Frequently Asked Questions
When does buffer pH equal the pKa?
The pH equals the pK<sub>a</sub> when the conjugate base and weak acid concentrations are equal, because log<sub>10</sub>(1) = 0. This is also the point of maximum buffer capacity.
Do the concentration units matter?
No. Only the ratio of conjugate base to weak acid enters the equation, so you can use any consistent units as long as both concentrations are measured the same way.
What does a base-to-acid ratio of 10 do to the pH?
A tenfold excess of conjugate base over weak acid adds log<sub>10</sub>(10) = 1 to the pH, so the buffer sits one pH unit above its pK<sub>a</sub>. A tenfold excess of acid instead subtracts 1, putting the pH one unit below the pK<sub>a</sub>.
How do I find the pKa or the base-to-acid ratio needed to hit a target pH?
Rearrange the equation. For the ratio, [A<sup>-</sup>]/[HA] = 10<sup>(pH − pK<sub>a</sub>)</sup>. To make an acetic acid buffer (pK<sub>a</sub> 4.74) at a target pH of 5.00, the ratio is 10<sup>(5.00 − 4.74)</sup> = 10<sup>0.26</sup> ≈ 1.8, so about 1.8 parts acetate to 1 part acetic acid. To solve instead for the acid you need, pick one whose pK<sub>a</sub> is within one unit of the target pH, then set the ratio. And if you know the pH and the ratio, pK<sub>a</sub> = pH − log<sub>10</sub>([A<sup>-</sup>]/[HA]).
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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.