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Topic: Titration - pH at equivalence point  (Read 43837 times)

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Offline Borek

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Re: Titration - pH at equivalence point
« Reply #30 on: June 30, 2006, 05:29:36 PM »
I am sorry but I am not able to understand what are you asking about, so I can't help you. Try to reword it.

Only thing I am sure about is that

pkb1= 14 - pka1
pkb2= 14 - pka2
pkb3= 14 - pka3

is wrong - check out these polyprotic dissociation constants definitions (check out indexes).
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Offline refid

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Re: Titration - pH at equivalence point
« Reply #31 on: June 30, 2006, 06:33:18 PM »
from the monoprotic acid titration curves is there relationship between the pka/pkb with pkw
like
14- pka = pkb

Offline Borek

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Re: Titration - pH at equivalence point
« Reply #32 on: June 30, 2006, 06:56:23 PM »
This relathionship doesn't have anything to do with titration curve - it describes dissociation of acid and conjugated base. For example for acetic acid dissociation constant:

CH3COOH <-> H+ + CH3COO-

Ka = [H+][CH3COO-]/[CH3COOH]

conjugated base is acetate, and its basic dissociation looks like:

CH3COO- + H2O <-> CH3COOH + OH-

Kb = [CH3COOH][OH-]/[CH3COO-]

(water concentration assumed constant and omitted).

You may check that Ka*Kb = Kw, thus pKa + pKb = pKw (it is also described in the lecture on Bronsted-Lowry theory).

So this is a general relationship - it holds for titration curve as well, but that's only a specific case, one of many.
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Offline refid

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Re: Titration - pH at equivalence point
« Reply #33 on: July 01, 2006, 07:19:19 PM »
Oh I see, thanks for your help and clear up that matter for me  :D

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