If this is the iodine clock experiment, as it appears to be, we did a similar one in class. Needless to say, it was a huge failure for our entire class. Our reactions were supposed to be first order, but they looked like second order when graphed. Which would completely throw off all calculations. I thought it was a horrible experiment as nothing played out as it should have. Sorry I didn't help, but I'm just as lost as I could possibly be on the whole thing.