Dissociation constants

Principle

The coloured indicator thymol blue  is a weak acid that is partially dissociated in aqueous solution, whereby  non-ionized and ionized forms show absorption maximums at different wavelengths in the visible range. Photometric measurements in the visible spectral range can therefore be used to advantage to determine the postion of the K_a and pK_a values of the indicator which characterize disscociation equilibrium.

Tasks

1. Experimentally determine the extinction (absorbance) of an aqueous solution of thymol blue (thymolsulphonephthalein) in dilute HCl, NaOH and a buffer of known pH values as a function of wavelength between 400 and 700 nm at constant concentration and constant temperature.
2. Calculate the dissociation constant (indicator constant) K_a from the measurement results.
   

What you can learn about

• True and potential electrolytes
• Strong and weak acids
• Law of mass action
• Dissociation constants and pK_a values
• Henderson-Hasselbalch- equation
• UV-visible spectrometry
• Lambert-Beer's Law
• Photometry

Literature for this article als follows

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