[QuantumSoda]

https://www.sigmaaldrich.com/catalog/substance/sodiumcitratedibasicsesquihydrate26311613205411?lang=en&region=US&attrlist=Brand|Assay

I'm doing a weak acid lab and this is on the list of potential things it could be. We got a pKa of around 5.9 and a molar mass of around 260, so I'm fairly certain it's this, but I can't find any information about it. Also, is it monoprotic? I'd think it is, with the dibasic in the name, but my lab partner isn't buying it.

[Borek]

pKa values for the citric acid are rather easy to google.

Two of the protons are already neutralized, so technically what you are dealing with behaves like a monoprotic acid. I wouldn't call it a monoprotic acid though, it is an acid salt with a single not neutralized proton.

[jeffmoonchop]

The central acid group is most acidic and most likely to form salts, check out my publication of citric acid isonicotinamide polymorphic cocrystals, interestingly, the salt form is the least stable compared to the 3 neutral polymorphs.

https://pubs.acs.org/doi/10.1021/acs.cgd.8b00597

[Babcock_Hall]

It may be worth bearing in mind that a thermodynamic pK_a is tabulated with respect to zero ionic strength.  On the other hand the pK_a that one might measure in an experiment is often obtained when the ionic strength is far from zero.