Your synthesis step certainly sounds interesting and somewhat similar to mine with Ammonia and Boric Acid, but there are several assumptions, which will not hold:
PS: Chloride fixes at very high concentrations, but not at the concentrations you will find in modified Sodium Thiosulfate based fixers. At lower concentrations it will actually slow down fixation. I had much better results with Ammonium Acetate.
- A fixer is a very concentrated solution, which means: ionic strength considerations must be applied. Adding extra ions to the mix will increase these effects, which translate into a slower fixer. This is the reason, why you can not add arbitrary amounts of ammonium salt to Sodium Thiosulfate based fixer and expect ever increasing fixing speeds. The correct amount of Ammonium Chloride will not be determined by molar ratio, but by ionic strength and impurity in actual chemical supplied you have. Nobody mixes fixer from ultra refined chems. The optimal amount of Ammonium Chloride also depends on how much Sodium Thiosulfate you have in your solution. Different people with different supplies find wildly different optimal amounts…
- The reaction mechanism during fixation is a lot more complex than "silver ion plus thiosulfate ions form soluble complex". There are mixed salts of silver ions, thiosulfate ions and whatever other ions happen to be present. There are particularly insoluble mixed salts of sodium/potassium, silver, thiosulfate and bromide/iodide, which do not form with ammonium ions. These poorly soluble mixed salts will form in exhausted fixer and lead to its premature failure. Sodium Thiosulfate based fixer pepped up with Ammonium Chloride will be fast, but its capacity will be no better than that of pure Sodium Thiosulfate fixer.
- Finally the biggest factor, which makes rapid fixer so fast, is the size of the counter cations: hydrated sodium ions are larger than hydrated potassium ions, which are again large than ammonium ions. The silver solvent character of Ammonia may also help, but remember, that Ammonia on its own can not even dissolve Silver Bromide. Potassium ions are a particularly weird case: Potassium Thiosulfate will give you a faster fixer than Sodium Thiosulfate, but it will exhaust yet earlier due to this poorly soluble mixed salt issue.
PS: Chloride fixes at very high concentrations, but not at the concentrations you will find in modified Sodium Thiosulfate based fixers. At lower concentrations it will actually slow down fixation. I had much better results with Ammonium Acetate.