Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: charleehorse on October 09, 2004, 02:47:32 PM
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Hi,
I am completely stuck on the following question:
when phosphoric acid is titrated ith NaOH to pH 8.0, how many protons are "removed" from the phosphoric acid molecule? Which of the following indicators would you use to detect the endpoint?
Alizarin Yellow R 10.0 to 12.0
Phenolphthalein 8.0 to 10.0
Methyl Orange 3.1 to 4.4
Quinaldine Red 1.3 to 3.2
I know that phosphoric acid has 3 H+ ions to donate, however, I also know that it acts as a buffer around the phenolphthalein endpoint (8.2). If this acts as a buffer, and stabilizes the pH, does it actually transfer any H+? My guess is that it doesn't. Since the endpoint is 8.0, I am assuming that phenolphthalein is the correct indicator to use, however , I know that NaOH has a pKa of 3.06 and phosphoric acid has a pKa of 2.12, so should I be using quanaldine red which has a range of 1.3 to 3.2 to overcome the buffering capacity?
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NaOH has a pKa much higher than 3. The pKa of water, the conjugate acid of NaOH, is about 15.7.
Furthermore, I don't think that phosphate will act as a buffer unless you add some kind of phosphate salt, so you don't really have to worry about that. You just need to find the indicator that will change color slightly after you've removed all three phosphate protons.
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I know that NaOH has a pKa of 3.06 - weak base ???. I do not bielieve!
H3PO4 - pKa1 = 2.2, pKa2 = 7.2 pKa3 = 12.4.
At pK = pH mixures of H3PO4 and NaH2PO4, NaH2PO4 and Na2HPO4 or NaHPO4 and Na3PO4 with molar ratio 1:1 exist, respectively.
pHs' of equivalent points depends on concentration and for 1 molar solutions are approximately at pH = 7 + 1/2pK. So at pH =8.2 a buffer solution exists with molar ratio Na2HPO4/NaHPO4 1:10. My advice is to use alizarin yellow R.