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Offline Meter

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Clarifying titration questions
« on: October 05, 2020, 03:54:41 PM »
We are finishing up a study in acid-base chemistry  for my UG and it dawned on me that I might have some misconceptions and that I would like to see if my "new" conclusions are correct:

1. At the equivalence point of a titration curve, the initial amount of substance of the titrand is equal to the amount of substance of the titrant, but physically speaking, all of the initial amount of substance has been converted, so this new, changed amount of substance is 0 (theoretically).

2. At the half equivalence point, half of the initial amount of substance of the titrand has been converted, meaning that there is only 50% left of the initial amount of titrand.

3. For something like glycine hydrochloride when titrated with a strong base, it would mean that the isoelectric point would be at its first equivalence point, as at this point, the initial amount of substance, (which is not isoelectric) has been wholly converted into the isoelectric intermediate (center compound), meaning that neither the left or right compounds exist (theoretically) at this point in the titration, but only before and after respectively.



If this is correct, then I think I properly understand titrations.
« Last Edit: October 05, 2020, 04:53:49 PM by Meter »

Offline Borek

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Re: Clarifying titration questions
« Reply #1 on: October 05, 2020, 05:00:40 PM »
1. At the equivalence point of a titration curve, the initial amount of substance of the titrand is equal to the amount of substance of the titrant

Not exactly. You added a stoichiometric amount of titrant, doesn't mean equal. Think NaOH + H2SO4.

Quote
but physically speaking, all of the initial amount of substance has been converted, so this new, changed amount of substance is 0 (theoretically).

Yes, but no. When you titrate a weak acid at the equivalence point some of it is still in the protonated form..

Quote
2. At the half equivalence point, half of the initial amount of substance of the titrand has been converted, meaning that there is only 50% left of the initial amount of titrand.

More or less. Again, equilibrium can do some tricks and slightly change the amount.

Quote
3. For something like glycine hydrochloride when titrated with a strong base, it would mean that the isoelectric point would be at its first equivalence point, as at this point, the initial amount of substance, (which is not isoelectric) has been wholly converted into the isoelectric intermediate (center compound), meaning that neither the left or right compound sexist (theoretically) at this point in the titration, but only before and after respectively.

No, at 50% there are still other forms present, just in (almost) equal and (typically) tiny amounts.

Quote
If this is correct, then I think I properly understand titrations.

I feel like you understand titrations better than equilibria.
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Offline Meter

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Re: Clarifying titration questions
« Reply #2 on: October 05, 2020, 05:09:56 PM »
1. At the equivalence point of a titration curve, the initial amount of substance of the titrand is equal to the amount of substance of the titrant

Not exactly. You added a stoichiometric amount of titrant, doesn't mean equal. Think NaOH + H2SO4.

Quote
but physically speaking, all of the initial amount of substance has been converted, so this new, changed amount of substance is 0 (theoretically).

Yes, but no. When you titrate a weak acid at the equivalence point some of it is still in the protonated form..

Quote
2. At the half equivalence point, half of the initial amount of substance of the titrand has been converted, meaning that there is only 50% left of the initial amount of titrand.

More or less. Again, equilibrium can do some tricks and slightly change the amount.

Quote
3. For something like glycine hydrochloride when titrated with a strong base, it would mean that the isoelectric point would be at its first equivalence point, as at this point, the initial amount of substance, (which is not isoelectric) has been wholly converted into the isoelectric intermediate (center compound), meaning that neither the left or right compound sexist (theoretically) at this point in the titration, but only before and after respectively.

No, at 50% there are still other forms present, just in (almost) equal and (typically) tiny amounts.

Quote
If this is correct, then I think I properly understand titrations.

I feel like you understand titrations better than equilibria.
I should probably have made it clear that we were titrating with NaOH, meaning that an equilibrium won't really happen anywhere near the beginning of the titration. Wouldn't this mean that all of the glycine hydrochloride is converted into its other forms as the reaction goes on? It is my understanding that a strong acid + weak base or weak acid + strong base is more or less unidirectional.

Offline Borek

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Re: Clarifying titration questions
« Reply #3 on: October 06, 2020, 02:52:34 AM »
Titrating with a strong base (or acid) doesn't mean you have switched off equilibria related to the weak acid/base properties of the titrated substance.
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Offline Meter

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Re: Clarifying titration questions
« Reply #4 on: October 06, 2020, 03:24:50 AM »
Titrating with a strong base (or acid) doesn't mean you have switched off equilibria related to the weak acid/base properties of the titrated substance.
No, but when you titrate with 1:1 amount of substance, wouldn't you have a full reaction?

Offline Borek

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Re: Clarifying titration questions
« Reply #5 on: October 06, 2020, 04:58:32 AM »
No, products are at equilibrium.

What is pH of sodium acetate solution? Is it neutral?
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Offline Meter

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Re: Clarifying titration questions
« Reply #6 on: October 06, 2020, 05:02:55 AM »
No, products are at equilibrium.

What is pH of sodium acetate solution? Is it neutral?
Should be basic as it dissociates into sodium and acetate in water forming some NaOH as well.

Offline Borek

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Re: Clarifying titration questions
« Reply #7 on: October 06, 2020, 06:15:14 AM »
NaOH - where do OH- come from?
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Offline Meter

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Re: Clarifying titration questions
« Reply #8 on: October 06, 2020, 06:25:48 AM »
NaOH - where do OH- come from?
Water after it reacts with acetate or more precisely when NaOH dissociates.
« Last Edit: October 06, 2020, 07:58:19 AM by Meter »

Offline Meter

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Re: Clarifying titration questions
« Reply #9 on: October 06, 2020, 12:53:18 PM »
I guess the point is that at the equivalance point, the added amount of titrant is equal to the initial amount of titrand meaning you can determine its concentration, but unless the titrand and titrant react fully (like a strong acid + strong base usually would), the exact chemical composition of the solution at the equivalence point requires some work to figure out, namely in terms of equilibrium constants, ICE tables, etc.




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