In a previous post an approximate equation to calculate buffer capacity of an acetate buffer is introduced. However, the following is an equation that can be used for exact calculations:
Where, β is the buffer capacity, C is the total buffer concentration, Ka is the acidity constant. Total buffer concentration is the sun of the concentrations of buffer components (acid and salt) in molarity. For a buffer composed from acetic acid and sodium acetate the total buffer concentration, C = [acetic acid] + [sodium acetate].
Example 1. Calculate buffer capacity of a solution composed from 0.2 M acetic acid ( Ka=
1.75 × 10-5) and 0.2 M sodium acetate at pH = 4.76.
Answer:
Example 2. Calculate the buffer capacity of the above solution after it is neutralized with KOH to pH = 7, assuming no significant change in volume after neutralization.
Answer:
It can be seen from examples 1 and 2 that the buffer capacity decreased as the difference between pH and pKa increased.
Example 1. Calculate buffer capacity of a solution composed from 0.2 M acetic acid ( Ka=
1.75 × 10-5) and 0.2 M sodium acetate at pH = 4.76.
Answer:
Example 2. Calculate the buffer capacity of the above solution after it is neutralized with KOH to pH = 7, assuming no significant change in volume after neutralization.
Answer:
Where does 2.303 come from?
ReplyDeleteThis number (2.303 or simply 2.3) appears during derivation of the equation. For further information you can follow the following link:
Deletehttp://www.jbc.org/content/52/2/525.full.pdf+html
Is the ka and [H3O] the same at all cases? why is that so?
ReplyDeleteIn the above example ka and [H3O+] are the same. However, the value of [H3O+] which is calculated from pH can be of different value.
DeleteHow do you get H3O+ with Ka
ReplyDelete[H3O+] is calculated from pH. If you know the acid-base components of the solution then pH can be calculated using appropriate equations. You can find some of them on this blog.
ReplyDeleteso, how about the base buffer? just change Ka and H3O+ to Kb and OH-?
ReplyDeleteGoing back to the derivation of the above mentioned equation by D. D. Van Slyke it appears that for basic buffers only Ka is replaced with Kb. See the following reference (pages 557- 558) on this URL: http://www.jbc.org/content/52/2/525.full.pdf
DeleteWhat are the units of buffering capacity?
ReplyDeleteAlso, what is the fractional term for 2.303?
This comment has been removed by the author.
DeleteAccording to the above equation it has units of concentration (Normality). Regarding the term 2.303 refer to the reply on the first comment on this page.
DeleteHi. I'm using this equation to calculate buffer capacity of different volumes(10,20,30,40cm3) of phosphoric acid buffer with same pH(2.23) and same pka. Using this equation I get the same buffer capacity regardless the volume because their concentrations are the same.
ReplyDeleteIs this right? I was expecting buffer capacity to increase as volume of buffer increases.
Can the buffer capacity formula be modified for base buffer?
ReplyDeleteare there units for the buffer capacity?
ReplyDeletebuffer capacity is the number of moles of base (or acid) that, when added to 1 liter of buffer solution, changes its pH by 1 unit.
Deletehttp://chemcollective.org/activities/tutorials/buffers/buffers5act1
Greetings Dr. Gharaibeh! Thank you for explaining this topic well. I have a question regarding one problem for buffer capacity:
ReplyDeleteThe concentration of Acetic Acid in buffer solution (AAA) is
changed from 0.1 to 0.3 N. Consequently, pH of the buffer solution
(AAA) was changed from 5.5 to 5.1. What is the buffer capacity of
buffer solution (AAA)?
For this problem are we going use the approximate formula for buffer capacity which is: ΔB/ΔpH , where ΔB= Normality, Δ= finite change and ΔpH= pH change and if yes is it correct to assume that the answer for the aforementioned problem is 0.5? Thank you again for making this topic understandable.
If it is an acetate buffer then the total buffer concentration, C, is equal to the concentration of acetic acid plus the concentration of acetate ion in solution. Firstly, calculate the acetate ion concentration using the Henderson-Hasselbalch equation . If acetic acid concentration was increased due to addition of a strong acid then that has to be accounted for and the total buffer concentration , C, remains constant. If the acetate ion concentration is constant after the increase in acetic acid, then the total buffer concentration is increased by a magnitude of 0.2. So at C the pH was 5.5 and at C+0.2 the pH became 5.1. Calculate [H3O+] from pH and Ka from pKa (4.76) and plug it in the above equation for both cases.
ReplyDeleteIf C remained constant then only [H3O+] will be different in the above equation.
ReplyDelete