[scientizscht (OP)]

Hello

When you have a buffered solution and you put a base or acid, does this become instantly neutralised by the buffer or you need to wait for it to diffuse into the whole solution and then become neutralised? In other words, how fast does the buffer react?

Thanks

[chiralSPO]

Often, acid-base reactions are diffusion limited. You'll need to stir it (or shake it) for best results.
(EDIT: one interesting thing about aqueous acid/base reactions is that the apparent diffusion rate can sometimes be much faster than the diffusion of the specific reagents, because of how protons can move through the network of hydrogen bonds in water. https://en.wikipedia.org/wiki/Grotthuss_mechanism but it is still diffusion limited)

[chiralSPO]

Out of curiosity, why would you think that an intermolecular chemical reaction could be faster than diffusion?

[scientizscht (OP)]

Out of curiosity, why would you think that an intermolecular chemical reaction could be faster than diffusion?

My rationale is this:

If you have an e.g. neutral pH solution which you want to electrolyse to produce hydrogen 2H+ + 2e- -> H2 then you have a very low [H+] that will limit diffusion alot.

If you have a same pH solution but it has additionally a strong buffer, the protons consumed in the electrode may be faster replenished by the buffer dissociating (where the buffer concentration is much higher than the [H+]) than what simple proton diffusion would allow given the same [H+].

Is that correct?

[Bored chemist]

Water is a buffer- albeit an unusual one.
If, by whatever means, you remove H+ ions from a solution, the water dissociates and produces more of them (and OH- as a by-product)

[scientizscht (OP)]

Water is a buffer- albeit an unusual one.
If, by whatever means, you remove H+ ions from a solution, the water dissociates and produces more of them (and OH- as a by-product)

Interesting thanks. Is this faster than diffusion? Is there a delay in the dissociation or in replenishes the proton gap immediately?

[chiralSPO]

I think the source of confusion here is that the diffusion of "H⁺" in water is special. An aqueous solution with a pH of 6 has a concentration of "H⁺" of 10^–6 M. But this is really the concentration of "excess" H⁺. Because water is H₂O, the concentration of H atoms is about 110 M (more than 8 orders of magnitude higher than suggested by the pH). (note that this higher concentration doesn't change the thermodynamics or stoichiometry of acid-base reactions, but as far as kinetics are concerned, the reactions are hastened because it doesn't really matter where the H⁺ comes from--they are all equivalent.) In contrast, if there were an aqueous solution of Fe²⁺ that was only 10^–6 M, that is all the Fe in the solution.