Here is what I have read: Photographers Formulary's TF-4 and Kodak's Rapid Fix are both based on Ammonium Thiosulphate and both are considered rapid fixers. TF-4 is non hardening therefore does not have to have an acid environment that the hardener needs to work. Kodak Rapid Fix can be a hardening fixer with the addition of the Part B Hardener solution, so I assume that the Part A solution is acidic. i) What component of a fixer formula causes it to be considered an acid fixer? ii) Can fixers based on Ammonium Thiosulphate or Sodium Thiosulphate be formulated to be either acidic or base? Thanks Todd Enjoying our 25 cm of fresh new snow (hopefully not acid snow)!