OK - so for a iodide emulsion, we would want to take all of the iodide out of the gelatin mixture and put it into the RS solution for the halide? Or since the iodide is going to immediately precipitate out regardless of where it is, can we leave it in the pot with the gelatin?
Also, if we say, a formula (like something from Baker, 2nd Ed, p 82, converted to KBr) that has in solution A:
Water - 50 ml
Gelatin - 2.5 g
KBr - 8.0 g
KI - 0.1 g
and a solution B:
AgNO3 - 5.2 g
Water - 5 ml
NH4OH (28%) - about 6 ml, as needed.
This give us about 0.0305 mole Ag+, and 0.0672 mole Br- and 6.02-4 mole I-, which is about 2% Iodide in relation to the Silver.
To convert this to a RS formula, we would need to have 0.0367 (0.0672-0.0305=0.0367) moles Br in the pot with the Gelatin, or about 4.37 g KBr, and 3.36 g KBr in the solution that will be added alongside the Silver solution.
For those that don't have the equipment to do additions of 2 different solutions at once, what about adding as large of an excess of KBr to the gelatin solution to make sure there is a large excess of Br floating around? What affect would this have? I know there will be practical issues like the solubility of the KBr in solution with the mix. It looks like KBr has a maximum solubility in water at 20C of about 40%, so for 50 ml of water you could get at most 20g KBr. That increases the concentration of Br in solution relative to the silver being added and will help with the changing concentration somewhat...
I suppose the KBr dropping out of solution when you chill set the emulsion would be a bad thing though...
Using ammonium bromide would allow us more solubility of the bromide and thus allow us to add more bromide, but then that means we need to not dissolve the silver nitrate with ammonia as we don't want too much ammonia during ripening, correct?
Finally, the amount of water used to make Solution A would need to be decreased as you want to control the ultimate amount of water at the end of emulsification. This then increases the concentration of the gelatin in the pot, which also has an effect on the size of the silver halide crystals during emulsification...
OK - not as simple as it seems.