Potassium Carbonate + Ascorbic Acid reaction

The pKa values of Ascorbic Acid are 4.17 and 11.57 respectively. This is not expected in an alcohol but is possible due to the enol structure of the fundamental compound with its double bond. This means that it acts like an acid with two acidic groups, one far weaker than the other.

So, I agree with you but again stress the precision factor. Due to the weakness of the neutralization, you could overshoot the target or undershoot it and pH is therefore a better measure of this than anything else. That is my contention. This is especially true in developers where there are a lot of other ingredients present.

I've learned this the hard way over years of R&D compounding developers, fixes, bleaches and blixes. It is why I have 2 pH meters and why I use them to tweak the final pH to the exact value with acid or base using a calibrated pH meter.

PE

Yes, the achievement of exact neutralization point is one side, but the number of neutralization step(s) is the second.

I don't polemize with the first thing, I agree with you fully, but I only protect my argument (btw. I've already talked about enol phenomenon):

H2O said:

AA has several hydroxyl groups in "enol" configuration and they are strong acid. Molecule behaves as monohydric acid [pK(a)=4,0]!

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Yes, I didn't point out medium potassium carbonate, but it was speach about it.

You "corrected" me:

Photo Engineer said:

Ascorbic acid effectively has 2 acid groups that can be neutralized, although only neutralizing one is most common...

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Ascorbic acid has only 1 acid group in potassium carbonate medium.

I am talking about practice and you are talking theory. Your theory is absolutely correct, but in practice it has "loopholes".

I will give an example. A developer contains Metol (which is a Sulfuric acid salt of a weak base), Sulfite, Carbonate and Ascorbic acid. This complex mixture is desired to be at pH 10.0 +/- 0.1 pH units at 20 deg C. In fact, adding the equivalent amount of base (carbonate) to achieve this is very difficult due to the mixture of components. Neutralizing the Ascorbic Acid is possible, but I have seen errors greater than 0.5 pH units due to all of the things I cited in my previous posts, particularly when one adds the rest of the ingredients.

Therefore, it is most useful to add the amount of Carbonate which we wish for the desired buffering capacity, then all of the remaining ingredients, (or vice versa in some cases, adding all of the other ingredients and then the carbonate) and then measuring the pH. If it is within range, then you tweak it with Sodium Hydroxide or Sulfuric Acid, otherwise something is wrong in either buffer capacity or the acidity of one of the ingredients.

Simply adding a mole of carbonate for two moles of Ascorbic Acid will probably be correct in getting the AA neutralized but leaves the question open of what pH has been achieved with the AA, and what will the effects of the Potassium salt of AA be on the pH of the remaining ingredients.

Of course, this gets even worse if one is working on a high contrast developer with a pH of 11.5 or so. Then you have the second pKa to contend with. But, as you say, you will not be at this pH with Carbonate.

In any event, my discussion centers around good practice in the lab which will yield consistent results without tedious calculations and the possibility of error or variability from batch to batch. We are saying the same thing, but from different angles. I hope you can see this.

And yes, I do know that you mention the enol form. I merely stress it to show that AA is quite complex and not a normal organic acid.

PE

OK.
I agree with you too. But intro question was:

Athiril said:

Being a theoretical chemistry newb, I was wondering to what ratio per gram do I add potassium carbonate to ascorbic acid to have total conversion to potassium ascorbate?
And how many grams do I get of potassium ascorbate?

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Nobody knew if Athiril wants to build the factory for producing potassium ascorbate (ONLY)
Without any remaining ingredients.

Photo Engineer said:

...Simply adding a mole of carbonate for a mole of Ascorbic Acid will probably be correct in getting the AA neutralized but leaves the question open of what pH has been achieved with the AA, and what will the effects of the Potassium salt of AA be on the pH of the remaining ingredients...

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And:
a mole of carbonate for two mol of AA will be correct ...
Sorry
Thanks for discussion.

Yes, two moles. Sorry and thanks for the catch of my typo.

As for the rest, I assume use in a developer due to the forum in which this was posted. Maybe that was presumptuous of me, but it tried to answer the question in the forum context which most often includes the use of AA in developers. And my advice was practical advice based on laboratory experimentation.

PE

Only short comments.
Every good, effective, successful experiment(s) in lab must be supported by good knowledge of theory. And any problems should be solved from more simply to more complex, both in theory and in a lab.
I thing that is clear for both of us.

Alan Johnson said:

K2C03 +.......... 2C6H8O6.....->2KC6H7O6........+H2O...+CO2
Pot Carbonate..Ascorbic Acid ...Pot Ascorbate...Water..Carbon Dioxide
138.2g............2x176.1g.........2x214.2g...........18g.....44g
138.2g+..........352.2g.........->428.4g............+18g....+44g
ie mixing 138.2grams potassium carbonate and 352.2 grams ascorbic acid gives 428.4 grams potassium ascorbate.
Unusual chemical,wonder what that is for.

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What about using potassium bicarbonate instead?