Sulphuric Acid and Calcium Carbonate reaction
>Subject: Bromate + iodide; mechanism?
>Date: Sun, 23 Feb 1997 19:18:57 GMT
>I'm an A-level student carrying out a project to determine the mechanism of
>the reaction between a bromate and an iodide, specificly potassium bromate
>and potassium iodide.
>The process I've been instucted to carry out for this is to see how rate is
>affected by bromate concentration by:
>Potassium bromate+potassium iodide+hydrochloric acid+water+starch+sodium
>This gives a blue colour due to the formation of a tunnel clathrate between
>the starch and the liberated iodine. I also have to carry out a second
>similar series of experiments to see how the concentration of the iodide
>affects the rate.
>So far so good.
>But... Things aren't working well, and to find out what's going wrong it'll
>be helpful to know what the mechanism is before I determine it (if you
>Unfortunately I can't find a decent book in either of my local libraries to
>help, so can any kind soul out there help me?
>Much appreciation, and thanks in advance.
> Spangly and thing...
KBrO3 + 6KI + 6HCl ---> 6KCl + KBr + 3I2 + 3 H2O
I2 + 2Na2S2O3 ---> 2NaI + Na2S2O6
then excess of I2 gives the blue color.
If you experience troubles, please check that the acidity in the system is not
too high, because that can cause Na2S2O3 to decompose (SO2 + S), and also not
too low, as you can see the reaction requires acidic conditions. You must have
excess of iodide in the system, because the formation of tunnel clathrate is
assisted by iodide. If you don't have enough iodide in the system, the
sensitivity of this color reaction will decrease by over 100 fold, which will
give a faulse impression of "slow" reaction.