Hi all,
Im curious to know the purpose and difference between these two chemicals for use in developers, and why one would use one of them instead of the other? And, are other chemicals more common instead of those two?
Cheers
Peter
They are both chelating agents that chemically sequester certain ions in the solution that might otherwise interfere with the photographic development (or fixing) process. Calcium, iron, copper, and other ions can interfere with developing agents, cause precipitation of scum onto the film, or catalyze the destruction of the developing agents in storage.
So these two substances and others (notably sodium hexametaphosphate “calgon” or etidronic acid “fotoplex”) are used to isolate those ions into harmless, soluble compounds.
The extent to which the chelating agents chemically bond with different ions, which ions you are trying to isolate, and the developing agents in the developer would affect the choices you make.
Notably, ascorbic acid and phenols can break down in a reaction known as a Fenton reaction (“sudden death”) that is catalyzed by iron or copper ions in solution. EDTA is a good chelating agent but does not isolate the iron enough to prevent this catalyzing behavior. In fact it appears to exacerbate it. So in those cases you see DTPA used, or Ryuji Suzuki used TEA and salicylic acid.