Ascorbate Chemistry 1

Alan Johnson · Aug 21, 2015

I have been curious to know how Xtol, Ilfosol etc work as details of the chemistry are not published. In 1968 Lee and Miller found regeneration of phenidone by ascorbate in solution appeared to be a simple reduction reaction:
(there was a url link here which no longer exists)
I borrowed from biology publications to make a theory applicable to photography.

Probably phenidone adsorbed on a grain surface just passes an electron from the ascorbate to a silver ion which is converted to metallic silver.Referring to the attached diagram it is suggested that this electron comes from an ascorbate ion which gives it up to become an ascorbate radical. This radical then converts to ascorbate and dehydroascorbate (called a dismutation reaction), really they should be shown as ions in a correct diagram.

Over a longer period of time than it takes to develop a film, other reactions can occur. The dehydroascorbate further decomposes (reactions not shown).In air oxidation of ascorbate can occur and in some cases is catalysed by metal ions (Fenton reaction).

The ascorbate radical has the structure of an ionised semiquinone, cf benzosemiquinone and there may be some similarity with hydroquinone chemistry.I have not considered the effect of sulfite.

Gerald C Koch · Aug 21, 2015

According to the Kendall-Pelz Rule ascorbic acid having hydroxyl groups on two adjacent carbons atoms joined by a double bond should be similar to catechol and also by inference hydroquinone in its photographic chemistry.

Alan Johnson · Aug 21, 2015

Mees and James, "Theory of the Photographic Process " 3rd ed p 284 discusses the oxidation of Hydroquinone (My abbreviations):
(1) HQ +2OH- -> HQ ion, (2) HQ ion -e -> semiquinone
Although this is the same reaction path as given for ascorbate above and could potentially donate an electron to a silver ion ( eg via adsorbed phenidone ) it is barely mentioned in the rest of the book. IDK if it was not understood at the time or if HQ is different.

Ian Grant · Aug 21, 2015

There was a US Patent by a Swedish company that covered developers like Xtol, it seems to be no coincidence that Kodak didn't release Xtol until the Patent expired. I have a PDF of it somewhere, I think there was a Phenidone/Ascorbate example.

Kodak also worked on Pyrocatechin/Ascorbic acid developers in the 1940's some using Ascorbic acid itself others using Sodium Ascorbate.

I don't know if I still have my Biochemistry notes on the properties of Ascorbic acid/Ascorbates but that's where the research on its properties lies, my niece had my books when she did her BSc in BioMedical sciences.

Kodak looked at the effects of developers containing 2 developing agents and Ascorbate/Ascorbic acid and in particular the 3 di-Hydroxybenzenes - Hydroquinone, Pyrogallol and Pyrocatechin (Kodak's use of the term di-Hydroxybenenes not mine).

I've always thought that the more interesting of the Hydroxybenzenes is Pyrocatechin a fine grain developing agent when used alone but even better when used in partnership with Phenidone (Dimezone), Metol and PPD. Johnsons Meritol a PPD/Pyrocatechin fusion was the basis of many excellent developers. It's a shame Johnsons pulled out of the chemistry market, they were the suppliers to Fox Talbot so the earliest Photo chemistry supply company still trading into the later part of the 20th C. The company still exists but only as an importer/distributor and service/repair center.

Ian

Photo Engineer · Aug 21, 2015

Alan, the problem is that all of the phenidone oxidation products can ring open and thus terminate the chain. Dimezone S, being hindered by the MeOH does not do this much if at all. If true, there should be significant differences in these mixtures.

PE

StephenT · Aug 21, 2015

Wow, shades of Organic Chemistry! It's been almost 50 years since I graduated from Ga. Tech - not sure I could pass Organic again! Not to mention Mechanics of Deformable Bodies.

Photo Engineer · Aug 21, 2015

Heterocycles and Steroids and Terpenes are not fun either!

I've been re-reading Fieser and Fieser.

PE

Alan Johnson · Aug 21, 2015

Photo Engineer said:
Alan, the problem is that all of the phenidone oxidation products can ring open and thus terminate the chain. Dimezone S, being hindered by the MeOH does not do this much if at all. If true, there should be significant differences in these mixtures.

PE

Thanks PE.In the original post 2nd paragraph phenidone is hereby deleted and replaced by Dimezone-S.What happens to the ascorbate is not changed.
It's curious that Mees and James don't discuss how hydroquinone regenerates the primary developer in more detail.
Sorry I don't have Haist.

Photo Engineer · Aug 21, 2015

Well, I may have to re-read Haist. He was the "in the lab and do the experiment type of guy". IDK who wrote the chapter for Mees or James. I'll have to look it up.

Grant wrote his entire 2 volumes himself, but Mess and James is a collection of "short stories", each one from a different author who was selected to represent that field.

I know that everyone was using the phenidone family for processing but few were doing fundamental R&D on its reactions, and the AA was not studied except in the EAA (Elon Ascorbic Acid formula) that I published here. It was well known at EK for being a clean working developer, but was only used for R&D.

PE

falotico · Aug 21, 2015

I've been re-reading Fieser and Fieser.

PE

I've been reading "Harry Potter". Probably not as helpful, but to me the chemistry of photography is magic.

Photo Engineer · Aug 21, 2015

"Harry Potty and the chamber pot of doom".

PE

Alan Johnson · Aug 22, 2015

Regarding Hydroquinone, Mason,"Photographic Processing Chemistry" 1975 pp 64-75 gives details of many free radicals that are or might be formed in HQ oxidation.
It seems to me possible that an electron converting Ag+ to Ag might (possibly via adsorbed Dimezone-S/phenidone/metol) originate from the HQ monoanion giving up an electron and converting to some free radical of this type.Mason does not express an opinion as far as I have read, on this type of superadditivity. IDK what might happen to such a free radical.
This path, if correct, is similar to ascorbate in the OP but more complicated.Still have not considered sulfite.

Photo Engineer · Aug 22, 2015

The most definitive work on this that I was privy to was that of Rex Pontius and Roland Willis. Anything you can find published by them would be worth reading and went on long after Grant published his book. The basis for the EAA developer came from Roland IIRC.

Superadditivity and Electron Pump or Electron Transfer Agent are different aspects of very similar effects in which an ETA only needs to be present in trace amounts, and the reaction can continue for many cycles.

PE

Alan Johnson · Aug 22, 2015

In theory Photo Sci Eng may be available from here: http://www.imaging.org/ist/publications/jist/
I have enquired the cost of the Willis & Pontius paper.

Alan Johnson · Aug 24, 2015

I received a copy of the Willis-Pontius paper from Rudeofus (thanks).
Unfortunately it is not the revelation I had hoped for as it deals with the (graphic arts?) situation of development in potassium ascorbate solution at high pH 10.8-11.8 with no primary developer (phenidone, metol etc).At high pH the electrons for the conversion of 2Ag+ to metallic silver 2Ag simply come from the dianion AA2- but what happens to the dianion after that is not discussed.

Rudeofus · Aug 25, 2015

My impression of the relevant patents is this: Ascorbate was discovered to be a substitute for some aspects of Sulfite, as it prevents aerial oxidation of some developers but doesn't destroy the tanning action of some oxidized developers: https://www.google.com/patents/US2415666. Then it was discovered that Ascorbate develops by itself or in superadditive mixtures with other developers, see Michael's first cited patent http://www.google.com/patents/US2688549. The Swedes added an interesting aspect: buffering action of Borax could be greatly increased if it is combined with polyhydric alcohols, and this increased buffering action could increase the capacity of such a developer by three or four times.

Now if we look at Xtol, neither its patent (US5853964), nor its various substitute formulas contain any such polyhydric alcohol, yet it is one of Xtol's striking features that it can be replenished with itself. My suspicion is that the Ascorbate ion works as polyhydric alcohol here, eliminating the need for an extra ingredient.

Alan Johnson · Aug 25, 2015

I have looked at what happens to hydroquinone when it gives up an electron.Here it forms a phenoxy radical:
http://www.chemgapedia.de/vsengine/...toff/oxidation_phenole/mechanismus.vscml.html
This is another name for benzosemiquinone:
http://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI:17977

Workers cited below found that at pH 4.6 it is the semiquinone anion that is present..
Two semiquinone anion radicals disproportionate to give hydroquinone and benzoquinone (equation 2).Ignore the stuff about iron it is not relevant to this.
http://www.researchgate.net/publica...quinone_radicals_and_reaction_with_iron_salts

This disproportionation is very similar to what happens to ascorbate in the OP.
One could speculate that all regenerants in superadditive development might involve such a disproportionation but the experimental conditions in this paper are not too close to those found in photographic development.
Still have not considered the effect of sulfite.

btw, imaging.org informed me that the cost of a pdf reprint from photo sci eng is $25.

Alan Johnson · Sep 8, 2015

Development in presence of Sulfite

With hydroquinone, Mason, p74 cites LuValle as concluding that sulfonate is formed by reaction of sulfite with semiquinone.Attachment is my version.
With ascorbate, I suggest a similar reaction path, ascorbate radical forming ascorbyl sulfonate.
In an experiment leaving buffered ascorbate solution in air (link, post 37), in absence of sulfite it went orange and there was a notable pH drop, probably due to formation of dehydroascorbate by the dismutation reaction discussed above, and subsequent oxidation products.
In presence of sulfite it went yellow( like Xtol does) this may be sodium ascorblyl sulfonate.
Presence of sulfite appears to stop the dismutation reaction and associated pH drop.
(there was a url link here which no longer exists)
There is no published work on ascorbate to refer.

Photo Engineer · Sep 8, 2015

I would guess that the ascorbyl monosulfonate would be unstable, otherwise it would have been isolated by now to prove the route. After all, the HQMS has been isolated and is a fairly good developing agent.

PE

Alan Johnson · Sep 8, 2015

I wonder if, in air, it might be oxidized to sodium ascorbyl sulfate, which is yellow.
http://www.saapedia.org/en/saa/?type=detail&id=1056

Rudeofus · Sep 11, 2015

Right away: I have no formal training in chemistry, so what I am about to write here may be completely baseless. I would welcome corrections or insights. Ok, here it comes: I am completely puzzled by the suggested sulfonation reaction which oxidized Ascorbic Acid is supposed to undergo, supposedly turning it into a Sulfate.

The sulfonation reaction, which we all know from oxidized HQ and most of its derivatives, will e.g. not happen if there is no free hydrogen available directly on the ring. Mees, in his book The Theory of the Photographic Process, writes that Duroquinone will not undergo sulfonation. Are we sure that sulfonation will happen onto a hydroxy-group, creating Sulfates?

My second point aims at this Sodium Ascorbyl Sulfate compound. If I look at literature, this seems to be an ester like compound, which sort of tells me it may or may not stay an ester in aqueous solution. It also tells me that it is formed by different reaction mechanisms than sulfonates.

Again, the above two points could be completely off base, don't believe a word of it until one of the chemists here (Alan, Gerald, Ron) confirm it. I wanted to bring up these points since I was unable to find an answer elsewhere, and it may be conductive to the ongoing discussion here.

Alan Johnson · Sep 11, 2015

Rudi,
From "Basic Photo Science" Walls & Attridge 1977 p 190:
"Most developing agents are by themselves in the first instance oxidized to substances with a quinonoid structure. Such substances are also catalysts for the reaction...However the sulphite ion reacts with quinonoid structures
and thereby removes the first products of oxidation from the reaction mixture...This at least is almost certainly true for hydroquinone which ,in absence of sulphite is oxidized to a quinone.The sulphite reacts with the quinone according to a rather complex schame, producing hydroquinone sulphonic acids.....A similar explanation probably exists for other developing agents , at least those which can be oxidized to a quinonoid structure."
Admitted this is rather a shorthand summary but Ascorbate can be oxidized to a quinonoid structure.

My attachment actually detailed the reaction in terms of semiquinones, it may be possible to track back to the original references given by Mason. "Photographic Processing Chemistry" p74 -LuValle and p166 Umberger ref 138a Phot Sci Eng 10 8 1966.
Even for Hydroquinone it is quite difficult to track back to the original papers.
On p166 Mason describes how the hydroqinone semiquinone reaction becomes autocatalytic leading to lith developers.The fact that weak lith developers can be made from ascorbates seems to support the claim that ascorbate semiquinones do exist.
Unfortunately no crumbs have dropped from the rich researchers table showing how they might get sulfonated.
However, experimentally it is found that sulfites definitely constrain the pH drop that occurs in their absence, leading to the yellow color of oxidized Xtol ,eg, rather than the orange color in absence of sulfite.
Hence sulfite does seem to interact with ascorbate semiquinone.To get to my attachment scheme was not a sophisticated process, I just copied what appears to be correct for the hydroqinone semiqinone.
It's possible there might be a better explanation gets from ascorbate semiquinone to the yellow color of oxidized ascorbate/sulfite.

Rudeofus · Sep 11, 2015

Alan, what I was trying to express was this: The classic Quinone sulfonation process replaces a hydrogen atom with the sulfonate group. This sulfonation will e.g. not happen with Duroquinone, because there is no Hydrogen directly attached to the aromatic ring.

Therefore I wondered whether sulfonation works onto an -OH group, as would be necessary in your drawn reaction scheme. To my great dismay I was unable to find whether Hexahydoxy Benzene can be sulfonated after oxidation. In my opinion there is a difference between Quinone sulfonation and your proposed reaction scheme.

Alan Johnson · Sep 11, 2015

I have drawn the HQ as semiquinone reacting with sulfite as I could not find the reaction scheme in any references but that is how it is usually described.
The ascorbate is drawn as semiquinone anion reacting to be consistent with the no sulfite dismutation mentioned earlier.
Sorry if any electrons are missing,the scheme just gives the general idea.
The sulfite radical does oxidize both hydroquinone and ascorbate etc,see p213:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1568601/pdf/envhper00447-0209.pdf
This is a bit vague but there are no publications relating to photochemistry, ie to sulfonation.

Alan Johnson · Sep 27, 2015

I made a new attachment based on sulfonation of a different radical ( AscH.), this is now the proposed scheme.
Now no -OH group is replaced by a SO3, to meet the point made by Rudeofus, and the sulfonation is made reversible to suit the point made by PE.
Now ascorbyl sulfonate acts as a store but over a long time can still convert to dehydroascorbate,The reaction with oxygen is not shown but assumed to take 1 electron like silver is shown.
There is no conversion to sulfate.
The radical (AscH.-) may be involved in ascorbate lith development, the fact that it can decompose by the reaction shown may be the reason why ascorbate is only a weak lith developer.

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