Question regarding the dissociation pKa(s) of NaHCO3 (Baking Soda)

[Silver_Is_Money]

Some sources list two pKa's at around 6.38 and 10.33 for Baking Soda (with some variability in these values). Other sources say that this is emphatically all wrong and that 6.38 is the pKa of Carbonic Acid, and therefore in an acidic environment the single pKa at 10.33 is the only one that should be used for Baking Soda. As far as I can tell, AJ deLange always promoted the use of both pKa's simultaneously as pKa1 and pKa2, but of late I'm thinking this to be incorrect, and just as for others, with specifically acidic environs in mind. If, OTOH, you are reacting Baking Soda and Sodium Hydroxide, which is a strong base, then for that case (and similar cases) Baking Soda assumes the role of a weak acid proton donor, and the 6.38 pKa becomes valid.

I'd like to hear your thoughts on this, because it makes a huge difference as to how basic Baking Soda really is when confronting an acidic environment. And if (for the case of an acidic environment) Baking Soda is best considered to have only a single pKa at 10.33 then at ballpark 5.4 pH it nearly fully dissociates.

The difference is such that (by AJ's way of looking at Baking Soda, or more properly how I perceive AJ's way, which admittedly may be wrong) at pH 5.4 its strength is about -10.775 mEq/gram, but if only the 10.33 pKa matters in an acidic environment its strength is about -11.904 mEq/gram. The error is at a magnitude of around 10%, and 10% error is totally unacceptable.

 

[Silver_Is_Money]

When I conducted my relatively recent experiments in neutralizing Alkalinity as induced into distilled water via the addition of a measured quantity of Sodium Bicarbonate (Baking Soda, NaHCO3), as accomplished via neutralization to a mildly acidic state of 5.58 pH via the addition of Citric Acid, I achieved an average neutralization to 5.58 pH only after making the presumption that Citric Acid was much stronger an acid than I had initially presumed (whereas before making that assumption I was overshooting and seeing noticeably lower pH's). But was I in fact measuring Citric Acid that was much stronger than initially presumed, such as I had concluded at that time, or was I actually witnessing Sodium Bicarbonate that is a much stronger base than I had initially presumed due to its primary pKa being ~10.33 instead of ~6.38? Or did I observe a combination of both?

I'm now concluding that it is likely a bit of both. What I observed in deviation from prediction (whereby at the time I called it an experiment gone awry) was (in my current way of looking at it) due to two initial presumption errors. The first presumption error being that Citric Acid's pKa3 was initially presumed to be 6.4, when I now believe it is more likely to be between 5.19 and 5.40, and the second presumption error being that NaHCO3 has a first pKa at 6.38 that is valid for use in both an alkaline and an acidic state.

When I set Citric Acids pKa3 to 5.3 (roughly splitting the difference between 5.19 and 5.4) and I abandon the 6.38 pKa for Baking Soda, leaving only the 10.33 pKa as relevant for common mash and Wort pH's, the dissociation based math model of the interaction between these two reactants highly matches and agrees with my experimental titrations that achieved a pH of (on average) 5.58.