Ascorbate Chemistry 1

[Rudeofus]

I finally found a publication which indirectly mentions interaction between Sulfite and oxidized form of aromatic rings which are fully substituted with -OH groups, just like oxidized Ascorbate. In this text the synthesis of Tetrahydroxy-p-Benzoquinone is described:

A solution or 400 g (3.17 moles) of anhydrous sodium sulfite and 150 g (1.79 moles) or sodium bicarbonate in 3 liters of water is placed in a 5-liter, three-necked flask fitted with a thermometer, an air inlet tube (1 cm diam), and a tube connected to a water aspirator. The solution is heated to 40 to 45 °C, and 600 g of a 30-percent, aqueous solution or glyoxal (3.1 moles) is added. A brisk stream of air is drawn through the solution for 1 hr without heating after which the mixture is gradually heated to 90 °C. Aeration is then stopped, and the mixture is heated to incipient boiling and cooled to room temperature.

So what do we have here? We have the oxidation product of Hexahydroxybenzene, plenty of air to provide further oxidation, and loads of Sulfite, and plenty of time at elevated temperature, yet you get at least 10% of the initial Glyoxal converted to Sodium salt of Tetrahydroxy-p-Benzoquinone. I would therefore suggest, that Ascorbate Sulfonate does not form in quantity from oxidized Ascorbate and Sulfite.

 

[Alan Johnson]

I only said ascorbyl sulfonate is COPIED from hydroquinone monosulfonate (formed by the reaction of sulphite with the semiquinone of hydroquinone, Mason p74).
If no ascorbyl sulfonate it would be necessary to find two oxide derivatives of ascorbate colored yellow (with sulfite) and orange (without sulfite) to fit the facts.

 

[Rudeofus]

There is a patent, WO2002051827A1, which describes the synthesis of Ascorbic Acid in the presence of Sulfite. The Sulfite (in the form of Sulfur Dioxide, or as Metabisulfite) is added for two purposes:

1. Acidify the processing liquid such that Ascorbic Acid is formed in quantity
2. Scavenge coloured side products

The patent text does not go into detail about what these coloured side products may be, and the precise composition of these coloured side products may not even be known. The patent does mention a range of compound classes which may contribute to discolouration of the Ascorbate created in this reaction. The patent was filed in 2000, so it's fairly recent. It was written by folks trying to efficiently synthesize Ascorbic Acid, so while a patent text is not necessarily scientific, it comes from people who presumably know Ascorbate well, and most likely far beyond published scientific literature.

I do think that these are about the same side products which form during aerial oxidation of Ascorbate in our developers. My conclusion from this patent text is, that there is not one single side product which causes discolouration of ascorbate developers, and that Sulfite engages in a range of reactions with these coloured compounds, turning them into (mostly) colourless compounds. This should not come as a surprise, as Sulfite has been used for bleaching (bleaching of dyes, not Silver) in many applications.

 

[Alan Johnson]

Ascorbates generally oxidize via dehydroascorbic acid to a mixture of oxalic and threonic acids:
http://authors.library.caltech.edu/11677/1/BORjbc37a.pdf
Oxalic acid has a pKa=1.23 and 4.19 compared to ascorbic acid pKa=4.1 and 11.8.It might be expected to cause a pH drop, which is what happens in absence of sulfite,The question is then how does sulfite stop the pH drop.Is it a buffering effect or a sulfite-oxalate compound (or ascorbyl sulfonate)?
I expect the production of oxalic and threonic acids to be the main reaction as indicated in the link,but we may need one mystery reaction.
There is no data for threonic acid.

 

[Rudeofus]

I am completely out of my league here, but from reading various tidbits from here and there, I could imagine a few reactions which could stabilize pH in this situation:

• Sulfite is both an alkali, and a buffer at pH 6-8
• Sulfite forming an adduct with ketones would raise pH. The reaction is suggested by the patent, and the pH rise is the reason why Acetone developer (discussed here a little while back) works.
• Sulfite forming an adduct with intermediary ketones could also prevent decomposition all the way down to Oxalic Acid, thereby preventing formation of the more acidic end products (again, see patent text).

@Threonic Acid: if you look at Hydroxyacetic Acid with pKa 3.83, one wouldn't expect pKa of Threonic Acid to differ by much. This page claims pKa = 3.4 based on some simulation. Whichever value is correct, we can safely assume that Threonic Acid is fully deprotonized in whatever mixture Xtol would turn into, and a stronger acid than Ascorbic Acid.

 

[Alan Johnson]

Dehydroascorbic acid (DHA) is a temporary product of development in ascorbate developers, but it appeared that it might produce development itself as it hydrolyses to diketogulonic acid and the oxidation of this to threonic and oxalic acis might reduce silver bromide to silver.A link from Rudeofus notes a reducing action:
http://authors.library.caltech.edu/11677/1/BORjbc37a.pdf
To check , I made a solution 30g/L sodium carbonate (anh) and 10g/L sodium ascorbate.This part blackens film in 5 min and fully in 10 min.
Next, DHA was made by adding to this solution 3.5 g/L or10 g/L potassium permanganate,which oxidizes the ascorbate.
The 3.5g/L sample only part blackened the film in 30 min and the 10g/L sample did not blacken it at all.
This result is consistent with blackening caused by residual ascorbate; I did not find any evidence for development by DHA derivatives.

 

[sfaber17]

Alan, I'm not sure about your conclusion, since permanganate is a powerful oxidizer, it would oxidize any DHA also. Wouldn't you have to carefully titrate the ascorbate with an exact amount to convert it to DHA, otherwise you are blowing away the DHA as well?

 

[Alan Johnson]

Alan, I'm not sure about your conclusion, since permanganate is a powerful oxidizer, it would oxidize any DHA also. Wouldn't you have to carefully titrate the ascorbate with an exact amount to convert it to DHA, otherwise you are blowing away the DHA as well?

Good point.
Difficulty is that I am not sure if the reaction could go directly ascorbate -> threonate/oxalate. It appears my test cannot prove no effect of DHA for this reason.
However, if it is found that the film is blackened rapidly,this would indicate DHA derivative action.
I hope to repeat the test with more but smaller increments of permanganate.
I did not find a supplier of DHA at consumer price.

 

[Rudeofus]

Alan, I'm not sure about your conclusion, since permanganate is a powerful oxidizer, it would oxidize any DHA also. Wouldn't you have to carefully titrate the ascorbate with an exact amount to convert it to DHA, otherwise you are blowing away the DHA as well?

Also, the document states quite clearly that DHA is a stronger reducer than AA, therefore we are not going to see much DHA in such a solution.

Sigma Aldrich sells a kilogram of AA in reagent grade quality for 121 Euros. A single gram of DHA can be had for 142 Euros, that's over thousand times the price! Something tells me that DHA synthesis is not a simple process ...

 

[Rudeofus]

PS: we have a somewhat similar situation with HQ: the first oxidation stage of HQ is a much more active developer than HQ itself, leading to super high contrast when it isn't scavenged by Sulfite (cf. lith development). We may be able to prove DHA action by introducing a compound which rapidly removes DHA but leaves AA untouched. No idea what would work for this purpose, though ...

 

[sfaber17]

On the other hand, earlier when I was doing iodine titrations of ascrorbic acid and it did lead to a clear endpoint with starch, so you would think that the iodine would have continued to take out the DHA if it is that good a reducing agent. Maybe I need to check the ratios of equivalents there. Guess I could repeat that experiment and see if the DHA, assuming there is any, can develop film. From what I remember, the DHA was supposed to take some hours to degrade to your threonic acid.

 

[Rudeofus]

From what I remember, the DHA was supposed to take some hours to degrade to your threonic acid.

The article Alan linked to states that DHA deterioration is very pH dependent and that at high pH as typically seen in developers DHA is gone within minutes. What was the pH you did your titration at?

 

[sfaber17]

I remember that paper you mention. I scanned it a bit again and believe the deal is that DHA is formed from 2 equivalents of iodine, so there is an intermediate oxidation state product at low pHs that is reversible and that product is the one that is the powerful reducing agent we were talking about. It says the DHA does not degrade via redox reactions. I agree it degrades faster in high pH, but the gone within minutes statement I think they were referring to in vivo. The paper is very old and the structures they give are just guesses and are wrong. I was doing titrations of pure ascorbic acid solutions so it was acidic. It is known that our bodies convert between ascorbic acid and the oxidized form.

 

[Alan Johnson]

I did try oxidizing the ascorbate/carbonate with iodine dissolved in isopropanol but it appears to coat the film with iodide which takes a very long time to fix so I changed to permanganate.
Attached, a more controlled experiment. 12g/L and 6g/L permanganate are thinner than ascorbate alone.So the 6g/L in particular shows that if DHA provides a developer at all it is less active than ascorbate alone.

 

[Rudeofus]

Alan, if you do the math, 1 mol of KMnO4 will oxidise 0.75 mol AA in your titration ( (158*10)/(176*12) = 0.748 ). If KMnO4 accepts three electrons per molecule (pH somewhere between 10 and 11), AA would donate four per molecule. This is a strong indication to me that DHA was not present in quantity at the time you had 12 g/l KMnO4 in your solution, since AA needs to donate only two electrons to form DHA as far as I remember.

 

[Alan Johnson]

3( C6H7NaO6 ) + 2 KMnO4 -> 3 (C6H5NaO6 ) + 2KOH + 2MnO2 + 2H2O
Ascorbate,,,,,,,,,Permanganate......Dehydroascorbate...etc
3x198.1.............2x158.0
594.3..................316
10g.....................5.3g
By this calculation, more than 5.3 g/L permanganate oxidizes beyond DHA.
That is assuming DHA does not immediately decompose within 15 min, the development time.
In my experiment however, holding the film to the light even 3g/L permanganate gave less density than none at all.
The solution deposits a brown precipitate at the pH used which is why I put Manganese Dioxide as a product.

Conclusion so far is still the same, no developing due to DHA detected, but with limited data.

 

[Rudeofus]

In my experiment however, holding the film to the light even 3g/L permanganate gave less density than none at all.

This fits nicely with Pat Gainer's observation that Phenidone plus Ascorbate's activity strongly depend on Ascorbate concentration at low Ascorbate concentrations, see this graph from this article. Your test doesn't use Phenidone, of course, but you don't try to measure film speed either. Your measurement is about activity of Ascorbate, therefore I think these two scenarios are somewhat comparable in this regard.

The solution deposits a brown precipitate at the pH used which is why I put Manganese Dioxide as a product.

I expected this, and this is why I wrote "If KMnO4 accepts three electrons per molecule". Note, that in acidic environment Permanganate would accept five electrons, and that Ascobate oxidation lowers pH, but with 30 g/l Sodium Carbonate in your soup I think this is a non-issue.

 

[Photo Engineer]

You are ignoring one fact. KMNO4 is a powerful oxidant for Silver and is used in bleaches. The KMNO4 could be bleaching silver at the high concentration.

I believe that Silver metal is being oxidized as fast as it forms. The rightmost sample looks like a partially bleached sample! Washing and redevelopment might reveal the Dmax, or fixing might clear those areas. IDK if that will help though.

PE

 

[Alan Johnson]

PE,
You have to wait for the video to come out. Undisclosed was the fact that the reaction of the purple permanganate with ascorbate to produce the brown precipitate of manganese dioxide is , with stirring, all over in a few seconds.For the rest of the 15 min development time the silver bromide is exposed only to ascorbate/carbonate (+dehydroascorbate?) solution.

 

[Photo Engineer]

My point was that any KMNO4, while present can have 2 functions here. We should look for 2 possible paths no matter how fast they take place and no matter to what extent. What if you lose 20% of the silver? IDK. I do know that KMNO4 can be very fast.

PE

 

[Alan Johnson]

I modified the test to try to avoid any solvent effect of KMnO4.The initial mix of ascorbate/carbonate solution and permanganate (added in solution) was stirred until the purple color was gone (a few seconds) before adding the film and commencing development. The attachment shows the density for various amounts of KMnO4 added to 30g/L sodium carbonate anh + 10g/L sodium ascorbate at 22.5 C for a development time of 10 min. Samples were photographed stuck on a window as this shows density differences better than the previously used lightbox.The KMnO4 additions were 0, 1.0, 2.0 and 4.0 g/L.

The conclusion is unchanged,with better data now.The fall off in density is probably due to the decrease in ascorbate concentration as it is lowered by the KMnO4 additions.
No effect of DHA derivatives as developers was found.

 

[Alan Johnson]

The orange color

Orange is the color to which ascorbate developers not containing sulfite oxidize in air.I noticed that the four solutions of 30g/L sodium carbonate + 10 g/L sodium ascorbate +0 , 1, 2, 4 g/L potassium permanganate started to go orange and photographed them after 4hrs and 28 hrs (attachments DHA 4hrs and DHA 28hrs).
Since these solutions were expected to contain dehyroascorbate (DHA) , and it is suggested in the link from Rudeofus in post 33 p245 table II this will slowly decompose to diketogulonic acid ion, the orange color may be that of diketogulonic acid.
It is known from recent work on ascorbate oxidation in the lens of the eye that other compounds can be formed but I have not investigated further.

 

[sfaber17]

endpoint

3( C6H7NaO6 ) + 2 KMnO4 -> 3 (C6H5NaO6 ) + 2KOH + 2MnO2 + 2H2O
Ascorbate,,,,,,,,,Permanganate......Dehydroascorbate...etc
3x198.1.............2x158.0
594.3..................316
10g.....................5.3g
By this calculation, more than 5.3 g/L permanganate oxidizes beyond DHA.
That is assuming DHA does not immediately decompose within 15 min, the development time.
In my experiment however, holding the film to the light even 3g/L permanganate gave less density than none at all.
The solution deposits a brown precipitate at the pH used which is why I put Manganese Dioxide as a product.

Conclusion so far is still the same, no developing due to DHA detected, but with limited data.

Sorry to go back so far, but I was wondering if your permanganate is quoted as final concentration after mixing? Also, the calculation should use the molecular weight of ascorbic acid, since that is what you started with at 10g/L, not sodium ascorbate.

 

[Alan Johnson]

Yes,KMnO4 concentration is g/L added to the ascorbate/carbonate solution.Sodium ascorbate not ascorbic acid was used throughout.
A new result is that if KMnO4 is added to the orange solution it becomes colorless (oxalic +threonic acid?) and deposits a brown precipitate (MnO2?).
This agrees but does not prove that the orange color is diketogulonic acid ion.

 

[Alan Johnson]

The yellow color

Yellow is the color of old oxidized ascorbate/sulfite developers eg Xtol.
In a quick experiment I dissolved 1/4 tsp each of sodium ascorbate, sodium sulfite and sodium carbonate in water and added potassium permanganate solution.
This produced a clear yellow solution with no precipitate.
Usually the ascorbate gets oxidized before the sulfite.
The yellow appears to be some kind of ascorbate-sulfite compound and stops the reaction going to the orange diketogulonic acid.