Saturday, February 18, 2012

IB Chemistry IA on Kinetics, Rate of reaction, Iodine clock reaction with H2O2 and peroxodisulphate measured using visible spectrophotometer

IB Chemistry, Chemistry IA, Kinetic, Iodine Clock, Visible Spectrophotometer
Conventional Way ( X cross Method )
  • Time for triiodide/starch to obscure a mark “X”.(subject to lots of Human Error)
Visible Spectrophotometer (Change in Absorbance at 608.7nm)
  • Time for triiodide/starch to form, causing an increase in absorbance.
Comparative study on both Methods can be done by students
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Iodine Clock Introduction.

H2O2 oxidises  into I3 and I3 will react with starch to form blue black.
H2O2 + 3 I  + 2 H+ → I3 + 2 H2O.
Triiodide formed will be reconverted to iodide by the thiosulfate. (Delaying mechanism) .
I3(aq) + 2 S2O32−(aq) → 3 I(aq) + S4O62−(aq)


Absorbance MUST increase immediately when blue black colouration forms.
Research Question?
How changing KI conc affect the rate ( by the sudden increase in absorbance ) over time.
Steps to follow
1. Prepare 2 fold serial dil for 0.1M KI
2. Add (0.5ml, 0.1M S2O3) , (0.5ml, 0.5M KI), (0.1ml 1% starch), (0.1ml, 0.1M HCI) into a cuvette.
3. Set visible spec at 608.7nm
4. Set data collection to 300s (Abs vs time)
5. Place cuvette containing solution in step 2
6. Add 1ml of 3%H2O2 and press collect button
7. Take time for absorbance to increase
8. See 2min video tutorial for clarification


Data done by IB students
Rate is measured by time it takes for an increase in absorbance using different KI Conc




Video, Iodine clock reaction using Visible Spectrophotometer

Advantage of Visible Spec and Tips to make it work
  • MUST produce a sharp increase in absorbance once blue black form
  • Reduce human error ( X method )
  • Micro volume is used, save reagents (total volume 2.5ml )
  • DO NOT NEED TO CALIBRATE but ensure wavelength at 608.7nm
  • MUST add H2O2 into cuvette and start data collection simultaneously
  • Sulphur clock may be adopted ( sulphur formation ) but may NOT BE ACCURATE cause sulpur formation may not produce a sharp increase in absorbance
Sample report of iodine clock using visible spectrophotometer

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Iodine Clock Reaction, Peroxodisulphate S2O82− and Potassium iodide, KI
Iodine is generated:
2I + S2O82− → I + 2SO42−
I + 2S2O32− → 2I + S4O62−
Same Method but using S2O82− as oxidising agent not H2O2
Steps to Follow
1. Prepare 2 fold serial dilution for 0.1M KI
2. Add 0.5ml 0.1M KI, 50ul 1% starch, 50ul 0.02M S2O3 into cuvette
3. Set visible spec at 608.7nm
4. Set data collection to 300s (Abs vs time)
5. Place cuvette containing solution in step 2
6. Add 0.5ml of 0.05M S2O82-and press collect 
7. Take time for absorbance to increase
8. See 2min video tutorial for clarification

IB Student data, Iodine clock with different conc KI
  • Visible spectrophotometric is accurate if it shows a sharp increase in absorbance
  • H2O2 is better compared S2O82-
  • Comparative study between X cross method, visible and light sensor can be investigate and error analysis can be done


Video, Iodine Clock using visible spectrophotometer
Sample report on iodine clock reaction using visible spectrophotometer

Thanks to all picture and video contributors for the above post

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