' t-ieiv has been confirmed by some experiments made recently in my laboratory. Leighton' showed that Fehling's solution could be reduced by red light provided a suitable amount of hydroquinone was added as depolarizer. Bennett' shoived that a copper sulphate solution is reduced by light when a dilute solutiori of phosphorus in ether is present. I n this case we do not get metallic copper as an endproduct because the copper reacts \\-ith the excess of phosphorus to form phosphide. Luther and JIichie" state that uranous sulphate in acid solution reduces copper sulphate to metal. By taking a dilute enough solution of uranous sulphate i t n-ould easily be possible to arrange the concentrations so that met:illic copper Ir-odd be formed in the light and not in the dark. \Then formulating r.he improved form of Grotthuss' first lam-. I used the n-ord radiations instead of light, intentionally because this enables me to include cathode rays, etc., which some people might object to classifying as light. They must be included because the same general lan-s apply in all these cases. JThile we 1 1 0 ~ -postulate that all radiations, ivhich are absorbed, tend to produce chemical change, we do not of course Jour. P h p . Chem., 17, 2 0 j (1913). Ibid.. 16,7 8 2 (1912). &it. Elektrochemie, 14:826 i 1908).
postulate that different \w\-e-lengths haL-e the same effect. I n the case of Fehling's solution. the ultra-yiolet rays are able t o cause deconipositioii without the addition of a depolarizer, while the red rays caiinot. The absorption of light by bromine is much more marked in the greenish-blue and the blue thari it is iii the yelloi\--jireen and the orange; but it is the rays corresponding to the latter absorption Ivhich are the most eft'ectix-e ill bringing about the reaction hetxveen bromine aiid tolueiie. ' 1 'ti? ei'ficieiicy o f particular \\-a\.e-lengths is a matter about \\-liicli !ye caii make iio predictions a t present. Until \ y e get some sort of theory iii regard to the relation betn-een the iiitensity of ai1 absorptioii aiid the change of chemical poteiitial, rye caii d o nothing more thari recognize the fact that all radiatioiis \vhicli are absorbed teiid to caitse chemical change : b u t tliat t h e efticieiicq- o f aiiy particular ivave-leiigtli of light depends upon factors i\-liich ha\-e not as yet lieen forinulated clearly. IVhile the htaterneiit t h a t light a c t s like :I \-oltaic cell has pro\-ed to be a11 excelleiit [vorkiiig hypothesis for cases in\.olx-iiig oxidation arid rediictioii. its usefuliiess is riot so striking in caseu iiivol\-iiix allotropic niodifications or polynierization. I t is true that Ikrtlielot' hay sho\vn that soluble sulphur caii lie obtained a t the aiiode by electrolyzing a solution of hydrogen sulphide aiid that iiiso1ul)le sulphur can he ohtaitiecl a t the cathode by electroly~,iiiga soltition oi sulphuric or siilpliuroiis acid; but thi.; is not really analogous to the action of light because light produces insoluble sulphur by acting on sulphur arid not oil hydrogen sulphide or sulphurous acid. I t is also l-ery probable t h a t \rhat one gets during electrolysis depends primarily on the conditions uiider \\,hirli the sulphiir is set free arid that insoluble sulphur is riot electropositiye siilphur in any proper w i s e of the term. - i t m e time that did not disturh me, but. a t t h a t time, I expected to lie able to prepare dianthraceiie electrolytically.:' 11.e Iiave since tried to do this
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* Schranini .1
and Zakrzc\\-ski: Monatshei't. 8, 2qc) (1x8; Ann. Chirti. Phys., 1,;) 49, 448 ( 1 8 5 ; ) . Trans. -\In,Electrochem. Soc.. 13, 2,s; , I ~ O S J .
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by dissolving anthracene in sulphuric acid and in other n a y s , b u t n-e have not been successful. Of course, it is possible t h a t somebody else may succeed later in making dianthracene electrolytically; b u t nobody has done so as yet, and I am willing to abide by the results of this, admittedly crucial, test. It is also not absolutely satisfactory to have to say, in the case of organic substances, that light produces the same decomposition products t h a t we should get if n-e could electrolyze the substance in question, I\ hich we know n-e cannot do. It sounds a little like the Gilbert and Sullivan problem: "If I can wheedle a knife or a needle, then Irhy not a silver churn?" There is another, more serious, objection to the Grotthuss generalization in its present form The selective action of light is much more marked than the selective action of the current JVe can take a solution containing cadmium and copper sulphates and we can precipitate metallic copper by regulating the voltage. JVe cannot, however, precipitate cadmium from that solution before the copper, though n-e can do i t if we change to a cyanide solution. Suppose that we have two dyes of different colors which are oxidized about equally readily. By changing the wave-length of t h e light n e can cause one dye or the other to oxidize in the air So far as I know the only analogy to this in electrochemistry is the reduction of nitrates and nitrites ' At a smooth platinum cathode nitrite is reduced more readily than nitrate while the reverse is true a t a spongy copper cathode One difference betm-een light and the electric current is that the current is a single reagent except in so far as the specific nature of the electrode may make a difference, while light is a mixture of reagents Light of one n-ave-length may act on one substance and light of another n-ave-length on another, or lights of different wave-lengths may cause the same substance to react in different ways This is not covered by the Grotthuss formulation and it is therefore necessary to make a more general statement Miiller and Spitzer: Zeit. Elcctrochemie, 11, jog (1905).
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I propose the following generalization as an improvement on the two laws of Grotthuss: All radiations which are absorbed by a substance tend to eliminate that substance. It is entirely a question of chemistry whether any reaction takes place or what the reaction products are. This is precisely analogous to the formulation n-hich I have given' for electrolysis. "In the case of electrolysis the only specific action which we have to attribute to the current is that it tends to set free the anions a t the anode and the cations a t the cathode. IJ-hat happens oyer and above that is a question of chemistry, depending on the reaction velocity and equilibrium relations in each particular case. ' ' The formulation which I have given for the action of light covers the cases of oxidation and reduction as well as does the formulation of Grotthuss. n'hat I n-rote four years ago,' is equally applicable today. "The chemical effect of the light is merely to eliminate, if possible, the substance absorbing the light. Whether the elimination takes place as a result of oxidation or of reduction is a matter which is quite independent of the light and which depends on the nature of the depolarizer. If the depolarizer is a sufficiently strong reducing agent,we get reduction by light. If the depolarizer is a sufficiently strong oxidizing agent, we get oxidation by light. If it is not sufficiently one or the other, we get no action by light. X very good instance of the variable action of light is to be found in the case of mercurous chloride. In the presence of a sufficiently powerful reducing agent, light reduces mercurous chloride to metallic mercury. In the presence of a sufficiently powerful oxidizing agent, light oxidizes mercurous chloride to a mercuric salt. If there is no more suitable depolarizer, the mercurous chloride itself acts as a depolarizer and is changed to mercury and mercuric chloride. " When we come to the case of oxygen, we now find it all plain sailing. Light which is absorbed by oxygen tends to l
__ Bancroft Trans. *\m. Electrochem. Soc., 8, 33 (190j). Ibid., 13, 246 (1908).
The Electrochem istry of Light
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eliminate the oxygen and we get the formation of ozone or of ions as the case may be. Light, which is absorbed by ozone, tends to convert the ozone back into oxygen. In this case \re get light accelerating both reactions, but not the same light. I t is easy to see, however, that we might have a case where the displacement of equilibrium in one direction by light might be small owing to low efficiency or to absence of certain n-ave-lengths, in which case light might apparently have very little effect on one modification, under the conditions of the experiment. This appears to be true with phosphorus Ultra-violet light converts white phosphorus into red phosphorus; but nobody has hitherto noted any photochemical change of red phosphorus into white phosphorus. Such a change must take place a t some temperature and with some wave-lengths of light. \t'ith the theory to guide him, Jlr. Leighton has succeeded in converting red phosphorus into white phosphorus, using green light. I t must be possible also to convert insoluble sulphur into soluble sulphur by means of light; but here we are hampered by experimental limitations in case the important rays are in the ultra-violet as they may well be. I know of 110 satisfactory method of obtaining approximately monochromatic light of any given wave-length and of high intensity. The amount of ozone obtained by the silent discharge is the difference between the amounts formed by waves shorter than 300/1(1 and that decomposed by waves longer than 300pp. If the chemical action of the waves longer than 300/1:1 had been somewhat more vigorous, or if that of the shorter n-aves had been somewhat less vigorous, we should not get any ozone by means of the silent discharge, even though ozone could still be made readily if the wave-lengths above 3 0 0 , q ~could be shut off. The phosgene equilibrium offers an interesting illustration of the principle involved. IVeigert' n-orked at too high a temperature (joo'), and in glass vessels which cut off the ultra-violet light. He found no displacement of equilibrium _--___-__ Drude's Ann , 24, j g , 243 (1907).
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by light and he norked out a theory to account for this. Coehn and Recker' worked a t room temperature n-ith quartz tubes and found that COC1, is decomposed chiefly by light having nave-lengths shorter than 265,pp. This was a particularly simple case because glass acted perfectly as a screen for the active rays while quartz did not. IVhat we need is a general method. IVhen n-e get a suitable way of getting ultraviolet light of any given wa\-e-length, we shall undoubtedly be able to demonstrate the photochemical conversion of insoluble sulphur and of dianthracene into anthracene The photochemical change of soluble sulphur into insoluble sulphur and of anthracene into dianthracene is covered by the formulation, which also foresees the possibility of certain rays causing anthracene to fluoresce' while others change it to dianthracene. The results of this paper may be summed up as follon s I . It is correct to say that only those rays nhich are absorbed produce chemical action; but i t is incorrect to add that some absorbed rays have no tendency to produce chemical action. 2 . The statement that light acts like a voltaic cell is not adequate to account for all the facts, though it has proved an admirable working hypothesis u p to a certain point. 3. The most satisfactory formulation of the chemical action of light is that all radiations which are absorbed by a substance tend to eliminate that substance. It is a question of chemistry whether any reaction takes place and what the reaction products are. 4. Different radiations may cause the same substance to react in different ways. j. \Ve have, as yet, no theory which will enable us to predict the relative efficiencies of lights of different wave-lengths Since the Grotthuss formulation has now been superseded, this paper is the last one of this series The future work on the chemical action of light will be published under a different title Cornel1 L'ntaerszty Ber. chem. Ges. Berlin, 43, 130 ( r y ~ o ) .
- Cf. Miss Stevenson: Jour. Phys. Chem
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15, 8.+j (1911).
