THE ELECTROCHENISTRY OF LIGHT. V B Y TVILDER D
BAANCROFT
T h e Problem of Solarization. Part I1
-1 summing-up of the problem of solarization from a single point of view has recently been gix-en by Luppo-Cramer.' T17hile one may not, and probablj- will not, agree with all that 1,uppo-Cramer says, the presentation is an interesting one and therefore i5 given at length. The oxidation theory of solarization assumes t h a t the sutxtmce of the latent image, formed by a brief action of light, is oxidized b)- the oxj-gen of the air t o a difficultly rediiciblc substance This theor!- is not onlj insufficient but ai50 1 i o p ~ l e s s l j~~ l b ~ u ta5 d far as it is applied to silver bromide. It i i quite remar1;able that the oxygen of the air should oxidi7e the tcsi rtnut sub-biowizde in the short period necessary for solarization in intense light especially when we renieriiber t h a t the same iub-bromide forming the latent image is so indifferent in the dark to oxygen and even to strong oxidizing agents. That the asiuniption of the participation of the oxj-gen of the air in the solariiation process is untenable, can be denionitrated liy quite simple experimmts. Guthrie? announccd that \il\-er chloride blackens under benzene exactly as it does in air The fact t h a t the blackening of silx-er chloride under naphtha takes place just a ? it does in the air was cited !b Care) Lea as an argument against the view that the blackeninq in liglit has anything to do with oxidation. I ha\-e extended these experiments and have found that silver bromide, silver chloride, mercurous b;oinide and mercurous iodide gelatine fillili shon no difference in photochemical behavior nhen exposed under benzene, alcohol or ether. ,-Us0 the Solarization of ordinary silver bromide gelatine "
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~ ~ ~ I ~ I ~ ~ IProhlcme ~ ~ ~ I ~I 1 I 8I ( iI i pI ; ~I ~ I ~ ~ - Eclti s !tilirl)ucli tlir I ' l ~ o t ~ ~ g r ~ i 5, ~ i 41: l i ~ e(1891 , 1 I
is not afi'ected in the slightest by benmne, alcohol arid ether.' The improbabilit? of the oxidation hypothesis in regard t o solariiation appears also from the self-ex-ident continuity in the action of light on silver bromide where there is not the slightest evidence that the cubstance of the normal latent image differ5 qualitatively' froin that of the latent solarized image. On the contrary i t appears that there is a cowtiizuous splzifi?ig-of/ u ) b?onii?iC, v h i c h is prowd by the appearance of the directlj visible image which every one knows cannot be reversed. The effect of reducing agents in preventing solarization has already been cited by Abne!- in favor of the oxidation theory; but T T C are reallj- dealinq n-ith the power of these substances to absorb halogen and not with their reducing power. In a poleriiic against Carey Le? in 1878, H. IT. \-oge13 brought out sufficiently the difference between the reducing action and the abilitj- to absorb ha'ogen. In opposition to Carey Lea's conception of sensitizers as reducing agents, T'ogel cited silver nitrate n hich is unquestionably an oxidizing agent and !-et \vhich is the best of all sensitizers on account of its strong absorptive power for the halogens. SIy guess, that a similar state of things vould hold true for solarization, has been confiriTlcr1 completely by the experimental evidence. DrJ- plates TT ere bathed for one minute in one percent solutions of nitrite, sulphite, hydroquinone, ethylene diamine, as uell a i in a solution containing one percent silver nitrate and t n o percent citric acid. The plates were dried, exposed for a few minutes t o daylight along n7ith check plates under a negative, \r ashed, and developed with metol-soda. 11-hile the check plates, which had not been bathed, g a w good, completely reversed solarization images, the solarization TT as completely eliminated on the j i
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t h e 111 ite i\11oiI-reaches the iiietal, I n spite of this it would he desirable to liax-e parallel I)!.
IV'iZder D . Bancroft
206
experiments with smooth electrodes polarized to high potentials." The parallelism [between the photo-electric and the photochemical phenomena] is not yet a t an end. In the field of photochemical reactions we find a strict analogy with the negative photo-currents which are superposed on the normal photo-currents at high electrode potentials and which finally become the more important. " I t has been observed that the silver halides become darker during the first stage of the exposure but that they become lighter again if the exposure is prolonged. This phenomenon is called solarization by the photographers. Abneyl made a careful study of it and found that it was conditioned on the presence of oxidizing agents. "Herschel was the first to obsen-e the reversal of a photochemical action as the result of a more prolonged exposure. He prepared a silver iodide paper with an excess of silver nitrate, exposed it to light until it was blackened, and soaked it in a solution of potassium iodide, whereupon a further exposure to light made the previous image disappear. I\-hen the experiment was repeated by Xbney, it appeared that the reversal took place xhen the solution had an acid reaction and not an alkaline one. This is quite natural because iodine is set free much more readily from hydriodic acid than from potassium iodide. Consequently the iodine pressure of the acid solution (which could be determined at a platinum electrode) is very much greater than that of the alkaline solution. To observe the same phenomenon with silver bromide or silver chloride an oxidizing agent must be added. " Potassium permanganate, bichromate, nitric acid, hydrogen peroxide and ozone all act in the same \yay. A11 the rays of the spectrum are capable, under certain conditions, of neutralizing a previous action of light, but the solarizing action of the red rays is much greater than that of the blue. Proc. Roy. S O C , 27, 291, 451 (IS:&). zoo (1880).
Plnl. Nag. [j], 5 , 61 (187R); IO,
The Electrochevzistry o/ Light
207
“Abney also showed that no bleaching takes place in presence of reducing liquids or gases. “NoTv one should notice the agreement between the conditions tinder which -Ibney obtains solarization and the conditions under which I observed the negative photo-currents. In both cases a high pressure of the oxidizing constituents was necessary and in both cases the phenomena were obtained more readily with silver iodide than with silver chloride or silver bromide. In view of all this, I feel that I am justified in calling the negative currents solarization currents, The solarization currents and the photochemical solarization phenomena stand in the same relation, one to the other, as the normal photo-currents and the normal blackening of the image when a picture is copied photographically. “The tendency of the silver chloride electrode to solarize especiallq- readily in yellow light finds its analogy in some well-known phenomena from the field of photography. With the exception of the Becquerell heliochromes, vhich owe their existence to stationary waves,2 all the directions for making silver chloride heliochromes require such a treatment of the silver chloride as to make solarization phenomena probable. The dry silver halide is exposed for an extraordinarily long time to the action of an intense, colored light,3so that a high pressure of the oxidizing substances may be assumed. (-1 long exposure of a paper4 silvered m-ith silver nitrate causes finally a high pressure of the oxidizing agent owing to the decomposition of the silver nitrate.) The same result is obtained much more easily by adding an oxidizing agent to the silver chloride after it has become dark violet in the light, and then exposing the silver chloride again to colored light, This method was first described by Poitevin’ and gives pic~
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Ann. Chim. Phys ( 3 ) , 2 2 , . + j I (184s); 2 5 , 417 (1849). Zenker: Die Photochemie (1S6b). IYiener IYied, A n n , 5 5 , Lipprnann Comptes rendus, 112, 2 7 4 (1391). Seebeck: Goethe’s Farbenlehre, 2 , 7 I 6. S. Flnurent: Phot Arch., 1891,30;. Cornptes rendus, 61, 1 1 1 1 (186j).
224
(189j).
2 IO
TVilder I]. B a m r o j t
protected portions became distinctly cloudy.’ It made no difference whether the glass side or the film side was exposed to the light, which does away with the objection that the absorption of light by the protecting layer might be a factor. It was also shown that light, which passed through glass plates partially coated with collodion, caused a uniform clouding on underlying sil\-er iodide plates. Since the light was just as effective in the places where it passed through the collodion film as in the places where it did not, it follows that the absorption in collodion is negligible. Since the index of refraction of collodion lies between those of silver iodide and of air, the result of exposing the film side of a silver iodide plate coated with collodion in spots will be that less light d l be reflected into the air and more light will pass into the silver iodide a t the places covered with collodion than at the bare spots. In spite of this somewhat greater intensity of light, the cha?Lge takes place very m u c h wove slox)ly wheiz the surface of the silves iodzde i s covesed. “Also if a glass lens with a radius of curvature of about 50 cm is pressed down tight enough, a disc of silver iodide, about 3 mm in diameter, will be protected from clouding while the remaining portions will become quite opaque. “There mas a chance of detecting the iodine set free, in case an illuminated silver iodide film would iodize the surface of an adjacent silver plate. Xfter two glass plates had been covered, one with silver iodide and the other with silver, they were placed with the two layers in contact and were held together by sealing-wax. Half of the glass side of the silver iodide plate was then exposed to light. The silver surface was somewhat changed, but scarcely more so than when the silver was exposed to an equallj- intense light without any silver iodide being present. KO distinct interference colors could be observed and the experiment is therefore inconclusive. Schultz-Sellack [Pogg Xnn , 143, 141 (i8;1)] explains the action of varnishes, n hich he himself noticed, by the assumptlon t h a t coating the particles of silver iodide pre) ents them from sep‘irating and occupying d larger
space.
T h e Electrocheniist?.y of Light
211
“If the clouding is really due to iodine being set free from the film by light, there will either be no clouding or a much weaker one if the sctti?ig-fyee o) iodine is m a d e more dificult b y the presence o) iodine vapor in the suirounding air. Two identical silver iodide plates were obtained by cutting a single plate in two, and were placed in two identical vessels of plate glass, one filled with pure air while the other also contained iodine vapor supplied by some solid iodine in the bottom of the vessel. The two plates were exposed to the electric arc under the same conditions of time and distance, but the result was not what was expected. The plate in the iodine vapor clouded surprisingly quickly and about ten times as long an exposure was necessary to produce the same result with the plate standing in air. This experiment proves conclusively that the cloudiTig 0 ) the silver iodide is not due to the loss 0 ) iodine. “Since the collodion layer does not prevent the clouding by checking the transfer of iodine from the film to the air, the only way in xyhich it can act is by keeping the air from the silver iodide. If this view is the right one, the change of the silver iodide must depend on the nature of the gas in which the plate stands during the exposure. ‘‘ X careful investigation along this line confirmed this idea completely. Pairs of practicallj- identical silver iodide plates were exposed under as nearly as possible the same conditions except that one plate stood in pure air and the other in an atmosphere of hydrogen or nitfogefi. It proved sufficient to drive the air out of the vessel for an hour, the opening of the vessel being turned down or up, depending on whether the gas was lighter or heavier than air. The plates standing in air clouded fairly rapidly but the changes were so slow and so slight in the other gases, especially when they had been carefully purified, that s i h e r iodide may be considered a s completely unchangeable when exposed zn absolutely pure gases of this type. “Since air helps the clouding and since this is not due to the nitrogen it must be due to the oxygen. As a matter of
fact a silver i o t l i c k p l a t v vloutletl ;I little, t11o~igI11 1 o t I I I I K ~ ~ I , faster in a11 ozj'gr'li crfiiiosblrc'i.r, t l i a i i dit1 tlie c.lieck 1)latc. i i i air^ We must tliereforv co~iclutlef i i c i f u i ' r i ) r ( . ) ~ u i , \ c sflit r e i f ( ' o j ,ing (z?zd that o s j y c - i l i ~ r f h c i i ~ i p o r f c i n j(ictor. i "The most prol)ahle assumption is t h a t illuiiiiiiatetl silver iodide takes up oxygen forming a n ox-ygc'~ r o ~ i r p u rzl ~t i . The clouditi:: might then he due to the tensions resultiiix from an increase of \-olumv when oxygen taken U]). If these tensions exceetietl the tenilc strengtli of the salt t l i c . whole film would be broken t o pieces. 'I'he film tnust I)ecoiiie thicker when exposed t o light. ( h i ex:iiiiining in so(liuiii light. the interference fringes formed I)!- a thin tilrii oi a i r enclosed b e t w e n the surface of the silver iodide anti a Iuperimposed glass plate. ;I displaeeinent of the iritigcs a t the edges of the illurninat et1 portion slio\vetl that the sil\.er iotlitlv film did 1)ecorne thicker \\hen exposecl to light. I t is not permissible, lion.c\.er, to deduce the existence of ;HI ox\-geri compound from this sivelliiig of tlie film. for an!- othcr change which disintegratetl t l i c sil1-c.r ioclide \voultl iieeessaril?- iiiake the film thicker. '' The possible ox>-ge~ic~oinpoundsare sill-t.r oxitle. biI\.er iodate, and si1i.c.r ox!,-ioclides. 'I'he first t i v o suhstaiices (Io not form the suljstanccb of tliv cloucled filrn I)ccauscb I ) o t 11 -Ig,O and - l g l ( ),, are rciatlil!. soluble i i i aiiiiiionia. '1'11t.y coulcl tliueiorc. I)e extracted frotii t h e illiiiiiinated silver iotlidc by iiieans of tliis sol\.c.nt' a i i t l coultl tlien he precipitatctl .1, hydrochlorie acitl ;is silver chloride. Repeated c.speriiiir)iits failed t o slioiv t lie iorniation of silver chloridt. e\'eii t hougli the concentrated aninionia solution. in \vhich the plates l a y for several hours. \vas filtered. and e1,aporatcd to dryness, hydrochloric acid being adclcrl a drop a t a time to tlit. aqueous solution of tlic residue. I;\~en under t h e microscope n o precipitatioii of silver chloride could be seen, tliougli i t \vas easy t o recognize tlie cliaracteristic crystalline ioriii of 111(silver iodide \vliicli had been dissolved by the aiiiinonia a n d (./lju(f
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