Principle

For weak acids HA, the position of the Ka and
pKa values that characterise the dissociation
equilibrium in the ground state can be determined from photometric
measurements in solutions having different pH values. Further to
this, the pKa* value for the excited state is accessible
from the spectrophotometric data.

Tasks


Record near UV absorption spectra of p-methoxyphenol in dilute
HCl, NaOH and 5 different borate buffer solutions of known pH.

Calculate the pKa and pKa* values for the
system.


What you can learn about


Ground and excitation states of molecules

Electron excitation spectroscopy (UV-VIS spectroscopy)

Spectroscopical energy and adsorption measurement

Jablonski diagram and Foerster cycle

pKa and pKa* values

Henderson-Hasselbalch equation

Van’t Hoff’s reaction isotherms

Gibbs-Helmholtz equation

Lambert-Beer law

Photometry