Iodide / Bromide Ratios in simple emulsions

[RogerHyam]

I think I've seen an answer to this before - or did I dream it - these lockdowns and dark winter days are getting to me. I can't seem to find it now though.

Inspired by Denise I just made my first Ammonium Bromide + Potassium Iodide based negative emulsion (see attached) which I'm quite pleased with. Previously I was making Potassium Bromide + Potassium Iodide based emulsion but I thought I'd see if I could get a bit more speed.

I still don't understand the role of Iodide in the emulsion. If I look at the ratio of Iodide:Bromide (molar wts)

Denise's AmBr 1:44
Kodak's AJ-12 1:35
My Latest: 1:23

If I reduce the amount of KI in my next batch to bring it more in line with others will I see things get faster/slower or more contrasty or less contrasty?

 

[Lachlan Young]

If I reduce the amount of KI in my next batch to bring it more in line with others will I see things get faster/slower or more contrasty or less contrasty?

I think this might be the post you're looking for. There's also quite a few posts from Ron about iodide placement in emulsions etc.

 

[RogerHyam]

I think this might be the post you're looking for. There's also quite a few posts from Ron about iodide placement in emulsions etc.

Thanks. That is a nice list. More Iodide = higher speed. But what's the downside? There is always a downside!

I really like the comments further down about the characteristic curve being a distribution of crystal sizes - an ah ha moment for me.

 

[Lachlan Young]

Thanks. That is a nice list. More Iodide = higher speed. But what's the downside? There is always a downside!

I really like the comments further down about the characteristic curve being a distribution of crystal sizes - an ah ha moment for me.

Regarding iodide addition - this should answer your question.

And regarding your comment about the distribution of crystal sizes, it should make clear why the move to multiple blended emulsions of increasingly controlled crystal habit (from single polydisperse emulsions of uncontrolled crystal habit) happened.

 

[laingsoft]

Thanks. That is a nice list. More Iodide = higher speed. But what's the downside? There is always a downside!

I really like the comments further down about the characteristic curve being a distribution of crystal sizes - an ah ha moment for me.

The downside is always fog and grain size.