Practical investigations
Read our general notes on Risk Assessment
Can you suggest what concentrations of peroxodisulphate(VI) ions I should use in my iodine clock experiment to find the order of reaction with respect to peroxodisulphate ions. I've used Salters Chemistry activity EP6.4 for the concentrations with respect to iodide ions, but am confused about working this out. And what volumes of each should I use? Also how would I justify these concentrations using calculations, and working out moles etc. Hope you can help. Thanks.
Igloo writes
I would try using solutions of potassium peroxodisulphate in the region 0.010 to 0.050 mol dm-3. If reactions are too slow try using more concentrated solutions, but you are limited of course by the solubility of potassium peroxodisulphate, which is not high. Also, more concentrated solutions might be more hazardous to use, so remember your risk assessment.
As regards the calculations, these are set out stage by stage on the worksheet EP6.4.
[1] To work out the concentration of iodide ions remember that in each case the solution has been diluted when it forms part of the mixture. For example, in the case of mixture 1, you are using 5 cm3 of KI solution, but since the final volume of mixture is 10 cm3 (as it is for all the other mixtures too), the concentration of iodide ions is (5/10) of the original concentration of iodide, i.e. one-half of the original concentration. You can work out the other concentrations of I- in a similar manner.
[2] As for the number of moles of I2 formed when the blue-black colour appears, you will need to work out the number of moles of thiosulphate used in each experiment. This will be (2/1000) multiplied by the concentration of sodium thiosulphate used. Realizing that one mole of iodine is destroyed by two moles of thiosulphate (I2 + 2Na2S2O3 ---> Na2S4O6 + 2NaI) you must halve this quantity to find the number of moles of iodine produced just before the blue-black colour appears.
[3] The initial rate of reaction must therefore be the value in [2] divided by the time taken in seconds. This will give an answer in mol sec-1
[4] I presume that you can plot the graph (rate against [I-])
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
back to Practical investigations
Rate this page or react
Share your views on this page, 13 ratings so far
, rated at 
updated: 20 January 2006
