A Kinetic and Sensitometric Investigation of Sulphur Ripening

Abstract

The kinetics of Ag₂S formation during thiosulphate digestion and the development of speed and fog have been investigated with monodisperse AgBr emulsions with either cubic or octahedric grain shape. Certain characteristic differences in the behaviour of the two different crystal types have been observed which are explained by application of general theories of crystal growth. According to these theories Ag₂S formation proceeds at (100)-faces relatively slowly via two-dimensional nucleation processes. The reaction is rapid at (111)-faces and proceeds here according to a first order reaction scheme. A calculation model for the reaction at (100)-faces has been developed which agrees well with experimental results. The photographic properties upon thiosulphate digestion depend also on the crystallographic shape of the AgBr grains. The observed desensitization of AgBr octahedra and the increase of speed of AgBr cubes during ripening can be explained by differences in the dispersity of the sensitivity centres. A rough estimation of the size of these centres has been made. No relation between speed and total Ag₂S amount has been found.