Clearing bath before Pyrochrome reversal bleach

Dinesh · Post by **Dinesh** » Thu Mar 17, 2011 4:51 pm

Jeff, first results.

I set up a single beam Denisyuk and made up some pyro:
A
15 gms pyrogallol
5 gms Metol
1 l water.
B
60 gms Sodium Carbonate
20 gms Potassium Hydroxide

Then I made up the van Ranesse bleach:
4 gms Pot Dichromate
4 mls Sulphuric acid (conc)
1 l water

Then the Nitric acid:
100 mls conc Nitric acid
200 mls water

I didn't add any sulphuric to the nitric, since I believe the Sulphuric is there to create an acidic environment for the Pot Dichromate and Nitric is already acidic.

The Van ranesse bleach went as usual. It started to darken slowly, starting with the least exposed parts and gradually making its way to the dark centre of the plate. It took about 2 minutes to bleach. The Nitric acid, however, didn't seem to be doing anything. I waited another 2 minutes and still nothing. I was about to give it up when the centre of the plate went clear very quickly. Then the claer area spread outward towards the outer, less exposed, edges. In other words, it went backwards, from dark area to light area. Initially, bleaching action was quite fast, then it slowed down. After about a minute, I noticed an odd smell (NO_2? NO ?) However, for the Cause of Science, I proceeded undaunted

If I should die think only this of me
That there is a corner of some foreign darkroom
That is forever Holographic

However, after a further couple of minutes, the bleaching action almost stopped and the smell was getting worse. Not wishing to do a Rupert Brook impression all the way to the end, I finally threw the plate in the wash. The outer (less exposed) edge was still black, but the centre was clear.

jeffblyth · Post by **jeffblyth** » Fri Mar 25, 2011 3:56 pm

Thanks for that very interesting info on your experiment Dinesh. It was really useful to know.

I thought about asking you Rupert Brookwise :
"...is there honey still for DE ?"
(You dont mention if the bleached bit was OK compared to the dichromate bleach).

That smell is a surprise I would not have thought there was enough Ag metal to create much of a pong of presumably NO2.

What is the explanation for it not clearing around the edge I wonder? Have you any thoughts on the physics of it? Your observation that the less the Ag metal the less it is inclined to bleach suggests that the silver is acting as a catalyst to oxidize itself and I have a vague memory of silver acting as a self catalyst in ancient photographic science papers. I think maybe if you could add a teaspoon of copper sulfate as a catalyst it might bleach faster .
But anyway, thanks for trying it out.
Jeff

Martin · Post by **Martin** » Sat Mar 26, 2011 8:29 am

jeffblyth wrote:Have you any thoughts on the physics of it? Your observation that the less the Ag metal the less it is inclined to bleach suggests that the silver is acting as a catalyst to oxidize itself and I have a vague memory of silver acting as a self catalyst in ancient photographic science papers. I think maybe if you could add a teaspoon of copper sulfate as a catalyst it might bleach faster .

Good to see you "on line" again, Jeff.

Regarding holographic bleaches, the only formula based on nitric acid, I'm aware of, is the one introduced by Nick Phillips:

- Potassium permanganate..............0.5g
- Nitric acid.......................................0.25ml
- Water.................................................1L

However, in that case the nitric acid doesn't seem to play the main part in the bleaching action...

jeffblyth · Post by **jeffblyth** » Sun Mar 27, 2011 4:13 am

Regarding holographic bleaches, the only formula based on nitric acid, I'm aware of, is the one introduced by Nick Phillips:

- Potassium permanganate..............0.5g
- Nitric acid.......................................0.25ml
- Water.................................................1L

However, in that case the nitric acid doesn't seem to play the main part in the bleaching action...

That is interesting , I am sure you are right Martin , with only a quarter of a ml of conc nitric in a litre it cannot be doing any oxidising itself, one could just as well use sulfuric acid or sodium hydrogen sulfate. That formula looks nice, it could be the best way to avoid using poisonous dichromate. Permanganate is such a good oxidant without being ultra toxic. I am not clear whether is is readily purchasable these days from chemists (drug stores), it is used for treating athletes foot infections as a foot bath and even as a gargle for mouth infections.

Martin · Post by **Martin** » Mon Mar 28, 2011 12:49 am

jeffblyth wrote:
That formula looks nice, it could be the best way to avoid using poisonous dichromate. Permanganate is such a good oxidant without being ultra toxic.
Yes, for reversal bleaching I always preferred permanganate to dichromate. At low concentration it seems to do little harm to the gelatin layer. So there are practically no side effects from local hardening.
Mixed with certain acids, one issue though used to be the poor keeping properties of the bleach. I assume that permanganate being such a powerfull oxidant, readily oxidizes a great many acids. Even with sulfuric acid the shelf life of the bleach is limited. That's why for photographic use the bleach is often prepared from two A (permanganate) + B (acid) stock solutions. By the way, another acid mentioned in the context of photography, was phosphoric acid. With regards to Phillips' bleach I wonder if nitric acid will improve its keeping properties.

I am not clear whether is is readily purchasable these days from chemists (drug stores), it is used for treating athletes foot infections as a foot bath and even as a gargle for mouth infections.

I'm afraid it may be equally hard to get as the dichromates.

a_k · Post by **a_k** » Mon Mar 28, 2011 5:09 am

In 2010 many compounds with oxidising properties got prohibited in germany with the intention to expand the prohibition to all EU countries. Those compounds, including nitrates, permanganates, perchlorates, H2O2 etc. may not be shipped and if you are lucky enough to find a place willing to sell them, your identity will be registered.

It is really sad to see how it is getting more and more difficult to even get solvents and an interest in chemistry is highly suspicious nowadays. Only a few years ago hundreds of households in germany got raided by the police because people had ordered legally obtainable chemicals by mail order. Having a bottle of Aceton and H2O2 in your house, it seems to be perfectly clear that your intention can only be to produce explosives.

Ed Wesly · Post by **Ed Wesly** » Mon Mar 28, 2011 9:10 am

Yes, I had to fill out a DEA form the last time I ordered potassium permanganate from Photographers' Formulary. And was limited to only getting 10 grams at a time! I for get if there was a HazMat shipping charge on top of that to add insult to injury!

What did the hologram bleached in Nitric Acid look like?