Potassium Thiocyanate Production

[Athiril]

Hi all,

 

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.

[John Cuthber]

This may seem like a silly question, but will sodium thiocyanate not do the job?

I can't easily see how it could make a difference to the grain size. I'm pretty sure that the whole of the silver halide crystal gets converted to silver during development so the grain size is set by the size of the halide crystals.

However if you want to you can make it from KOH and NH4SCN band then remove the ammonia by blowing air or nitrogen through the solution.

[Athiril]

I thought I typed up a reply.. lost it.

 

Anyway, I can use sodium thiocyanate, or even sodium thiosulphate, they at least perform the function of clearing the film density, so the reversal is opposite of the negative, not the negative + a whole lot more density that never got used.

 

In any case, iirc, potassium thiocyanate does give finer grain. Which is why I wanted it, my photo supplier only has sodium thiocyanate, and it's pricey, and I already have that anyway.

 

 

 

Well, the problem then is, how do I get to NH4SCN? I can get to a thiocyanate from thiosulphate and a cyanide, so I assume I use potassium ferricyande for the cyanide source. But I am unsure of the reaction, and whether say ammonium thiosulphate will give a mix of ammonium thiocyanate and potassium cyanate or just one or the other?

[Athiril]

I thought I typed up a reply.. lost it.

 

Anyway, I can use sodium thiocyanate, or even sodium thiosulphate, they at least perform the function of clearing the film density, so the reversal is opposite of the negative, not the negative + a whole lot more density that never got used.

 

In any case, iirc, potassium thiocyanate does give finer grain. Which is why I wanted it, my photo supplier only has sodium thiocyanate, and it's pricey, and I already have that anyway.

 

 

 

Well, the problem then is, how do I get to NH4SCN? I can get to a thiocyanate from thiosulphate and a cyanide, so I assume I use potassium ferricyande for the cyanide source. But I am unsure of the reaction, and whether say ammonium thiosulphate will give a mix of ammonium thiocyanate and potassium cyanate or just one or the other?

[John Cuthber]

I don't think that ferricyanide will do; you would have to use potassium cyanide and that's not nice stuff or easy to get.

The traditional way to make thiocyanates is from sulphur and cyanide but that''s not something for an amateur to play with.

 

I think your best bet is the photographic suppliers or ebay.

[Ziconotide]

Melt potassium cyanide with an excess of sulfur. Extract it with water and filter it. Now you have a solution of KSCN. You can get KCN by melting ferrocyanide with potassium carbonate, extracting the mixture with as little water as possible and precipitating the cyanide with ethanol. Caution ! KCN is toxic.