Kinetics of rehalogenation bleaches

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Title: Kinetics of rehalogenation bleaches
Author: Harrison, Kenneth A.
Abstract: A study was made to determine if a correlation existed between the silver density at 318 nm and the actual amount of silver present in photographic film. Based on twenty (20) samples of bleached and unbleached film, a linear correlation was found, for specific film/ developer combinations, with a correlation coefficient in excess of 0.99. The above correlation was then used to evaluate the bleaching kinetics of several rehalogenation type bleaches. It was postulated that there is a charge barrier effect between the photographic gelatin and the positively charged oxidizing agent in the bleaches. This was caused by the pH of the bleach solutions being below the isoelectric point of the gelatin, giving the gelatin a net ill positive charge. This charge significantly affected the diffusion of the bleaches through the gelatin, and was observed as a decrease in bleach rate with time. Presoaking the films in either water or bromide solutions had no significant effect on the bleach rates. It was also shown that the addition of potassium bromide to the CuCl bleach greatly increased the bleach rate. This was partially due to the increased amount of negatively charged halide present and partially to the lower solubility product of silver bromide. These results were confirmed on two types of Eastman Kodak film, Fine Grain Positive (type 5302) and High Contrast Copy film (type 5069). 2 At bleach rates approaching .013 g/M /sec, the bleach reaction became diffusion limited and the highest observed bleach rate was .014 2 g/M /sec.
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Date: 1980-09

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