Chemistry · BEGINNER LESSON
Buffer
A buffer is a solution made from a weak acid and its conjugate base, or a weak base and its conjugate acid, that resists changes in pH when small amounts of acid or base are added. pH is a measure of how acidic or basic a solution is.
What a buffer is and what it measures
A buffer is a solution that helps keep pH nearly constant. pH is the scale chemists use to describe how acidic or basic a solution is. Buffers are usually made from either a weak acid plus its conjugate base, or a weak base plus its conjugate acid. A weak acid or weak base is only partly able to give up or take up protons, which are hydrogen ions. The measured effect of a buffer is not that pH never changes, but that the change is smaller than it would be without the buffer.
[1] MITOCW | ocw-5-111-f08-lec23_300k [2] The Old West analogy for acid-base buffering - PMC [3] [PDF] Lecture 22: Acid-Base Equilibrium: Salt Solutions and Buffers
How buffering works
A buffer works by reacting with added acid or added base. If strong acid is added, the weak base part of the buffer can accept protons from it, reducing the number of free protons left in solution. If strong base is added, the weak acid part can donate protons to neutralize hydroxide ions, which are the basic particles added by the strong base. This two-way action is why both parts of the pair are needed.
[1] MITOCW | ocw-5-111-f08-lec23_300k [2] The Old West analogy for acid-base buffering - PMC [3] [PDF] Lecture 22: Acid-Base Equilibrium: Salt Solutions and Buffers
What makes a buffer effective
Two main factors matter: the ratio between the two buffer forms and the total concentration of the buffer. The Henderson-Hasselbalch equation links pH, pKa, and the acid-to-base ratio; pKa is a property of the acid that helps describe where the weak acid and its conjugate base can coexist well. The sources say buffering is strongest when pH is near pKa, often within about one pH unit, and that higher concentrations give greater resistance to change. If concentrations are too low, the buffer may not work well.
[1] MITOCW | ocw-5-111-f08-lec23_300k [2] The Old West analogy for acid-base buffering - PMC [3] [PDF] Lecture 22: Acid-Base Equilibrium: Salt Solutions and Buffers
Limits and examples
Buffers do not make a solution perfectly stable, and they only work over a limited range. A solution with only a strong acid or only a strong base is not a good buffer. The sources also note that real biological systems can contain several buffers at once; blood is buffered around pH 7.35 to 7.45. The Old West analogy is useful for understanding the basic idea, but it does not cover all parts of whole-body acid-base physiology.
[1] MITOCW | ocw-5-111-f08-lec23_300k [2] The Old West analogy for acid-base buffering - PMC [3] [PDF] Lecture 22: Acid-Base Equilibrium: Salt Solutions and Buffers
SEE THE IDEA
How a buffer resists pH change
Both forms must be present so each can react with something added.
The buffer’s weak base binds incoming protons and reduces free protons.
The buffer’s weak acid donates protons to neutralize hydroxide ions.
pH changes less than it would in an unbuffered solution.
The buffer pair removes added acid or base before it can strongly change pH.
A buffer acts like a chemical sink and source for protons, but only within a limited range and only if enough material is present.
TRY IT YOURSELF
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PUT IT TOGETHER
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.