Hello Some my research with well known Agfa-304 formula for rapid fixer without Ammonium thiosulfate: Water at 52°C – 750ml Sodium Thiosulfate – 200g Ammonium Chloride – 50g Potassium Metabisulfite – 10g Water to make 1 litre Idea is that Sodium Thiosulfate + Ammonium Chloride --> Ammonium thiosulfate. So we can write reaction: Na2S2O3·5H2O + 2NH4Cl = (NH4)2S2O3 + 2NaCl + 5H2O molar mass: MW(Na2S2O3·5H2O) = 248.18 MW(NH4Cl) = 53.49 100g of Sodium Thiosulfate + 43.1g of Ammonium Chloride --> 59.7g of Ammonium thiosulfate so in Agfa-304 formula only part of Sodium Thiosulfate convereted to Ammonium thiosulfate: 200g of Sodium Thiosulfate + 50g of Ammonium Chloride --> 84g of Sodium Thiosulfate + 69.26g of Ammonium thiosulfate for full reaction we need 86g of Ammonium Chloride. and final formula will be: Water at 52°C – 750ml Sodium Thiosulfate – 200g Ammonium Chloride – 86g Potassium Metabisulfite – 10g Water to make 1 litre I don't known why Agfa-304 rapid fixer contains less Ammonium Chloride than required. Second thing is that Ammonium Chloride can be replaced with Ammonium Sulphate (NH4)2SO4 (in my chem.store A.Sulphate in twice cheap than A.Chloride). Reaction will be next: Na2S2O3·5H2O + (NH4)2SO4 = (NH4)2S2O3 + Na2SO4 + 5H2O molar mass: MW(Na2S2O3·5H2O) = 248.18 MW((NH4)2SO4) = 132.14 100g of Sodium Thiosulfate + 53.24g of Ammonium Sulphate --> 59.7g of Ammonium thiosulfate rapid fixer formula with Ammonium Sulphate: Water at 52°C – 750ml Sodium Thiosulfate – 200g Ammonium Sulphate – 107g Potassium Metabisulfite – 10g Water to make 1 litre Other known formula with Ammonium Chloride/Sulphate is Kodak F-7/F-9: Kodak F-7 Water, at 125F (see note below) 600.0 ml sodium thiosulfate, crystals 360.0 grams Ammonium Chloride 50.0 grams Sodium Sulfite, anhydrous 15.0 grams Acetic Acid, 28% 48.0 ml Boric Acid, crystals 7.5 grams Potassium aluminum sulfate 15.0 grams Water to make 1.0 liter F-9 is the same except for substituting Ammonium sulfate 60.0 grams for the ammonium Chloride above. in both case as result we have mix of Sodium thiosulfate and Ammonium thiosulfate (~100g) in fixer solution, because amount of Ammonium salt is low! As we known Agfa-304 fixer is slow than true Ammonium thiosulfate rapid fixer. I found state that NaCl salt decrease speed of fixing. But maybe problem in low amount of ammonium salt?