Repeating this experiment, making measurements of the amounts involved...
Note: I usually calculate solution strength in moles / kg solution, not moles / liter since I can more accurately and conveniently measure mass than volume. This is similar to molality, but not quite (I count total mass of solution, not mass of solvent.) I'm not sure of the right name for my unit...
I have previously measured 1 drop of H2O as 0.055 ml, for the iron solutions involved I assumed that 1 drop then was 0.055 g of solution, this may be inaccurate.
Fe3+ / thiosulfate:
To 0.75 g of 0.26 moles/kg sodium thiosulfate (0.195 mmol), I added 1 drop of 0.247 ferric ammonium sulfate (in 1.781 moles/kg H2SO4), this is approx. 0.0135 mmol Fe3+.
I observed a purple color which faded quickly, after c. 10 sec it was mostly gone, after c. 30 sec completely gone. After a little more time, I observe a yellowish/white cloudy suspension, assumed to be precipitated sulfur.
Before this, I did the same thing with twice as much thiosulfate, I saw still the purple color but not as intense.
Fe2+ / thiosulfate:
Now I repeat this using ferrous ammonium sulfate, nominally 0.487 moles/kg, with 0.162 moles/kg of H2SO4. Since I had observed a slight purple color earlier with this solution (see earlier posts) I had added some iron filings to this (I heard somewhere this improves Fe2+ solution keeping properties.) This was a day or so ago, immediately after adding the Fe filings, I got lots of bubbling (assuming this to be H2), by now this has mostly settled down, but my solution is now presumably overstrength.
Anyways... To 0.65 g of the same 0.26 moles/kg thiosulfate (0.169 mmol thiosulfate), I added 4 drops of the ferrous solution. (At least 0.107 mmol Fe2+, probably somewhat more.)
I observed no color change at all. The resulting solution was very light green. After 15 minutes or so, there is a slight cloudiness (possibly sulfur, not sure).
Just for fun, I then combined approximately equimolar amounts of Fe3+ and thiocyanate (as KSCN), and observed the familiar blood red color, which does not fade. (At leat in 30 sec or so, I just did this.)