Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: xe phantom on December 12, 2009, 04:40:00 AM
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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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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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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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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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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!