Specialty Chemistry Forums > Other Sciences Question Forum

pH of weak acid after neutralizing

<< < (2/2)

chenbeier:
In excess of NaOH you will have mostly PO43-. Borek wrote the concentration already.

yene:
Thanks for the reply. so is the answer "yes"? Does the weak acid eventually fully dissociate?

Also, Borek said that in the pH of 13-14 range we will see mostly phosphate ions.
I guess I should have clarified. In my situation, I have excess of sodium hydroxide, but not very much. It's a very diluted NaOH solution with pH of 10-11.

Borek:

--- Quote from: yene on February 03, 2020, 11:50:10 AM ---The Ka for H2Po4 dissociation and HPO4 dissociation is way lower than that of the H3PO4 dissociation, therefore my understanding is that those subsequent reactions are negligible in this situation regarding Hydrogen production.

So just neglecting those subsequent reactions for now: in a solution with excess OH-, does the weak acid eventually fully dissociate?
--- End quote ---

You can't neglect them. Concentrations of all forms depend on the pH, this in turn depends on the amount of base added.

To get exact concentrations you will need to solve full set of equilibrium equations (dissociation steps, mass and charge balance). For some acid/base combinations approximations can be used, but they are never trivial if you don't know how to solve whole system.

Alternatively, there is a software that allows such calculations: https://www.chembuddy.com/?left=BATE&right=pH-calculator

yene:
For my system, I am not looking to bring the solution to an exact pH. I'm just looking to bring the solution to a pH somewhere within 5.5 and 9.5.

So for my application, do I really need to consider these subsequent reactions? Especially if the solution is never expected to get above pH of 12 or so?

And just for my education: if we had a weak monoprotic acid (i.e. that didn't have subsequent reactions), like nitrous acid, would all of the hydrogen be eventually dissociated and consumed in a neutralization with NaOH?

Thanks.

Borek:
You will always have some equilibrium concentration of the protonated acid, no matter what the pH is. It can be so low it is negligible for most applications, but it will always be there. You don't need to worry abut it if all you need is a given pH range.

Just neutralize first proton completely and the second to 50%, you will get a buffer with pH=pKa2.

Navigation

[0] Message Index

[*] Previous page