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Offline schnorch

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pH buffer ingredients questions
« on: October 08, 2020, 01:15:38 PM »
I know that I need a weak acid and its conjugate base for making a pH buffer (or a weak base and its conj. acid).
For example citric acid and sodium citrate.

My first question is: Can I make the same buffer from citric acid and sodium hydroxide? Of course I know that I need different amounts for making this:

Solution A: 10mol citric acid + 3mol sodium hydroxide 
Solution B: 9mol citric acid + 1mol (tri-)sodium citrate

Solution B needs three H2O molekules that the numbers add up.
Citric acid is C6H8O7, sodium hydroxide is NaOH and (tri-)sodium citrate is C6H5Na3O7

Do both solutions behave identical?
I think, yes they do. But perhaps there is a detail I am missing?


Second question:
What happens to the Na+ in such a buffer solution? Does it connect to citrate anions? Or remains it mainly dissociated?


Thank you in advance.

Offline Meter

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Re: pH buffer ingredients questions
« Reply #1 on: October 08, 2020, 02:02:09 PM »
I know that I need a weak acid and its conjugate base for making a pH buffer (or a weak base and its conj. acid).
For example citric acid and sodium citrate.

My first question is: Can I make the same buffer from citric acid and sodium hydroxide? Of course I know that I need different amounts for making this:

Solution A: 10mol citric acid + 3mol sodium hydroxide 
Solution B: 9mol citric acid + 1mol (tri-)sodium citrate

Solution B needs three H2O molekules that the numbers add up.
Citric acid is C6H8O7, sodium hydroxide is NaOH and (tri-)sodium citrate is C6H5Na3O7

Do both solutions behave identical?
I think, yes they do. But perhaps there is a detail I am missing?


Second question:
What happens to the Na+ in such a buffer solution? Does it connect to citrate anions? Or remains it mainly dissociated?


Thank you in advance.
The solubility of trisodium citrate is quite high so the majority of the salt will dissociate.

Offline Babcock_Hall

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Re: pH buffer ingredients questions
« Reply #2 on: October 08, 2020, 03:03:43 PM »
IIRC species such as (pyrophosphate)4- have measurable dissociation constants with mono cations.  It would not surprise me to learn that sodium ion associates with citrate3-, but I am not certain.

Offline Borek

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Re: pH buffer ingredients questions
« Reply #3 on: October 08, 2020, 04:26:49 PM »
Doesn't matter if you prepare buffer from acid and salt or acid and base, as long as the final composition is identical.

The more concentrated the solution, the higher tendency of cations and anions to combine in solution and create ion pairs, google "ion association". Note that it is a rather advanced concept that doesn't change much in the theory of acids, bases and buffers (stability constants for ion pair being orders of magnitude lower than dissociation constants).
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Offline schnorch

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Re: pH buffer ingredients questions
« Reply #4 on: October 09, 2020, 10:01:30 AM »
Thank you.

Yes, it makes sense.
I had found a few opinions online like "you can't make a buffer with NaOH, you need the konj. base of citric acid" what made me skeptical. There is much contradicting information about chemistry topics in internet unfortunately. Even with basic questions like this.

Offline Borek

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Re: pH buffer ingredients questions
« Reply #5 on: October 09, 2020, 12:07:42 PM »
Chances are it was about something else - people mistake "base" in the buffer with NaOH and attempt to calculate pH using concentrations of (for example) acetic acid and NaOH used for preparation. That is definitely incorrect.
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Offline AWK

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Re: pH buffer ingredients questions
« Reply #6 on: October 09, 2020, 12:44:35 PM »
Solution A: 10mol citric acid + 3mol sodium hydroxide 
Solution B: 9mol citric acid + 1mol (tri-)sodium citrate

You can check by simple stoichiometry that both solutions have the same mole ratio of citric acid to monosodium citrate.
AWK

Offline Borek

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Re: pH buffer ingredients questions
« Reply #7 on: October 09, 2020, 12:55:59 PM »
Solution A: 10mol citric acid + 3mol sodium hydroxide 
Solution B: 9mol citric acid + 1mol (tri-)sodium citrate

You can check by simple stoichiometry that both solutions have the same mole ratio of citric acid to monosodium citrate.

Hardly surprising, he designed these solutions to be this way :)
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Offline schnorch

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Re: pH buffer ingredients questions
« Reply #8 on: October 09, 2020, 02:29:28 PM »
For example the comments in a youtube video about citric acid buffers for alcoholic fermentation:

Quote from: Me
Is it possible to replace the sodium citrate with sodium hydroxide?
The hydroxide would also form anions with the citric acid, right? I think the same anions. Of course it would need a different ratio of acid and hydroxide compared to acid and citrate. But it should be possible to get exact the same buffer solution this way.
Or is there something I am not aware of?
Quote from: author of the video
No. The reason why buffers need their conjugates, is because the conjugate don't really react with the weak acid/base, and they don't react with the water itself. If you mix citric acid and sodium hydroxide,  the sodium hydroxide will react with the citric acid, and give you sodium citrate, and more water. You also want a base that slowly disassociates as needed, not a strong base like NaOH that will disassociate all at once.
Quote from: Me
But isn't that exactly the same? For example:
2mol citric acid + 3mol sodium hydroxide => 1mol citric acid + 1mol sodium citrate (+ 3mol H20)
So what is the difference between mixing 2mol citric acid with 3mol sodium hydroxide and mixing 1mol citric acid with 1mol sodium citrate?
Quote from: author of the video
You will be wastinrg sodium hydroxide, why not just add sodium citrate. Also, you don't know which form the sodium citrate will take.

Shall I post the link?

Offline Borek

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Re: pH buffer ingredients questions
« Reply #9 on: October 09, 2020, 03:42:13 PM »
Nah, sound like the guy has no idea what he is talking about. Happens.
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