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Dissociation constants

Dissociation constants

Item no.: P3031101

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  • Scope of Supply
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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.


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


  1. 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.
  2. 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
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Required Accessories
Document     Filesize
p3031101e.pdf Experiment guide, English 398.74 KB

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