Thursday, June 2, 2011

IB Chemistry, Chemistry IA, Kinetics, Iodine Clock Reaction

IB Chemistry, Chemistry IA, Kinetic, Iodine Clock Reaction  
Light Sensor to measure the Rate of Reaction ( Iodine Clock )

Conventional Way ( X cross Method )
  • Time for triiodide/starch to obscure a mark “X”.(subject to lots of Human Error)
Light Sensor Method  (Accurate )
  • Time for triiodide/starch to form and immediately causes drop in light intensity.(Accurate)
Comparative study can be conducted to decide which method is more accurate.

Iodine Clock Introduction.



H2O2 oxidises I- 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)


Reaction Mechanism






Light Intensity MUST drop immediately when blue black colouration forms.
Research Question?
How will changing KI conc affect the rate of Iodine Clock Reaction ( monitored by the sudden drop in light intensity ) over time.

Steps to follow
1.Prepare 2 fold serial dilution 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.
2. Light sensor (0-6000) lux range, 20W bulb  
3. Place cuvette above the sensor
4. Set up light sensor for 300s
5. Add 1ml 3%H2O2 using micropipette into cuvette and press data collection button simultaneously





7. Blue black will cause light intensity to drop
8. Rate measured by time taken for light intensity to drop.
9. Triplicate for each Conc and take average time.






Advantage of light sensor and Tips to make it work
  • Light intensity MUST immediately drop once blue black form
  • Reduce human error ( X method )
  • Micro volume is used, save reagents (total volume 1.5ml )
  • Must use light source above the cuvette
  • Set light sensor to most sensitive range ( able to sense slight change in light intensity)
  • Dont need to calibrate sensor and can start at any light intensity ( we measure only drop in intensity )
  • Sulphur clock may be adopted ( sulphur formation ) but may NOT BE ACCURATE cause sulpur will not block the light well and it will not be a drastic drop in light intensity
Iodine Clock light sensor (accurate if you perform it correctly )


IB students are risk takers, innovators, creative, adopt new techniques for iodine clock instead of X method
Other iodine/sulphur clock that can be measured using light sensor
  • Sulphur Clock ( acid with thiosulphate  for sulphur formation )
  • Iodine Clock between Peroxodisulphite S2O82− and potassium iodide, KI
..........................Next posting on Visible spectrophotometer for Iodine Clock reaction................

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