[kirsch]

Hello all,

If I remember correctly from Gen Chem, the discussion of buffers began with something called "The Common Ion Effect."  For instance, if a simple buffer is prepared (say sodium acetate/acetic acid), there is a common ion once the ions are disassociated (in this case, the common ion is acetate).

However, I was seeking help today in my research lab, and a technician told me that I can make a "phosphate buffer and titrate it with HBr" for the current project on which I am working.  And I thought, well, yeah, if I need to lower pH I can always add acid, but... in that case, where's the common ion?  Wouldn't I want to use something like monosodium phosphate and phosphoric acid? 

I understand that the H+ generated from the disassociation of HBr in water would cause a decrease in pH (which is what I need to do)--that's pretty obvious.  But, I thought the whole point of common ions was to make use of the equilibrium involved in weak acid disassociations.  By making use of the equilibrium, the common ion can "suck up" or donate protons as necessary.

Am I missing something here?

thanks,

kirsch

[Loyal]

Think about it.  What are you making by adding the acid?   None other than the conjugate base of Phosphates. 

H⁺ + PO₄^3-     HPO₄^2-

So by adding the acid you are making the conjugate base right there in the solution.

This is actually the way a lot of labs prepare buffers since you can grab one reagent off the shelf and simply add a strong acid/base till you reach the buffer area on the titration curve. 

[Borek]

What you need is a pair of conjugate acid and base. These form buffer by definition.

[kirsch]

ah--I got it now.  guess I forgot the part about how buffers can be prepared in several different ways.   thanks!

[Borek]

Take a look at buffer calculator

[Loyal]

A slight correction on what I was saying.  By adding the acid you are making the conjugate acid not base.  I slipped up on my terminology.

[kirsch]

So, if you can simply titrate a salt or acid one way or another with something like HCl or NaOH, what's the point of the "two solution" method (where you mix proportions of acid and conjugate base to get the desired pH)? 

[Borek]

I don't have a pH meter here... but I can easily prepare pH 5.0 buffer.