[xe phantom]

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

[Schrödinger]

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.

[xe phantom]

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

[Borek]

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. pK_a3 = 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.

[xe phantom]

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. pK_a3 = 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!