Gracja Kowalska   2IB



  • Thermometer [0-110 ºC]
  • Boiling tubes [10 cm3 ±0.05]
  • Test tubes
  • Potassium iodine solution 1 mol dm-3 [15 cm3]
  • Potassium peroxodisulphate (VI) 0.04 mol dm-3 [10 cm3]
  • Sodium thiosulphate  0.01 mol dm-3 [10 cm3]
  • Starch solution [5 cm3]
  • Stopwatch [±0.5 s]


Eye protection must be worn.

Potassium peroxodisulphate (VI)- harmful and flammable


  1. First you are going to make up reaction mixture 1 from Table no 1, and measure how long it takes for the blue iodine-starch colour to appear.

In each experiment use 2cm3 of sodium thiosulphate, 2cm3 of potassium peroxosulphate. Volume of each solution is 10cm3. To the first mixture add 5 cm3 of potassium iodide solution and do not add water. To second mixture add 4cm3 of potassium iodide solution and 1cm3 of distilled water. In third experiment pour 3cm3, in fourth 2cm3 and in the last one 1cm3 and add appropriate volume of water.

  1. The five mixtures differ only in the concentration of iodide ions. Water is added to keep the total volume of solution constant, so the concentration of everything else is the same in each mixture.

You can place the potassium iodide, sodium thiosulphate, starch and ( for later mixtures ) distilled water straight into a beaker using a measuring cylinders. Measure out potassium peroxodisulphate(VI) solution ( CARE Harmful. Oxidiser ) into a separate container, such as a test tube.

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  1. When you are ready to start the experiment, pour the potassium peroxodisulphate(VI) solution into the mixture in the beaker. Immediately start timing, and carefully stir to ensure that everything is properly mixed. Record the time taken for the blue colour to appear.

  1. Repeat the procedure with mixtures 2 – 5, and record the time taken for the blue colour to appear in each case.


Amount of substances needed for carrying each measurement were recorded in Table 1.

Later on the experiment was carried and the ...

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