Peroxide-based reversal bleach and Thiocyanate-containing First Developers

[adycousins]

I've been using a perecetic acid bleach composed of 1 litre of 9% hydrogen peroxide (food grade) and 28ml of acetic acid glacial (98%). pH is about 2.5 and if temperature is kept between 19-25°C It works quite well, with minimal emulsion damage.

Recently the safety of using peracetic acid bleach with First Developers containing thiocyanates has been questioned. The worry is that traces of thiocyanate will remain in the emulsion following the first wash and will then react with the peracetic acid to form potentially lethal hydrogen cyanide.

I'm obviously still alive after repeatedly using First Developer's containing thiocyanates followed by peracetic acid bleach but I wondered what thoughts others had on potential risks.

Would it be better to use another silver solvent in the first developer when using peroxide bleaches, or is it safe to assume that a proper wash would remove any thiocyanates before the bleach stage?

Food grade peroxide, acetic acid and thiocyanate are easy to come by in UK. Alternatives to thiocyanates such as DTOD seem much harder to find.

Examples of peracetic acid reversal bleach:
Plus-X Super 8:

Kodachrome 40 16mm (found footage):

 

[Rudeofus]

If you use about 1 g/l KSCN and leave 10 ml developer in your film as carryover, then that's 10 mg KSCN. If you convert this into HCN with 100% yield, you get less than 3 mg HCN. These 3 mg HCN make up a volume of about 2.5 ml at room temperature and ambient pressure. Even in a tiny room of 2m x 2m x 2m this would be 0.3 ppm, which is orders of magnitude below dangerous levels.

If you are still anxious, wash the film after development and before bleaching, this should reduce KSCN to extremely low levels.

 

[Photo Engineer]

What he said! However, I doubt that this would even take place.

I would be more concerned over your comment "minimal emulsion damage". That is one strong bleach with a very low pH. That and the bleach itself is not good for you. Peracetic acid is a powerful oxidizing agent and very toxic. Take care. I did a lot of research in grad school using Peracetic acid.

PE

 

[peoplemerge]

The first video of the two was particularly stunning, particularly for super 8.

 

[adycousins]

Thanks so much for your comments on this, it's reasurring to know that I'm not about to poison myself (or others), though I had suspected that a good wash would remove pretty much all of the the thiocyanate (I use 2- 3 mins under running water with couple of changes).
when I refer to 'emulsion damage' I mean the blistering that occurs in the emulsion when the reaction is over-vigorous (vesiculation), I may have got the pH wrong (flakey cheap pH measuring device).
I'm also not clear whether simply adding 27ml of acetic acid to 9% hydrogen peroxide results in peracetic acid. Its a very handy bleach - I've just used some that I made six months ago and kept in a light-and-air-tight container and it worked very well.

 

[flavio81]

I'm also not clear whether simply adding 27ml of acetic acid to 9% hydrogen peroxide results in peracetic acid. Its a very handy bleach - I've just used some that I made six months ago and kept in a light-and-air-tight container and it worked very well.

That was something I was going to ask.

Peracetic acid takes time to be formed. So a freshly-made mixture of H2O2 and acetic acid will have little actual peracetic acid in there. There's another thread here discussing this. It seems you need to wait until the peracetic is formed. How much, I don't know...

 

[Raghu Kuvempunagar]

I've been using a perecetic acid bleach composed of 1 litre of 9% hydrogen peroxide (food grade) and 28ml of acetic acid glacial (98%). pH is about 2.5 and if temperature is kept between 19-25°C It works quite well, with minimal emulsion damage.

@adycousins: what is the capacity of 1 litre of this bleach? How many rolls of 35mm film?

 

[Raghu Kuvempunagar]

@adycousins: what is the capacity of 1 litre of this bleach? How many rolls of 35mm film?

I found Adrian on Facebook earlier today and asked him the same question. He kindly responded as follows:

"I don’t really know for sure, it degrades with exposure to light and air, if you’re referring to fresh 9% peroxide + 27ml of acetic acid (ie not been stored for more than 24hours) it’s about 200’ of 16mm, so approx 30m of 35mm, roughly 18 x 36 exposure films."

Sharing the information in this thread so that someone might find it useful.