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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: QuantumSoda on April 04, 2019, 10:56:04 PM

Title: Can anyone tell me anything about this weak acid, its pKa value, any info?
Post by: QuantumSoda on April 04, 2019, 10:56:04 PM
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.
Title: Re: Can anyone tell me anything about this weak acid, its pKa value, any info?
Post by: Borek on April 05, 2019, 02:48:59 AM
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.
Title: Re: Can anyone tell me anything about this weak acid, its pKa value, any info?
Post by: jeffmoonchop on April 05, 2019, 11:57:10 AM
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

Title: Re: Can anyone tell me anything about this weak acid, its pKa value, any info?
Post by: Babcock_Hall on April 05, 2019, 12:35:03 PM
It may be worth bearing in mind that a thermodynamic pKa is tabulated with respect to zero ionic strength.  On the other hand the pKa that one might measure in an experiment is often obtained when the ionic strength is far from zero.