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

Scope of Supply
Device NameItem no.Qty
Cell for spectrophotometer, optical glass35664-002,00
Ethyl alcohol, absolute 50030008-501,00
Thermometer -10...+50 °C38034-001,00
Volumetric pipette, 1 ml36575-001,00
Microspoon, steel33393-001,00
Hydrochloric acid,0.1M 100048452-701,00
Pasteur pipettes, 250 pcs36590-001,00
Beaker, DURAN®, tall form, 150 ml36003-003,00
Buffer solution, pH 9 100030289-701,00
Rubber caps, 10 pcs39275-031,00
Volumetric pipette, 5 ml36577-001,00
Wash bottle, plastic, 500 ml33931-001,00
Shimadzu UV-1280 Spectrophotometer 190-1100 nm35655-991,00
Volumetric flask, Borosilicate, 50 ml, IGJ12/2136547-003,00
Weighing dishes, square shape, 84 x 84 x 24 mm, 25 pcs.45019-251,00
Volumetric pipette, 10 ml36578-001,00
Funnel, glass, top dia. 50 mm34457-001,00
Graduated cylinder, Borosilicate, 250 ml36630-001,00
Volumetric flask 1000ml, IGJ24/2936552-003,00
Thymol blue indicator31896-021,00
Caustic soda sol.,0.1M 100048328-701,00
Pipette dish36589-001,00
Water, distilled 5 l31246-811,00
Downloads and Documents

Document  Filesize
p3031101e.pdfExperiment guide, English398.74 KB