In my experiment, the concentrations stays the same in the mixture,which was made up of acetone, sulphuric acid and iodone. The mixture was titrated against sodium thiosulphate.

Aliquots were withdrawn every 7 minutes. We withdrew six aliquots.

The endpoint or the reaction between the iodine present and the thiosulphate,starch indicator, occurred more quick and more quick after each aliquot.

The concentrations stays the same and nothing is heated to cause the reaction to react more quick, so my question is howcome does the reaction react more quick the longer the mixture stands?:(

Well, without thinking about this too much, is there maybe a secondary reaction going on that is forming a catalyst?

If you could provide chemical equations for the reactions that are going on, that would help me to visualise what is going on.
It's been a long time since I've done chemistry :)