The authentic receiving layer in the Dye Transfer process was a Thorium salt in gelatin. The Thorium precipitated the acid dye and therefore created a concentration gradient across the Matrix film into the receiver sheet. This method cannot be used today, but others have found ways to simulate this using Aluminum salts. The ph effect is there, but is a minor actor in the face of a mordant which actually precipitates the dye and totally removes it from action.