Dr Gerrard

10+ Year Member
7+ Year Member
Feb 16, 2009
1,044
1
Status
Medical Student
How can you calculate the amount of concentration of acid or base needed to significantly change the pH of a buffered system.
 

G1SG2

10+ Year Member
5+ Year Member
May 2, 2008
1,454
2
Status
Pre-Medical
How can you calculate the amount of concentration of acid or base needed to significantly change the pH of a buffered system.
Maybe you can use the info in the titration curve?
 
May 8, 2009
289
1
Status
Pre-Medical
pH = pka - log(Base/Acid)

That henderson guy. Of course you need to know you concentrations of base and acids.
 

G1SG2

10+ Year Member
5+ Year Member
May 2, 2008
1,454
2
Status
Pre-Medical
pH = pka - log(Base/Acid)

That henderson guy. Of course you need to know you concentrations of base and acids.
I think that just tells you the pH of the buffered solution, since it's already a buffer, the pH would just equal the pKa.
 

kentavr

10+ Year Member
Nov 13, 2006
103
1
Status
Pre-Medical
I would do the following. First, let assume that the buffer is a week acid (base will be similar).

Then to the right side from the buffer will be the equivalence point where concentration of H+ is equal to added OH- from strong base. That part of the chart almost vertical and represent the drastic change in pH. So, to reach this point you have to add the equivalent amount of base.

On the left side it is not so clean, if we add strong acid and assume(not exactly) that all new H+ react with conjugate base A- pushing base ions back to weak acid (or A- + H+ --> HA) then there will be no more acidic buffer capacity. Which gives us an estimation from the left side. Add as much H= as you have [A-].

To be exact we have to define what is the "significant change" and solve the equation where the first derivative of equilibrium is equal to the value for "significant change".
Is there are any other ways?