Hi DJ,
In my experience, a new oil should give a more consistent set of results, ie lower relative standard deviation than a used oil. The main reason, if one goes into the details is that the end point is more tightly defined and as the oil degrades so the end point becomes more diffuse, ie occurs over a wider "pH" range. The more tightly defined the end point the easier it is to determine. Consequently, one needs to be careful about trying to apply correction factors. If your RSD on your new oil is greater than 15% then you need to do some root cause analysis. I would check the following:
1) Not getting air bubbles into your titrant delivery tubing
2)Ensure that you are massing enough sample
3)Make sure the oil and solvent are well mixed before the titration begins
4) Ensure electrodes are properly maintained - check the OEM literature on this - you need a big enough mV change that happens fast enough. Many problems arise in potentiometric titrations because of poorly maintained electrodes.
5) Ensure your titrant's concentration is accurately known - concentrations can change over time, depending on how you store your reagents. You can always re-standardise your titrant with a primary standard such as potassium hydrogen phthlate.
Hope this helps,
Paul
