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This question already has an answer here:

A buffer consists of a weak acid and its salt or weak base and its salt.

When the ratio of weak acid and its salt in a buffer (or the ration of weak base and its salt) is equal to 1, we say that the buffer capacity is maximum. How does this tell us that buffer capacity is maximum?

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marked as duplicate by Mithoron, Tyberius, Jon Custer, airhuff, Todd Minehardt Nov 9 '17 at 1:17

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  • $\begingroup$ This wikipedia page, and this page are good places to learn this. It addresses all elementary questions about buffer capacity. $\endgroup$ – Satwik Pasani Jan 2 '14 at 18:07
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    $\begingroup$ @SatwikPasani Feel like you could give an overview of what is in the links and provide an answer? $\endgroup$ – Nicolau Saker Neto Mar 8 '14 at 0:33
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With a buffer, you're trying to maximize your ability to neutralize acid or base. The log dependence of the pH on the ratio of interest in the Henderson-Hasselbalch equation is why the buffer works well:

$$\mathrm{pH} = \mathrm{pK}_{a} + \log\frac{\ce{[A-]}}{\ce{[HA]}}$$

Now, your ability to neutralize acid is determined by the amount of the conjugate base $\ce{A-}$. Your ability to neutralize base is determined by the amount of the acid $\ce{HA}$. When the ratio between these is 1, you have equal ability to neutralize either. If you're skewed too far one way or the other, then you will not have much of the respective species around to neutralize added acid or base.

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  • $\begingroup$ Shouldn't the ability to neutralize base is determined by by amount of $H^+$.Why have you written "ability to neutralize base is determined by the amount of the acid $HA$"? $\endgroup$ – pranjal verma Jan 20 at 11:08
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    $\begingroup$ Everything is in equilibrium, but you can still only provide as much proton as there is acid. @pranjalverma $\endgroup$ – Zhe Jan 21 at 0:27

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