Sciencemadness Discussion Board

Acetic acid not neutralised by ammonia?

optragon - 18-1-2016 at 07:03

Hola amigos,

I'm puzzled by acetic acid residue that should have been neutralized by ammonia. Working on a Liquid-liquid extraction and to the aqueous acetic acid solution NH3 is added to reach a pH of 9.5. Strangely, after distilling off the organic phase, there is quite some acetic acid left over (determined by smell). How can that even be at pH 9.5?

I can only think of the pH electrode being inaccurate, but I don't have a new one at hand until next week. What do you think? Any other possibility?

hissingnoise - 18-1-2016 at 07:26

Ammonium acetate actually does smell of acetic acid, as a result, probably, of its hygroscopicity . . .

Anyhoo, welcome to scimad!


optragon - 18-1-2016 at 08:02

Aha! That might be exactly what is happening. I was thinking it doesn't quite smell like clean acetic acid... Thanks a lot for the input!

blogfast25 - 18-1-2016 at 08:05

Quote: Originally posted by optragon  
Hola amigos,

I'm puzzled by acetic acid residue that should have been neutralized by ammonia. Working on a Liquid-liquid extraction and to the aqueous acetic acid solution NH3 is added to reach a pH of 9.5. Strangely, after distilling off the organic phase, there is quite some acetic acid left over (determined by smell). How can that even be at pH 9.5?

I can only think of the pH electrode being inaccurate, but I don't have a new one at hand until next week. What do you think? Any other possibility?


By all means check your meter (regularly) but ammonium acetate undergoes hydrolysis:

NH4<sup>+</sup> + H2O < ==== > NH3 + H3O<sup>+</sup>

OAc<sup>-</sup> + H3O<sup>+</sup> < ==== > HOAc + H2O

Both perfectly neutralised ammonium acetate solutions and solid ammonium acetate always small of vinegar (acetic acid, aka ethanoic acid).

Praxichys - 18-1-2016 at 09:28

Also, ammonium acetate readily loses water under gentle heating to form acetamide.

PlatinumLab - 22-1-2016 at 15:53

Heating ammonia salts, or even other salts where one component is a gas, can cause that gas to leave the solution. In principle it is extremely common. I think something like this was one of the very first labs I had done in class years ago.

DrMethyl - 27-1-2016 at 13:23

Quote: Originally posted by optragon  
there is quite some acetic acid left over (determined by smell)


^^

Welcome to sciencemadness ;)

[Edited on 27-1-2016 by DrMethyl]