The Diffusion of Potassium Bromide through Gelatin Gels

Abstract

The diffusion coefficient of potassium bromide in gelatin gels was determined by setting the gel, containing a little silver chromate os an indicator, in a capillary tube, and observing the movement of the boundary of silver bromide formed when one end of the capillary tube was immersed in 0·1 M potassium bromide solution.

The diffusion coefficient was independent of intrinsic properties of the gelatin but depended on the concentration of the gel. This conclusion is likely to apply equally to the diffusion of other low-molecular-weight substances that do not interact strongly with gelatin. The rate of diffusion decreased linearly with gelatin concentration up to a value of 40 per cent at which concentration it was 0·35 times the value in water.

Sodium sulphate and sucrose added to the gelatin only, or distributed equally in gel and solution, reduced the diffusion rate by an amount approximately equal to that of an equal weight of gelatin. Cyanamer had a rather greater effect than gelatin.

It is concluded that hardening of photographic films by reduction of swelling will decrease the diffusion coefficient within the layer, but will increase the rote of penetration of solutions into the layer due to the reduction in diffusion path length.