Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: diana on 09/04/2006 16:48:55
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what are the exact qualifications for an acid-base titration to be a buffer?
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What are they?
Buffers are solutions in which the conjugate base of an acid and that acid or a conjugate acid of a base and that base are in solution together. A buffer is defined as a solution that resists changes in pH. If you have a buffer solution and you add a little bit of acid, the pH does not change very much. If you add a little bit of base, the pH does not change very much. In a normal, neutral aqueous solution, the addition of just a small amount of a .1 molar strong acid will change the pH by factors of hundreds of thousands. If you have a buffer, however, the addition of that same amount of acid would not change the pH appreciably.
You can find tons of links if you google the subject buffers.
[http://library.thinkquest.org/C004970/acidbase/buffers.htm]
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A buffer minimises the change in pH when small amounts of acid or alkali are added to a solution. They are made by mixing a weak acid with one of its soluble salts (eg ethanoic acid and sodium ethanoate). An equilibrium is set up: HA <=> H+ + A-. In a buffer, the concentrations of HA and A- are large ([HA] and [A-]), but the concentration of H+ ([H+])is small and known, so the pH is fixed. If small amounts of acid are added, then [H+] increases, so the reaction H+ + A- -> HA occurs to return [H+] to approximately its original value, keeping the pH constant. If small amounts of alkali are added, the reaction H+ + OH- -> H2O occurs (adding alkali is adding OH-), which causes [H+] to drop, so the reaction HA -> H+ + A- occurs to return [H+] to approximately its original value, so the pH remains constant.
Hope that helps a bit.
"I just set fire to the table!"
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So an acid-base titration can only be used as a buffer
-if an acid was titrated with its conjugate base
or
-if a base was titrated with its conjugate acid?
which would probably not work because strong acids have weak conjugate bases and strong bases have weak conjugate acids.
It all has to do with the pKa of the buffer and the pH of the solution. (Henderson Hasselbach Equation)
correct?
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You don't generally make a buffer through titration. A buffer is simply a solution composed of a weak acid and its conjugate base. A general purpose buffer will have equimolar amounts of each, and the pH of this buffer will be equal to the pKa of the weak acid. The pH is independent of the amounts of materials used (as long as the ratio remains the same), but the resistance towards added acid or base (the buffer capacity) does depend on amounts.
To make an acetate buffer, you would mix acetic acid and an acetate salt (e.g., sodium acetate). The pH of the buffer will depend on the relative amounts of each.
DrDick