THERMODYNAMIC POSSIBILITIES OF T H E SILVER SULFIDE BROMINE-ACCEPTOR HYPOTHESIS O F LATEXT IMAGE FORhIATION* WIGHTMAN**
BY R. H. LAMBERT ASD E. P.
Introduction The sensitivity substance present in the gelatin of high-speed photographic emulsions has been traced by Sheppard to organic compounds containing sulfur, which react under certain conditions with silver halide to form silver sulfide.'] 2 Such silver sulfide appears to be concentrated in minute specks on or imbedded in the surface of the halide grains.3! The sensitizing action of these centers has become the subject of some interesting speculation. Those interested are referred to the papers of Sheppard and co-workers on the hypothesis of orientation and crystal strain, 2, and to the earlier work of others before the nature of the specks was known.+ Recently a new conception of the role of the speck in sensitivity was advanced by Hickman.6 He proposed that silver sulfide acts as a halogenacceptor, and in so doing, that it also becomes the agent for the deposition of more metallic silver than would ordinarily be produced simply by the action of light on silver halide. I n other words, he proposes a chemical theory which may either supplement, or even replace the orientation-strain theory. The mechanism of Hickman's hypothesis is essentially as follows. When light acts on silver bromide, the bromine set free, a t first in the atomic condition, probably attacks the silver sulfide speck or other halogen-acceptor before it has had a chance to become molecular, since it is liberated at the interface between the solid silver halide and the sensitivity speck. Two possible alternative series of reactions are suggested as the result of this attack: Series ( I ) is regarded by him as the more probable since under light action it is unlikely that silver bromide would again be formed even by way of sulfide attack. *As opposed to the idea of sulfur being formed, Renwick,' in a criticism of Hickman's paper, points out that sulfur actually acts as a desensitizer. We shall not attempt to deal with this criticism, but will have something more to say presently as to the probability of the above series of reactions. 1j
'Communication S o . 312 from the Research Lahoratory of Eastman Kodrtk C o . **Paper presented at the American Chemical Society meeting, Richmond, April, 1927. * S. E. Sheppard: Colloid Symposium Monograph, 3, 76 (1925). S.E. Sheppard: Phot. J., 65, 380 (1925); 66, 505 (1926). S. E. Sheppard and H. Hudson: Paper to be published shortly. S.E. Sheppard, A. P. H. Trivelli and E. P. Wightman: Paper in press. Reference is made here to previous work, including that of Svedberg, Toy, and others, on the existence of sensitivity specks. S. E. Sheppard, A. P. H. Trivelli, and R. Loveland: J. Franklin Inst., 200, 51 (1925!. t Referred to in the above-mentioned papers of Sheppard and co-workers. 6II. C. D. Hickrnan: Phot. J., 67,34 (1927). 7 F. F. Renwick: Phot. J., 67, 41 (1927).
R. H. LAMBERT AXD E. P. R’IGHTUBN
I250
SERIESI
SERIES2
z AgBr
z Ag
+ z Br
4 Ag
+ S2BrZ
1
1
+ 2E + AgzS
2
2
Ag
2
1
+ Ag?S +S + 4H20
AgBr 6H
1
IO
IO
+zE
1
+8H20
IOH
6 Ag
IO
TOTAL = 16 Ag metal
1
+ z Br
2
+ z HzS04 + zHBr + AgBr Ag HBr -+
AgBr
~
+ H?S04
+ 6AgBr
+ 6 HBr
TOTAL = 8 Xg metal
It is not claimed by Hickman that these particular chain reactions are the ones involved; he regards the above formulations as only symbolical. However, he gives experimental evidence which indicates that the sulfide does act as an acceptor, and moreover, that a greater amount of silver is formed than would be expected from the photochemical decomposition of the silver halide. He finds that a photographic film or paper treated with a soluble sulfide and then exposed to light until a strong print-out image forms, gives a greater visual photo-product than one untreated. He shows further that this is not caused by light decomposition of the‘sulfide, but that the sulfided portion is bleached by the light action. This, he concludes, is due to the action of the bromine set free by the photochemical decomposition of the surrounding silver bromide. Since a decrease of free energy indicates the possibility of a reaction proceeding, and since in some cases rtt least the magnitude of the decrease is a measure of the tendency of the reaction to occur, we have calculated the free energy changes of the above reactions and also of some other suggested procedures in order to find out whether or not any of them are possible, and if posszble which would be the more likely to occur. Free Energy Data Free energies of formation of substances incolued. Most of the free energy of formation data which we have used have been taken from Thermodynamzcs by Lewis and RandalL8 These as well as some others to be discussed are summarized in Table I. Free energy of formation of SzBrP and S J Z . Very little data could be found from which free energies could be calculated for sulfur bromide and iodide. Spring and Lacranier stateg that a t equilibrium at room temperature, in a mixture of the compound with its components, sulfur bromide is 2 7 7 dissociated and sulfur iodide 90%. 8 G. N. Lewis and Merle Randall: “Thermodynamics and Free Energies of Chemical Substances,” (1923). W. Spring and A. Lacranier: Bull., 45, 867 (1886).
~
SILVER SULFIDE BROMINE-ACCEPTOR HYPOTHESIS
1251
TABLE I Free Energies of Formation Substance
Hdg) H (g) HzO (1) HBr HBrO (Aq) HI (Ad Br2 (1) Brz (g) Br (a) Brz ( A d
AFZg8 in cal. O*
37,730* - 56,560* - 24,595* - 19,680* - 12,361* O*
755* 18,250* 977*
1 (5)
O*
I (g)
I59470* 3,926*
12 (Ad
Substanre
SS rhombic Si,,, (1)
A F z ~ins cal.
(s)
S (9)
Sz (9) S2Brz (1) SZIZ (1) HzS ( A d HzS03 ( A d HzSO, ( A d AgBr (s) AgI (s) Ag2S (SI
NE
O*
93 * 30,240* 18,280* - 3,040** 4,316** -6J490* - 126,330* - 176,500* -23,730 t -15~767t -6,355ti 76,900
*Lewis and Randall, cf. ref. 8. **Calcd. from dissociation values of Spring and Lacranier, cf. ref. 9. t T . J. Webb, cf. ref. 1 1 . ttcalcd. from data given by Xoyes and Freed, cf. ref. 13. Calcd. from photochem. equivalence, ie , one mol-quantum of light energy.
In the reaction
S,+ I/naBrz % r/2aSnBrz
(1) all the substances are liquid. Lewis and Randall8 consider liquid sulfur a t 25' to consist usually of an equilibrium mixture of Sg and S6 which they designate as Si,,, Since according to them there is only about 0.8% S,, Le., SO, in the mixture we may neglect it and consider a = 8. So we obtain for the formation constant of reaction (4) the value 198. Since AF = - R T In k
(2) where k is the formation constant, and R and T have their usual significance, the value of AF can easily be calculated. For S2Br2,AF298 = -3,133 cal; and for S&, AF2#8= 5143 cal., the elements being in the liquid state. The free energy of transfer of Si,, to S(rhomb8e) is given by Lewis and Randall as AFs8 = 93 cal. Hence, the free energy of formation of S2Br2 from liquid bromine and solid sulfur a t 25OC is -3,040 cal. Since the free energy of transfer of liquid to solid iodine is, according to the same authors, -920 cal., the free energy of formation of S21zfrom the solid elements is 4,316 cal. Ogier'O obtained-2,000 cal. and o cal. respectively for the heats of formation of sulfur bromide and iodide ; therefore, it may be assumed that the free energy of formation of the bromide as calculated above is not greatly in error. The value for the iodide, however, seems rather high. But, in the absence of any more reliable data we shall have to accept it. P. 525. LOOgier: Compt. rend., 92,923 (1881).
I2 j 2
R . H. LAMBERT AND E . P. W I G H T M A S
As a matter of fact, were the value for the bromide fifty, even a hundred per cent, in error it would not, as will be seen later, alter seriously the final results and conclusions a t which we have arrived. Free energy of formation of AgBr and Agl. The free energy values for silver bromide and iodide are given by T. J. Webb." The value for the former was calculated by him from the data of Lewis and Starch'? and Lewis and Randall.8 That of t,he latter is a very reliable one which he obtained by averaging five independent values very closely in agreement. Both values are given in Table I. Free energy of formation of Ag1S. Noyes and Freedla determined the electromotive force of the hydrogen-silver sulfide and hydrogen-silver iodide cells. From these the free energy of formation of silrer sulfide can be calculated. This value turns out to be -6355 cal. Solubility and specific heat data, which are available, can also be used to calculate the free energies, but the value so obtained is much less reliable than that given above. Free energy o j I mol-quantum of lighl. Finally, we shall consider the energy of light producing photochemical decomposition of the silver halides. The energy for one quantum of light is given by the equation E = hc,/X in which h is Planck's constant, 6.547 X IO-?? erg sec.; c is the velocity of light, 2.999 x I O ~ O cm./sec.; and X is the wave-length of light. Choosing X = 370 mp, a wave-length to which both silver bromide and iodide are highly sensitive under ordinary conditions, then E = 5.307 X 1 0 - l ~erg or 1.268 X 1 0 - l ~ cal. If we assume, as Hickman has done, that one molecule of silver halide (i.e., 1/4 of a crystal molecule according to Bragg) requires one light quantum for decomposition, N would be the energy necessary to decompose one grammol, where N is Avogardo's number, 6.062 X ;oZ3. N is then 76,903 cals. or, in round numbers, 76,900 cal. Free Energy Change in Various Reactions The photochemical decomposition of the silver halides. In determining the free energy change in the photochemical decomposition of silver bromide or iodide, two important assumptions had to be made. First, that at least one AgBr pair in the silver bromide lattice-and the same is true for the iodideis decomposed for each quantum of light energy absorbed, as claimed by Eggert and Noddack," and secondly, that the photochemical energy can supply the free energy of decomposition in the case of a light sensitive substance like silver bromide. "T. J . Webb: J. Phys. Chern., 29, 816 (1925). 1*G,N. Lewis and H. Storch: J . Am. Chern. SOC.,39, 2544 (1917). 13A. A. Noyes and E. S. Freed: J. Am, Chern. SOC.,42, 476 (1920). "J. Eggert and W. Noddack: Sitzungsber. preuss. Akad. Kiss., 39, 631 ( 1 9 2 1 ) ; 41, 1 1 6 (1923); W. Nernst and W . Soddack: 41, I I O (1923).
SILVER SULFIDE BROMINE-ACCEPTOR HYPOTHESIS
‘253
Granting these assumptions, we then get AgBrfs) T E ~=XAg(s) Br(g); nF298 = - 34,920 cal. (4) a.AgBr(s) f 2xt3io = zAg(s) Bra(l); AFzg8= -106,330 cal. (5) depending upon whether we have the formation of gaseous atomic or liquid niolecular bromine. The corresponding reactions for silver iodide are AgI(s) Kt370 = Ag(s) I(g); AFzg8= - 45,663 cal. . (6) z.AgI(s) 2XC3iO = nAg(s) Il(s); AFag8= - 1 2 2 , 2 6 6 cal. (7) It appears from this that silver iodide should be more sensitive t o light than silver bromide. This is known to be true, but the relative developability in ordinary developers is in the reverse direction. If it is assumed that bromine or iodine when set free goes immediately into solution in the water in the emulsion-present usually to the extent of 5 to IO? by weight of the dry emulsion-before it can be taken up by an acceptor, the free energy decrease of the reaction corresponding to equation (4) would be - j2,682 cal. This assumption, however, seems rather improbable when a halogen-acceptor like silver sulfide is in immediate contact with the silver halide lattice. The AgpS-bromine-acceptor reactions. We are now prepared to test Hickman’s and other series of reactions or postulates:
+
+
+
+
+ +
+
Postulate 1 , via SzBrz aXgBr(s) 2 x 6 = 2Ag(s) zBr(g); AF298 = - 69,840 cal. zAg?S(s) nBr(g) = 4Ag(s) S2Br?(1); AFzgs= - 26,830 cal. S2Br2(1) 8Hz0(1) = nHBr(Aq) 3. 2H2S04(Aq) 1oH(g); AF298 = +430,630 cal. IoH(g) IoAgBr(s) = IoAg(s) IoHBr (Aq); AF298 = -385,950 cal. Adding these four equations and dividing by 2 : 6AgBr(s) Ag?S(s) 4H20(1) Kt = HzS04(Aq) bHBr(Aq) 8Ag(s); AFZQB = - 25,995 cal.
+ +
+
+
+
+
+
+
+
+
+
+
+
H,SO,(Aq)
+
(c) (d)
+
Postulate 2, via S(atom,C) zA4gBr(s) 2Ne = zAg(s) zBr(g); AFzss = - 69,840 cal. S(g); AFm = - 47,365 cal. Aigzs(s) zBr(g) = aAgBr(s) s ( g ) 4-4H20(1) = HzSOa(4q) 6H(g); AFm = +245,880 cal.
+
(a) (b)
+ +
+ 6HBr(Aq) + bhg(s);
AF298
=
-102,985 cal.
(8)
(a) (b) (c)
(9)
First of all, it is seen that in Postulates I and 2 of Hickman that reaction (c) in each case gives an enormous positive free energy, even greater than the sum of the negative free energies of the first two reactions. It appears unlikely then that the latent image formation could follow such a procedure. Even if the atomic hydrogen in each case were to form molecular hydrogen
R. H. LAMBERT A S D E. P. WIGHTYAN
I254
first, before acting on the silver halide, the free energy would still be positive, +53,330 cal. in Postulate I and +19,600 cal. in Postulate 2 . We discuss this matter more fully below. Hickman was careful to state that he did not limit himself to these two series of reactions but he did not mention any other specific ones, except to say that the oxidation might not proceed to sulfuric acid but might stop short a t sulfurous acid. Even this does not give a negative free energy. Slater Price,I5 in a criticism of Hickman's hypothesis, mentioned the fact that bromine reacts with water to give hypobromous acid. On the basis of this suggestion we have Postulate 3.
Postulate 3, via H BrO
+
bAgBr(s) 8 Ne 8Br(g) 4HzO(l)
+
8Ag(s) 8Br(g); 4HBr(Aq) qHBrO(Aq); AgzS(s) 4HBrO(Aq) = nAgBr(s) aHBr(Aq) H2S04(Aq);
+
+
+
=
+
=
+ + +
+
(a)
AFZAE = - 96,860 cal.
(b)
AFm
=
-188,o;j cal.
(c)
AFzss =
- j64)zgj cal.
(IO)
+
6AgBr(s) Ag,S(s) 4Hz0(1) 8 Se = HzS04(Aq) 6HBr(Aq) 8Ag(s);
+
AFZM= -279,360 cal.
Here, since all the intermediate reactions show a decrease in free energy, the process appears thermodynamically possible. I t is to be noted, however, that no silver results except by the photochemical decomposition, which has to be considerable in order to furnish sufficient bromine for the reaction to proceed. In other words, only large exposures would be expected to produce this result. I n this case, the moisture in the plate serves as the bromine absorber and the silver sulfide acts only indirectly. There are other modifications of this procedure of course. For instance, it is unlikely that so much bromine would be formed a t once and remain in the atomic condition before acting. This would cause reaction (b) to have a positive free energy, thus Postulate 3 A , vza H B r O and lzquzd Br
+
+
8XgBr(s) 8 Ke = 8Xg(s) 4Brz(l); AFSg8= -425,360 cal. (a) 4HBrO(Aq); AF298 = 49,140 cal. (b) 4 B r ~ ( l ) 4 H ~ 0 ( 1 )= 4HBr(Aq)
+
+
+
Since reaction (b) in Postulates I and 2 are both possible, let us consider if we can find some other way in which the sulfur bromide or the sulfur may react with the water or silver bromide, the most likely things for them to act upon. We have not found any statements in the literature as to just how sulfur bromide acts with water, although we know it does. The following postulate suggests itself (Postulate 4). 'ST.Slater Price: Phot. J., 67,40
(1927).
SILVER SULFIDE BROMINE-ACCEPTOR HYPOTHESIS
1255
Postulate 4, via S2Br2and H 2 S
+ = zAg(s) + zBr(g); AF29s = -69,840 cal. + = 4Ag(s) + S?Br2(1); A F z ~= s -26,830 cal. + = H?S(Aq) + (Ad + zHBr(Aq); AF298 - 9,290 cal. H2S(Aq) + aAgBr(s) = Ag2S + zHBr(Aq); AFZgg= - 1,595 cal. 4AgBr(s) + Xg?S(s) + 3%0(1) + 2Ne = zAgBr(s) s S e zAg2S(s) aBr(g) S2Br2(1) 3H?0(1)
=
+
+
H2S03(Aq) 4HBr(Aq) 6-M); and similarly where S is first formed:
AFQg,
= -107,jjj
cal.
Postulate 5, oia S and S B B r 2 = 6.&g(s) 3Br2(l); AF2gs= -319,ozo cal.
+ + = qAgBr(s) + S2(g); AFzg9= - 63)930 cal. + = S2Br2(1); = - 21,320 cal. + H?S(Aq) + H2S03 (Aq) + zHBr(Aq); A F 2 9 8= - 9,290 cal. H2S(Aq)+ zAgBr(s) = Ag&(s) + zHBr(Aq) AF298 - 1,595 cal. qAgBr(s) + Agss(s) + 3H20(l) + 6Ke = HzSOs(hq) + qHBr(Aq) + 6Ag(s); AF295 = -415,155 cal. +
6AgBr(s) 6Ne zXg2S(s) zBr2(l) Sdg) Brdl) S?Br2(1) 3H20(l)
AF298
(a) (b) (c) (d) (6)
(a) (b) (c)
=
=
(d) (e) (11)
In an atmosphere of oxygen the sulfurous acid formed would likely soon be oxidized to sulfuric acid, thus: AF2,, = - 100,340 cal. z H ~ S O ~ ( A ~Oz(g) ) = 2HzSOa(Aq); (f) Hickman is of the opinion that, even in a chain reaction such as postulate 4 or j, H2S cannot form even if only instantaneously in the presence of H 2 S 0 3 but that free sulfur would be formed. If this were true, then we should have one of the two following. 2S2Brdl) ~ H z O ( I= ) 3S(g) 4HBr(Aq) H2S03(-&q); AF = + 4 1 $ 7 7 0 cal. (c’) zS2Br2(1) 3H?O(I) = 3 8 Sg (rhombic)(s) 4HWAq) H2S0dAq); A F = -48,950 tal. (c”) Reaction (c’) gives a positive free energy and is therefore unlikely and (c”) seems improbable unless sufficient SzBr2were formed to give a whole instead of a fractional molecule of sulfur. The action of atomic sulfur on water we have already considered. There are also some other possibilities which we shall not attempt to give here. Xotice that in the case of Postulate 4, 4 atoms of silver are produced from z silver sulfide, but none from silver bromide except that due to light action, some silver sulfide being regenerated. AI1 the intermediate reactions show a free energy decrease. In Postulate 5 , which is a variation of 2 , we again have a decrease of free energy throughout, but only silver due to light action is produced. So far, we have been unable to find any series of intermediate reactions which would produce silver aside from that caused by the photochemical decomposition itself, except as in Postulate 4, and as noted below. The fact that all the intermediate reactions in Postulates 3, 4, and 5 show a fairly large
+
+
+ +
+ +
+
1256
R. H. LAMBERT A S D E. P. T I G H T M A Y
negative free energy would indicate that they are admissible thermodynamically, and hence, that these postulates are more plausible than Postulates I and 2 . To be sure, if molecular hydrogen were to be formed instantly in Postulate 2 reaction (c), and if sulfurous instead of sulfuric acid were formed, the energy supplied by the photochemical decomposition of XgBr and by the formation of S2Br2or of S would be greater in each case than that of reaction (c), (the oxidation of the S2Br2 or S) and the sum of free energies of the first two reactions might be sufficient to force reaction (c) to take place, especially when the hydrogen so formed would be instantaneously disposed of according to reaction ( d ) , (the reduction of silver bromide).
Postulate 6 , via S2Rr2 and €€$OS a.-igBr(s) 2h’e 2Ag(s) nBr(g): AF298 = -69,840 cal. (a) 2Ag2S(s) nBr(g) = 4Ag(s) S2Br2(1); AF298 = -26,830 cal. (b) 6H20(1) = aHBr(Aq) S2Brd) 2H2S03(As) 4- 3Hdg); AFz98 = +41,550 cal. (c) 3Hdg) 6AgBr(s) = 6Ag(s) 6HBr(Aq); AFZ99= - 5,190 cal. (d) 8AgBr(s) nAg2S(s) 6 H 2 0 n?;e = HnSOdAq) 6HBr(hq) IzAg(s); AF2g8 = -60,310 cal. ( 1 2 ) Postulate 7 , via S (atomzc) and H$03 2AgBr(s) 2Ke = aAg(s) zBr(g); A F z= ~~ -69,840 cal. (a) zBr(g) = aAgBr S(g); AFZB= -47,365 cal. (b) AgzS(s) s(g) 3H20(1) = HzSOZ(Aq) 2 H ~ ( g )AF298 ; = +13,11o cal. (c) 2Hi(g) 4AgBr(s) = 4Ag@) qHBr(Aq); AF2g8 = - 3,460 cal. (d) Ag2S(s) 3H20 (1) P N E= bAgBr(s) HzSOdAq) 4HBr(Aq) 6Ag(s); AF299 = -107,555 cal. (13)
+ + + + +
+
+
+
+
+ +
+
+
+
+
+ + + + +
+
+
+
+
+
There is one way suggested to us by Hickman by which silver might be formed from silver bromide aside from the photochemical decomposition. This is by oxidizing sulfurous to sulfuric acid. Reductzon of AgBr by H 2 S 0 3 ( A q ) HISO,(Aq) H20(1) = H2S04(Aq) H d g ) ; AF298 = +6,390 cal. (a) Hz(g) zAkgBr(s) = Ag nHBr(Aq); AFZ96= -1,730 cal. (b) If the molecular sulfurous acid has a chance to become ionized we get instead 2s03-H20(1) = zH+ 2SO4-of (a) 2H+ H z k ?; AF238 = -3,260 cal. (a’) The positive free energy of (a) is not large and might easily be overbalanced by the energy set free in the preceeding intermediate reactions. We might liken this effect, which applies also to the formation of H in Postulates 6 and 7, to that of falling water. In A , Fig. I the water always falls from one step to the next. In B it would fall the first two steps, but would probably not come back over the third step until the depression had been filled becausc it would have to rise too far, but in C‘ the rise in the third step would slow it down and lessen
+
+
+
+
+
+
+
+
SILVER SULFIDE BROMINE-ACCEPTOR HYPOTHESIS
'257
its force but would not stop it from falling the remainder of its journey. If the hump in B were higher than the source, no water would fall and in the same way with the series of reactions, nothing would happen. I t must be remembered that in the formation of the latent image the quantities of substances reacting are of the order of a few molecules a t the most, and it may be that thermodynamic relationships which have been worked out on the molar basis where millions of molecules are involved are somewhat different from the former. Any statement concerning the possibility or impossibility of a reaction proceeding must therefore be very guarded.
FIG I
I t should be noted that Hickman found that silver sulfide formed in a photographic emulsion is bleached by the action of light, and silver was formed greater in amount than that produced on an unsulfided portion of the emulsion. This could easily be explained by such a procedure as t h a t in Postulate 4 in which sulfur bromide was formed from the silver sulfide and this was disposed of through reactions, all of which showed free energy decreases. There is one suggestion which we offer which seems reasonable a t first sight, since it appears quite possible thermodynamically. This is that light acts on silver sulfide as well as on the silver bromide; Sheppard has already pointed out' that AgzS has an electron affinity less than AgBr. I n this case we should get something like the following, 2AgzS(s) ZSE = 4Ag(s) 2S(g); AF298 = -80,620 cal. (a) zAgBr(s) f 2S(g) = 2Ag(s) S2Brz(1); AFZg8 = -16,006 cal. (b) and the S2Br2 could react as in one of the preceding postulates. Here, considerable Ag2S and AgBr are both destroyed and silver is produced t o an even greater extent than where AgBr is the photoactive substance. The objection to this is that the spectral distribution of the light energy in latent image formation appears to be definitely that due to absorption by silver bromide. The spectral distribution in the blue violet remains the same
+
+ +
R.
I258
n. LAMBERT
AND E. P. WIGHTMAN
for sensitized and desensitized plates, and Toy and Edgerton foundtEthat the number of centers formed per grain was proportional to the light-absorption by silver bromide a t that wave-length. The only possibility of meeting this objection would be to suppose that the silver bromide is acting as an optical sensitizer' for the szlver sulfide a t the same time that the latter is a chemical sensitizer for the silver bromide. Summary and Conclusions It would appear from the data which we have given that Hickman's hypothesis that the silver sulfide can serve as a halogen acceptor in the formation of photographic latent image is sound thermodynamically. The two specific reactions which he proposes tentatively as the mechanism by which t o arrive a t the latent image, while in their summation showing free energy decreases, contain in each case a reaction with a positive free energy higher than the sum of the free energies of the preceeding reactions. This throws doubt on the particular intermediate reactions in question, but does not invalidate the hypothesis as a whole. In other words, whatever the route by which we arrive a t the final result, it, the final result, appears to be thermodynamically admissible. Following the absorption of the bromine by the silver sulfide, there are numerous series of reactions which may lead to a final equilibrium condition. One of Hickman's fundamental assumptions is t h a t more silver is formed than the photochemical decomposition alone would account for. Most of the reactions which have been investigated do not give rise to more silver than that produced by the photochemical decomposition or from the sulfide by the absorption of the bromine. However, Hickman has suggested to us the reduction of the silver bromide by the sulfurous acid formed. This appears thermodynamically feasible if the surplus energy of the preceeding reactions is taken into account. Finally, it is suggested that silver sulfide itself in the presence of silver bromide is photochemically decomposed, the silver bromide in this case serving probably as an optical sensitizer of the sulfide, as Sheppard has previously suggested, and as the sulfur acceptor giving rise to sulfur bromide and silver. I n this case, much more silver would result than otherwise. The free energy of formation of silver sulfide and of sulfur bromide have been calculated, the one from the electrochemical data of Noyes and Freed, and the other from the work of Spring and Lacranier on the dissociation of sulf'ur bromide. These are respectively - 6,3 5 5 and - 3,040 calories. Rochester, S e w Yorl; M a y 5, 1927 1eF. C.
Toy and J. A. Edgerton: Phil. Mag., 48, 937 (1924)
