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Author Topic: Equivalent Weight Phosphoric Acid  (Read 8923 times)

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xe phantom

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Equivalent Weight Phosphoric Acid
« on: December 11, 2009, 10:40:00 PM »

I understand that the equivalent weight of an acid is equal to the molecular weight divided by the basicity. By this definition Phosphoric acid should have an equivalent weight of 98/3 = 32.67 but standard methods (such as the one given in US pharmacopeia) state the equivalent weight to be 49. I tried searching for exceptions but couldn't find any reason. Any help/explanation would be great!

xe
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Schrödinger

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Re: Equivalent Weight Phosphoric Acid
« Reply #1 on: December 12, 2009, 12:56:48 AM »

There is more than one phosphoric acid. (ortho, meta)

If you are talking about H3PO4 :
Equivalent weight of a compound mainly depends on the rxn that it undergoes.
You cannot expect orthophosphoric acid (H3PO4) to be tribasic always. There will be certain rxns where it releases only 2 H+ ions.


Please correct me if I am wrong.
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xe phantom

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Re: Equivalent Weight Phosphoric Acid
« Reply #2 on: December 12, 2009, 01:27:45 AM »

There is more than one phosphoric acid. (ortho, meta)

If you are talking about H3PO4 :
Equivalent weight of a compound mainly depends on the rxn that it undergoes.
You cannot expect orthophosphoric acid (H3PO4) to be tribasic always. There will be certain rxns where it releases only 2 H+ ions.

Please correct me if I am wrong.


You're correct about the varying number of H+ ions released depending on the reaction but what I want to know is what equivalent weight to assume in a simple acid base reaction with let's say Sodium Hydroxide. A reference literature indicating corresponding equivalents for different reactions would be great!

Cheers
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Borek

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Re: Equivalent Weight Phosphoric Acid
« Reply #3 on: December 12, 2009, 01:31:38 AM »

You cannot expect orthophosphoric acid (H3PO4) to be tribasic always. There will be certain rxns where it releases only 2 H+ ions.

And that's the correct answer. pKa3 = 12.35.

You're correct about the varying number of H+ ions released depending on the reaction but what I want to know is what equivalent weight to assume in a simple acid base reaction with let's say Sodium Hydroxide.

Define "simple acid base reaction". Third dissociation constant is so low you will not observe third end point in water titration.
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xe phantom

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Re: Equivalent Weight Phosphoric Acid
« Reply #4 on: December 13, 2009, 04:56:45 AM »

You cannot expect orthophosphoric acid (H3PO4) to be tribasic always. There will be certain rxns where it releases only 2 H+ ions.

And that's the correct answer. pKa3 = 12.35.

You're correct about the varying number of H+ ions released depending on the reaction but what I want to know is what equivalent weight to assume in a simple acid base reaction with let's say Sodium Hydroxide.

Define "simple acid base reaction". Third dissociation constant is so low you will not observe third end point in water titration.

I think I got it. Thanks!
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