Why don't effervescent vitamin C tablets react with the vitamin C?

Question

I have a packet of effervescent Vitamin C tablets. When placed in a glass of water, a tablet will fizz and dissolve. They have the following ingredients (all included for completeness):

• citric acid
• ascorbic acid
• sodium hydrogen carbonate
• sorbitol
• sodium carbonate
• inulin
• potato starch
• zinc citrate
• tricalcium phosphate
• maltodextrin
• riboflavin phosphate sodium
• sweetener & flavours
• tocopherol

I'm guessing that the citric acid is intended to react with the sodium hydrogen carbonate to effervesce.

Given that vitamin C (ascorbic acid) is also an acid, surely it could also react with the sodium hydrogen carbonate, removing the vitamin C from the finished drink?

Does this happen to any extent? If so, in what proportions? If not, why not?

The ingredients state how much vitamin C is present in the tablet, not in the finished product. This is also specified as a proportion of government recommended recommended daily amounts, which suggests that one would receive that amount if one took a dose as directed. If the vitamin C does react, does this mean that the label doesn't indicate how much vitamin C is available?

(I last studied chemistry at GCSE level in [high] school, so please excuse the lay question. A simple answer would be most useful!)

Answer 1

Acid-base reactions are fast and reversible (barring any additional reaction that occurs). That is, once you move the conjugate base of ascorbic acid to a low-pH environment (e.g. your stomach), it rapidly reverts to the protonated form. (Ascorbic acid does not undergo any pH-dependent further reactions, as opposed to carbonate, which when protonated to carbonic acid can decompose to carbon dioxide and water.)

Actually, as the pKa (roughly the "halfway pH" of acid/base reactions) of ascorbic acid is 4.2, the moment the protonated ascorbic acid enters your bloodstream (roughly pH 7.3), it immediately loses that proton again, reverting to the deprotonated ascorbate ion.

So in a practical sense, it really doesn't matter which form you take your Vitamin C in - either (protonated) ascorbic acid or (deprotonated) ascorbate acts in you body the same way, because they can rapidly interconvert based on the pH of the solution they happen to be in at the time.

(Because they typically work in buffered aqueous environments, biochemists typically play fast-and-loose with the difference between the protonation states of molecules. Unless you're explicitly dealing with pH-dependance, they're typically lumped all together, with the implicit understanding that at a given pH they'll adjust to the appropriate relative concentration. This can get confusing if you're a stickler for nomenclature.)