Ascorbate - active form?

[alanrockwood]

This might be a good question for PE. What is the active form of ascorbate in an ascorbate buffer? Also, is it the same form for regenerating a primary developer, such as metol (or p-aminophenol) in a mixed developer containing ascorbate?

Hypothesis: It is the doubly ionized form that is effective in direct development, but the singly ionized form would be active in regenerating the primary developer.

This could explain why ascorbate is active as a direct developer at very high pH, but at lower, e.g. around pH 9.5 or lower it would not be active as a direct developer but could act to regenerate the primary developer in a mixed metol/ascorbate or p-aminophenol/ascorbate developer.

 

[Alan Johnson]

Did they close the labs before this was investigated? Here's a guess, AH-
http://www.tandfonline.com/doi/full/10.1080/05704928.2014.952431#inline

 

[sfaber17]

That paper above shows a lot about the structures of the acid-base forms of AH2 but not the oxidation mechanisms. A search on the reduction potentials of AH2 pulled up some original paper from the 30s where they measured the reduction potential where it was reversible below pH of 4, but at high pH, it was unstable. The dehydroascorbate was found to decompose within minutes. They were just guessing at the structures of the products back then, but later works found that at the low pHs, the AH- would oxidize to the semidehydroascorbate form that was reversible, but to work as a developer you need the more basic environment where there is more A-2 that then oxidizes to dehydroascorbate - where both OH go to double bond O. That is what you read from most chemistry class experiments online, but apparently the dehydroascorbate is in fast equilibrium with water where the middle double bond O changes to 2 OH and that decays to oxalic acid and threonic acid (4 carbon sugar acid) with a rate constant in minutes.