[savagefarmer]

Hello everyone,

Today I witnessed the titration of a sodium pentobarbital solution with 0.1M HCl. At first, a lot of white precipitate was formed. When more HCl was added the white precipitate disappeared again completely.

Now I am trying to figure out what is going on. As I understand it the reaction is:

C₁₁H₁₇N₂O₃^- + H₃O⁺ -> C₁₁H₁₈N₂O₃ + H₂O

And the white precipitate is C₁₁H₁₈N₂O₃. What I don't understand is why the precipitate disappears again when more HCl is added and the pH drops.

Thanks for any advice!

-sav

edit: typo

[Arkcon]

To solve a problem such as this, you have to first abandon the standard chemical notation:

C₁₁H₁₇N₂O₃^- + H₃O⁺ -> C₁₁H₁₈N₂O₃ + H₂O

And the white precipitate is C₁₁H₁₈N₂O₃. What I don't understand is why the precipitate disappears again when more HCl is added and the pH drops.

And instead really look at the structures (you linked to one.)  Try to see what it has turned into at each step, structurally, that explains why its insoluble or soluble.

[savagefarmer]

To solve a problem such as this, you have to first abandon the standard chemical notation:

And instead really look at the structures (you linked to one.)  Try to see what it has turned into at each step, structurally, that explains why its insoluble or soluble.

Thanks for you reply. I know for a fact that the white precipitate is Pentobarbital (C11H18N2O3), but I have no clue what the 'next step' is. I don't know what the Pentobarbital turns into that is soluble. Is there another acid/base reactions with Pentobarbital as the base? How would I go about figuring this stuff out?

Thanks,
sav