equivalence point and indicators

[Meredith92]

TBR says in an answer..
To predict the pH near the equivalence point, which is what youre doing when you choose an indicator, you should use the henderson-hasselbach equation. at 2 pH units beyond the pKa of the acid, the ratio of conjugate base to acid is 100:1, which means that the reaction is almost at equivalence. This is the point at which the indicator should start to show some color change.

Can someone help explain this to me?
-I thought we should only be using the henderson-hasselbach equation for the bufferzone- why are we using it for the equivalence point?
- why when the ratio of conjugate base to acid is 100:1 does it mean that we're at equivalence? is it because most of the acid has been deprotonated and we assume that now the moles of H+=moles of OH-?

Thanks for your help! This has always been so confusing for me

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[Meredith92]

Another question I have is about TBR's method of estimating the pH of the equivalence point, they say to take the average of the pKa of the acid being titrated and the pH of the titrant. This makes sense for when a weak acid is being titrated with a strong base...

however, there is a problem (question 86 in chap 5) where a weak base is being titrated with a strong acid. To estimate the pH of the equivalence point they still use the same formula except for the conjugate acid!! How does this make sense?
For example, they say that KCN is being titrated with HCl .. in order to estimate the pH at the equivalence point they took the average of the pKa conjugate acid (HCN) and the pH of the titrant.

So you can average either the pKa of the acid (in a weak acid titration) OR the pKa of the conjugate acid (in a weak base titration) with the pH of the titrant to get the pH at equivalence point?

Thanks again for your help!

 

[gettheleadout]

TBR says in an answer..
To predict the pH near the equivalence point, which is what youre doing when you choose an indicator, you should use the henderson-hasselbach equation. at 2 pH units beyond the pKa of the acid, the ratio of conjugate base to acid is 100:1, which means that the reaction is almost at equivalence. This is the point at which the indicator should start to show some color change.

Can someone help explain this to me?
-I thought we should only be using the henderson-hasselbach equation for the bufferzone- why are we using it for the equivalence point?

The H-H equation is being used to approximate the curve just beyond the buffer zone, where the pH has steeply upturned to approach the equivalence point. They're choosing 2 pH units and a [A-] : [HA] ratio of 100:1 as an approximating point because the usefulness of the equation starts to break down any higher than that. Recall that the buffer zone ends at about the 10 : 1 ratio of [A-] to [HA]. That's a reasonable enough assumption though, because it's likely there would be at least 101 total molecules of weak acid initially. You're right though, you can't use H-H to calculate the pH at equivalence, because at equivalence the ratio of [A-] : [HA] is infinity since [HA] = 0. Of course it's not actually 0, because the conjugate base won't form a neutral salt, but this can't be taken into account with H-H either.

- why when the ratio of conjugate base to acid is 100:1 does it mean that we're at equivalence? is it because most of the acid has been deprotonated and we assume that now the moles of H+=moles of OH-?

Thanks for your help! This has always been so confusing for me

Yes, it's because if <1% of the reagent remains as weak acid, then nearly all has been converted to conjugate base. Since equivalence is when all of the weak reagent has been converted to conjugate base, we're 99% there.

Another question I have is about TBR's method of estimating the pH of the equivalence point, they say to take the average of the pKa of the acid being titrated and the pH of the titrant. This makes sense for when a weak acid is being titrated with a strong base...

however, there is a problem (question 86 in chap 5) where a weak base is being titrated with a strong acid. To estimate the pH of the equivalence point they still use the same formula except for the conjugate acid!! How does this make sense?
For example, they say that KCN is being titrated with HCl .. in order to estimate the pH at the equivalence point they took the average of the pKa conjugate acid (HCN) and the pH of the titrant.

So you can average either the pKa of the acid (in a weak acid titration) OR the pKa of the conjugate acid (in a weak base titration) with the pH of the titrant to get the pH at equivalence point?

Thanks again for your help!

I believe TBR explicitly says in the chapter that the pKa in the equivalence pH estimation equation is supposed to belong either to the weak acid being titrated or the conjugate acid of the weak base being titrated. So yes, you can use either, just make sure you're using the one that's appropriate for a given situation.

 

[Meredith92]

Thank you that was really helpful!