The Washing of Thiosulphate from Gelatin Layers

Abstract

The transport of fixer salts from a gelatin layer during washing proceeds via a fast stage of exponential loss followed by a slower stage during which thiosulphate appears to be lost by exchange for anions from the wash water. The conductance of the layer falls to a minimum value at the point where the second stage of washing becomes dominant. At this point the fixer residue, determined as silver sulphide, may be high or low in concentration depending on the pH of the fixing bath. The presence of alum in the fixer is equivalent to a large drop in pH value in its effect on thiosulphate retention. Inasmuch as the residual thiosulphate at the second stage does not significantly contribute to the conductivity of the wet layer it may be considered to be bound to the gelatin.