That's a duel at sunrise!
Originally Posted by Photo Engineer
Seriously, here's my first bullet. Snipped from my tests and narrative posted:
"2. TF-3 and Superfix both start with a 20% ammonium thiosulphate working strength. That seems pretty typical ammonium thiosulfate working strength over a number of formulas. Time to clear, 16 seconds. pH 7. See Experiment #9 and the note. I came back to this, thinking that something was amiss. Sure enough, with film fresh out of the bulk loader plus some 3-8 minutes in water, the times changed to 8 and 9 seconds with two trials.
(First and very important observation: Testing by time to clear has variables. The time the film was left in the light and the light intensity seems to be very critical. Perhaps the water soak, too. “Develop” your own procedure and stick to it. You can use dry film for your own fixer life span checks. I even had film clear after a day left soaking in plain water that had previously soaked film in it. Might be the iodide dissolved out, it is a silver reducer.)
3. I then added 10 g each thiourea and ammonium thiocyanate to get the core Superfix. Surprise! Time to clear, 20 seconds! I added 10 more grams of each, and the time went up to 27 seconds! This is exactly the opposite of all expectations. I’m willing to admit that this could be procedural fault on my end. Since this formula wasn’t my prime interest, I went on. pH: 8+, just as Ron says without the acetic acid. See #2 and my revisit.
Revisited: With the carefully weighed thiourea and ammonium cyanate, and “fresh” film from the bulk loader two times, my new times were 10 and 9 seconds. I then added 5ml of a 14% total solution of thiourea and ammonium cyanate and got 12 and 10 seconds. I added 10ml of the above and the times increased to 12 and 11 seconds."
In brief, despite all expectations - yours and mine! - adding urea and thiocyanate slowed the reaction time. I can't explain it, but there it is.
Also as I pointed out elsewhere, whether the film is dry or wet is a huge variable, something that threw off Kodak researchers into the 1930's; inconsistent results. As I discovered, it also matters A) how fresh the film is from the bulk loader, and B) how long it has been soaking.
So, yes, many variables, but once I learned them, I tried to be consistent.