Ascorbate Chemistry 1

[sfaber17]

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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.

So you add some permanganate to get the orange color, and then more permanganate makes it colorless.
It would be nice to know when all the ascorbate was converted but can't compute anything without the
starting volumes and the volumes of permanganate solution that were added.

 

[Alan Johnson]

So you add some permanganate to get the orange color, and then more permanganate makes it colorless.
It would be nice to know when all the ascorbate was converted but can't compute anything without the
starting volumes and the volumes of permanganate solution that were added.

Yes, a quantitative test could confirm complete oxidation to dikeotgulonic acid if the reaction follows the equation but for the time being I would like to find out more qualitatively about the production of the yellow color.
Manana.

 

[Alan Johnson]

I repeated the test done for the orange color, except I added sulfite. Bicarbonate not carbonate buffer was used to give a pH similar to that of solvent ascorbate/sulfite developer.
The solution was now Sodium Ascorbate 10g/L, Sodium Bicarbonate 30g/L, Sodium Sulfite anh 50g/L.
When potassium permanganate solution was added there was at first a brown precipitate, presumed Manganese dioxide, equation in post 52, but this soon disappeared to give a yellow colored solution and a white cloudy precipitate (not identified).
The yellow color was deeper with more permanganate added, which may simulate progressive change with time for the oxidation of ascorbate/sulfite developers in air.The yellow did not change in depth from 4hr to 28 hr (attachments).

This is my suggestion for the formation of the yellow color:
(1) DHA formed due to oxidation reacts with sodium sulfite to give sodium bisulfite plus sodium hydroxide.
I don't have a reference for this but experimentally the pH of the sample with the highest amount of permanganate added (8g/L) was about 1 unit higher than the sample with zero permanganate addition after 4 hrs.
It may be similar to what happens with acetone/sulfite, eg in Bishop's developer.
(2) DHA reacts with sodium bisulfite.DHA is a lactone and I believe unsaturated on the carbon in the ring connecting to the side chain COH. According to this reference, unsaturated lactones react with bisulfites:
https://books.google.co.uk/books?id...h3vogbO#v=onepage&q=lactone bisulfite&f=false

From this I conclude that the yellow color may be a DHA-Bisulfite compound.