M. R. V. Sahvun lmog r g Reseorcn .obororory 3M Ccmpony St. Paul, Minnesoto 55101
I
I I
Mechanisms
The study of the chemistry of so-called conventional photography, based on the photosensitivity of silver halides, has been a part of chemical research for nearly 150 years, yet it still has many unanswered questions to deal with. In the teaching of chemistry, the study of the photographic process provides a useful framework for introducing concepts from a variety of disciplines: solid state science; kinetics and catalysis; photochemistry; electrochemistry; coordination chemistry; and organic chemistry, particularly the chemistry of dyes. Its study also illustrates therefore the importance of interdisciplinary efforts in the develo~mentof a socially and economically meaningful technolbgy. A number of useful monographs on the chemistry of photography are available, among which that edited by Mees and James ( I ) has come to be considered the standard reference work. It is not the intent of this review to overlap these treatises except as necessary to introduce a particular topic. Rather, this review will be concerned with illustrating themes of particular current research interest in photographic chemistry, and considering some of the recent developments therein. The silver halide photographic materials generally consist of a so-called silver halide emulsion coated on a suitable s u.. o ~ o r.t ex.. . film base or paver. This emulsion is not actually an em&ion at all, b k really a dispersion of micron to sub-micron sized silver halide microcrystals (commonly called grains) in a polymer matrix, usially gelatin. Although commercial photographic emulsions are quite complex in their production, an emulsion of quite reasonable photographic properties can he made in straightforward fashion, for example, as described by Hill ( 2 ) . The chemistry of photographic emulsion-making has been reviewed in detail in monographs by Duffin (3) and by Zelikman and Levy (4). The silver halides useful in photography include silver bromide and the mixed halides, especially non-stoichiometric chlorobromides and hromoiodides. Although it has recently been demonstrated to be photographically active (5) silver fluoride has not been commercially exploited. On exposure to light or other electromagnetic or ionizing radiation the silver halide photolyzes to form an invisible, latent image of silver ( 6 ) in the microcrystals. The latent image can then function as a catalyst for the further chemical reduction, i.e., deuelopment, of the grain containing it to metallic silver, or, alternatively, catalyze its dissolution by complexation. In the former case, the unreacted silver halide must he removed after development, again by complexation, to fix the photographic material, i.e., stabilize it against the effects of further irradiation. These steps are outlined in Figure 1, each of them represents an area of considerable current research activity as will he illustrated below. Owing to the fact that a real photographic emulsion comprises: (a) grains of various sizes, hence presenting various cross-sections toward actinic radiation; and (h) among grains of a given size, grains of various levels of photosensitivity; different degrees of exposure to which a photographic material may be exposed yield different optical density levels on development. This provides the hasis for photographic reproduction of various tones 72
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Photographic Chemistry
Figure 1. Steps in image formation in a conventional silver halide photographic material.
(shades of gray); and a given material is usually eharacterized by a plot of developed optical density (D)as a function of exposure ( E ) , on a logarithmic scale, as illustrated in Figure 2. Photochemistry
Before entering into discussion of recent advances in our understanding of the photoeffects occurring in silver halide microcrystals, it is necessary to review briefly the prevailing, accepted notions concerning the mechanism of formation of the latent image (6). These are derived, primarily, from the proposal of Gurney and Mott (7). The silver halides exhibit n-type photoconductivity, i.e., with electrons as the principal charge carriers. Conceptually, the events occurring in silver halides on absorption of radiation may be treated by means of the conventional hand model of semiconductors, as shown in Figure 3. The essential features of the model include two continua of electronic energy levels: the valence band, in which electrons are localized on particular atoms; and the conduction
Log E Figure 2. Typical characteristic ( D versus log E) curve of a negative-acting photographic material.
sensitivity depends not only on the high quantum yield for electrons in the conduction hand in the silver halide crystals and the effectiveness of the latent image as a catalyst for the subsequent development reaction, hut also on the following factors which are unique to the silver halides, among photoconductors which have been studied (9, 10)
Figure 3. Energy band model of a photoconductive silver halide crystal
1) Long diffusion range for electrons in the conduction band 2) mobility of positive holes in the valence band 3) mobility of silver ions as interstitials 4) the ability of successively produced silver atoms to accumu-
late at the same spot in the crystal lattice. band, in which the electrons are mobile and contribute to electrical conductivity. Between these two bands are found isolated, localized energy levels which can function as electron or hole traps, and which are identified with defects in the crystal lattice or impurities, including silver(0). Absorption of a quantum of radiation by the silver halide crystal occurs with transfer of an electron from the valence band to the conduction band and creation thereby of a positive hole in the valence band. Note that this positive hole corresponds to a "free" halogen atom. The mobile electron may he trapped in one of the localized levels found in the hand gap; this, however, does not guarantee its survival. The hole, unlike the electron, in the valence band is also somewhat mobile, and can, given time, react with the trapped electron. On the other hand, the trapped electron can react with an interstitial silver ion, one of the silver(1) ions which are diffusing about the crystal lattice in smaIl numben, insteaid of remaining in their proper places. This reaction leads to formation of an atom of silver, which, in the silver halide lattice, is itself a electron trap or a hole trap. A photoproduced electron, trapped at the site of this silver atom is then thought to react with another interstitial silver ion t o form a two-atom aggregate, and so on, until a four-atom aggregate results, which is believed to be the smallest effective latent image usually encountered in a silver halide grain (80). The process is summarized in Figure 4. If silver halide photolysis is continued beyond the level required for latent image formation until microscopically detectable amounts of silver are formed, it can he seen that the process has resulted in selective decoration of the crystal defects ( 8 6 ) . Silver halide photography has come to he the dominant imaging technology because of the exceptionally high photosensitivity (photographic speed) which characterizes its materials. From the above model, it can be seen that this
4-2
e- +
TRAP
e;,Ap,E,
4&g0 Figure 4. Steps in latent image formation. according to the proposal of Gurney and Molt (7). Reactions 4-1. -2, -3. -5. end -7. lead to latent image formation, while reactions 4-4, -8 and the reverse of reactions 4-2. -3, and -5 are sources of inefficiency.
In addition, it is important to consider the significance of the fact that not one but four (more or less) separate photo-events are necessary for latent image formation. Thereby, it is possible to store "active" silver halide materials for useful periods of time (months, years) without the grains becoming spontaneously developable as a result of isolated events. wherebv an electron eets into the conduction band as a result of thermal excitaGon (9). On the other hand. the comolex seauence of events outlined in Figure 4 prdvides numerous bpportunities for inefficiencv. The nrincinal ones are the recombination of the photoproduced electrons and holes and back-reaction of the silver atoms to hole-electron ~ a i and n interstitial silver ions. The latter process is, as indicated in Figure 4, primarily of significance for aggregates of less than four atoms. The reduction of these inefficiencies has been the goal of a half century of research, which has led to techniques in which the silver halide microcrystals are treated with minute quantities of sulfur (or selenium), reducing agents, or heavy metals, e.g., gold. While the last of these techniques, known collectively as chemical sensitization, is still-not well understood, considerable recent progress has been made in understanding.sulfur and reduction sensitization of silver halides. Chemical Sensitization
An understanding of the most recently suhstantiated conceot of chemical sensitization of silver halide ~ h o t o l v sis by sulfur requires consideration of some additional features of silver halide microc~stals.We have alreadv mentioned the existence of inteistitial silver(1) .ions, able to diffuse about in the crystal lattice. Now it is easier at ambient temperatures to remove a silver ion from its lattice site a t the surface of the crystal, where its coordination sphere is incomplete, than from a site in the bulk of the crystal. On the other hand, the interstitial ion, itself, is more stable within the bulk of the crystal, This distribution of interstitial ions and vacancies leads to a space charge in the crystal; it is negatively charged at the surface, and positively charged below the surface (10). As a result, photoproduced electrons are repelled from the grain surface, while their counterpart positive holes are attracted to the surface, where they can be neutralized. This situation, on the one hand, contributes to silver halide photosensitivity by providing a mechanism for separation of the electrons and holes. On the other hand, it leads to formation of latent image, by the mechanism of Figure 4, within the bulk of the crystal, where it is of limited utility as a catalyst for the subsequent development reaction; it has been shown that the development reaction must take place at a surface of the grain ( l l ) , even though the latent image may not originally have been formed at a surface. Sulfur sensitization is believed to involve formation of silver sulfide at the surface of the sensitized crystal (12). In practice, this is effected through digestion of the liquid photographic emulsion in the presence of micromolar concentrations of a labile sulfur compound, notably a thiosulfate salt. Under these conditions, thiosulfate adsorbs to the silver halide crystal surface, where it is unstable with ~~~~~~
~
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Figure 6. Two proposed models far spectral sensitization of silver halide photolysis by adsorbed dyes; the energy transfer mechanism (left); and the electron transfer mechanism (right): (after West and Giiman ( 1 6 ) ) . Figure 5. Mechanism of sulfur sensitization of the photolysis of silver halide (after Faturm and Coppo ( 7 3 ) ) : see text for furlher description.
-
respect to formation of silver sulfide (12). By means of di&0,2 2AgBr + H?O S0cZ + Ag2S 2HBr (1) electric constant measurements, Fatuzzo (13) has shown that deposition of silver sulfide on the surface of a silver halide crystal leads to a reversal of the space charge in the crystal. This means that where silver sulfide has been deposited on a silver halide crystal, photoproduced electrons are attracted to the surface; i.e., whereas the rest of the crystal surface carries a negative charge, a t the point of silver sulfide deposition, it carries a positive charge (14). Thus, an exploitable latent image forms a t the surface of the silver halide microcrystal where silver sulfide has been deposited. Fatuzzo's model is illustrated in Figure 5. The mechanisms of reduction sensitization have been studied extensively by Tani (15). He hypothesizes that digestion of a liquid photographic emulsion in the presence of a reducing agent, e.g., stannous chloride (experimentally analogous to the process whereby sulfur impurities are introduced into the silver halide crystals) can introduce two types of impurity centen. One type of center can he considered to he a sub-latent image quantity of silver(0) itself which, as we have seen (above), functions as an electron trapping site intermediate in the process of Figure 4. The other type of center is thought to destroy, by reduction, the positive hole produced in the primary photoprocess, and which can be thought of as an oxidizing species. Removal of the positive hole increases the deeree oi irrerersihility associsr&l wirh the steps of Figure 4, 'And hence the overall effir~encyof tht, process.'
+
+
Spectral Sensitization
Significant absorption of light by silver chloride does not extend into the visible portion of the spectrum, i.e., silver chloride is a colorless substance; silver bromide absorbs only blue light, out to -500 nm, although addition of small quantities of iodide can extend the adsorption edge to -540 nm. Over the past century however, techniques of spectral sensitization, whereby the range of wavelengths useful for effecting photolysis of silver halides is extended to include the entire visible spectrum and part of the near infrared, have been developed. These involve adsorption of appropriate dyes to the silver halide microcrystals which comprise the photographic emulsion; light absorbed by these dyes can then he used to effect the photolysis of the silver halide. Two mechanisms have been proposed to explain this effect, one involving energy transfer and the other involving electron transfer between the dye and the silver halide; considerable current research effort is being devoted to the confirmation of one or both of these mechanisms (16). These two models are diagrammed in Figure 6. The energy transfer process involves transfer of energy from a molecule or aggregate of molecules of dye, in an excited state by virtue of light absorption, to the silver halide 74
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Journal of Chemical Education
crystal to excite an electron into the conduction band, e.g., according to the resonance theory of Forster (17). In order that this energy transfer process he not prohibitively endothermic, the electron liberated into the conduction hand must come out of a trap energetically somewhere in the silver halide crystal band gap. Several lines of evidence lend plausibility to the energy transfer mechanism (18); its feasibility was apparently demonstrated in the elegant experiments of Kuhn's group (19a). They deposited monolayers of an inert substance (cadmium arachidate) on the surface of vapor deposited silver bromide as "spacers" of known thickness, and then deposited typical sensitizing dyes thereupon. They found that photoexcitation of the dye could lead to formation of a developable latent imaee in the silver bromide laver at senarations un to 55 A, consistent with F~rster's theory. These exper;mental conditions rendered electron transfer from the excited dye to the conduction hand of the crystal highly improbable. There are, however, several difficulties with Kuhn's work which has engendered some reluctance towards its acceptance as a model for the process of spectral sensitization of photographic emulsion grains. The most critical of these is the possibility of monolayer breakage under the experimental conditions (196). The electron transfer model for spectral sensitization has been developed in its most sophisticated form by Tani, in a series of papers over the last five yean (20). According to this "modified electron transfer mecha~ nism," only dyes whose "quasi-Fermi l e ~ e l , "%(Eh, El,) in the notation of Figure 6, lies higher than the corresponding Fermi level (analogously defined) for the silver halide crystal, will sensitize, i.e., transfer electrons to the crystal with a higher probability than holes, from their excited states. In practice, this quasi-Fermi level is obtained from values of E,, and E l , determined polarographically, spectroscopically, and by Hiickel molecular orbital calculations. This mechanism has been shown to have excellent predictive powers for the identification of what dyes will or will not spectrally sensitize silver halide photolysis. The utility of this model suggests that it provides a reasonable description of the mechanism of spectral sensitization in silver halide photographic emulsions of practice, despite the demonstrated feasibility, in a model system, of the energy transfer mechanism.
+
Development Mechanisms I-Catalysis
of Reduction
As noted above, the photographic speed of silver halide imaging materials depends not only on the inherent pho-
'The experimental bases for distinguishing these two types of reduction sensitization eentem are, unfortunately, derived by techniques which are beyond the scope of this article, and, hence, will not be considered here. 2The quasi-Fermi level is a measure of the electronegativity of the dye in the molecular state, corresponding to the Fermi level of the crystallinestate of the dye (20).
tosensitivity of the silver halides, hut also on the opportunity to exploit the latent image as a catalyst in a suhsequent development step. According to predominant practice. the silver(0) . . latent imaee. - . formed in a silver halide microcrystal by action of light or other electromagnetic or ionizine radiation is used to catalvze the reduction of the rest o f i h e silver(1) ions in the crystal to metallic silver. This process is generally believed to involve progressive enlargement of the latent image, by deposition of additional silver(0) thereupon. Historicallv, the development reaction has been desciihed in terms or two limitinicases, "direct" and "physical" development. In the former of these, additional silver(0) is deposited directly from the silver halide solid state; whereas in the latter, the reaction is carried out in the presence of reagents capable of complexine silver(1) in solution, and the deposition of silver(0) from this solution occurs a t the solutibn-metal interface. Modern workers, however, have come to realize the arhitrariness of this distinction, along with the fact that neither limiting case is likely to obtain absolutely in practice (21, 22). A great variety of organic and inorganic reducing agents may he used to effect the development reaction; the most commonly used ones include hydroquinone, p-aminophenol, p-phenylenediamine, ascorbic acid, 1-phenyl-3-pyrazolidone and their derivatives (structures given in Fimre 7), normally employed in alkaline soluti& c u r r e n t research on the chemistrv of photoeraphic development centers on the mechanismjs) by which ihese agents affect silver halide, and particularly, the mechanism(s) by which the latent image exerts its catalytic effect. The hypothetical mechanisms have been reviewed by Jaenicke (21) and include 1) the
charge barrier theory, whereby the negative surface charge on the silver halide crystals provides a harrier to the transfer of electrons thereinto, except at the latent image 2) the electrode theory, which treats the developing grain as a short circuited electrochemical cell, with the "metallic silver" latent image as bath cathode (silver-silver halide interface) and anode (silver-solutioninterface) 3) the triple-phase theory, which assumes that the site of reduction is the triple-phase theory, which assumes that the site of reduction is the triple-phase boundary of latent image, silver halide and solution 4) the electron injection mechanism, according to which the beginning of development involves injection of electrons from the reducing agent into the conduction band of the silver halide crystal The first of these possibilities is considered to he inconsistent with the activitv of uncharged or cationic reducing agents in selective reduction of exposed silver halide crystals. Jaenicke (11) has derived a mathematical model of the last hypothesis, and concluded that electron injection was incapable, by orders of magnitude, of supporting experimentally ohsewed rates of latent image enlargement.
ASCORBIC ACID IIOcforr formi
I-PHENIL-3-PYRAZOLIWNE lPhcnldaneR 1
Figure 7. Structures of typical photographic developing agents.
However, the dark electronic conductivity of silver halide microcrystals, assumed by Jaenicke is lower by -1013 than a recently calculated value ( 2 3 ~ )validation ; of Jaenicke's conclusion must therefore await better experimental determination of this number. A further difffculty with Jaenicke's model lies in the fact that it is an oversimolification of the originally proposed electron injection process, insofar as it presumes conduction of the injected electrons to the latent images uia the conduction hand uisa-uis, e.g., a tunnelling process (236). I t is possible to follow the kinetics of enlargement of the latent image hv electron microsco~v(24). These data are entirely consistent with mathema&al fokulations of the electrode theory, whereby the development reaction, in its early stages, ought to he autocatalytic. The reaction has also been ohsewed to he autocatalvtic in a recent investigation in which reduction of silver-halide emulsion grains was followed spectrophotometrically (25) using a specially constructed instrument capable of measuring optical density changes as low as lo-=. On the other hand, not all evidence is consistent with the electrode theory. The observation that certain dyes which adsorb to silver halide, hut not to metallic silver, inhibit development, supports the formulation of the triple-phase theory (26). James (271, and later Mason (28), sought to avoid some of the difficulties with both of these theories by suggesting that the triple-phase theory describes the early stages of the reduction when the silver nucleus is still relatively small; subsequently, the electrode theory applies. Recently it has been observed that addition of compounds such as 6-nitrohenzimidazole or henzotriazole, which chemisorh strongly on silver halide; but not metallic silver, surfaces, to the solution of the reducing agent leads to inhibition of the early, autocatalytic phase of development, hut not the later no longer autocatalytic phase which continues after the latent image has been enlarged to a certain, critical extent (29). This result seems to confirm the operation, consecutively, of two mechanisms of development, specifically as proposed by James and Mason. It should he noted in passing that the characteristics of the development reaction lead to the individual silver halide grain's functioning as an all-or-nothing radiation detector; either it develops or it does not. This situation leads to a real limitation of the efficiencv of silver halide materials as photodetectors (30). A major challenge facing, photoeraphic scientists is. therefore, the desian of photographic emulsions characterized by "multi-1&el" grains, i.e., arains whose individual. rather than collective. response on development is in some way proportional td the amount of radiation they have received. Development Mechanisms Il-Catalysis
of Complexation
The latent image in a silver halide microcrystal may also he a catalyst for dissolution of the silver halide by complexation; or, more specifically, the process of latent image formation may also produce a complexation catalyst. This phenomenon is the basis for photosolubilization photography, in which light exposure of the silver halide photographic material leads to enhanced solubility of the exposed grains in a solution of a silver(1) complexing agent, e.g., a thiosulfate salt (31). Thereby a positiue image comprising uncomplexed silver halide remains in the unexposed portions of the material, uis-a-uk the negatiue silver image obtained in the exposed portions of conventionally, i.e., developed by reduction, treated material. The photosoluhilization and conventional processes are compared in Figure 1, and the mechanism of silver halide complexation reactions will he discussed in a later section. In order to obtain good selectivity in the preferential dissolution of the exposed silver halide grains, it is necessary Volume 51, Number 2, February 1974
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to treat the emulsion prior to exposure with an "insolubilizina agent" which chemisorbs stronglv ... (em. . .~(2)) . . to the silver hafide grain surface; the insolubilizing agent is usually an organic mercapto compound (32). It is believed that the insoluhilizing agent encapsulates the microcrystalline grains and protects them from attack by the complexing agent which can effect dissolution. The process of latent image formation (see above) generates positive holes Co) in the valence band of the silver halide crystal which, it has been proposed, attack and oxidatively destroy (eqn. (3)) the insolubilizing agent (33). (Insolubilizing agent) RSH AgX AgSR + HX (2) AgSR
+
- + +
p
AgBr
112 RS-SR
(3)
Commercial exploitation of photosolubilizatioli photography has been limited in part hy the fact that photographic materials require substantially greater exposure for imaging by the process, compared to conventional development by reduction. However, a hybrid process, capable of photographic speeds as great or even greater than obtained with conventional processing, termed "solubilization by incipient development" (SID), has been disclosed hy Land (34). In this process, dissolution of silver halide grains, treated with the insolubilizing agent, in the exposed material is effected in the presence of a reducing agent capable of carrying out conventional development. Thereby, a latent image of conventional size is enlarged by chemical reduction, until such a point is reached that it (or a byproduct of its formation) catalyzes the complexation of the silver(1) from the exposed grain. No mechanism for SID has been satisfactorily demonstrated. In one hypothesis, she chemisorbed insolubilizing agent is displaced by the halide (bromide or especially iodide) liberated in the initial stages of development, in order to maintain crystal stoichiometry. Circumstantial evidence for this mechanism comes from studies on the kinetics of conventional development in the presence of inhibitors similar to the insoluhilizine aeents used in SID: disolacement of the chemisorbed s;bstances by the iodide liberated in the initial stages of development is proposed to explain the observed results (%a). More recently the behavior of anodically halidized silver surfaces had been studied (35b). Image-wise light exposure of such a silver halide layer leads to preferential etching of the exposed areas, competitive with silver reduction, in a thiosulfate containing developer solution, thus demonstrating the catalytic effect of the latent image on silver complexation. Conventional development of silver halide grains produces metallic silver in the form of fine filaments. An electron micrograph of typically developed silver is shown in Figure 8. This filamentary character is, in part, responsible for the relatively high extinction coefficient of photographic image silver. Filaments formed in the early stages of the reduction reaction have been examined chemically and by electron microscopy (36a). They are reportedly reactive toward silver(1) complexing agents but, unlike silver metal, not towards oxidizing agents; similarly, like silver halide, the material in these filaments radiolyzes during microscopy., On the basis of these observations a mechanism of development by reduction, alternative to those described in the preceeding section, has been proposed in which the latent image catalyzes dissolution of the silver halide; the dissolved silver halide recrystallizes to form filaments which are then reduced in situ ( 3 6 ~ ) . From this viewpoint, then, it is impossible to distinguish between the latent image's behavior as a reduction catalyst and a complexation catalyst. At this juncture it is important to reemphasize the role played by defects and impurities in the chemistry of the silver halide crystal. A rigorously pure, crystallographically perfect silver halide crystal, for example, would be photochemically inert. Indeed we have seen that impurities 76
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Journal of Chemical Education
Figure 8. Electron micrograph of metallic silver developed by chemical reduction of a light-exposed silver halide microcrystal (16000 X ) .
are deliberately introduced in the preparation of photographic emulsion grains in order to increase the efficiency of their photolysis. Likewise, the presence of defects enhances the photolytic reactivity of the crystals; the interstitial silver ions, which are essential in latent image formation according to the mechanism of Figure 4, are one example of such defects. We also saw, above, that the photolytic silver is deposited preferentially a t crystal defects. It is furthermore thought that surface defects are the sites a t which spontaneous reduction of silver, i.e., fog formation, may begin, and at which anti-foggnnts, compounds which selectively inhibit fog formation relative to the catalytic reduction of exposed silver halide grains, chemisorb to silver halides (3.50). The assumotion that the reduction reaction preferentially begins a t specific defect or imouritv sites on the silver halide crvstal surface is implicit'in one model of the early stages bf development by reduction, which treats the reaction as a corrosion process (366). There is also evidence from the observation under the microscope of the dissolution'of silver halide crystals that this process, along with photolysis, reduction, and chemisorption, preferentially begins a t surface defects ( 3 6 ~ ) .Photosolubilization photography (see ahove) depends on preferential dissolution of silver halide at sites where defects have been introduced into the protective hull formed by the insoluhilizing agent (33). In most general terms, therefore, it i s possible to view the process of latent image formation in silver halide as the photochemical creation of surface defects at which the chemical reactions of silver reduction or complexation may preferentially begin. Complexation
Complexation of silver(1) to dissolve the silver halide grains in photographic materials is an integral part of the post-exposure treatment, whether such complexation is part of the latent image amplification process or merely a means of rendering the materials inert towards radiation, so that thev will he stable on storaee. The chemistm of formation o"f soluble silver(1) complekes from the halides has been the subject of less intensive research than, for example, latent image formation or the mechanisms of develo~ment137). From the earliest davs of ohotomaohv. " . ". thiosuifate salts' have been the com&exi& agents of choice, owing to their reasonable cost and availability, high solubility of the silver(1) complexes, rapid rate of reaction with silver halides, and low toxicity. The thiocyanate salts are probably the next most important com-
9-1
~ g (crystal) '
Ag ( S 2 0 d -
(adsarbed)
1
1-1
Determining I-te
Ag(SCN1; Figure 9. Reaction sequence for dissolution of silver halides by formation of lhiosulfate c o m p l e x e s of silver(]) (from ref. (40) by permission of t h e Society of Photographic S c i e n t i s t s a n d Engineers).
plexing agents in photography, although some effort has been devoted to finding novel organic complexing agents of photographic utility (38). It seems now to be reasonably well established that the rate of complexation of silver(1) by thiosulfate salts from silver halide grains in practical photographic materials is limited by the rate of diffusion of the thiosulfate anion through the gelatin matrix in which the grains are distrihuted (39. 40). This means t h a t i t is imnossihle to ohtain ~-~--meaningful information on the mechanism(~)of the com~lexationreaction from the observable kinetics of dissolution of the silver halides out of an ordinary photographic film or paper in a suitable thiosulfate salt solution. However, the processes of adsorption of thiosulfate anion onto silver bromide microcrvstals and of desor~tionof silver thiosulfate complex anions therefrom have'been followed by means of microcalorimetry (41). Salts of the comolex anion, (AgSz03)3Br4-, have been isolated and characterized from photographic fixing solutions, and their presence in the emulsion layers of photographic materials, as intermediates in the fixing process, has been subsequently confirmed by X-ray diffraction (42). On the basis of these and other studies, the sequence of reactions outlined in Figure 9 has been proposed to describe the pathway by which thiosulfate anion converts silver halide to a soluble silver(1) complex and halide ion (40). The corresponding reaction sequence for thiocyanate complex formation has been less firmly established. Although the rate of complex formation in photographic materials does not in this case appear to be diffusion limited, the kinetics are rather complex. One reaction scheme proposed to kinetic observations is shown in Figure 10. Note that two alternate pathways are involved; which one is actually followed depends, among other things, on the bromide concentration in the environment of the dissolving grain (43). ~
~
~
~
~
~
lsaluble)
(in roiulionl Figure 10. Reaction sequence for dissolution of silver halides by farmation of t h i o c y a n a t e Complexes of silver(1) (from ref. (43) by permission of t h e S o c i e t y of Photographic S c i e n t i s t s a n d Engineers).
Acknowledgment I would like to take this opportunity to thank Dr. W. R. Workman, Director, Imaging Research Laboratory, 3M Co., for his encouragement; my colleagues, especially Dr. F. A. Hamm, for suggestions and helpful discussion; and Mr. A. J. Dmbe, for Figure 8. Literature Cited
~
Conclusion we have considered some of the mechanisms thought to he involved in the photochemistry, reduction, and dissolutio,, by comp~exation of the silver halide microcrystals which comprise conventional photographic materials. Specifically, we have seen the importance of crystal de. .fects in all of these aspects of silver halide chemistry. It is hoped that in illustrating the variety of chemical mechanisms which apply to the making, use, and processing of conventional photographic materials this review can help in establishing, thereby, the place of photographic ,.hemistry in a chemistry curriculum. I have tried to show that it is an area of vigorous research activity, and not confined within any one traditional sub-discipline of chemistry.
I11 Mecs, C. E. K., and James. T. H.,"The Theor, of the Photographic Pmcess." Ed. 3, TheMaemillan Co., New York, 1966. I21 HiI1.T.. J. CHEM. EDUC., 43.492 11966l. 131 Duffin. G. F., '"Photographic Emulsion Chemiatry,"Foeal Press, London, 1966. 141 Zelikman. V. L., and Lev;, S. M.. "Making and Coating Photographic Emulsions," (English Ed.]. FocalPreas, London. 1964. 151 Lubin. P.D., andPehaeh, J. E.,U.S.PatentNo.3,537,855(1970). 161 Hamilton. J.F.. andUrbech, F.. Ref Ill. Chapters. 171 Gurney, R. W., and Motf. N. F.. R o e . Roy. Soc. (London), Ser. A, 164. 151 119381; Mott. N. F., and Gurney, R. W.. "Electronic P m ~ insIonic Crystals," &fordUniverjity Press. London. 1948. 181 1s) Marriage, A,, J . Photogr. SC., 9, 93 119611. (hl Hedgw, J. M., and Mitchell, J. W., Phii Mag. (7). 44,223, 357 (19531. 191 Bird, G. R., Jones, R. C., and Amer. A. E.,Appl. Opt., 6,2389 11969). I101 Slikin. L., McGawan. W., Fuksi. A,, and Kim, J., Photogr. Sci. Eng., 11. 79 (1967): Kliewer, K., J Phy* Chem Solids, Z7,705,719119661. 111) Jsenieke, W.. J . Phofogr. Sri., 20.2(1972). , 119721and referencescited therein. I121 Cash, D. J.. J. Phologr S C L20,107 (131 Fatuzzo. E.,andCoppoS.. J. Phofogr. Sei. 20.43 (1972). 114) Mitchell. J. W.. RoportsonPlagressinPhyaica, 20. 433 (1957). (I51 Tani. T.,Phatagr. Sci. Eng. I5.28,181 (19711; 16.3511972). 1161 West, W., and Gilman, P. B., Photogr. S e i En#. 13.221 119691. I171 Fbmer, Th., DbeussionFaroday Soe.. 27.7 119591. 1181 Terenin, A., andAkimw.I..Z. Phyir. Chem., 217,3U7(1961). (19) la1 Szentpsly, L. V., Mobius. D., and Kuhn. H . , J Chem. Phys., 52.4618 119701. lhl O'Brien. D. F.. Phorogr Sei. Eng. 17.226 (1973). 1201 Tani, T.. Kikuchi. S., and Honda. K., Photogr Sci: Ens., 12. 80 (1968): Tani, T., 13, 231 (1969): 14.63.72, 237119701: 15, 21 (1971). (21) Jaenicko, W.. Phofogr. Sci. EM.. 6,185 (19621and rofereneeseifed therein. I221 Met%.,H.J.,J Photon. Sci. 20, 111 119721. 1231 (a1 L o w B., andLindncy, M., Phofogr. Sei. Ena., 16.389l1972l.IblSfovcna,J . P s sona1 communication. I241 Pontiw, R. B.. Willis. R G..andNewmillel. R. J., Photon. Sci. Eng 16. 406 119721;Pontiua.R.~..end willin, R.G.. I?. 21119731 1251 Rieser,H.. and Sehles~inper,M..J Photogr. Sci., 20,19211972). 1261 James T.H.,and Vaneslaw, W., J. Phya Chem.. SS,894(19541. ,I J,m,:T.H.,~.~hys. them., 66,2416119621. 1281 Mason. L.F.A., J Phofogr Sci.. 16,177 (19681. I291 Sshwn. M. R. V., pacer presented at SPSE Annvsi Conference. Roeheater, New York, May, 1973. 1301 Shsa, R.,Photogr. Sci. Eng. 16.192.395119721. I311 Blake. R. K.. Photwr. Sci. Eng.. 9. 91 (19651; Haugh. E. F., and Strange, J. F., 12.22119681. 1321 cahen, A. B.. celeste. J.R., and F~",R. N., motogr sci. ~ " g . ,9.96 (1965). 1331 Hsugh. E. F., Celpate. J. R., Chinholm, R. C., Cahen, A. B.. Hunt. H. D.. and Sinicius, J. R., Phologr Sei. Eng., 9.116 (1965). 1341 h n d . E. H.. F a m e y , ~ . ~a. .n d ~ o n e M . . M . , P ~ o ~ o sS~C . ~EW., . 15,4119~1). I W 1.4 Sahyun. M. R. V.. Phbfogr Sci. E n s , 15.48 119711. ( b f ~ o f f m s nA,. , ~ ~ D.P., WRghf,M., andBilllng.,B.,Phot~grSci.Eng, 17,272(19731. 1361 (a) M U ~ I W. I ~E~. ,, P ~ ~ ~sri. o ~ E,,~., P . 15, 369 11971); 17.94(1973). ( b ~ A., and Riedman, F.,~ h o r o g rSci. EW, 16,203 1197%~ o f f ~A,,a paper ~ , presented at 1973 Annual confcrenec, society of ~hofographiescientists and ~ ~ICI Jones. D. A., andMitchd1. J. W., Phil. Mag, (8) 2.1047 (1957). (37) E ~ ~ o G.T.. ", ~ e i( .i 1 , ~ ~ . 3 ~ 7 f f . (381 Haist. G. M.. ~ i n gJ., R.,and wage, L. H.. Photogr s e i EW. 5,198 (19611. (39) Gerssimova, T . M., and Bmmberg. A. V., Zh. Nouch,i Pl11. Foton. K i ~ m o f o g r 12.13611967). I401 S8hyun.M. R.V.,Phofogr. Sci. E w , 17.171 (19731. (41) Csnh, D.J.. J Phologr Sci., 20.19 (19721. 1421 ,B ,, E. R., ,d ~ i ~H.,~J h t o, g r sCi., 13.301 (1965). (43) S8hyun.M. R.V.. Phofogr. Sci. Eng, 17.174l19731.
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