Potassium Iodide as a restrainer for paper

[Richard Searle]

Is anyone using Potassium Iodide as a restrainer instead Potassium Bromide or Benzotriazole? I'm thinking of using it for fogged paper.

 

[Gerald C Koch]

The restraining power of the three potassium halide salts is roughly proportional to the solubility product the corresponding silver salts. Therefore the amount required is 1/1000 that for potassium bromide. This is a very small amount and would be hard to measure. In addition potassium iodide solutions are unstable.

 

[Rudeofus]

It is easy to measure if you start with a 0.1% stock solution (= 0.1g KI in 100ml water), and most color developers indeed use iodide together with bromide as restrainer.

The biggest problem is leaching out of iodide: AgCl + I- <===> AgI + Cl-, AgBr + I- <===> AgI + Br-
Color developers are matched to color film such that iodide released by development more or less balances iodide leached out, but this is not the case with B&W paper. Unless you want to use your paper developer single shot, you'd have to carefully devise a replenishment regime to maintain constant iodide level.

 

[Gerald C Koch]

There is another problem that solutions of potassium iodide only keep for only a short time. So when you may think you are adding iodide ion you are actually adding iodine in other oxidation states which will not work.

But the real question is why seek a complicated and unworkable solution when potassium bromide works just fine. A perfect example of the KISS principle -- Keep It Simple Stupid. However in all honesty I was not aware of the problem until I was corrected by PE several years ago But then too Geoffrey Crawley of the FX developers advocated adding KI to film developers. If you have an analytical balance then I guess you can measure out a few milligrams of the solid. However KI solutions do not keep.

 

[NedL]

There is another problem that solutions of potassium iodide only keep for only a short time. So when you may think you are adding iodide ion you are actually adding iodine in other oxidation states which will not work.

I don't want to take this thread off-topic, but I'd like to understand this a little better. When I make a calotype, the paper is first "iodized" in KI solution, dried, and then "senisitized" in AgNO3 solution. I made up a batch of iodizer about 4 years ago, which contains about 6% KI and about 2% KBr. Originally, they added a little elemental iodine as an "indicator": during sensitizing, you can tell when all the KI has been converted to AgI when the iodine color disappears. I don't want to deal with elemental iodine, so I've left that out. So, I guess 500ml contains about $10 worth of KI. As I've used it, I have simply topped it off with fresh solution occasionally. About a year and a half after I made it, after an iodizing session, it turned a violet color, but it still seems to work well and I haven't had problems with my calotypes. Am I being "penny wise and pound foolish"? Or maybe the "other oxidation states" will still make silver iodide and I don't need to worry? I've been wondering if it's time to mix a fresh batch!

 

[Gerald C Koch]

What happens is a process called disproportionation. The iodide ions simultaneously undergo a reduction and a oxidation as follows in the following equation which has not been balanced.

I-1 --> I0 + IO3-1

The following article describes the mechanism.

https://en.wikipedia.org/wiki/Disproportionation

Using potassium iodide to create an emulsion probably accounts for the difference.

When you are boiling water for pasta and add iodized salt you can smell the released iodine.

 

[NedL]

Thank you. Almost 40 years late, I wish I'd taken more than 1 semester of chemistry in college!

 

[Rudeofus]

While iodide is very prone to oxidation, this is a non issue when it is added to photographic developers, which will reduce any iodate straight back to iodide.

 

[Richard Searle]

Thanks for all your replies. I'm getting the feeling that using KI as a restrainer isn't viable.