Iodide Clock

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How does a catalyst in the form of a metal ion affect the rate of reaction in an iodide clock reaction with potassium peroxydisulphate ions?

Background
When peroxydisulfate ions (S2O82-) react with iodide ions (I-) in the presence of a starch indicator, they produce a dark blue solution. Reaction (1)
S2O82- + 3I-  I3- + 2 SO42-
(peroxydisulfate ion) (iodide ion) (iodine ion) (sulfate ion) To determine the rate law for this reaction a series of changes in the concentration of peroxydisulfate, [S2O82-], and a series of changes in the catalyst while mean the corresponding changes in time, t, needs to be observed and measured. To measure these changes in concentration over time, a chemical "clock" is needed to change the dark blue color back to a colorless solution. This chemical clock is created by adding a series of measured volumes of thiosulfate ion, S2O32- , to Reaction (1). Thiosulfate ions quickly react with iodine ions, I3- , to produce iodide ions. Reaction (2)

I3- + 2 S2O32-  3I- + 2 S4O62-
(iodine ion) (thiosulfate ion) (iodide ion) (tetrathionate ion) This in effect rewinds the clock for another time interval by changing all of the iodine back to iodide. In my reaction I will be using different metal ions to see which the best catalyst for this reaction is. I will also change the concentration of the potassium peroxydisulphate used in the reaction to see whether the catalyst works quicker with higher or lower concentrations of potassium peroxydisulphate.

Apparatus

Burette
Pipettes
2 250ml beakers
Stop clock
Safety Goggles
Lab coat
Scales accurate to 2dp
Thermometer
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