Question

I'm confused when I can use Henderson equation for pH calculation? it seems my professor use it for both calculating ph of titration and for buffer solution too. but can I use this equation for both ??

Answer 1

Hendorson-Hasselbach equation basically gives the pH of a system in which a weak acid/base is in equilibrium with its salt or conjugate acid/base. It will get clear with this example.

Let us suppose you want to studying a titration of CH₃COOH versus NaOH. CH₃COOH is a weak acid whereas NaOH is a strong base. CH₃COOH + NaOH   CH₃COONa + H₂O.

As you add NaOH to the acetic acid solution, the above reaction shows that it ends up forming the conjugate base of acetic acid CH₃COONa. There exists an equilibrium between CH₃COOH and CH₃COONa. This is essentially called a buffer as it resists small changes to pH that may occur upon addition of an acid or base. The range is called buffer capacity. When NaOH completely neutralizes CH₃COOH, the titration is complete but also there is a buffer existing between CH₃COOH and CH₃COONa. The pH calculated at this point would be the pH for titration as well as that of buffer solution. Thus, depending on your system, Henderson-Hasselbach equation can calculate pH of titration as well as buffer.

pH = pKa + log([CH₃COONa]/[CH₃COOH])

is the Henderson-Hasselbach for the buffer between acetic acid and its salt. Ka is dissociation constant of acetic acid.