I have a problem with TAN analysis of resistive oil. When I am titrating one and the same oil sample with KOH I got 3 results-2 normal and in the 3 try I noticed something very strange- in the middle of the titration the pH stop to increase. I add more then 3 ml KOH and pH changed with pess then 1. I try another run with the same sample and it was OK. Did anyone know the reason for such a behavior? And is there any way to change this?