Fixers decrease in stability by a predictable amount based on sulfite content and pH. Those at alkaline pH are most stable and become less stable as pH becomes more acid. It begins to become most unstable below about pH 5, and will decompose instantly on mixing below 4.
Therefore, TF4 is most stable, Kodak fix is in the middle and KRLF is least stable if everthing else were equal (they are not because each fixer has a different formula). I have seen KRLF ripoffs that were stored side by side with KRLF and the ripoff went bad before I could even use it but the KRLF is still there.
Now, this applies to both concentrate and working solution. However, some fixers come with 2 parts, and this does not hold for them. Also, the concentrate is sealed. Once opened, oxidation starts.
Sulfite is there as a preservative. It is oxidized by the air before the hypo is. It also represses the decomposition of the hypo into sulfur and hydrogen sulfide.
If sulfur forms, it can get into the film and leave trapped sulfur particles that look like fine white dust spots on the final print. They cannot be removed and that is why fresh fixer, or clear fixer is to be desired. Filtration can fix this up if you do have cloudy fix.
TF4 dissolves completely when mixed with water to make working solution.
Bottles with white sulfur scale can often be cleaned out with the use of strong sodium hydroxide (lye) solution. Protect your hands with gloves and your eyes with goggles. Add the lye solid to water when you make it up, or the solution might boil over. Never add water to solid lye.
PE
