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 04-22-2012, 09:59 PM #1 2K Member   Join Date: Apr 2010 Posts: 2,406 Equivalence point SDN Members don't see this ad. (About Ads) I know equivalence point is pKa1 + pKa2 / 2 I don't remember why this is though. Can I please have a proof? thanks.
 04-22-2012, 10:01 PM #2 2K Member   Status: Pre-Medical Join Date: Mar 2010 Location: NY Posts: 2,184 Do you mean pI? __________________ sector9, mauberley, flodhi1, flatearth22, MedPR, Neuronix, Catalystic, LizzyM, PharMed2016, Fencer, DrMidLife, nadaba, Gnomes, thlaxer, [04/28/12 MCAT]: Without them, I could not be where I am now. The most f'ed up, psychotic thing I've ever read on SDN.
 04-22-2012, 10:09 PM #3 8-16-13-39-42-45     Status: Pre-Medical MDApps: View Profile Join Date: Jun 2010 Posts: 9,303 I have never seen that equation before. __________________ Summer Research Data | Med School Info & Thread | Med School Data & Thread | SDN Mobile for iPhone/iPad or Android | Donate for perks! MCAT Flashcard Count: 888
04-22-2012, 10:10 PM   #4
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Quote:
 Originally Posted by gettheleadout I have never seen that equation before.
Amino acid pI. Zwitterionic point.

04-22-2012, 10:15 PM   #5
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Quote:
 Originally Posted by pfaction 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

 04-22-2012, 10:16 PM #6 2K Member   Status: Pre-Medical Join Date: Mar 2010 Location: NY Posts: 2,184 At 8.3, the molecule will be 100% HCO3-. <6.3: majorly or all H2CO3. Above 10.3: Majorly or all CO32-
04-22-2012, 10:17 PM   #7
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Quote:
 Originally Posted by pfaction 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

04-22-2012, 10:22 PM   #8
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Quote:
 Originally Posted by chiddler 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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 04-22-2012, 10:22 PM #9 2K Member   Status: Pre-Medical Join Date: Mar 2010 Location: NY Posts: 2,184 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].
04-22-2012, 10:25 PM   #10
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Quote:
 Originally Posted by pfaction 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.

04-22-2012, 10:27 PM   #11
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Quote:
 Originally Posted by typicalindian better go make a trip to your nearest lab then
oh man. i have to break in again?!

 04-22-2012, 10:27 PM #12 2K Member   Status: Pre-Medical Join Date: Mar 2010 Location: NY Posts: 2,184 You know I think you're right, I may have been translating a monoprotic acid into diprotic acid.
04-22-2012, 10:33 PM   #13
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Quote:
 Originally Posted by chiddler 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

04-22-2012, 10:39 PM   #14
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Quote:
 Originally Posted by typicalindian 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*

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

 04-22-2012, 10:44 PM #16 2K Member   Join Date: Apr 2010 Posts: 2,406 lol
 04-22-2012, 10:45 PM #17 Senior Member   Status: Pre-Medical Join Date: Jul 2011 Posts: 236 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??
 04-22-2012, 10:49 PM #18 1K Member   Status: Pre-Medical Join Date: Jan 2012 Posts: 1,480 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?
04-22-2012, 10:52 PM   #19
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Quote:
 Originally Posted by rjosh33 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.

Quote:
 Originally Posted by SaintJude 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.

04-22-2012, 10:52 PM   #20
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Quote:
 Originally Posted by pfaction Amino acid pI. Zwitterionic point.
Ah, well I know absolutely nothing about that. Glad I wasn't just forgetting something...

 04-22-2012, 10:53 PM #21 Senior Member   Status: Pre-Medical Join Date: Jul 2011 Posts: 236 Nm Last edited by rjosh33; 04-22-2012 at 10:57 PM. Reason: Wrong, lol
04-22-2012, 10:56 PM   #22
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Quote:
 Originally Posted by chiddler 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.

 04-22-2012, 10:59 PM #23 2K Member   Join Date: Apr 2010 Posts: 2,406 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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