Absorption spectra and pKa values of p-methoxyphenol

Principle

For weak acids HA, the position of the K_a and pK_a 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 pK_a* value for the excited state is accessible from the spectrophotometric data.

Benefits

• Modern spectrophotometer with display
• Spectrophotometer can be used for many other applications
• Detailed operating instructions

Tasks

1. Record near UV absorption spectra of p-methoxyphenol in dilute HCl, NaOH and 5 different borate buffer solutions of known pH.
2. Calculate the pK_a and pK_a* values for the system.

Learning objectives

• Ground and excitation states of molecules
• Electron excitation spectroscopy (UV-VIS spectroscopy)
• Spectroscopical energy and adsorption measurement
• Jablonski diagram and Foerster cycle
• pK_a and pK_a* values
• Henderson-Hasselbalch equation
• Van't Hoff's reaction isotherms
• Gibbs-Helmholtz equation
• Lambert-Beer law
• Photometry