I'm currently studying pKa in my chemistry class, and the mathematical definition $(-log(ka))$ doesn't really explain it tangibly to me. Can you please try to simplify what pKa means and how is it useful when studying buffers.


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The notation $pSomething$ is somewhat old and probably dates back to the use of logarithm tables.

The $p$ notation simply means "Take the negative logarithm of what follows the p".

Example: let the acid constant $K_a$ of some (weak) acid be $1\times 10^{-5}$, then the $pK_a$ is:

$$pK_a=-\log K_a=-\log \big(1\times 10^{-5}\big)=5$$

$5$ is of course a more manageable number than $1\times 10^{-5}$.

That is really all there is to it. We can take a $p$ value of anything, like a solubility product, or some equilibrium constant $K$. It is particularly helpful when the $Something$ is really small ($<<1$).

So taking the $p$ value is essentially a way of converting unwieldingly small numbers into something much easier to deal with. Buffers deal with weak acids and bases, so you'll find $p$ values frequently used in that field.

Of course you can convert a $pK$ value back to its $K$ value:


  • $\begingroup$ Well this is not surprising since chemistry/physics always do this kind of things, create a whole new something just to simplify calculations. Thank you though. $\endgroup$ – Abbkey Nov 19 '17 at 21:36
  • $\begingroup$ Yes. Although a bit old, $pK$ values still have their uses. I still use them a lot. Upvote? :-) $\endgroup$ – Gert Nov 19 '17 at 21:44
  • 1
    $\begingroup$ OP doesn't have enough rep. to upvote you, but can accept your answer. $\endgroup$ – Mithoron Nov 19 '17 at 22:04

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