Iso-ascorbate vs. 'normal' ascorbate and their developing properties.

[Formulahunter]

In basically all of the published Patents and MSDS's the isoascorbate vesrion or the isoascorbic acid is mentioned. There is a paper mentioning that isoascorbate is faster acting then ascobate. https://pubs.acs.org/doi/pdf/10.1021/ja01229a026
Does anyone know more about this topic? Until now I thought isoascorbate was only used due to it beeing cheaper in larger quantities since it has less uses, and beeing a byproduct of vitamin-C production.
Any Info is apreciated.

 

[Donald Qualls]

If I understand correctly, isoascorbate is the same as erythorbate -- it's the chiral opposite of ascorbate (i.e. the molecule is a 3D mirror image). Ascorbate, like most biologically active molecules, is "left handed" (in solution, it rotates polarized light anticlockwise), while erythorbate/isoascorbate rotates polarization clockwise.

As noted, synthesis of Vitamin C produces both in equal quantity, and they're then separated; they work equally well as antioxidants outside an organism, but humans and other primates can only prevent scurvy with the left-handed version. It's possible the right-handed version develops a small amount faster (or slower) at the same pH, since the methods used to separate the two isomers usually depend on either subtle differences in solubility or in activation energies for some reaction -- in other words, they aren't chemically identical, but require sensitive and subtle methods to distinguish.

 

[Brendan Quirk]

Ascorbate functions as an electron donor to reduce silver. In a non-biological process, I would not expect the chirality to matter. I could be wrong - see above.

This caught my interest, because I have wondered if people appreciate that the way developers work is to provide electrons to reduce silver ions to silver metal. Hence, the enormous variety of developing agents. Anything that can provide electrons is a potential developer. Coffee anyone? Such simple chemistry that anyone can appreciate!

 

[Formulahunter]

The above mentioned Paper sates there is a difference in acitvity at a pH of 10.3 yet none observed in more caustic enviroments.
My main Question is mostly: Is this difference neglactable. And the main reason the Iso product is used in commercial developers is the lower price, while the higher activity is mostly a a welcome side effect. Or is it integral for Formulas like XTOL, Wich Im trying to mix myself from raw, according to the Patent.
Most if not all the DIY Formulas mention Vit.C, ascorbic acid, or sodium ascorbate. This is probbably down to it beeing vitually impossible to get a holf of the iso forms in low quatntitys for a reasonable price.

 

[Alan Johnson]

I have made homebrew Xtol from the formula in the patent quoted in the film developing cookbook, using ascorbic acid from a health food store along with alkali to convert it to the ascorbate. Results were indistinguishable from the real thing.
I used EDTA instead of DTPA which is hard to get. Since this is less effective to prevent oxidation I stored the solution in a glass bottle under inert gas.

 

[albada]

I have made homebrew Xtol from the formula in the patent quoted in the film developing cookbook, using ascorbic acid from a health food store along with alkali to convert it to the ascorbate. Results were indistinguishable from the real thing.
I used EDTA instead of DTPA which is hard to get. Since this is less effective to prevent oxidation I stored the solution in a glass bottle under inert gas.

Alan, could you post the formula you used? Or can you provide a link to it if you already posted it?

Mark

 

[Alan Johnson]

Hello Mark,

Homebrew Xtol [calculated from the formula of Xtol given eg on p61 of the Film Developing Cookbook 2020].

Demin water................................................850ml .......at 40C
EDTA.....................................................................1g
Sodium Sulfite anh.......................................85g
Sodium Metabisulfite...................................3.5g
Sodium metaborate.......................................4g
Dimezone-s.......................................................0.2g.................can be substituted by Phenidone.
Ascorbic acid...................................................10.7g
Sodium Hydroxide..........................................2.4g
Demin water to..................................................1L.................pH~8.2............store in glass bottle under inert gas.

I made this up out of curiosity in 2016, results were not distinguishable from Kodak Xtol powder solution.

12 g sodium isoascorbate =10.7g ascorbic acid + 2.4g sodium hydroxide
198.1...................+18 [water]=176.1..................................40

 

[albada]

Hello Mark,

Homebrew Xtol [calculated from the formula of Xtol given eg on p61 of the Film Developing Cookbook 2020].

Demin water................................................850ml .......at 40C
EDTA.....................................................................1g
Sodium Sulfite anh.......................................85g
Sodium Metabisulfite...................................3.5g
Sodium metaborate.......................................4g
Dimezone-s.......................................................0.2g.................can be substituted by Phenidone.
Ascorbic acid...................................................10.7g
Sodium Hydroxide..........................................2.4g
Demin water to..................................................1L.................pH~8.2............store in glass bottle under inert gas.

I made this up out of curiosity in 2016, results were not distinguishable from Kodak Xtol powder solution.

12 g sodium isoascorbate =10.7g ascorbic acid + 2.4g sodium hydroxide
198.1...................+18 [water]=176.1..................................40

Thanks, Alan.

Long ago, somebody figured out that one could change ascorbic acid into isoascorbate by adding sodium bicarbonate (baking soda). The problem I had with that is that it takes around five minutes for the bubbling to stop, as CO2 is released. I like your Sodium hydroxide idea better. I suppose one could also use Sodium carbonate.

Mark

 

[GLS]

If I understand correctly, isoascorbate is the same as erythorbate -- it's the chiral opposite of ascorbate (i.e. the molecule is a 3D mirror image). Ascorbate, like most biologically active molecules, is "left handed" (in solution, it rotates polarized light anticlockwise), while erythorbate/isoascorbate rotates polarization clockwise.

Isoascorbate and erythorbate are different names for the same compound yes, but it is not the mirror image of ascorbate. Only one of the stereocenters is inverted; i.e. they are epimers, not enantiomers. In chemical nomenclature a common use of the "iso" prefix is to denote an epimer. If they were indeed enantiomers then their properties re: photographic development would be identical.

Having said that, in an achiral context such as this the difference in redox properties between them will be small, hence the result in the linked paper in the OP.

 

[Formulahunter]

This is btw. the part of the paper mentioned above, that compares the activity of the two forms

 

[Formulahunter]

Hello Mark,

Homebrew Xtol [calculated from the formula of Xtol given eg on p61 of the Film Developing Cookbook 2020].

Demin water................................................850ml .......at 40C
EDTA.....................................................................1g
Sodium Sulfite anh.......................................85g
Sodium Metabisulfite...................................3.5g
Sodium metaborate.......................................4g
Dimezone-s.......................................................0.2g.................can be substituted by Phenidone.
Ascorbic acid...................................................10.7g
Sodium Hydroxide..........................................2.4g
Demin water to..................................................1L.................pH~8.2............store in glass bottle under inert gas.

I made this up out of curiosity in 2016, results were not distinguishable from Kodak Xtol powder solution.

12 g sodium isoascorbate =10.7g ascorbic acid + 2.4g sodium hydroxide
198.1...................+18 [water]=176.1..................................40

if living in Europe/Germany one can buy 40% DTPA Na Solution from Fototechnik Suvatlar.

 

[Alan Johnson]

View attachment 339967
This is btw. the part of the paper mentioned above, that compares the activity of the two forms

The commonly quoted "Xtol Patent", US 5756271 does indeed specify isoascorbate in example 1 p7:

https://patentimages.storage.googleapis.com/1b/ac/0e/b54d27cb273759/US5756271.pdf

Does this mean that Xtol made from health food store ascorbic acid would be slightly less active?

 

[GLS]

The commonly quoted "Xtol Patent", US 5756271 does indeed specify isoascorbate in example 1 p7:

https://patentimages.storage.googleapis.com/1b/ac/0e/b54d27cb273759/US5756271.pdf

Does this mean that Xtol made from health food store ascorbic acid would be slightly less active?

Tbh I don't think the paper linked in the OP is actually all that relevant, as they are testing the developing properties of ascorbate/isoascorbate in isolation, not as a superadditive mixture with a phenidone-type developer, as in Xtol. Also the working pH is very different (~8 vs ~10).

You'll notice in the patent they also list several formulations which exclusively use ascorbic acid, but the described testing is all done with the dry isoascorbate formulation. I suspect the reason for this is that in practice it makes little or no difference which is used in this case.

 

[Formulahunter]

Tbh I don't think the paper linked in the OP is actually all that relevant, as they are testing the developing properties of ascorbate/isoascorbate in isolation, not as a superadditive mixture with a phenidone-type developer, as in Xtol. Also the working pH is very different (~8 vs ~10).

You'll notice in the patent they also list several formulations which exclusively use ascorbic acid, but the described testing is all done with the dry isoascorbate formulation. I suspect the reason for this is that in practice it makes little or no difference which is used in this case.

my though also was, that if one can't see any differece, then the superadditive effects may oversaddow the differenece in isolated activity. Also this might be a hint that the main development comes from the Dimezone, and the ascorbate 'recharges' the phenidone. Even though the XTOL patent says that they believed the ascorbate to be the main developing agent

 

[GLS]

my though also was, that if one can't see any differece, then the superadditive effects may oversaddow the differenece in isolated activity. Also this might be a hint that the main development comes from the Dimezone, and the ascorbate 'recharges' the phenidone. Even though the XTOL patent says that they believed the ascorbate to be the main developing agent

Yes, the superadditivity is no doubt key.

One nitpick though, with my chemist's hat on: it is not Dimezone used, but 4-hydroxymethyl-4-methyl-phenidone (Dimezone S).

 

[Donald Qualls]

Does this mean that Xtol made from health food store ascorbic acid would be slightly less active?

Even if it does, you'll always wind up adjusting your times to match your methods and thermometer and exposure habits, etc. If your developer comes out 10% less active, you're still within the smallest adjustment most would make.

 

[titrisol]

If I understand correctly, isoascorbate is the same as erythorbate -- it's the chiral opposite of ascorbate (i.e. the molecule is a 3D mirror image). Ascorbate, like most biologically active molecules, is "left handed" (in solution, it rotates polarized light anticlockwise), while erythorbate/isoascorbate rotates polarization clockwise.

As noted, synthesis of Vitamin C produces both in equal quantity, and they're then separated; they work equally well as antioxidants outside an organism, but humans and other primates can only prevent scurvy with the left-handed version. It's possible the right-handed version develops a small amount faster (or slower) at the same pH, since the methods used to separate the two isomers usually depend on either subtle differences in solubility or in activation energies for some reaction -- in other words, they aren't chemically identical, but require sensitive and subtle methods to distinguish.

Ertyhorbate is used in sausage making, I got a 1kg sample many years ago.
In my limited experience works the same as ascorbate.

 

[Formulahunter]

Ertyhorbate is used in sausage making, I got a 1kg sample many years ago.
In my limited experience works the same as ascorbate.

the question was mostly out of curiosity. The fact that with a 'normal' bw development process you'll probbably get bigger variation from run to run is to be assumed.