Eddie Rising


There is considerable speculation in literature as to the mechanism of photographic emulsion stabilization by 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene, (Aza). Aza is important as a stabilizer because, unlike the heterocyclic mercaptans, it causes no desensitization when the emulsion is processed in the commonly used MQ developing agents. The mercaptans are strongly adsorbed, whereas Aza is weakly adsorbed on silver halide. Therefore, there has been speculation that the stabilization mechanism of Aza is distinctly different from that of the mercaptans. For this work, Aza was tested as an antifogging agent and development restrainer along with KBr, as a reference, in two developers which function at emulsion pH, (6 to 7): (1) Amidol at pH 6, 7, and 8, and (2) the ferrous ethylenediaminetetraacetate (EDTA) complex ion at pH 4.5, 7.0, 8.5, and 10.0. KODAK Fine Grain Positive Film, 5302, was used with both developers, and KODAK PANATOMIC-X Fine Grain Film, 5060, was used only with Fe-EDTA. Aza and KBr were compared sensitometrically for effects on fog, contrast index, and exposure index. This sensitometric evaluation comprises Section 1 of this work. The evaluation yielded evidence that at the pH of photographic emulsions, (near 7), and in concentrations of Aza normally used in the emulsions, (1 to 2 grams/mol Ag X), Aza, like KBr, was both a restrainer and an antifoggant during the development of the 5302 Film by both Amidol and Fe-EDTA. PANATOMIC-X Film, 5060, processed in Fe-EDTA gave similar results. The restraint of development by Aza decreased at pH values above 8.5. This evidence and the fact that Aza is adsorbed on the grains of the emulsion to the same extent as the mercaptans, when present in amounts normally used, remove the necessity for assuming two distinctly different mechanisms of emulsion stabilization. The fact that Aza caused no desensitization or restraining action during development at the high pH of the commonly used MQ developers is explained by its desorption from the development centers. Section 2 of this work consists of a study in determining if there was an increase in the adsorption of Aza on silver bromide as a result of any effective increase in the concentration of Aza after drying the emulsion. An adsorption equilibrium exists between 1, 1'-diethyl-2, 2'-cyanine iodide dye and Aza on silver bromide in the emulsion. Because the displacement, or deaggregation, of dye by Aza was evidenced by a decrease in the J-band strength of the dye, this decrease in J-band absorption at 575 nm was a convenient measure of the equilibrium. Aza not only decreased the J-band strength of the dye but also caused the appearance of two bands, identified as unaggregated dye in gelatin, one with maximum at 495 nm and the other near 530 nm. This is evidence that Aza not only deaggregated the dye but also displaced it from the AgBr. The analysis was made with a Beckman DK-2A Spectrophotometer which measured total reflectance and transmittance by means of an integrating sphere over the wavelength range 380 to 730 nm. From these data, absorption was calculated for the dye in the presence and the absence of Aza both wet and dry. The results of Section 2 of this work are evidence that the adsorption equilibrium between Aza and dye on silver bromide did not significantly change on drying the emulsion in spite of the change in the concentration of Aza from the wet to the dry emulsion.

Library of Congress Subject Headings

Photographic emulsions

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School of Photographic Arts and Sciences (CIAS)


Carroll, Burt


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