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Old 04-22-2012, 09:59 PM   #1
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I know equivalence point is pKa1 + pKa2 / 2

I don't remember why this is though. Can I please have a proof?

thanks.
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Old 04-22-2012, 10:01 PM   #2
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Do you mean pI?
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Old 04-22-2012, 10:09 PM   #3
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I have never seen that equation before.
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Old 04-22-2012, 10:10 PM   #4
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I have never seen that equation before.
Amino acid pI. Zwitterionic point.
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Old 04-22-2012, 10:15 PM   #5
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Do you mean pI?
yes it does mean pI. but i meant equivalence point! is that incorrect?

i'm trying to translate this concept to, say, carbonic acid a diprotic buffer.

what does pKa1 + pKa2 / 2 tell us about carbonic acid? to help visualize, pKa 1 is 6.3, pKa2 is 10.3. both / 2 = 8.3
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Old 04-22-2012, 10:16 PM   #6
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At 8.3, the molecule will be 100% HCO3-.
<6.3: majorly or all H2CO3.
Above 10.3: Majorly or all CO32-
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Old 04-22-2012, 10:17 PM   #7
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At 8.3, the molecule will be 100% HCO3-.
<6.3: majorly or all H2CO3.
Above 10.3: Majorly or all CO32-
there we go. it is the first equivalence point.

thanks.

wait, i still need proof lol
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Old 04-22-2012, 10:22 PM   #8
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Originally Posted by chiddler View Post
there we go. it is the first equivalence point.

thanks.

wait, i still need proof lol
better go make a trip to your nearest lab then
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Old 04-22-2012, 10:22 PM   #9
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No, it's not the first equivalence point, that's H2CO3->HCO3- at 50/50. pH = pKa1 at that point. Equivalence point is when [H]=[OH].
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Old 04-22-2012, 10:25 PM   #10
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No, it's not the first equivalence point, that's H2CO3->HCO3- at 50/50. pH = pKa1 at that point. Equivalence point is when [H]=[OH].
when it's 50/50, then it's pH = pKa1 which is 6.3. we're looking at 8.3.

equivalence point is when you add one equivalent of base to the acid. since carbonic acid is weak, it becomes slightly basic which is observed with 8.3.
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Old 04-22-2012, 10:27 PM   #11
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better go make a trip to your nearest lab then
oh man. i have to break in again?!
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Old 04-22-2012, 10:27 PM   #12
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You know I think you're right, I may have been translating a monoprotic acid into diprotic acid.
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Old 04-22-2012, 10:33 PM   #13
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oh man. i have to break in again?!
Rookie mistake. My PI likes me enough to give me the keys to our lab and 24/7 access ID badge
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Old 04-22-2012, 10:39 PM   #14
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Rookie mistake. My PI likes me enough to give me the keys to our lab and 24/7 access ID badge
i am so jealous! you get to go to lab at NIGHT?

all the fun i'm missing out on! *swoon*
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Old 04-22-2012, 10:41 PM   #15
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i am so jealous! you get to go to lab at NIGHT?

all the fun i'm missing out on! *swoon*
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Old 04-22-2012, 10:44 PM   #16
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lol
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Old 04-22-2012, 10:45 PM   #17
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The pH of the equivalence point between the pKa1 and pKa2 of a diprotic acid is actually computed from a pretty complicated formula:

[H+] = [(Ka1Ka2F x Ka1Kw) / Ka1 + F]^1/2

You need to know the starting concentration of the acid as well as the base (in order to get the formal concentration, F) along with the Ka's (both 1 and 2) and the water dissociation constant.

Was a problem asking you for the pH at the equivalence point of a diprotic acid??
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Old 04-22-2012, 10:49 PM   #18
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Ok, so,

So pH of the 1st equivalence point = pKa1 + pKa2 / 2 . And this equals the pI for an amino acid that has a neutral R group, yeah?
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Old 04-22-2012, 10:52 PM   #19
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The pH of the equivalence point between the pKa1 and pKa2 of a diprotic acid is actually computed from a pretty complicated formula:

[H+] = [(Ka1Ka2F x Ka1Kw) / Ka1 + F]^1/2

You need to know the starting concentration of the acid as well as the base (in order to get the formal concentration, F) along with the Ka's (both 1 and 2) and the water dissociation constant.

Was a problem asking you for the pH at the equivalence point of a diprotic acid??
no i was just trying to find a better way of remembering the equation. i thought understanding its derivation would help.

if it is indeed this complicated, then thanks and nevermind. i must have remembered incorrectly that i had known the proof before.

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Ok, so,

So pH of the 1st equivalence point = pKa1 + pKa2 / 2 . And this equals the pI for an amino acid that has a neutral R group, yeah?
for acidic amino acids. for basics, it is pKa2 + pKa3 / 2.
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Old 04-22-2012, 10:52 PM   #20
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Originally Posted by pfaction View Post
Amino acid pI. Zwitterionic point.
Ah, well I know absolutely nothing about that. Glad I wasn't just forgetting something...
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Old 04-22-2012, 10:53 PM   #21
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Nm
Last edited by rjosh33; 04-22-2012 at 10:57 PM. Reason: Wrong, lol
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Old 04-22-2012, 10:56 PM   #22
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Originally Posted by chiddler View Post
no i was just trying to find a better way of remembering the equation. i thought understanding its derivation would help.

if it is indeed this complicated, then thanks and nevermind. i must have remembered incorrectly that i had known the proof before.



for acidic amino acids. for basics, it is pKa2 + pKa3 / 2.
Ok, I just went back and looked it up, and you're right. Turns out the equation I wrote earlier can be further simplified to what you said, pH = pKa1 + pKa2 / 2. Sorry about the confusion.
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Old 04-22-2012, 10:59 PM   #23
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that equation is complicated enough that it won't really help, anyway.

is ok. thanks for the responses. discussion about the equation will have helped memorizing it so i'm satisfied.
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