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Why does a buffer work most effectively when the concentration of salt and acid is 1:1?

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  • $\begingroup$ 1) Not all salts are basic. Example: sodium hydrogen sulfate (acidic salt). NaCl - salt which exerts no effect on water solution pH. 2) Are we talking about conjugate pairs of acids/bases or just acids and bases in general? $\endgroup$ – Dissenter Jun 8 '14 at 4:07
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    $\begingroup$ @Dissenter: I am referring to acidic buffers e.g. ethanoic acid and sodium ethanoate $\endgroup$ – Eliza Jun 8 '14 at 8:38
  • $\begingroup$ It would be nice if you could add some more information, so that we know where exactly you are stuck. (Please edit it into the question.) For example, do you care about a specific buffer? $\endgroup$ – Martin - マーチン Jun 8 '14 at 11:15
  • $\begingroup$ @Martin, it is valid for acidic buffers, I guess. $\endgroup$ – RE60K Jun 9 '14 at 3:22
  • $\begingroup$ @Aditya This website is no place for guessing - questions and answers should be opinion free. And I have no Idea what your comment is targeting. I asked to improve the question, as it is worded poorly, broadly and in a way, I do not want to do my own research. $\endgroup$ – Martin - マーチン Jun 9 '14 at 3:42
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Using the Henderson-Hasselbalch-Equation: $$\mathrm{p}\ce{H}=\mathrm{p}{K_a}+\log\frac{\left[\ce{salt}\right]}{\left[\ce{acid}\right]}$$ and buffer capacity: $$\phi=\frac{\text{number of moles of acid/base added to 1L solution}}{\Delta\mathrm{p}\ce{H}}$$ where: $$\Delta \mathrm{p}\ce{H} = \left|\log\frac{\left[\ce{salt}\right]}{\left[\ce{acid}\right]}\right|$$ it is minimum when $$[\ce{salt}]=[\ce{acid}]$$

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It will take an equal amount of acid or base to change the pH of the buffer the same amount. If the buffer system was established with 1:2, it would be the same as a 1:1 system after adding an equivalent of a base (or acid).

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Support: Just after equation 1 on this page:

Since a buffer is most effective when about half is in the acid form (Buffer H+) and half in the base form (Buffer)...

The buffer concentration must be sufficient to maintain the pH within acceptable limits with the changes in proton concentration expected to occur.

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  • $\begingroup$ Would those people who gave me a -1 explain, please. $\endgroup$ – LDC3 Jun 8 '14 at 13:22
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Just look at a titration plot of a weak acid or weak base. When you are half way to the equivalence point the rate of change (slope) of pH vs mL titrant is near zero. That point is when you are at 1:1 for salt:acid.

enter image description here

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