Silver bromide emulsions

This is a high-tonnage chemical and one of the most important of the bromide salts commercially. High-purity grades are required in the formulation of silver bromide emulsions for photography. The compound, usually in combination with hypochlorites, is used as a bleach, notably for cellulosics. The production of sodium bromide simply involves the neutralization of HBr with NaOH or with sodium carbonate or bicarbonate. 

Emulsions prepared with gelatin that had been treated to minimize sensitizing impurities have relatively low sensitivities for exposures made in air, particularly at low irradiances. They show pronounced low intensity reciprocity failure (LIRF). The pure silver bromide emulsions do not show high intensity reciprocity failure (HIRF) for direct development (65,66), but may do so for physical development (66). Faelens obtained HIRF in silver chloride emulsions for both physical development and the same developer he used for direct development of his silver bromide emulsion (67). Silver chloride emulsions, however, are more prone to unintentional chemical sensitization than silver bromide, and it is uncertain to what extent some chemical sen-... 

FIGURE 1. Reciprocity curves for an unsensitized silver bromide emulsion coated at pAg 9.0. (A) Curve 1, exposure in room air ... 

FIGURE 2. Dependence of sensitivity on the concentration of dry oxygen and 45% RH oxygen unsensitized silver bromide emulsion exposed 100 s. 

Although S-sensitization decreases low intensity reciprocity failure it usually does not eliminate it. In our experiments with monodisperse fine-grain silver bromide emulsion, vacuum outgassing of the S-sensitized emulsion eliminated the LIRF, just as it did for the unsensitized emulsion. Moreover, the sensitivities of the two emulsions under vacuum were nearly the same. Whatever may be the role of S-sensitization in this emulsion, it became inconsequential for exposures made under vacuum. However, the degree of increase in sensitivity caused by S-sensitization of the fine grain emulsions for exposures in air is much smaller than can be achieved with coarse-grain poly-disperse emulsions. 

The combined (S + Au)-sensitization usually is applied during the manufacture of the emulsion, but sensitization can be achieved also by bathing a coated S-sensitized emulsion in an aurous thiocyanate solution before exposure (161). Reduction sensitization can be combined with (S + Au)-sensitization under some conditions. Collier (162) found that reduction sensitization either before or after (S + Au)-sensitization increased both sensitivity and fog in a l-ym octahedral grain silver bromide emulsion. The largest increase in sensitivity and lowest increase in fog were achieved when reduction sensitization was applied after the S+Au. Hydrogen hypersensitization likewide is effective (108) and can produce large increases in sensitivity for some emulsions. 

In our experiments with the low-temperature sensitivity of monodisperse silver bromide emulsions having 0.5 ym cubic grains, the sensitivity of the (S+Au)-sensitized emulsion differed from that of the S-sensitized emulsion both with respect to the dependence on the degree of sensitization and the correlation with microwave photoconductivity measurements (16). 

They arrived at a size of the order of 100 silver atoms for a bare metal nucleus, but only 10 atoms for a nucleus embedded in gelatin. Moisar and associates (205) estimated the minimum size as four silver atoms. Ronde (206) revised their calculations and obtained 10 for an unsensitized silver bromide emulsion and six for a S-sensitized emulsion. The validity of the Gibbs-Thomson equation for such small sizes, however, is questionable. 

Gilman (263) subsequently used a series of dyes covering an Er range from -1.35 to -0.11 V. Only one of the ten dyes with ER less negative than -0.86 spectrally sensitized the silver bromide emulsion for exposure in air, and that one sensitized only slightly. Under vacuum, all dyes in the range -1.35... 

Evans FJ, Gilman P (1975) Comparison of the spectral sensitization of surface and internally sensitized core/shell octahedral silver bromide emulsions. Photogr Sci Eng 19 344-351... 

Xhiry, H. Preparation and properties of ultraline grain silver bromide emulsions. J. Photogr. Sci. 1987, 35, 150-154. 

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