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AP®︎/College Chemistry
Buffer range
AP.Chem:
SAP‑10 (EU)
, SAP‑10.C (LO)
, SAP‑10.C.1 (EK)
A buffer has an effective pH range of one pH unit on either side of the pKₐ value for the weak acid. If the pH of a buffer goes out of this range, the buffer will no longer be effective at resisting large changes in pH. Created by Jay.
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How do we know it's not effective anymore if one of the concentrations is more than 10x present in the solution?(1 vote)
Again the purpose of a buffer is to resist large pH changes with the addition of more acid/base to a solution. So the most obvious indication that a buffer is no longer effective is when we do see large pH changes when we add acids/bases to the solution. The Henderson-Hasselbalch equation just gives us a reason why it is so; the concentrations of the acid/conjugate base are too dissimilar.
Hope that helps.(3 votes)
Video transcript
- [Instructor] Buffers consists
of a significant amount of a weak acid, which
we will represent as HA and the conjugate base to the weak acid, which we will represent as A-. Buffer solutions resist
large changes in pH. However, buffers are only effective over a certain range of pH values. We are going to use the
Henderson-Hasselbalch equation to find the effective
pH range of a buffer. Looking at the
Henderson-Hasselbalch equation, the pH of the buffer solution is equal to the pKa of the weak acid, which would be HA, plus the log of the concentration
of the conjugate base divided by the concentration
of the weak acid. And it's this ratio of the concentration of the conjugate base to the
concentration of the weak acid that determines if a
buffer is effective or not. Buffer solutions are most
effective at resisting a change in pH in either direction when the concentration of the weak acid is equal to the concentration
of the conjugate base. And when the concentrations
are equal to each other, the ratio is equal to one, and the log of one is equal to zero. Therefore, when the concentrations
are equal to each other, the pH of the buffer
solution is equal to the pKa of the weak acid plus zero. So we could just say that
the pH is equal to the pKa when the concentration of the weak acid is equal to the concentration
of the conjugate base. So we usually try to choose
a buffer with a weak acid that has a pKa value
close to the desired pH of the solution. So buffers are effective at
resisting large changes in pH when the pH is approximately equal to the pKa of the weak acid. However, if the concentration
of one component of a buffer is more than 10 times the concentration of the other component in a buffer, buffers are not effective at
resisting large changes to pH. Therefore, to find the effective pH range, we're gonna use the
Henderson-Hasselbalch equation to calculate the pH when the concentration
of the conjugate base is 10 times the concentration
of the weak acid, and also to calculate the pH when the concentration of the weak acid is 10 times the concentration
of the conjugate base. Doing these two calculations gives us the upper and lower limits of the effective pH range. So let's calculate the
pH of the buffer solution when the concentration
of the conjugate base is 10 times the concentration
of the weak acid. Looking at the
Henderson-Hasselbalch equation, if the concentration of the conjugate base is 10 times the concentration
of the weak acid, the ratio is equal to 10 over one, and the log of 10 is equal to one. Therefore, the pH of the buffer solution is equal to the pKa value
of the weak acid plus one. This value for the pH
represents the upper limit of the effective pH range. Next, let's calculate the
pH of the buffer solution when the concentration of weak acid is 10 times the concentration
of the conjugate base. Looking at the
Henderson-Hasselbalch equation, if the concentration of HA is 10 times the concentration of A-, the ratio is equal to one over 10, and the log of one over 10
is equal to negative one. Therefore, the pH of the buffer solution is equal to the pKa value
of the weak acid minus one. This value for the pH
represents the lower limit of the effective pH range. By the calculations that we've just done, we've seen that the effective
pH range of a buffer is plus or minus one of the
pKa value of the weak acid. Let's use these concepts
of an effective pH range to choose a buffer solution. Let's say we want to buffer a solution at a pH of 9.00 at 25 degrees Celsius. And suppose that we have two choices, we could either choose an
acetic acid-acetate buffer or we could choose an
ammonium-ammonia buffer. Because the effective pH range of a buffer is plus or minus one the
pKa value of the weak acid, we don't wanna choose the acetic acid-acetate buffer solution. Because at 25 degrees Celsius, the pKa value for acetic
acid is equal to 4.74. Therefore, this buffer
would only be effective at a range of plus or minus one from 4.74, so about 3.74 to approximately 5.74. The ammonium cation has
a pKa value equal to 9.25 at 25 degrees Celsius. Therefore, the ammonium-ammonia buffer is effective plus or minus
one of this pKa value, so approximately 8.25 to 10.25. Since our pH of nine
falls within that range, we would choose the
ammonium-ammonia buffer.