GRAIN SIZE AND T H E QUANTUM THEORY O F PHOTOGRAPIIIC EXPOSUEE BY MALCOLM C. HYLAN
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Introduction In a paper1 entitled “Dispersity of Silver Halides in Relation to their Photographic Behavior” published by the author and Dr. F. E. E. Germann it is shown that theoretically we should expect fine-grained photographic emulsions to be faster than coarse-grained ones, altho in practice the order of sensitivity seems usually the reverse. This discrepancy between theory and fact is explained on the basis of adsorption of retarding soluble halide, the fine-grained emulsions adsorbing relatively more than the coarse-grained ones, and thus reversing the purely dimensional effect. They back up their theoretical conclusions by experimental evidence, actually preparing emulsions of large and of small grains, and comparing their speeds both before and after sensitization. The sensitization was by a process calculated to remove adsorbed retarding halide. Before sensitization they found the large-grained emulsion the faster, but after sensitization, the small-grained one the faster. Their assumption that their sensitizing process removed the adsorbed retarding halide was based on evidence described in a previous article2 on the “Photographic Sensitiveness of Silver Iodide”. That the sensitizing process does remove adsorbed retarding halide is not to be doubted, but the increase of sensitivity of the small grains over the large ones might be due, not to their size alone, but to the fact that the sensitizer, which consists of a solution of metol, pyrogallol, hydroquinone and sodium sulfite, may itself be adsorbed relatively more by the small than by the large grains, and thus produce an unequal accelerating effect. We might say, then, that the evidence by which they support their theoretical conclusions is merely ‘(circumstantial”. That is, their results are a necessary but not sufficient condition of proof of their theory. The “direct” evidence in support of their theory could be obtained only by the comparison of speeds of emulsions of different grain size, known to be free of either adsorbed retarder or adsorbed sensitizer. Such results would depend purely upon the dimensional effects of the grains. In their discussion Germann and Hylan showed that the “effective area” of a given amount of silver halide is greater with greater dispersity, and argued that from either the continuous wave theory of light, or from the light-dartquantum theory, a larger portion of the light incident upon the plate would be absorbed by the silver halide in small-grained emulsions than in largegrained ones, hence the speed of emulsions should be increased with increased dispersity. In a criticism of their theoretical conclusions S. 0. Rawling3 J. Phys. Chem., 28, 1924, 450-456 (1924). J. Am. Chem. SOC., 45, 2486-2493 (1923). Brit. J. Phot., 71, 401 (1924).
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points out that Silberstein’, in an article on the “Quantum Theory of Photographic Exposure”, has shown that the number of grains hit and affected by exposure to these light quanta is given by the equation k
=
S ( l - e-na)
where k = number of grains hit, K = number of grains per unit area of plate, n = number of light quanta impinging, and a = average area of grains. Rawling goes on to show that on the basis of this equation the total “effective area” of grains hit, in emulsions containing equal amounts of silver halide, will be greater for large-grained emulsions than for small-grained ones. Both Rawling and Silberstein proceed on the assumption that every grain hit by a light quantum is rendered developable, that its developability is un affected by absorption of further light quanta, that the amount of silver deposited by any grain is proportional to its effective area, and therefore that the total amount of silver deposited is proportional to the total effective area of grains hit, regardless of how many or how few times they have been hit. Germann and Hylan, on the other hand, assume that the developability of a grain is proportional to the amount of light it absorbs, and that the amount of silver deposited is proportional to the total amount of light incident upon the silver halide grains. To the author, the latter view seems the more reasonable. It is a well known law of photo-chemistry that only the light absorbed has any photochemical effect. The converse, that the photochemical effect is proportional to the light absorbed, seems reasonable, If this be true, a silver halide grain absorbing two light quanta should be twice as developable as one of the same size having absorbed but one. Perhaps even, it requires the absorption of a certain minimum number of light quanta per unit mass of silver halide in the grain to render that grain developable. There is experimental evidence in support of this last assumption. If the view of Rawling and of Silberstein is correct, we should expect the density-exposure curve to pass thru the origin, or at least come infinitely close to it, for the absorption of a single quantum by any grain should cause some silver deposit, It is well known, however, that there is a certain finite minimum value of exposure up to which the density of the silver deposit on development is zero. This can only mean that a certain finite amount of light must be absorbed by the silver halide to render it developable. Also, in certain “ripening” processes the emulsions are exposed to a subdued light, especially in the case where an emulsion is being rendered sensitive to light of some definite color, where it is exposed to light of that color. Xow if the absorption of a single quantum of light rendered a grain developable, some of these grains should absorb a quantum of light during ripening and should be developable without further exposure. The present investigation was undertaken with a view to seeing if “direct evidence” in support of the theoretical conclusions of Germann and Hylan Phi!. Mag., 44, 257-273
(1922)
GRAIX SIZE AXD QUANTUM ‘THEORY
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could not be obtained, and also to see if the behavior of emulsions free of adsorbed impurities would not furnish evidence as to which of these two views, that of Germann and Hylan, or that of Ramling and of Silberstein, was more nearly correct.
Experimental Equimolar quantities of potassium iodide and of silver nitrate were accurately weighed on an analytical balance, and emulsions of silver iodide prepared as follows : EMVLSION I Solution ii Solution B potassium iodide 2 , 7 6 7 gms silver nitrate 2.830 gms gelatin 0 . 5 gms water I5 cc 15 cc water Warmed A to dissolve the gelatin, added B with stirring and added the mixture to a solution of 40 gms of gelatin in 2 2 0 cc of water.
EMULSIOS I1 potassium iodide 2 . 7 6 7 gms gelatin 40 gms water 25 cc Warmed to dissolve the gelatin then added silver nitrate (crystals) z , 8 3 0 gms Slides of these emulsions were prepared and examined under the microscope. Emulsion I was found to have the larger grains, being on the average 50% to 100% larger in diameter than those in Emulsion 11. This result is the reverse of that obtained by Germann and Hylan in their work, and the discrepancy will be explained in the discussion. Cards were painted with these emulsions, and after drying, were exposed and developed. Emulsion I1 was the faster. Fifty cc from each of Emulsions I and I1 were decanted into clean beakers and j gms of gelatin added to each, and these modified emulsions labelled I-G and 11-G respectively. Cards painted with these, exposed and developed showed them faster than I and I1 but 11-G faster than I-G. Forty eight cc of 2 0 % ammonia was added to each of Emulsions I and 11, and 1 2 cc to each of Emulsions I-G and 11-G, and the resulting emulsions labelled I-A, 11-8,I-G-A, and 11-G-A respectively. Cards painted with these showed them to be faster than I, 11,I-G, and 11-G, but 11-A faster than I-A, and 11-G-A faster than I-G-A. In order to approximate as closely as possible the conditions in an emulsion as usually prepared with an excess of soluble halide, cards from these emulsions were soaked five minutes in a 1% solution of potassium iodide. After drying, exposing, and developing, Emulsions I and I-G were ncw the faster.
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Discussion In every case, the emulsions free of adsorbed impurities showed the finegrained emulsion faster than the coarse-grained one. On soaking in 1% potassium iodide the order of sensitivity was reversed, as was to be expected, for the fine-grained emulsion should adsorb more retarder and be slowed dow7n more than the coarse-grained one. These results offer direct evidence in favor of the theory of Germann and Hylan, and against the theory of Rawling and of Silberstein. Germann and Hylan in their work found that the emulsion prepared by adding silver nitrate crystals to a solution of potassium iodide gave them much larger crystals of silver iodide than by mixing solutions of the two salts, while the author found in this investigation the reverse to be true. The key t o the difference in the two cases is found in the ratio between the speed of reaction between potassium iodide and silver nitrate, and the speed of solution of silver nitrate. Germann and Hylan used large crystals of silver nitrate, some being as much as one centimeter in diameter and a millimeter thick, In this case the speed of reaction was greater than the speed of solution, leaving consequently, always a very low concentration of silver nitrate in solution. The author this time used finely ground silver nitrate in order to facilitate accurate weighing on the analytical balance, In this case the speed of solution was much greater, probably producing in the neighborhood of the crystals a fairly high concentration of silver nitrate in solution, with a corresponding decrease in size of silver iodide gains. Summary I. The experimental evidence offered by Germann and Hylan in support of their theoretical conclusions as to the relation between grain size and sensitivity in photographic emulsions might be regarded as merely “circumstantial”. 2. Direct evidence in support of their conclusions can be obtained only by a comparison of emulsions known to be free of all adsorbed “impurities”, either retarding or accelerating. 3 . The quantum theory of photographic exposure as expounded by Silberstein, combined with the assumption that a silver halide grain is rendered developable by the absorption of a single light quantum, was shown by Rawling to lead to conclusions directly opposed to those of Germann and Hylan. 4. The theory of Germann and Hylan is shown to be more in accord with known facts and principles of photo-chemistry than is that of Rawling and of Silberstein. 5 . Emulsions free of adsorbed “impurities” have been prepared and their speeds compared, with results which support the theoretical conclusions of Germann and Hylan. Hale Physical Laboratory, Unsuersity of Colorado, Boulder, Colorado.
