Understanding Buffers: Definition and Composition

[Qube]

Homework Statement

What's a buffer?

Homework Equations

The most general definition is that a buffer resists changes in pH. However, my textbook and ChemWiki by UC Davis maintain that a buffer is composed of conjugate pairs (i.e. a base and its conjugate acid).

The Attempt at a Solution

Is a buffer necessarily composed of its base and its conjugate acid or acid and its conjugate base?

I think no ... can't some salts be buffers? Such as ammonium sulfate?

 

[jedishrfu]

Here's the definition of a buffer that defines what it is:

http://en.wikipedia.org/wiki/Buffering_agent

and as a solution:

http://en.wikipedia.org/wiki/Buffer_solution

 

[Yanick]

You may want to consider the type of conjugate acid/base pairs which make useful buffers. For example, which would make a useful buffer 1) HCl/Cl^- or 2) HOAc/OAc^-?

 

[Borek]

Buffer doesn't have to be made of an acid and its conjugate base. Sadly, it was repeated so many times in so many places, now everyone thinks it is the only way.

For pH between - say - 2 and 12 - acid and conjugate base are a way to go. Outside it is enough to have just a strong base or just a strong acid to have a solution with buffering capacity high enough to resist pH changes.

 

[Qube]

You may want to consider the type of conjugate acid/base pairs which make useful buffers. For example, which would make a useful buffer 1) HCl/Cl^- or 2) HOAc/OAc^-?

The pairs should be weak. Having a strong acid wouldn't do you much good in a solution since the reaction between the acid and any base that enters the system would go to completion under most circumstances. Not much of an equilibrium going on between the acid and base the case of HCl and chloride ion, as the hydronium ion would simply be consumed by any base added to the system. Additionally, chloride ion is a very weak base - actually weaker than water. Not much use for chloride ion as a buffer in water solution.

H_{3}O^{+} + SO_{4}^{2-} \rightarrow H_{2}O + HOSO_{3}^{-}

In the case of acetic acid, however, a reaction such as this might occur, but the system would be able to resist pH changes better because 1) the first below reaction isn't large extent and 2) the second reaction below can still occur because the conjugate base of acetic acid is being produced despite the fact that HAc is being depleted. This differs from the above case in which hydronium ion was being depleted and no chloride ion was being made.

HAc + SO_{4}^{2-} \rightarrow Ac^{-} + HOSO_{3}^{-}

HAc + Ac^{-} \leftrightharpoons HAc + Ac^{-}

 

[Qube]

Buffer doesn't have to be made of an acid and its conjugate base. Sadly, it was repeated so many times in so many places, now everyone thinks it is the only way.

Okay, that makes sense. The definition of a buffer as an weak acid and its conjugate base or vice versa is definitely valid but limited in scope. Thanks for confirming my suspicions :).

 

[MathsMind88]

Hey, hopefully you have found the answer to your question by now but just for future reference be very careful when using Wikipedia as a reference source. It has often been found to be highly inaccurate and the references used are not verified for accuracy. As an undergraduate chemistry tutor both myself and many other tutors/lecturers actually refuse to accept any Wikipedia references made in assessments. This is not a criticism. :)

 

[Qube]

I'm not sure how you concluded I was using Wikipedia at all.

 

[jedishrfu]

Hey, hopefully you have found the answer to your question by now but just for future reference be very careful when using Wikipedia as a reference source. It has often been found to be highly inaccurate and the references used are not verified for accuracy. As an undergraduate chemistry tutor both myself and many other tutors/lecturers actually refuse to accept any Wikipedia references made in assessments. This is not a criticism. :)

Many people know this so we read the article before posting to decide if its accurate enough to post.