simple titration question

does anyone know why the Samples should immeidately titrated after they have been withdrawn from the reaction mixture ?

I am guessing you are talking about a titration to find out concentration in order to work out reaction rate.

If I'm right, it is so that no more reaction takes place.

Or, perhaps one of the chemicals can be oxidised by the air and you therefore don't want to leave you mixture sitting around.

Perhaps some more information might help us.