Nitric acid for reversal bleach?

[Donald Qualls]

The thought clicked in my head the other day, thinking about ways to solubilize reduced silver in film without removing the undeveloped halide, that silver nitrate is quite soluble (its solution is used in most methods of applying silver halides, because silver halides are mostly insoluble, hence precipitate on formation).

Of course, even weak nitric acid requires some care in handling -- ideally a vent hood and heavy impermeable gloves, plus the usual face protection and apron -- but there are ways of obtaining useful levels of nitric acid without (at least) actually storing nitric acid (so no possibility of spilling the strong stuff and nitrating things that don't need to be extra-flammable).

What I don't know is how gelatin reacts to nitric acid. There has to be a good reason why nitration isn't the simplest reversal bleach, and I suspect this is it. Anyone with experience or better chemical knowledge than me on this?

 

[koraks]

No, not better chemical knowledge at all, but like you, I'd be concerned about the nitrogen breaking down the peptide strands, weakening or even completely dissolving the emulsion. Calling in @fgorga and @nmp who are much better at home than me in this.

 

[Donald Qualls]

Thanks!

Alternately, I wonder if there's a nitrate salt that could dissolve the silver and produce soluble products without the very low pH?

 

[koraks]

I really wouldn't know! I suppose if there was an easy fix, we'd all be using it. I suppose that whatever is aggressive enough to steal an electron from silver would generally come with other drawbacks as well, such as being damaging to film or utensils, being somehow highly unpleasant/dangerous to work with, or simply unaffordable.

 

[Donald Qualls]

being damaging to film or utensils, being somehow highly unpleasant/dangerous to work with, or simply unaffordable.

Well, you've covered permanganate (softens emulsion -- Foma may fringe or just slide off, though most other brands are hard enough they're just scratch prone in/after the bleach bath), dichromate (carcinogenic and banned for most uses in EU), copper sulfate (itself toxic, though not so much it isn't sold in hardware stores, but the ammonia bath to remove the silver chloride is both slow and very bad to breathe). Not sure what's in the last category; EDTA/PDTA are for color processes where you'll remove all the silver at the end anyway.

 

[koraks]

Well, there you go. It's a tricky proposition!

 

[DeletedAcct1]

The thought clicked in my head the other day, thinking about ways to solubilize reduced silver in film without removing the undeveloped halide, that silver nitrate is quite soluble (its solution is used in most methods of applying silver halides, because silver halides are mostly insoluble, hence precipitate on formation).

Of course, even weak nitric acid requires some care in handling -- ideally a vent hood and heavy impermeable gloves, plus the usual face protection and apron -- but there are ways of obtaining useful levels of nitric acid without (at least) actually storing nitric acid (so no possibility of spilling the strong stuff and nitrating things that don't need to be extra-flammable).

What I don't know is how gelatin reacts to nitric acid. There has to be a good reason why nitration isn't the simplest reversal bleach, and I suspect this is it. Anyone with experience or better chemical knowledge than me on this?

Please stop spreading dangerous suggestion. Nitric acid is dangerous. Nitric acid is forbidden to sell. It's an explosive precursor.
Stick with the proper reversal method and call it a day.

 

[DeletedAcct1]

Well, you've covered permanganate (softens emulsion -- Foma may fringe or just slide off, though most other brands are hard enough they're just scratch prone in/after the bleach bath),

Never had such issue. Fomapan 100 R or Fomapan 100 DON'T fringe or whatever. I reverse b&w material from 2004, it's been 20 years, I know what I say.

 

[koraks]

Please stop spreading dangerous suggestion. Nitric acid is dangerous. Nitric acid is forbidden to sell. It's an explosive precursor.

It's fine to discuss possibilities. We also routinely discuss dichromate for instance. Or indeed permanganate, which as you know is also easy to fashion into an explosive. @Donald Qualls was explicit in the need for precautions called for when handling this material - precautions which also apply to many other materials we discuss here, such as glacial acetic acid.
As to legal realities - these differ across the globe. Whether you can buy (or sell) nitric acid depends on where you are, who you are and what concentration acid it is - as I trust you know.

So with a (repeated) note about the dangers associated with handling certain chemistry, we can end this diversion and continue discussing this and associated topics.

 

[Donald Qualls]

Nitric acid is forbidden to sell.

None the less, it's easy to make in a form that might be photographically useful from ingredients sold at home improvement stores. If it is photographically useful, I can make it in small quantity at low concentration for that purpose and never have any on hand between reversal sessions. If it's not, I don't even want/need to buy the ingredients.

Fomapan 100 R or Fomapan 100 DON'T fringe or whatever.

Well, happy to be wrong. Fomapan isn't the softest emulsion I've used (that would be Fortepan), but it's softer than most. If it holds up in permanganate bleach, good deal.

 

[DeletedAcct1]

None the less, it's easy to make in a form that might be photographically useful from ingredients sold at home improvement stores. If it is photographically useful, I can make it in small quantity at low concentration for that purpose and never have any on hand between reversal sessions. If it's not, I don't even want/need to buy the ingredients.

About the emulsion damage: it would be useful if some light is shed definitively on the matter.
It's not the emulsion that melts, it's the subbing layer beneath, a sort of adhesive layer that hold in place the emulsion to the film base.
If the subbing layer fails there's no hardener, no special bleach that can be helpful. It means that simply the film isn't designed to be optimally reversed. The solution is to switch to another film. As simple as that.

 

[MattKing]

A note from the moderation team.
We note and re-emphasize the safety concerns that have been expressed earlier in the thread.
In perhaps an excess of caution, and in case someone may have missed it earlier, to reiterate earlier concerns - handling nitric acid can be very dangerous, and ought only to be undertaken by those who are knowledgeable, well trained and very well equipped to deal with its hazards.
With respect to the moderation actions earlier in the thread, we always appreciate it when members respectfully raise issues of safety.
We also always appreciate it when people refrain from repeatedly re-posting the same information over and over.
And we are always likely to intervene if that repetition tends toward anger or the argumentative.
If you are unsure whether a safety concern is not made clear, go ahead and post your concern respectfully. But do not ignore earlier cautions in the thread, that have already been made clear.

 

[MCB18]

Please stop spreading dangerous suggestion. Nitric acid is dangerous. Nitric acid is forbidden to sell. It's an explosive precursor.
Stick with the proper reversal method and call it a day.

Nitric acid is not banned for sale in a lot of places, in fact it’s a pretty regularly available reagent in most places. I know that USer’s get a lot of crap for forgetting the rest of the world exists, but that also applies to… the rest of the world. The EU’s (in my opinion overly restrictive) policies regarding chemicals are very much not the norm in the rest of the world. I can pick up 4 liters of 70% nitric acid from a biodiesel company for $150.

This isn’t to say that people should buy it, however I think that yelling at someone for simply asking a question (and also bringing up concerns with safety in the OP) isn’t a very useful contribution to the discussion.

 

[halfaman]

Nitric acid is not forbidden in EU, but it is a retricted substance only for professional use. This means that you need to fill a form probing that you are a company or a professional with an activity that justify its use (you can't get nitric acid if you are a writter) in order to buy some.

 

[Rudeofus]

I would personally avoid Nitric Acid for a multitude of reasons, but we can still experiment with an equivalent without scratching on "up to no good" territory. Sodium Nitrate is not suitable for "up to no good", so this is where I'd start from. Add some acid, e.g. Sulfuric Acid (aka battery acid) and you should (as far as film is concerned) have something very similar to Nitric Acid, and again without the "up to no good" factor attached to it.

I would be very surprised, if Nitric Acid or nitrates attack the emulsions, since most professional color bleaches contain it in quantity to fight corrosion of stainless film. If emulsion was unhappy about its acidity, then it would be just as unhappy about "Sulfuric Acid plus Potassium Permanganate".

I do, however, warn against unjustified optimism here: while nitrate is certainly a strong oxidizer, I have never seen it bleach film. It's also not mentioned in patent literature, so it may simply not work.

 

[koraks]

I would be very surprised, if Nitric Acid or nitrates attack the emulsions

This is a valid point, with the caveat that concentration will likely matter (a lot). However, concentrations that would be sensible for a bleach (if this route were to work in the first place) would likely be 'emulsion safe'. So I retract what I said earlier, although I still expect the emulsion to break down rapidly in concentrated nitric acid.

 

[Donald Qualls]

Sodium Nitrate is not suitable for "up to no good", so this is where I'd start from. Add some acid, e.g. Sulfuric Acid (aka battery acid) and you should (as far as film is concerned) have something very similar to Nitric Acid, and again without the "up to no good" factor attached to it.

This is exactly the route I had planned; I have no need or desire to buy and store nitric acid, but this mixture produces enough nitric acid in solution to be usable for some "up to no good" purposes even so. However, starting with 30% sulfuric (aka battery acid) limits the strength of the nitric/sulfuric mixture you can produce this way.

 

[Quiver2]

This is exactly the route I had planned; I have no need or desire to buy and store nitric acid, but this mixture produces enough nitric acid in solution to be usable for some "up to no good" purposes even so. However, starting with 30% sulfuric (aka battery acid) limits the strength of the nitric/sulfuric mixture you can produce this way.

Um, aside from being hydrophilic how is sodium nitrate different from potassium nitrate? I'd think that they could both be used as an oxidizer, and that has a bunch of "up to no good" potential.

 

[Ivo Stunga]

Never had such issue. Fomapan 100 R or Fomapan 100 DON'T fringe or whatever. I reverse b&w material from 2004, it's been 20 years, I know what I say.

Your mileage may vary. Mine however is this:
- I had emulsion leaving me with it's own reversal kit and with almost all 10 films until I thought of diluting the bleach or reducing bleach time. Only then I had some neat shots with R100 in its native kit.


And I've had some funk in Ilford Reversal, PQ Universal, so I reduced the bleaching time next time around and problem went away



No need to scream about your experience being the only viable one, though. Truth will find its way without all that extra effort

 

[Donald Qualls]

aside from being hydrophilic how is sodium nitrate different from potassium nitrate?

Yes, both are oxidizers that can, among other things, be used to make black powder (sodium nitrate based powder is much harder to keep dry, however). Even with that being the case, one or the other is sold in home improvement stores in the US as stump remover.

 

[DeletedAcct1]

Your mileage may vary. Mine however is this:
- I had emulsion leaving me with it's own reversal kit and with almost all 10 films until I thought of diluting the bleach or reducing bleach time. Only then I had some neat shots with R100 in its native kit.

Was it the Foma kit of some times ago, the one that the permanganate came in powder in sachets?

 

[Ivo Stunga]

Was it the Foma kit of some times ago, the one that the permanganate came in powder in sachets?

A and B in small plastic bottles and instructions telling to bleach for 8 minutes.

 

[Yezishu]

Thanks!

Alternately, I wonder if there's a nitrate salt that could dissolve the silver and produce soluble products without the very low pH?

I came across this topic by chance. I have an answer that doesn’t from photography: ferric nitrate.
It’s used in the jewelry and microelectronics industries for etching silver and producing silver nitrate. Its pH is about 1.3 (due to hydrolysis of iron ions), which is roughly equivalent to 0.5% dilute nitric acid.

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I suppose if it hasn’t been widly used before in photography, there’s probably a reason for that. It could be that the required concentration is too high and affects the emulsion, or perhaps iron ions tend to leave residues at high concentrations. But some patents do mention Ferric nitrate.

US4322493A:

Bleaching Solution (A) +Bleach Accelerator
Distilled Water 700 mL
Cerium Sulfate 50 g
Sulfuric Acid (conc.) 8 g
Distilled water to make 1 L

Bleaching Solution (B) +Bleach Accelerator
Distilled Water 500 mL
Ferric Nitrate (9H2O) 600 g (so much!)
Sulfuric Acid (conc.) 100 g
Distilled water to make 1 L

 

[Raghu Kuvempunagar]

There is a fairly well-known, at least in holography, Ferric Nitrate based bleach called Phillip's Safe Solvent Bleach (PSSB). It is very effective but in my limited explorations with PSSB, I saw the film getting damaged. Perhaps a more diluted bleach, reduced agitation and lower temperature might help minimise the harm.

 

[Yezishu]

There is a fairly well-known, at least in holography, Ferric Nitrate based bleach called Phillip's Safe Solvent Bleach (PSSB). It is very effective but in my limited explorations with PSSB, I saw the film getting damaged. Perhaps a more diluted bleach, reduced agitation and lower temperature might help minimise the harm.

It appears that potassium persulfate may be responsible for damaging the gelatin. Have you heard of Piranha solution, which is used in the microelectronics industry? I suggest conducting a small comparison test on discarded film: one without potassium persulfate and one without iron salt, to determine which component is causing the damage.

If potassium persulfate is indeed the reason (which is my suspicion), reducing its concentration should help protect the film. Since only a small amount is needed to oxidize or regenerate the iron salt during bleaching, the solution can be prepared at a lower concentration and supplemented with fresh persulfate as needed before use.