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 position of the Ka and pKa values of the indicator which characterize dissociation equilibrium.

Tasks


Experimentally determine the extinction (absorbance) of an aqueous solution of thymol blue (thymolsulphonephthalein) in dilute HCl, NaOH and a buffer of known pH value as a function of wavelength between 400 and 700 nm at constant concentration and constant temperature.

Calculate the dissociation constant (indicator constant) Ka from the measurement results.


What you can learn about


True and potential electrolytes

Strong and weak acids

Law of mass action

Dissociation constants and pKa values

Henderson-Hasselbalch- Equation

UV-vis spectrometry

Lambert-Beer’s Law

Photometry


Necessary accessories


Precision balance 620 g / 0,001 g