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Topic: Acid-Base calculations giving me a headache  (Read 383 times)

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Offline Mnemonic Entity

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Acid-Base calculations giving me a headache
« on: June 18, 2022, 08:26:14 PM »
I have a rather simple problem, but I was never very good at acid-base chem math--I am more of an algebra guy and so I have trouble remembering when to switch one variable out with another, etc.

I am trying to make a 10% N-acetylcysteine solution, pH corrected with NaOH to a pH of 6.5. I have already done the math and a 10% NAC solution is around .613M, which yields a solution with a pH of 1.73. Now, I just need to figure out how to account for the reactions between the NAC conjugate base and the NaOH.

As a type this post, I'm suspecting that, as with much in acid-base math, I can just ignore the [A-] at such a low pH? I mean, if the pKa is 3.24, then there should be at least some sort of [A-] value, right? I can never feel confident with my conclusions on acid-base math, darnit!

Offline Borek

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Re: Acid-Base calculations giving me a headache
« Reply #1 on: June 19, 2022, 03:12:51 AM »
pH 6.3 is pretty close to neutral and so far from the pKa of 3.24 that the only sure way of making such a solution is to use pH meter. You are in the area where adding just few drops more of NaOH changes pH by several units.
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Offline Mnemonic Entity

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Re: Acid-Base calculations giving me a headache
« Reply #2 on: June 19, 2022, 02:58:36 PM »
I've seen that there are ways, though, of calculating the pH of solutions after they've been added. Surely there is a way of reverse-engineering those calculations to get the amount of base needed from for a specific pH, right? This is math, and I have all of the other variables solved for

Don't I? or is it that I can't know for sure the dissociation of the NAC as I add NaOH? Is the problem that this is a multivariate question and I'm thinking of it as univariate?

Offline Borek

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Re: Acid-Base calculations giving me a headache
« Reply #3 on: June 19, 2022, 03:25:45 PM »
You are right it can be calculated, trick is, accuracy of the volume measurement makes the calculation result an art for art's sake.

I just did a quick check using BATE, pH calculator. 10.0000 mL of 0.1000 M acid of pKa 3.24 mixed with 9.9944 mL of 0.1000 M NaOH yields solution with pH 6.5. Great, exactly what you want! But if you take 10.00 mL of the same NaOH solution final pH is almost 8 (7.96). There is no way you will be able to measure volume with accuracy high enough. And that was all done under assumption we know exact concentrations. We don't.

Do you see why trying to prepare the solution based on the calculation result is a waste of time? In this particular case if what you need is a solution of a given pH forget about math, just titrate against pH meter. Much faster, much more accurate, much better.
« Last Edit: June 19, 2022, 06:50:07 PM by Borek »
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Offline Babcock_Hall

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Re: Acid-Base calculations giving me a headache
« Reply #4 on: June 20, 2022, 08:14:41 AM »
Obviously I agree with Borek, but I would like to add that there is also the pKa of the sulfhydryl group to consider.  By the way, once you prepare this solution, be sure to minimize its exposure to oxygen so that the sulfhydryl group does not oxidize.

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