Athiril
Harmless
Posts: 24
Registered: 8-5-2011
Member Is Offline
Mood: No Mood
|
|
Methods of Potassium Thiocyanate Production?
Hi all,
I really need to take some kind of course rather than learning on a case-by-case basis..
I already have sodium thiocyanate which I can use for the purposes I want potassium thiocyanate (additive in b&w developer to make b&W or
colour positive images in a reversal process on photographic film), but potassium thiocyanate gives finer grain.
I was wondering how to get to potassium thiocyanate, online the most I can find is a reaction between a thiosulphate and cyanide. So a thiosulphate
and potassium ferricyanide in some kind of function may give me what I need?
Is it as simple as a 1:1 molar ratio of thiosulphate to ferricyanide? Do I need the potassium salt to be on the thiosulphate side as well? Or only on
the ferricyanide side?
As I have ammonium thiosulphate, but adding KOH would give potassium thiosulphate and ammonium hydroxide, correct?
Just want some general pointers (a link, or otherwise) on the best approach to potassium thiocyanate and purification.
Sorry if I sound stupid, my chemistry knowledge is only practical towards analogue photography, so I pretty much only know most things on a
case-by-case basis rather than on general principles.
|
|
|
Nicodem
|
Thread Moved
3-9-2011 at 11:18 |
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
It would be better firstly to establish whether KSCN really does give better results than NaSCN or whether this is eseentially 'an old wives
tale' (a superstition). Some salts have historically been prepared mainly as the potassium salt because in those days potash (K2CO3) was more readily
available than sodium based alkalis.
Often for those reasons poatssium salts are 'preferred' bu that really doesn't mean they actually work better, especially if whatever they are
supposed to do is a function of the anion, not of the cation.
Synthesis of thiocyanates isn't something I would recommend to any beginner. Search the forum for any ideas, if you must...
'General pinters' won't really do: with that kind of synthesis the devil really is in the detail (trust me, I'm a chemist!  )
[Edited on 4-9-2011 by blogfast25]
|
|
|
Athiril
Harmless
Posts: 24
Registered: 8-5-2011
Member Is Offline
Mood: No Mood
|
|
Thanks for the reply, the cation is actually very important in photographic use.
Potassium salts are generally preferred as they have higher solubility. But in the case of B&W slide production using thiocyanate in the first
developer in small amounts to 'clear the highlights' (the developed image is nowhere ever near half of the available silver halide, without it, your
second developed image (positive) is basically black).
KSCN results in finer grain for this purpose.
Potassium salts are also more photographically reactive generally speaking.
Other than that, the wrong cation can mess up the process too in some bathes and cause fogging, or incomplete action etc.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
