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    hi! i came across a section in my chemistry notes concerning "iodine-thiosulfate titrations" and i think it must be important cause it came out for my CTs =(. I don't understand most of what the notes say, and searches on the internet have proved futile.
    what i do gather is that:
    1)iodine is very volatile and in pure form, that is, I2, it will evaporate into the air,
    2)it is a back titration
    3)it involves the reaction of iodine and "a few drops of starch"

    Does anyone know more about this titration? i need help on the following questions:
    1) what is the purpose of this titration?
    2) why is starch added?

    thanks!
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    This titration is used to find the concentration of oxidising agents. Using the ionic equations of thiosulphate and knowing the oxidation numbers of the "to be tested" oxidising agent (O.A.), you can derive the concentration of the O.A. Starch is added near the end point of the titration because starch turns black in the presence of iodine (it is very sensitive to iodine presence) and since iodine is faint yellow and it is difficult to see the end point when pale yellow becomes white. Therefore starch is added (just before the end point for accuracy reasons which i can explain if u need them) so that a black colour forms and it is easier to determine the end point more correctly as black and white are rather different colours!

    hope this helped
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    thank you! yes, this helped quite a lot! =)
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    (Original post by anonymousboss)
    This titration is used to find the concentration of oxidising agents. Using the ionic equations of thiosulphate and knowing the oxidation numbers of the "to be tested" oxidising agent (O.A.), you can derive the concentration of the O.A. Starch is added near the end point of the titration because starch turns black in the presence of iodine (it is very sensitive to iodine presence) and since iodine is faint yellow and it is difficult to see the end point when pale yellow becomes white. Therefore starch is added (just before the end point for accuracy reasons which i can explain if u need them) so that a black colour forms and it is easier to determine the end point more correctly as black and white are rather different colours!

    hope this helped
    i also have a bit of a problem with this topic, you answer has helped a lot but it has also raised another question for me, how is the concentration of the O.A. found from the oxidation number because i can see how it is related?
 
 
 
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